Mechanical behavior of Hauzenberger granite and Orient Beige limestone under high-pressure triaxial testing

Understanding the mechanical response of rocks under very high confining pressure (80–650 MPa) is essential for designing deep mines, underground reservoirs, and CO2 sequestration systems. This study investigates how confinement up to 650 MPa influences stiffness, strength, and failure mechanisms in two lithologies, Hauzenberger granite and Orient Beige limestone, tested at the decimetric scale. Hydrostatic loading up to 650 MPa and triaxial compression at σ3 = 80 MPa and 160 MPa were performed. Under hydrostatic loading, both rocks show an initial pore-closure phase followed by stiffness stabilisation. Under triaxial compression, peak deviatoric stress rises with confinement (from 545 to 667 MPa for limestone and from 675 to 935 MPa for granite), and the post-peak response evolves from brittle to more ductile-like, especially for the limestone at σ3 = 160 MPa. The transition from compaction to dilatancy in the volumetric strain occurs close to the stress peak and is used as an additional indicator of failure. Mohr–Coulomb fitting yields cohesion c and friction-angle φ pairs of (89.5 MPa, 31.8°) for limestone and (77.1 MPa, 41.1°) for granite. Increasing confinement markedly enhances rock strength and promotes a shift from localised brittle cracking to distributed, ductile-like deformation.