DISSCUSSION FORUM: Rockfalls, rescures, recoveries

Direct by Design 41 workers were trapped for 17 days and nights behind a 50m rockfall some 250m from the excavation portal of the Silkyara road tunnel portal in northern India until rescued on 28 November

DISCUSSION FORUM

Rockfalls, rescues, recoveries Shani Wallis, TunnelTalk

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s the situation at the Silkyara road excavation project in northern India was unfolding, professionals of the underground construction industry were gathered in Mumbai for the Tunnelling Asia Conference and the 9th Edition of the ITA Tunnelling Awards. Everyone was keeping up to date with progress at Silkyara, particularly via the media reports by Arnold Dix who was expected to officiate at the Mumbai event as the current President of the ITA, International Tunnelling and Underground Space Association, but was instead on site in Uttarakhand offering support, expertise and advice in his private professional consulting capacity to the rescue efforts of the 41 tunnellers

November 2023 held on the far side of the rockfall that blocked portal access to the 2.5km heading of the road tunnel excavation.

With reports coming through, there was much discussion at the conference about what might have happened to have caused the rockfall from the crown of the excavation some 250m from the portal and 2,100m back from the advancing face of the 4.5km long tunnel. The collapsed section of the heading will have been standing in its primary-lined state of shotcrete and lattice girders, for many months but, according to on-site reports, there was no indication, immediate or historic, that the tunnel at that

NATM excavation was progressing under an EPC form of contract

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Contract and incident details

According to a Ministry of Road Transport & Highways statement, construction of the 4.5km long two-lane, bi-directional road tunnel at Silkyara, under a budget of 1,383 crore rupees, was sanctioned in March 2018 to provide an all-weather connection and reduce the current 26km, 50 minute, snow-affected mountain route to a 5 minute, 4.5km tunnel pass. In June 2018 the National Highways & Infrastructure Development Corporation Limited (NHIDCL) signed a contract with Navyuga Engineering Company on an EPC (engineering, procurement, construction) basis for 853.79 crore rupees. Work on site commenced in July 2018 with an intended completion day of July 2022.

Work was delayed, with current progress at about 90% complete - 2,350m from the Silkyara portal and another 1,710m from the opposite portal, and about 477m remaining. Breakthrough had been anticipated

by May 2024 but will be further delayed by the recent incident. It was on 12 November 2023, at 05.30 hrs, that 41 workers were in the tunnel undertaking reprofiling work in the section 260m to 265m from the portal when the ravelling collapse occurred from 205m to 260m cutting off access completely for the workers. point was going to collapse so completely as to block the entire 30m-span of the underground space.

The forensic investigations will have to review the logs to know of the ground conditions encountered initially at that section of the advance and of the observations and any remedial works carried out at this point. The need for reprofiling work just ahead of the collapse zone suggests high and continuous deformation of the tunnel profile with time.

Following the collapse, shotcrete was used to reinforce the standing tunnel on the portal side but attempts to remove the fall initiated additional falls and that was suspended leaving a length of about 50m between the portal and the workers. First contact with the workers was via a 6in pipe forced through the rubble to establish camera communications and a link for passing through food and water. With a 2,000m heading of supported tunnel ahead of them, the workers were in no immediate danger. They were also in an environment that they knew well, going into the long heading each day for the 10-12 hr long working shifts. Having them safely on the portal side of the collapse was only a matter of time, and things moved fast.

An auger pipejacking machine, with its 900mm diameter steel pipes for a 50-60m drive plus its experienced operating crew, was airlifted to the high mountain-side site and set up to bore a connection through the rubble. The unpredictability of any tunnelling operation was however demonstrated on this effort when the auger machine hit metal obstructions in the heading (lattice girders of the collapsed zone and perhaps also buried excavation equipment) and was damaged irreparably. The auger and its flights had to be cut up and extracted from the pipeline.

The damaged front end of the pipe being jacked also had to be cut back and the last 10m or so of the 900mm diameter heading would have to be completed by manentry hand-excavation. All cost the operation days, and in the meantime a vertical auger boring rig had been flown to site to start a vertical rescue access of about 60m deep from the top of the overburden. Excavation of the last 477m of the main route excavation also progressed from 2

the opposite portal. With three efforts underway, one was going to succeed, time would tell which one.

