Experimental study on the pore structure and permeability characteristics of clay soil under freezing–thawing cycles

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.