TECHNICAL | SHAFTS, CAVERNS
TBM preparation for launch to drive offshore.
Below and bottom: Seabed shafts were excavated by an offshore drill with capacity to overream the liner structural shear key zone; and, liner installation.
Once the full cavern connection
was constructed in SCL and with rock bolts, a permanent lining was installed. The Permanent Works include the adit, the 90° elbow structure and the vertical shaft above it. The design accounted for differing pressures as the shaft passes through different geological units, from the upper sediments down into more competent rock.
Design and software David said modelling for the connections was complicated: “We needed that critical reassurance that during construction we wouldn’t have any failures of temporary works.” So, they ended up using multiple different software packages, including PLAXIS 3D, UnWedge, RS2, 3DEC, and SAP2000. He added that each package had its own benefits. PLAXIS 3D was useful for modelling displacements of
the vertical liners and the interactions of the shear keys. It was less useful for determining rock-bolt requirements. “We also had to understand how temporary loads would affect the Permanent Works. That was another key modelling requirement.” 3DEC was used to develop a robust yet constructable
rock-bolting arrangement. RS2 and PLAXIS 3D had suggested a need for far too many rock-bolts to be constructable, he said, so the design was optimised using different software packages. The jamb frame was modelled in SAP2000. “It was
also important to interact between the different types of software packages.” SAP2000 was used for modelling the Permanent Works of the reinforced concrete structures. All permanent structures had a 100-year design life and
were nuclear-classified with seismic loading requirements. David handed over to Barry to discuss construction, both onshore and offshore.
CONSTRUCTION Barry began with an overview of the project and works:
18 | September 2026
● Onshore excavation and primary lining (including TBM launch adits, outfall tunnels and shafts, water transfer tunnels and maintenance tunnels);
● TBM drives – 3No (build, transit, lift, launch, drive, bury & strip out);
● Onshore Permanent Works – secondary lining, varied profiles, complex formers;
● Offshore works – offsite fabrication, dredging, head placement, shaft drilling and placement;
● Offshore connections – grouting, enabling work, break out, excavation, liner location, liner drain down, back mining, jamb frame
● Offshore secondary lining – for passages, shafts and elbows
● Triple point structure – connecting the outfall structures, filling the shaft.
Both the GI and probing were performed by BAM
Richies. Waterproofing works were by Renesco and materials were provided by Sika. For SCL, materials provision onshore was by Hansen (on-site batching) and offshore by Cemex (dry silos). Workforce resources were supported by TG Tunnelling.
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