TECHNICAL | BTS MEETING/NORTH BRISTOL RELIEF SEWER
Right, figure 6: TBM drive site Autumn 2020 – Full Production
grout port, into the surrounding rock mass. These
bleeders were then connected to a pipeline that ran the length of the tunnel and discharged clean water into the water treatment system (WTS). These bleeders would act like well points and passively reduce the water pressure in the surrounding rock mass. This meant that during the excavation and ring-building cycle of the TBM, less ground water would reach the excavation chamber, which remained at atmospheric pressure, and therefore the consistency of the spoil would be improved. Figure 11 shows the depressurisation system developed for the project. Further to the measures being used at the face,
solutions were also implemented in the tunnel, such as using multiple trains per advance to account for the loss of volume in the spoil skips being taken up by water instead of spoil. By not allowing the skips to overflow with slurry it would improve the working conditions on the TBM and also help to minimise any effects to the logistics system. In addition, there was a continuous cleaning regime required to clean the inverts in the tunnel of any slurry that had built up. A specialist vacuum pump and a small conveyor belt were used to minimise manual handling. The last section of the water management strategy
was to apply measures on the surface to handle and process the slurry spoil that was being produced
Original
from the TBM. The first part of these measures involved mixing drying agents (cement, polymers, and bentonites) into the slurry spoil that would absorb the free water and make the spoil suitable for disposal from the site. This additional stage proved very effective and enabled the project to continue removing spoil from the site. However, it took time and space to carry out efficiently. To improve this process an additional spoil handling bay was constructed on site to allow this stage of the spoil management plan not to limit the production rate of the TBM. In addition to the spoil management on the surface,
the WTS was frequently overwhelmed by the volume of slurry it was required to process and this would mean the TBM had to stop production until the WTS had been cleaned. The WTS was not sized or designed to be processing the actual volume of slurry being produced. To assist with this problem a small slurry treatment system was established to process the slurry originally entering the WTS. This allowed for the discharge standards to be maintained, meaning that the TBM production rate would not be limited by the WTS capacity.
Challenge 3 The third significant challenge faced by NBRS was that of the global, COVID-19 pandemic. The pandemic struck
Concept As-built
Right, figure 7: TBM cutterhead modification
Opening ratio 29.5%
Opening ratio 25.5%
22 | November 2023
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