Confinement-dependent characteristic stresses, energy evolution and pre-peak brittleness of granite under triaxial compression

Understanding the pressure-dependent cracking and energy evolution of granite is essential for evaluating the stability of deep tunnel surrounding rock under complex in-situ stress conditions. In this study, uniaxial and conventional triaxial compression tests were conducted on granite specimens under confining pressures of 0, 5, 10, 15, and 20 MPa to investigate the evolution of mechanical behaviour, characteristic stresses, energy partitioning, and the pre-peak energy-storage state. The crack volumetric strain method was used to identify the crack closure stress, crack initiation stress, crack damage stress, and peak stress, while a stage-based energy framework was adopted to quantify the total input energy, elastic strain energy, dissipated energy, energy ratios, and the energy-storage-based pre-peak brittleness indicator at different characteristic stress states. The results show that confining pressure markedly enhances the strength and deformation capacity of granite. As the confining pressure increases from 0 to 20 MPa, the peak stress, elastic modulus, and peak strain increase from 82.95 to 293.62 MPa, from 40.83 to 56.13 GPa, and from 0.2001 to 0.6122%, respectively. The crack closure stress, crack initiation stress, and crack damage stress also increase systematically, reaching 88.02, 147.27, and 209.77 MPa at 20 MPa confinement. Although the absolute characteristic stresses are strongly pressure-dependent, the normalized stress ratios remain relatively stable under triaxial compression, with crack initiation occurring at approximately half of the peak strength and crack damage occurring at about 70–75% of the peak strength. Energy evolution is strongly stage-dependent, with irreversible energy dissipation becoming more pronounced after crack initiation and accelerating after the crack damage threshold. Confinement increases the absolute magnitudes of total input, elastic, and dissipated energies and modifies their relative partitioning most evidently near peak failure. The energy-storage-based pre-peak brittleness indicator generally decreases after confinement is applied, indicating that a smaller fraction of the total input energy remains recoverably stored at the characteristic stress states; however, the peak-stage indicator becomes more responsive under high confinement. The interval-dependent sensitivity analysis further demonstrates that the response is jointly governed by the initial confinement level and the cracking stage. The 15–20 MPa interval generally produces the largest sensitivity coefficients for the characteristic stresses and absolute energy components, while peak-stage energy partitioning and the associated pre-peak energy-storage indicator exhibit the strongest high-confinement response. These findings indicate that confining pressure substantially elevates the stress and energy thresholds required for crack evolution while largely preserving the normalized sequence of cracking stages. The proposed stage-based framework provides a quantitative basis for assessing granite failure tendency and surrounding-rock stability in deep underground engineering.