TunnelTalk Special Edition: UK Focus and Case Studies

Direct by Design

SPECIAL EDITION FOCUS: UK CASE STUDIES

BRENNER BASE TUNNEL, AUSTRIA – ITALY

PUSHING LIMITS WITH VIGOROUS ENGINEERING The new brenner base tunnel represents a next level of trailblazing hard-rock-tunnelling. With a total length of 64 km and maximum overburden of 1,300 meters this high-impact project sets new records.

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Contractors: 1 x Gripper TBM: Consortium H33 Tulfes-Pfons 3 x Double Shield TBM: Consortium Mules 2-3 2 x Single Shield TBM: Consortium H51 Pfons-Brenner

TUNNELTALK SPECIAL EDITION UK FOCUS AND CASE STUDIES FROM THE EDITOR’S DESK

CONTENTS

THE PAST LEADING THE FUTURE

TUNNELTECH

B

5 Intricate excavation upgrades London metro station 7 Thames Tideway – Navigating the challenges

methods and technologies, many imported from Europe and

10 Underground communications, safety, productivity

ritain has long been considered the founder of the art and science of soft ground excavation and tunnelling, a lead

that continues today in many respects. The application of new further afield, keeps current projects in the UK at the highest of international standards. From TBM tunnelling on the Tideway

11 TBM main bearing grease with CSR credentials

CSO project and the mega-TBM drives for the Silvertown and Lower Thames Crossing under river highways, to the TBM drives and excavation of large underground spaces for the Crossrail, HS2 and London Underground station upgrade projects, the UK continues to push the frontiers of the industry

UK CASE STUDIES 13 First NATM contract in Britain guarantees watertight drifts

and its contractual and management structures.

Arriving at this point has been a journey across the years and decades, perhaps the most significant decade being the

17 Caution key to Cornish road tunnel 18 Penmaenbach: hand mining saves rock tunnel drive

ten years between mid-1980 and mid-1990. It was in these years that major projects were undertaken and some very hard lessons were learned towards making the advances that have established the norms of today.

As a new recruit journalist to the industry in 1982, I visited and reported on the most significant projects and developments of the time, including the Channel Tunnel, the London Water Ring Main, the Heathrow Express Rail Link, and the London Underground Jubilee Line Extension. A revisit of those articles and projects as case studies in this special UK focus edition of TunnelTalk indicates how the experiences shaped the future we are living today.

have

progressed

diligently,

23 Stansted: Tunnel taxies under airport runway 26 West Runton long sea outfall 29 Geared up for London Ring Main drive and beyond 30 London Water Ring Main feeder 33 UK development of a vacuum segment erector 34 Heathrow baggage tunnel

Like all visits to the past, there are examples of how developments

20 Carsington Aqueduct: Man against nature

including

the

introduction of steel fibre reinforcement for shotcrete and concrete segments, the advance of closed-face pressurised TBMs and greater attention to settlement and water ingress control, as

37 Fullfacer vs roadheader: an investment in speed 39 Killingholme: Britain’s first underground liquefied gas store 42 Heathrow rises from the mire and shines

well as tremendous improvements in health and safety and occupational wellbeing of the underground industry workforce.

There are other lessons that could yet be fully appreciated. n Shani Wallis Executive Editor

Direct by Design www.TunnelTalk.com Order copies of the 2010-2020 issues of the Annual Review online

47 Heathrow failures highlight NATM (abuse?) misunderstandings 51 Jubilee Line Extension: Triumphs and tribulations 58 Channel Tunnel in hindsight

Copyright© TunnelTalk 2021. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic or otherwise, without the prior permission of the copyright owner. All views expressed in this journal are those of the respective contributors and are not necessarily the opinions of the publisher, neither do the publishers endorse any of the claims made in the articles or the advertisements.

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Underground spaces created by SCL ready for construction of the inner architectural and M&E finishes

Intricate excavation upgrades London metro station • Removal of load bearing foundation piles of four high-profile stakeholder buildings; • Development of a sprayed concrete radial joint, without connecting reinforcement, to eliminate the need of workers entering the face; • Optimised SCL lining thicknesses to reduce the risk of fallouts and improve construction cycles; • Change of proposed hand mining operations to sprayed concrete linings for improved workability; • Application of 3D ground modelling; and • Adoption of a risk-based approach to design and construction.

During excavation, the exterior of several existing London Underground assets were exposed. No internal propping within existing assets was required.

The Option A SCL sprayed radial joint between the top heading and bench excavations eliminates the requirement

Sebastian Kumpfmueller, Senior SCL Engineer & Senior Associate, Dr Sauer & Partners Donal Kelly, Tunnel Manager, Dragados SA UK & Ireland for the installation of reinforcement bars.

The joint is formed fully using mechanised means, eliminating the need for the workforce to access close to the face and avoiding the need for a temporary SCL sealing layer of the face. The stepped SCL lining application used the robotic concrete spraying equipment with the excavator breaking out the SCL lining toe for the joint preparation and cleaning (Fig 2). This innovation is a significant advancement for the SCL tunnelling industry by making the works considerably safer.

The 19m2 heading of the new Northern line running tunnel progressed as a full-

T U N N E LT E C H : N AT M / S C L

U

pgrade of interchange Bank Station on the London Underground network in the UK has been an exercise in intricate design and construction of underground structures in the heart of the financial centre of the city and within a complex of existing underground infrastructure, all of which had to remain in operation throughout the works.

In a successful collaboration between Dr Sauer & Partners, Dragados and the London Underground project management team, the upgrade project began with award of the £642 million design-build contract in July 2013 and had progressed to substantial completion of the civil works in July 2020. With the aim of boosting capacity of the underground station by 40%, the new structures include a new Northern line platform and running tunnel to convert the existing centre-island platform to two separate platform tunnels.

To complete the complex SCL (sprayed concrete lining) works, several technical achievements to improve constructability and protect the workers were developed and implemented successfully. These have included: • Uphill and downhill excavation of 30 degree inclined escalator barrels in a top heading and combined bench/invert sequence;

Fig 1. Plan (left) and 3D image of working site and access shaft

SCL advances through London Clay

Fig 2. Design of a fully mechanised joint between top heading and bench eliminating need of workers in the face

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From left: Working from the SCL canopy between exposed operating platform tunnels; Unavoidable uphill excavation of escalator 4; Downhill excavation of escalator 2 face excavation on 1.2m advances. It was a pilot heading for the platform tunnel and was enlarged to full platform cross section of 10.5m wide at spring line x 9.6m high.

Load transfer of piles

During excavations, four 20m deep x 1.8m diameter end bearing foundation piles supporting an existing 1970s office building required interception.

A load transfer structure was designed to transfer the load of up to 10MN of each cut foundation pile to an arched reinforced concrete structure bridging over the new tunnel to deliver the loads to a reinforced concrete invert and diffused to the ground.

A set of intricate excavations was required to create connecting adits to existing and operational parts of the station. The preliminary design of handmined timber headings was redesigned to a sprayed concrete lined access tunnel and canopy. During excavation,

the narrow space between operating platform tunnels was opened up, with the lean squareworks ground support system maximising the space between the back of the platform linings. Movements to both platform structures during adit excavation were as predicted and below 40mm on the diameter.

Escalator barrel excavations

Escalator barrels of about 7m x 6m high and on a 30 degrees decline progressed top down in a sequence of top headings and combined bench/inverts. Excavators and spraying machines were hooked to a winch at the top and personnel wore harnesses to ensure health and safety. The Option A sprayed radial joint was especially beneficial on the escalator tunnels.

Escalator 4 required uphill excavation between the existing and operational Central line platform tunnel assets and with no access from the top. Close collaboration between

design and construction was required to complete excavation successfully.

The construction works at Bank posed unprecedented risks and pushed the limitations of tunnelling equipment and the use of the SCL technique. In the end, five different types of excavators and four different types of SCL spraying robots were required. Due to the complexity of the works, the excavation advances were drawn as graphical images for the workers.

Following primary supported excavation the underground structures were finished with either a spray applied or a plastic sheet waterproofing membrane and steel fibre reinforced sprayed or cast concrete secondary linings.

In July 2020, after approximately 1,200m of SCL excavation, 1,840 advances, 1,195 daily review meetings and 995 required excavation and support (RES) sheets, the SCL excavation works for the Bank Station upgrade were successfully completed. n

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26/01/2017 19:23

Thames Tideway – Navigating the challenges The new Tideway interceptor sewer currently in construction and running beneath the River Thames in London is designed as a gravity system, falling on a 1:790 gradient across its 25km route from Acton in the west to the Beckton treatment works in the east (Fig 1). After approval of the scheme in 2014 and a start of construction in 2016, excavation of the 7.2m i.d. 25km alignment by four TBMs commenced in 2018. VMT provided the vital systems needed to keep the West contact TBM on accurate gravity flow line and level.

T

TBM engineer using the total station joystick function on the TUnIS.mobile app to determine a new backsight prism for the moving station Fig 1. The 25km long Tideway project will be a gravity operated system from Acton in the west to Beckton in the east

unnelling on four main TBM drives for the Thames Tideway supersewer in London has progressed 24hr/day to erect a precast primary segmental lining within which a secondary in-situ concrete lining will be cast to ensure water-tightness and contain high internal pressures when the combined sanitary and stormwater system is running full.

From a depth of 30m below the surface at Acton to more than 60m deep at the reception shaft at Beckton, precise line and level accuracy to within millimetres is vital to the success of the gravity-operated system over the whole project.

T U N N E LT E C H : T B M N AV I G AT I O N

VMT News Release

Thames Tideway West

As part of the tunnelling works, Herrenknecht supplied an 8.1m o.d.

EPBM to the BMB JV for the 6.9km long West contract C405 running from Acton to Wandsworth. Specialist guidance system designer and manufacturer VMT (based in Bruchsal, Germany) provided several tunnelling support systems to guide and aid the TBM on its high-precision journey.

With the VMT systems on board, the TBM launched on the 6.9km long west section on 26 April 2019, with breakthrough on 24 September 2020.

Supporting and simplifying the dayto-day activities of the surveyor and TBM engineers was the latest development of the VMT navigation range. The TUnIS. moving station determines and calculates all data and information that is necessary

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T U N N E LT E C H : T B M N AV I G AT I O N

Mounted total station inside the laser window for a fixed distance to the video target with a solid bracket for the AD12 on the gantry essential for easy use of the moving station; Hard-wired cables at the moving station with an easy switch back to the standard TBM laser system, with the total station mounted on the tunnel wall for navigating the TBM along a tunnel axis. The high information content of the data displayed ensures optimum control of the machine position, thus helping to maintain a uniform shield run with minimal deviations from the desired route. The position and tendencies are continuously displayed to the shield operator, allowing vertical or horizontal curves to be easily and precisely controlled.

Three prisms installed on the tunnel

wall with an active electronic laser target in the shield of the TBM are used for exact determination of position. In the first step, the exact current position of the total station is determined using the prisms. In the second step, the current position of the laser target is determined, thus defining the position of the TBM to within millimetres.

This new system undertook its earliest and very effective trials on the London

Tideway drive, in parallel with the VMT initial primary navigation system. So successful were the trials, the full tunnelling operation was moved onto the new TUnIS.moving station system, replacing the primary VMT standard navigation system after 560m of the advance.

Also tested on the site was the new SLuM.Connect hardware central box, as part of SLuM, the existing automatic tailskin clearance measurement system

The package of equipment and software supplied by VMT for the guidance operations included the following systems: TUnIS Navigation TBMLaser As the initial primary navigation system, TUnIS Navigation TBMLaser can be used on EPB, slurry shields and hard rock TBMs. It is based on the use of a total station and a target unit installed within the TBM shield to determine the current advance position of the TBM. When using a small or limited laser window, the TBM position is continuously calculated during short-term interruptions. The system offers high reliability as it provides temporary navigation through the thrust cylinders of the TBM.

TUnIS.moving station This newly introduced assistance system enhances a laser and target-based navigation system in large diameter TBMs. With TUnIS. moving station the total station is not mounted on the tunnel wall, but travels on the gantry of the TBM.

TUnIS Office Providing real-time navigation and ring data from one or more TBMs, the TUnIS Office system allows the operations to be monitored directly from the site office. The user may immediately analyse and document current and historical data, and all persons involved in the project may track the tunnelling process on the machine, in the site office, through web server, or on mobile terminals. Several users may simultaneously access the entire functionality of the navigation system in real-time without affecting the construction process.

VDMS.Process This web-based data management system is developed by VMT to handle complex construction projects. The system processes and correlates data from a variety of different sources, analyses them and displays them in user-friendly form so that they can be made available to project managers worldwide in a wide range of output forms. VDMS ensures full information transparency and supports the efficient control of both the TBM and many other operating processes within the tunnelling project.

Setup of the TUnIS Navigation TBMLaser system and presentation of the data on the user’s screen

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Normal prism mounted on tunnel wall (left) and glued back sight prism (right), for when it is difficult or not permitted to drill into segments; TUnIS.moving station uses three prisms for the free stationing and can work with a minimum of two prisms used for precise ring build on TBM drives erecting precast concrete segmental linings. To determine optimum installation of each new ring, it is necessary to observe the current position and orientation of the TBM, plus the position of the last ring built. SLuM enhances the ring build process by measuring the reference ring position relative to tailskin axis with millimetre accuracy. The TUnIS.mobile app was also widely used

by West contract site engineers and surveyors, allowing remote operation of

the navigation system from outside the TBM control cabin.

Roy Behrend, Chief Surveyor Tunnel for the BMB-JV said: “Communication was really good with VMT and their site engineers for the testing and implementation of the new TUnIS.moving station system. There was always immediate ongoing support for the VMT navigation systems and the TUnIS software. The TUnIS software package is well developed and its menu systems allow for ease of use. VMT Support

Engineers also had remote access to the system, which allowed for easy system maintenance and testing. I would not hesitate to recommend or use VMT products again.”

With the last of the four main TBMs still in action on the eastern most drive to Beckton and the other three drives complete, the plan is to have all excavation works finished in 2023 and all testing and commissioning works finalised for the sewer to go into operation by 2025. n

Smooth Operator Operate the total station from the palm of your hand and on safe ground with TUnIS.mobile app.

For accelerated workflow and increased safety. Available for smartphones, tablets, iOS and Android.

www.vmt-innovations.com

The Strata mission is to improve safety in underground works; Strata safety refuge chambers available in different types, sizes and configurations

Underground communications, safety, productivity

T U N N E LT E C H : U N D E R G R O U N D S A F E T Y

T

he advent of Strata Tunnelling as a new division of Strata Worldwide is not a new one but rather an evolving one. With 29 years of experience, the company is a knowledgeable partner for safety and productivity in underground working environments.

With Strata Tunnelling, the organization takes a deliberate and strategic step forward within the underground construction industry, with investment and commitment to this community of infrastructure owners, design and construction engineers and the contractors who build the shafts, tunnels, caverns and diverse underground spaces that serve society.

The mission of Strata Tunnelling is to provide advanced products and technologies that are designed to keep working environments in the tunnelling and underground construction industry safe and productive. This is underpinned by committed service on the job and a significant number of hands-on, underground experienced people in its workforce, armed with knowledge and expertise to apply its technologies safely and efficiently at the job site.

In this way, Strata Worldwide is a purveyor of advanced technologies, both internally manufactured and thirdparty sourced, with the ability to train and support contractors who bid and build underground works. The company centers of activity are built around three categories: • StrataProtect™ • StrataConnect™ • StrataGeotech™ At the core of StrataProtect is the safety of people in the underground environment. The line of Strata emergency refuge chambers offers a choice of portable steel-sided or inflatable units or

in-situ safe rooms if required by a project.

The proprietary HazardAvert system, which is designed for vehicle proximity detection and collision avoidance, is built on an electromagnetic platform providing Level 9 capability.

The Strata line of emergency and rescue breathing apparatus includes the Biomarine range of state-of-the-art self-rescuers and Trolex gas monitoring equipment. Strata remains the ideal technology partner for safeguarding underground personnel.

StrataConnect comprises a variety of tools and functionality that brings state-of-the-art surface communication to the underground environment. The range of equipment encompasses voice communication, personnel tracking, data transmission and overall connectability.

The Strata networking options cover a variety of features that permit integration into existing networks to expand coverage and capabilities, including digitization of analogue systems. These technologies facilitate real-time telemetry and automation.

As a network backbone, StrataConnect also offers seamless coordination with the StrataProtect offerings.

StrataGeotech is centered on the strata encountered in the underground working environment and includes a variety of durable and non-durable technologies for areal support of excavations with shotcrete, point support with hydraulic prop elements, modification of the groundmass properties to enhance stand-up time or fill voids, and control of groundwater either in groundmass or in the excavation work areas. These systems are supported by our specialist partners GCP Applied Technologies, US Concrete Products and RE-Systems Group Americas.

Strata Tunnelling News Release Through an established international network of qualified distributors, Strata Worldwide has a presence in USA domestic and in countries of the world. The organization is committed to expanding its presence in international civil underground projects and is actively seeking interested parties who have the vision and credentials to join it on the journey.

Vice President of Strata Tunnelling, Mike Rispin, is a seasoned underground professional engineer of 38 years with experience and expertise in ground control, construction chemicals, explosives and blasting, heavy underground mobile equipment, mechanized tunnelling and the drill+blast excavation cycle. He has been committed over the years to the International Tunnelling and Underground Space Association (ITA) and is currently Vice Chair of the Underground Construction Association (UCA) in the USA, becoming Chair for a two-year term in June 2021.

Strata Worldwide is a Supporting Member of the ITA is a corporate member of the: • UCA of SME Underground Construction Association in the USA • Tunnelling Association of Canada • Australian Tunnelling Society The company will continue to participate in and expand its support of such organizations consistent with the growth of its global footprint.

Strata Worldwide is committed to being a part of the international civil tunnelling community and its representatives look forward to the opportunity to work with you in addressing the challenges of individual projects within the industry. n

ADVERTORIAL

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In 2018, CONDAT advanced its CSR policy by joining the international ECOVADIS platform, making available its performance for annual assessment annually by an independent body according to 21 criteria divided into four themes: • • • •

Environment Human resources and human rights Business ethics Responsible purchasing policies

For 2020, CONDAT was awarded the ECOVADIS Silver level, recognizing its far-reaching sustainability and CSR policy. This achievement places CONDAT among the 25% best rated global companies by ECOVADIS and among the top 6% of companies in the lubricants economic sector.

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Email:

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T U N N E LT E C H : T B M S U P P L I E S

A

s a leader in the development and supply of industrial lubricants, CONDAT of France shares a conviction of continued innovation to respond to contemporary challenges including climate change, diminishing resources and environmental pollution. Supporting customers and partners towards responsible performance to combine social and environmental issue, corporate social responsibility (CSR) is a main objective for the company.

Initiated several decades ago, this responsible commitment is one of the fundamental values of the CONDAT Group.

Founded in 1854 and guided by this historical value, the company launched its first biodegradable lubricants 20 years ago and continues to pursue a long-term strategy to develop products with a reduced environmental impact, without compromising either their performance or the safety of users.

For the tunneling industry the new BTG 4602 grease is designed specifically for the protection of the main bearing and the lubrication of the associated seals in TBMs.

Self-scoring eco-designed system To provide customers better readability and total transparency on its products, CONDAT established in 2020 the innovative Lubriscore® self-scoring system that weights its product properties based on the life cycle criteria: • Raw materials and design

• Production • Transport

• Use • End of life

This rating includes impacts on people and the environment and takes into consideration the state of the knowledge at the time of the assessment and changes to regulations. A bonus is applied for compliance with a label or certification, as, for example, for an ECOLABEL certified product.

Based on a point system and on the answers given to about 20 questions, the Lubriscore® grants, or not, the product with an associated number of stars. The more eco-designed a lubricant, the higher the number of stars, the range being from 0 to 3 stars. By choosing a product with at least one star, you benefit from an eco-designed alternative!

The Lubriscore® rating is available for a wide range of CONDAT lubricants, including tunnelling products, hydraulic oils, metalworking fluids, wire drawing soaps, die lubricants, cooling lubricants, hardening/quenching fluids and maintenance lubricants.

Developed in 2018, the BTG 4602 main bearing grease is a 2 stars product. It is formulated from high viscosity synthetic esters and lithium/calcium soap, to provide an high-rated resistance to The new BTG 4602 main bearing grease water washout and will give an efficient barrier to water/soil ingress. The polarity of the esters helps meets high CSR credentials the creation of a uniform and resistant lubricating film protecting the seals from premature wear.

The specific anti-wear and extreme pressure performance will offer protection even if the load suddenly increases. The high viscosity index of the esters ensures that the viscosity of the oil remains stable even when temperature varies, providing a consistent and reliable protection.

BTG 4602 is readily biodegradable, meeting the OECD 301B test criteria, and is formulated without molybdenum. As such it will not negatively impact the excavated soils and will allow their reprocessing. BTG 4602 is self-extinguishing and will not propagate flame in case of a fire outbreak, ensuring safety of the operators and protection of the equipment. n

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Watch video

CASE STUDY: NATM had a chequered introduction to the UK in the 1980s and 1990s. After rejection of the method for the Saltash road tunnel in Devon, despite a successful trial of the concept at the site ahead of contract award, to the calamitous collapse of station works at Heathrow for the express rail link in 1994 (p42), this first project application of the construction method and the design and contract management concepts in the UK at Barrow-upon-Soar in the mid 1980s demonstrates the advantages with the twin drifts still performing to specification for the continuing operation of the gypsum mine today. The technique continues to be applied with great success in the UK (p5) albeit under the name SCL (sprayed concrete lining) and with much of its on-site flexibility overtaken by prescribed design requirements now specified in detail in the contract documents.

SITE VISIT REPORT: SEPTEMBER 1987

First NATM contract in Britain

guarantees watertight drifts limestones bounded by impermeable Lias and Rhaetic mudstones above and below (Fig 1).

The local strata varies from river bed and glacial deposits at the surface to moderately strong to strong Keuper Marls immediately overlying the strong, massive gypsum and anhydrite horizon. Water would, with time, degrade these formations and cause problems for a mechanised mining operation. The required lining would not only have to cope with variable, if sometimes relatively low, overburden pressures, but also prevent water ingress and withstand the consequential build up of substantial water pressures.

Like most mining operations in the UK, British Gypsum’s own mine workers will take over operation of the mine after green fieldsite development by an independent contractor and in mid-1984, British Gypsum invited bids from six well known British tunnelling contractors.

Instead of the more usual constructto-­ the-client’s-design type tender, British

Fig 1. The first 300m of each drift passes through waterbearing strata

Twin drift access to British Gypsum’s new mine at Barrow-upon-Soar

• Client: British Gypsum • Consultant: Dr Gerhard Sauer • Contractor: Thyssen (GB) in JV

with Östu of Austria Gypsum decided to let this particular job as a design-and-construct contract. “We considered that this would be the most cost-effective method of completing the work,” explained Walter Deacon, Deputy Mineral Resources Manager for British Gypsum. “We believe that a design-andconstruct contract encourages a closer co-operation between contractor and client.

Both are more appreciative of each other’s aims and as such, we believe that frivolous claim situations are less likely,” he said.

When preparing its tender, each contractor had to consider four fundamental aspects; ground treatment for the control of water ingress during construction; drift excavation; final lining; and a water ingress guarantee of no more than 25 litre/min in each of the completed drifts as specified by the client.

Tenderers proposed a variety of different methods of dealing with these matters. For example, ground freezing, deep well dewatering and grout injection either from the surface or as a rolling operation from the tunnel face were some of the methods proposed for ground water control during construction. Drill+blast and mechanical diggers in shields were excavation alternatives with either mass concrete or bolted concrete segments for the lining.

The water ingress guarantee presented the greatest challenge with some contractors unable to comply and others making unacceptable qualifications.

“It was also necessary to devise the most efficient means of integrating ground treatment, excavation methods and lining design to ensure funds and resources would be deployed only according to necessity,” said Deacon.

Thyssen (GB) in JV with its sister company Östu of Austria offered the only NATM alternative. The proposal was to excavate the tunnel by roadheader, support the tunnel with arches and shotcrete and

UK FOCUS CASE STUDY

F

or existing gypsum mines, the struggle against water ingress lasts the lifetime of the mine and represents a large proportion of the mine’s operating costs.

When British Gypsum, the largest gypsum mining organisation in the UK, decided to develop a new mine at Barrow­upon-Soar, controlling ground water ingress was a top priority. Incorporating new methods of controlling water ingress in the early stages of development would affect the long term profitability of the mine by reducing water pumping and maintenance costs.

Two 510m long x 17m2 internal cross section parallel drifts, 24m apart on a 1:6 down gradient provides access to the new gypsum mine 80m beneath open fields near the village of Barrow-uponSoar. Early mineral exploration of the area indicated significant quantities of water in the Lower Lias Limestone Series between the surface and the productive gypsum horizon. Extensive subsequent investigation confirmed fissure flows under sub-artesian pressure within the strong

13

UK FOCUS CASE STUDY

meet the final water ingress guarantee by installing a PVC waterproofing membrane between the primary shotcrete lining and the final unreinforced in-situ concrete lining which is of a watertight quality in the invert (Fig 2).

“While this tender may not have given the lowest initial bottom line price, we found that the maximum risk could be assessed and contained by the method and, after final analysis, provide the cheapest overall solution,” explained Deacon. No independent consultant was engaged to assess the competing bids although Faithful & Gould, the Chartered Quantity Surveyor, was retained to handle contract documentation, administration and cost management.

After having established that the system would fulfil all other British Gypsum fundamental requirements, the ability of the NATM to guarantee the water exclusion specification within a predetermined cost clinched its decision to award the £3.5 million contract (including additional access road works, drainage and muck shifting) to the Thyssen/Östu JV. There was however one proviso, that an experienced NATM designer be engaged. This position was awarded to Dr Gerhard Sauer of Austria who has extensive experience having designed several NATM tunnels for Deutsche Bundesbahn in West Germany and for the Washington DC Metro in the USA among others.

The design for the Barrow-upon-Soar drifts incorporates two different excavation cross sections. A 26m2 excavation at the start of the drifts, through the waterbearing top layers, will allow the application of a PVC waterproofing membrane behind the final in-situ concrete lining. In the lower dry strata where the waterproofing membrane is not required, the excavated cross

section is reduced to 19m2 to provide clearance for the 10cm thick shotcrete lining only.

“Trying to prevent ground water seeping into an excavation by making the surrounding ground impermeable is often a futile exercise,” explained Sauer. “Water will inevitably, with time, find or create its own path into the tunnel. The most cost effective action in many cases is to allow permanent but controlled drainage of the ground water through the tunnel.” But for British Gypsum this is not acceptable. The Barrow-uponSoar drifts have to be watertight.

Sauer went on to explain that the immediate layer of shotcrete, a characteristic of the NATM, has a certain degree of impermeability and a limited water controlling effect by allowing only minimal ground exposure. Any leaks through this shotcrete shell during construction are captured and drained to a pump sump outside the section to be lined. Once the final unreinforced in-situ concrete lining has been cast over the PVC membrane and it has acquired sufficient strength, usually after 28 days, the drainage pipes will be blocked off by grout injection. With a watertight concrete invert closing the circle and a grout injected concrete collar at the lower end of the waterproofed section the drifts will become completely watertight and subjected to the full hydrostatic pressure of up to 5 bar.

In addition to this waterproofing system, excavation and support of the tunnels by the NATM design is also quite different from the more traditional techniques. This is the first time the method has been used in the UK although some say many of the principles were used during excavation of the Dinorwic pumped storage hydroelectric scheme in Wales in the 1970s.

Below: Steel ribs and wire mesh form the skeleton of the false tunnel portals Inset: An ITCSchaeff backhoe machine (top); Inset: Paurat E134 roadheader (bottom)

14

This way is the only way

“We are now eight months into actual excavation of the drifts here at Barrow­ upon-Soar and believe that the lining technique adopted is the only possible way to handle the job successfully given the onerous final inflow limit,” explained John Hughes, Chief Mining Engineer for Thyssen (GB).

