Fixed-axis shear characteristics of anisotropic calcareous sand under variation of principal stress axes

In order to explore the shear characteristics of anisotropic calcareous sand, the GDS hollow cylinder torsional shear apparatus is used to compare and analyze the anisotropy of calcareous sand and quartz sand through the fixed-axis shear tests in different directions of large principal stress to reveal the effects of inherent anisotropy on strength and pore pressure. The results show that both the calcareous sand and the quartz sand exhibit anisotropy in strength. As the large principal stress direction angle α (the angle between the large principal stress direction and the vertical direction) increases, the strength decreases first and then increases, and reach the minimum when α = 70°. The anisotropy of calcareous sand is more pronounced, and the magnitude of change in strength of calcareous sand in different directions of large principal stress is about 1.4 to 1.5 times that of quartz sand. The pore pressure values of the calcareous sand and quartz sand both increase with the increase of α, and the state of failure also gradually changes from tending to dilatancy to shrinking. Different from that of the quartz sand, the maximum value of Henkel's pore pressure coefficient appears at α = 70° instead of α = 90°. The radial strain ε_r at the time of failure has the smallest amplitude change with the direction angle of the large principal stress, and the torsional shear strain γ_zθ has the largest one.