Tunnelling | A royal success
Barhale has completed refurbishment of inlet and outlet tunnels at the Queen Elizabeth II reservoir in England.
Right: Oulet temporary access shaft site
THE QUEEN ELIZABETH II (QEII) Reservoir is a raw water reservoir in South West London that is owned and operated by Thames Water. The reservoir holds over 19 million litres of water, equating to 10% of the raw water storage for London. Once treated, this provides clean drinking water for millions of people across Surrey and London. Barhale was engaged by Thames Water to complete the £11 million tunnel relining contract to guarantee the integrity of the inlet and outlet pipes. The goal of the project was to increase resilience by strengthening the pipes and preventing any leakage from what is a crucial clean water asset. The project involved relining 2x2.5m internal
Below: Site at Walton Treatment Works (inlet tunnel)
diameter tunnels. Before works could begin, the reservoir needed to be drained down and the pipes fully isolated for safety. Ultimately, the water level
was reduced by 11m, leaving 6m of water in place to sustain the wildlife in and around the water.
Relined The first tunnel to be relined was the 1060m long inlet
tunnel, which draws water into the QEII Reservoir from the River Thames, via Walton Raw Water Abstraction Pumping Station. The second tunnel was the 826m long outlet pipe, which takes water from the QEII to Hampton Water Treatment Works for treatment before being fed into trunk distribution mains and the Thames Water Ring Main network. Both were constructed in the early 1960s using an unbolted wedgeblock technique.
Both tunnels required two access points. For the inlet tunnel, this was at Walton Treatment Works, where a surge tower pipe was removed for access and refurbishment. Additional works here included adding a 600mm butterfly valve to futureproof the surge tower. The secondary access point was a temporary 6m internal diameter access shaft constructed at the North West boundary of the reservoir. The access points for the outlet tunnel were at an
existing, permanent shaft at Boormans Field site (in a nearby residential area) and at a new 6m diameter temporary access shaft within the boundaries of the reservoir near its north east entrance. The temporary access shafts were constructed
through a combination of caisson and underpin techniques. Both shafts were sunk to a depth of around 18m, using a 40t excavator with a telescopic pole grab and clamshell bucket. The team then swapped to under-pinning, using a 3t excavator at the base of the shaft to muck out to formation level that encompassed the existing tunnels.
14 | January 2022 |
www.waterpowermagazine.com
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