An experimental investigation on the macroscopic and microscopic fracture behaviour of brittle granite using a novel true triaxial apparatus

Abstract

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.