TECHNICAL | SHAFTS, CAVERNS


Top left and right: Adit tunnelling to reach the shaft location, which holds a liner inserted into the shaft drilled into the seabed.


Below: Access into the liner.


The design went back and forth between online


and offline connection of tunnel-to-shaft during early development, he said – “which was really interesting to be involved in.” He added that this was a first-of-its-kind connection and so the heads were being designed, the shafts were being designed, and the TBM drives were also developing. There were a lot of unknowns about what would come first, when and how and such. “The best way to get round this was to de-link these


altogether,” Barry said, and so the offshore campaign and TBM drives could progress independently. Safety was another major reason for going for an


offline tunnel-to-shaft arrangement, being 3.6km out under the bed of the Bristol Channel. Mining underwater is not anything new but breaking out to invite water in is something you try to avoid in tunnelling, he said. Steel secondary bulkheads were also installed in each


Intake Tunnel, at the shore end, to mitigate inundation risk, as was also done within the outfall galleries – which therefore covered such need for the Outfall Tunnel. These were secondary bulkheads to the primary ones, which were hydraulic flood doors in the internal ring propping (IRP) – a steel segmental ring built to support the main tunnel where miners would break-out and excavate an adit toward the shaft bottom, containing the liner.


Below: Pilot enlargement excavation upon offshore liner opening.


Below right: Excavation and removal of the lower section of offshore liner.


Offshore – shafts, liners, heads The offshore marine engineering and structures campaign was huge and of many parts, and would deserve its own presentation, Barry said. For the offshore installation works, the casings and


then steel sleeves were driven into the seabed, and left protruding above water for subsequent steps in the installation process. Far below the seabed, at the time,


miners were in the tunnels. “When the offshore team first started hammering on a night shift, the vibration reached the tunnel canteen. Plates and cutlery started rattling,” and stories spread of the thudding footfalls of approaching dinosaurs. After that, the offshore team made sure to tell the miners before they started hammering. Shaft drilling was performed from jack-up barges. “The


drill bit used for the offshore shaft works was itself very large. As it drilled, it also reamed out a section to form the shear key,” he said. The jack-ups also inserted a steel liner (with the domed


bottom) into each shaft, along with an Isolation Cap. The execution was performed by New Wave Solutions. At Avonmouth, the liners were stood vertically in a row.


There was some precasting for the Permanent Works, including the reinforced concrete Isolation Cap supports, pre-formed as it would not be possible in-situ, offshore. Locally, Avonmouth was also where the four huge Intake


Head reinforced concrete structures and the two Outfall Head structures were constructed. Each of the four Intake Heads was 4650 tonnes,


including 775 tonnes of rebar plus 125,000 individual reinforcing bars, and 1600m³ of concrete. The two Outfall Heads were smaller but still huge, at 3500 tonnes with 250 tonnes of rebar plus 52,000 individual reinforcing bars, and 1100m³ of concrete.


Tunnels: connections, Temporary Works Balfour Beatty’s digital team produced a sequencing model of the construction steps to create the tunnel-to-shaft connections. The sequences included: the initial GI and grouting works; depressurisation; the grouting campaign;


Above: Liner front opening sequence. 20 | September 2026


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