Experimental Study on Direct Shear Mechanical Behavior of Unsaturated Loess-Steel Interface Considering Freeze-Thaw Cycles

The strength characteristics and stress-strain relation of the interface between soil and structure are the key parameters to determine the bearing capacity and security of foundation engineering. Especially in cold regions, periodic freeze-thaw cycles have repeatedly changed microstructure and physical-mechanical properties of shallow unsaturated soil, which has directly affected the interaction between soil and structure. In order to better serve the practical project, the mechanical behaviors of unsaturated loess-steel interface were studied considering freeze-thaw cycles, water content and the matric suction of loess based on the direct shear test. The results show that with the increase in freeze-thaw times, the cohesion (c) and internal friction angle (phi) of interface first increase and then decrease and gradually tend to be stable, while the matric suction gradually decreases and becomes steady. When the vertical load is low (not more than 200 kPa), brittle failure occurs near the loess-steel interface both before and after freezing-thawing. As the vertical load increases, the failure mode of interface gradually changes from brittle failure to plastic failure. The constitutive model of shear stress and displacement as well as freeze-thaw cycles on the loess-steel interface is tried to be established based on experimental data, and Gompertz model is turned out to be a good expression of the relationship between shear stress and displacement of the loess-steel interface considering freeze-thaw cycles. The aim of the study is to reveal how freeze-thaw cycle has affected the strength characteristics and stress-strain relation of unsaturated loess-steel interface, hopefully to provide reference for the numerical calculation and engineering design of the foundation bearing capacity in seasonal frozen soil regions. And the research results have theoretically filled the gap in the research field of the effect of freeze-thaw on mechanical behaviors of unsaturated loess-steel interface, and provided scientific basis for the design and safety evaluation of geotechnical structures in seasonal frozen soil regions.