Shear Mesodeformation Characteristics and Damage Evolution of the Frozen Soil–Concrete Interface Zone

The mesodeformation characteristics of the frozen soil–concrete interface are of great significance for revealing the mechanism of shear damage evolution, which directly affects the stability and durability of engineering structures in cold regions. In this study, an improved visual direct shear apparatus combined with digital image correlation technology was employed to perform shear tests on frozen soil–concrete interfaces without predefined failure planes. The results indicate that (1) shear failure at the frozen soil–concrete interface can be classified into three modes: sliding failure mode of contact surface (CSSM), interface-frozen soil interactive shear dislocation failure mode, and internal shear failure mode of frozen soil (ISSM). The ISSM exhibits a greater shear band thickness than CSSM, with both modes showing a sharp reduction in thickness during failure; (2) the interface shear strength is positively correlated with normal stress, water content, and surface roughness. Under constant water content and roughness, the failure mode transitions from CSSM to ISSM as normal stress increases; (3) the dilatancy effect of the interface is positively related to surface roughness and water content but negatively related to normal stress. Differences in dilatancy behavior across failure modes are primarily attributed to significant reductions in soil density near the failure surface; (4) the established constitutive relation of interfacial shear damage can well simulate the whole process of stress–displacement curves of different failure modes. The proposed damage factor D can accurately describe the damage evolution process at the frozen soil–concrete interface. The research results can provide a basis for pile-soil theory and numerical analysis in the seasonal frozen area.