Crown Estate, Thetford, Norfolk

A ground investigation was carried out by RSA Geotechnics for The Crown Estate in relation to installation of two 1200mm diameter pipes for foul and surface water sewers, beneath a railway cutting at a site in Thetford, Norfolk.

The proposed pipe installation method involved construction of pits on opposite sides of the railway cutting and tunnelling using a tunnel boring machine (TBM), for both pipes from the base of one ‘thrust’ pit to the ‘reception’ pit on the other side.

The site was in the north-western outskirts of Thetford, in a rural location, adjacent to the A11 bypass and a railway cutting.

To the north and south of the railway cutting there were open level areas covered in long grass and occasional trees. Network Rail boundary fences were present along the crests of the railway cutting side-slopes on each side and a road bridge was present over the cutting just to the north of the site which carried the A11. The railway cutting was approximately 4m deep with side-slopes at an angle of approximately 1(vertical):2(horizontal) and included two non-electrified tracks.

The British Geological Survey 1:50,000 scale geology map for the area (Thetford, Sheet 174, Bedrock & Superficial Deposits, 2010) showed that the solid geology at the site comprised the Lewes Nodular Chalk Formation and no superficial material was shown above the Chalk. Information from a previous ground investigation carried out by others, however, showed that a superficial layer of granular material was present above ‘structureless remoulded’ chalk at the location of the proposed pipes. Natural and man-made mining cavities in the chalk were also known to be present in the vicinity, although there were no records or obvious evidence of such features at the site of the proposed sewer pipes.

RSA Geotechnics drilled two percussive sampling boreholes to a depth of 11m, one on each side of the railway cutting.  The invert level of the proposed sewer pipes was at around 8m below ground level. The drilling was carried out using a Comacchio Geo 205 tracked rotary soil sampling rig.  In order to monitor ground gas emissions and groundwater levels, 50mm diameter wells were installed in the boreholes. Soakage tests were also undertaken in the boreholes to provide an indication of infiltration rate.

The boreholes disclosed a superficial covering of glacial granular material above the chalk which was present to the full depth of the boreholes. Based on the groundwater monitoring and also desk study information, the groundwater level was anticipated to be at some depth below the base level of the pits.

RSA Geotechnics provided advice in relation to construction of the ‘thrust’ and ‘reception’ pits, tunnel boring and pipe jacking, determination of settlement resulting from the tunnelling and the soakage characteristics of the chalk.