“Our initial idea was to excavate the drifts using shield-mounted roadheader booms and erect a bolted concrete segmental primary lining behind. Our experience so far shows us that such an operation would have already run into serious problems. The shield would have had to have been more than 6m diameter, to provide the clearance required, and keeping such a machine on line and level on this steep downhill gradient in such variable, wet ground would have been difficult. Furthermore, it would have been hard to justify the cost of such a shield for the expected length of hydrostatically lined tunnel. Rendering the concrete segments watertight by caulking and back grouting would also have been difficult to guarantee even with a secondary in­ situ concrete lining,” explained Hughes.

It was after discussing the job with representatives of the Austrian sister company Östu and visiting tunnelling projects on the continent, that the NATM design and method of excavation became a more feasible and attractive alternative.

In addition to the initial investigations carried out by British Gypsum, the JV, as recommended by Dr Sauer, commissioned an extra almost US$100,000 core drilling and pumping programme to establish the geological and hydro­geological conditions.

From the findings, ground conditions were divided into five categories for immediate support purposes. All are based on the application of steel arches, and wire mesh reinforced shotcrete in various configurations to suit the prevailing conditions. “The beauty of the method is its infinite flexibility,” explained Hughes. “The space between the arches, the thickness of the shotcrete, the overlap of the wire

mesh, can all be altered from one metre to the next if necessary, either to increase support in deteriorating conditions or reduce it in more competent ground.”

However, as all agree, knowing when to make changes is a matter of knowledge and experience of the managers, the consultant, and the tunnelling crews. For this reason, a team of six experienced Austrian tunnellers and engineers from Östu have come to Britain to oversee the job and transfer their particular knowledge and experience to the British crews especially in relation to the shotcreting operation.

Britain in the past was the leading nation in soft ground shield tunnelling techniques and successfully transferred this knowledge throughout the world.

It is now, at least at Barrow-uponSoar, showing the same commitment to technological advance and is being as open and receptive to adopting new methods developed by others.

Few of the British tunnellers have worked with the materials and techniques used in the NATM approach. Some of the materials are not available in Britain as yet and either had to be imported or made specially for the job. For example, lattice girders, a more effective alternative to heavy arched steel beams when used in conjunction with shotcrete, are being used at Barrow-upon-Soar and had to be imported from West Germany from Tunnel Ansbau Technik (TAT).

Sharing knowhow

Lattice girders are not difficult to manufacture, the lattices being either hand-welded or welded on a production line assembly. Hollybank, the British steel fabricating company in Tuxford, proposed manufacturing and supplying an alternative design whereby the three steel bars of the arch are connected with a welded-in spiral. However, this design did not meet the specifications according to Sauer. Hollybank is still considering investment in new machinery to make these girders if research indicates that the market will support the move. The 150mm

x 150mm x 6mm wire mesh is also of nonstandard design and had to be purpose made for the job.

In one of the drifts, a Paurat E134 roadheader was achieving a production rate of about 30m3/h and in the second tunnel an imported ITC-Schaeff backhoe type tunnel excavator, capable of adapting quickly and easily to various ground conditions, is producing a steady average of 15m3/h. Mucking out behind the roadheader is via continuous conveyor with dump trucks working behind the backhoe.

“Shotcreting is perhaps the most misunderstood operation in that the skill required to apply it properly is so often underestimated,” commented Jon Thomson, Project Manager for Thyssen/ Östu. “There is so much more to it than spraying concrete from the end of a nozzle at the tunnel wall, as the Austrian tunnellers are showing us,” he said. The skill of the operator is to achieve a uniform layer of shotcrete over the mesh and arches, filling the crevices behind the mesh and between the girders, while at the same time minimising rebound.

Experience is also required in controlling the mix of the cement and aggregates with the accelerator. At Barrow-upon-Soar, dry mix is batched and transported to a holding hopper with belt conveyors feeding a Meynadier GM090 shotcrete machine in each drift. Sprayset powder accelerator is added to the mix on the conveyor. From the shotcrete machine the mix is pumped up to 250m via a 65mm diameter steel pipeline with a flexible hose at the nozzle and at changes in pipeline direction. More accelerator is added for spraying overhead and in wet conditions.

The nozzleman controls the rate of water addition and rebound was reported at less than 20%.

During a site visit in early June 1987, 350m of the first drift and 230m of the second had been excavated and it was time to make the crucial decision of when to reduce the tunnel cross section marking the end of the watertight section and the

continuation with just the final shotcrete lining. As it has a significant influence on the cost of the tunnel, it was an important decision to be considered and discussed by all parties including Dr Sauer. After a 3h meeting at the British Gypsum head office, the decision was made.

“The intention is to take the PVC waterproofing well into the dry marl and create a watertight seal at that point with a groutable concrete collar,” explained Sauer.

“After blocking off the ingress drainage system, all inflow should be stopped and on-going water control limited to infrequent pumping out of the sump.”

Realising the high potential for large volumes of water, some 50,000m3/24h, leaking into the drifts from the limestone acquifer layer 17m below the surface, it was decided to limit this inflow by forming a 27m deep grout injection box around the portals. This curtain was built as a sub-contract by Thyssen Geotechnical using the split spacing technique. While it is difficult to calculate how effective such a grout curtain will be, water ingress of up to 40 litre/sec has been recorded which is slightly higher than anticipated.

“The contract specifications require us to guarantee a water ingress of no more than 25 litre/min in each drift,” explained Hughes. “To achieve this we must have the co-operation of the client and a flexible construction method, both of which we fortunately have. British Gypsum is anxious to have as dry a tunnel as possible and is eager to co-operate to that end and the NATM offers the flexibility to make decisions at the appropriate time,” he said.

“Unfortunately, there has been more water-bearing ground to go through than we expected and the larger cross section to accommodate the PVC membrane has had to continue beyond the original point.

We still hope to be close to our original tender price however, and the client has been party to all the decisions which affect the eventual cost increases as work progresses, which is another advantage of NATM,” he added. n

UK FOCUS CASE STUDY

From left: A PVC membrane behind the final lining of the walls and vault; Fig 2. Watertight concrete invert

15

Briti ritis sh Tunnelli nellin ng Societ y

BTS Annual Gala Dinner

Each year, the British Tunnelling Society holds its Annual Gala Dinner at the Brewery in London.

TunnelTalk has sponsored the official photographer since 2010, recording the festivities.

The event provides an enjoyable social opportunity to celebrate the achievements of the underground engineering industry, look ahead to future projects, and network with colleagues.

The next event will be held on Friday 6th May 2022.

Visit www.britishtunnelling.com for more information.

CASE STUDY: After a successful first application at the Barrow-upon-Soar gypsum mine, it took several attempts yet for NATM to be applied for road, rail and metro projects in the UK. In these early days the offer-therecord comment in the Buttery Bar after a BTS evening meeting was that it was proving difficult to incorporate the design criteria and the flexibility of the method to the ICE forms of contract and to apply UK construction supervision and acceptance practices being used in the UK at the time. The Heathrow Express railway line and the London Underground stations of the Jubilee Line were the first large scale use of NATM in Britain.

Following the Heathrow NATM works collapse (p42) and the revision of the method as the SCL method, the North Downs double track tunnel on the Channel Tunnel Rail Link Phase 1, opened in 2003, and the Hindhead road tunnel on the A3 in the south of England, opened in 2011, were applications of the method ahead of more recent Underground station upgrades, the Crossrail project stations, and the coming HS2 works, always with a membrane or the alternative spray-applied waterproofing system.

SITE VISIT REPORT: JULY 1987

Caution key to Cornish road tunnel a water-jet assisted Titan El69 roadheader, drill+blast being prohibited in this urban environment. The trial tunnel also trialled the application of NATM as a construction and excavation method.

Findings from a 40m long horizontal bore taken from the west portal were correlated with the conditions encountered when excavating the 40m long x 10m2 trial tunnel on the same line. Two horizontal bore holes then drilled along the line of the proposed tunnel, 185m from the west end and 170m from the east end, theoretically provided a core of the entire tunnel length.

These cores indicate that the ground conditions consist of soft weathered slate in the shallower areas of the portals with more competent slate and occasionally very hard volcanic dykes, up to 150MPa compressive strength, under the maximum 16m cover.

While it is arguable just how much detailed information can be deduced from a 75mm diameter core, even by the most skilled and experienced, the logs and reports did highlight areas of soft, potentially difficult ground that would require careful excavation.

In light of this, Mott Hay & Anderson specified the excavation of a pilot heading first. The detailed design of the pilot was left open to the contractor although Mott Hay & Anderson recommended that it be driven in the crown. With a concrete drainage duct either side of the road level, the consultant expected the contractor to excavate two small side wall drifts next, install the concrete footings and then excavate up towards the completed pilot.

However, Balfour Beatty, who won the contract in April 1986 for £7 million as lowest of five competing bids, the second bid being

• Client: UK Department of Transport • Consultant: Mott Hay & Anderson • Contractor: Balfour Beatty almost £2 million higher at £9 million, chose a different course of action. After completing a 3.4m high x 6m wide (18m2) pilot heading through the crown, it is widening this either side to the 5.1m high x 12m wide (48m2) top heading before excavating the 6m high bench to form the full 9.5m high x 13.75m wide (120m2) tunnel section (Fig 2).

Immediate support requirements are divided into two categories. In the poorer rock with minimal cover immediate support is provided by 254mm x 254mm x 132kg steel arches on 1m centres. In the more competent rock maximum 2m rounds are excavated and supported immediately by a 1.5m pattern of 2m dowels installed within 2.5m of the face followed by a 5cm thick initial layer of shotcrete to which the 100mm square wire mesh is attached by spit nails.

A second 2m round is then excavated and as the dowels are being applied, a 1.5m pattern of 5m fully-bonded rock­bolts (which must be applied within 5m of the face) is installed in the previous 2m. A final layer of shotcrete creates a 10cm total thickness.

The resin-bonded rock­bolts are tensioned to a 100kN pull (Fig 2).

Some 61% of the tunnel is expected to be in the Type 2 better quality rock with 144 arches required in the remainder.

The immediate support is not designed as part of the permanent structural support.

A final 60cm thick unreinforced in-situ concrete lining will carry the full load, making the embedded immediate support

UK FOCUS CASE STUDY

S

altash, on the Cornish coast of England across the Tamar River from Plymouth, had been awaiting a solution to serious traffic congestion for many years. Since the three-lane Tamar Bridge was built in the early 1960s, commuter traffic has had to pass through the town via the two-lane A38.

As it approaches the bridge, the A38 passes through a residential area of the town built over a small hill. With no opportunity of widening the existing road, the client, the UK Department of Transport, had three options. Either purchase the homes over the proposed route and put the road in an open trench or cut-and-cover tunnel or, avoid the cost of buying so many homes and excavate a bored tunnel through the hill.

The latter was considered the most acceptable despite the fact that the fixed position of the Tamar Bridge and the prevailing topography dictates a very demanding overburden of 3.5m to 16m maximum above the 340m long x 120m2 tunnel with a 47m cut-and-cover section under the A38 at the west end (Fig 1).

Excavation of such a shallow tunnel in a dense residential area was further complicated by suspected poor ground conditions, the hill being composed largely of a cleaved and fractured slate, known locally as shillett, with volcanic intrusions, sills and dykes.

Realising the potential difficulties, Mott Hay & Anderson, the Consulting Engineer, commissioned an additional site investigation trial tunnel to supplement information obtained from earlier bore holes.

This £120,000 contract was concluded by the Thyssen-Taywood JV early in 1986 using

From left: Fig 1. Saltash bypass tunnel; Fig 2. Type 2 immediate support

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UK FOCUS CASE STUDY

The Titan roadheader driving through poor ground in the pilot; Enlargement of the pilot to the 48m2 top heading in type 2 ground measures redundant. This lining will be cast concurrent with top heading enlargement and benching.

At 15 months into the 27-month contract, work proved more difficult than expected for Balfour Beatty.

Pilot excavation required 85 arches with a further 18 needed in a patch of very bad ground 170m from the west portal where two 3m3 roof falls occurred, with 30 in an area 25m from the east portal where 20m3 fell following a face collapse.

Enlargement of the top heading requires replacement of the smaller arches.

Subsequent excavation of the bench involves working with bracing needles in the crown to distribute the load from the suspended arch to its neighbours. Moving these needles is a slow, laborious job and a disadvantage of working from the top down.

Working with the needles also places a great deal of pressure on the weakened bench face. To help withstand this pressure, Balfour Beatty is installing four 7m long sacrificial rockbolts into the 15m2 bench

faces either side of the central top heading access ramp.

Having decided not to excavate small headings for the footings of the tunnel, Balfour Beatty had to excavate a drainage trench either side as part of the benching operation and installing large, heavy-duty steel footings to hold the arches. The concrete drainage duct will be cast as a later operation.

In the two areas of very soft weathered ground where cleavage of the slate is more horizontal, stand-up time is limited and cross beams have had to be rock bolted to the pilot arches to help hold the ground.

In the very bad ground area at the east end, the pilot is being enlarged by careful and laborious hand work in order to complete this difficult section ahead of the main advance.

The ground is relatively dry although during and after heavy rainfalls there is noticeable leaching in certain areas. The pilot tunnel is effectively dewatering the ground during construction but the tunnel

Bracing needles support the suspended arch during benching

design does not include a full water­proofing system. 1m wide drainage strips applied at 1m intervals behind the final lining will relieve hydrostatic pressure and channel water down to the drainage ducts. Final lining concrete with a 50% slag content has been specified to reduce cracking and the joints between pours will be caulked to help prevent seepage. Finally, the concrete lining will be painted.

An alternative bid based on NATM included a full PVC waterproofing membrane but despite allowing a thinner final lining the bid was said to be more expensive than conforming tenders.

Installation of mechanical and electrical equipment is planned to start in March 1988.

To meet this hand over deadline, Balfour Beatty has a long way to go and ways of increasing production such as continuing the hand work at the east end or introducing a second roadheader are being examined.

Lower bench production rates of about 8m/wk were restricted by careful underpinning needs and the heavy concentration of activity in a small space.

As the distance between the top heading, bench and lining activities open up, progress should improve and in the Type 2 areas, progress rates of 6m/24h day are expected on the bench.

With a 36h striking period, Balfour Beatty expects to get two pours/wk behind the 8m shutter which will eventually fall 10 weeks behind the top heading and about 24m behind the bench. However, in such potentially difficult conditions, small, careful steps are believed to be the only way to avoid a calamity and achieve steady progress. n

SITE VISIT REPORT: OCTOBER 1988

Penmaenbach: hand mining saves rock tunnel drive

S

oft ground at one end of the basically hard rock road tunnel on the A55 road improvement project in North Wales created a challenging situation for the designer and contractor alike. While drill+blast excavation on a top heading and bench programme started from the west end in strong rhyolite rock, the east portal was opened as a multiple face hand-mining operation to get through the 90m section of moraine. Immediate support in the hard-rock section of the

18

660m long tunnel comprised mainly 3.5m and 7m long fully resin-grouted rockbolts on 1.3m and 2.2m centres tensioned to 10 tonne, followed by 25mm of unreinforced shotcrete, depending on the support classification. Temporary support in the soft-ground section was left to the discretion of the contractor to choose its preferred system. Whatever system was chosen, it would have to suit the heavily reinforced final lining specified to withstand possible loads in this section.

• Client: The Welsh Office • Consulting Engineer: Travers Morgan in association with Howard Humphrey • Contractor: Balfour Beatty Final lining in the hard-rock section is a minimum 250mm layer of unreinforced in-situ concrete cast for lighting, ventilation, smooth finish and safety purposes. The minimum excavation to accommodate

this lining is 82m2. To accommodate the thicker 125kg/m3 reinforced in-situ lining and provide the same finished dimensions in the soft ground the designed excavation cross-section is larger at 125m2.

One tunnel inside another

Soft ground temporary support exercise and took six months to complete.

The moraine required careful control with timbers and straw packing, particularly after rainfall, when water inflow increased.

A 3-boom Maximatic Tamrock drilling jumbo was used to tunnel through the strong rhyolite of the Ordovician Age with unconfined compressive strengths of between 100MPa and 300MPa. Extreme care had to be exercised to control blastinduced vibrations as the old, generally unlined, parallel railway tunnel constructed in the 1840s is only about 60m inland from the new tunnel. Continuous rail traffic through this tunnel and the need for visual inspection of the old tunnel after every blast limited excavation in the new tunnel to one carefully timed blast/day.

Three rock classifications were identified, with most of the length in rock Type 2. In these conditions, the 4m long charge holes produced a 3.8m pull in the 52m2 top heading. Underbreak and overbreak in the rock tunnel was checked by an Amberg profilometer, believed to be the first use of this technique in Britain.

Results and performance of this system and other measurement and monitoring techniques used on the project were the subject of the October 1988 meeting of the British Tunnelling Society in London.

Final in-situ lining

Balfour Beatty started work on the £4.6m tunnelling contract in January 1986 and finished excavation in September 1987.

The final in-situ concrete lining was cast behind an 8.76m long Stelmo form. It

was cast in three pours/wk with a striking period of 18h.

The rock section is above the water table and does not require any special waterproofing. In the soft ground section, ICI Siltram IBZ material is sandwiched between the inner 250mm thick unreinforced lining and the outer reinforced structure to prevent water seepage. Balfour Beatty is expected to be off site, and the electrical and mechanical contract completed, for the tunnel to open in April 1989.

This two-lane tunnel will ultimately carry westbound traffic but will initially carry two-way traffic while the existing road is closed for refurbishment. Once reopened, the existing two-way road will carry eastbound traffic. Unfortunately, the road alignment around the rhyolite outcrop through which the tunnel passes is not considered adequate to carry two lanes of traffic in the same direction. Traffic will therefore be reduced to one lane around the bluff. This situation is expected to be adequate for the next ten years or so but a second tunnel will be required, provision for which is made in the statutory processes.

UK FOCUS CASE STUDY

The problem arose with the contractor’s choice of 305kg/m3 steel arches on lm centres as temporary soft-ground support.

The steel arches were not accepted by the designer as anything but temporary works, despite the fact that their removal would be virtually impossible prior to finishing the tunnel. They were not acceptable as an integral part of the permanent lining reinforcement either since they could not be fully protected against corrosion.

The excavated cross section, therefore, has had to be larger still at 135m2 to accommodate these 254mm temporary steel arches which are sacrificed at the contractor’s expense behind the design specified heavily reinforced final lining. In effect, one tunnel inside another.

Perhaps a more appropriate method in this section would have been a softground NATM approach. Had such a method been adopted, it would have resulted in a more elegant tunnelling solution and may have helped to avoid possible claim situations. Instead of the heavy steel arches, the contractor could have used lattice girders fully encased in shotcrete as immediate support. These lattice girders would eliminate the barriers and joints in the shotcrete created by the I-beam arches and could then have been incorporated in the permanent lining without the need for such a large amount of additional excavation.

As it happened, excavation by Balfour Beatty started with a 35m2 handexcavated pilot tunnel in the crown. This was supported by crown bars and timbers and the central section of the heavy steel arches in the vault. A second and third face expanded this to either side, the legs of the steel arches supported on concrete footings in the invert. Two further faces opened up headings in the sidewalls, again the undermined arches being supported by horse heads back to the rear arches. The retained central bund of ground, besides providing restraint, also enabled access to erect lining reinforcement in the crown.

Excavation of this 90m x 135m2 section entirely by hand was a painstaking

Footnote: The North Wales Expressway also includes an east-bound two-lane carriage tunnel through the Penn-y-clip headland, started in four years after the Penmaenbach tunnel and opened in 1994, and the 1,060m immersed tube crossing of the Conwy River estuary, the first immersed tube in the UK, that opened in 1991. The second tube at Penmaenbach is still awaited. n

East portal of the road tunnel

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CASE STUDY: The 10.5km long Carsington aqueduct was a salient experience of how decisions made during design can go seriously wrong. Contractors bidding for the project predicted that the roadheaders in the shields would be on the hillside before Christmas. They were right when harder rock and more water than anticipated forced three out of the four drives to give up their roadheaders and turn to hand mining or drill+blast. The remaining face, which kept its roadheader shield, was advanced under compressed air to keep large amounts of water under control.

SITE VISIT REPORT: MARCH 1983

Carsington Aqueduct:

UK FOCUS CASE STUDY

W

hat started as a £15.1 million contract to build a 10.5km (6.5 mile) long aqueduct at Carsington in Derbyshire has run into severe geological difficulties, and completion of excavation may be as much as two years behind the originally planned date and may end up costing twice as much.

The tunnelling contract, awarded by Severn Trent Water Authority to contractor John Mowlem, calls for two tunnels, one 3.6km and the other 4.8km, connected by a 1.6km x 1.6m diameter pipeline (Fig 1).

There is also a shorter 140m tunnel at the downstream end. The consulting engineer is G H Hill & Sons of Manchester.

Site investigation for the scheme was carried out during 1979 and included sinking of deep boreholes along the line of the aqueduct, generally at 1m intervals, with several additional shallow holes in the

Man against nature

vicinity of the intended portal sites. The maximum depth of the tunnels is about 160m. Core samples were taken and kept in store for examination by the eight firms tendering for the tunnel contract.

The borings indicated that the tunnels would be driven mostly through the Ashover Grit series with underlying mudstones, these dipping at shallow angles along lengths of tunnel. The main series contained interbedded strata of mudstones interbedded with siltstones and sandstones, and vice versa. A geophysical survey was also carried out at the same time to confirm aspects of the borings. During the design stage, following the main site investigation, supplementary borings to explore in more detail the ground adjacent to two of the portals were made. One portal was moved downhill some distance from its originally planned site when an area of surface

Fig 1. Carsington Reservoir and Aqueduct scheme

• Client: Severn Trent Water Authority • Consulting Engineer: G H Hill & Sons • Contractor: John Mowlem & Co instability was identified, causing the tunnel alignment to be adjusted slightly to avoid the affected area.

Fullface TBMs could not be used because of the large amount of water expected, and the lack of access to the face to do the specified pre-grouting.

The use of drill+blast, except in isolated circumstances, was also specifically excluded from the bids. In view of the extensive problems expected due to faulting, it was decided to specify a shieldbased excavation method. Roadheader booms were considered the best option, and all equipment would need to be flameproofed to National Coal Board standards as the tunnels are going through Coal Measures strata, with the possibility of methane being encountered.

The Engineer also specified the grouting regime to ensure sealed, stable ground conditions through which to tunnel and to deal with the expected amount of ground water. This is done in three stages: advance grouting in advance of driving the tunnel; grouting as the rings are built; and radial grouting after the lining has been completed.

Four consecutive drives

Fig 2. Longitudinal and cross sections of the 10.5km Carsington Aqueduct

20

Of the eight firms that bid the job, Mowlem won with the second lowest tender. Work started in September 1981 and was designed to be completed in four years.

The Mowlem plan was the only one to drive the main tunnels from the four portals using Anderson Strathclyde roadheaders mounted in Markham and Grosvenor Steel shields. Tunnelling started at Portal C in February 1982, and at Portals B and D in March, and finally at A in April.

It was decided at an early stage, in view of the predicted ground conditions, to formulate a comprehensive plan incorporating the method of excavation, roof support and permanent lining. With the agreement of the Employer, the main recommendations of CIRIA Report 79 ‘Tunnelling - Improved Contract Practices’ were followed and the general approach incorporated in the contract documents.

Although sections of the tunnel were to be driven in competent rock, the need for immediate roof support was expected near

Lining

The tunnels were designed to be lined with concrete segments. Where the depth below ground is sufficient to contain the internal water pressure, no reinforcement was envisaged. At the portals, the tunnels will be lined with 2m diameter x 16mm thick welded mild steel pipe, with the annulus backfilled with concrete.

Tunnel segments near the portals are standard 2.44m i.d. Buchan Smoothvert bolted rings, but further inside these are changed to non-bolted concrete segments made by a local contractor to the design of John Mowlem and stored on site.

The segments have a specified thickness of 200mm and an internal diameter of 2.34m. The units have plane radial joints and do not depend on ties or bolts for integrity.

The tunnel grades are between 1:50 and 1:70, except at Portal B where there is a considerable length which is almost flat, 1:1000 for the first 1,000m.

Major set-backs

It was not long before these well defined design plans fell foul of what was considered at the time to be an adequate and comprehensive site investigation. With only about 1.8km of the 10.5km of tunnel excavated, the ground conditions proved too difficult for excavation to continue according to plan.

The Portal A drive was 315m into the hill when after 14 weeks it was decided by the contractor that the roadheader would have to be taken out. Increasing quantities of water were pouring into the tunnel and it was occupying space that affected the time taken for geotechnical processes. The tunnel is now proceeding using hand held pneumatic clay spades. A week of tunnelling is followed by a week of ground treatment.

The shield was still in place and Buchan rings were being erected with the face advance, which was averaging 7.5m/week. It is hoped to speed this up in due course.

The site investigation revealed that Portal B could pose problems due to the lack of cover and the knowledge that the

area was prone to landslips. The portal was subsequently moved to avoid the unstable conditions and it was specified that the steel pipe inner lining would extend to a length of 1,100m, the longest of all the portals.

But these precautions could not guarantee optimal tunnelling conditions and from the outset the presence of water was a major problem. At times the flow reached more than 1,000 litre/min through 50mm diameter bore holes. Then in August 1982, a sudden flood washed away the face completely and submerged the equipment. Fortunately all the miners escaped without injury.

It was several days before the tunnel was clear of water and equipment could be retrieved. The solution here was to continue the drive with the roadheader, but under compressed air at 1.75 bar pressure. It took a further two weeks to install the horizontal airlock which had to be long enough to take an eight skip train and loco.

This is the only drive of the four still being carried out with the originally recommended machine. Before the flood, progress had been about 40m/week, but since then it has been considerably less. During the site visit, 365m had been completed.

Least progress

Portal C was the first to start but has made the least progress to date, only 265m. The roadheader was removed after 12 weeks and 210m of the drive, again because of the excessive amount of water pouring in at the face and the hardness of the ground.

Tunnelling here has had to continue by drill+blast using Atlas Copco pneumatic hand drills. The Markham shield is still in the tunnel and is being pulled through following the excavation. The concrete segment lining is also being installed according to the original design.

As is the case at all the faces the amount of grout being injected at this face has increased considerably. Advanced grouting holes are now being drilled to 20m depths. Colin Mackenzie, Mowlem’s Tunnelling Director, explained during the site visit that: “Ground conditions were so bad in one section of the Portal C tunnel

Shield and Anderson Strathclyde roadheader during manufacture

that for every 100 tonne of rock removed, 120 tonne of cement was put in.”

The tunnelling cycle has changed to a two week pre-grouting process followed by one week of excavation, achieving about 15m, making the average advance about 5m/wk. Jim Swanson, Mowlem’s Project Agent, stated that the rock is expected to tighten up after about 300m, meaning that the grouting time would be considerably less. He hoped that grouting could be reduced to less than a week for each week’s tunnelling.

Most changes

Although Portal D was making the best progress of the four drives at that time, it was the one that experienced the most changes. The roadheader was taken out after 195m of excavation and drill+blast is being used instead.

The 750mm long Buchan Smoothvert bolted segmental rings which were supposed to continue for the first 550m were replaced after 200m by shorter 610mm segments. After a further 45m, segments were abandoned altogether and steel arches were placed approximately 1m apart for the next 15m. Here the support changed yet again to arches and shotcrete with mesh and rockbolts. During the visit, the miners commented that the ground here, although it does weather rapidly, is competent during excavation and that they had enough confidence in it to decrease the number of rock bolts to speed up the working cycle. Swanson agreed with the workers, and a decision to do this was subsequently taken.

Progress at this portal is averaging 25m/wk with about 3 x 1.8m pulls being achieved in the 24h cycle. The specified final grouting and lining will be installed on completion of excavation. At the time of the site visit, 575m had been driven.

