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Project Focus: Trenchless sewer rehab at Wessex Water

Trenchless techniques are being used by Wessex Water and its contractors for both large scale operations and small diameter networks

Constructing the timber heading in the Bournemouth Rising Main projectConstructing the timber heading in the Bournemouth Rising Main project

by Ian Clarke

Over 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?’.

What follows are two interesting examples of how this thought process now runs through the company psyche, from larger scale operations to smaller diameter networks.

Redcross Lane, Bristol

Redcross Lane, just off Old Market Street 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 300 mm diameter vitrified clay sewer pipe, which was around 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.

Working with Onsite Central Ltd, Wessex Water’s Framework Contractor, a plan was developed to utilise an ultra-high-pressure (10,000 psi) 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 circumstances of the old sewer were such that just before the damaged pipe section the diameter of the pipe changed from 300 mm to 225 mm, which meant that the pipe had to be 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 1.8 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 after 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 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 52m 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 4m.

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 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 45 degree bend and a 17½ degree bend had to be removed from within the cliff. To enable this work a 1.2m square and 2m 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.

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