Effect of advance drainage on tunnel face stability using Limit Analysis and numerical simulations

Abstract

Positive pore water pressures ahead of tunnels constructed under the water table influence the stability of the tunnel face, and they may considerably affect the safety of tunneling operations. Advance drainage is often necessary, since traditional support measures may be insufficient to provide the required face support pressures. This work presents an Upper Bound Solution, in the framework of Limit Analysis, to assess the stability of a tunnel face considering the influence of advance drainage boreholes in two different arrangements, upper and lower. Results of the proposed Limit Analysis approach are compared with the results of a numerical model in FLAC3D for a tunnel with a D=10m diameter, demonstrating that the critical pressures and failure geometries computed with the analytical solution correctly reproduce the FLAC3D results when the height of the water table above the tunnel crown is lower than 5 tunnel diameters. Similarly, our results are compared with the results of a similar face stability analysis conducted with a Limit Equilibrium mechanism, suggesting that the Limit Analysis approach gives lower collapse pressure values, with relative differences of up to 30% (mainly depending on the water level and on the drainage configuration), and with results that generally agree better with the numerical simulation results. Finally, the influence of the borehole layout at the tunnel face is analyzed considering different water table positions. Results confirm that the water level has a significant influence on face stability (both on critical pressure values and on failure geometries), that the advance drainage considered significantly reduces the support pressure required for stability, and that the advance drainage configuration can also affect the shape of the critical failure mechanism.