As it happened it was the hand-excavation in the horizontal jacked pipeline that succeeded first and all 41 tunnellers were each lead to safety. As the operations progressed, safety of the 41 workers in the tunnel relied on them staying well back from the collapsed zone and under a wellsupported and stable part of the tunnel, as well as on the safety of the workers progressing the rescue excavations and of the many additional officials and experts and the media and members of the workers’ families who congregated at the site and around the portal, amid the movement of heavy construction vehicles, ambulances, and private transport vehicles around the site. It is a tribute to all involved that all 41 workers are rescued and that no one was hurt during the 17-day operation.

But excavation through difficult and sensitive geological conditions is necessary. It is vital for development of connectivity through mountains, under seas and rivers, and beneath cities and dense urban areas. There can be no question as to whether the tunnel should have been attempted. This is not the first heading into sensitive geology to collapse – be it by TBM, drill+blast or NATM far out in the mountains or in the city centres – and it will not be the last. All types of construction carry risk. Success is a question of how that risk is managed and who bares the risk responsibility.

Adopting the L1 lowest bidder for awarding the contract is a risk. It opens the way for taking short cuts and adopting inferior materials or processes to save money.

Passing the risk of the geology entirely to the contractor is another major risk, for who then is responsible for controlling quality of construction and the supervision of the works to ensure compliance with design and ultimately for the safety of the workers.

These inherent risks to any tunnelling project have been discussed so many times, as have safe working practices, and the need for experienced design and construction of underground works, particularly in regions of known difficult geological and/or logistical conditions. And while there have been many, many, many difficult geological and logistical conditions faced on projects around the world, all have been managed successfully, overcome and eventually the underground infrastructure completed placed into service.

The Silkyara road tunnel will also be completed and will go into operation. The tunnelling engineers involved will recover the collapsed zone and the final stretch of excavation will be completed – the amount of extra time and extra money to achieve that is now for the negotiations between the contractor and the owner.

Rothang experience

One of the first road tunnels excavated using the NATM concept high in the Himalayas was for the 8.9km long single-tube, bi-directional Rohtang tunnel on the ManaliLeh route between the States of Himachal Pradesh and Jammu and Kashmir in the 2010s. Awarded to a joint venture of Strabag and Afcons for a contract price of about €250 million, the excavation process encountered all manner of rockfalls, high deformations, inrushes of saturated material and collapses. Through the excavation period, started in 2010 from both portals, all the tools

All the tools in the NATM toolbox were needed to complete the 8.9km long single tube Rohtang road tunnel high in the Himalayas from 2010 to 2017

in the NATM excavation toolbox had to be engaged to achieve successful breakthrough in September/October 2017. The tunnel was inaugurated into service by Prime Minister Modi of India in October 2020.

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to a variation of the RMR geomechanical classification Classification method did not cover failure mechanisms that usually occur in deep tunnels Findings for a pilot tunnel experience where deformations of more than 10-15% of the excavation diameter were encountered were not properly incorporated into the main tunnel excavation process Anisotropic or time-dependent rock mass behaviour was not considered, nor the relationship between the rock mass deformation and the support, nor the effect of the construction sequence Geomechanical classification did not consider relationship between maximum and minimum in-situ stresses and the relationship with the type of rock and

At the Tunnelling Asia conference in Mumbai and as a 2023 finalist of the ITA Tunnelling Awards, we heard of • the experience of the 7.6km long highway tunnel on the twin-tube Sociedad Concesionaria Vespucio Oriente (AVO1) highway to improve connections in the southeast of Chile. Excavated between 2018 and early 2022 the • works encountered extremes of squeezing ground, high deformations, rockfalls, long delays and cost overruns.

The shortfalls of the operation, as related to the delegates of the conference by Andres Marulanda of INGETEC, included: Principle of yielding elements to support squeezing ground • Much larger loads on the concrete lining than those considered in the design • No provision for reactivation of squeezing due to rock creep • Incomplete installation of the primary support elements – steel arches and shotcrete – and incomplete round inverts in zones of squeezing conditions • Deficient geological registers with contradictions between design and as built records and incomplete instrumentation data, and • Deficient quality controls • The support system design was based purely on empirical methods related

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Design of the prestressed anchor cable support and its application and the deformation experienced before and after the application of the new technology

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the geometry of the excavation and Awareness that heavy rigid steel sets become impractical to stabilise high load pressures was ignored.

One of the most important observations of the experience was the importance of incorporating a shotcrete lining with yielding support elements and rock bolts to cope with high deformations. The new highway opened in July 2022 with the tunnels a vital element on the link.

