Thermally induced displacements and post-thermal shear behavior of soil-structure interfaces using a modified triaxial apparatus

The mechanical behavior of energy pile–soil interfaces is significantly influenced by temperature variations, especially under cyclic thermal loading. This study presents an experimental investigation using a modified triaxial apparatus to evaluate the thermally induced structural displacement and subsequent shear behavior of sand–structure interfaces under in-service stress conditions, simulated by applying an initial shear stress. Three representative thermal cycles between 6 and 4 °C, two interface roughness conditions (smooth and rough), and two types of vertical loading: constant vertical stress and constant vertical stiffness were applied. The results show that cyclic thermal loading induced irreversible cumulative downward displacement along the shear direction, which tended to stabilize after several cycles. Displacement accumulation increased with temperature amplitude, and rough interfaces exhibited significantly larger value than smooth ones. Thermal displacement was also substantially lower under constant vertical stiffness conditions compared to constant vertical stress. Although peak shear strength remained largely unaffected by thermal cycling, interface shear stiffness increased significantly in all cases. For rough interfaces, this increase in stiffness markedly reduced the displacement required to reach peak shear stress, to approximately 33–80% of the corresponding value at room temperature. In contrast, this reduction was minimal for smooth interfaces.