True triaxial compression failure of fine-grained granite and brittleness characterisation based on a damage-energy index

True-triaxial stress states can strongly influence brittle failure and excavation-damaged zone (EDZ) in crystalline host rocks. This study presents a systematic true-triaxial experimental investigation of the brittleness behaviour of fine-grained granite, motivated by stress conditions relevant to deep openings. Two loading series were performed, with varying intermediate and minor principal stresses. Characteristic stress thresholds were identified from pre-peak responses using volumetric strain and crack volumetric strain criteria, and the associated pre-peak energy evolution was quantified. The pre-peak response demonstrates pronounced stress-path dependence, with σ2 exerting a distinct non-monotonic influence on brittleness-related responses, whereas increasing σ3 generally enhances confinement and promotes more stable deformation. To enable practical, pre-peak quantification of the observed behaviour, an auxiliary damage-energy index Bcd is introduced based on the ratio of total energy density at σcd to that at peak stress, avoiding reliance on unstable post-peak data. Independent morphological evidence from fracture-angle measurements and 3D laser-scanned fracture surfaces shows that the joint roughness coefficient varies non-linearly with stress path, consistent with the experimentally observed brittleness trends. The established relationships among σ2, σ3, characteristic stresses, energy evolution and fracture morphology provide laboratory-scale constraints that may support stress-based zoning and stability assessment of excavation-induced brittle damage and EDZ development in granitic repositories.