Direct Shear Behavior of Silty-Loam–Concrete Interfaces Subjected to Freeze–Thaw Cycling: An Experimental and DIC Investigation

Freeze–thaw cycling can alter soil–structure interface response in seasonally frozen regions. This study investigated the direct-shear behavior of a commercially sourced silty-loam–concrete interface subjected to sealed freeze–thaw cycling. A 25-condition mixed-level design based on the standard L25(56) orthogonal array (25 runs with six available five-level columns) considered normal stress, nominal interface roughness, moisture content, and freeze–thaw-cycle number; each main condition was tested once, so the results are interpreted descriptively. Shear strength and shear-induced vertical contraction were measured, and digital image correlation (DIC) was used to characterize surface deformation localization. The level-wise mean shear strength increased with normal stress and approximately linearly with roughness. It changed little between 14% and 18% moisture content and decreased at 22% and 26%; the measured plastic limit was 19.2%. With increasing freeze–thaw cycles, the level-wise mean strength decreased to three cycles, then recovered and approached stabilization. These results provide laboratory-scale evidence under the tested closed-system conditions rather than directly transferable pile-design parameters.