With the increasing construction demand for tunnels in soft silty clay layers, it is important to effectively control the face stability of a tunnel. One of the key issues for maintaining face stability is addressing how to accurately obtain the limit support pressure of a tunnel face excavated by the benching method in a soft silty clay layer. Therefore, based on the limit equilibrium method, an improved three-dimensional face stability analysis model is presented in this paper considering the advancing support effect of small pipes. In the model, the sliding failure zone is formed by a log-spiral line. The upper load of the sliding failure zone is shaped by the gravity of the soil, and it can be calculated by a two-parameter elastic foundation beam model when an advanced small pipe is utilized. By comparing these results with the limit support pressure predicted by existing models, the improved model is shown to be reasonable. Finally, the influence of the soil parameters on the limit support pressure is also assessed. The proposed failure mechanism improves the stability analysis of the tunnel face excavated by the benching method in the soft silty clay layer.

To obtain the critical collapse pressure of a circular tunnel driven by a pressurized shield for purely cohesive soils, a two-dimensional analysis model is presented based on limit analysis and slip-line theories. When the ratio of the cover depth C to the tunnel diameter D equals 0.5, there are three regions, Zones I, II, and III. When the C/D value is greater than 0.5, the mechanism involves up to four regions, i.e., Zones I, II, III, and possibly IV. Zone II is a transition zone that satisfies the normality condition. Zone IV is a Rankine zone subjected to the influence of the vertical soil arching effect that appears at the top of Zone III. Under Terzaghi’s theory of relative soil pressure, the contribution of Zone IV, including the weight and cohesion of the soils and the uniform surcharge loading, to the critical collapse pressure of the tunnel face is equivalent to an additional load. The proposed failure mechanism improves the existing upper-bound solutions for the face stability of circular tunnels in purely cohesive soils.

Based on the slip line theory and the upper-bound limit analysis theorem, a three-dimensional (3D) slip–rupture mechanism to design the shield tunnel face stability is proposed. This model is defined by using a spatial discretization technique. According to the stress arching theory, the vertical earth pressure at the top of the slip zone is calculated. The critical support tunnel face pressure is then deduced by using this additional load on the upper part of the slip damage zone. A series of finite difference numerical calculations based on the limit shear strain theory were performed to find the slip failure surface and obtain the critical support pressure. The results indicate that the soil-arching effect significantly affects the magnitude and the vertical stress distribution on front of the tunnel face. The good concordance obtained by the comparison of the results with the numerical simulations and other existing researches shows that the proposed model has a good applicability in both purely cohesive and frictional soils. At the same time, the critical tunnel face failure pattern is close to the numerical calculations one.