Abstract:The study on the shear characteristics and soil migration law of clay-cement soil interface is important to deeply understand the bearing and deformation behaviors of cement-soil piles and walls. A series of direct shear tests for clay-cement soil interfaces with different roughness were performed to study the mechanical properties of the interfaces in terms of the shear strength, shear failure law and deformation behavior. The test results showed that the shear failure of the interface followed the Mohr-Coulomb strength criterion. The friction angle of the clay-cement soil interface increased with the increasing roughness, while the growth rate then gradually slowed down. The cohesion of the clay-cement soil interface varied similarly to that of the clay. In addition, the curves of the normal strain versus shear displacement for the clay-cement soil interfaces exhibited a shear contraction pattern, where the magnitude of the shear contraction increased with the increasing normal stress and interface roughness. The normalized effective coefficient (E_s) of the interface strength increased with the increasing roughness, and the E_s became larger than 1.0 as the roughness R exceeded 1.0 mm, indicating that the shear strength of the clay-cement soil interface was improved and became larger than that of the clay itself due to the existence of “passive resistance”. Moreover, the curves of the shear displacement versus shear stress under different normal stresses and roughness experienced an exponential relationship, from which a unified exponential model was established to describe the relationship among the roughness, shear stress, shear displacement, normal stress, friction angle and cohesion of the interface. Besides, the shear failure zone gradually spread from the shear forward direction to the shear rear as shearing. The research results are helpful in revealing the mechanical behavior of cement-soil interfaces and can also provide references and guidance in the design of cement-soil piles and walls.