Tunnel face stability considering drainage and surface settlements

Tunnels excavated under the water table may need a face support pressure to ensure its stability. When such support pressure is too high to be provided by traditional support measures, advance drainage is required. But drainage lowers the water table, hence increasing the surface settlements and their effects on nearby structures. This work studies the surface settlements induced by tunnel face advance drainage boreholes, and their relationship with the tunnel face stability, using a numerical model of a shallow tunnel with different values of the applied support pressure. Results show that advance drainage produces a “more stable” situation, as less face support pressure is required for stability, while increasing surface settlements and, consequently, the risk to nearby structures. The critical pressures computed with a rotational Limit Analysis failure mechanism are also analyzed, preliminarily suggesting that they correspond to the asymptotic values of the stand-up time.