Dual-failure-mechanism model for face stability analysis of shield tunneling in sands

摘要

This study proposes an improved model for face stability analysis with a dual-failure-mechanism based on the limit equilibrium method. The failure mechanisms consist of rotational failure and gravitational failure. Thus, the slip zone defined by the model is composed of two parts: a rotational failure zone (lower zone) and a gravitational failure zone (upper zone). The rotational failure zone is bound by a log-spiral-shaped slip surface. It forms mainly because of insufficient support pressure and tends to rotate. The gravitational failure zone is shaped solely by soil gravity and is described by Terzaghi's theory of soil arching. Failure zone and limit support pressures estimated by the proposed model are only depended on soil parameters and geometry and position of tunnels. The model summarizes the soil parameters and geometric parameters of the limit support pressure for face stability as three parameters: N-c, N-r and N-q. These three parameters can be easily calculated with the values of J(s), J(v) and J(p) provided in this paper. Limit support pressures and failure zones predicted by this model are compared to the results of other typical models and the finite difference method. The results suggest that the proposed model is reliable. For project applications, the limit support pressures predicted by the model are also compared with Rankine's earth pressures. Rankine's earth pressure is an insufficient support pressure for tunnels in soft soils with thin earth covers, and the model proposed in this paper is more practical for project engineering. In addition, the proposed model can be also employed for tunnels in aquifers by considering the principle of effective stress. The limitations of the proposed model are also discussed.