A thermo-mechanical constitutive model for fine-grained soils under cyclic and monotonic loading

Geothermal systems and geostructures, as sustainable energy sources, undergo daily and seasonal temperature fluctuations that significantly influence their mechanical response. Reliable prediction of thermally induced deformations therefore requires advanced thermo-mechanical constitutive models. Existing approaches often address constant elevated temperatures but fail to capture multiple thermal cycles or the coupled effect of mechanical cycling under heating. This study presents a hypoplastic thermo-mechanical model enhanced with the extended intergranular strain anisotropy concept to reproduce small-strain behaviour. Experimental evidence shows that normally consolidated fine-grained soils, when subjected to repeated thermal cycles, exhibit a transition to an overconsolidated state after the first heating–cooling cycle. To capture this, the model introduces a temperature-dependent preloading surface, enabling the evolution of the three-dimensional overconsolidation ratio under thermal cyclic loading. In addition, the original viscous strain-rate mechanism at ambient conditions is preserved, ensuring a consistent representation of rate effects under coupled thermal and mechanical actions. The proposed model is validated against diverse thermo-mechanical loading paths, including monotonic and cyclic scenarios, across different soil types. The results demonstrate its capability to capture key aspects of the complex response of fine-grained soils under combined thermal and mechanical loading, indicating its potential applicability to energy geotechnical problems.