Another finalist in the 2023 ITA Awards from China highlighted similar geotechnical difficulties that were overcome on the 15km long twin tube Muzhailing road tunnel on the Weiyuan-Wudu Expressway through mountainous geology under an overburden of about 630m. A new high-strength prestressed anchor cable support system was introduced to control high-stress deformation in the soft rock. Based on an ‘anchoringbefore-supporting’ process, instead of the traditional ‘supporting-before-anchoring’ process, the new system: • highlights the concept of timely and active support, • reduces the unsupported time of excavated surfaces and • strengthens the control effect of pre-stress on deformation of squeezed surrounding rock. A primary support system for squeezing ground tunnels and its design theory and method have been proposed to form a supporting technology integrating rapid prestressed support and anchoring before supporting to achieve an important step in prevention and control of large deformation disasters in ultra-high ground stress soft rock tunnels from a system of multiple strong support and brace to a one-off successful support.

A final high-quality example of NATM excavations achieved in India are those NATM structures excavated and constructed for the current Line 3 Metro project in Mumbai. In particular, the NATM construction of the 4

Sahar Road crossover cavern, the winner of the 2023 ITA Award for the category Beyond Engineering.

The original design called for a conventionally reinforced, cast-in-situ secondary lining, through a given number of geometrical sections. As an alternative, Mumbai Metro Rail Corporation opted to explore the use of sprayed waterproofing membranes and a permanent steel fibre reinforced sprayed concrete lining to cope more successfully with the multi-geometrical cross sections and development of a drained regulating layer system to allow application of sprayed waterproofing membranes in wet conditions. This is a first for any metro project in India, and perhaps an international first for the design in all its elements. The effort required the full collaborative commitment of the Client (MMRC), its Engineer (AECOM/ Maple JV), the main contractor (J Kumar), the specialist subconsultants (BEDI Consulting) and the material supplier (Bekaert) and sprayed concrete specialist (Normet). All had to work together to assure that the innovative technology could be delivered to international standards. This was especially the case given that no experience, codes, or standards existed for permanent sprayed concrete linings or sprayed waterproofing membranes in India.

So back to the incident in the Silkyara tunnel. There is no doubt that top-quality, international-standard NATM excavation can be undertaken and delivered in India, in all locations and given logistical situations and with success despite the difficulties encountered on the journey. In the India Case Studies Special Edition publication of TunnelTalk, produced for the Tunnelling Asia conference in Mumbai, there are articles about the development of tunnelling – both NATM and TBM excavation – in India through the years and on different projects. From the difficult short, soft ground tunnels around Goa on the Konkan railway project in the 1980s, to the difficult NATM

Design of the permanent sprayed concrete lining with its drained sprayed waterproofing system for the Mumbai Metro crossover cavern, in its primary lined state (left) and the final permanent fibre reinforced sprayed concrete lined structure (right)

and TBM works for the early Phases of the Delhi Metro and other metro systems in India, the articles recount how, despite setbacks and treacherously difficult geological conditions, the projects have all been completed successfully and are in operation to this day and will be for their design lifetime.

But the lessons of the Silkyara collapse and rescue of the tunnellers trapped ahead of the rockfall must be thoroughly investigated and new standards and processes developed and applied to ensure successful excavation of underground infrastructure and the safety of the workers who build them. It has been reported that each of the 41 workers are to be paid a compensation of 1 lakh rupees ($1,200 USA) following their 17 day ordeal which is a lot of money for them, but not a lot of money when so many workers on tunnelling projects around the country and elsewhere around the world are at risk in their workplace every day.

In his address to the bauma ConExpo exhibition and conference in Delhi in 2023, Shri Nitin Gadkari, Minister of Road Transport & Highways (reported on p46 of the TunnelTalk India Case Studies Special Edition) spoke of the tremendous amount of public infrastructure investment and construction to be achieved in India over the coming years and also of the challenges ahead: the need to convert to less carbon fuel powered construction equipment; to manufacture more construction equipment in India; of how construction is entry level for the unskilled into the economy “we have a duty to train them for safe construction work”; of the need to attract the growing pool of educated Indian professionals to the construction industry; and that “corruption must reduce”.

There was also discussion of more safety regulation, worker insurance and compensation schemes, and higher occupational health and welfare standards for

an underground construction work force that is, and will grow further, into the many thousands over the coming years. The article on p48 of the Special Edition, recalls the ITA World Tunnel Congress hosted by India in Agra in 2008 and the discussions in the Open Session about contractual practices worldwide with contributions from Brazil, Switzerland, France, Germany, Australia as well as from India itself. All sage advice which is still yet to be adopted universally. n

References TunnelTalk India Case Studies Special Edition

Direct by Design

SPECIAL EDITION FOCUS: INDIA CASE STUDIES

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