Shear failure behaviors and degradation mechanical model of rockmass under true triaxial multi-level loading and unloading shear tests

The redistribution of three-dimensional (3D) geostress during underground tunnel excavation can easily induce to shear failure along rockmass structural plane, potentially resulting in engineering disasters. However, the current understanding of rockmass shear behavior is mainly based on shear tests under 2D stress without lateral stress, the shear fracture under 3D stress is unclear, and the relevant 3D shear fracture theory research is deficient. Therefore, this study conducted true triaxial cyclic loading and unloading shear tests on intact and bedded limestone under different normal stress σ n and lateral stress σ p to investigate the shear strength, deformation, and failure characteristics. The results indicate that under different σ n and σ p , the stress–strain hysteresis loop area gradually increases from nearly zero in the pre-peak stage, becomes most significant in the post-peak stage, and then becomes very small in the residual stage as the number of shear test cycles increases. The shear peak strength and failure surface roughness almost linearly increase with the increase in σ n , while they first increase and then gradually decrease as σ p increases, with the maximum increases of 12.9% for strength and 15.1% for roughness. The shear residual strength almost linearly increases with σ n , but shows no significant change with σ p . Based on the acoustic emission characteristic parameters during the test process, the shear fracture process and microscopic failure mechanism were analyzed. As the shear stress τ increases, the acoustic emission activity, main frequency, and amplitude gradually increase, showing a significant rise during the cycle near the peak strength, while remaining almost unchanged in the residual stage. The true triaxial shear fracture process presents tensile-shear mixture failure characteristics dominated by microscopic tensile failure. Based on the test results, a 3D shear strength criterion considering the lateral stress effect was proposed, and the determination methods and evolution of the shear modulus G , cohesion c jp , friction angle φ jp , and dilation angle ψ jp during rockmass shear fracture process were studied. Under different σ n and σ p , G first rapidly decreases and then tends to stabilize; c jp , φ jp , and ψ jp first increase rapidly to the maximum value, then decrease slowly, and finally remain basically unchanged. A 3D shear mechanics model considering the effects of lateral stress and shear parameter degradation was further established, and a corresponding numerical calculation program was developed based on 3D discrete element software. The proposed model effectively simulates the shear failure evolution process of rockmass under true triaxial shear test, and is further applied to successfully reveal the failure characteristics of surrounding rocks with structural planes under different combinations of tunnel axis and geostress direction.