Ashbourne is a picturesque market town in the Derbyshire Dales with a population of just over 7,000. The Ashbourne Flood Alleviation Scheme has been designed to reduce 16 flooding problems within the central area of Ashbourne, Derbyshire. Hydraulic analysis of the sewer network revealed that the primary cause of the sewer flooding within the centre of Ashbourne, affecting 14 out of the 16 flooding locations, was the lack of spill capacity from the existing combined sewer overflow (CSO), situated near the town centre, and the hydraulic capacity of sewers passing flow to it.

To increase the hydraulic capacity in the sewer system, the existing CSO was relocated to a position downstream of its current location to an area of public open space and a short length of upstream sewer was upsized to enable unrestricted flows to reach the new CSO location. A new outfall sewer was constructed from this CSO to enable flows to be discharged much further downstream than the existing CSO outfall position, where additional hydraulic outfall capacity in the Henmore Brook had been gained.

The scheme solution involved the upsizing of several lengths of both the surface' and combined sewer system together with the rationalisation of three existing CSOs into one new overflow, and the provision of a new surface water outfall structure.

The construction of the 675 metre, 900 mm diameter outfall sewer formed a major element of the project solution in that it provided an unrestricted surface water outlet from the Town Centre. This sewer incorporated the 32 metre long no-dig crossing of the Station Road – King Edward Street junction.

Also included as part of the project were sewer rehabilitation works on the combined sewer upstream of the existing CSO location and the surface water sewer in Dig Street, Ashbourne.

Pipe jacking prevents disruption

The pipe jack was constructed beneath the Station Street – King Edward Street traffic siganalised junction. This road junction is the primary access points to one of Ashbourne’s two main car parks, which is also the car park to the town’s Sainsbury’s Supermarket store. The Project designers felt that although the new sewer could be constructed across this road junction using an open cut construction method, doing so would cause considerable disruption to Ashbourne’s traffic system and its car parking, as well as unacceptable disruption to the supermarket.

The main project contractor was Whitehouse Construction. Kiamm Ltd was the specialist subcontractor for the pipe jacking work. The pipe jack constructed in Ashbourne was a 1,200 mm hand driven open face pipe jack.

The pipe jack was constructed across the road junction between thrust and reception pits situated on either side of the junction over a distance of 32 metres. The depth to the sewer at the thrust and reception pits was just over 4 metres, with a cover to the pipejack of approximately 2.8 metres over its whole length. The ground conditions primarily comprised slightly sandy, gravelly clay. There were no problems experienced during construction and the pipe jack was driven successfully over a period of six hour working shifts.

No-Dig on Dig Street

The lining process used in Ashbourne was OnSite’s Premier-Pipe lining system.

Premier-Pipe is a CIPP renovation process, which is installed without the need for costly and disruptive excavation. Premier-Pipe liners are individually manufactured to suit the dimensions of the pipes to be lined; the lining thickness being determined by individual design requirements. The lining tube is made of polyester felt with an outer coating of polyurethane, which is impregnated thoroughly with a liquid resin chosen to suit the working environment of the pipe.

Two main sewer runs were relined as part of the Ashbourne Flood Alleviation Project. The first sewer being a 640 mm x 430 mm brick egg combined sewer of a total length of 135 metres and the second sewer being a 600 mm x 660 mm rectangular brick surface water sewer of 125 metes in length. Both sewer lengths were situated on Dig Street in Ashbourne, which is one of the towns main commercial streets. Once impregnated with resin, the Premier-Pipe was installed using the scaffold tower inversion method from existing manholes situated on Dig Street.

The relining was carried out on both of the sewer lengths to resolve structural problems discovered during investigation of the town's flooding problems and also to improve the hydraulic perforation of the sewer lengths. Due to the location of the proposed work, the relining was carried out under a ten day road closure. This time period was, however, vastly reduced than the time period that would have been required for conventional open cut replacement.

Severn Trent supplies water and sewerage services to more than 3.7 million households and business customers in a region stretching from Mid Wales to Rutland and from Bristol Channel to the Humber. The water authority is responsible for 54,000 km of sewers and 1,000 sewage treatment works.