Hook WwTW (Ejector Wells) Case Study

Case Study

Hook WwTW (Ejector Wells)

Client: J N Bentley
Location: Hook, Pembrokeshire
Duration: February 2017 to March 2018

Project Overview

To enable construction of new within 33m x 25m area. The structure was constructed on a slope, which required steep open cut with rock anchoring on the northern face grading down to a more open-cut batter to the south.

Dewatering was required for active depressurisation of the nailed slopes to provide improved slope stability. Control of groundwater up-flow was achieved by an array of passive pressure relief wells within the excavation, bleeding into a drainage blanket, with any ingress removed by sump pump arrangement.

Ground conditions consisted of South Wales Lower & Middle Coal Measures Formation. With groundwater apparent in the upper weathered zone f-m Sandstone beds.

• 34no x 12.5m deep ejector well dewatering system (200mm Ø bore/125mm ID well liner)
• 5no x 12m deep passive relief wells
• 3no x 12.5m monitoring wells
• Provision of sump pumping/boost pump system

Individual twin-pipe ejectors were connected to a supply and return main positioned around the perimeter of the excavation. In turn this is connected to a pumping station consisting of a duty and standby 415V high head pumps (22 kW star/delta soft start) with pumping/inspection tank. Abstracted water was discharged using 100mm Ø polybauer discharge line to exiting site outfall.