Ashford Wastewater Treatment Works

Technically challenging groundwater control projects are the foundation of WJ’s ongoing success story. An example of a successful project in particularly difficult ground conditions was for Southern Water at their wastewater treatment works in Ashford, Kent.

Location

Ashford, Kent

Client

Black & Veatch (later BTU)

Year

2016

Features

Deep well systems, rivers and canals

A new sludge onsite dryer building was to be constructed on ground consisting of variable deposits comprising comparatively highly permeable River Terrace deposits, an overlay of alluvial deposits, and the silty Sandgate Formation. The excavation depth was to be taken to approximately 2 metres below the existing groundwater level. Due to these low permeable soils, it was initially the intention of the main contractor to undertake the groundworks with the aid of localised sump pumping. However, this method failed to bring groundwater levels down to within acceptable parameters. Also, the pumped groundwater contained a level of fine suspended solids that was too high to be discharged.

WJ’s groundwater control solutions

At this stage, the client assigned WJ to provide it’s practical and consultative expertise. A dewatering system with 110 drilled wellpoints was installed from a reduced level around the perimeter of the battered excavation using a rotary drilling rig with water flush, with wellpoints at 1.5-metre 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 eliminating the loss of finer materials through pumping.

Success was achieved by pumping steady-state abstraction flows of up to 6 litres per second (l/s) from the 110 wellpoints, resulting in vastly improved ground conditions which reduced uplift pressures acting on the base of the excavation, as well as improved slope stability.

The improvement in surface conditions, slope stability and discharge quality enabled the contractor to complete the structures ahead of programme and the added benefit of reducing the volume of discharge water that was required to be disposed of through the plant itself.

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