UK FOCUS CASE STUDY

the portals and also at intervals along the tunnel where major faults occurred.

Probing ahead

Probing ahead, carried out in all tunnels at the weekends, sufficient for one week of advance work, was at first one probe hole driven to 50m in each face. In practice this was found to be insufficient and has been increased to three probes per face fanning out to 3m distance outside the tunnel profile, but to a depth of 25m rather than 50m.

These are the methods by which each of the tunnels is advancing at present and despite the relatively slow rates of progress, the solutions are working. As Paul Brooks, Project Engineer for Hill said: “No other consulting engineer or contractor faced with the same problems would have come up with better solutions than we have.”

A technical assessment group has been set up headed by John Waterfall (Mowlem) bringing together Engineers from the contractor and from the Consulting Engineer to look at the various methods required to solve the long term problems on the contract.

Back-up expertise from Soil Mechanics and Edgar Morton & Partners (geologists advising Hill) is also available to the group. Mowlem’s senior mechanical engineer, Peter Remnant, is also a member of the group.

A feature of the valuation of the

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a main discharge tunnel 190m long x 6.7m in diameter under the dam, and constructing a 25m long forebay tunnel of 4.75m diameter. This tunnel is lined with precast concrete bolted segments and was completed in February 1982.

Pumped water storage

UK FOCUS CASE STUDY

Rings of the 750mm wide x 2.4m i.d. concrete Smoothvert segmental lining works is the use of a rock classification system based on rock strength and presence of discontinuities. This results in the tunnel face being identified as one of six categories, and hence establishes payment level. The effectiveness of this system in identifying degrees of difficulty for tunnelling remains to be seen.

A laboratory has been set up on site at Portal C to assess the strength of rock samples so that they can be assigned to the correct category for payment. An assessment of discontinuities has also to be made before payment level can be decided. However, Swanson said that in practice about another three categories may be needed where rock strengths exceed 50MN/m2.

Claims for extra costs, although still to be defined, are expected to be quickly paid under the straightforward contract arrangements incorporated in the CIRIA recommendations. Swanson summed up: “The final cost of the tunnels is almost entirely dependent on how much time is spent on grouting. We seem to have moved from a tunnelling contract with some grouting to a grouting contract with some tunnelling.”

Mucking out

There are two contrasting mucking out systems in use at Carsington; a batterypowered electric loco and skips in Tunnels B and D where the loaded train travels downhill, and the Mowlem Shuttlecar in Tunnels A and C where the load has to be hauled uphill to the portals.

The Mowlem Shuttlecar in use at Carsington is based on and is a development of the Shuttlecar introduced at the Mowlem Falconbrook sewer contract in London. A power unit runs off 1.1kV breck track mounted at the side of the tunnel interior, rather like the old trams. It powers hydraulic motors which provide power to travel, and to work the chain conveyor which stretches from end-to-end of the openended Shuttlecar. Muck is loaded on the end nearest the face by the loader, and

22

is wound back on the continuous chain belt till the Shuttlecar is fully loaded. At the portal, the chain is wound on again depositing the muck out of the other open end.

Each Shuttlecar can carry a load of 11 tonnes, and at Carsington was designed to remove the muck at the rate of one car load per ring of excavated tunnel.

Due to space limitations, Portal A was advanced at the bottom of an 8m deep shaft with special mucking arrangements. The Shuttlecar empties its load into a large hopper situated below the track. After the car has been moved, the rails hinge out of the way and the hopper is lifted out by crane.

In addition to the two main tunnels, there is a shorter 140m long drive from Portal E to Portal F. It was started in July and completed in October 1982, with a two week stoppage while compressed air with a boiler lock was installed when the drive hit running sand at ring 24.

Another tunnel, not included in Mowlem’s main contract, was driven by the company under the dam as sub-contractor to Shephard Hill, main contractor for the dam. This sub-contract included making an access shaft 19m deep x 10.67m (35ft) in diameter, driving

The Carsington Reservoir scheme is being developed so that more water can be taken from the River Derwent for public water supplies. When the supply in the river is plentiful, water will be pumped via a new pump station at Ambergate back through the aqueduct to the reservoir formed by the dam which is being built

by Shephard Hill. In dry weather, when there is insufficient flow in the river, water will be released from the reservoir to flow under gravity through the aqueduct back into the river. In practice, the draw-down of reservoir levels in summer will be made good during the following winter.

The scheme will make an additional 50 million gal/day available for future supplies.

The dam is built of selected shale with a clay core and is built across the valley of the Scow Brook, a small tributary of the River Dove. It is 1,150m long x 35m maximum height and will carry a road when completed. The reservoir will provide 35 million m3 of storage capacity and cover about 295 hectares (725 acres) at its normal top water level of 199m (654ft) above Ordnance Datum.

Slow but sure progress

The original site survey was further supplemented with intermediate boreholes that confirmed the roadheaders to be no longer practical. Although drill+blast was specifically excluded from the bids, it was adopted as the principal method of excavation from Portals C and D. “We are keeping the roadheaders handy but feel that conditions will remain as they are and that the roadheaders will not be re-introduced at a later stage,” said Swanson.

The problems at Carsington are so daunting that all three parties are right when they say that the only way to overcome them is for everyone involved to combine their resources to get the tunnel through in the most economical and sensible fashion. n

Mowlem Shuttlecar, with a capacity of 13.5m3

CASE STUDY: It is inconceivable today to suggest an open-face shield would be permitted on a drive under vital infrastructure. It was the case however back in the mid-1980s when a refurbished enlarged shield-mounted backhoe was used to excavate the rail link under the runway and under shallow cover into the new Stansted Airport terminal. Permitted settlement was set at a maximum 20mm. Until the improved development of full-face, soft-ground TBMs, open-shield backhoe and roadheader booms were the norm for drives with segmental linings. Curved bolts on the radial and longitudinal segmental joints were also the trend at the time. These too have long since been superseded.

SITE VISIT REPORT: MARCH 1983

Tunnel taxies under airport runway

S

tansted, to the north-east of London, is destined to become the capital’s third major international airport, along with Heathrow to the west and Gatwick in the south. In preparation for a new 8 million passenger/year terminal complex opening in early 1991, an express service rail link is being built to connect it with London’s Liverpool Street mainline railway terminal.

Services will run on existing track for the first 54.6km of the 60km distance with a new 5.4km long spur leading into the terminal complex. After investigating several options, British Rail, the owner and operator of the new service, decided to take the shortest, least expensive and most environmentally suitable route between the existing line and the terminal. However, this has complicated matters in that the main runway of the airport lies directly across its path. After 2.7km of surface track, a 1.6km long tunnel is needed to take the line under the runway. It then emerges to run 1.1km on the surface into the terminal’s groundfloor station area.

“The tunnel alone will cost £11.5 million of the new £45 million rail spur,”’ said Mike Bowles, Senior Resident Engineer for British Rail Anglia Region. “It was the cheapest option and is also more costeffective and environmentally acceptable than taking a surface track around the runway. The closest contender to the 5.4km tunnelled route was an 11.7km long surface route that would have cost £52 million, included eight road crossings and was more environmentally intrusive.”

Straightforward tunnelling

Technically, the tunnel is not a difficult undertaking. Site investigation data predict a minimum 15m layer of good quality chalk boulder clay beneath a thin layer of alluvium and overlying London Clay. Sir William Halcrow & Partners, engaged by British Rail for the bored tunnel section of the project, selected the vertical alignment so that the tunnel was within the boulder clay.

One potential danger lies in the presence of a layer of water-bearing pressurised sandy gravels between 1m to 2.5m deep known as the Kesgrave Beds, sandwiched between the chalk boulder clay above and the London Clay formation below (Fig 1). These have a permeability of about 10-5m/s and are under up to 1.6 bar pressure. In some areas, particularly over the first 500m of the drive, the tunnel invert comes close to these sandy gravels and precautions are needed to prevent them from boiling up into the tunnel. “If this should happen, we could have difficulty steering the shield and preventing it from sinking or taking a nose dive,” said Jon Dale, Site Agent for J Murphy & Sons, which won the £11.6 million contract in April 1988 as the lowest of six bidders.

A second potential hazard is the presence of isolated pockets of pressurised waterbearing gravels within the clay which the contractor must be prepared to deal with if encountered in the tunnel face.

Despite these potentially difficult conditions, none of the contenders for the job felt that the risk was great enough to warrant the use of a fullface soft ground tunnelling machine and the tender documents make no reference to the design of the tunnelling machine to be used.

“The contract would not cover the cost of a new machine and there was no time allowed in which a new machine could be built,” said Peter Jaques, Manager of the Murphy tunnelling division. “In the event, we chose a used shield with a mounted backhoe which was initially built by James Howden, then Grosvenor Tunnelling, for the Singapore Metro.” The machine excavated 800m in Singapore before being shipped back to the UK y where it was refurbished

by Stelmo for the Stansted project.

The principal refurbishment required was to increase the shield diameter from 5.9m o.d. to 6.2m o.d., an increase of 300mm. Stelmo achieved this by inserting three 100mm packers into the shield and

• Client: British Rail Anglia Region • Consulting Engineers: Sir William Halcrow & Partners as sub-consultant to Bristish Rail for the bored tunnel.

Bristish Rail for all other associated works.

• Contractor: J Murphy & Sons rolling a new skin. “This has slightly distorted the circle of the shield,” said Jaques, “and is probably the largest increase many would consider to modify a used machine for a new job.” Stelmo also built a new set of trailing gantries for the shield.

Modification and refurbishment took three months before the shield arrived on site in late November 1988. Prior to this, work had to be completed before launch of the machine. The new terminal is to have a new dual-carriage access road and this is being built under a separate contract concurrently with the new rail spur. A branch of this road passes over the east end of the tunnel and a 130m length of cut-and-cover work. “In the contract, this expensive length of 10m deep cut-and-cover was 210m but as a substantial cost saving alternative, we offered to shorten it by 80m and extend the tunnel drive,” said Jaques. “This not only

UK FOCUS CASE STUDY

Stansted:

Fig 1. Tunnel route under the runway

Fig 2. Geological section of the 6.08m o.d. rail tunnel underpassing the runway with a minimum 2.5m and maximum 10m cover

23

UK FOCUS CASE STUDY

Left: The powerful backhoe of the refurbished shield rotates 360° and is mounted on two slides at shield axis Right: The refurbished shield was increased in diameter by 300m gave the necessary road building access on schedule, but also reduced considerably its construction time.”

Murphy achieved another edge over its competitors by modifying the construction of the temporary 8m deep tunnel access shaft. This shaft was required because the road works prevented a portal launch and working access from the east and because difficult access and the railway construction earthworks prevented tunnelling from the west. “This shield launch chamber was designed by British Rail as a 9.5m x 9.5m box extension of the cut-and-cover tunnel,” said Roger May, Resident Engineer for Halcrow. “Murphy offered to sink a 12m diameter concrete segmentally lined shaft as a cost-saving alternative, which was accepted.”

Preparatory work also included a programme of dewatering. “To reduce the potential risk presented by the pockets of wet gravels in the chalk Boulder Clay, provision for probing and pregrouting from the tunnel face has been allowed for in the contract,” said May. “The contract also calls for compressed air installation as a billed item but we hope that it will not have to be applied. To help avoid having to apply compressed air, the programme of dewatering will reduce the water pressure in these gravel pockets and reduce the risk of underlying sands breaking up through the invert.”

Six 250mm deep dewatering wells were installed, two directly into two

identified pockets of gravel in the tunnel face and four into the Kesgrave Beds, each borehole equipped with pumps over a distance of about 150m. These wells will be progressively taken off once the shield is about 75m ahead Dewatering cannot take place under the runway and compressed air may have to be applied to stabilise poor ground conditions.

It would take about two hours to bring the tunnel up to 1 bar of compressed air. Also, the contract calls for 24h/7 day/wk working during the 300m run under the runway and taxiway to keep the shield on the move.

In addition, the shield was fitted with five mechanical breasting plates which, when engaged, will cover and support the top half of the face. Face jacks can hold timber support over the remainder if necessary.

Maximum settlement under the runway is specified at 20mm.

At the time of the site visit in early 1983, tunnelling had advanced 240m in a face of homogeneous Boulder Clay with a long stand-up time that was allowing a best advance rate of 6m/10h shift. “As long as this persists we will hole through on schedule in October 1989, approximately 40 weeks after we started on the 1.6km drive in January 1989,” said Dale “but we are not taking these conditions for granted and are prepared for the possible hazards.”

Although a tail seal was not vital to the success of the shield, since it was not expected to drive continuously through waterbearing ground, Murphy did have a

Sir William Halcrow & Partners design of the curved bolted concrete segmental lining

24

rubber lip tail seal fitted to the top of the shield to hold back the grout. However, soon into the drive, this rubber lip seal failed and was removed. “I am personally of the opinion that wire brush seals with continuous grease feed are the best, particularly if a reliable seal is vital to the successful performance of the shield,” said Dale. “The experience here confirms this. We did not replace the seal on this machine and are using a fluffer instead during grouting.”

Curved bolts

The tunnel is being lined with precast bolted segments designed by Sir William Halcrow & Partners and manufactured by C V Buchan.

From top: The air lock 120m from the east portal ready for compressed air work; Segments are lifted by a mechanical pin in the threaded grout hole

There are eight segments to a ring. The largest, the flat- bottomed invert segment, weighs 2.25 tonne. Instead of the more usual straight bolt connection, Halcrow has designed a curved bolt connection.

Although curved bolts have been used on other jobs such as the Villejust TGV rail in France, the Singapore and Sao Paulo metros and are being used exclusively on the French part of the Channel Tunnel project, this is believed to be the first time they have been used in the UK. The use of curved bolts avoids the weakness of a conventional bolted lining where the flanges cannot take the high shield ram thrusts. The bolt pockets can be reduced in size, allowing more uniform reinforcement of the segments.

Curved bolts have the added advantage of avoiding durability problems associated with cast-in fixings.

There are six bolt pockets in each of the six, 1m wide main segments, two in each circular joint and two on the transverse joint with a 470mm long x 24mm diameter bolt threaded through. The 220mm thick x 5.64m i.d. ring is designed to withstand the maximum live loading of a fully laden 568 tonne Boeing 747 on the runway just 9.5m above.

Reforming ring

Similar ring designs were used on the Singapore MRT contracts and each was well documented in a paper presented by Terry Hume, Project Manager, and Peter Copsey, Chief Engineer of the MRTC, to the British Tunnelling Society in January 1988.

“On our contract, we used curved bolts on the transverse joints and straight bolts on the circular joints,” said Dale who previously worked in Singapore. “Here we have curved bolts on both joints. On contracts that used curved bolts on both connections, it was said that the curved bolt on the circular joint allowed some movement, causing slight deformation or squat in the ring.

Being prepared for this, we have added a reforming ring to the Stansted shield just behind the erector. We are grouting two rings at a time and before we start, the reforming ring, which runs on its own set of rails on the shield sledge, is expanded against the joint between the two rings to hold them firmly in place.”

A further innovation in the ring design is the casting in of a nib along the circular edges of the key segment. This fits snugly into corresponding recesses in the adjoining top plates and prevents the familiar problem of nodding keys. The invert segment has been designed to allow for the use of a continuous track bed paving machine.

Nibs have been cast in to the segments to support the rails in their temporary position prior to passage of the paving train.

Back grouting

Each segment is fitted with an EPDM rubber gasket supplied by Victaulic and the lining is back grouted from four injection points with a specially designed Pozament GP grout.

The shield has an over-cut of between 50mm and 100mm to allow for the minimum 740m radius curve at the east end and this is taking about 1m3 of grout/ring. “Because

we cannot retract the rams to build new rings while the grout behind the two previous rings is going off, we have looked hard to find a fast-setting grout,” said Dale.

“After experimenting we are satisfied now with the Pozament cement/fly ash grout with a mixture of silica accelerators. This is allowing us to achieve average progress of 50m/5 day week on a regular basis.” The grout must be mixed on the machine as the acclerators would not allow transport from the surface.

Once the tunnel is completed, a track bed will be cast before the tunnel is handed over to McGreggors, Murphy’s subcontractor, for track installation. Electrical and mechanical finishing will be completed

by British Rail.

Initial demand is expected to require only a single track carrying trains in both directions. This may be doubled later with a second single-track tunnel parallel to the first under the runway, to accommodate future increased demand.

This is the first major tunnelling work undertaken and funded by British Rail for some time. It should provide valuable experience for British Rail personnel who will be engaged on tunnelling projects that British Rail, as a client, is scheduled to embark upon to complete a new rail link between London and the Channel Tunnel.

At Stansted, Murphy is expected to have completed its 112 week contract

by June 1990 and trains are expected to be carrying passengers safely under the runway to and from the new Stansted international airport by January 1991. n

CASE STUDY: The West Runton outfall project in the mid-1980s was an early application of full face EPB mechanised excavation in the UK. It followed success of several Lovat TBMs on the London Water Ring Main of the same period (p29). The experience on the London Ring Main and here at West Runton indicates the scope for the significant development of the technology in the years since.

SITE VISIT REPORT: DECEMBER 1993

West Runton long sea outfall

UK FOCUS CASE STUDY

O

riginally, the 2km long West Runton Long Sea Outfall out under the North Sea was designed as a pulled pipe in a shallow trench and covered by 2m long concrete blocks and rock armour. Of six selected contractors who tendered for the job in November 1992, the contract was awarded by Anglian Water, the client, to Sir Robert McAlpine on the basis of its tunnelled alternative.

The seabed pipeline included a short length of tunnel and open cut work to take the pipe down the steep 20m cliff face to a point just beyond the low water tide mark where the seabed pipe would emerge and be pulled the final 1.5 km. McAlpine, with Howard Humphreys, its design engineer, offered to continue tunnelling and install the 660mm diameter outfall pipe in a 2.24m i.d. undersea bore.

“We studied the tunnelled option”, said Mr Paul Mitchell, Projects Manager for Anglian Water, “and although it offered several favourable advantages, including reduced impact on this environmentally sensitive area and elimination of the weather factor on the construction phase, we selected the seabed pipeline as its estimated cost was considerably less than our tunnel estimate. McAlpine’s alternative, however, at about £5.5 million was within the same range as the tenders received for the seabed pipeline and about £2 million less than our budget estimate. After careful study of the alternative together with Gerard Pakes Consultants, our independent tunnelling consultant and construction supervisor, we accepted McAlpine’s tunnel alternative. Notice to proceed was granted in April 1993 and the contract period was extended from an original completion date of November 1993 for the seabed pipeline to June 1994 for the pipe laid in the tunnel.”

EPB tunnelling

In preparing its alternative, McAlpine chose the EPB tunnelling technique to work through the upper chalk geology off the Norfolk coast near Cromer. For most of its length, the 2.2km long tunnel is subject

to the hydrostatic pressure of the North Sea above with the tunnel a minimum two tunnel diameters (5.3m) beneath the seabed and a further 18m below sea level at high spring tides.

The chalk of the area is classified as Grade 4/5 with areas of Grade 3.

Grade 5 is completely weathered chalk without bedding or jointing. Grade 4 is less weathered and is friable and rubbly with some defined bedding and jointing while Grade 3 is pockets of unweathered chalk within the weathered zones which is moderately strong, rubbly to blocky chalk with some washed joints which have opened up due to the action of frost. The permeability of the upper chalk was defined as variable with K values ranging between 10-3 to 10-7, depending on the grade.

The chalk is also characterised by its irregular bands of flint. The geological forecast predicted flint bands of up to 3m thick containing cobbles of about 150mm diameter with thicknesses between bands being about 600mm to 1.6m apart. There is also the possibility of meeting very large flints or paramoudras which can measure up to a metre or so in diameter.

When in the market for its TBM, McAlpine considered using one of the Lovat EPB TBMs owned by Thames Water and used on the London Ring Main Project. However, at 2.93m diameter, these are larger than needed at Cromer and the cutting heads needed modification to cut through the flintbands. In addition, a larger machine would use more power, require a larger segmental lining and would extract more spoil. The cost of the used machine proved only marginally less than that of a new machine.

After approaching various manufacturers, McAlpine chose a new 2.64m o.d. TBM from Dosco. Well known for its shield mounted roadheaders, this is the first full face TBM and the first EPB machine designed by Dosco. The TBM is fitted with a 380mm diameter x 5m long screw conveyor passing through the centre of the bulkhead into the excavation

• Client: Anglian Water • Contractor and Consultant for the design-build contract: Sir Robert McAlpine and Howard Humphreys chamber. The cutterhead is dressed with bullet-type picks and spade-type tools. It is also equipped with hydraulically controlled flood doors that can seal the excavation chamber almost watertight when closed.

The 5m long articulated shield is powered by two electro-hydraulic motors.

The 185kW motor drives the cutterhead while a 75kW auxiliary motor powers the screw conveyor, belt conveyor, shove rams, articulation rams, and the segment erector.

By removing the screw conveyor and extending the belt conveyor into the excavation chamber, the TBM can operate in the open non-pressurised mode. For West Runton the machine was delivered with the screw in place ready to be launched in the closed pressurised mode.

With a tight tunnelling programme of 26 weeks in the 58 week contract, delivery of a new machine was a major consideration for McAlpine. The TBM needed to be delivered to site within 14 weeks of placing the order. This Dosco achieved, delivering the machine in July 1993. When it arrived, the TBM was lowered nose down into the shaft in one full piece of about 45 tonne weight.

The 32m deep shaft lies about 100m from the cliff edge and was sunk 21.5m into the chalk as a caisson and taken to full depth by underpinning. The 23m long back shunt tunnel, which facilitates muck train movements, was excavated by hand.

After the normal two to three week launch and commissioning period, routine tunnelling from the bottom of the 6.1m i.d. x 32m deep shaft started in midAugust 1993. From the 350mm x 350mm guillotine gate in the screw conveyor casing, extruded muck is transferred by

The tunnelled outfall alternative is part of a waste water management system which links five coastal communities to a new treatment works in Cromer and discharges treated effluent 2.2km out under the North Sea

26

Rock hard sponges

The delay has been due in large part to the high concentration of flint. Flints were formed by the digestive action of sponges which extract silica from their marine environment. After millions of years, their remains become fossilised as nodules of very dense and brittle silica deposited as irregular layers in the chalk.

“Although the site investigation involved shallow bore hole cores for a seabed trench, we were aware of the presence of these flints,” said Mike Duggan, Senior Tunnelling Engineer for McAlpine. “Large boulders of flint were retrieved from the shaft excavation and from other excavations in the area. As such, we accepted variations in geological conditions as being our risk under the ICE 5th Edition contract.”

An open faced shield-mounted roadheader boom would have been ideal for working around flints but the potential risk of running into swallowholes precluded an open machine. The disadvantage with

the closed EPBM is that easy access to the face and the cutterhead is lost. This is counterbalanced by the security of operation provided by the EPB technique when faced with water emanating under considerable pressure from fissured rock, and its ability to excavate very soft as well as hard ground.

In chalk with reasonable flint content, the EPB has performed very well. An effective plug of material has been established in the 2.5m3 excavation chamber and through the 0.6m3 capacity of the screw tube without the need for modifying agents such as bentonite or polymer muds. Careful control of the Archimedes screw through its 0-29 rev/ min rotation and skilled operation of the 350mm x 350mm guillotine gate in the screw casing maintains the earth pressure balancing plug of muck that holds back the water content and allows a steady extrusion of homogeneous paste. Once established, the plug of muck can be maintained for days at a time including over weekend breaks when the TBM has stood with the flood doors open and the guillotine gate closed.

Advance slows down drastically when large concentrations of flint are encountered and it is under such conditions that the EPB plug of muck can be lost. Unless the situation is controlled by reducing the speed of the screw and partially closing the guillotine gate, too much muck is extracted, the plug breaks down and an inrush of water escapes through the screw.

“In largely chalk material, we can cut a 1m stroke in about 10 minutes”, said Duggan, “but when the flint content rises, it takes much longer. On occasion, it has taken almost a full shift to cut a stroke through what appears, from the extruded muck, to be a face almost entirely of flint.

The largest piece we have retrieved from the screw conveyor is 250mm diameter and 125mm thick with the knapped edges indicating that it has been broken away from a much larger flint.”

When slow rates were first encountered, the concern was that the teeth and cutter head may have experienced excessive wear. To avoid having to apply compressed air to the whole tunnel and execute a safe man entry, McAlpine deployed fibre optic

camera and video recording equipment developed for the oil industry to inspect the cutterhead.

After backing up the TBM about 150mm from the face, compressed air was applied through a port in the bulkhead and the small camera lens was passed through another and out beyond the opened flood doors.

This revealed that the cutterhead and teeth were in good shape and that slow going was attributable to cutting through the flints.

When cutting flint, the TBM shove pressure can rise to the maximum afforded by the 8 x 55 tonne rams. Torque also rises toward the maximum of 676kNm at 1.7rev/min.

“This slow going through frequent bands of flint has limited our average advance to about 4.7m/day,” said Brady.

“To increase production we have introduced a third tunnelling crew to allow continuous 24h/day, 7 day/wk operation and we are discussing with the client methods by which we can expedite the finishing works and meet the due completion date.”

Results of this change were promising but when all appeared to be under control, chance threw up two events associated with the risk element of tunnelling. First, progress suddenly reduced and the cutterhead stalled. Following this a piece of bent and folded steel about 500mm x 250mm emerged from the screw conveyor.

Laboratory examination of the steel indicated that it is of a grade commonly used for borehole casing, despite the fact the nearest site investigation borehole in that area should have been 20m distant from the tunnel axis.

Since running into the steel casing, it has been difficult to maintain an effective plug of muck and progress has slowed.

“My fear is that a piece of the casing is either wrapped around the teeth or is being dragged along by the edge of the shield”, said Duggan. “In addition, a sharp increase in the volume of grout needed to backfill the annulus behind the lining suggests that a snagged piece of steel could well be increasing the overcut. This would allow more water to run down the annulus around the TBM into the excavation area which could be preventing the creation of the necessary EPB plug of muck.”

Secondly, after a week of wet working conditions, it appeared that condensation

UK FOCUS CASE STUDY

the intermediate belt conveyor into trains of three, 2m3 Decon muck skips hauled by four tonne BEV and Clayton battery driven locos. At the pitbottom, skips are lifted singly out of the shaft and muck is stockpiled on adjacent green field space ready to be used as backfill material around the outfall pipeline in the completed tunnel.

By early November 1993, the EPB tunnel had advanced some 450m out under the North Sea achieving a best to date of 20 rings or 20m in a two 12h shift day, erecting rings of a bolted tapered trapezoidal precast concrete segmental lining supplied

by Charcon.

Each 1m wide ring comprises six segments which are fitted with a hydrophilic water sealing gasket supplied by Heinke.

“Our schedule for the 58 week contract is based on a tunnelling average of 8 rings (8m) per shift (16/day)”, said Mr Chris Brady, Project Manager for McAlpine. “However, since starting in mid-August on a two 12h shift/day, 5.5 day/week routine, tunnelling has progressed slower than predicted and we have slipped behind our optimum programme. It is hoped however, that the higher rates recently achieved can be maintained over the remainder of the drive.”

The cutterhead of the Dosco EPB TBM is equipped with flood doors between the three excavating arms and is fitted with a 380mm diameter x 5m long screw conveyor

27

UK FOCUS CASE STUDY

By maintaining an effective plug of muck in the excavation chamber, material extrudes through the screw conveyor and off the intermediate belt conveyor into the muck skips as a manageable paste in the main cutterhead drive motor caused it to fail. During the downtime while the motor was away being rewound, it was intended to shut the flood doors, remove the screw conveyor and attempt a manentry inspection in free air. Unfortunately, up to the time of writing, efforts to shut the flood doors have failed indicating further that these maybe obstructed by a piece of the borehole casing. With the TBM stationary under about 2 bar of pressure and with up to 1,000-1,500litre/ min of water expected to flow from the face when the flood doors are fully open, man entry has not been possible. As such the use of compressed air on the tunnel is being considered. Under compressed air, any possible borehole casing debris will be removed and any worn tools will be replaced ahead of the remaining 1.7km of the drive.

