Abstract: Through unidirectional gradient cooling tests on indoor soil columns, we analyzed changes in temperature distribution, water replenishment, water redistribution, and frost heave deformation under two soil types. We also examined how median particle size affects interlayer water migration in coarse-grained railway subgrade fillers. The results indicate that altering the lower layer soil while keeping other conditions constant during unidirectional freezing results in a similar temperature change process and no significant difference in freezing depth under the same temperature boundary. Focusing on the median particle size d₅₀ of the upper and lower layers of soil, we propose the ratio F₅₀ of their median particle sizes to assess the impact of particle size variation between these layers on interlayer water migration. When the ratio of median particle sizes between the upper and lower layers falls within the range of 1.40−3.24, water replenishment is relatively limited, leading to water accumulation at the interlayer interface. With an increasing ratio, water replenishment decreases further, enhancing interlayer accumulation, impeding water movement from the lower to the upper layer, and reducing frost heave deformation in the soil column. Additionally, notable disparities exist in flow rate and density of water migration pathways between the upper and lower layers, with the upper layer exhibiting lower density and the lower layer higher density. Water migration is slower in the lower layer but faster in the upper layer. The extent of interlayer water accumulation is significantly influenced by the density of water migration pathways. Extreme ratios of median particle sizes between the upper and lower layers can result in water breakthrough across interlayers during upward water migration within the soil column.

Key words: crude granular packing, one-way freezing, double-layered soil, moisture migration