Negotiation of poor ground in an undersea tunnel

Abstract

The 2ṡ65 km Gwithian outfall tunnel formed part of the tunnelling operations under the ‘Clean Sweep’ sewage/sewerage distribution system upgrade within the south-west region of the UK during the 1990s. The 3ṡ0 m high by 2ṡ8 m wide, rectangular, tracked tunnel was constructed by Trafalgar House Construction using drill and blast techniques to intercept a series of pre-drilled diffuser units 25 m below the seabed in St Ives Bay. In view of the close proximity to the seabed, and the risk of water ingress, systematic probe drilling was performed at regular intervals during construction. Where necessary, in order to reduce the water-make to within pre-defined limits, cementitious grouting of the ground ahead of the advancing face was undertaken. One major fault zone required extensive grouting, as initial probe holes were making in the order of 200 gallons per minute (909 l/min). Tunnel advance through ‘exceptionally poor’ ground required modification to the excavation methodology and implementation of additional support measures. Evaluation of geotechnical data from the undersea tunnel suggests that the Q-system provided a sound basis for assessment of rock quality and for guidance on associated support requirements. Good correlation was obtained between mapped Q-values and tunnel advance rates. Importantly, engineering judgement informed final support recommendations.