In the meantime, it was decided to move the splash proof drive motors out from within the lower part of the shield to a more protected position under the belt conveyor and moving the transformer from there to a position on top of one of the trailing gantries. Tunnelling is now under way again (as of mid-November) and the decision on whether to apply compressed air is being reserved.

Tunnel finish

Despite the ground conditions, the tunnel as built is of good quality. The hydrophilic sealing gaskets provide for a virtually dry tunnel which is important on what is a maximum 1 in 90 downhill drive. McAlpine would have preferred to drive the tunnel on a level or slightly uphill grade, but with the seabed dipping away sharply from the cliff edge, the launch shaft would have to have been twice the depth to effect an uphill drive on the 2.2km length. Water make, principally through the screw conveyor in the absence of an effective muck plug, is pumped out by Flygt submersible pumps to a three-tank settling system on the surface. With the assistance of chemical flocculents, chalk particles are removed to the 200 parts per million maximum before water is discharged back into the sea.

Substantial completion of the contract is governed by the May/June 1994 commissioning date of the new treatment works at Cromer from which treated effluent will be pumped to the outfall. To compensate for slower than expected progress on the tunnel, Anglian Water has re-examined the merits of McAlpine’s first proposal to fill the tunnel around the 660mm diameter HPPE (high pressure polyethylene) outfall pipe with water rather than backfilling the space

Flints appear as horizontal firmly embedded bands in the chalk face of the hand-excavated back shunt tunnel

28

with the excavated muck. This has the added advantages of allowing access for subsequent maintenance and repair of the pipe and of providing valuable underground space in which to install a further pipeline for increased outfall capacity or other purposes.

In accepting the proposal, the tunnel is upgraded from a temporary works facility to a permanent structure. The minimum 150mm thick reinforced segmental lining meets this requirement adequately. Any damaged segments will be repaired and the occasional leak due to displacement of the hydrophilic gasket will have to be grouted up. In addition, the 13,000m3 of chalk spoil will be transported to a suitable landfill disposal site.

Progress through the flint laden chalk is difficult to predict but, nonetheless, it is expected that tunnelling and pipe installation will be complete in time to meet the system commission date next summer.

In the meantime, McAlpine has installed the five outfall diffusers at the far end of the tunnel. The past summer season has been particularly stormy and work from the jack-up drilling rig has taken longer than hoped, but this is now complete and remains off the critical path.

When the TBM reaches the end of the drive, the components will be removed and the shield left in place. The connection to the offset diffusers will then be achieved by hand-excavated headings through grout-consolidated ground and under compressed air if necessary.

For Anglian Water, the outfall is part of the larger North Norfolk Waste Water Management project which is designed to meet European Community directives for effluent discharge. Under the £21 million capital investment programme, five seaside communities will be linked to the new treatment works at Cromer via some 15km of open trench pipelaying and five new pumping stations.

Once in operation, the system, with a treatment works capacity of 440litre/sec or 38,000m3/day, will eliminate existing short sea outfalls of raw sewage and provide much improved management of waste water generated by the regional winter population of 19,000, rising to about 40,000 in the summer holiday season.

“Of the outfalls we operate along the coast line in our district, this is the first to be installed in tunnel in recent times,” said Mitchell for Anglian Water. “It has brought with it problems inherent with tunnelling but it has overcome major difficulties associated with the seabed pipeline alternative. The waters off the Norfolk coast sustain an important crab and fishing industry. Limiting the detrimental impact of a seabed operation on this marine environment was recognised as a major challenge and the potential for a seabed operation to be weathered off over this past summer could well have seen construction work continuing through next summer, delaying commissioning of the project for a year or more. Weighing up the pluses and minuses, we are pleased to be working under the seabed rather than on it and Anglian Water remains in support of the cost effective EPB tunnel alternative.” n

CASE STUDY:

The Thames Water London Ring Main water supply project was a project of major advance for tunnelling in the UK. Under the leader ship of a progressive client management team, several firsts were achieved on the project that led to the start of a period of more rapid development in machine technology and procurement, lining design and contract procurement. At the end of the project, the full project of the 80km long programme of tunnelling was reported as being completed and delivered two years ahead of programme and under budget.

SITE VISIT REPORT: DECEMBER 1990

• Client: Thames Water

Geared up for London Ring Main drive and beyond

• Contractor: Segmental lining designed jointly by Charcon and Miller Construction

said Chris Hughes, Tunnel Manager for Miller Construction. “This ruled out the use of compressed air as the pressures required would be too high.”

Today, compressed air tends only to be used where absolutely necessary. It has instead been successfully usurped by modern tunnel boring systems. “We looked at several soft ground tunnelling systems,” said Hughes, “including those from Germany, Japan and the UK. But it was the Lovat EPBS system that we chose. At £1 million, it was not the lowest cost machine offered, but it is a robust, well designed, flexible machine with good service back-up.

Lovat gave us the best delivery of four months from placing the order and in addition to this, the system has a proven track record.”

After submitting a tender based on the Lovat EPB machine, Miller won the contract in October 1989 with the lowest bid price of £11.2 million and bought the 99th TBM to leave the Lovat factory in Canada.

It is among 14 Lovat TBMs sold to UK contractors, three of which are working or have worked on the Ring Main project.

The 3.38m o.d. machine is of the basic Lovat design with the EPBS extras.

It operates as an open, non-pressurised shield when in stable conditions and can be converted to a pressurised EPBS system when in wet, unstable ground. In addition to the familiar hydraulically operated flood doors on the cutterhead, the TBM is fitted with the Lovat pressure-relieving gates.

These are positioned at the top of the stationary muck ring inside the excavation chamber. As muck is scooped up by the rotating cutterhead and dropped on to the

hydraulically operated gates, earth pressure must build up sufficiently to overcome the pressure of the hydraulic rams keeping the doors closed.

EPBS mode

Should conditions deteriorate further, the flood doors are closed to provide a nearly watertight bulkhead, and the shield is converted to the EPBS mode. This involves removing the central belt conveyor and replacing it with a single 48cm i.d. x 7m long screw conveyor bolted to the conveyor portal in the bulkhead between the working area of the shield and the excavation chamber in front. Under such conditions, the muck chamber is completely filled and muck is extruded through the screw conveyor.

The tunnel is being lined with bolted precast concrete segments. Design of the lining was not specified by the ICE Fifth Edition form of contract let by Thames Water, other than it must be bolted. Instead, it was designed jointly by Charcon and Miller Construction.

The approved primary lining is quite unusual compared to designs used so far in the UK. There are five segments and a key in each 1m wide x 210mm thick ring. These are fastened on the circumferential joint by Charcon Fastlock dowels and by 220mm long curved bolts on the cross joints. All five segments are identical, with a tapered key (Fig 2). The segments are bolted together as they are positioned, starting in the invert.

It takes about 15 minutes to build one 1m wide ring. The identical design of each segment eliminates the need for specific stacking and transportation.

Each 0.75 tonne segment is lifted into

UK FOCUS CASE STUDY

S

ubstantial capital investment in new tunnelling equipment is paying off for Miller Construction. Work on its 142 week tunnel contract for Thames Water was in its 50th week and excavation of the 3.8km long x 3.38m o.d. tunnel was set to finish by mid November 1990. Much of this satisfactory progress is credited to the purchase of new equipment and the establishment of a well controlled tunnelling operation, according to Tommy Talbott, Managing Director of the tunnelling division of the family-owned Miller Construction contracting company.

“When we tendered for this length of the London Water Ring Main project, the company decided to invest in the latest modern equipment including locos and muck skips,” said Talbott. “It was considered that this would provide the best opportunity not only to complete this contract satisfactorily, but also give us an advantage when tendering for many of the future tunnelling projects that are planned by the water authorities and other clients.”

As in many urban situations, the 3.8km long tunnel from New River Head to Barrow Hill, between 40m and 75m under the streets of London, would include tunnelling through soft, waterbearing ground. The geological report predicted sand lenses in the Woolwich and Reading Beds of the first third of the run, Thanet Sands in the middle third, and Woolwich and Reading Beds again in the final third, with a short length of good quality London Clay before finishing at Barrow Hill (Fig 1).

“The contract specifications required that the tunnelling method be able to cope with up to 4 bar of ground­water pressure,”

• Lining Design: Charcon and Miller Construction

Left: Fig 1. Alignment of the 3.8km long New River Head to Barrow Hill section for the London Water Ring Main; Right: Filling of the muck wagons to be empties into the shaft hopper for hoisting to the surface

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UK FOCUS CASE STUDY

Left: Fig 2. Bolted segments supplied by Charcon and designed by Charcon and Miller Construction; Right: Segments positioned by the erector arm before being bolted together position by the Lovat TBM segment erector arm. A threaded inset cast into the centre of each segment acts as a receiver for the lifting ball. It is also the grout hole through which the cement/PFA grout is injected to fill the annulus. The grout is mixed on the 60m long Lovat designed and manufactured trailing backup.

Grout injection is carried out immediately the new ring is erected. A thick, fibrous grease supplied by Mobil is constantly and positively fed into the space between the double-row wire brush tail seals and their wire filaments to prevent ingress of groundwater or grout. The main bearing is constantly fed with the same grease.

In-situ concrete lining

The segmental lining is largely non­gasketed.

In wetter conditions, Hydrotite is fitted to the segments as required. Once the tunnel is excavated, it will be finished to the required 2.54m i.d. with a 130mm thick x 200kg/m3 reinforced in­situ concrete lining as specified

by Thames Water.

“Having invested in a high-speed tunnelling shield, we needed the necessary back-up equipment to ensure that maximum progress rates could be achieved and sustained,” said Bruno Speed, Project Manager for Miller. “For this reason, we bought two new 6.5 tonne battery locos from Omam of Italy, which supplied the units within four months of placing the order, and a fleet of new 2.5m3 side tipping muck wagons from Decon of the UK. We also installed an efficient muck hopper system for speedy service through the 50m deep working shaft.” Loaded muck cars are emptied into the 20m3 muck hopper set into

the floor of the shaft which is then hoisted to the surface by a specially noise-proofed 80 tonne diesel hydraulic crane. It takes one train to muck out each ring and the shaft hopper holds one train load.

California switches

The TBM was assembled in a week and started tunnelling in March 1990. It is expected to complete its 3.8km long single heading drive by mid-November 1990.

Working on a two 10hr shift/day, five days/ wk, the best advance rate achieved was 16m/shift. To keep pace with the TBM, California switches were installed at 800m, 1,800m and 2,800m and the number of muck trains was increased to five.

Although the TBM did pass through some very wet ground, the EPB screw conveyor was never required, since the flood gates and the pressure relieving gates on the muck ring were capable of coping with the situation by themselves.

From the 50m deep access shaft at New River Head, the line of the tunnel has taken the TBM through three gentle curves, the tightest being 170m long on a 500m radius. It has also taken it under the King's Cross main line railway station with only 10m cover to the Northern Line of the London Underground system. In this urban environment, settlement is a major consideration. Despite the fact that the tunnel is on average about 45m below the surface, precise monitoring stations have been established throughout the alignment.

Another major consideration was the possibility of encountering dangerous gases. Strict 24h/day monitoring to detect any poisonous gases such as methane or

SITE VISIT REPORT: DECEMBER 1992

London Water Ring Main feeder

E

fficient muck removal, mechanised segment handling, and a well organised routine by Costain has brought about safe, record time completion of the 7.8km long Moor Lane to Ashford Common connection as part of the London Water Ring Main project (Figs 1 and 2). Costain completed the 7.8km x 2.59m i.d. tunnel in two drives of 4.5km and 3.3km each. Despite these distances, average progress was 260m/week on the first drive and 300m on the second. All tunnel excavation was completed in nine

30

months. As many as 51 rings of 1m wide x 180mm thick precast concrete wedge block segments were erected behind the Lovat TBM in a single 12hr shift. Best advance for a single 24hr day was 100m with 442m being the best advance for five non-consecutive working days or 10 shifts totalling 112 hours.

The tunnel runs an average 50m below the surface and entirely through Blue London Clay, a competent material that allows effective excavation by the Lovat machine and installation of the cost effective precast

dangerously low levels of oxygen is required by the contract for the 3.8km single heading drive. With no intermediate access points, fresh air is drawn in from the working shaft and conveyed along the tunnel in 400mm diameter ducting. Before entering the tunnel, everyone is logged on the tally system and equipped with a self-rescue kit.

Miller Construction prides itself on running tight safety procedures as well as a strict quality assurance programme.

Miller is one of the first tunnelling contractors to approach the BSI for quality assurance accreditation.

No hole through

The shafts at either end of the tunnel were both completed under earlier separate contracts. The connecting shaft at Barrow Hill is already fitted with Ring Main pumping equipment. The Lovat TBM cannot therefore hole through. Instead, Miller will drive to within 25m of the shaft and excavate a chamber around the machine, where it will be dismantled for withdrawal backwards to the working shaft. To accomplish this, the TBM is the first double-skinned Lovat TBM that is bolted instead of welded together. The remaining 25m of tunnel and breakthrough into the shaft will be accomplished by hand mining methods.

Once tunnelling is finished and the TBM is out, Miller will cast the in-situ secondary lining and carry out finishing works in the New River Head access shaft to complete its contract. Meanwhile, with a new outer skin, the Lovat TBM and all the associated equipment, that has performed to specification and beyond, will be ready to move to the next job. n

• Client: Thames Water • Design-build Contractor: Costain • Design to Contractor: Charles Haswell & Partners concrete wedge block segmental lining.

Like all the current Ring Main contracts, the TBM and the segments for the lining are supplied by the client, Thames Water.

The Lovat TBMs are driven essentially by electrical and hydraulic motors and have an installed power capacity of 650hp with two 250hp motors driving the pumps for the cutterhead and one 125hp motor driving the pumps for the ancillary hydraulics including the muck conveyor. The TBMs

came with a screw conveyor, muck ring, tail skin and triple tail seals for conversion of the tunnelling system to the bolted precast lining system should ground conditions deteriorate and prevent effective application of the expanded wedge block lining – a facility never needed on the Costain job.

Along with the TBM systems, Thames Water as the client, also provided to each contract all cables, hoses, consumables (including hydraulic oil and grease) and all necessary spare parts. As owner of the TBMs, Thames Water, in conjunction with Lovat, has specified a routine maintenance programme for all the machines. The hydraulic oil, supplied by Condat of France, is an environmentally safe, biodegradable, water-based product.

Under a £10 million contract with Thames Water, Taylor Woodrow supplied the segments of the precast concrete segmental lining for the last 33km of the Ring Main. For the most part, the wedge block lining is unreinforced, except for immediately adjacent to shafts. Designed to carry some 1,000 million litres of water/ day with a gravity flow pressure of about 6 bar, the finished Ashford to Moor Lane tunnel will be tested to this standard for working pressure.

Unlike more traditional designs, the wedge block ring designed by Thames Water is wider, at 1m instead of 686mm, and has fewer segments per ring, eight instead of 12 including the key. This has contributed substantially to the increased progress rates being achieved. More than 1km of tunnelling has been completed by three Lovat machines for Thames Water in one week, an impressive achievement by any standards.

The new wider rings of segments have, however, created their own particular set of

considerations. Perhaps most importantly, more unsupported ground must be exposed before the 1m wide ring can be erected. In good quality clay this is not a problem, but should the clay deteriorate or become blocky, problems can arise.

As this was the first of the three final sections of the Ring Main project to be let, Costain was involved in the design and specification of the Lovat TBMs procured

by Thames Water. Recognising the potential problem of the length of exposed clay, Costain engineers recommended a set of seven 1.5m long x 10mm thick fingers on the back of the shields to provide partial support until the ring is built. A plough ahead of each finger cuts a channel to prevent them from becoming trapped behind the expanded ring.

The larger, heavier segments also require more mechanised handling.

Segments are handled entirely mechanically from the delivery truck to installation at the tunnel face. At the pit bottom a mechanised automatic segment handler, designed jointly

by Bennett Associates and Costain, lays the segments in correct ring build sequence on the bogies. In the tunnel, the integrated segment handling system, including the segment carrying bogies on the service trains, was designed jointly by Lovat and Thames Water. At the face, a mechanical handling system lifts the segments, turns them simultaneously into circular position, and moves them forward for lifting and placing by the Lovat erector arm.

Overcut

A critical consideration when using the wedge block system of segmental lining is the exact cut diameter of the tunnel.

The integrity of the wedge block system is derived from using the key for expanding

the non-bolted segments out against the ground. Too large a cut diameter and the segments will not remain in stable compression. Too tight a cut diameter and the wedged key cannot be forced all the way into the ring. The actual size of the bead, or over cut, on the front end of the shield can only be determined once tunnelling is underway. On the Costain contract, it has taken several attempts to arrive at the correct overcut requirement.

This exercise was exacerbated by using a long shielded TBM as opposed to an open-faced shield.

The Lovat machines are designed with all power packs in the shield rather than on trailing gantries. This leaves ample space in the back of the shield for segment handling and ring building, but it also results in a larger shield length-to-diameter ratio, compared to open-faced shields, which delays build of the lining at the end of each excavation stroke. In that time, the clay has sufficient time to begin to relax. This reduces the cut diameter of the tunnel making build of the wedge block lining more difficult.

Initially the 2,952mm diameter shield was supplied with a 17 piece x 6mm bead and a spare 12mm bead. “The front end of the shield is especially machined for the cutting bead to be attached by 51 cap-head screws,” said Andrew Smith, Project Manager for Costain. “But even after fitting the 12mm bead, the cut was too tight for correct ring build. Keys had to be shortened while larger beads were manufactured. Squeezing in of the clay, coupled with the length of the TBM, required a 33mm bead,” said Smith. “After fitting the 33mm bead, no further problems were experienced with driving the keys home at the correct pressure.”

The clay on the second drive was poorer than on the first according to Smith.

UK FOCUS CASE STUDY

Left: Pit bottom setup with muck trains emptying into the shaft hopper installed behind the aluminium safety gate Right: The 2.59m i.d. water tunnel is lined with a one-pass wedge block segmental lining tested to a maximum 9 bar internal pressure

Left: Lovat designed segment handling to ensure rapid ring building behind the TBM; Right: A set of seven 1.5m long trailing fingers provide partial support of the exposed crown of London Clay until the lining is in place

31

UK FOCUS CASE STUDY

Above: Fig 1. Plan of the Costain 7.8km contract for Thames Water; Right: Fig 2. Route of the 80km long London Ring Main

32

“It has a higher moisture content at 22% as opposed to 17% or so. It is also more fissured and blocky at this depth of 45m of 50m below the surface. The softer clay tends to ball up in the excavation chamber slowing progress and the softer crown will squeeze on. Also, the slick sided clay blocks will drop from the crown.”

During the site visit, the TBM had almost finished the second drive and had been standing at full stroke for an hour or so for a minor repair. In that time, blocks in the crown of the exposed clay had dislodged proving the worth of the trailing fingers on the shield.

Cycle times

The Costain schedule for the contract is calculated on a 15 min/1m cycle. It was actually achieving an average of about 12 min/cycle with 6 min to excavate a 1m stroke and 4 min to build a ring.

Interruptions in the muck hauling cycle, connecting new lengths of cable and ventilation duct, and other routine stoppages account for the remainder. “Our estimated long average was for 200m/week working two 12hr production shifts Monday to Thursday, with two 8hr shifts on Friday”, said Smith. “We have actually achieved a production average of 300m/wk including driving curves of 800m and 1,200m radius and maximum 600m in length requiring frequent repositioning of the laser and prism beam deflectors.”

The muck hauling system, always a significant factor in production rates, is based on trains of 7 x 2m3 side tipping skips from Decon, hauled to the shaft bottom by 5 tonne Clayton electric locos. Each train carries the 13m3 of muck/1m ring (about 7.25m3 solid) while the locos are supplied with high capacity batteries which allow a maximum 42km between charges for the given drawbar pull and gradient.

During construction, the single haulage track was divided every 800m with a switch.

Towards the end of the first longer drive, seven muck trains and four switches were in service and trains were programmed to travel between crossings in 6 min. For safety’s sake, the 40m-long trains were always pulled, never pushed. As such, the locos changed ends on a switch towed behind the TBM and another at the shaft bottom.

At the shaft, the side tipping skips are emptied individually into a 20 tonne hopper which is lifted to the surface for onward muck disposal. Two tower cranes service

the working shaft. One 30 tonne unit is dedicated to the muck hopper giving a turn round in the 50m deep working shafts of about 6 min. The second smaller 12 tonne crane lowers segments and supplies.

The three 50m deep shafts on the contract 10.3m i.d., 9m i.d. and 12.5m id. will ultimately house large diameter steel connecting pipe work and high integrity valves. The shafts were sunk as caissons with steel cutting edges through the 6-8m of flood plain gravel at the surface, and underpinned to full depth using precast concrete segments supplied by Charcon with some from C V Buchan, all to Thames Water’s order.

The 400mm ventilation ducting, supplied

by Flexadux, is paid out in 200m lengths from cassettes on the TBM trailing gantries. A rigid GRP duct in a flat oblong shape of equivalent cross section allowed safe passage of the filled skips at the slightly elevated crossings.

Muck in the skips was levelled automatically before leaving the TBM.

Safety

Given the long distances between relatively deep shafts and the high level of activity from shaft top to tunnel face, safety is a major consideration. As such, and in conjunction with Thames Water, Costain has produced a site-specific safety manual.

All employees, including management, are acquainted with the safety procedures described in the manual, and regular induction and refresher sessions remind all of safe working practices as well as correct emergency procedures.

Because employees and tunnelling crews must be constantly aware of the underground working environment, special attention is paid to safe means of entry to, and exit from, all confined spaces.

Gas monitoring is undertaken as a routine procedure and anyone entering the tunnel carries a self-rescuer and an identity tag.

Key personnel, such as crane drivers and banksmen, are only appointed once they have demonstrated a competency in all safety procedures and, where required, can produce the necessary registration certificate.

In the event of a major incident, the site is well prepared. A well equipped rescue

bogie, designed specially by Costain, stands on a siding in the pit bottom ready at all time for immediate use. It has space for a stretcher and medical attendant and is equipped with a comprehensive first aid kit, resuscitation equipment and multiple breathing sets using umbilical hoses. In addition, Costain has arranged with the nearby Ashford Hospital to utilise its emergency rescue service. Known as BASICS (the Berkshire and Surrey Immediate Care Service), this provides a fully equipped doctor to an accident or emergency site within 15 min.

The fact that no accidents of a serious nature have occurred on the contract to date is witness to the emphasis that Costain and Thames Water place on safety and is a credit to the working routine established by Costain on a tunnelling job where such high advance rates have been achieved.

Target cost contracting

Similar to the other Ring Main contracts for Thames Water, Costain is completing its almost 8km long contract as a designconstruct target cost contract based on the I.Chem.E. Green Book form of contracts. As Costain’s engaged designer, Charles Haswell & Partners completed the detail design not only of the tunnel but also of all the complex connection into the Ashford treatment plant and the Wraysbury reservoir. It also completed the site investigation study. This included bore holes every 200m to 5m below tunnel invert to confirm the predominance of London Clay at tunnel alignment. A number of shorter interim holes were sunk to determine the upper gravel/clay interface and to identify, if possible, the presence of scour holes, a potential anomaly at the outer edges of the London Clay basin.

After launching the Lovat TBM in July 1991, the final breakthrough occurred in early May 1992. After completing the finishing works, Costain expects to hand over the finished job on schedule in June 1993.

Top quality projects with clean safety records such as this, elevate tunnelling from a perceived dangerous occupation to a dedicated trade and profession, undertaken by informed and safety conscious employees. n

CASE STUDY:

A team of site engineers working on a tunnelling project in the UK some 25 years ago, brainstormed the concept of a vacuum segment erector to solve damage cause by a mechanical gripper erector. Since then the concept and technique has become an indispensable element of modern, TBM-excavated, segmentally-lined tunnels.

SITE VISIT REPORT: MAY 1983

UK development of a vacuum segment erector or spalling the edges with increased gripping force. The problems were exacerbated by the different geometry of the ordinary, top and key segments.

Andrew Draper, Thyssen’s contract Agent at the time, said that it was ‘’while sitting in the car at the Datchet level-crossing on the way home’’ it occurred to him that a vacuum lifting device might work.

A quick bit of mental arithmetic, based on segments weighing approximately 375lbs and using a vacuum force of 12psi, a vacuum pad would require an area of about 31in2 for a successful lift. For practical purposes, and with a factor of safety of 4, the pad area would increase to 125in2. On arriving home Draper said: “I jotted down a few calculations that confirmed my original thoughts.’’

The following morning, he set up a meeting with his Thyssen colleagues on site, Brian Smith, mechanical engineer, Rob James, sub-agent, and Danny Travis, foreman fitter.

Draper recalls; “some, including the machine’s manufacturer, were sceptical of the idea but it was decided to set up an on-site trial. Danny was brilliant and in no time he had cannibalised a vacuum pump from and old Sykes sump pump and had made a crude vacuum pad glued to a curved steel plate using draught sealant. This was rigged up in the site workshop and tried out with the help of a forklift truck. Somewhat to the surprise of bystanders, it worked! First time. Even with someone standing on the segment!’’

Things moved rapidly from there. In less than a week, the first vacuum erector with site-made vacuum pads was fitted to the shield.

“The lads on the machine thought it brilliant,’’ said Draper. Each segment needed a small hand brush sweep to remove any dirt, but unlike the steel finger mechanical erector, the vacuum pads avoided gripping the edges and needed no precise locating on the segment for operation. As a result, production rapidly

• Client: Three Valleys Water Company • Contractor: Thyssen (GB) Ltd • Machine Manufacturer: Markham reached the target of 25 rings/shift.

Development of purpose-made suction pads increased production rates further.

Following these demonstrations, shield manufacturer Markham designed and produced a factory-made unit which enabled frequent production rates of 40 rings (90ft or 27m) per shift and a best of 54 rings, 122ft or 37m in a single 12hr shift.

At the time, a patent application was made and subsequently granted to Thyssen (BG) and naming Andrew Draper and Brian Smith as co-inventors.

Following this application, Markham developed vacuum erectors for new tunnelling machines in the UK. The first was for Miller Construction for its North London Flood Relief Tunnel in Islington for Thames Water that was under construction at the same time as the Three Valleys project. With the patent pending, Thyssen (GB) agreed this license.

In years to come, the technique has been perfected to lift ever heavier segments. The first Herrenknecht machine with a vacuum segment erector was also for a UK project - the backhoe shield manufactured for the Docklands Light Rail tunnel connection to the Bank Underground Station in London in 1988.

Vacuum lifting devices have been used in sheet metal industries and in precast concrete factories for many decades. In the tunnelling industry, vacuum segment erectors are now an industry norm and are vital for lifting the increasingly more common steel-fibre reinforced segments and for building rings of enormous segments behind the mega-diameter TBMs of today.

And it all began in the UK as the possible solution to an irritating problem and in the Thyssen (GB) workshop, on the Three Valleys tunnel project in 1983. n

UK FOCUS CASE STUDY

I

t was while excavating a 6.2km long tunnel near Heathrow Airport in 1983 for the Three Valleys Water Company that a new segment build handling system was needed. The 100in (2.54m) internal diameter tunnel was being built by Thyssen (GB) Ltd through London Clay and was being lined with 27in (685mm) wide rings of wedge block or expanded precast concrete segments, each ring comprising nine ordinary plates or segments, two top segments and a wedge-shaped key at the crown to expand the ring tightly against the clay. The system works without any bolting or fixings on the circle or longitudinal joints and without any back grouting.

Shield manufacturer Markham of Chesterfield was engaged to supply an open-face tunnelling shield with a backhoe excavator and conveyors to discharge muck into rail-bound skips, and an erector to lift and place the segments.

