Lake Mead No 3 Intake Tunnel - Geotechnical Aspects of TBM Operation

Abstract:

The new Lake Mead No 3 Intake Tunnel will be constructed using a hybrid TBM (both slurry shield and open mode operation are possible) mostly through tertiary sedimentary rocks. Due to the very poor quality of the ground and the high pore pressures prevailing in the 4 km long subaqueous section of the tunnel (up to 14 bar, the highest pressures seen to date in closed shield tunneling worldwide), particular attention must be given to the risk of shield jamming or face collapse during boring or during the performance of maintenance activities in the working chamber. The paper outlines the expected geological-geotechnical conditions and discusses their potential impact on the operation of the hybrid TBM (e.g. mode of operation, face support pressures), as well as proposed auxiliary measures (e.g. advance drainage, grouting) and decision-making during construction.

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