Thermally induced displacement of soil–structure interface at saturated and unsaturated conditions

The behaviour of soil–structure interfaces under cyclic heating–cooling is crucial for analysing the serviceability limit state of energy geostructures (e.g., energy piles). To date, there has been only one laboratory study investigating thermally induced displacement at soil–structure interfaces under saturated conditions and limited stress conditions. The current work conducted comprehensive heating–cooling tests on saturated and unsaturated interfaces under constant normal and shear stresses using a new direct shear apparatus. The interface shear and normal displacements were measured during thermal cycles in the temperature range of 8 to 42 °C under various net normal stresses, suctions, and shear stresses. The results indicate that the forward displacement (i.e., movement in the same direction as increasing shear stress) induced by cooling is about ten times larger than the displacement induced by heating. This suggests a more critical relative movement of geostructures under cooling, likely due to the reduction of interparticle contact force and collapse of strong force chains. The irreversible forward displacement at 100 kPa net normal stress and 200 kPa suction is 60% smaller than that at 50 kPa net normal stress and 0 kPa suction. Moreover, the relationship between irreversible displacement and shear stress ratio (i.e., the shear stress normalised by the shear strength) suggests the existence of a threshold value of shear stress ratio (i.e., around 0.5 for the tested interface), above which there is significant irreversible movement in this study. It is important to maintain the stress ratio at soil–structure interfaces below the threshold value to minimise relative movement.