Seaforth Passage Widening
Catagory: Groundwater Control
Location: Liverpool, UK
Client: Farrans Construction
Sector: Transport, Water
FARRANS Construction Ltd was awarded the contract by Peel Ports to widen the Seaforth Passage from 40m to 60m to increase efficiency for the transit of vessels within the Royal Seaforth Dock, Port of Liverpool.
As part of the project, a 3m diameter siphon, connecting the Liverpool storm drainage system to an existing outfall was to be extended beneath the passage.
BCI Awards 2016 – HIGHLY COMMENDED
Ground Engineering Awards 2017 – HIGHLY COMMENDED
OGI Involvement
The structural design required construction of a combi-wall cofferdam tied into the existing diaphragm wall (with a proposed excavation level approximately 18m below the adjacent dock water level), whilst ensuring that the original Seaforth Passage remained active throughout.
An original dewatering design specified an external deep-well system , however, site investigation revealed this approach was neither practical nor economical due to higher than expected Sandstone permeability, perched groundwater conditions, excessive costs and spatial requirements.
As such, OGI were appointed to review the original dewatering strategy and develop an alternative solution.
OGI conducted substantial investigation and finite element analysis from which a design specification was produced that comprised a combination of deep active wells and vertical and inclined passive dewatering and pressure relief wells:
OGI Dewatering Strategy
Numerical modelling supported an alternative design, comprising a system of passive pressure relief wells, combined with a small number of active pumping wells.
Nested piezometers were installed to monitor performance and groundwater pressures were compared during the operation with mathematical predictions.
This design addressed risk of heave, and mitigated risk of structural collapse in the event of a sudden rise in groundwater pressure (such as due to power or pump failure).
In addition, replacing pumping wells with passive wells resulted in considerable cost and space savings compared to an active system.
Project Success
Replacing pumping wells with passive PRWs resulted in considerable cost savings and equipment took up far less space than would otherwise have been the case, reducing site congestion and disruption, and the project was completed on time.
The successful temporary groundwater control system designed for the Seaforth Passage Widening project was only made possible by the team approach which included full collaboration between OGI, the Main Works Contractor (Farrans Construction) and the Structural and Geotechnical Designer (Royal Haskoning).
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