Energy evolution and fracture mechanisms of single-fissure rock-like specimens subjected to pore water pressure

The deformation and failure of fissured rock masses under groundwater pressure are governed by fissure orientation, effective stress variation and crack propagation. To elucidate the macroscopic degradation and mesoscopic fracture mechanisms under coupled fissure inclination and pore water pressure, rock-like specimens with a single through-going prefabricated fissure were tested. Hydro-mechanically coupled triaxial compression tests were conducted at a confining pressure of 3 MPa, with seven fissure inclinations (0° − 90°) and four pore water pressures (0.7–1.3 MPa). Based on stress-strain response, volumetric deformation and energy partitioning, the effects on strength, stiffness, dilatancy threshold and energy storage capacity were analysed. PFC2D discrete element simulations were performed to interpret crack initiation, propagation and coalescence. Results show that fissure inclination governs the interaction among shear slip, tensile cracking at fissure tips and matrix damage. Peak strength, elastic modulus, dilatancy stress, total input energy and elastic strain energy first decrease and then increase with fissure inclination, reaching minima near 30°, the most unfavourable orientation under present test conditions. Increasing pore water pressure reduces effective normal constraint on the fissure plane and particle contacts, weakens interfacial friction and energy storage, and consequently advances the onset of dilatancy, dissipated energy growth and macroscopic failure. PFC2D results further demonstrate that the failure mode evolves with increasing fissure inclination from tensile failure dominated by wing-crack growth at low inclinations, to mixed tensile-shear failure at intermediate inclinations, and finally to composite failure involving shear slip, local compression-shear damage and axial splitting at high inclinations.