Ashford Wastewater Treatment Works

Technically challenging groundwater control projects are WJ’s forte, and one of the most successful of these projects were the wastewater treatment works in Ashford, Kent.

A new sludge dryer building was to be constructed in variable deposits comprising comparatively high permeability River Terrace deposits (where permeabilities range between 2.5×10^-3m/s to 3.6×10^-5m/s) overlying  significantly lower permeability Alluvial deposits and the silty sand of the Sandgate Formation (where permeabilities ranged between 1.2×10^-7m/s to 4×10^-8m/s), with the excavation to be taken to approximately 2.0m below the existing groundwater level. Due to these lower permeability soils, it was initially the intention of the main contractor to undertake the groundworks with the aid of localised sump pumping.

This sump pumping proved to be unsuccessful with the pumps unable to adequately draw the groundwater levels down, and with pumped groundwater containing to high a level of fine suspended solids to be discharged.

WJ were fortunately on hand to provide our expertise and experience. A drilled wellpoint dewatering system was installed from a reduced level around the perimeter of the battered excavation using a rotary drilling rig with water flush, with wells at 1.5m intervals and powered from centrally located piston wellpoint pumps. Wells were installed with a high-quality sand filter to ensure that a vacuum was maximised and the system could run efficiently to achieve the required drawdown as well as minimising the loss of finer materials through pumping.

Pumping achieved steady state abstraction flows of up to 6l/s from the 110no wellpoints that were installed and resulted in dramatically improved ground conditions, reduced uplift pressures acting on the base of the excavation and improved slope stability over the 5 month programme of works. This meant that the Ashford Wastewater Treatment project was a success, and one of many interesting case studies of dewatering in low permeability soils.

• LOCATION: Ashford, Kent
• CLIENT: Black & Veatch (later BTU)
• MAIN CONTRACTOR: Southern Water