Unified model for shear strength of soft and hard rock masses based on in-Situ direct shear test parameters

The nonlinear evolution of rock mass shear strength and its coupling with structural plane morphology represent a critical scientific challenge in geotechnical stability analysis. Existing shear strength models based on in-situ direct shear tests lack universality for both soft and hard rock masses, failing to meet rapid stability assessment needs in engineering practice. To address this gap, the study improves the Mohr-Coulomb criterion by incorporating a normal stress modulation coefficient (κ) and a shear plane undulation coupling coefficient (λ). A unified shear strength model for soft-hard rock masses, accounting for normal stress effects, is proposed. The model's validity is confirmed using in-situ direct shear test data from engineering sites. The results demonstrate excellent fitting of experimental data. Hard rocks exhibit characteristics of normal stress suppression coupled with undulation enhancement, while soft rocks display normal stress strengthening and undulation weakening. Structural plane specimens exhibit normal stress inhibition resulting from plastic deformation, while residual interlocking maintains positive contributions from undulations. The model reveals the differential governing mechanisms of normal stress and shear plane undulation on shear strength in soft-hard interbedded rock masses, providing a theoretical foundation for rapid stability assessments in rock mass engineering.