Effect of unloading paths on fracture activation: Insights from true triaxial unloading tests

Underground excavations can activate fractures, leading to dynamic geohazards like fault-slip rockbursts, while how true triaxial unloading paths affect fracture activation remains unclear. This study investigates the influence of four unloading paths (unloading σ 3 (U3), unloading σ 3 while loading σ 1 (U3L1), unloading σ 2 (U2), and unloading σ 2 while loading σ 1 (U2L1)) on fracture activation in pre-fractured granite. True triaxial unloading tests with different σ 3 values were also conducted under the U2L1 path. The results show that specimens break through the intact part in the U2 path, while fractures are activated under the U3 and U3L1 paths. In the U2L1 path, fracture activation occurs when σ 3 =10–30 MPa, while fracture activation and intact-part breaking occur when σ 3 =40 MPa. The lower unloading ratio ( η ) for the paths with σ 1 loading (U3L1, U2L1 vs. U3, U2) and σ 3 unloading (U3, U3L1 vs. U2, U2L1) indicates a higher risk of fracture activation. The strain increments Δ ε 1 and Δ ε 2 (from the start of unloading to the final failure) are greater along σ 2 unloading paths (U2 and U2L1) than those along σ 3 unloading paths (U3 and U3L1), while Δ ε 3 is comparable between σ 2 unloading paths and σ 3 unloading paths. For the four unloading paths, tensile cracks are dominant in the initial unloading stages, while shear cracking becomes prominent as failure approaches. Along the U2L1 path, the increase in σ 3 from 10 MPa to 30 MPa promotes fracture activation, crack propagation on the σ 2 rock surface, and strain increments (Δ ε 1 , Δ ε 2 , and Δ ε 3 ) during the unloading stage.