A Unified Theoretical Thaw Expansion and Settlement Coefficient Model for Frozen Soil

Thaw settlement and expansion of frozen soil pose significant risks to railway construction and maintenance in cold regions, particularly due to the greenhouse effect and extreme climate events. To address the thaw settlement and expansion of silt, coarse-grained materials, and weathered rock permafrost, a unified theoretical model for the thaw expansion and settlement coefficient has been developed. This model considers mass conservation and volume consistency at both the beginning and end of thaw settlement. It uses initial water content and initial dry density as fundamental indices while introducing four new indices that account for the initial moisture phase and mass distribution, moisture retention capacity, overall expansion degree of the particles and skeleton, and the soil skeleton structure’s ability to maintain stability in permafrost. Analysis using this model on measured and collected data sets reveals the following findings. (1) Permafrost of a specific classification, despite originating from different regions, exhibits a common characteristic in which moisture retention capacity decreases linearly with the thaw settlement coefficient when volume changes induced by various factors operate at full capacity. However, for specific types of permafrost, moisture retention capacity varies based on the fine grain content. (2) When soil particles and the skeleton expand, even with the same structural integrity, moisture retention capacity does not maintain a linear relationship with the thaw deformation coefficient. (3) If there is no expansion of soil particles due to thermal changes or of the skeleton due to thermal changes and prestress release, and if moisture drainage does not occur, thaw deformation is primarily attributed to moisture phase change. The final thaw expansion or settlement results from a compromise between moisture drainage and the phase change from ice to water, which contributes to settlement, and the expansion of particles and skeleton, which contributes to expansion. The interactions among the expansion of soil particles and skeleton, moisture change, and structural integrity collectively influence the final thermal deformation. (4) A lower absolute thaw deformation coefficient consistently corresponds to a strong soil structure maintenance property. Overall, the model provides a theoretical and unified framework for describing thaw expansion and settlement, although further research is needed to refine the determination of the indices. Currently, probability tables for specific thaw expansion and settlement coefficients have been proposed.