Development of a ground model, targeted ground investigation and risk mitigation for the excavation of an open face cross passage on the underground Elizabeth Line, London

Abstract The new London Crossrail running tunnels for the future London Rail Elizabeth Line were driven by Tunnel Boring Machine (TBM) and linked at regular intervals by cross passages, constructed with open face techniques. This paper describes the development of the ground model and depressurisation required for a cross passage link between the running tunnels in East London, through the analysis of the existing data and a subsequent targeted ground investigation. The cross passage was 14 m long and included a central sump with a formation level at 48 m below ground surface. Cross passages encompass two risks: Programme risk as they run along the critical path of the project and construction risk, since they are excavated with traditional open face techniques. For cross passage CP6, a pre-construction borehole indicated that the ground conditions at the cross passage horizon comprised predominantly cohesive Lambeth Group soils. The well-known variability of the Lambeth Group, often with sudden lateral and vertical change in facies, make the prediction of ground conditions extremely challenging and critical for safe open face tunnelling. The ground model evolved from the design to the construction stage, using the information provided by: the project ground investigation; TBM drive data and sampling; followed by a targeted probe drilling investigation conducted from within the running tunnels. During the latter investigation, more extensive water bearing sands were identified, than previously identified by the surface borehole, which required a revision to the ground model and subsequently a change in approach to the cross passage excavation. The new ground model allowed the Contractor to manage the ground risk appropriately, which included a change in approach to groundwater control in the form of in-tunnel well-points depressurisation and surface ejector wells.