Farringdon Station SCL Design – Reducing risk at the heart of Crossrail

Summary

The ground conditions expected at Farringdon Station are amongst the most challenging on Crossrail, exacerbated further by four TBMs all terminating in reception chambers required to facilitate their extraction. The Sprayed Concrete Lined (SCL) tunnels are expected to be within the Lambeth Group, but crucially subject to intermittent, water bearing sand channels that could unexpectedly occur anywhere in the vicinity of the tunnels. This uncertainty, verified through various ground investigations, has dominated the design with five years worth of detailed design carried out to ensure the construction risk is ALARP. Moreover, Farringdon is situated within a highly faulted area and these risks have also been managed in the design. Extensive work has demonstrated how to safely construct the platform tunnels at Farringdon station from within which other tunnelling works (such as cross passages and ventilation adits) can subsequently be managed. The SCL design process was closely scrutinised through the use of CDM registers, comparative risk registers, peer review challenge teams and client expert panels; adding to the robustness of the proposed design. C121 worked closely with the client (CRL) to reduce the risks associated with excavating tunnels in this highly variable, challenging ground using ALARP principles. Particular focus was given to the geotechnical challenges underpinning the design as well as the potential different tunnelling options for the station works. Farringdon station is unique and the work carried out by C121 (Contract 121 Sprayed Concrete Lining design for all Crossrail structures) shows the specific approach taken and how C121 deviated from the standard design adopted for the majority of SCL tunnels on Crossrail to produce a tailored solution for a very challenging station.