Influence of Fabric and Drying Shrinkage on Heat Transfer in Unsaturated Soils: Experimental and Microscale Numerical Study

The thermal conductivity of unsaturated soils is critical for thermal management in shallow energy infrastructures. Despite growing recognition that soil fabric and drying shrinkage can substantially affect thermal behavior, the underlying physical mechanisms remain poorly understood. This study combines laboratory experiments and numerical simulations to investigate these effects. Experiments were conducted on completely decomposed granite (CDG) and Nanyang clay (NYC), using reconstituted specimens (RS) and compacted specimens (CS) to represent different initial fabric conditions. Results show that RS consistently exhibits higher thermal conductivity than CS at the same saturation and void ratio. Along the drying path, CDG displays a monotonic decrease in thermal conductivity with decreasing saturation, whereas NYC exhibits a nonmonotonic trend, with conductivity increasing at intermediate saturation and peaking at low levels. To interpret these trends, a physics-constrained quartet structure generation set method was developed for pore-scale simulations using experimentally constrained structures and a simplified geometric representation of drying shrinkage. The method captures both the particle-aggregated, dual-porosity structure of CS and the uniform particle arrangement with unimodal pore size distribution found in RS, as well as their distinct fabric evolution induced by drying shrinkage. Simulation results show that RS achieves higher thermal conductivity through well-connected “solid particle chain” pathways, whereas CS depends on more tortuous “solid–liquid bridge–solid” paths caused by aggregates and larger interaggregate pores. The differing thermal conductivity trends in CDG and NYC are further attributed to their distinct shrinkage behaviors, which drive different fabric evolutions and thermal path reconstructions during drying. Furthermore, the anisotropy of shrinkage leads to distinct directional variations in conductivity, though its effect on the average remains limited.