Experimental investigation on the failure and deformation mechanisms of tunnels in horizontally stratified-interbedded rock masses

A number of typical cases in China show that for tunnels in stratified rock masses, the tunnel structure and surrounding rock are often exposed to damage, which affects the safety of tunnel construction and operation. In this study, the failure and deformation mechanism of tunnels, in the horizontally stratified rock formation are investigated by a large-scale model test. The stratified rock mass and elaborated tunnel structure in the model test were carefully designed and produced to be more in line with the actual site conditions. The test results indicate that the failure of horizontally stratified rock mass under high horizontal ground stresses is mainly concentrated in the tunnel bottom and vault. The gradual failure process started with the interlayer separation, followed by the bending failure of rock layers, and finally resulted in the shearslip failure of the rock mass. After obvious interlayer separation and apparent rock layer bending cracks, the rock mass formed a dominant failure surface which allows shear-slipping to occur along the dominant failure surface under the compression of horizontal ground stress. The stratified rock layers show a bending failure towards the tunnel centre with the shear-slip line inclined approximately 60 degrees to 80 degrees to the horizontal. After the stratified rock mass starts to damage, the increase of the lateral pressure coefficient will intensify the floor heave behaviour of the tunnel. The tunnel structure failures in the test mainly include crush failure at the vault, longitudinal penetration cracks at the central gutter, and tension penetration cracks at the sidewalls. In the stratified rock stratum with high ground stress, attention should be paid to preventing the tunnel floor heave, strengthening the inverted arch if necessary, and paying attention to the crack detection of the tunnel structure during the operation.