SHAFTS, CAVERNS | TECHNICAL


Onshore works Onshore, there were multiple tunnel structures for the cooling system, including: ● Outfall galleries (5m-ID discharge tunnels, transferring spent water to the main offshore outfall tunnel);


● Three launch adits, each about 60m-long (two for the Intake Tunnels, one for the combined discharge tunnel, or Outfall Tunnel);


● A liaison gallery (a shared tunnel allowing water to be transferred between the plant’s two generating units during maintenance);


● Pond shafts (to bring water towards the outfall); ● A triple-point structure (a large connection linking the outfall galleries and the main offshore discharge tunnel itself); and,


● The diversification galleries. “The outfall shaft was the first structure we built,”


he said. The 17.5m-diameter shaft was 40m-deep. Below the shaft collar and guard ring, the top 12m was segmentally underpinned, the main body was constructed with SCL (sunk in 1m advances), and the lowest 10m was flared to accommodate the three soft eyes for the galleries and launch itself. “Something different” for the outfall shaft was a


drained solution on the plug, achieved by installing a French drain around the perimeter with a series of passive wells. It allowed the shaft plug thickness to be reduced “by quite a lot,” Barry said. The liaison gallery was a fairly simple straight tunnel,


he said, at 144m-long and constructed with 200mm-thick SCL and friction rock bolts. The secondary lining turned the single excavated tunnel into two separate galleries using a 1m-thick central spine wall. Casts were in 12m-long bays, creating two separate tunnels that would deliver water if and when needed. The diversification galleries were among the smallest tunnels on the project but their secondary lining was complicated. Each gallery had two dogleg bends, a full waterproofing membrane system, complicated reinforcement, a Faraday earthing cages and inclined throughout. “Almost everything that could make a small tunnel complicated was present,” Barry said.


The two outfall galleries, he said, snake around the


deep excavations in the forebay of the Balfour Beatty area, which is why they are on radius. They were excavated to 6m-diameter with ground support by SCL and systematic rock bolting. The secondary linings were cast using a hydraulic Kern shutter (radial steel panels on travelling falsework): invert first, then dropped back to cast the crowns. The shutter was reconfigured for the bends: 6m-long bays on the straight, reducing to 4m for the wider radius (50m) bends, and down to 2m for the tight bends. “A good bit of kit,” he said. The outfall galleries had steel bulkheads installed as


additional mitigation against inundation risk into the main construction site onshore. The 20m-deep pond shafts are smaller: two shafts,


although each about 10m-diameter primary lined in SCL, then cast in reinforced concrete. The secondary lining had to accommodate a particularly large transition opening – a 5m-diameter transition from shaft to tunnel via a 90° elbow by a large former wrapped in reinforcement. Complex formers were looked at, including Cordek, GRP and timber solutions that previously have been used as complex formwork in some hydro schemes in Europe, with craftmanship almost like boatbuilding, he said. “I contacted the British boatbuilding industry, shared the shape, and then spent the next two years being contacted by people wanting to build me boats.” The eventual solution came through special formwork,


in the Midlands: steel laser-cut pans when bolted together formed the transition elbow. It was built in the supplier’s facility and “the accuracy was spot on”. It was too big to get the cap on “and we had great fun flying drones through it”.


Tunnel drives and connecting offshore The project used three Herrenknecht earth pressure balance (EPB) machines to drive the tunnels offshore, with the tunnel crowns about 20m below seabed: ● Intake Tunnel-1 is 3.5km-long and 6m ID, and was driven by TBM ‘Mary’;


● Intake Tunnel-1 is 3.5km-long and 6m ID, driven by TBM ‘Beatrice’; and,


● Outfall Tunnel-1 is 1.8km-long and 7m ID, driven by TBM ‘Emmeline’.


Schematic of a tunnel-to-shaft connection to be constructed under the seabed


Internal ring propping (IRP) was installed in main tunnels to enable adit breakout for excavation to the shaft location.


September 2026 | 19


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