Effect of Support-Induced Stress on the Failure Characteristics of Deep Tunnels Based on True-Triaxial Compression Tests

The stability of tunnels during excavation is closely related to the strength of the surrounding rock and depends on the three-dimensional stress they experience. This work explored the effect that a radial stress gradient considering support stress (σ_r = σ₃) has on the failure of granite from the Daxiagu Tunnel in southwestern China. A series of true-triaxial compression tests were conducted using a constant intermediate principal stress (σ₃ = 25/50 MPa) and a variety of minimum principal stress levels (σ₃). The main objective is to investigate the effect of σ₃ on the failure mode, deformation characteristics, strength properties, and energy storage characteristics of the rock. The resulting stress–strain curves indicate that increasing σ₃ reduces the brittle failure characteristics of the granite. Furthermore, the final fracture angle is negatively correlated with σ₃. The deformation of the granite is evidently anisotropic and increasing the value of σ₃ affects the deformation in the direction of the maximum principal stress (ε₃) most strongly. However, the strength of this influence gradually diminishes as the difference between σ₂ and σ₃ decreases. The characteristic strength parameters (crack initiation stress σ_ci, crack damage stress σ_cd, and peak stress σ_p) also increase with increasing σ₃. As σ₃ is varied, the σ_ci: σ_p and σ_cd: σ_p ratios remained in the ranges 40%–60% and 65%–85%, respectively. The values of the energy storage limit calculated using the true-triaxial strain energy formula show that σ₃ can significantly increase the energy storage limit of the rock, thereby enhancing its resistance to failure. Our findings enhance our understanding of the effect of minimum principal stress on the failure of the rock surrounding deeply buried tunnels. It also helps identify the optimal measures that should be employed to support the rock.