TECHNICAL | SHAFTS, CAVERNS
Left: Spraying SCL to profile below the offshore shaft.
Right: Back mining.
and erection of the IRPs – which had the bulkhead doors against inundation. The segmental tunnels also had deluge systems, which were basically a series of large pumps and 6” lines running throughout, as flood mitigation. In each of the three main tunnels, near the ends the construction logistics were served by having a platform deck. It was about 200m-long and gave a wide and long working floor as it ran between the two adit turn-offs, where the IRPs were located, plus a little longer. Break-out would be through an IRP. The first excavation would move 5m out from the main tunnel and immediately go into a bend. The tunnelling activities would then include pilot headings, bends, probing ahead, rock bolting and mining towards the liner. Of course, executing the tunnelling work is always far more challenging, even when going well – as it did.
Connections: probing and grouting On the coast, on Lilstock beach, the geological formation comprises Blue Lies (old and deeper) and Charmouth mudstone (more recent and shallower). The bedding planes have about a 10° dip. Essentially, both formations are made of alternating claystone, mudstone, limestone plus shale in one area. Of the two formations, the Blue Lies is the stronger (higher limestone content). The rock is weathered. The same geology is encountered offshore but in a less weathered state, Barry said. The offshore connections are almost entirely excavated in the Charmouth Mudstone, which contains two different Members – Calcareous (stronger) and Shaley (weaker). Before excavation, the GI (probing) campaign from the segmental tunnels informed whether grouting was needed in connection areas. There were also central cores all the way to the liners to give rock strength and characteristics. The grouting campaign was extensive for three of the six adit drives, he said. BAM Richies did the GI and grouting.
Mining for the liner A service platform was put in place along each segmental tunnel about the distance between IRPs, through which an adit would be mined to reach the adjacent shaft with its liner, located in the seabed. The platform, therefore, served both adits. It held refuge chambers, plant, dry-mix shotcrete to be available when needed, and provided dust extraction. “We also installed a turntable on the platform,” Barry
said. He added: “I had not seen a turntable on a tunnel platform before, and the miners enjoyed filming themselves going round and round on it slowly before turning plant” – safely, of course. Following tunnel break-out through the IRPs, to drive the adits, the geometry was challenging with various awkward shapes. There was understandable nervousness around approaching the liner, he said, so they probed ahead, going in small steps (1.2m advances). The rock-bolt design made systematic use of Swellex bolts, which utilises high-pressure water to take the shape of the hole and “knits joints together”. It is better than grouted or resin bars, he commented. Probing was always done. There was a surprise one
time. A bar had hit something far down inside the hole – far too early, and far too deep for the liner. But it sounded metallic, in surrounding quietness. Like striking a bell. Everything stopped, only for it to turn out to be a very tight limestone band. With concern around the liner, the important point is that the team stopped, he said, and made the call for checks. “The ground was blocky,” he said, and added it was
quite a different experience to tunnelling on Crossrail where London Clay could be mucked with a profile arm. Here, it needed hammering. Every day, the geology was face-mapped, profiles checked, silo capacities confirmed, toolbox items on the working deck reviewed and monitoring data assessed. Shape arrays, convergence monitoring and other monitoring systems which fed into the Permit-to-Work process. The liner when found “looked like a giant spaceship,” he
Jamb frame installation 22 | September 2026
said. It had a side-entry panel and a person-entry panel. Pressure-relief holes were drilled below the liner against risk of uplift from the seabed once water was removed, for it had to be drained down. The team then installed
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