Effects of the hydraulic capacity of advance drainage boreholes on tunnel face stability

This paper analyses face stability with limit equilibrium computations that take account of the numerically determined hydraulic interaction between the boreholes and the ground. Pipe flow in the boreholes is modelled on the basis of an equivalent hydraulic conductivity model which takes account of the characteristics of the drainage boreholes (roughness, diameter etc.). Casings are considered by modelling the geometry of several slotted or perforated borehole screens. The computational results 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 greater demand for face support or may even necessitate ground improvement in addition to the advance drainage.