A case study of construction dewatering in Northern Province Chalk

This case study provides a rare insight into the hydrogeology of Northern Province Chalk Group. Installation of a new 6 km services pipeline under the Humber Estuary required the construction of a launch pit and a reception shaft for the TBM drive each side of the estuary. This paper covers the design and performance of the temporary works groundwater control arrangements used to facilitate excavation of the launch pit and associated ramp structure. The launch pit is located on the estuary south bank and was founded in the chalk aquifer in an area where groundwater is already fully committed for water supply. Preliminary studies suggested the likely presence of a superficial horizon of potentially highly permeable chalk ‘bearings’ (clast dominated frost shattered chalk) which are a feature of the Northern Province Chalk Group. A pumping test was undertaken which was designed to investigate the variation in chalk permeability with depth and the practicality for recharge. The groundwater control arrangements for the launch pit included a partial cut-off to control inflow from the chalk ‘bearings’, abstraction from an array of internal chalk wells and recharge of the pumped groundwater to the chalk aquifer via external recharge wells.