An investigation into efficient drainage layouts for the stabilization of tunnel faces in homogeneous ground

• Various advance drainage schemes greatly improve tunnel face stability.
• The effects of geometric parameters for advance drainage are quantified.
• The most efficient advance drainage arrangements are presented.
• A design equation and nomograms provide a quick assessment of face stability with advance drainage.

This paper investigates the effectiveness of various advance drainage schemes with respect to face stability. The latter is analysed through limit equilibrium computations taking account of the modified seepage flow conditions prevailing in the ground after the implementation of drainage measures. The seepage forces are determined numerically through steady-state, three-dimensional seepage flow analysis which takes account of the characteristics of a given drainage scheme. A suite of computations was carried out to quantify the effects of the geometric parameters for the drainage layout, i.e. the number, length, spacing and location of drainage boreholes or the diameter and location of the pilot tunnel used for drainage. The computational results provide useful indications about optimum drainage arrangement with regard to face stability. A dimensionless formulation of the required support pressure (or the required cohesion of the ground, respectively) is developed in order to produce dimensionless design nomograms, which can provide a quick assessment of face stability in cases involving partial or complete pore pressure relief in advance of excavation.