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  Effectiveness of advance drainage boreholes for the face stability of subaqueous tunnels crossing highly permeable ground at great hydraulic head gradients

  Subaqueous tunnels close to the seabed but far below sea level are characterized by very high hydraulic gradients, which in combination with weak ground may endanger face stability. Advance drainage improves stability (Zingg and Anagnostou, 2016), but becomes less effective in the presence of casings or where the water inflow volumes are so high that pipe flow develops within the boreholes. Both pipe flow and casings result in water pressure development along the boreholes, thus impeding pore pressure relief in the surrounding ground. Limit equilibrium computations taking account of the hydraulic interaction between boreholes and ground show that the adverse effects of high-permeability ground, rough borehole walls and sparsely slotted or perforated screens on pore pressure relief may result in significantly increased demand for face support or may even necessitate ground improvement in addition to the advance drainage.

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