This was selected, in preference to a fullface rotary type excavator, as being the most economical solution for the length of tunnel, the planned outputs, and the anticipated ground conditions.

Historically, tunnels of this type and size in the UK had been driven successfully using full-face rotary type excavators, such as the Kinnear Moodie Drum Digger, and with segments erected manually. Potential clay pug in the face of a rotary type excavator promoted the choice of a open face excavator shield and the weight of the individual segments on the Three Valleys contract. A desire to reduce manual labour wherever possible led to design of a segment transport system and a mechanical erector for the machine.

The rotary-type erector supplied with the shield had steel claw grippers designed to pick up the segments from the invert and rotate them into position. It quickly became apparent however, that the detail would not achieve the planned production rates.

Despite some modifications, the gripper system was either dropping the segments

From left: The project’s backhoe shield in Markham’s factory; The vacuum erector prototype in action; Right (top): Purpose-made suction pads; The engineered erector in trials and holding a segment plate

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CASE STUDY: The Heathrow baggage tunnel was the introduction of steel fibre as concrete reinforcement for precast segments and the further development for reinforcement for shotcrete. Today steel fibre reinforcement for segmental linings is widely accepted and adopted for projects in the UK with further work required on establishing codes and standards for their wider international application.

SITE VISIT REPORT: OCTOBER 1984

A

s part of its design and construct contract for the 1.5km long x 4.5m diameter baggage tunnel under the runways of Heathrow Airport in London, Miller Civil Engineering seized the opportunity to use steel fibre, as opposed to rebar cages, to reinforce the segments of the tunnel's precast concrete expanded wedge block lining.

"Reinforcement in the lining is not required to provide extra load bearing capacity of the lining", explained Colin Eddie, Chief Tunnelling Engineer for Miller Tunnelling. "We are passing through good quality London Clay at Heathrow and unreinforced precast concrete segments have been used on several projects to line tunnels in London Clay, most notably, the recent work for the Thames Water Ring Main. But for the baggage tunnel, we have designed a 1m wide ring of fewer and thinner segments which require reinforcement principally for handling purposes and to withstand the thrust pressures of the tunnelling shield. Where a 1m wide ring of unreinforced segments for a tunnel of similar diameter (4.5m i.d.) would require 11 or so segments plus a key and be some 200mm thick, the steel fibre reinforced ring for the baggage tunnel is 25% thinner at 150mm and comprises seven segments and a key."

Although rebar is less expensive than steel fibre by weight, there are significant savings to be had by eliminating the cost of making the rebar cages and taking them out of the mould set up and production line.

The most tangible economic advantage at Heathrow is that with fewer segments/ring, each ring is quicker to build and provides faster tunnelling rates overall. In the good quality London Clay and using an open faced tunnelling shield from Dosco, Miller hopes to sustain and better a target average of 100m/ week over the 1.5km long tunnel, working two 10h shifts/day, five days/week.

Contract award

The Heathrow baggage tunnel is a £40 million project financed and managed by Heathrow Airport Ltd, part of BAA, the owner and operator of most of Britain's largest

airports. The project is being provided for British Airways, the largest airline operating from Heathrow. When in service, baggage in transit from flights arriving at Terminal 4 and leaving again from Terminal 1, 2, or 3, and vice versa, will be transferred through the tunnel rather than by road as at present.

The civil contract for the project was bid in late 1993 with Miller Civil Engineering receiving a letter of intent in December 1993.

With a tender price of £6.5 million, Miller believes it was innovative design which gave it the edge over its five competitors. Ove Arup & Partners is lead design consultant for BAA, and Bovis Construction is supervising the construction phase.

As set out by Ove Arup, the tunnel is divided into two drives, both of which must be driven from an airside central access shaft near Terminal 4. From here the two tunnel drives comprise a short run of 250m back to Terminal 4 and a longer drive of 1,200m under the main runways to the Central Terminal Area.

When the Dosco shield arrived in April 1994, it set off in early May to complete the short 250m drive first. After achieving modest learning curve advance rates and a best of 13 rings (13m) in one 10h shift, the shield broke through into the reception shaft on 23 June 1994. Following a speedy turn of only 16 days, the machine was relaunched from the central working shaft on 12 July to complete the 1,200m drive.

To late September, the tunnel had advanced 967m and was expected to breakthrough on schedule in mid October.

According to Chris Hughes, Manager of the Miller tunnelling division, the drive average has remained consistent at about 100m/ week with a shift best of 16 rings (16m) in 10h. The tunnel had also successfully passed under the London Underground Piccadilly Line with no significant movement recorded in the existing structure.

Segment production

Being part of a design and construct contract, Miller has broken with tradition in the design and procurement of its segmental

• Client: Heathrow Airport Ltd • Consultant: Ove Arup & Partners • Supervising Engineer: Bovis Construction • Contractor: Miller Tunnelling lining. Normally, tunnelling contractors in the UK purchase their segments from one of the three of four well established precast concrete segment manufacturers.

On this occasion Miller has teamed up with Crendon Concrete of Aylesbury on its first job producing tunnel segments.

In the meantime, Miller had approached Bekaert of Belgium, manufacturers of Dramix, for the supply of the steel fibre. Bekaert welcomed this move by Miller to introduce the technique to the UK, and contributed valuable advice, support and technical data to the project.

Among other points, Bekaert was instrumental in answering doubts about the durability and possible corrosion of the steel fibres in the precast segments over time. In fact, experience on the Continent has shown that steel fibre reinforced precast concrete suffers less from corrosion attack than segments reinforced with rebar cages. The path of possible corrosion is not as defined as the path offered by rebar through concrete and the area of concrete deterioration around a corroding steel fibre is many times less than that around the cross-section of corroding rebar. Together, Miller with Crendon and Bekaert have advanced the design and production of steel fibre reinforced segments in the UK and have created a lead which others are sure to follow.

"There are no codes or standards of practice for steel fibre reinforced segments in the UK, so we have worked with the Building Research Establishment at Watford to produce the design for the lining", said Eddie. Together with Mott MacDonald, who provided the check on the ring design, a programme of full scale load tests was devised to verify the performance of the ring. Through calculation and tests, the necessary performance specification was met using 30kg of steel fibre for each cubic metre of concrete. An equivalent of about 80kg/m2 of rebar would be required to meet

UK FOCUS CASE STUDY

Heathrow baggage tunnel

From left: Plan of the baggage tunnel; The working shaft within the airport boundaries where crane heights and movements as well as site access is strictly controlled

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UK FOCUS CASE STUDY

From left: The 4.8m o.d. Dosco shield fitted with a telescopic roadheader boom and vacuum segment erector; Mucking out and transport of segments and supplies is by a self-propelled Shuttlecar; Test rig at Crendon Concrete where the fibre reinforced segments were cast the same specification. The design also calls for a minimum concrete strength of 50N/mm2. Crendon is maintaining a consistent 60N/mm2 in its segment production. As another time and labour saving modification, the central drainage channel is cast into the invert segment of the lining eliminating the need to build this as a second pass in-situ concreting exercise.

Over the length of tunnel built to date, damage to segments either during handling at the factory, during transportation, or during ring build or shield pushing has been low and well below industry averages. The knuckle joints between segments in each ring avoids spalling of the edges as the ring is expanded and packers between rings ensure even distribution of thrust pressures.

The one key in each ring is located in the crown as opposed to other expanded rings of similar size which have one or two keys at knee or axis position. The axis key design is favoured on some projects as it avoids the possibility of crown keys falling out should they become loose over time. However, experience shows that the axis key rings are more difficult and are time consuming to build. The gaps left by the shorter wedge keys in the expanded lining are filled with high strength non-shrink mortar as part of the finishing works.

The seven segments and crown key in each 1m wide x 4.5m i.d. ring are being erected in about 10 minutes on average.

The 4.8m o.d. Dosco shield is fitted with a 360° vacuum segment erector and it applies about 3,000kN of its total 7,500kN thrust capacity to advance each 1m stroke. To accommodate the thin (150mm thick) edges of the lining, the shoes on the shield's 15 thrust rams are offset to direct the pushing forces squarely into the body of the lining.

The non-articulated tunnelling shield is 5m long and has an adjustable bead on the leading edge to assist machine steering and efficient ring build. The competent London Clay is readily cut by the shield's telescopic roadheader boom and lifted by its chain conveyor back to the muck transfer belt and so into the Shuttlecar hired from Specialist Plant for the job. The self-propelled diesel driven Shuttlecar has a carrying capacity of 16m3 and takes two trips to muck out each 1m ring or stroke. The car remains stationary under the muck conveyor as its chain conveyor moving floor draws back the load until full. At the shaft, the car off-loads into the 22m3 shaft hopper for hoisting to the surface. The Shuttlecar carries the segments into the tunnel on its return journey. "Waiting for the Shuttlecar on its round trip is unlikely to create a delay in the cycle time until the far end of the longer 1,200m drive," said Eddie.

Shafts and NATM

In addition to the central working shaft, there are five further shafts on the project, two at either end of the tunnel and an on-line ventilation shaft. With the exception of the ventilation shaft, all five shafts are 10.3m i.d., are between 18 and 21m deep and have been sunk using a combination of traditional and NATM techniques. Through the top 5m of wet gravels, they were sunk as jacked caissons using eight 80 tonne hydraulic thrust jacks instead of kentledge to apply the downward pressure. Once into London Clay, the shafts are continued down in 1.2m steps using sprayed concrete as the primary and permanent lining.

This sprayed concrete, which is being applied as a subcontract for Miller by

VIDEO INTERVIEW: AUGUST 2020

Colin Eddie, UK tunnelling engineer, and Marc Vandewalle, a pioneer in the field of steel fibre reinforced concrete, speak of the introduction of steel fibre reinforced shotcrete and precast concrete segmental linings in the UK in 1994 and on UK and international projects since then. Vandewalle speaks of his own career in the development of the technology in a Face-to-Face interview.

36

Sprayed Concrete Ltd of Lenham, Kent, is of the wet mix method and is being applied in several layers. On excavation, a flash 50mm thick layer of 40kg/m3 steel fibre reinforced shotcrete is followed by a 150mm thick layer of bulk shotcrete which is also reinforced with a similar quantity of Dramix steel fibre.

The following day, a finishing coat of 75mm thick plain shotcrete is applied over wire mesh reinforcement. A maximum advance of 1.2m was specified by Miller for safety and settlement reasons.

NATM work continued at the bottom of the two shafts at either end of the tunnel where large chambers are required for the operation of the baggage transfer equipment.

These 4.5m long x 4.5m diameter chambers will also be lined with the same shotcreting system as in the shafts.

This use of NATM and steel fibre reinforced shotcrete as the permanent lining in London Clay is also at the leading edge of tunnelling technology in the UK. For its application as part of the design and construct contract at Heathrow, Miller has drawn on its joint venture partnership with Beton-und Monierbau of Austria for technical and practical know-how.

Settlement

Surface settlement above the baggage tunnel is a major consideration given its 12 to 25m deep alignment beneath the main runways and taxiways of one of the world's busiest international airports. A tolerance of 20mm maximum is allowed and settlement is monitored extensively. The Piccadilly Line underpass is at just 11m between each, axis to axis, or a clearance of 1.5 times the tunnel diameter. The expanded lining, erected as it is directly against the ground and expanded by the key for a tight ground contact, effectively limits surface settlement.

The short 5m length of the Dosco shield with its small bead also ensures that the lining is applied quickly giving the ground little time to relax, thus limiting face loss and subsequent surface settlement.

Having led the way on its baggage tunnel contract, Miller is now seeing certain techniques being picked up by contractors working in London Clay on the Jubilee Line Extension and the PaddingtonHeathrow Express Railway Link. NATM has been approved for the construction of underground stations on the Jubilee Line and Heathrow Express projects and both are now investigating the use of steel fibre rather than wire mesh reinforced shotcrete.

In addition, proposals to use steel fibre to reinforce the segments of the alternative expanded lining designs for the Jubilee Line running tunnels in London Clay are being examined using the experience on the baggage tunnel as a benchmark. n

CASE STUDY: Application of a TBM for development of one of the twin spine roads to link five coal mines in North Yorkshire was the first use of a TBM on a mining project in the UK. The 5.8m diameter Robbins TBM demonstrated the significant advantages of the technique over traditional roadheader excavation and Robbins went on to supply TBMs to mine operations the world over. Most recently Robbins has developed a new non-circular TBM for excavating rectangular cross sections.

SITE VISIT REPORT: JUNE 1983

Fullfacer vs roadheader:

an investment in speed • Client: UK National Coal Board • Consultant: National Coal Board • Contractor: Cementation 1983 when the 78m deep x 1.8m diameter bore hole reamed with a Wirth reamer were complete and ready to drop coal production onto a conveyor in the cross adit for transferring onto a conveyor in either of the spine roads and hauled to the surface at Gascoigne Wood (Fig 2). When all five mines are producing at full capacity by the late 1980s, Selby will deliver 10 million tonne of coal a year, mostly for use by the Central Electricity Generating Board at nearby power stations.

In the interim the conveyors in the spine roads will transport both coal from the mines as they come on line and muck from behind the tunnelling machines in their deadend headings. The Anderson Strathclyde conveyor in the South Tunnel incorporates 57 x 16mm steel rope cords across the full width of the 30mm thick belt. The Anderson Strathclyde Cable Belt conveyor in the North Tunnel is a steel rope reinforced belt conveyor with two steel ropes of 57mm in diameter supporting a lightweight 1in-thick belt. The steel ropes are spliced together to form an endless rope at each side.

UK FOCUS CASE STUDY

T

he Selby coal mine complex in Yorkshire was designed by the UK National Coal Board (NCB) to connect five separate mine sites - at Wistow, Stilling Fleet, Riccall, Whitemoor and North Selby. Each would have a pair of shafts for transport of men and materials and for ventilation, with coal brought to the surface via two 14.5km long spine roads constructed in hard rock well below the coal bearing strata.

A conveyor system will bring the coal up the two 1-in-4 drifts to emerge at the Gascoigne Wood site where all the coal handling, storage and loading facilities are located (Figs 1 and 2).

As soon as spine roads reached the first mine at Wistow, coal production from it began with tunnelling of the spine roads continuing to connect the remaining four mines.

The south drift tunnel is being driven with the 5.8m diameter Robbins TBM with an average of about 100m/wk. The north tunnel, some 70m parallel, is being excavated using a Titan 134 roadheader making about half the rate of progress at about 50m/wk. Coal cannot be extracted until the cross adit connecting the two spine roads and the bore hole connecting the mine above is completed.

The Wistow mine came online in April

Robbins TBM doubles the speed of roadheader excavation for the Selby spine roads At its final 14.5km length, the conveyor will be the longest of its kind in operation in Europe.

TBM vs roadheader

Since the introduction of the TBM, the progress rates in the South Tunnel have

Fig 1. Plan of the Gascoigne Wood drifts and the five mine shaft sites; Inset: Fig 2. Main design features

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Assembly area in the south drift tunnel for the Robbins fullface TBM Robbins TBM specifications Machine diameter

5.8m

Cutting discs

42 x 14” dia.

Maximum recommended total thrust 572 tonnes Cutterhead horsepower

900hp

Boring stroke

1.52m

UK FOCUS CASE STUDY

Hydraulic system: - Pumps and motors

2000psi

- Cylinders

3000psi

Electrical system: - Motor circuit

1100V

Power supply

6600V

Transformer capacity

1500kVa

Weight (excluding pantechnicon)

240 tonnes

far outstripped those of the road header in the North Tunnel. During a visit to the spine road project in March 1983, the TBM had completed 6,470m and was at 460m depth below surface and the Titan roadheader heading had reached the 5,834m mark and was at 430m deep.

At their end spine roads will be at about 1,000m below the surface.

Having passed the connection with the Wistow mine, driving the spine tunnels is no longer on the critical path and the Titan will not be pushed to keep up with the TBM. During the site visit, the Titan was making a 45° turn to drive another 70m long cross passage to link the spine roadways.

Although the two roadway tunnels are being driven through similar ground, they are being treated quite differently not only in excavation methods but also for support and finish.

The most important difference is in

the profile of the tunnels. In the open, main beam gripper TBM tunnel, circular steel arches are being installed on 1m centres for support, and the invert is filled with excavated material to create a flat service roadway. In the horseshoeshaped roadheader tunnel, colliery steel arches support the roof and walls leaving a flat floor.

The horseshoe profile, being the weaker profile, is being subjected to floor blow as ground pressures increase with depth. As a result, the floor is having to be remined and levelled in many places using Hausherr dinting machines.

Steel arches of 6in x 5in are being used in both tunnels and as the depth increases these will be set at closer intervals. Extra strength may be achieved also by using higher grade steel.

The TBM, being the faster excavating method of the two, is expected to complete the South Tunnel in mid 1985, while the roadheader is expected to have completed about half the 14.5km length.

Because of its fast excavation rates and particularly because it eliminates the problem of floor blowing, the TBM will be dismantled at the end of the South Tunnel, brought back, reassembled in a new enlarged assembly chamber in the North Tunnel and used to finish the North Tunnel. This could mean that the roadheader and the TBM might be driving, the same tunnel in the same direction at the same time until the roadheader holes through into the TBM assembly chamber.

Due to stringent NCB safety standards, the TBM had to be modified in two fundamental ways before delivery.

First, cutter changes would not be undertaken by sending technicians through to the front of the cutterhead.

The cutters have to be changed from the back. Secondly, all components of the machine had to be bolted together, rather than welded, as welding is not permitted by the UK Mines Inspectorate in a working mine. A spin-off advantage of this is that once the South Tunnel is finished the machine can be dismantled and more easily taken back along the tunnel. This will be the only way of retrieving the TBM at the end of its work.

The TBM is operating on a 24h cycle x 5 day/wk and is averaging 100m/wk.

Eighty-six cutters have been replaced during the operational period of 18 months, profile cutters being replaced more often

than face cutters. This is considered light in the ground conditions met to date.

During the site visit, the TBM was unfortunately on stop having worked into an area of badly faulted ground. This was not unexpected, since during work on the Wistow roadways some 78m above, these same faults had been encountered and were likely to reappear in the spine road drives. These faults lie on various inclines which intersect the crown first and disappear in the invert ahead. When the TBM first struck the disturbed zone, it was found that the machine was excavating the loose ground in front stalling advance and creating a large cavity above and behind the cutting head.

It was decided that the machine must stop and that a pilot tunnel be hand excavated above, and in front of, the TBM to assess the extent of the fault (Fig 3). The timber supported heading extended up and away from the face for about 12m to the point where competent ground was once again present. From here, ground treatment was carried out.

Dywidag rockbolts were inserted and the ground grouted with a water sealing cement. These installations, particularly the rock bolts and timbers, have been placed so that they will not interfere with the TBM once the drive resumes. On completion of the ground treatment, the pilot tunnel will be filled with a Pozament cavity filling grout so that the TBM can continue. The machine however, had stopped halfway through the faulted area and bad ground had settled on top of the shield again, holding the machine fast. This ground had to be excavated to allow the hydraulics to lift and correct the position of the cutterhead before advance could resume (Fig 4).

Despite the hold up to deal with the fault zone, the TBM had shown its advantages when compared directly with roadheader excavation, driving through the same type of ground at the same time. The NCB said that the £2 million investment in the TBM is proving to be “a sound investment” and that it is “very satisfied” with its performance to date. “It has taken the driving of the spine roads off the critical path and has pulled both drives back onto the construction schedules.

The machine has many working years ahead of it and although the Board has, as yet, no definite plans for it after the Selby job, a number of interesting possibilities are being considered.” n

Fig 3. The work carried out to investigate the extent of the fault in which the TBM is held up; Fig 4. Necessary excavation of the crown to overcome ground relief stresses and free the machine

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CASE STUDY: Excavation of the first ever purpose built LPG storage caverns in the UK in the 1980s were built at an estimated half the cost of creating the same storage capacity in tanks on the surface. Underground storage also provided greater security for these vital infrastructure facilities.

SITE VISIT REPORT: MARCH 1984

Killingholme:

Britain’s first underground liquefied gas store and loading terminals for rail and road tankers at Immingham.

• Client: Calor Gas Ltd

Geology

• Contractors:

The geology of the site is “amazingly simple”, according to Tom Paterson, Resident Mining Engineer. The top 20m consists of alluvial and glacial deposits and the next 200m is the Upper Cretaceous chalk which stretches east under the North Sea and into Denmark and West Germany.

From six exploratory boreholes, it was found that the chalk contains several aquifers and bands of marl and very hard flint (Fig 2).

When choosing a site for an LPG storage facility, the geology is perhaps the most important consideration. Its success depends to a great extent on natural geological properties. Petroleum gases, more easily stored in liquefied form, are liquefied by either lowering their temperature or by compression. On the surface, it is usual to store them in the familiar refrigerated tanks rather than in pressure vessels. Cheaper still is to store them under pressure in larger capacity underground facilities. Liquefied gases have been successfully stored under pressure in leached cavities in salt deposits for many years but finding suitable geology for a purpose-built underground facility is no easy task, as Paterson explained: “The geology must be competent enough to allow extensive excavation and of course the more limited the support requirement, the more economic the

• Consulting Engineer: Geostore Ltd • Thyssen/Pigott Consortium: Shafts • Boldon Drilling: Service boreholes • Fairclough: Cavern tunnels construction. It is also necessary to use the natural ground water pressure to contain the LPG.” The principle is that a higher ground water pressure, in this case 285psi, acts against the 100psi needed to liquefy propane or the 30psi to liquefy butane, thus containing it. The presence of substantial aquifers is essential. “Unlike almost all other tunnelling jobs, we welcome the sight of water seeping into the works and pray it will continue!” confessed Paterson.

The art of site selection lies in finding a stratum for the caverns whose water inflow is sufficient to contain the LPG but not so great as to make the cost of pumping seepage water from the cavern prohibitive.

In some cases this may require extensive grouting operations and alteration of the groundwater regime around the caverns by the construction of water galleries above, and water curtains between caverns. At South Killingholme, neither grouting nor a water gallery is expected to be necessary.

Because the geological properties of the site are so important, a geotechnical bay has been established, equipped with piezometers to measure ground water

UK FOCUS CASE STUDY

A

labyrinth of unlined closed-circuit tunnels, driven in chalk some 190m beneath the flatland on the south bank of the River Humber, will be used as the first mined underground liquefied petroleum gas (LPG) storage facility in the UK. The £20 million project, a joint undertaking of Calor Gas, with a 60% interest, and Conoco, with the remaining 40% share, has moved through the planning stages and on to construction remarkably quickly.

In January 1981, planning permission was granted by Glanford Borough Council for the excavation of the two approximately 115,000m3 caverns under land owned by Conoco for the storage of liquid propane and butane gas (Fig 1). Site investigation work began immediately and later a £2 million contract to sink four access shafts, two for each cavern, was awarded to the Thyssen/Pigott Consortium. Nine services boreholes for each cavern were drilled and cased by Boldon Drilling using a conventional oil well type drilling rig.

The £4.5 million tunnelling contract was awarded to Fairclough, the lowest of five tenders. Geostore, a licencee of the French company Geostock, which has designed and supervised the construction of several LPG caverns, including one in chalk in northern France, is the consulting engineer. Associated contracts include the construction of a new LPG jetty on the River Humber, and pipelines which will also connect the facility to the Conoco refinery

A Titan E205 roadheader excavating the 9m wide x 5m high top heading, with visible bands of marl and flint present in the chalk

39

avoided. With both caverns at the same depth, each can be used to store either propane or butane.

With viability of the project accepted and planning permission granted, tender documents were prepared. In the specification, blasting was excluded as an excavation method. The surrounding ground was to be left as undisturbed as possible in order to retain its geological competence.

Shaft drilling

The Thyssen/Pigott contract called for the drilling of two access shafts for each cavern. One shaft, fitted with a high speed winder of 120tonne/h capacity for mucking out and transporting heavy items of equipment. The other, fitted with an electric Alimak rack and pinion hoist, will transport men and small items of plant. This hoist has a double deck cage capable of carrying 12 men at a time and taking five minutes to travel up or down the 200m shaft.

Of the nine service boreholes, up to 20in in diameter, one will act as the filling line and the other eight will service two 10m diameter sumps, four to each. The sump boreholes will house product and water pumps, instrumentation lines and a vent line.

At the time, the largest Fig 2. Geological section through the cavern and machine available in the UK to access shaft areas (m) drill the shafts was the 2.4m diameter full face Titan drill pressures, and 18m long extensometers, stressmeters, flat jacks and a convergence designed and manufactured by Pigott array, to monitor in-situ stresses, rock Foundations. With a final i.d. of 2m, this movement or swelling of the marl. Results was deemed to be adequate. During the to date show little sign of any deformation. drilling process, cuttings were removed by The caverns are in fact twice as deep as reverse circulation of the drilling mud. On necessary as the ground water pressure the way down, the drill encountered three would have been adequate at half the significant aquifers, the largest between depth. But it was decided to go within 20m 20m and 30m deep, one at 90m and the of the bottom of the chalk because at this third at 150m.

When the hole reached the required depth the chalk is of a better quality for constructing unsupported caverns, and depth, a steel tubular lining tapering from thicker beds of flint at higher levels are 40mm thick at the bottom to 20mm thick From left: Fig 1. Plan of the caverns with shaft and sump areas

40

at ground level was floated into position.

The first 12m section of the lining has a base plate. The can was filled with water to force the lining down the hole. The next 12m section was welded on while about 2m of the previous section was still above ground. Each shaft has two sections of bolted steel segmental tubbing to provide easy access to the surrounding ground at a later stage. One at cavern level permitted break out into cavern excavation, and the other, some 10m above, where a massive wedge shaped concrete plug will seal off the caverns.

With the lining in place, the annulus was grouted from the surface and the water inside the shaft pumped out. During initial works on the muck shaft plug tubbing for the Riverside cavern, the first to be worked on, one of the plates failed and the shaft flooded overnight. Ground water poured in at a rate of about 600 to 1000gal/min. This was to prove costly. It took four months and some 25m3 of grout to rectify.

Grouting, pumped from the surface and carried out from a working stage inside the shaft, was injected below each of the major aquifers to seal the annulus and a concrete ring was constructed in the annulus at the top of the plug tubbing. The shaft was then watertight. Grouting in the manshaft was carried out in a similar way as a safety precaution.

With the two shafts completed, the high speed winder and Alimak hoist were installed. The flanges of the segmentally lined sections in the manshaft had to be removed to clear the Alimak cage, and joints in service pipes and guiderails had to be made up. Excavation of the caverns could then begin.

Cavern excavation

The caverns themselves form a grid of 10m wide x 10m high elliptical horseshoe shaped tunnels. The Riverside cavern has two x 250m long north-south tunnels and seven x 130m long cross tunnels. The inland cavern will have two 200m long north-south tunnels and six 180m long cross tunnels. In each cavern a drainage tunnel will connect the two 10m diameter sumps (Fig 1).

Fairclough won the contract with its proposal to drive a complete 5m high x 9m heading and to follow with one benching operation to bring the tunnels to their full 85m2 cross section. Two company owned 50 tonne Titan E205 roadheaders, which had been fitted with a larger boom and turret to increase the original 4.5m vertical reach to 5m, would be used. These machines have a total installed power of 165kW each and are electrically driven from an 1100V supply. Mucking out is via a network of conveyors to the muck shaft, where a four tonne measuring flask empties into the shaft skip for hoisting to the surface.

As the chalk is not required for local construction projects, it is being used to reclaim land adjacent to the site.

Before the Titans could get started, the 25m separating the two shafts was to be excavated by hand, but this proved too slow. The chalk at this level was found to

excavated muck is stockpiled in the galleries. A JCB backacter mounted on the opposite tunnel wall transfers muck from the muck bin to the flask.

With the introduction of diesel powered equipment and the use of explosives, the ventilation system had to be revised. “The problem is not so much the fumes caused by the explosives these disperse in about ten minutes but the continuous use of diesel plant,” explained Paterson. To begin with, a Woods 125hp fan forced fresh air down the manshaft with the spent air exhausting through the muck shaft. When the tunnel under the sump areas was opened up, a further 125hp fan was fitted to two of the 20in diameter service boreholes with 20in diameter ducting supplying fresh air to each working face.

