Experimental Study on the Mechanical Characteristics of Saturated Granite under Conventional Triaxial Loading and Unloading Tests

It is essential to study the mechanical properties of saturated rock under different loading and unloading paths for strength calculation, safety assessment and disaster prevention; however, current literature rarely mentions conventional triaxial loading and unloading conditions. To analyze the mechanical properties, strain energy evolution characteristics and failure mode, a series of conventional triaxial unloading tests (with axial loading rate v a of 0.06–6 mm/min and circumferential unloading rate v u of 0.1–10 MPa/s) and conventional triaxial compression tests were carried out on saturated granite. The test results showed that the damage sources of specimens in the conventional triaxial unloading test were mainly related to circumferential deformation, while in the conventional triaxial compression test, it was related to the axial deformation. Under the same v a , the confining pressure and axial stress at the failure point decreased with the increase of v u , and the stress coordinate of the failure point was located outside the conventional triaxial compression envelope of σ 1 –σ 3 . As v u increases, except for the variation of circumferential strain energy ΔU c decreasing slowly, the trend of strain energy changes must be determined together with v a . As v a increases, the relationship between the magnitude of each energy changes from ΔU a > ΔU > ΔU d > ΔU e > ΔU c to ΔU d > ΔU a > ΔU > ΔU e > ΔU c , while the change of dissipated energy is dominated by v u and v a together to become dominated by v a . In addition, with the increase of v u and v a , the damage pattern of the specimen also changes from shear damage in a single shear plane to mixed damage with tensile strain failure and shear plane during which the dilation angle of the specimen increases in total except for v u = 10 MPa/s, v a = 0.6 mm/min and 6 mm/min.