- Project Wigan Civic Centre sheet piled cofferdam
- Main Contractor Shepherd Construction
- Key Benefit Bespoke design used for a complex excavation
In early 2005, work was beginning 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. Alongside 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.
The excavation support was installed by Galt Civil Engineering. The trench sheets were installed by the push and dig method. First the upper frames were laid out and used as a guide to pitch the sheets as far into the ground as possible to provide some rotational stability.
The first 1.0m of earth was now excavated and the top frame was lowered into place and secured.
The sheets were then pushed in further and more earth was excavated out, ensuring that some toe-in remained to provide intermediate stability.
When the required level was reached, the second frame was installed and the sheets pushed down to formation level.
Excavation now continued to formation level, despite the lack of toe-in, as the lower frame now provided rotational stability.
A concrete base was cast and once it had achieved adequate strength, the bottom frame was removed to allow the construction of the permanent works to commence.
The scheme was quick and easy to install in unpredictable ground conditions. Working room was kept to a maximum throughout the excavation, without the need for installing thrust blocks to prop off.
Water ingress was contained more efficiently with this type of excavation than it would have been with a single sided excavation.