The porosity and permeability characteristics of clay are critical indicators for evaluating the stability of clay structures. Nuclear magnetic resonance (NMR) and standard variable head permeability tests (SVHPT) were conducted on Jilin ball clay to investigate the relationship between pore structure changes and permeability in seasonally frozen regions. This study analyzed how freeze–thaw cycles affect the microstructure and permeability properties of ball clay specimens across three groups of dry densities (1.60, 1.65, and 1.70 g/cm³) and two groups of saturation degrees (100% and 81.24). The experimental results revealed that freeze–thaw cycles substantially increased the void ratio in saturated specimens, whereas changes in void ratio under unsaturated conditions were negligible. The rate of void ratio change caused by freeze–thaw cycles increased with higher dry density, the growth rates of void ratio are 19.61%, 25.74%, and 33.23%, respectively. Under saturated conditions, freeze–thaw processes induce particle aggregation, leading to an exponential increase in permeability coefficients. This increase correlates with both the number of freeze–thaw cycles and the void ratio. Furthermore, the three-dimensional permeability coefficient surface model underscores the necessity of precisely controlling soil dry density and moisture content to effectively regulate the permeability properties of clay soil in seasonally frozen regions. These findings provide theoretical insights and empirical evidence essential for the design and construction of compacted clay soil in such regions.

A modified parallel model for estimating the thermal conductivity of unsaturated sand was proposed in this study. The heat conduction in the solid phase of sand depends mainly on the form of contacts between solid particles, while water bridges at the particle contacts increase the contact areas and remarkably enlarge the transfer paths of heat conduction in sandy soils. However, the thermal conductivity of the solid particle itself (λ_s) cannot describe the influence of the form of contacts and water bridges on heat conduction through the solid phase. In this study, the equivalent thermal conductivity of the solid particle (λ_es) was presented which reflected the influence of the form of contacts and water bridges between particles under dry conditions or a low degree of saturation, respectively. The relationship between λ_es and degree of saturation was described by hyperbolic expression. The modified model was calibrated using measured values of the thermal conductivity from published datasets, including those for 41 types of sand from 15 studies. Numerical analyses of the temperature field of the energy pile were performed and validated against laboratory measurements. The results illustrated that the modified model was more applicable than the original model for predictions of sand thermal conductivity.

To investigate the distribution and evolution characteristics of the internal temperature field in a geosynthetic-reinforced soil (GRS) retaining wall, this study conducted a model test of a GRS retaining wall under temperature cycling based on the climatic conditions of the Taklimakan Desert, and established numerical model. Following validation of the numerical model, a full-scale model considering solar radiation effect was further developed to analyze the evolution characteristics of the temperature field inside the wall over five annual temperature cycles. The results show that the poor thermal conductivity of dry aeolian sand causes a lag in the extreme temperature values at monitoring points relative to the ambient temperature, with the lag time increasing as the number of cycles accumulates. As the number of temperature cycles increases, both the vertical frost depth and horizontal frost thickness within the wall gradually increase. During a complete annual cycle, the temperature field transitions from hyperbolic to frozen core and back to hyperbolic distribution as ambient temperature rises. Conversely, it shifts from hyperbolic to heated core and back to hyperbolic distribution during cooling. The temperature curves on horizontal cross‑sections within the wall can be used to identify temperature‑sensitive zones, though their specific characteristics are influenced by factors such as ambient temperature and thermal parameters of the backfill material. This study provides a theoretical basis and data support for the design and maintenance of reinforced soil retaining walls in desert regions.