PDL Delivering Excellent Wellpoint Dewatering on Recent Projects

Fordingbridge phase 2

Introduction

Dewatering was required for four launch and receptor pits near Fordingbridge. Each pit had a perimeter of 16 metres and extended to a maximum depth of 6 metres. Groundwater was encountered at 2 metres below ground level (mbgl) within running sands.

The Works

PDL proposed a wellpoint dewatering system. Wellpoints were the preferred method, as vacuum-assisted pumping promoted flow to the wellpoints within the very silty sands. The main limitation of a vacuum pump is its ability to lift water to a maximum of 6 metres, meaning a reduced dig around the pits was required to allow the wellpoints to reach sufficient depth. The wellpoints were spaced at 1-metre centres and extended 6 metres from the reduced level.

To maximise groundwater capture during directional drilling, PDL advised the installation of several wellpoints along the drilling line. The project has since concluded, with the dewatering system delivering excellent results and enabling the client to complete their works for this section.

​Woods Meadow

Introduction

PDL was awarded the dewatering works to facilitate the excavation of an attenuation basin. The basin had an approximate perimeter of 206 metres and extended to a depth of 2.7 metres at its deepest point. The borehole logs provided noted the strata sequence as sandy topsoil over slightly gravelly, clayey sand (Corton Woods Sand and Gravel Member). Trial pits conducted by the client near the basin recorded silty, sandy, loamy clay over stiff clay at a depth of 4.5 metres. The groundwater level was assumed to be at 1.2 mbgl.

The Works

PDL proposed the use of a wellpoint dewatering system for this project. Wellpoints were installed around the perimeter of the basin at approximately 1.5-metre spacing. In addition, a central line of sacrificial wellpoints was also installed to assist in achieving and maintaining the required drawdown, to be removed or destroyed as part of ongoing groundworks.

All wellpoints were installed to a depth of up to 5 metres. The system was successfully commissioned and achieved effective drawdown. PDL is in regular communication with the client to ensure the dewatering system remains operational throughout the course of the groundworks.