Ejector Dewatering - Islington Pumping Station

Temporary dewatering for the construction of a pumping station as part of a flood defence system

Project Overview

Islington Pumping Station is part of a critical infrastructure project for flood defence located on the banks of the River Great Ouse near Kings Lynn, Norfolk.

Objective

The dewatering objective was to reduce the porewater pressures and groundwater levels temporarily below final excavation levels to allow the construction of the Islington Pump Station.

Solution

An array of ejector wells was installed to accommodate the variable low permeable ground conditions and the close proximity of the River Ouse. A conceptual dewatering design was developed and refined on a site observational basis.

Scope of Works

The dewatering objective was to reduce the groundwater level temporarily below the formation level of the new pumping station. Ground conditions consisted sandy silt and fine sand deposits with a permeability range of 7.5x10-7m/s to 2.1x10-6 m/s.

Ejector wells (22 no.) installed at approximately 5 m centres and up to a depth of 15 m. Each deep ejector well would comprise of a 250mm Ø bore/125mm ID well liner located along the external excavation perimeter.

Individual twin-pipe ejectors were connected to a supply and return main positioned around the perimeter of the excavation.

In turn this is connected to a pumping station consisting of a duty and standby 415V high head pumps (22 kW star/delta soft start) with pumping/inspection tank. Abstracted water was discharged using 100mm Ø polybauer discharge line to outfall.