Abstract: As a typical geological structure, the layered composite structure of surrounding rock is formed by weak interlayer and hard brittle rock, and it significantly affects the stability of tunnel surrounding rock. In the past, the research on composite rock with weak interlayer focused on uniaxial, biaxial or conventional triaxial stress paths, and the mechanical properties and failure characteristics of composite rock at the free face of tunnel under true triaxial stress path were lack of analysis and discussion. In this study, the influence of the thickness of weak interlayer on the mechanical response and failure characteristics of surrounding rock at the free face is discussed based on the composite rock samples with different thicknesses of weak interlayer. The results show that: (1) The thickness of weak interlayer significantly affects the peak stress and strain of composite rock samples. With the increase of thickness, the sliding deformation of rock blocks above the weak interlayer gradually increases, and the compression deformation of the weak interlayer decreases gradually. (2) With the increase of thickness of the weak interlayer, the failure mode of the rock element near the free face of the composite rock sample gradually changes from mixed tension shear failure to tension failure, and the number and failure range of macro cracks gradually decrease, while the rock element far away from the free face gradually changes from shear failure to basically undamaged fracture. (3) The failure areas of composite surrounding rock of side wall with different thicknesses are concentrated on the weak interlayer and the surrounding rock above it, while the surrounding rock below the weak interlayer is basically stable. In terms of stress distribution, with the increase of thickness of the weak interlayer, the maximum compressive stress gradually transfers to the deep weak interlayer, and the tensile stress area gradually decreases, while the depth of the tensile stress area gradually increases.

Key words: weak interlayer, sliding deformation, mixed tension and shear failure, maximum compressive stress, tensile stress