Abstract:  The research on determining the stability of the circumferential excavation face of a tunnel in the nonhomogeneous clay layers primarily focuses on circular cross-section tunnels, and the action law of different cross-section shapes has important significance. Therefore, the stability of the circumferential excavation face of a tunnel in a clay layer is investigated as a function of the factors influencing the stability, such as various cross-section shapes, buried depth ratios, soil weights, and heterogeneous strength that increases linearly with depth. The upper limit finite element method of rigid translational motion element is used to investigate the relationship between the critical load ratio and potential failure mode. The results show the following. (1) For the case of the same height of tunnel, the critical load ratio under the failure limit state for the straight wall of the horseshoeshaped tunnel is slightly lower than that of the circular tunnel. However, the upper limit solution obtained for the elliptical tunnel decreases with increasing span. The critical load ratio of tunnel failure is positively related to the soil heterogeneous strength and negatively correlated to the depth of soil. (2) The tunnel′s buried depth is affected by the strength of soil heterogeneity and the soil weight. Moreover, it is noted that failure range is determined by the shape of cross-section and buried depth, and it is considerably less sensitive to other factors from the failure mode of the sliding line obtained using the upper limit finite element method. Therefore, the upper limit solution with the critical load ratio agrees well with the centrifuge test results. This demonstrates that the upper limit method of the moving element and the fitting formula of the upper limit solution can be used to evaluate the stability in the circumferential excavation face of tunnels in the nonhomogeneous clay layer.

Key words: tunnel, nonhomogeneous clay, crosssection type, circumferential excavation face stability, failure mode, upper limit finite element method