Top: Steel reinforcement in place Centre: Relining works underway Bottom: Relining completed
In longer tunnels, where suction is not possible
(because of velocity loss through the length of the suction pipework), the suction pipework is ‘broken back’ to a distance where the suction velocity is still good. In this instance, waste is transported manually (for example by scrapers, barrows or trollies – although efforts are always made to mechanise the operation) from the location of the engineering teams back to the head of the suction pipes. Mussels were also a minor problem in the outlet
tunnel at the QEII Reservoir in South West London, where we were called in to carry out an £11m tunnel relining contract to strengthen the aging 1.06km-long inlet and 0.9km-long outlet tunnels to prevent leakage and to reinforce the integrity of the reservoir. The inlet and outlet tunnels run under the
embankments of three critical reservoirs, busy roads and property so the integrity of these tunnels is of the highest importance to Thames Water. Submersible ROV CCTV and Lidar surveys were used
to survey 72” internal pipework at the base of the reservoir that formed part of the outlet system. These digital surveys helped Thames Water to understand the hydraulic modelling of the reservoir pipework. Temporary shafts had to be constructed over the
existing tunnels for access. By creating 3D CAD models the temporary works team was able to break down the phases of work and specific requirements, helping to deliver a more efficient and safe solution. The models also provided visualisation of the works for the operations team and allowed a simpler interface between the temporary and permanent works to be designed such as the support required for breaking into the two tunnels. To protect our workforce relining the tunnels, we
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. The molluscs and debris were cleared before both
of the tunnels were relined with poured concrete, with steel fixing done ahead of the concrete work. Shutters were installed insitu by the team, with concrete poured and cast in place. Operational challenges are not confined to the
condition of tunnels and their residents. Sometimes internal valves can form an obstacle in the tunnel. Gate valves are not so much of an issue as, when open, they leave nothing within the tunnel bore. Butterfly valves on the other hand, cannot be passed safely without having
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