Special Works
Wessex Water – meeting rehabilitation challenges head-on
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 some 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 what appears to be an old, narrow and partly cobbled alleyway for want of better description 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 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 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 damage 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
The narrow access of Redcross Lane is clearly seen outside the building beneath which the damaged sewer runs.
LEFT: The sewer beneath Redcross Lane blocked with stones brick and other hard materials.
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. Commenting on the project Stuart Soutar said: “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 had luck on our side as well, given the bridging of the defect by some of the materials above the pipe. Ultimately the trenchless
The stairwell beneath which the Redcross Lane blockage was to be found.
22 drain TRADER | September 2015 |
www.draintraderltd.com
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