The original Bullring shopping centre, completed in May 1964, was demolished in 2000 to make way for the total redevelopment of the 40 acre site which includes 110 000 m2 of retail space for over 140 shops, cafes and restraints, including the iconic Selfridges building.
The large undercroft area provides over 3000 new car parking spaces as well as a service yard and plant rooms. This area, some 60m by 150m in plan and 6m deep, was excavated inside a secant piled coffer dam. During the initial works main contractor, Sir Robert McAlpine, encountered ground water within the mudstone and sandstone sequences at a level of 3m below ground level, much higher than anticipated.
The Result
Initially Dewatering Services Ltd carried out a pumping test using an array of piezometers and two pumping wells. This enabled us to calculate the coefficient of permeability for the mudstones and so design a dewatering system of 30 wells around the perimeter of the site, each drilled to a depth of 15m below ground level using rotary drilling rigs. This successfully lowered the ground water level to below the undercroft invert so allowing the excavation works to proceed in dry conditions.