Since the site visit, the Tamrock jumbo, delivered from Finland, has been introduced. To get it down the 2m diameter shaft, it had to be cut into manageable pieces and welded together again underground. Production is now at the necessary seven linear metre/shift. “With this rate, I am confident we will finish on time,” said Paterson.

Progress rates

By late January 1984 some 900m of the top heading had been driven by roadheaders, 450m of the top heading enlarged to full width and 200m of the bench excavated by drill+blast. “The jumbo has performed extremely well since its introduction at the beginning of November 1983,” stated Paterson.

Commenting on the project as a whole, Paterson confessed that he had suffered more than one sleepless night.

“Even though this Northern Province chalk extends deep into Europe, we believe this is the first time anyone has tunnelled in it. I often wish we had been second! The strength of the chalk is similar to that of a good quality concrete with an unconfined compressive strength of 30 to 60MPa. “This is fortunate in that we have been able to leave the caverns completely unsupported so far, but it has played havoc with the excavation plan. If the roadheaders had been able to meet

Fig 3. Plan of the muck pit adjacent to the muckshaft the expected progress, we would have completed the top heading with them but I am not sure we would have used them on the bench. Economic considerations would be against them.”

The first cavern is expected to be completed during summer 1984 and excavation of the second will begin immediately. “Following the experience gained on the first cavern, progress of the second will hopefully be uninterrupted,” said Paterson.

When the caverns are completed and in operation, there will be very little evidence that some £30 million worth of LPG, at current prices, is stored 200m below.

There will be a control room, a plant to treat seepage water pumped out of the caverns and the surface pipeline network.

There is, in fact, no reason why Conoco should not build further installations on the surface. But the facility is, due to the large volume stored, considered a high risk area and as such the UK Health & Safety Executive has been involved in its design and construction and will monitor its future operation. A major safety feature is that a minimum quantity of LPG will be contained in surface plant and pipelines. Another is that the service boreholes have been designed and equipped so that they can be sealed off near cavern level.

The economics of the £20 million purpose built underground LPG storage caverns project do add up as it would cost an estimate of up to £40 million to build the equivalent storage capacity as surface refrigerated facilities. n

UK FOCUS CASE STUDY

be much tougher and more compact than expected. The restriction on blasting had to be relaxed and the connection driven using explosives and hand held drills.

The first Titan was then transported down the muck shaft in pieces and assembled underground. Its first task was to drive the access ramp to the top heading. This completed, the second Titan was introduced and both began driving cross gallery or tunnel B in opposite directions. Effective use is made of multiple laser beams to mark the course and profile of the faces. A tolerance of 200mm maximum is acceptable. A minimum of 10m forward probing is maintained to warn of the presence of fissures connecting aquifers to the workings.

As excavation got under way, it became clear that the initial working procedure would have to be changed.

Because the chalk is tougher (some three times stronger than chalk encountered in the south of the UK) and there are few, very tight joints, it requires more mechanical working to excavate. This, plus the blunting of tungsten carbide picks by flints in the chalk, made the average three linear metre/12h shift production from the two roadheaders far from adequate. The required production rate for the five day/ week, two x 12h shifts/day was equated to 100% utility of the 120 tonne/h capacity of the muck shaft. This was far from being achieved. During the site visit in October 1983, only the top heading for gallery B and half of the gallery A loop section had been excavated. On top of this, the bands of flint were resulting in very high pick wear. Between 20 and 30 tungsten carbide picks were being replaced every 12h shift.

In light of this and following favourable results of blasting trials, the decision to include drill+blast had been taken. During the visit, Fairclough was awaiting the arrival of a Tamrock three boom electrohydraulic jumbo with hydraulic drills and a charging basket. With the jumbo, four faces will be worked simultaneously. The roadheaders will now excavate only half of the top heading. The jumbo will drive the other half and the bench, working two alternate faces. The Titans would then be used for final profiling. Production rates are expected almost to double, to seven linear metres/12h shift, which will keep the muck shaft working at capacity.

Other changes were also evident during the site visit. The weight of the roadheaders had been increased to 60 tonne to prevent them slewing on the slippery chalk floor during operation. The conveyor mucking system had already been taken out and Benford 3.5 tonne capacity dumpers were transporting muck from the roadheaders to the muck shaft. When the jumbo starts, a CAT 910 wheel loader and a Drott track loader will be added to the fleet. To keep the journeys of these diesel powered machines as short as possible, a new muck bin from the top heading level to the shaft level was being excavated (Fig 3). During excavation for the muck bin and downtime on the muck shaft,

The three boom Tamrock jumbo drilling charge holes to excavate half the top heading and the bench

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CASE STUDY:

Collapse of the NATM station excavations on the Heathrow Express Rail Link in October 1994 had high impact ramifications for the tunnelling industry in the UK and across the world. The criminal prosecution trial of the contractor and its design-build engineer four years later, after working on to recover the works and complete the project, added insult to injury.

SITE VISIT REPORT: JUNE 1996

Heathrow rises

from the mire and shines

W

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hatever jinxed the job in the beginning, all appears to be well under control after collapse of station excavation works at Heathrow in October 1994. The tunnelling now being carried out for the project looks faultless.

The application of men and machinery and project resources is impressive and the attention to detail is meticulous.

While an event on the scale of the

collapse that occurred on the project in October 1994 will always focus the mind, the question remains - if it is being achieved now why was it not back then? The answer to that is the subject of an in-depth investigation by the UK Health &Safety Executive (HSE), the report of which is unlikely to appear for several months yet.

Material evidence is still being collected from the site and the possibility of legal

60m diameter rescue shaft avoiding the need for props, whalings or struts and ease core excavation

• Client: BAA • Consulting Engineer for the owner: Mott MacDonald • Construction Manager: Taylor Woodrow • Design-build Contractor: Balfour Beatty proceedings as a result remains highly likely for the parties involved.

Therefore, it was made clear to journalists attending an all-party-organised site visit and technical briefing in late June 1996, that questions about the causes of the collapse and the costs thereof would not be answered. The journalists however were treated to a thorough briefing on the project's current progress and to a visit underground into various sections of the on-going works. Knowing exactly how the current quality of workmanship and construction supervision differs from what was going on before the collapse may never be known, but it was explained in some detail how the work now being progressed is different from the original.

Giant shaft rescue

Fig 1. Trace of the 60m diameter x 30m deep rescue shaft and the four outer access shafts superimposed on the original 19m diameter access shaft, central concourse and outer platform tunnels, all of which were under different stages of excavation when the works collapsed

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To start, the most obvious, most extensive and clearly most expensive, modification to the original scope of works is the 60m diameter x 30m deep cofferdam built in the Central Terminal Area (CTA of the airport Terminals 1, 2 and 3) to almost totally encompass and remove the extent of the collapse and the disturbed ground. Only by standing at the edge of this huge excavation could its scale be appreciated. "The upper 20m through soft water-bearing ground and gravels is supported and stabilized by 1.2m diameter Secant piles which interlock on 1.06m centres," explained Alan Powderham, Managing Director of the Foundations and Geotechnical Division of Mott MacDonald and the lead designer's representative on the so-called Solutions Team convened immediately following the collapse to engineer a safe and economic way forward. "The London Clay below is supported by 900mm diameter contiguous piles which step in from, and continue a further 20m below, the Secant piles. "A substantial capping beam at ground surface forms a rigid concrete collar linking all the piles together.

"Originally, recovery ideas were instinctively based on excavating a large box of about 65m x 65m," said Powderham.

"However, a rectilinear structure would have required between 5-6,000 tonne of propping. The circular concrete-lined cofferdam eliminated this propping and reduced the amount of excavated material by a substantial 30,000m3."

Once the giant cofferdam was chosen as the method of recovering the collapse situation, piling by specialist subcontractor Stent Construction began in early October 1995. Piling went well, it was said, and was completed in early February 1996. Central excavation in 1m lifts began in early March and was completed by mid-July 1996. Once exposed, each lift was lined with 1m thick in-situ concrete lining, the central dumpling of earth contributing to shaft stability until later excavation.

Excavation of the core progressed so well that the initial 1m lifts were increased to 1.2m to speed progress. The construction teams also reduced the original 7-day cycle per lift to 5 days. Another technique adopted to expedite the programme was the idea of portalisation. Immediately outside the cofferdam perimeter, four 32.5m deep access shafts were sunk to station level. The two to the south are 10.7m in diameter and are permanent ventilation shafts. The two to the north are 15m in diameter and were added to allow early access for continuing station excavation at the CTA. Possible permanent uses for them

were being studied. All four were sunk to continue station excavation without having to wait for full excavation of the cofferdam.

This was important in efforts to gain early access for the track laying progress.

At the end of June 1996, core excavation of the large diameter cofferdam shaft had reached lift 19 of 25. The top of the portal structures of the two station platform tunnels from the south were exposed and the equipment left buried in the 70n/mm2 concrete and foam concrete used to immediately backfill all the excavated space after the collapse was being uncovered. As the cofferdam excavation had gone down, the original 28m deep x 19m diameter access shaft had been broken out and removed and it was now time to unearth the collapsed tunnels and uncover the collapse evidence. Teams of independent observers representing the client added to the activity in the bottom of the excavation, watching as each bucket load of dirt was removed and logging all possible indications as to why the tunnels collapsed in the first place.

Given the size, cost, quality of construction and time taken to build it, surely the cofferdam was not going to be back-filled and forgotten like a structure that is but should never have been?

No. It is to be incorporated into the final design of the CTA station. In addition to

accommodating the station's lifts and escalators, the huge excavation provides potential for some 5,000m2 of valuable space, which is being safeguarded for possible development of underground car parking or office space. The 255 anchor piles to be sunk into the floor of the cofferdam to hold down its base slab are also designed to carry the weight of a possible seven-story building above. If BAA (British Airports Authority), the private owner of Heathrow Airport and principal client for the current Express Link Project, decides, this could be a top-quality hotel or office development built right in the heart of the airport and on the rapid rail transport link into London. The potential is there and development studies are underway according to Rod Hoare, Group Rail Strategy Director for BAA.

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Left: NATM bell-out at the bottom of an access shaft to continue station excavation while work on the collapse recovery progressed Right: Fig 2. Plan of the Heathrow Express rail link project with 8.8km of running tunnel, two underground stations at the Central Terminal Area and Terminal 4, a turnout for future extension to the proposed Terminal 5, and several intermediate temporary and permanent shafts

Tunnelling culture

The underground visit began in the main crossover of the twin-tube railway tunnel alignment, a structure that will be largest span NATM cavern built in London Clay to date. At its widest the 98m long cavern will be 13.33m high x 16m wide. At its narrowest it is 9m high x 16m wide.

Located on the south side of the CTA station, the crossover was accessed from the 6m-diameter shield-driven and precast concrete segmentally lined running tunnel

From left: Open face shield used to excavate the running tunnels; Front of the Howden shield once it had completed one of its drives

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Fig 3. Sequences of excavation for the T4 turnout (left) and the double crossover chamber, the largest NATM structure on the project at 16m wide x 12.33m high and finished with a 400mm thick layer of steel-fibre shotcrete which was driven from the Sealand Road access shaft. The sequenced excavation of the chamber to full cross section then progressed from the enlarged central section. The NATM excavation phases progressed in 1m rounds on a top headingbench, top heading-bench and double invert sequence. Average advance at each of the many concurrent NATM faces was said to be 1m/day working two 12hr shifts/ day, seven days/week. While excavation continues, one of the running tunnel shields is working its way towards the crossover from one of the access shafts outside the cofferdam perimeter. Once it has broken through it will be turned and relaunched to complete the parallel running tunnel back toward the cofferdam.

As a result of the collapse, many NATM design and excavation sequence modifications were imposed. Together with increasing substantially the robustness of the original designs, the changes have also removed a large portion of the risk, which is, or was, expected to be monitored, managed and controlled under the original NATM design and excavation phases.

The collapse appears also to have introduced a greater appreciation of the method among all concerned, from senior company management to those at the face. There is undoubtedly greater attention to detail not only among the operatives, but also towards the batching and quality control of shotcrete; the recording, collating and interpretation of geotechnical instrumentation data; and the presence on site, at all times and in sufficient numbers, of qualified and experienced NATM engineers.

A major point emphasized during the visit and briefing to the technical journalists was the culture developed for the project, perhaps from the beginning but certainly re-enforced since the collapse. This culture, it was explained, centres on five

main principals: • A single team approach, to eliminate the lateral them and us demarcations between the parties involved; • Supportive management to eliminate the horizontal divisions between levels of management from foremen and workforce to the most senior managers; • A pro-active and continual process of risk management not just at the beginning of the job but reassessment throughout the full construction period; • Team building with front line focus groups dealing with any problems which may arise on a daily basis and bringing more serious problems more immediately to the attention of more senior staff; and • Promotion of total can do positive thinking. The single-team approach to project completion was described

by Hoare as "totally unusual and very different to approaches used in the past."

If there were any rifts between the major players after the collapse, they have certainly been pulled together. It is a united project from which responses to questions about the collapse by the many journalists and given that the collapse may well yet be the subject of legal proceedings. The continuation of the construction civil works is the result of a confidential agreement between BAA, the client, and Balfour Beatty, the UK construction company which won the tunnelling contract for the project back in January 1994 as the lowest of six bidders and for a contract award price of £60 million. Media speculation has put the cost of the tunnelling contract now at more than £200 million but this has not been substantiated. It is also reported that Allianz, the all-party, all-risk project insurer will cover this extra cost but again this is not confirmed. The Balfour Beatty contract is extensive covering the total 8.8 km of shield-driven, segmentally lined

running tunnels and the two underground stations at CTA and Terminal 4. Tarmac Construction has completed the 50m of surface works from the tunnel alignment to the existing Great Western mainline railway into Paddington Station in London from the west. Tarmac is also the tracklaying contractor and the Laing Bailey joint venture has been awarded the mechanical and electrical equipment installation contract.

Claims from third parties arising from the consequences of the collapse were said to be minimal. "A few private citizens claimed compensation for having missed their flights due to the fact that one of the car parks was closed immediately after the event," explained Hoare for the Heathrow Express (HEX) project team.

"There is also a claim currently being considered from London Underground arising from the fact that the Heathrow extension of the Piccadilly Line to Terminal 4 was closed for 10 days, the services terminating at the CTA station instead. Other than that, the airport did not actually close at all and there was no impact on airlines or other third party operators in the airport."

The unified response to the collapse by the major parties was made possible, it was said, by the New Engineering from of contract adopted for the project

by BAA. The terms and conditions of the NEC (New Engineering Contract) document eliminate the clauses which, under other more commonly used contract documents, create an adversarial climate among the parties involved. The comment was made that under an ICE 5th Edition type of contract, for example, the work at Heathrow might still be on hold waiting for some kind of break-through which would allow things to move forward. This is perhaps an exaggeration but the point is clear. "The NEC allowed a common sense

From left: Heightened attention to installation of the wire mesh, lattice girders and the application of the initial shotcrete layers in the invert joints; Final steel-fibre reinforced permanent shotcrete lining applied in 2.5m wide bays and between metal strip stop ends

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NATM redesigns

Restart of NATM works at Heathrow after the collapse,as was also the case with the works at waterloo and London Bridge Stations for the Jubilee Line Extension, was as a result of an extensive review of the technique and its construction sequences by the HSE. Not until a series of design and construction sequence modifications were adopted would HSE allow NATM work to resume by submitting its now familiar 'letter of no objection'. These reviews and revisions took some time and it was not until September 1995, some 11 months after the collapse in October 1994, that NATM at Heathrow resumed.

According to Alan Finch, Project Director for Mott MacDonald, the main changes fall into seven categories.

• The thickness of the initial immediate shotcrete support was increased by 50mm on average.

• The shape of the tunnel profile was changed slightly and the lattice girder rings extended through the invert • The quality of the shotcrete was re-examined to confirm consistency of the mix and the quality of the end product • The excavation sequences were reviewed and consecutive excavation of three parallel NATM tunnels in the stations areas was prohibited. The excavation of station tunnels at CTA have reverted to shield-driven and handexcavations through the disturbed and concrete-filled NATM excavations of the collapse. At T4 the two outside station platform tunnels must be complete with their in-situ concrete final lining installed

Two 9m diameter Dosco open-face shields with a backhoe in the invert and a stage for hand mining in the upper level were used to excavate the platform tunnels at the CTA station after the collapse before excavation of the central concourse tunnel is allowed to begin.

• Initially, face bolts were introduced to stabilize larger face excavations. In a 9m high x 8m wide NATM station platform face at T4,for example, a pattern of about 20m long face bolts were installed. The tunnels then advanced 8m to 10m before the next set of face bolts were installed. Initially, fibre-glass bolts were used but the splinters of fibre-glass produced as the excavator broke them out produced a spoil that required special disposal according to UK regulations. Steel bolts were then substituted and the ends burned off as they were uncovered by the advancing excavation. Face bolting became no longer necessary as works progressed.

• The application of compensation grouting was reviewed and more strict guidelines imposed.

• A more structured system of collecting, collating and interpreting recorded geotechnical data was introduced.

NATM advances

Although some might suggest that the above changes place unnecessary constraints on the basic philosophy of NATM and its use in soft ground, despite this, BAA as a buyer of a NATM construction has allowed some major advances with the technique in the UK. The most significant is the use of shotcrete not only as the immediate but also as the final lining. Wet shotcrete applied using MBT admixtures and equipment provides the final lining in the main crossover chamber, in the turnouts of the running tunnels and in other non-public areas of the stations.

The final permanent lining shotcrete is designed to the same load bearing criteria

and specifications as an in-situ concrete lining. It is 350-400mm thick and reinforced with 40kg/m3 of Dramix steel fibre, with the fibre is included principally to help limit shrinkage cracks rather than increase loadbearing capacity. A trip to the T4 turnout from the CTA station saw the application of permanent shotcrete lining in action.

To control the application, the tunnel length is divided into 2.5m wide bays using metal stop-end strips fixed to the inner shotcrete shell. The steel-fibre reinforced final shotcrete is then built up in sections around the profile starting with the first sidewall blocks up from the in-situ concrete floor slab. Surprisingly the 350-400mm thickness of shotcrete over a 2.5m x about 3m area was going on all at once without any mesh or reinforcement other than the steel fibres. The final lining has little by way of additives except those required to assist wet shotcrete pumpability. Despite this, Alan Myers, Project Director for Balfour Beatty, explained that the final steel-fibre reinforced shotcrete was acquiring early strengths of some 40N/mm2 within 7 days and continued to gain strength achieving some 54N/mm2 at 28 days and 61N/mm2 at 120 days.

In the T4 turnout, the visitors - at the end of the shotcrete cycle - witnessed the operatives grading the shotcrete by pulling a leveller over the finished section using the stop ends as guide rails. On to this is sprayed a 50mm thick layer of cement and sand gunite or mortar to achieve a smooth final finish and cover the protruding steel fibres.

Onto this is sprayed a liquid curing agent to ensure a crack-free set of the mortar.

For its wet mix operations, Balfour Beatty is using both MBT equipment and MBT admixture. There are seven Suprema

UK FOCUS CASE STUDY

approach to contracting," said Hoare, and all parties to the contract were involved in the Solutions Team established within days of the event, and resumed after a brief pause for thought.

The current ”seamless” approach to project management builds on an organisation shuffle undertaken back in June 1994, four months before “the event”.

In the early days, Taylor Woodrow was the construction manager to BAA for the project.

This role was since absorbed into the HEX (Heathrow Express) project team in which there are some 40 to 50 Taylor Woodrow employees. BAA also strengthened its involvement in the team by increasing its staff numbers. Mott MacDonald continues in the team as the principal designer for the job and Balfour Beatty is the team's tunnel construction partner.

Geoconsult, the specialist NATM tunnel designer involved on the project is also part of the team. Originally Geoconsult was engaged by Balfour Beatty as the designer of the primary shotcrete lining support when this was considered as temporary works and therefore the responsibility of the contractor. The responsibility for the primary lining design since shifted from Balfour Beatty to the HEX team. Mott MacDonald is now responsible for the design of the primary lining. As such Geoconsult is now a specialist sub-consultant to Mott MacDonald. There is a team of Geoconsult engineers in the on-site engineering staff with one appointed to each NATM shift.

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UK FOCUS CASE STUDY

shotcrete pumps now on the project and two Robojet nozzle booms. For the primary shotcrete, the company is using the Delvocrete hydration control stabilizer and the Delvocrete S-51 sodium aluminate with the Rheobuild 3520 superplasticiser. The final permanent lining is a richer mix of Meyco SA 145 alkali-free non-caustic liquid activator with the Rheobuild 2000 PF superplasticiser and the Delvocrete stabliser.

Balfour Beatty initially mobilized with SBS dry-mix shotcrete equipment and Shotax additives from Lafarge. Wet-mix using MBT technology was introduced in the last quarter of 1995 and is now the principal method used on the project.

According to Balfour Beatty Site Agent for the shotcreting works, David Ingle, the change to wet-mix shotcrete,for the primary lining initially, was promoted by several factors. "Wet mix shotcreting is more highly productive. We are applying +6m3/h now instead of the 3m3/h achieved with the dry mix method. There is less rebound at about 10% as opposed to 20 to 25% for hand held spraying. There is minimal dust when using the wet mix and with the new MBT admixtures now available the quality of the product is superior. High early strengths of up to 25N/mm2 in 24h are possible and the shotcrete continues to gain strength with time which is another major advantage of the wet mix over the dry. These cost, heath and quality advantages of the wet mix supported its adoption as the permanent lining where appropriate."

With 350mm to 500mm of immediate shotcrete, reinforced with three layers of wire mesh, plus 350mm to 400mm of final steel fibre reinforced shotcrete,the thickness of the lining in the larger excavations is now about 700mm to 900mm in total. However, where appropriate and in smaller NATM tunnel cross sections, a combined initial plus final lining design will be used. Such a composite final lining is being used on the concurrent Heathrow baggage tunnel facility between T4 and the CTA terminals.

For that project, the 8m diameter NATM chambers at either end of the shield-driven tunnel, are finished with a CombiShell in which a certain capacity of the initial support shell is considered contributory to the total 275mm final lining. For that project, the running tunnel is lined with rings of expanded steel-fibre reinforced precast concrete segments.

Settlement control

During the media briefing at Heathrow, Finch of Mott MacDonald went on to discuss the compensation grouting techniques used to limit ground movement and settlement as the tunnels came through under major airport structures including airport runways and taxiways and roads, railways and car parking facilities. At T4, the platform station tunnels pass 5m under the floors of the underground ticketing hall of the London Underground Piccadilly Line station and 2m under a sewer pumping station.

"Under all structures, we have limited the horizontal tube-a-manchette arrays for compensation grouting to a minimum of 5m above the advancing Express Link tunnels,"

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Two 9m diameter Dosco open-face shields with a backhoe in the invert and a stage for hand mining in the upper level were used to excavate the platform tunnels at the CTA station after the collapse explained Finch. "However, because of the limited distance between the tunnel and the Underground ticketing hall, we decided not to use concurrent compensation grouting.

Instead we injected grout through the arrays before tunnel excavation began and preheaved the floor by 15mm. The systematic face bolting was used to provide greater face control during excavation of these tunnels. There was a contingency to use concurrent injection if necessary but this was not required. Following excavation of the tunnels,the floor of the hall has returned to its original ground level.

Settlement under all other structures, it was said on several occasions, "has been less than that predicted".

Settlement, and the instrumentation installed to monitor the inevitable movement caused by ground excavation, is now an obvious focus of project attention. How this differs from the situation before the collapse was not discussed, but the instrumentation installed to monitor movement during the excavation of the huge cofferdam, for example, is comprehensive. All readings from these instruments are logged daily and, for some, more frequently. The data is gathered at a central on-site monitoring station and meetings are held daily to discuss the readings and their interpretation. Data gathered from an extensive array of instrumentation installed for excavation of the cofferdam indicated a maximum movement of the piled wall of 14mm once excavation had reached about 24m deep. The capacity of the piled wall, according to Powderham of Mott MacDonald is 70mm. An amber level of 50mm had been set for the point at which contingency measures

would have been implemented had they been necessary.

In short

To recap, all is now going extremely well at Heathrow. A better standard of NATM work and workmanship overall would be very difficult to find anywhere in the world. A full team effort attitude towards completing the project is evident. The hourly paid work force on the tunnelling job in May 1996 is at its peak of some 900 (up from 400 workers in October 1995) which, when added to the senior management staff and project employees appointed by BAA, Balfour Beatty, Mott MacDonald and other companies associated with the project, totals about 1,250.

By June 1996, 800m of tunnel was left to excavate. The two 2.8km long twin tube running tunnels from the PaddingtonReading main railway line to the CTA station are complete. The 2.2km long single track running tunnel from the double crossover south of the CTA station to the T4 station is also complete. The only tunnelling left is the two running tunnel legs to link the large cofferdam structure to the double crossover facility and the second 900m running tunnel link from the T4 turnout into the T4 station.

Meanwhile, by June 1996, all NATM excavation of the two platform tunnels for the T4 station are complete with excavation of the central NATM concourse about to start. Excavation of the T4 turnout is complete and NATM excavation of the double crossover chamber was about 75% complete. In the CTA area, the two station platforms are complete and excavation of the concourse through the concrete-filled collapsed zone has started.

The section of tunnel adjacent to the Sealand Road shaft near T4 incorporates the NATM trial tunnel which was designed and supervised by the Dr Sauer Company, excavated by the Kier/ KunzJV, and completed three years ago to substantiate the choice of NATM for the CTA and T4 stations.

It has often been said that it takes a collapse for all parties involved to appreciate

the classicism of NATM and the limitations of any type of tunnelling or civil engineer. It would be nice to believe that the approach to the project currently evident at Heathrow was there before the collapse but there is no doubt that 'the event' has certainly focused the minds. It can only be hoped that the current standards of work and attitude toward civil engineering design and contracting can be sustained and promoted throughout the

UK tunnelling industry without any relaxation.

Further lessons to learn may become evident if the collapse becomes the focus of a public investigation or court case. n

References

1. Safety of NATM with particular reference to London Clay: The UK Health & Safety review 2. Sprayed concrete linings (NATM) for tunnels in soft ground. The Institution of Civil Engineers (ICE) guidelines

COURT CASE REPORT: MARCH 1999

Heathrow failures highlight

NATM (abuse?) misunderstandings

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n February 1999 the heaviest fines ever imposed following prosecution by the British Health & Safety Executive were levied against Balfour Beatty and Geoconsult following collapse of NATM station tunnels on the Heathrow Express Rail Link Project at Heathrow Airport in London in October 1994. Balfour Beatty, tunnelling contractor on the £440 million project, pleaded guilty to violating two counts of the country’s Safety at Work Act 1974 and was fined £1.2 million plus £100,000 toward prosecution costs and were given 14 days to pay.

Geoconsult, as specialist designer to Balfour Beatty for the NATM works on the project, denied the charges but was found guilty by a jury of 12 after a 26 day criminal trial at the Old Bailey Criminal Crown Court in London and was fined £500,000 with £100,000 towards costs and was given two-and-a-half years to pay.

The two charges laid against each company were for “failing in their duty to ensure the safety of their employees” during construction of the NATM station tunnels at the CTA (Central Terminal Area), and “failing to ensure the safety of those not in their employ” (i.e. the general public), by exposing them to danger. For Geoconsult the charge was in connection with its technical supervision of the works.

