Water & Wastewater Treatment

Project focus: sewer networks excavation site being within the footprint of the two new-build houses at a depth of 1.8 m. To complete an excavated repair this would have meant removing the stairway of one house, supporting the separating wall between them and excavating under this wall to the pipe depth within the confines of the house. This would have been both a difficult and expensive option given that the residents would need to be rehoused for the duration of the works and extensive make-good works would need to be completed in the house a•er the pipe work was finished. The narrow alleyway and the location of the excavation also meant that access for excavation equipment would be virtually impossible. Stuart Soutar, Head of Wessex Water's Repair and Maintenance Team, comments: "Given the alternative, we had nothing to lose in trying to use the trenchless option here. The costs involved with the excavation option were prohibitive and the disruption that such works would have caused to the residents would have been immense. To some extent we had luck on our side as well, given the bridging of the defect by some of the materials above the pipe. Ultimately the trenchless option cost us less than £10,000 as disruption and cost in excess of £950,000. So it was decided that a rehabilitation solution for the existing rising main was preferable. 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 'dual 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 provided 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 17½ 0 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. 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. These are just two of the challenging projects that have been undertaken by the teams at Wessex Water over recent times. They show the diversity of expertise as well as the open-mindedness of the teams in finding the creative solutions needed by the range of defects and circumstances that the teams have to manage when the call comes in about what to the caller is a simple sewer problem. 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 mindset was the repair of a burst on an existing 100 mm diameter cast iron rising main from Fisherman's Walk SPS (Sewage 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. Installing a new rising main using directional drilling would have been high risk as the steep cliff face was prone to movement. The only alternative replacement option was to take the discharge to a different part of the sewerage network by installing a 950m long rising main within the Bournemouth promenade which would have caused widespread A view down the cliff face beneath which the Fisherman's Walk SPS rising main runs. Constructing the timber heading 22 | OCTOBER 2015 | WWT | www.wwtonline.co.uk Installing the 100 mm diameter lining

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