Deep Well Dewatering - Paull TBM Reception Shaft

Dewatering of chalk for the construction of a tunnel boring machine reception shaft beneath the River Humber

Project Overview

The National Grid needed to establish a new gas pipeline beneath the River Humber by constructing a 5-kilometre-long segmental tunnel to house the steel high-pressure gas pipe. Stuart Wells established two separate aquifer dewatering systems to dewater the superficial deposits and to depressurise the chalk aquifer and mitigate risk of potential for uplift during excavation. 4no x 22m deep wells were installed internally together with 5no x 42m external chalk wells at ~12m centres around the external perimeter of the shaft.

Objective

Stuart Wells was contracted to design a system to dewater the Paull shaft to allow a base slab tobe installed. This would ensure the stability and dryness of the shaft to receive the tunnel boring machine.

Solution

Pumping tests on-site suggested a poor connection between the Chalk and the overlying superficial deposits requiring two separate dewatering systems. Remote monitoring of groundwater levels and abstracted flow recorded data and acted as an alarm system in the event of pump failure.

Scope of Works

It was initially thought that by dewatering the lower chalk aquifer it would underdrain and have a beneficial effect on groundwater in the overlaying deposits. However, pumping tests undertaken on site suggested poor connection between the chalk and the overlying superficial deposits. Therefore, separate aquifer dewatering systems were established to both dewater the superficial deposits (Glacial Till) using 4no x 22m deep wells were drilled at ~12m centres around the internal perimeter of the cofferdam; and to depressurise the Chalk aquifer and mitigate risk of potential for uplift when excavating through the superficial deposits by pumping from 5no x 42m external deep wells.

Remote monitoring of groundwater levels and abstracted flow was set up to both record data and act as an alarm system in the event of pump failure.