National Grid cable tunnels substation shaft – Hackney

Project Involvement

Specialist subcontractor for the design, installation and maintenance of a dewatering system to control groundwater levels in a lower aquifer (made up of Thanet sand and chalk) during construction of 12.5m diameter shaft with a depth of 27m, supported by diaphragm walls with a drawdown of 18m.

Introduction

Project Dewatering Ltd (PDL) was awarded the contract via Costain Skanska JV for the design, installation and ongoing dewatering system maintenance at the National Grid’s cable tunnels substation shaft in Hackney, East London.

The Works

This essential dewatering project required us to develop and drill six 40m deep chalk pumping wells with a finished diameter of 200mm. The wells were located around the external perimeter of the shaft. Wells were developed and fitted with 7.5kW borehole pumps each delivering up to 4 l/s as the ground conditions at this location had relatively moderate permeability. A monitoring well was installed to prove the drawdown providing the client with confidence to proceed with excavation.

A series of groundwater pumping tests were carried out on each well during commissioning to determine individual well yields, validate the design to allow for the optimisation of the dewatering system.

The final dewatering solution was operated and maintained for a period of six months to allow the successful construction of the substation shaft.