SHAFTS, CAVERNS | TECHNICAL


dewatering equipment. The first thing to do was to pressure test the Isolation Cap to make sure the double seal on top was not breached, then air was introduced and water removed steadily, with gauges monitoring pressure and volumes against predicted volumes. A salinity test was also used “as we got into it” to confirm no breach. The offshore installation accuracy was impressive,


Barry said. “All the liners were within about 50mm, and when we removed the bottom saw the offshore GI core was bang in the middle of the shaft. The surveyors had done a fantastic job.” But how to remove a large steel liner from under the


seabed, at a tunnel connection? Burning gear could not be used in that location, he said. Plasma cutting was impractical and unsuitable from an occupational health perspective, he added. The two practical options were track saw cutting or high-pressure water jetting (no sparks, hot works, fumes or close proximity working). To choose, scale mock-ups were built and the chosen method was high-pressure cutting, firing garnet at high pressure, able to cut through steel. As part of the cutting sequence, probe- drilling of the annulus grout was performed to confirm competence. It was particularly high strength (more than 100 MPa). Pressure-relief holes were then drilled at the back of the liner to enable the front parts to be removed. Shape arrays and precise levelling monitored any displacement. Also, elastic-sensor monitoring was developed during the progress of works to remove liners and provided for real- time monitoring of very small movements. The liner was removed in sequential cuts – top cut, then


open up the front, mine around the can, muck behind the plates, enlarge the cavern, pull the plates, trim and SCL spray the profile – and slowly they made their way around. At that stage, the liner end bearing was removed and the liner was solely dependent on the shear keys, and leaving the liner dome at the bottom. Looking up, the Isolation Cap was visible – “keeping us dry”. Above it was the sea. With the shaft gone, there was the large dome to remove


the bottom. It sat like a chalice on top of the invert, he said. They mined the benches and cut it “into pizza slices” for removed it in sequence, then carried on to the invert and completed the profile. Having started with a pilot profile, there was back-mining


to do to enlarge the opening for the jamb frame to be installed to the extrados of the segmental tunnel. Being at a dead end, that was done in two halves with a dividing


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wall, giving a safe route for mucking and access. The wall position in the arrangement was relocated as progress on the enlargement proceeded, first undertaking works on the left side and then on the right. The full back-mined opening was nearly 12m across. The opening set was wide and the jamb frame so large,


due to the angle where the adit meets the tunnel. It was transported in large pieces. “We backfilled the invert and placed a temporary slab so that the frame pieces could be brought in a handling frame,” he said. The 11 tonne, 9m-long lintel was installed in its permanent


position, the sill only moved into its final position (from a temporary position) after the backfill was removed. The banana beams, or jambs, were jacked up and installed. The lifting points used 4m-long resin CT bolts to provide a lot of capacity to raise 11 tonnes. Once installed, the frame was cast in concrete before the Permanent Works were done and as it transferred load the IRP could be removed from the main tunnel. The Permanent Works then proceeded through the invert reinforcement, crown reinforcement and elbow section – which was cast using a similar pan-design to the initial elbow, onshore. The final connection included a splayed wall (cast, mass filled) that followed the same 45° profile as the adit. Buried just beyond was the TBM, having been left in place, stripped and then back-filled. Back onshore, the three-way outfall shaft – or ‘triple


point structure’ – had even bigger formwork for its combined transition work. He said: “These were like benching manholes on steroids - enormous structures built on the surface, dropped in and cast in concrete.” With that, Barry finished his description of the construction works and the presentation.


Below left: Barry


Gilsenan inside the cast triple point structure prior to cap installation.


Below: ‘Triple point’ formwork about to be lowered into the onshore shaft leading to the Outfall Tunnel.


Above left: Offshore shaft lining, showing rebar installation for the elbow at a tunnel-to- shaft connection.


Above right: End


connection completion with splayed head wall.


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