Following a quite extraordinary trial - which had actually left no party to the proceedings unscathed - Geoconsult lodged an appeal against both the conviction and the sentence. It claimed that HSE “breached its paramount legal obligation to prosecute fairly” and that rulings on procedural orders “resulted in prejudice” against the company. Against the sentence it claims that the fine was

It was on the night of 21 November 1994 that the Central Terminal Area station tunnels collapsed causing extensive damage but fortunately no injuries or fatalities excessive and overlooked guidelines that fines should reflect the gravity of the offence but should not imperil the existence of the company. The total £600,000 fine for Geoconsult was levied against an original contract value of £1 million and against a medium sized consulting engineering company. The $1.3 million total fine for Balfour Beatty was set against an original contract value of £60 million and against a company within a large corporation that accounts for about 10% to 15% of the group’s £2 billion annual turnover. The fact that Balfour Beatty admitted guilt, the judge said, mitigated in its favour although it was

made evident during the trial that Balfour Beatty “must bear greater responsibility for the collapse”.

One of the major losers in this case was the prosecuting agency itself. The HSE was severely criticised by the trial judge for its conduct of the case and was awarded only £200,000 of its £880,000 claim for costs. In the opening days of the trial and by chance, the defence team found that much of the prosecution’s main expert witness report had been directed, and in some cases written, by the leading HSE inspector. The report could not be relied upon as being independent and

Section of the excavation works completed at the time of the collapse of the Central Terminal Area

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alfour Beatty’s lowest bid £60 million contract was to build 8.8km of shield-driven running tunnel and various NATM caverns including the underground station complexes at CTA and Terminal 4 on the estimated £440 million Heathrow Express Rail Link Project. Geoconsult was specialist NATM design engineer to Balfour Beatty.

NATM was specified for design of the stations following a successful trial of the method in London Clay at Heathrow in 1992. The trial was designed and supervised by the UK office of Austria’s Dr Sauer company and was built by the Kier/Lilley/Kunz UK/German joint venture. This was the first use of NATM in London Clay and the method was subsequently adopted for the Waterloo and London Bridge stations on the Jubilee Line Extension of London’s Underground. At no time in the trial did HSE or prosecution suggest that NATM as verified by the trial tunnel or designed by Geoconsult for the Heathrow Express was defective in any way. Prosecution and the HSE in its report “Safety of NATM with particular reference to London Clay” stated that NATM is a safe and appropriate tunnelling method in London Clay “providing enough care is taken” and that the Heathrow NATM structures now open to the public since mid 1998, are safe.

unbiased. As a result the prosecution withdrew the report in its entirety and the court allowed the prosecution to depend on the alternative expert witnesses of Sir Alan Muir Wood and of Prof John Hutchinson. These reports were introduced by prosecution at a preliminary hearing in November 1998 it was said, to support the original and subsequently withdrawn expert witness report submitted by Guy Lance of WS Atkins. In response, defence made application for the case to be dismissed on the grounds of “abuse” and applied also for the expert witness of Muir Wood and Hutchinson to be disallowed as substitute. Both applications were denied and the trial proceeded. This matter was to be re-examined in Geoconsult’s appeal. So although HSE obtained a conviction, it was effectively fined £680,000, not including the cost of the Lance report.

The trial

During the trial, evidence revealed extremely poor workmanship; major flaws in construction management; and questionable contractual arrangements.

Miners in the tunnelling gangs told of difficulty in constructing the invert joint of the NATM side-wall drift excavation sequence; of problems with blockages and delayed delivery of shotcrete; of regular changes in the personnel in the gangs and of little continuity of routine. Construction foremen and shift engineers told of pressures from management to improve

productivity and make up for delays Site and supervising engineers raised concerns about over-excavation of rounds by the miners; inexperience and lack of skill in the crews; and higher than expected surface settlement leading to inspection and repair of inverts where shotcrete was 50mm instead of 250mm to 300mm thick; where there was insufficient overlap or no starter bars at all in construction joints; and where shotcrete was sprayed over rebound. Senior project management engineers questioned the apparent lack of an experienced supervision team; the apparent disregard of the many CARS (corrective action requests) raised by any engineer noticing poor workmanship; apparent disregard of SDNs (system defect notices) raised by the project management team for the attention of senior contract managers; the application of “self certification” as part of the construction contract; and early relaxation of the already disfunctioning auditing role by HEX of the contractor’s self-certification system.

All of these flaws culminated in October 1994 in the collapse of the CTA NATM tunnels. Cracks in the shotcrete started to propagate farther and faster and grew wider during the early hours of the night shift (started at 7pm) and teams were being moved from one area to another to try and repair cracks and replace lumps of shotcrete as they fell and wire mesh burst through the green shotcrete of earlier repairs.

Eventually the teams were ordered to

vacate the tunnels at about 12am amid the sound of cracking and the sight of shotcrete particles raining down. At about 1am the abandoned tunnels started to collapse and continued over the two following days creating a hole on the surface and causing extensive damage to surface structures. All was said to be stabilised by the third day and with much of the underground cavity filled with concrete.

The prosecution case

HSE as the prosecuting agency claimed that the collapse “was the worst civil engineering disaster in the United Kingdom in the last quarter of a century” and that it was only by chance that no-one was seriously injured or killed.

As well as endangering the lives of the workers, prosecution suggested that the consequences for the public could have been disastrous, if the collapse at one of the world’s busiest airports had occurred during the day, and if the concourse tunnel had “unzipped” to beyond the Piccadilly Line tunnels of the London Underground system that run above the Express Rail Link tunnels and some 70m out in front of the collapsed zone (Figs 2 and 3).

Prosecution claimed, on the basis of expert witness, that monitoring data indicated that the collapse was evident up to seven days before the event and that Geoconsult was “the watchdog that did not bark”.

Sir Alan Muir Wood for prosecution stated that in his opinion, the collapse was caused by the defective invert. He claimed that recorded monitoring data and other contemporaneous evidence indicated that collapse was imminent up to 14 days before. He criticised the NEC form of contract as inappropriate for tunnelling contracts and said that self-certification as a method of quality control had failed and that (under cross examination) “it is very unfortunate for Geoconsult to find itself where it is” and that “morally it is very hard luck for them.” He said that “the way in which the whole management of this contract was proceeding meant there was a great break between engineering and construction which, to my mind, is the fundamental ill effect which led to all

Plan of the excavation works completed at the time of the collapse of the Central Terminal Area and location of a compensation grouting array

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AA, known in the UK as a progressive client, chose the New Engineering Contract (NEC) for the project and to adopt a system of “self certification” whereby contractors certify their own work and keep their own quality assurance records. To manage the project, BAA established the HEX team comprising engineers from Mott MacDonald, BAA’S project engineer and designer of all the permanent works; Taylor Woodrow, appointed before contracts award as construction manager; and members of its own engineering and management staff, in addition, Mott MacDonald was appointed by BAA and under separate contract to monitor and control all surface settlement issues during construction of tunnels under the airport facilities. It undertook the same responsibilities for Balfour Beatty under a different contract and Keller was engaged as the specialist contractor to undertake compensation grouting to control surface settlement under sensitive structures. Following the collapse, all parties to the project remained engaged and in a truly single minded project oriented approach, some quite remarkable engineering and construction work, including NATM work in the stations, was undertaken to restore the situation and complete the project just 6 months later than schedule - but at what cost depends where you start counting. on inverts [in section back to the shaft] still on-going. You might as well take the whole section out.” In his evidence Muir Wood suggested that removal of a large section of the tunnel’s infill clay over the defective invert (rather than in 2m leapfrog sections as instructed and carried out for the earlier repairs), together with starting the parallel platform tunnels, initiated the collapse.

The case for the defence

Of all the collapse reports produced, perhaps the most thorough was that produced by defence expert witness Prof.

Reinhard Rokahr of Hanover University. Sir Alan Muir Wood admitted that his report was prepared in a week and used “rough back of an envelop” calculations; the Guy Lance expert witness report was withdrawn; an expert witness report produced by Profs.

Mair and Potts of Cambridge University for Balfour Beatty was not made available given Balfour Beatty’s guilty plea; and the Incident Report, called for by the UK Government and being prepared as a statutory duty by HSE with input from Guy Lance of WS Atkins, Martin Thurgood of HSE, and Sir Alan Muir Wood among many others, was to be published.

Although Prof Rokahr criticised several aspects of the work, including the lack of on-site NATM authority at Heathrow; lack of experience among the field engineers, tunnelling foremen and the crews; poor quality of the monitoring instrumentation data; lack of full time geotechnical geologists within an NATM supervision team that already number too few; limited instrumentation data available; serious omissions in the installed instrumentation regime; and of the “inadequate” computer

software system provided by Balfour Beatty for processing the instrumentation data; he did state that there was sufficient recorded data to indicate that: “In my view, the causation of the collapse, in all probability and I think bordering on certainty, is a large scale slide of the clay mass in the zone between the downline platform tunnel north heading and Camborne House (Figs 2 and 3).” The collapse, he claimed, started as horizontal shear cracks in the walls of the advancing sidewall drift of the downline platform tunnel. After losing support in the area of the downline platform tunnel, a monolithic block of clay created by a number of bored piles near the working shaft and acting like stiff dowels, bore down on the concourse tunnel which, with its defective invert, could not act as a closed ring. This imparted heavy loads onto the upline platform tunnel where the new shotcrete could not carry the exceptionally high loads and failed.

Thereafter the block subsided further and caused the defective invert of the concourse tunnel to fold in on itself and the upper part of the concourse to settle by approximately 2m but without leading to a complete collapse of the crown. The defective invert, said Rokahr, and its ongoing repair could not, on their own have triggered the collapse. The report by Mair and Potts he said, declared that even with part of the invert completely removed, the tunnel, as designed, should had stood up for up to 80 days. He suggested that a softening of the clay body caused by starting the two parallel platform tunnel headings, and a soaking of the clay through the collapsed eye of an escalator box (excavated overhead and under a earlier contract) could have initiated the landslide. He claimed that deformation

UK FOCUS CASE STUDY

these things happening”. He said that liability for the collapse “should extend to those who were putting these sorts of contracts together”.

Jonathan Allen, CTA Area Manager in the HEX (Heathrow Express) management team for BAA (the private owners of Heathrow Airport) said that he was “concerned with the lack of understanding about self certification, in that Balfour Beatty felt HEX should not be involved”.

He felt, he said, that Balfour Beatty were paying “lip service” to self certification. In his read statement Bob Gee, Engineering Design Manager in HEX, said other than increasing the initial prediction of 9.34mm of settlement over the concourse tunnel at CTA, the HEX team were satisfied with Geoconsult’s NATM design and the baseline criteria and assumptions used. There was concern however of apparent lack of supervision and lack of experience in the construction teams as work started and a major concern that the process of self-certification and its audit by the HEX team was not working.

He said that while compensation grouting had been used in August 1994 to counteract higher than expected (18mm) settlement under Camborne House above the CTA concourse tunnel as it passed underneath, the HEX team was not alarmed by monitoring data they had seen to date. “We did not spot the lead up to the collapse,” he said, and that at the last progress meeting between HEX and Balfour Beatty on the day of the collapse, “neither settlement nor on-going repairs in the invert of the CTA concourse had been raised”, (in his evidence Sir Alan Muir Wood said on this point that “Nero was fiddling while Rome was burning”.) In defence, it was made clear that Geoconsult was acting as part of Balfour Beatty’s own engineering department under manager Ian Massey; and that Geoconsult personnel were not invited to project meetings; were not on the distribution list of letters or meeting records; and were not party to various decisions including, it was claimed, the programme of compensation “jacking” under Camborne House in August 1994, nor the decision to stop concourse tunnel invert repairs in August rather than continuing back to the access shaft; nor the instruction to open the two parallel upline and downline station platform tunnels in late September to make up for delays of 28 days and 80 days on these headings. Further evidence read by prosecution from the Balfour Beatty section engineers log book prior to the collapse gave an indication of the climate on the site. Entries read: “Alan (more muck) Myers [Balfour Beatty’s construction project director] demands we double production by the end of the week. He demands that we open all six faces. Panic all round”. “Cracks in the temporary wall - expert foreign sounding gentleman to inspect and instruct.” “Oh dear, oh dear, oh dear. Horizontal cracks in concourse tunnel. The witch hunt starts now.” “Big movement in the concourse crown at chainage 31m to 40m. Time to open up the invert for a look see.” “Work

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and settlement readings taken under Camborne House in August 1994, with an equally defective invert, were higher and did not cause collapse than those recorded in the area that did collapse in October. All readings, including the last taken before the collapse, he stated, were well within the design’s critical level criteria.

Rokahr’s expert evidence was based on the theory that some unforeseeable and completely unpredictable behaviour or geotechnical mechanism in the clay body was the only explanation for the collapse given the circumstances of the work, the evidence, and the geotechnical data available. He said that the speed of collapse could not be predicted and that workers were not exposed to risk until immediately before the collapse.

Hutchinson, professor of geology at Imperial College and as an expert witness for prosecution, stated that London Clay was a well documented, largely homogeneous, uniform and extensive body of over consolidated sedimentary clay with very few discontinuities and none identified in the area of the CTA that could have caused a landslip such as the one described by Prof Rokahr. He did however criticise the site investigation carried out

by Mott MacDonald as being “preliminary” and “surprising if a tunnelling expert did not carry out a further investigation”.

In a reverse of usual criminal prosecution proceedings, the burden of proof was on the defence to prove that all reasonably practicable was done by their staff to avoid the collapse and avoid endangering the lives of workers and the public. To this end Geoconsult argued that its responsibility on-site was greatly reduced by Balfour Beatty in that its original proposal for three full time employees (an experienced site engineer and two full time NATM construction foremen) had been reduced to one site engineer whose responsibility was to act as site liaison with Geoconsult’s design office in Austria, prepare the NATM working sheets as required, interpret monitoring data,

and advise Balfour Beatty accordingly.

They further argued that the one on-site Geoconsult engineer, Adrian Kattinger, tried to exert the type of authority usually invested in the NATM site Engineer on the Continent and through a full team of equally skilled and experienced field engineers, but against all the odds.

Kurt Laubichler, Geoconsult’s head of NATM work at HEX, said in an interview with HSE that Geoconsult was “requested in writing by Balfour Beatty, after award of contract I think, that we should propose one man to work within the framework of the services description”. He said Geoconsult had no choice other than to agree. Balfour Beatty claimed that it had sufficient NATM experience from the Channel Tunnel and other projects and that it could provide all the necessary site foremen, shift engineers and field engineers. With only one Geoconsult engineer on-site, defence claimed, it was implicit that Balfour Beatty had assumed the role of construction quality control supervision. This was confirmed in a letter from Alan Myers, Project Director for Balfour Beatty, to the HEX management team a month before the collapse that: “The primary function of the NATM engineers was not quality control. Quality control was the responsibility of the construction teams. This was in accordance with our self-certification scheme, we repeat we had continuous NATM engineering supervision and we do not have projectwide quality problems as you suggest.”

Three engineers were provided by Balfour Beatty to assist Geoconsult’s engineer in the field but these had little or no on-site or NATM design experience and had to cover all NATM areas of work on the full project.

In his efforts Adrian Kattinger warned of poor workmanship and the possible consequences to Balfour Beatty’s engineering and management staff and to his colleagues in head office in Austria.

He raised several CARs, and had his instructions overruled - a point clearly illustrated in the trial evidence when the

VIDEO INTERVIEW: OCTOBER 2013

Geoconsult settles the Heathrow case As a leading geotechnical design engineer, Geoconsult of Austria has introduced modern tunnelling techniques to many countries of the world. Chairman and Co-founder Johann Golser speaks about the journey and confirms that the company aim is to bring its expertise back to the UK after settling with the authorities following the controversial Heathrow collapse court case more than 12 years ago.

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NATM field engineer on night shift asked of his fellow engineer “was it agreed on the day shift that there is no need to progress invert repairs in the concourse any further back toward the shaft?” A “NO!” in Kattinger’s hand writing indicates the answer, but regardless, repairs of the concourse tunnel invert (following excessive settlement and compensation grouting under Camborne House during August 1994) was stopped at chainage 54m. It was during subsequent repair of the invert from chainage 54m back to the working shaft in October that the tunnels collapsed.

The verdict

After closing statements the jury of lay citizens hearing the criminal case was dismissed to consider its verdict. In his instructions, the judge asked the jurors to consider: Did Geoconsult endanger the lives of its workers and those of the general public and did they do every thing “reasonably practicable” to avoid such risks? If yes to the first part and yes to the second part - not guilty. If yes to the first and no to the second – guilty.

It took the jury 10 hours - and a return to the court room to hear further evidence about the collapsed escalator box and the 13m3 of concrete pumped in to fill the collapse void- before it returned a unanimous verdict of guilty against Geoconsult.

In the aftermath of the trial, the UK technical trade press had carried numerous letters and articles on various aspects of the case including criticism or support of the New Engineering Contract and of selfcertification while in the meantime the actual cause of the collapse remained a point of professional dispute.

In addition other serious issues were being discussed. For example was a criminal court really the most appropriate venue for investigating such events? In an open letter to a UK technical trade magazine Geoconsult stated that, had the collapse occurred in Austria, an official investigation would have been conducted but “it is improbable that criminal proceedings would have resulted in a case where nobody was injured or killed”. The criminal case against the four construction site managers following the NATM tunnel collapse in Munich just weeks before the Heathrow collapse, and which killed four people, was stopped (near the end of the Heathrow trial) by the judge on the grounds that even though the four accused were not on the site when they clearly should had been, the event and its consequences could not have been avoided. Equally, was judgement by a lay jury of such complex technical matters appropriate? The industry itself knows how difficult it can be to discuss technical engineering principals even among skilled professional engineers.

So, although the trial was over, the debate continued. First the result of Geoconsult’s appeal was awaited and the international tunnelling industry also awaits, after more than four years, sight of the official HSE report into the incident itself. n

CASE STUDY:

JLE, the London Underground Jubilee Line Extension project, lived through exciting as well as turbulent times. Excavation progressed under or alongside some of the most important landmarks of the city, including the Big Ben clock tower and the Houses of Parliament; created new metro stations under the Waterloo and London Bridge mainline railway stations; and passed under the River Thames four times to bring metro services to areas of south and east London, which until then had remained outside the technical construction limits of the Underground system. Controlling excavation and settlement and managing the impact of the station excavation works collapse on the concurrent Heathrow Express Rail Link project in October 1994 were among the most critical challenges together with managing the budget and the M&E installations.

SITE VISITS AND ABRIDGED FROM TWO JLE SUPPLEMENT: JULY 1994 AND FEBRUARY 1996

Jubilee Line Extension:

Triumphs and tribulations JLE budget as well as on its programme schedule. Application for more funds to cover the associated cost increases were made to the UK Government Department of Transport through which the majority of the project’s public funding is granted.

“One of the most important of several modification clauses we introduced into the contract document based on ICE 5th conditions, is a stipulation that the contractor would be bound to accept and implement an acceleration programme providing it was shown to be practicable and that the cost estimates were accepted as reasonable,” said Mike Smith, JLE’s Project Manager for Construction. “We cannot allow the project programme to be jeopardised by events that are within our control. The acceleration clause has proven its contractual usefulness over the months when the recovery programmes were being negotiated.”

Settlement control

Ground movement induced by excavation has been one of the most critical concerns for the JLE project team. The Line, running as it does beneath some of the oldest, historically most important and prestigious, areas of London, is subjected to the most stringent damage limitation tolerances ever before imposed on a tunnelling project in the city.

Assessments of all buildings within the predicted 1mm settlement contour along the 12.4km underground route defines a

• Client: JLE Management team for London Underground Ltd • Contractors and their design-build engineers: See following pages surface corridor of up to 90m wide, the corridor being equivalent to approximately three times the alignment depth.

Of the many methods available for controlling ground movement and minimising settlement, nearly all have been used on JLE. To compensate for areas where predicted settlement may still prove too high, grout injection ground treatment methods come into their own. Two effective grouting methods used on JLE include permeation grouting, to stabilise and strengthen loose water bearing ground, and compensation grouting for mitigating the effects of predicted volume losses and intercept ground movements as they occur.

UK FOCUS CASE STUDY

W

hen the Jubilee Line Extension Project team, appointed by London Underground to manage construction of the new 16 km-long extension, set a 53-month programme from start to public opening, it knew it had set itself a tough task. As soon as the first of the 14 large civil construction contracts was let in late 1993, signs of major excavation and construction work along the alignment started. By the end of January 1996, the construction work was in week 117 of the total 230-week programme and excavation of the underground section was about 75% complete.

The JLE Project team is supervising construction while appointed technical contractors or consulting engineers completed the original detailed designs and prepared the contract documentation.

On a project of this size and complexity, there have been unforeseen difficulties.

One of the most adverse was the collapse in October 1994 of NATM excavation work on the unrelated, but concurrent, Heathrow Airport Express Rail Link tunnel project. As an immediate and responsible reaction, the Jubilee project team voluntarily instructed its contractors to stop NATM work at Waterloo and London Bridge to review the integrity and safety of the works. About 1km of NATM work had been completed for JLE at that time and although revised NATM work restarted in due course, the delay had a damaging effect on the

Compensation grouting

Compensation grouting is principally a complex exercise in multi-level management and co-operative all-party decision-making based on information provided by extensive geotechnical instrumentation and computer-based analysis. “The aim,” explained David Harris, a Director of the company GCG as the principal geotechnical adviser to JLE on compensation grouting, “is to keep the

Compensation grouting arrays under the Westminster area installed from five temporary grouting shafts

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buildings as undisturbed as possible. Not to lift them with heave, or allow them to settle and then ‘pump’ them back up again once settlement has occurred, although this too is possible if the magnitude of movements are sufficiently small.”

The theory of compensation grouting in practice is to create an array of horizontal or vertical grouting injection tubes through which controlled quantities of grout are injected at a point about half way from the top of the tunnel to the top of the clay or at least one tunnel diameter (about 5m for the running tunnels) above the tunnel crown.

To cover the surface area to be protected, the grout pipes or TAMs (tube-amanchette) are installed in fanned arrays to create an ideal spacing between injection points of about 2m. The sleeves on the 70mm diameter TAM pipes are typically at 1m centres and TAMs of up to 65m long have been required to reach all areas from a given access point.

Having installed the arrays, the three phases of grout injection takes place.

• Phase 1 pre-treatment to stiffen the

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ground to a slight heave of 2-3mm before the tunnelling arrives.

• Phase 2 concurrent with tunnel advance and • Phase 3 to compensate for any residual settlement caused as the disturbed ground settles.

“By injecting grout within the ideal distance from the tunnel crown, the volume required is typically between two and three times the estimated total volume of predicted surface settlement,” explained Richard Wright, Project Director for the Amec/Geocisa JV responsible for permeation and compensation grouting operations at Westminster and Waterloo Station works. “Say, for example, the volume loss experienced for a 5m diameter tunnel with a cross section area of approximately 20m2 translates to the equivalent of 700 litres. The volume of grout required to compensate for this loss might range from between 1,200 to 2,800 litres, depending on the type of ground and its stress history, the size and depth of the tunnel, the method of excavation, and the

weight and location of structures above.

“Throughout the operation, the objective is to limit settlement,” said Harris “It cannot be ‘prevented’. The experiences to date on JLE demonstrate that compensation grouting on such a wide scale is possible and has been achieved.”

Tunnelling machines Eleven tunnelling machines of five types and from different manufacturers have excavated the 22.2km (2 x 11.1km) of nominal 4.4m i.d. running tunnels which link the eight new JLE underground stations.

Two shield mounted backhoe excavator machines from Howden Wirth are completing the 4.45m i.d. running tunnels between Waterloo and Green Park passing under the Thames and assuming a stacked running tunnel alignment through the Westminster Station area and under Bridge Street past Big Ben and the Houses of Parliament.

A major advantage of the machines is their ability to excavate the face and build the rings concurrently with the shield pushing off the completed ring. A 20mm

stacked alignment drive as its depth at +30m was expected to be deep enough to limit settlement caused by the predicted 2% volume losses to within the tolerances.

Actual settlement through the Westminster area however, was higher than expected at up to 20mm. For the second drive through Westminster, the same volume losses at the shallower level of the upper tunnel alignment would cause additional and unacceptable settlement. To limit this, a comprehensive operation of compensation grouting was programmed. To limit volume losses tunnel advance was also slowed substantially by excavating each 1m shove in 1/2m steps, setting the breasting plates between each step. During the sensitive three-week run down Bridge Street past Big Ben, under the operating District & Circle Line, past the Treasury Building and other important structures on Great George Street, tunnel progress was slowed to about 10m/24h on average and as little as 2m/shift on occasion.

Once past this area at the end of December 1995, progress increased again for the run beneath St James’ Park and

gardens before slowing again for the drive under the expensive Pall Mall properties into the junction with the existing Green Park Station.

When the first running tunnel was finished in May 1994, the components of that shield, including the backhoe and the forepoling and breasting plates, were stripped from the sacrificial skin and rebuilt into a new larger shield to excavate of the first of the two 140m-long platform tunnels at Westminster Station. These will be built by dismantling the 4.45m o.d. running tunnel pilots and enlarging the excavation to the 7.3m o.d. (7m i.d.) of the station platform tunnels. As the concrete segments are broken out, the SGI segments for the permanent station platform lining are erected. To provide increased protection against excessive ground movements above the platform tunnel drives, a canopy of seven 1m diameter horizontal microtunnel piles, infilled with concrete, formed a pipearch roof over the station tunnel area.

For the two 1.2km long running tunnels from London Bridge through Southwark and

UK FOCUS CASE STUDY

bead on the leading edge of the 5m-long non-articulated shields assists steering and prevents them becoming trapped in the clay tunnel, the last 150mm of which is trimmed by the cutting edge to ensure a precise cut for the accurate installation of the expanded concrete segmental lining.

With average rates of about 2m or two rings/hr, the two drives from the Jubilee Gardens access shaft under the river to Green Park met the conservative 50m/wk averages envisaged by JLE in its overall tunnelling progress programme. On several occasions progress also exceeded the 160m/wk programme envisaged by the Balfour Beatty/Amec JV and beat the 200m/wk mark on occasions. Factors that limited the overall average included the routine need to fully box and grout up the face with heavy timbers at the end of the last production shift before the two-day weekend and take this all down again at the beginning of the Monday morning shift, each step taking about five hours.

No specific compensation grouting was undertaken with advance of the lower

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Wirth Howden backhoe shield for enlargement of the platform tunnels at Westminster Station on to Waterloo the Costain/Taylor Woodrow JV, chose a 4.8m diameter roadheader boom shield from Dosco. As it advances, muck is fed onto the muck conveyor and back over the trailing gantries into trains of refurbished 12m3 Mühlhäuser sidetipping muck wagons pulled by Schoma diesel locos. At the back, a simple birdwing erector winches the precast concrete segments of the expanded lining around the roller bars and into place. The segments, manufactured by Charcon, are lightly reinforced with 30kg/m3 of Dramix steel fibre giving them the strength to resist the total 1,300 tonne thrust pressure generated by the 13 x 87 tonne shove rams of the shield.

Having started excavation on 26 April 1995, the tunnelling crews finished the first 1,193m-long drive some 19 weeks later-breaking into NATM work at Waterloo Station on 21 September 1995. “Weekly production rates averaged 87m/week working two 12h shifts/day, five days/ week,” said Stephen Thompson, JLE’s Assistant Supervising Engineer for the contract’s running tunnels. “The maximum advance in one day was 30m and best advance in one week was 111m.”

EPB TBMs

The two 2.8km long running tunnels on Contract 105 between London Bridge and Canada Water Station were expected to be amongst the most difficult on the project.

Bolted lining following the Mixshields drives

The tunnels pass predominantly through the Woolwich & Reading Beds of which little was known. From materials with a high clay content, a tunnel heading can burst without warning into lenses of loose watercharged sand.

In early 1993, when the £70 million contract, including Bermondsey Station, was awarded to the Japanese/French joint venture Aoki:Soletanche, it chose the Kawasaki EPB tunnelling system, which complied with the specification, and planned to use special lubrication muds and foams to cope with anticipated ground conditions. The first of the four TBMs working on the contract was imported direct from Japan. The other three were fabricated by FCB in Lille, France, Kawasaki’s licensed manufacturer in Europe. The back-up sets were designed and supplied by Decon of the UK in an FCB/Decon joint venture.

