Innovative geotechnical risk management for SCL tunnels

Following the construction of the Crossrail Project, Farringdon will become one of Britain's busiest train stations connecting three major networks – Thameslink line, Crossrail and London Underground. The station tunnels are constructed using sprayed concrete lining techniques. The geotechnical conditions were the principal challenge, governing both design and construction processes. The tunnels were excavated mainly within the Lambeth Group, a formation comprising various units, mainly stiff to very stiff overconsolidated clays with interbedded sand lenses of unknown size, hydraulic properties and continuity. Moreover, four major geological faults cross the footprint of the station, effectively changing the position of the risk-imposing sand lenses in relation to the tunnelling works. A robust and innovative geotechnical risk management approach was adopted from the design to the construction phase, utilising additional surface investigation, in-tunnel investigation and a ‘live' three-dimensional ground model. This provided a framework for effective on-site decision making and a cycle of risk reduction related mainly to the presence of multiple faults and potentially high water pressures within sand layers at the tunnel face. This approach is demonstrated in various stages of the project. This ‘best practice' model could be applied to other projects with challenging geotechnical conditions.