WET News

10 WET NEWS SEPTEMBER 2015 ProjEcT SPEcS • Unblock and repair a sewer pipe • Install pipe lining • Rehabilitate and reinforce an existing mains • Investigate trenchless options for a pipe near collapse • No matter the scale of operations, Wessex Water's preferred method for dealing with sewer problems is through trenchless technology where possible. But this process is not always straightforward Getting out of a hole Installing the St Paul's Road liner which added structural strength to the new pipeline ONSITE TRENchlESS TEchNology • An ultra-high-pressure water jet cutter was used to remove blockage material • To complete an excavated repair would have meant removing the stairway of one house • A 'dual skin', steam cured, glass-fibre reinforced, 3.5mm thick liner was reinforced by the introduction of a 2mm thick non-reinforced liner • The underpinning design comprised 37 concrete-filled steel piles tied into a number of cast in-situ concrete beams • "given the alternative, we had nothing to lose in trying to use the trenchless option here. The costs involved with excavation option were prohibitive and the disruption that such works would have caused to the residents would have been immense" Stuart Soutar accurately surveyed and the liner designed with a taper to fit exactly when it was installed. Furthermore, the liner had to be installed with access only from one end of the sewer as the exit point of the sewer was not accessible to the contractor. Had this renovation option not been available the only other alternative was difficult with the excavation site being within the footprint of the two new-build houses at a depth of 1.8m. 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 re-housed 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. Soutar says: "Given the alternative, we had nothing to lose in trying to use the trenchless option here. The costs involved with excavation option were prohibitive and the disruption that such works would have caused to the residents would have been immense. To some extent we O ver the past few years the name Wessex Water has become somewhat synonymous with the use of trenchless technology in the UK water industry, particularly in its dealings with the wastewater sector. Looking at some of the varying works that have been undertaken over recent months it is easy to see how this ethos of looking to use trenchless techniques has permeated the wastewater planning and construction hierarchy of the company. It was not so many years ago that if a report of a blockage or sewer failure came into the office that the first question was 'how big will the dig be?'. Now, at least within Wessex Water's sewer teams, this approach has changed significantly with the first thought being 'is there a suitable technical solution to this problem using trenchless technology?'. This thought process now runs through the company psyche, from larger scale operations to smaller diameter networks. Redcross Lane, in central Bristol, is an old, narrow and partly cobbled alleyway lined by new-build terraced houses. When a sewer problem was reported to Wessex Water the response team investigating found that it was no ordinary blockage. Having tried to jet the blockage clear with no result, investigation with CCTV found that the 300mm diameter vitrified clay sewer pipe, which was probably something in the region of 100-years old, was indeed blocked but not with the usual, easily jettable materials but with fill, stones and brickwork materials from a collapse at the crown of the pipe. This is where Stuart Soutar and his R&M team sprang into action. Working with Onsite Central, Wessex Water's framework contractor, a plan was developed to utilise an ultra-high-pressure (10,000psi) water jet cutter to remove the blockage material alongside traditional jetting equipment to remove the spoil created. This process, whilst removing the initial blockage, also enabled the CCTV operator to see the subsequent condition of the defective pipe. Over the crown of the pipe there appeared to be a further collapse of material including some form of lintel or kerbstone across the pipe section that was preventing further collapse of material into the pipeline. This was somewhat to the advantage of the contractor as it enabled a full lining to be installed through the sewer subsequent to final removal of the original blockage. However, the lining itself was not an easy option. The location and circum- stances of the old sewer were such that just before the damaged pipe section, the diameter of the pipe changed from 300mm to 225mm which meant that the pipe had to be NEEd To kNoW 1 The liner had to be designed with a taper so that it would fit exactly when installed 2 Utilising more traditional techniques would have required a diversion route 3 Pipe-bursting was ruled out due to the forces required and the potential for foundation movement 4 Use of a timber heading to install the new pipe would not have been safe

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