As one of the first uses of EPB technology in the UK, a great deal of investigation and a programme of trials was need to arrive at the most appropriate mix for the ground conditioning foam. “This comprises a 5% dose of surfactant into the water supply and a target air injection to expand the foam in the range of 1000 to 1500%,” said Paul Haines, Contracts Manager for Aoki. “In addition to preventing the balling of the clay, the lubricating effect of the foam reduced torque on the cutterhead and screw conveyor by about 20% and improved advance rates during the cutting cycle by some 300%,” said Haines. The TBMs operated under a maximum of about 1.8 bar EPB pressure. Maintaining a steady progress rate of at least 50m/week/TBM working two 12h shifts/day, 5 days/week will maintain programme. “Any more than that would choke the muck bins in the central working shaft of the four TBM drives, since we can only haul muck away from the site in dump trucks between 8am and 6pm on weekdays and 8am to 1pm on Saturdays.”

Slurry Mixshields

The quality of the finished running tunnels excavated by the two Herrenknecht Mixshields between Canada Water and Canary Wharf on Contract 107 is among the best on the project, according to Hadyn Davies, Senior Tunnel Engineer for JLE. Despite the treacherous nature of the waterbearing Thanet Sands through which the 4.4m i.d. tunnels pass, the TBMs and their tunnelling crews have maintained line and level to well within the ±100 mm

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tolerance and have ensured a precise build of the bolted segmental lining supplied

by Charcon and fitted with Heinke rubber gaskets to provide both primary and final finish for the tunnels. In addition, the ability of the slurry Mixshields to control the face and immediately backfill the annulus through the tail skin has resulted in minimal surface settlement.

When the Sir Robert McAlpine/Wayss & Freytag/Bachy JV was awarded the £32 million contract in late 1993, it chose the two 5.13m diameter Herrenknecht Mixshields specifically to cope with the potential 3 bar ground water pressures likely to be encountered in the Thanet Sands. As the two Mixshields advance, the bentonite slurry carries the excavated material back to the separation plant at the Durand’s Wharf working site. The cushion of compressed air in the plenum, or excavation chamber, regulates the operating pressure of the slurry to compensate for variations in the surrounding groundwater pressure and effectively limit surface settlement by limiting over excavation volume losses.

Throughout the first two drives from the central access shaft at Durand’s Wharf to the Canada Water Station box, and without any ancillary ground treatment, surface settlement some 20-25m above tunnel axis has been limited to between 5-10mm and more typically at the 5mm end.

Dual mode TBMs

The first of the 11 tunnelling machines on the project to start are the two Lovat TBMs working from the North Grenwich Station box to complete four drives of the 2.5km running tunnels. When it came to choosing machines for the job, the McAlpine/Wayss & Bachy JV needed a tunnelling system capable of operating in the open mode in London Clay and of coping with the 3 bar water pressures in the Woolwich & Reading Beds and Thanet Sands. The Lovat machines with their hydraulic flood doors, muck-rings and hydraulic pressurerelief gates (PRGs), are easily converted from the open and semi-open mode, to the closed EPB mode by replacing the conveyor belt with a screw conveyor.

When the two 5.2m machines arrived at North Greenwich in July 1994, they were first set to drive the two 1.1km long tunnels from the east end of the North Greenwich Station box to the Canning Town transition portal north of the river. The machines were launched in the open/semiclosed mode in which the familiar Lovat

Jubilee Line Stations Of the eight stations on the new JLE line, the mined stations at London Bridge and Waterloo were originally designed by the JLE technical contractors as traditional hand-mined, shield supported SGIlined excavations. However, while the successful contractors for each (Costain/ Taylor Woodrow for London Bridge and Balfour Beatty/Amec for Waterloo) based their tenders on conforming bids, they also submitted NATM alternatives.

The sequential excavation of the NATM technique, finished with an in-situ concrete lining, exhibited cost savings of up to 10% over the traditional designs. The alternatives, as proposed, were also faster than the traditional methods to build; were better suited to excavation of the complex station geometries; and contributed

positively to settlement limitation.

The method was proposed following the successful NATM trial of 1992 in advance of adopting NATM in London Clay as the specified method for excavation of the underground stations for the Heathrow Airport Express Rail Link project.

In anticipation of NATM alternatives, JLE engaged the Austrian consulting joint venture ILF/Laabmavr as its specialist adviser. In turn, Costain/ Taylor Woodrow engaged the London office of Austria’s Dr Sauer Company for the NATM alternative for London Bridge, and Balfour Beatty/ Amec teamed with Austrian consultant Geoconsult for the NATM design for Waterloo Station, Geoconsult working also with Balfour Beatty on the Heathrow Express Rail Link contract.

In the UK, NATM is considered a ‘temporary’ support system. The strength of the initial shotcrete, wire mesh and lattice girder or steel arch support, is not included as a contributing element in the design of the final in-situ concrete lining which is designed to carry the full overburden and hydrostatic design load criteria.

A plastic membrane waterproofing system behind a thinner (250mm) largely unreinforced in-situ concrete lining was proposed by the Dr Sauer Company for the NATM works at London Bridge. However because reinforcement had to be included to meet the full overburden and hydrostatic loading criteria, JLE decided to increase the reinforcement steel and the specification of a thicker in-situ concrete lining to provide the necessary resistance to water ingress and so removed the need for a membrane.

Following a positive start to NATM excavations on JLE, all was thrown into a state of confusion and long delay when NATM excavations on the Heathrow project collapsed in October 1994. The immediate effect was to stop all NATM work at Waterloo and London Bridge. “We had no cause to doubt the workmanship or quality of our NATM structures as already built, but the immediate concern was that there might be a fault in the design criteria adopted for NATM in London Clay,” said Hugh Doherty, Director of the JLE project team. “We were using the technique under two of the busiest mainline railway stations in the country and it was agreed with

London Underground that the responsible reaction was to halt the work and revert large sections of NATM back to the original segmental ring designs.”

In addition, JLE authorised the contractors to order traditional handmining shields and large stocks of SGI lining segments to pre-empt the possibility that NATM for the longer station platform tunnels would fail to obtain HSE ‘no objection’ status. The shields were ordered and supplied but none were ultimately needed with NATM permitted to progress once again under extensive redesign and changed working practices.

NATM revisions

In summary, the redesigns proposed by JLE and which were given statements of ‘no objection’ by the HSE to resume NATM work were based on the following points.

• The load criteria used in the design of the temporary support have been increased while the allowable stress criteria in the temporary shotcrete shell have been revised downwards.

To meet the higher load criteria, the thickness of the ‘temporary’ shotcrete linings for larger diameter tunnels has been increased by 50mm to 100mm resulting in shell thicknesses of up to 400mm for the largest tunnel cross sections. Criteria for the final lining designs remained unchanged.

• Side drift excavation, originally proposed for 9m diameter platform tunnels, and the double side-drift excavation for the 11m diameter concourse tunnels, have been abandoned in favour of near-circular or circular pilot tunnels and subsequent enlargement to full cross-section. This eliminates the radial construction joints of the vertical wall in the crown and invert of the outer shell which, because of awkward access, are comparatively more difficult to construct and supervise than the radial joints of the crown/bench and bench/ invert transitions. The change was also expected to expedite ‘no objection’ clearances for the large diameter NATM tunnels from the HSE in order to take advantage of the programme benefits and mitigate against further delay.

• The near-circular or circular sections

UK FOCUS CASE STUDY

flood doors and the pressure-relief gates would regulate and control the ingress of excavated material or close off the face completely in unstable conditions. As the TBMs advanced, the tunnels are lined with the bolted segmental lining supplied by Charcon and guided on their alignments with the assistance of guidance systems supplied by ZED Instruments of the UK. The TBMs set off in August and September 1994 and broke through some 18 and 19 weeks later, having achieved average advance rates of 11m/machine/ day. After breaking through, the backups were reversed through the completed drives and the TBM shields were returned by road to North Greenwich for the second two 1.4km long drives, also under the Thames, in the opposite directions towards Canary Wharf.

These drives start in a short length of London Clay with gravel in the crown before entering the wet changeable conditions of the Woolwich & Reading Beds and the more permeable Thanet Sands in which water pressure predictions were as high as 3 bar. The tunnels also pass closely beneath the Blackwall road tunnels. To ensure total control over the excavation and to minimise settlement beneath these tunnels to keep them open to traffic at all times, both TBMs will be fitted with their EPB screw conveyors for working in the closed pressurised mode throughout.

From left: Installing a pressure cell prior to applying the shotcrete; Development of NATM junction with NATM running tunnels

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UK FOCUS CASE STUDY 56

Top from left: Pilot redesign to avoid side wall drifts at London Bridge and enlargement of the pilot to station platform tunnel Bottom from left: Cross section of the tunnelled complex at Waterloo where the upper levels reverted to conventional SGl-lined, hand mined excavation and the lower levels continue under revised NATM work including enlargement of pilot to platform tunnel of the pilot and the enlargement tunnel sequences are more easily achieved than the shapes for the side drift geometry. Full round steel arches have been extended into the invert of the central pilots of the Waterloo Station revisions to help maintain accurate profiles. The pilot tunnels are designed and built to the same specification as the standard 5.3m diameter shotcrete lined NATM running tunnels which are excavated on full-face sequences progressing on 1m rounds with the top heading advancing by four rounds before a 2m section of combined bench invert is excavated and the invert is closed. As was always intended, sealing shotcrete is applied to all exposed surfaces, including the faces, before the arches, wire mesh and further shotcrete layers are installed.

• There is now an increased emphasis on the relationship between compensation grouting and tunnel advance resulting in a more co-ordinated and co-operative approach between the tunnelling and grouting teams particularly when considering the work schedules and monitoring arrangements of each operation. The shape of the pilots and the subsequent enlargements have the added advantage of reacting more favourably than the oddly shaped side wall drift geometry to any possible grouting pressures.

• Although the exact effect of compensation grouting on the NATM method of construction will only be fully understood following extensive

back analysis of data recorded on the project, the excavation and construction sequence must take into account potential grout loads or pressures.

Compensation grouting on JLE is injected between 3m and 10m above the tunnel crowns and in the redesign, wide exclusion zones of up to 10m ahead, behind and either side of an advancing NATM face have been specified.

• Although concurrent advance of adjacent NATM structures was never anticipated on JLE, the construction of two large adjacent tunnels, within one tunnel diameter of each other, is only allowed providing the permanent lining is installed throughout the full length of the first before the second begins. This was considered an HSE requirement for obtaining statements of ‘no objection’.

• The revision of NATM had a major impact on the programme and led to the decision at London Bridge to build SGI segments as the final lining within the shotcrete immediate support shells of the station platform tunnels. Erecting SGI segments could start earlier and could progress concurrent with tunnel advance, thus saving time.

• At Waterloo Station, the platform tunnels adjacent to the lower concourse tunnel will be lined with their in-situ concrete lining before enlargement of the concourse begins.

The consequences of the Heathrow collapse cost JLE dearly. The exact additional costs are not yet finalised but they run into millions for each contract,

according to Mike Smith, JLE’s Project Manager for Construction. But as Doherty commented: “If we had not persisted with our efforts to restart the NATM, it would have been lost to the UK tunnelling industry for many years, if not forever, especially in urban areas. By persisting, we have saved this cost effective technique for future UK projects. Through the HSE review and our own redesign specifications we have ‘Anglicized’ the technique or modified it to better suit British tunnelling practices.”

As work on the mined stations managed delays of up to a year before restart of NATM works, intricate work progressed on combined open-cut and mined enlargement of running tunnels at Westminster, Southwark and Bermondsey Stations; for the junction with the existing underground station at Green Park and in the large open box cut-and-cover structures for the Canada Water, Canary Wharf and North Greenwich Stations.

For all involved, the excavation works for the new Jubilee Line Extension was an outstanding accomplishment. The installation works of the mechanical and electrical works was a different story of delays and cost over runs. However in February 1996, 28 March 1998 remained the opening date of the new line. “It has been asked many times whether the project will open on time and my answer is yes,” said Doherty. “We do not underestimate the challenge that lies ahead and we cannot afford any more setbacks, that is true, but I am confident that the task is do-able and that the JLE team and the contractors are capable of meeting that challenge.” n

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CASE STUDY: The Channel Tunnel, completed and opened to public rail services in 1998, provided a role model for the innovative planning, organization and funding of future mega infrastructure projects. Its outstanding construction successes, as well as its well documented problems, stand as benchmarks against which others will avoid similar mistakes.

REPORT PUBLISHED: MARCH 1999

Channel Tunnel in hindsight

UK FOCUS CASE STUDY

F

rom a technical point of view, the undersea Channel Tunnel to connect Dover in England to Calais in France was not a difficult project, as compared to the contemporary Seikan Strait crossing in Japan for example. The chosen scheme, comprising two 7.6m i.d. rail running tunnels and a 4.8m i.d. central service tunnel, was aligned in the Chalk Marl stratum, which is generally an impermeable, weak, homogeneous rock highly suitable for rapid tunnelling, although fractured zones with high water inflows were to be anticipated.

Following the Chalk Marl stratum takes the tunnels to a maximum 115m below sea level and 75m beneath the seabed. Tunnelling systems highly suited to working in this geology were well tried and readily available.

What had changed remarkably was the technological advances which allowed record breaking high speed tunnelling on the UK side of a maximum 428m/week (24h/day, 7 days/week with a maximum 8h downtime maintenance shift/week) building a fully non-bolted concrete segmentally lined undersea tunnel of o.d. 8,036m. The best overall average was 179m/week on the UK marine running tunnel south.

On the French side, modern slurry and earth pressure balance (EPB) pressurised

tunnelling systems were introduced from Japan to tunnel through the wet fissured conditions of the upper chalk at the start of their undersea and landward headings.

These systems were not readily available to European contractors for the early 1970s attempt and the French would have had great difficulty passing through the fractured conditions with the equipment chosen at that time.

Two other notable achievements are: • Excavation of the large 55m diameter x 66m deep access shaft on the French side as the principal working access for its three undersea and three 3km long landward tunnel headings. All muck was transported to a large sump in the bottom of the shaft where it was slurried with water and pumped out for disposal in a large 4.5 million m3 settling pond about 1km distance.

• Application of NATM on the UK side, used initially for rapid excavation of the complex chambers and tunnels of the underground marshalling area at Shakespeare Cliff. This formed the principal working station and launch site of the six TBMs used to complete 22km of the marine service tunnel, 17.36km and 18.77km each

Geological section of the 53km long Channel Tunnel from portal to portal

Howden TBM in manufacture for the UK Service Tunnel heading

58

• Client: Eurotunnel • Contractor and its Designers: Trans-Manche Link (TML) Consortium (p56) • Consortium Tunnel Designers: France: BETU (Bureau d’Étude Tunnels) UK: Mott McDonald marine running tunnel and the 8km long UK landward tunnels. NATM was then used in spectacular fashion for excavation of the 164km long x 21.2m wide x 15.4m high (252m2 section) UK undersea crossover chamber. It was used again for excavation of short 500m long land rail tunnels under Castle Hill soft in gault clay en-route to the UK Channel Tunnel terminal site.

Worthy of special mention also is: • The logistical accomplishment of running the necessary service trains, particularly the three long UK undersea tunnels. These continued while at the same time access from the advancing service tunnel allowed concurrent excavation of the cross over chamber, which had to be almost complete before arrival of the two tunnel machines. Similarly, excavation of the cross passages and pressure relieving ducts also had to share muck transportation times with the advancing marine running tunnel headings.

• The hand mining work which accomplished the many lateral cross passages and pressure relief ducts linking the three main bored tunnels.

• The advanced surveying techniques and guidance systems employed to ensure that the six undersea tunnel heading did actually meet under the sea. In fact junction of the UK and the French service tunnel headings was within 350mm on line and 60mm on level.

• The establishments of the marine lagoons on the UK side which created at the foot of the white chalk cliffs a disposal site for some 3.75 million m3 of muck in what is an small and tightly confined working site.

• The segment casting factories in both countries where thousands of high precision tunnel lining segments were produced at high rats. The UK factory on the Isle of Grain had a potential output of 1,152 segments per day and produced the required 4,42,755 segments within 3.5 years.

• The installation now going on of the many thousands of units and lengths of electrical cabling, track and other

Claims

It is difficult to pin point exactly where and how such an enormous overrun has

NATM excavation of the UK crossover cavern occurred and perhaps the calls for its justification will get buried and (some may say hopefully) forgotten with the passage of time. However, as Sir Miur Wood stated at a meeting about the project in London in February 1993 for the Institution of Civil Engineers, that the cost of building a structure is as much a part of the engineering as design and construction.

While some specific over runs are identifiable, others are less obvious and a cause of circumstance. For example, criticism can be levied against the method and the speed by which the construction concession was awarded.

From invitation in April 1985 to close of tender in October 1985, promoters had only seven months in which to prepare their design, construction and financial proposals. The governments then gave themselves only three months to assess each proposal and announce the winner in late January 1986. Some say that the speed was essential to preserve momentum and secure a positive go-ahead. Many now accept that more time spent in the early planning stages would have avoided many costly problems to come.

The concession consortium advancing the project were interested principally in the construction and banking opportunities presented rather than with the prospect of owning and operating a transport system.

The ten contracting partners therefore funded the establishment of Eurotunnel, a small organization that would act initially as a financial institution to raise the necessary working capital during the construction phase and gradually grow into a owner and operator of the completed system. The ten contractors then formed Trans-Manche Link (TML), which wrote and agreed the seven year design-and-construct contract to build and commission the full system

by May 1993 for a contract price of £2.66 billion (in 1985 prices).

This was based on: • A £1,286 million of target cost element for the tunnels and associated civil works with Eurotunnel and TML sharing any cost-over-or-under runs; • A £1,070 million lump sum for the terminals and the fixed mechanical and electrical equipment for the terminals and tunnels; and • A £245 million as a procurement

UK FOCUS CASE STUDY

fixed equipment required to operate a safe and efficient rail system. There are many published papers that describe in detail the technical and engineering accomplishments of the Channel Tunnel work and give credit to the Channel Tunnel work and give credit to the designers, consulting engineers, machine manufacturers and others involved in various aspects of the project.

However, very little is said about the actual cost of the engineering accomplishment and its organisational and management model. The 1980 successful attempt to provide a fixed link under the Channel between England and France was a proposal initiated by the construction industry, with a Member of Parliament asking the UK government to support restart of the 1970s attempt that was aborted due to lank of UK public resources to continue its financial commitment.

Famously, Prime Minister Margaret Thatcher stated in Parliament that if the construction company wanted to build the link it could do so but with not a penny of public money and with no government guarantees on borrowed funds either.

The construction industry needed no further encouragement and in 1986, a group of ten construction companies (five UK and five French) with five national banks (three French and two UK) proposed to design, build, fully equip and commission the fixed rail link after 4½ years of construction work in May 1993, and at an estimated total cost of £4.87 billion. It was on this basis that the group raised nearly £1,000 million equity from individual shareholders and £5,000 million in loans from more than 40 international banking and financing institutions. Of the nearly £5 billion cost, construction was estimated at £3,000 million, with £650 million for corporate costs, £450 million as an inflation contingency, and £1,000 million for net financing costs.

The most recent predictions of April 1992 put the cost at the time of opening (now tentatively rescheduled to December 1993) at £8.1 billion excluding on-going interest payments. (Note: The project finally opened in 1997 and at a reported cost of £13 billion.)

Left: The 55m diameter x 66m deep working shaft serviced the three undersea and three landward headings on the French side; Centre and right: French marine running tunnel TBM and its vacuum segment erector and assembly

59

UK FOCUS CASE STUDY 60

Trans-Manche Link (TML), a consortium of 10 large construction contractors - five UK and five French – secured the concession to design, build, own, operate and finance the construction of the Channel Tunnel. Eurotunnel the client had to be created by these companies. fee and cost reimbursement for the system's rolling stock and locomotives.

The establishment of a Maitre d'Oeuvre was a requirement of the French and British Governments to provide project management and contract administration and to protect the interests of banks and other third parties to the contract between TML and its 'client' Eurotunnel.

However, the significance of an independent and strongly identifiable 'client' became obvious in October 1986 when financial commitments by up to 40 investment institutions controlling a syndicate of up to 200 banks worldwide (Equity 2), was made conditional on the promoters giving up control of Eurotunnel.

Nine months after award of the concession Eurotunnel "acquired a mind of its own" according to Sir Alastair Morton, appointed co-chairman with André Bénard of France of Eurotunnel in April 1987. In doing so, Eurotunnel inherited a construction contract and a term sheet for a credit agreement that it had not negotiated.

Again according to Morton, Eurotunnel has had to live with the consequences of its delayed birth.

With its independence, Eurotunnel took over of the daily project management and commercial responsibilities from the Maitre d'Oeuvre, including the staff. This strengthened Eurotunnel as the owner and client and left the Maitre d'Oeuvre

responsible for overall monitoring and audit of the project and to be expert advisers to Eurotunnel. Equity 2 also effectively reduced the holding of the 10 founding shareholders of TML to a minority of a combined £50 million.

Today this is about a 2-3% shareholding in Eurotunnel.

Through this evolution, the original promoters were relegated to working for a client strengthened further by the appointment of experienced financiers and a management team from Bechtel of the USA. Thus were sown the seeds for a confrontational and contractual attitude toward the construction work.

The well-publicized contractual posturing between TML and Eurotunnel has led to many management and organisational restructures and the many claims and counterclaims between the two have kept the project's disputes panel of experts and its arbitrators busy.

Many of the claims and increases in cost can be directly related to the fasttrack approach adopted to complete the 'turnkey' project as quickly as possible. While this does improve financing arrangements, it also has major financial disadvantages. When detailed design and construction are progressing simultaneously it is very difficult to estimate costs and keep a tight control on budgets.

There is no argument about having to pay

the price when new design directives are specified during the construction phase.

A major cause of delays and cost overruns are related directly to design changes imposed by the intergovernmental commission (IGC), the project's impartial regulatory body and safety watchdog. Major design changes have been demanded by the IGC to improve the fire safety aspect of the car and freight wagons of Eurotunnel's Shuttle service. Many were imposed after award of procurement orders with suppliers. The IGC also ordered a wider gap between the opened doors in the cross over chambers which allow trains to pass from one track to the other for maintenance purposes. During normal operation the heavy steel doors are closed to separate the two tunnels for both safety and ventilation reasons.

TML has lodged large claims for such design changes many of which remain unresolved. In return, Eurotunnel has indicated that it may lodge claims against the IGC for changes legally outside the criteria of the original concession requirements or beyond the agreed concession's specifications. This could also include claims against IGC for the notorious delays and mismanagement by the UK government to provide a dedicated high-speed rail link between the Channel Tunnel and London. The provision of national infrastructure is central to acquiring for the nation and its people the greatest benefit from the fixed link. This has been recognized and acted upon by the French government. The UK government appears not to share the same commitment or vision.

The most recent breakdown of Eurotunnel's project capital costs were published in the prospectus for the 1990 rights issue. Since then, the high number of unresolved issues and uncertainties about the exact opening dates as well as the phased introduction of a full service due to late delivery of rolling stock, make forecasting for the general public unwise.

At that time, forecasts for TML's final contract price had risen from £2,660 million to £3,897 million with an additional £239 million set aside as a claims contingency fund.

£13,000 per meter of tunnel

In the forecast, the target cost tunnelling element had increased from the 1985 £1,286 million contract price to £2,009 million and represents largest portion (54%) of the total forecast increase. Given three tunnels of 50.5km long from portal to portal, and given the complexity of the project and the different tunnelling techniques employed, this gives a rough tunnelling cost of £13,000 per linear meter.

This does include design costs and client management costs but, compared to other current tunnelling costs, most European tunnelling contractors consider this high and hard to justify. The reimbursable cost forecast for the rolling stock had risen from £245 million to £583 million and the lump sum forecast had risen from £1,070 million to £1,305 million (£491 million for the terminals and £814 million for the fixed

mechanical and electrical equipment). By August 1992 and assisted by disputes panel rulings, all issue in the target cost works and two terminals had been settled.

It is the increases in the fixed equipment and the rolling stock that remain the most contested dispute between Eurotunnel and TML. In spring 1992, the disputes panel of experts ordered Eurotunnel to pay some £50 million per month as 'interim funding' to TML without limit until either both agreed otherwise, or both agreed prices for the work or both asked the panel to set them. Eurotunnel disagreed with the ruling and took the issue to arbitration. In the meantime it offered TML a conditional cash sum as part settlement which increased the accepted cost of the fixed equipment and rolling stock to £980 million, 60% more, in 1985 money, than the original £620 million.

TML rejected the offer and asked the panel to set the price.

Then in September, the arbitrators found that the disputes panel was wrong in law to order the 'interim funding' and the £200 million paid by Eurotunnel to TML by August 1992 is held on account toward ultimate settlement sums. Accountability to its more than 600,000 private individual shareholders commits Eurotunnel to regular blow-by-blow accounts of these battles, albeit from a biased Eurotunnel point of view having effectively banned TML from speaking to the media. It is a pity the drama of the claims and the confrontational relationship between Eurotunnel and TML is overshadowing the actual positive settlements and work achieved and could well lead to protracted litigation lasting years after completion of the contract.

on corporate costs. A letter to shareholder in October 1992 stated that settlement of claims associated with the civil costs of the tunnels and the terminals added less than 0.1% in 1985 values to the forecast £8 million total cost to project to completion in late 1993, including inflation and interest.

Some of the borrowed money is on fixed interest rates but much is floating and therefore subject to the interest rate changes. The responsibility of following the markets and making decisions about how best to profit from these trends consumes the full time efforts of the extensive Eurotunnel finance department.

That Eurotunnel has survived despite zero financial support from the Governments, could well stand as a negative point. Other major infrastructure projects, such as the Storebaelt fixed link project in Denmark, have adopted the Eurotunnel model of raising venture capital on the open international money markets, however, while the basically autonomous client is responsible for raising and managing its own finances, it does enjoy the support of government guarantees on borrowed money. This gives it a major advantage when negotiating loan agreements as well as an average 4% interest rate advantage, paying, for example, 8% against Eurotunnel's comparable 12%.

The question of commissions charged by banks also fit into this category

and requires constant scrutiny as does the robustness of Eurotunnel's traffic forecasts. These forecasts are the sensitive blue prints against which Eurotunnel's potential income and commercial viability is judged. Current economic statistics do not support optimistic predictions about traffic flows particularly in the first two or three years of operation.

In the meantime, while Eurotunnel financial managers juggle with fantastic sums of money, individual shareholders must continue to believe in the ideal of contributing to one of the greatest engineering fetes of our time since they will not see a return on their investments for many years. The joint chairmen of Eurotunnel in their letter of October 1992 admitted to shareholders that after opening, virtually all Eurotunnel's income for several years will service bank debt.

Given current revenue forecasts, cash flow after interest rates is not expected to turn positive until 1996-97 with the first dividends to shareholders expected - hopefully - by the end of the Century.

They go on to say that they will "have to seek further funds from various sources, including our shareholders, in 1994 or 1995" to help it through its most vulnerable infant years. We shall have to wait and see if the sense being part of something really big and visionary will encourage private shareholders to part with more cash to support this wayward child. n

UK FOCUS CASE STUDY

Scheme of the fixed undersea rail link comprising the central service tunnel the running tunnels with cross passages and pressure relieving ducts

Fit-out of the UK NATM excavated crossover cavern

Financing

As it is, all the capital costs and the claims, both settled and outstanding, pale in significance when stacked against the cost of borrowing the necessary working capital and servicing interest payments.

Envisaging a maximum 5:1 debt to equity ratio at the start, the ratio is now about £7.3 billion debt against an equity of just £1.6 billion.

According to the last interim report, published in October 1992 for the half year to 30 June 1992, Eurotunnel had a loan facility of £7,276 million and had spent £3,701 million on financing costs and net interest expenses of £842 million

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