Shear behavior and progressive failure of grout–rock interfaces in soft–hard interbedded rock: Role of soft-layer proportion

The shear behavior of grout–rock interface in soft–hard interbedded rock mass is governed by complex interactions between material heterogeneity and interfacial mechanical properties; however, the underlying mechanisms remain insufficiently understood. This study systematically investigates the influence of soft-layer proportion ( α ) on the shear response of grout–rock interface in soft-hard interbedded rock mass through laboratory direct shear tests, numerical simulations, and analytical modeling, where α is defined as the ratio of the soft layer thickness and the total rock thickness. The results show that: (1) the shear stress–displacement response exhibits a transition from a single peak pattern under homogeneous conditions ( α = 0 or 1) to a pronounced double peak behavior in composite systems (0 < α < 1). With the increasing α , both shear strength and shear stiffness decrease nonlinearly, while the residual strength increases due to enhanced surface roughness. Compared with α = 0, the peak shear strength at α = 0.25, 0.5, 0.75, and 1.0 decreases by approximately 23%, 40%, 48%, and 51%, respectively. (2) Numerical results reveal that this nonlinear behavior is governed by normal stress redistribution, which leads to stress concentration on the grout–hard rock interface. The double-peak response is attributed to the sequential failure of grout–soft and grout–hard rock interfaces, accompanied by progressive load transfer. A critical α is identified, beyond which the governing shear strength shifts between the two peak values. (3) An analytical model is further proposed to provide rapid prediction of interfacial shear strength, showing good agreement with experimental and numerical results. Parametric analyses indicate that a larger elastic modulus contrast and interfacial shear stiffness contrast jeopardizes the cooperative load-bearing capacity of the interface, thus reducing in overall shear strength. These findings clarify the mechanical origin of α -dependent strength evolution and double-peak failure behavior of grout–rock interface, providing a theoretical basis and practical guidance for evaluating and optimizing anchorage performance in soft–hard interbedded rock masses.