Effect of Temperature on the Cyclic Behavior of Clay–Structure Interface

The shaft capacity of foundations highly depends on the monotonic and cyclic loads applied to the soil–structure interface. In energy geostructures that exploit the heat of soil using earth-contact elements, the interface is subject to cyclic thermomechanical loads. Monotonic and cyclic constant-volume equivalent-undrained (CVEU) direct shear tests were performed on clay–clay and clay–structure interfaces at different temperatures (22°C and 60°C). An effective vertical stress of 300 kPa was applied to the samples and the cyclic and average shear stress ratios (${\tau }_{cy}/S_u^{Ds}$ and ${\tau }_a/S_u^{Ds}$, respectively) were varied between 0.35 and 0.57. The tested soil was a kaolin clay (plasticity $\mathrm{index}=24$) prepared in a normally consolidated state. The results showed that the number of cycles to failure for the clay–structure interface test was lower than that for the clay–clay case in the same range of cyclic and average shear stress ratios. In cyclic clay–structure tests, decreasing the cyclic stress ratio increased the number of cycles to failure; however, decreasing the average shear stress ratio decreased the number of cycles to failure. Increasing the temperature decreased the rate of strain accumulation, and the number of cycles to failure increased by two to three times. The rate of degradation (degradation parameter, $t$) decreased by 16% with heating from 22°C to 60°C for the different cyclic stress ratios tested.