Location: Paull (Hull)
Duration: June 2018 – February 2020
As part of the National Grid Gas Transmission – Feeder 9 Replacement Project a 22m Ø x 13.5m deep secant pile shaft was constructed to allow reception of the TMB machine once it had tunnelled under the River Humber. External dewatering was required to depressurise the sub-artesian head in the Chalk aquifer together with internal dewatering of the superficial deposits to enable shaft construction. In addition to external dewatering to enable TBM entry into the shaft in dry and stable conditions.
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.