High damage potential, relatively high pore pressures and limited pre-construction accessibility are the main features of subaqueous tunnels. Potential hazards include high water inflows or even a complete flooding of the tunnel in the case of a hydraulic connection to the seabed. In subaqueous tunnels high pore pressures may occur at small depths of cover, i.e. in combination with low effective stresses and low shear strengths of the ground, thus having a particularly adverse effect in terms of stability and potential deformations. The paper examines some of the geomechanical issues relating to subaqueous tunnels (face stability in fault zones, the limits of open mode TBM operation in weak sedimentary rocks and the effect of advance drainage in squeezing ground) by means of three case studies: the recently completed " Melen 7" Bosphorus tunnel, the soon-to-commence Lake Mead Intake No 3 tunnel construction and the future Gibraltar Strait tunnel project.

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