Depressurisation for the excavation of Stepney Green cavern

Abstract

At over 50 m long, 17 m wide and 15 m high the Stepney Green Cavern is one of the largest caverns ever built using sprayed concrete lining techniques in central London. The east- and westbound caverns lie approximately 20 m below the existing ground level in a constrained urban site and are located within the London Clay and underlying Lambeth Group, which is a variable series of clays, silts and sands; the higher permeability sands contain high pore pressures that are hazardous to sprayed concrete lining construction without depressurisation. Successful implementation of a practical depressurisation scheme was therefore crucial to safe construction. The overall strategy adopted for investigating these water-bearing sands within the Lambeth Group is outlined and the ground investigations and pumping tests required to determine typical material permeabilities and the response of the sands to pumping are described. The paper also reviews how that information was developed into the outline depressurisation scheme and how this was subsequently developed into the final contractor-designed depressurisation scheme that was implemented. Changes to the scheme during construction owing to ground conditions are discussed, together with the response of the ground during pumping.