Influence of arch foot crack on stability of surrounding rock mass in horseshoe-shaped tunnels

In order to clearly investigate the effect of a crack at the tunnel arch foot on the stability and failure patterns of horseshoe-shaped tunnels, the cracks are classified into two categories: one is that the cracks are distributed counterclockwise in the areas of A(0°<$\alpha$≤90°), B(90°<$\alpha$≤180°) and C(180°<$\alpha$≤270°) with the arch foot junction as the center, and the other is that the crack dip angle α is fixed as 127° between the tunnel bottom and the crack, and the ratio of crack length β is different. The disadvantageous factors of the arch foot crack are analyzed by comparing the laboratory test results with the numerical simulation ones, and then green sandstone is chosen to make tunnel model samples to obtain the effect of crack on the stability of the surrounding rock mass. The stress intensity factor at crack tip and the damage evolution nephogram of the surrounding rock mass are calculated by finite element program. The following significant conclusions are drawn: (1) When a crack is located at the tunnel arch foot with different angles of $\alpha$, the damage degrees of the tunnel model failure distribution zones can be arranged in a sequence: B>C>A. (2) As the dip angle α ranges from 120°~135°, the stability of the tunnel is the weakest. (3) For a tunnel model sample under biaxial compression loading, the failure modes are mainly the tensile and shear failures at the crack tip and sidewall as well as the tensile failure at vault. (4) The influences of crack on tunnel stability are inversely proportional to the crack length ratio β.