Anisotropic Deformation and Fracture Mechanism of a Deep Marble under True Triaxial Stress: A Multiscale Investigation

True triaxial compression tests were conducted on deep-buried marble (deep marble for short) using a true triaxial disturbance unloading test system under different initial stress conditions. The effects of initial confining pressure ratio (b) on the strength, deformation, and failure characteristics of the deep marble were thoroughly investigated. The results show that the marble samples exhibit stress-induced anisotropy under true triaxial stress conditions. When the intermediate principal stress (σ2) is constant, the stress drop in the postpeak stage decreases obviously with the increase of the minimum principal stress (σ3), accompanied by a gradual weakening of strain-softening characteristics. The minimum principal strain (ε3) shows the most pronounced variation, transitioning from initial contraction to subsequent expansion. When σ3 is constant, sample damage intensifies with the increase of b. Failure modes include tensile-shear composite failure along the σ2 direction and predominantly split-tensile failure in the σ3 direction, featuring extensive rock spalling pits. The plastic volumetric strain curves evolve through three distinct stages: rapid accumulation, steady growth, and gradual decline, with their inflection points corresponding to the crack initiation stress and peak stress, respectively. Acoustic emission (AE) analysis reveals that the cumulative damage at the end of initial preloading first decreases and then increases with the increase of σ3, while it shows an inverse trend with the increase of σ2. The maximum cumulative energy of AE decreases with the increase of σ2, but follows a rise-then-decline pattern with the increase of σ3. The AE activity concentrates primarily during initial preloading and axial pressure loading stages. Higher b-values promote more active AE counts, facilitate main crack propagation, and enhance post-peak brittle failure characteristics.