Three-dimensional analysis of surface settlement in soft ground tunneling

Abstract

A parametric study was performed to evaluate the effects of different construction variables on surface settlements above tunnels in soft ground. The finite element program "PLAXIS 3D Tunnel" was used to carry out the numerical analysis for the earth pressure balance (EPB) shield tunneling method in a saturated, normally consolidated clay. The two construction parameters that were varied were the face pressure and the grout pressure. Longitudinal settlement profiles were obtained for many different face pressures and several different grout pressures. Results show that the ground surrounding the tunnel is very sensitive to changes in grout pressure in terms of surface settlement and failure of the soil body, while a wide range of face pressures can be accommodated without failure. Furthermore, minimum surface settlement is achieved for a certain face pressure and becomes larger as the pressure is increased or decreased from that particular value, which for this analysis corresponded to an overload factor of one. Also, results show that surface settlement decreases with increasing grout pressure, as expected.