Staged damage evolution and failure mechanisms of layered rocks under different confining pressures

• Innovative Hybrid Methodology Combines strain energy theory and ultrasonic monitoring to achieve multi-stage damage characterization (compaction, plastic development, brittle failure), resolving ambiguities in deformation staging. • Critical Bedding Angle Identification Identifies θ = 60° as the orientation maximizing damage sensitivity due to shear stress concentration , validated through experiments and physics-based models. • Confining Pressure-Driven Failure Delay Demonstrates that higher confining pressures ( e.g. , 20 MPa) suppress microfracture propagation, delaying failure onset and enhancing plastic deformation. • Mechanism-Integrated Constitutive Model Develops a damage evolution equation incorporating dip angle ( θ ) and confining pressure ( σ ₃) , validated with R² > 0.90 and explicit physical significance. • Field-Relevant Failure Mechanisms Links laboratory-observed mechanisms (tensile-shear, splitting) to tunnel stability and slope hazard mitigation , providing actionable guidelines for engineering practice. Layered rocks, which are prevalent in engineering geological conditions, are characterized by strong anisotropy and complex damage mechanisms, posing significant research challenges. Traditional research methods often failed to accurately reveal internal microscopic damage evolution. To overcome these limitations, energy theory was integrated with ultrasonic monitoring technology in this study. Additionally, stress-ultrasonic synchronous tests were performed on the slate with various dip angles (0–90°) and confining pressures (0–20 MPa) to examine their influence on damage evolution. The staged damage process of layered rocks was systematically investigated under confining pressures, providing a multidimensional characterization of the entire damage process in layered rocks at different dips. Our findings suggest that shear stress concentrated on the bedding plane as the dip angle increased, accelerating microcrack development. Damage was most pronounced at 60°, whereas bedding planes parallel (0°) or perpendicular (90°) to the principal stress axis exhibited minimal shear stress effects and slower damage progression. Besides, increased confining pressure suppressed microcrack expansion, bringing about further slowed damage. Damage evolution demonstrated stage-by-stage characteristics as follows. Wave velocity effectively characterized damage in Stage I (compaction), while energy accurately reflected Stages II–III (plastic development and brittle destruction). Furthermore, a comprehensive damage constitutive model accounting for dip and confining pressure effects was developed to accurately describe the entire damage process from microcrack compaction to destruction. This study elucidated intrinsic damage mechanisms and provided new theoretical perspectives and experimental evidence for engineering stability assessments under complex geological conditions.