Pregrouting combined with double shield TBMs

Direct by Design

Pregrouting combined with double shield TBMs Of the 36km of 9.9m o.d. (9.1m i.d.) segmentally lined double-shield rock TBM excavation required on the twin tube Follo Line railway between Oslo and Ski in Norway, nearly 8,000m or 22% required systematic pre-excavation grouting to lower the pore pressure and ground water levels in the host geology and protect against the surface settlement that alteration of the groundwater table can induce. This detailed review of the pregrouting operations developed, optimised and applied by the Acciona Ghella JV during the project is extracted from a paper presented by Acciona at the 2020 WTC World Tunnel Congress, Malaysia.

T

he Follo Line construction contract established strict criteria to avoid settlement and damage to environment and any existing structures and building along the alignment of the 20km long twin tube TBM bores on the new railway connection

between Oslo and Ski. To meet the criteria, the contract specified two main methods for protecting the natural ground water regime and to avoid water ingress during excavation: • A watertight precast concrete, bolted and gasketed segmental lining and

Acciona reporting from a paper presented at WTC 2020, Malaysia • Systematic and specified pre-excavation grouting as a mandatory requirement at specific locations and in others, a requirement for the contractor to determine the extent of pre-excavation grouting required to maintain the ground water levels.

The contract established and defined grouting requirements, and also provided information on grouting concepts, with the contractor required to further develop these concepts and the chosen methods of construction to maintain the existing ground water levels. Assessing the potential for water ingress into the headings was via probe drilling by drill rigs installed on the first backup gantry close to the shields and drilling 44m long probe holes. The contract pre-established the number of probe holes that were required in areas previously classified as highly sensitive.

Pre-grouting was triggered by ingress of ground water through the probe holes.

Initially, Bane Nor, the client, estimated pre-grouting quantities in the TBM headings as about 1,600m in total or some 4.4% of total 36km of bored tunnel length when excavation of the parallel tubes by four double shield TBMs from a central access adit towards the portals began their work in 2016. When both tubes and the last two TBMs completed their drives in February 2019, the total length of pre-excavation grouting executed was in fact 8,000m or some 22.22% of the total bored length. Preexcavation grouting was a major part of the excavation process and required team work with the client to optimise the process, meet the water control criteria and maintain the project programme.

The contract described the drilling equipment to be installed on the TBMs to comply with the probing and pregrouting requirements. Based on this, the following equipment was installed on each machine: • Two Atlas Copco COP3038 drill rigs with 185º rotation capacity • A TC 42 short twisted splins shank adapter • 12ft x 39mm round TC 42 extemsopm rods • A C12ft x 46mm round TC 42 RND 46 speed guiding rod

Above: A two-hole probe drilling concept with the litres of water per minute from each triggering the need for pre-excavation water control grouting Below: Drilling equipment on the first gantry of the TBM backup allowed for full 360˚ grout injection arrays

T U N N E LT E C H

Underground in Action U

Rigs 1, 2 and 3 of the mobile pregrouting equipment

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Permanent installation of grouting equipment on the back of the TBM gantries • 64mm ballistics retracting button bit drill bits and • 38 drill ports and positions in the TBM shields With this configuration the TBMs had the capacity to comply with the contract requirements for achieving a valid pregrouting round of 20 drill holes in each 44m long array.

The injection equipment was mobile and was parked at the end of the TBM backup to be mobilized forward and close to the TBM shield area when pregrouting was required.

The injection system was comprised of three main sets of equipment:

T U N N E LT E C H

Rig 1: Four pumping units with a capacity

of 60 litres/min at 2 bar to 100 bar

Two agitator units.

Rig 2: Two units of mixer and pumping

transfer equipment

Additive pumps, two units per mixer

A 9m3 storage silo with automatic

cleaning filter systems

Two screw conveyor mixer

discharge units Rig 3: Two 16 tonne silos on the mobile

grouting platform

A compressed air unloading

system, one per silo The pre-excavation grout mix comprised microsilica, superplastifier, water and CF 20 cement. Two formulas were designed with different water:cement ratios of between

0.7 and 1, depending on the pressure of the water ingress measured at the drill holes, and a cement content of between 250kg and 300kg with water in the 240kg order plus 25kg of microsilica and 3.5kg of superplasticiser.

Learning curve and optimization

The probe drilling and pregrouting concept developed during progress of the four TBMs. Also, through the experience and lessons learned from day-to-day activities, the contractor and client agreed on a new assessment of pregrouting trigger values. The new evaluation parameters were based on: • The number of probe holes executed; • New consolidated sensitivity criteria for specified pregrouting areas; and • Water leakage measured over a 10 min interval.

Once the criteria for pregrouting was stablished, and after 33 rounds of pregrouting, it was clear that the pregrouting cycle had to be optimised to improve the time required for each cycle. After analyzing a lot of parameters, main areas for improvement were detected: • Pregrouting on TBM headings had not done before on this scale of magnitude and the crew TBM crew was inexperienced in the process.

• The drilling equipment required a lot

Table 1. Revised trigger levels for pregrouting No of probe holes with Trigger value leakage >10l/min Small - moderate-sensitive Very sensitive 1 2 3 4

15l/min 25l/min (total) 33l/min (total) 40l/min (total)

10l/min 20l/min (total) 20l/min (total) 25l/min (total)

Markedly improved pregrounting cycle times following the equipment modifications

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of maintenance due to hard work and demanding hours.

• The drilling and injection works were not overlapped, so there was no efficiency in the cycles rotations.

• The mobilization time for each pregrouting operation was too high.

After working sessions between the different TBM operating teams including those involved in the safety, production, quality, plant and equipment and logistics aspects, a new concept and construction method statement for pregrouting was developed and agreed. It was based on: • More trained and experienced drilling activities workers were incorporated to manage the pregrouting works.

• Modification of the maintenance and production shifts with an optimized crew for both activities.

• Electrical and hydraulic modification of the pregrouting system, including the refrigeration concept.

• Overlapping in the injection hole drilling and grout injection activities.

• Installing fixed equipment on the TBM backups instead of basing all on the mobile rigs.

With the new construction methods applied, optimisation of the cycle achieved a reduction in the cycle times in some cases by half. The reduction in the pregrouting cycles allowed the TBMs to improve their productivity. The main pregrouting productivity improvements obtained included: • Average drilling increased from 7.81m/ day to 12.13m/day; • The full grouting cycle decreased from 2.96 days to 1.66 days; • Drilling time reduced from 1.27 days to 0.75 days; and • Demobilization time after each grouting cycle improved from 0.11 days to 0.03 days.

Through a collaborative work model between Bane NOR (the client) and the ACCIONA – Ghella JV Team, this innovative methodology was executed in a very productive and safe manner. A total of 7,639m of pregrouted 9.9m o.d.

TBM excavated tunnel was completed, impacting the TBM excavation process in a much reduced manner than ever before and allowing the pregrouting cycles to be fully integrated into the TBM excavation cycles. The innovations are part of a complex underground project and have minimized the environmental hazards and protected the urban landscape for sustainable urban development. n

PREGR O UT I N G FR OM T B M

I NN OVATIV E S OLUTI O N TO M I NI M I ZE E NV I R O NM E N TA L HAZ A R DS I N T U NNE L I NG P R OJ E CT S