BTS MEETING/NORTH BRISTOL RELIEF SEWER | TECHNICAL


lecture, and related paper in T&T, where Jake presented how the ML works and the outputs it provided to the site team. It is important to note, however, that whilst


these techniques helped to improve production rate forecasting, it was found that they weren’t perfect. Differences were still encountered between these models and the existing ground conditions that required the production team to react during tunnelling. This also highlights the need to continue developing technology and solutions for improving forecasted tunnel production rates. The Maxwell model and ML were not the only


solutions to the first challenge – the TBM was still required to bore through the larger than anticipated zones of conglomerate, sandstone and siltstone. To optimise advance rates and minimise cutter tool wear, the lacing pattern of the tools on the cutterhead was changed multiple times to suit the varying geology. Generally, tools changes occurred at 700m intervals in the mudstone, 70m in the conglomerate and 15m in the sandstone (40MPa -70MPa).


Challenge 2 The second significant challenge was the higher than anticipated ground water flows experienced between 1,500m and 3,000m into the tunnel drive. In this area, the TBM struggled to advance using EPB pressure to balance ground water with the use of ground conditioning chemicals, and, as such the spoil quality deteriorated into a slurry, creating tough working conditions. Prior to this zone, spoil conditioning was managed as


had been planned, using a range of ground conditioning products. The slurry was also causing problems on the surface


as the spoil was not suitable for disposing off-site via road lorries without additional measures to make the spoil more cohesive. Due to this, a complex water management strategy was required to continue


excavation and increase the production rates of the TBM. The strategy can be broken down into measures required at the face, in the tunnel, and on the surface, respectively. The measures at the face consisted of three main


options, the first to create a grout block behind the TBM shields. To do this, it was required to stop the TBM advance and back grout the last 20m-30m of tunnel to fill any fissures in the area that the primary grouting had failed to seal. The grout would then be left to cure for approximately 5hrs and then production could resume. Whilst this method initially proved effective at controlling the water flows and improved the quality of the spoil, the benefits were short-lived. When the TBM resumed excavation, it would intersect new fissures and the spoil condition would begin to deteriorate again. The benefits from the grout block would typically last 10m-15m, which was approximately a day’s worth of production. This method was eventually discounted as it did not provide a large enough benefit to counteract the loss in production required to create it. The second option developed at the face was to


alter the primary grout in strength, consistency, and its properties to prevent wash out and maintain refusal pressure. Alternative admixtures, such as limestone fillers and polymers (anti-wash-out additives), were developed into a suitable set of mix designs that could be used with the TBM’s grout system. These alternative mix designs proved effective at maintaining a refusal pressure on the primary grouting. However, to achieve this, at times the grout volume required would be three times higher than the theoretical amount. During these trials, there was little relationship seen between the different grout mixes and the quality of the spoil the TBM was producing. The third option used at the face was a


depressurisation system installed as close to the excavation face as possible. A series of bleeders were drilled through the tunnel lining, using the existing


Above, figure 5: Tunnel face during anticipated ground conditions November 2023 | 21


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