Crossrail Sprayed Concrete Lining Depressurisation at Stepney Green Caverns
Crossrail includes approximately 10km of Sprayed Concrete Lining (SCL) tunnels. One of the largest of these tunnels is being constructed at Stepney Green, East London to facilitate the construction of a sub-surface junction, allowing Crossrail trains from Maidenhead and the west to travel to Shenfield and Woolwich in the east. These caverns are the largest span underground excavations constructed below London to-date.
The east- and west-bound caverns lie approximately 20 m below existing ground level in a constrained urban site and are located within the London Clay and underlying Lambeth Group. The Lambeth Group is a variable series of clays, silts and sands and the higher permeability sands contain high pore pressures which are hazardous to SCL construction without depressurisation. The successful implementation of a practical depressurisation scheme was therefore crucial to safe construction and compliance with the zero harm Crossrail ethos.
This paper outlines the overall strategy adopted for investigating these water-bearing sands within the Lambeth Group and then focuses on the ground investigations and pumping tests which were required to determine typical material permeabilities and the response of the sands to pumping.
The paper reviews how that information was developed into the outline depressurisation scheme and how that was subsequently developed into the final contractor-designed depressurisation scheme that was implemented. Changes to the scheme during construction due to ground conditions are discussed together with the response of the ground during pumping. The safety critical nature of the scheme required close monitoring and implementation of a system of triggers and actions.