Project: Wigan Civic Centre Sheet Piled Cofferdam at Grand Arcade, Wigan
Customer: Wigan Civic Centre
Main Contractor: Shepherd Construction
In early 2005, were beginning work on a £52 million project developing a new shopping complex in the centre of Wigan. The geography of the site was complicated, cutting into the side of a steep hill, dotted with existing buildings, some of which were recently demolished to make way for the new multi storey shopping mall and car park. Along side the existing 4 storey Wigan Civic Centre, ran an existing retaining wall which needed replacing with a reinforced concrete retaining wall which would form part of the basement of the new shops. A 5.0m deep (maximum) excavation was required within close proximity of the building.
The ground conditions varied across the site from granular made ground in some areas to clays in others. Traditional cantilever sheet piling was not an option as the level of rock head was generally at formation level and there would be little chance of achieving a toe in. Similarly, raking supports as part of a propped cantilever pile design were not feasible due to drivability issues and furthermore, the provision of thrust blocks at lower levels to props off would have created more temporary works problems for Shepherds.
MGF moved away from the idea of supporting only one face of the excavation and instead opted for a trench style excavation with no toe-in. The new retaining wall did not follow a straight line and was stepped in some places, so rectangular bays were designed at suitable offsets to accommodate the correct layout.
The design specified MGF KKD 600/6 Trench Sheets with no toe-in and two levels of MGF Heavy Duty Tank Brace in each bay. In the bays with long spans, MGF Mechanical (540kN) Superstruts were installed to reduce the bending moment and deflection on the frames.
Download the full case study PDF here: Shepherd Construction Grand Arcade.pdf