Preston Shafts, Preston England

The Preston Shafts Project, officially known as the Preston Unsatisfactory Intermittent Discharge (UID) Scheme, was a major infrastructure initiative aimed at modernizing the city’s aging sewer system and improving water quality in the River Ribble and surrounding areas.

The client for the project was United Utilities (UU), the water company responsible for drinking water and wastewater for the north west of England. WJ have a long heritage of providing dewatering services to significant infrastructure for UU and their predecessor, North West Water, such as the Davyhulme Wastewater Treatment Works in the 1980s and the Fylde Coast Sewers Scheme in the 1990s. Working for the contractor, KMI Water (a joint venture of Kier, J. Murphy & Sons, and Interserve) WJ were brought in after the excavation of the Terminal Pumping Station (TPS) at Penwortham encountered highly permeable sands and gravels, leading to shaft flooding during excavation. WJ’s solution was to install an array of dewatering wells into the complex underlying strata.

By targeting each strata, the well design optimised the flow and drawdown, reduced the risk of cross contamination of aquifers and ensured flexibility to control groundwater flows into the structures throughout the construction phase. Wells were operated by submersible pumps, and in the lower permeability strata a vacuum was applied to assist the drawdown. Eventually, over 110 pumping wells and monitoring wells were installed across the Penwortham site. For additional security and assurance for the construction of the base plug and slab in the TPS, a ring of inclined, drilled wellpoints were installed through the diaphragm wall close to its base.

The scheme was in operation by 2014 and has helped to significantly improve the quality of bathing waters along the Fylde Coast and Southport areas.