| Tunnelling
Right, top: QE2 solar aerial shot (during construction)
Right, bottom: QEII Inlet aerial after draindown (Barhale compound to rear left)
Reinforcing ribs were installed inside the tunnels
to ensure their structural integrity was maintained during the shaft construction. Once the crowns of the tunnels were reached, the existing wedgeblocks were carefully removed to facilitate the lining works. Each shaft progressed to around 2m below the invert of the tunnels, with a final depth of 44m. The temporary shaft required for the inlet tunnel was located at the base of the QEII Reservoir’s embankment. This access shaft was between the reservoir embankment and Walton Road, which runs parallel to the northern side of the reservoir and is a well-used route between residential areas. To gain access safely and at the correct location over the tunnel, it was necessary for the shaft to encroach on the road. Initially it was essential that the road was closed, but as soon as it was safe to do so, one side of the road was reopened to minimise disruption to the travelling public as much as possible.
Collaboration To protect its workforce while relining the tunnels,
Barhale worked collaboratively with Thames Water to complete and implement a robust and thorough isolation procedure. This included making sure that every valve and outfall into the tunnels was double isolated – with primary and secondary physical and electronic isolations. In total there were over 30 isolation points, with the primary physical isolation being at the location of the valve or outfall, and the secondary electronic isolation occurring at the remote control room. Additionally, there were a number of large valves within the reservoir that needed to be isolated. Specialist divers worked with the Barhale team to install blanking plates. These were 3.5m in diameter and 2.3 tonnes each, so required careful fitting and placement to guarantee full isolation.
Once shaft construction at the inlet tunnel was complete, the team moved straight onto the outlet tunnel temporary shaft which was within the grounds of the reservoir. At the location of the outlet shaft, an existing disused sampling building was demolished to enable works to progress. While relining works were underway on the inlet tunnel, the outlet tunnel was being cleaned to enable its relining to progress. This involved the removal of silt and zebra mussels which had settled along the f
Below from left to right: Steel in preparation for relining
Relining underway The relined tunnel
www.waterpowermagazine.com | January 2022 | 15
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