SHAFTS, CAVERNS | TECHNICAL
the tunnel-to-shaft connections that were among the most complex tunnelling works on the project. Joe took on the role of Engineering Manager about a year
before the lecture. Prior to that he was Principal Engineer and co-head of the Temporary Works and Methods team, specifically for the tunnel-to-shaft connections, having had that focus since 2021 in the concept phase. For any question tunnel-related question on excavation or preparing for the next construction sequence, “we go to Barry”, whom, Joe said, was “the main driving force” behind the tunnel construction works. Joe said David is the “go-to” person on design for the tunnels. David spoke next, on the challenges of the complex tunnel-to-shaft connections.
DESIGN Overview David explained some of the complex design work behind the tunnel-to-shaft connections. Offshore, and primarily for risk-mitigation reasons,
the shaft and tunnel connections are not in-line with each other but are positioned in an offline arrangement. The principal construction risk was inundation of the works and, through the tunnels, back to the main onshore construction site. The selection of the offline arrangement was the outcome of an iterative, collaborative process over several years. In plan view, the tunnel-to-shaft connection sits at
about 45° to the segmental tunnel, which is a geometry selected to balance constructability with limiting
hydraulic head losses once the tunnels convey water during operation of the power station, David said. At the top of each shaft structure is a casing, put in
place on the seabed and hammered down into the top of the bedrock. An offshore rig then excavated within the casing to form the hole and then install a prefabricated, thick-walled steel liner (effectively as steel vessel, sealed off with a dome at the base), almost reaching the bored tunnel elevation. Also, about halfway down the liner is a shear key - which David explained was “a critical part of the system” as it would take all the vertical loads on the structure once the bottom of the dome was cut, during works to link the tunnel to the shaft. An adit linked each segmental tunnel opening to the shaft. There is a large jamb frame/ opening set at the juncture, formed from high-strength steel and able to take the hoop stresses from cutting into the bored tunnel. An adit was constructed in a temporary case using
rock-bolted design with a 200mm-thick Sprayed Concrete Lining (SCL). The ground to be supported comprises horizontally bedded limestone and mudstone. Intact it has low permeability, but the bedding and jointing planes give the rock mass much higher permeability – which called for significant ground investigation (GI) and a grouting campaign from the segmental tunnels. The horizontal bedding also created concern that groundwater pressure could cause the SCL to fail. The solution was to use regularly spaced pressure-relief holes, the pipes of which were subsequently back-grouted.
Construction of the outfall gallery, showing rebar and casting shutter.
Pond shaft large elbow former for the secondary lining.
September 2026
| 17
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45
