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  Geotechnical Risk Management for Sprayed Concrete Lining Tunnels in Farringdon Crossrail Station

  The proposed Crossrail station will comprise two ticket halls, two escalator/concourse tunnels, two platform tunnels, numerous cross passages and ventilation tunnels. All station tunnels are constructed using sprayed concrete lining (SCL) techniques. The geological conditions were the principal challenge for the design and construction of the Farringdon station tunnels, excavated mainly within the Lambeth Group, a heterogeneous formation comprising stiff to very stiff, overconsolidated clays with interbedded sand lenses of unknown orientation, size and water pressure regime, in an area crossed by four major geological faults. A comprehensive geotechnical risk management approach, adopted from the design through to the construction phase, making use of additional surface investigation, in-tunnel probing and a “live” 3D ground model, providing a robust framework for effective on-site decision making is presented in this paper. Initial studies assessed the probability of encountering water bearing units during tunnelling. This enabled a strategy for the implementation of toolbox items to be developed and refined for the construction phase. The report demonstrates how dynamic modelling of the geological features encountered has reduced project risk by continuously increasing the level of knowledge and understanding of the complex ground conditions at this station.

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