Innovative approach saves time and money on sewer rehab

A team from Lanes and NMCNomenca took a ‘risk versus cost’ approach to bring a Severn Trent Water (STW) sewer rehabilitation scheme in six weeks ahead of schedule. At the same time, they reduced disruption for the water company’s residential and business customers, and optimised the client’s budget.

Advanced techniques, such as pipe re-rounding, patch linings, and concrete repairs to a box culvert were employed, alongside mini dig-downs to minimise inconvenience on a residential estate and on Tata Steel’s Scunthorpe site. By adopting this innovative strategy full-scale open trench repairs were avoided, yet the project still met all client’s targets. It also allowed works to be completed in double-quick time.

In total, 4100m of STW’s Grade 4 and 5 sewers were in danger of collapse. No-dig methods had been specified wherever possible to reduce traffic and customer disruption, though any sewers too severely damaged — such as one running under rear gardens on a residential estate — had been earmarked for replacement via excavation, which would mean dismantling and reinstating gardens, sheds, garages and paved areas. However the Lanes/NMCNomenca team came up with a new approach and proposed using patch liners which would exert enough pressure to push the pipe outwards, ‘re-rounding’ it sufficiently to allow a full-length liner through.

Risks were reviewed and accepted by the client, and the plan was implemented with great success. The patches re-rounded the pipe so that a full liner was installed. Deformation was kept to levels undetectable on post-CCTV survey, and 76-metres of 375mm diameter surface water sewer were rehabilitated in three days, without a single building, shed or patio being touched!

Further, the risk money set aside for compensation and reinstatement was then used to resolve other previously unidentified problems within the catchment, so providing still more benefit to STW and its customers.

At the front of the same houses, a 300mm diameter sewer on the kerb line had been identified for open cut repair of 44-metres, and dig-down repairs on the remaining 56-metres. This would require a five-week road closure. No-dig methods, of course, would have been preferred, but the risk was deemed too great: a failed liner could potentially block the sewer further.

Again, the team came up with an alternative strategy: isolated dig-down repairs at the worst affected locations, with patch repairs used, as before, to ‘re-round’ less damaged areas, before installing a full liner in the usual way. And, once again, STW’s customers were saved prolonged and unnecessary disruption.

During the same project, exposed re-bar in a concrete box culvert at TATA Steel’s site had been identified for hydro-blasting to remove loose concrete, before applying resin cement. Each working area would be sandbagged off from the culvert’s main flows to capture concrete debris, which would be pumped into a settling chamber to avoid polluting a downstream watercourse.

Setting up and taking down temporary works for this sub-task would take longer than carrying out the actual work, so Lanes proposed a technique it had used elsewhere: applying resin concrete directly to the affected area, without having to remove loose material, so that there was no need to isolate the working area.

Prior to signing off this solution, NMCNomenca assessed whether the concrete spalling had affected the integrity of the culvert and concluded that the level of damage was minimal as the culvert sections had been cast in-situ and were >600mm thick. However the repairs would improve the structural integrity of the asset, reducing the risk of further deterioration and extending its life by 50+ years (in line with SRMv4).

Steve Wood, Project Engineer for NMC Nomenca, said that Severn Trent Water had been delighted with the team’s performance: “Not only did we come in way ahead of completion date, STW’s customers did not register a single complaint during all the works.

“This project is an example of positive partnership in action: Lanes provided technical capabilities and understanding of no-dig technology, and NMCNomenca ensured that all client’s targets were met.”