Mechanical response, macro and micro fracture characteristics of jointed rock under true triaxial lateral unloading- axial loading condition

True triaxial lateral unloading-axial loading (LU-AL) tests were conducted to study the effect of joint properties, including joint roughness coefficient (JRC), strike, inclination angle ( α ) on the mechanical characteristics of jointed rock. Under LU-AL conditions, the disappearance of lateral confinement stress perpendicular to the joint strike ( σ D ) intensifies the weakening effect of joint, leading to obvious post-peak drops in stress-strain responses. After lateral unloading process, the stress parallel to the joint strike becomes the new intermediate principal stress. That leads to the peak maximum principal stress ( σ 1m ) of specimens with joint strike parallelling to σ 2 (FX2 strike) is higher than that of specimens with joint strike parallelling to σ 3 (FX3 strike). As σ D decreases, the influence of JRC and α on failure modes becomes more evident. For jointed rock with α = 45° and 60°, mixed joint shear slip combined with matrix tensile splitting occurs, while a mixture of matrix shear and splitting governs the failure for jointed rock with α = 30° and 90°. The CT analysis further reveals that joint morphology redirects and hinders the propagation of matrix cracks, altering their spatial distribution and development paths. Fracture volume show significant variation with α , taking specimens in FX2 strike with JRC = 14.74 as example, the fracture volumes ranged from 4064 mm 3 to 9089 mm 3 with increasing α . These findings provide new insights into the coupled effects of joint properties and unloading conditions on the crack evolution of jointed rock.