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A view down the cliff face beneath which the Fisherman’s Walk SPS rising main runs.
BELOW: Constructing the timber heading at the Fisherman’s Walk SPS.
Pumping Station) in Bournemouth during February 2015.
The 52 m long rising main, laid in 1977, runs within a steep cliff face with the SPS located at the bottom between the cliff face and the promenade, adjacent to a toilet block and a number of beach huts. The 27 m high cliff face is prone to movement and land slips, which is believed to have been the likely cause of the burst which lay some 6 m into the cliff face from the wet well end of the rising main and at a depth of 4 m.
Initial thoughts were that a new rising main could be
installed using directional drilling but knowing that the steep cliff face was prone to movement could have made such an operation high risk and potentially dangerous as the drilling may have caused a significant land slip. Such a solution would also have involved significant temporary works, further adding to the costs.
The only other replacement option was to take the discharge to a different part of the sewerage network by installing a 950 m long rising main within the Bournemouth promenade which would have caused widespread disruption and cost in excess of £950,000.
So it was decided that a rehabilitation solution for the existing rising main was preferable as it would not affect the surrounding soils thereby minimising potential for cliff instability.
Wessex Water approached Onsite with a view establishing how this could be achieved. Working closely with the Wessex Water design team, Onsite was able to bring together a liner design that would be installed from the wet well at the base of the cliff that would not only repair the burst in question but also provide a reinforcement of the rising main into the future. The in-house design comprised a ‘duel skin’, steam cured, glass-fibre reinforced, 3.5 mm thick liner, which was further reinforced by the introduction of a 2 mm thick non-reinforced liner which allowed for easier installation and provide additional stiffness once cured. This combination liner not only enhanced the overall stability of the liner but also provided high resilience to potential pressures fluctuations which might be experienced within a rising main of up to 3.5 bar.
Due to major corrosion of the rising main at the wet well and to enable the liner to be installed, both a 45o bend and a 17½o bend had to be removed from within the cliff. To enable this work a 1.2 m square and 2 m long timber heading was driven into the cliff face from the side of the brick wet well chamber. This also allowed an access platform to be erected in the bottom of the wet well.
option cost us less than £10,000 as compared to something in the region of £100,000 for the excavation option and all its additional customer care costs.”
BOURNEMOUTH RISING MAIN
Another recent problem solved using this mind-set was the repair of a burst on an existing 100 mm diameter cast iron rising main from Fisherman’s Walk SPS (Sewage
ST. PAUL’S ROAD, SALISBURY
Following routine CCTV survey, a 600 mm diameter concrete surface water sewer pipe, running beneath a Victorian
Lining was achieved from within the wet well. Construction of the timber heading started in the week of 16 March and the liner was installed during the week of 23 March. The liner, being a composite twin-wall liner, was not inverted into the host pipe but winched through. It was then inflated and steam cured. On completion, the timber heading was backfilled with concrete and voids surrounding the timber heading grouted. The opening of the brick wet well chamber was rebuilt with glazed brickwork by 14 April with the project cost being around £100,000 in total about 10.5% of the alternative option.
Installing the 100 mm diameter lining at Fisherman’s Walk SPS
Installation complete at Fisherman’s Walk SPS
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September 2015 | drain TRADER 23
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