An experimental investigation on the macroscopic and microscopic fracture behaviour of brittle granite using a novel true triaxial apparatus
摘要
True triaxial compression (TTC) tests were conducted on granite specimens from a depth of 1000 m to investigate the effects of σ₂ and σ₃ on both macroscopic and microscopic fracture behaviour. Under true triaxial stress conditions, the specimens exhibited pronounced directional fracturing. The macroscopic fracture planes were parallel to the σ₁–σ₂ plane, while microcracks were mainly distributed perpendicular to the σ₁ and σ₂ directions. The results showed that σ₂ and σ₃ exert different influences on the fracture behaviour of the specimens. When σ₂ was held constant at 40 MPa, increasing σ₃ from 0 to 30 MPa caused a transition in the initial slope of the post-peak stress–strain curve from positive (Class II behaviour) to negative (Class I behaviour). Concurrently, the angle between the macroscopic fracture plane and the σ1 direction increased from approximately 0° to 30°, indicating a shift in the failure mechanism from extensional to shear. In contrast, when σ₃ = 0 MPa, increasing σ2 from 10 to 40 MPa did not change the macroscopic fracture angle (about 0°) but reduced microcrack number and density. For example, on thin section perpendicular to the σ1 direction, the number of microcracks decreased from 391 to 127, and the crack density dropped from 0.68 to 0.11 mm⁻1. These findings suggest that higher σ2 promotes the release of elastic energy and increases the risk of strain bursts, especially near excavation boundaries.