Deformation damage mechanism and strength criterion of columnar jointed rock mass under true triaxial condition

To accurately understand the anisotropic mechanical properties of columnar jointed basalt distributed in the Baihetan Hydropower Station, indoor true triaxial loading physical model tests were conducted with different joint angles (β = 0°–90°) to study the strength deformation and failure mode of columnar jointed rock mass (CJRM). The results indicate that the strength anisotropy characteristics of CJRM under different stress environments are significant. The peak strength shows a U-shaped (σ2 = 2 MPa, σ2 = 4.5 MPa), W-shaped (σ2 = 3 MPa), and linearly increasing (σ2 = 5 MPa) rule of change. The increments in peak strength of CJRM varied significantly with the intermediate principal stresses, with average increments of 29.0% (σ2/σ3 = 1.5), 48.8% (σ2/σ3 = 2.25), and 45.4% (σ2/σ3 = 2.5); ε3 increases and then decreases with the change of inclination angle β, and the shear expansion phenomenon increases, ε2 decreases and then increases with the β, and the deformation characteristics of CJRM are that the expansion deformation transforms into compression deformation. The increase in σ2 shifts the rock features from moderate anisotropy to low anisotropy. According to the damage characteristics, the CJRM are classified according to three damage modes: (a) structurally controlled, (b) stress-controlled, and (c) stress-structurally controlled. Finally, Mogi–Coulomb strength criterion and Drucker–Prager strength criterion, which are applicable to true triaxial stress conditions, were used to verify the experimental results. The Mogi–Coulomb strength criterion fitted the strength parameters c and φ values, which showed a high degree of agreement with the in situ test results on site, indicating good applicability. The research results can scientifically and accurately predict and evaluate the stability of the Baihetan project.