| Frozen soil is a multiphase complex composed of soil particles,ice,liquid water and gas,in which water is transformed between solid and liquid phases under the influence of ambient temperature,resulting in strong temperature sensitivity and rheology of the mechanical properties of frozen soil.This mechanical property of frozen soil leads to frost heave and thaw settlement of subgrade soil,which brings great technical problems to the engineering construction in cold regions.The frozen soil area in China accounts for about75% of the land area,of which the seasonal frozen soil accounts for 53.5% of the land area.In recent years,more and more large-scale engineering projects in China will be carried out in the frozen soil region,which has important guiding significance for the experimental and numerical simulation research on the moisture and heat transfer and the stress-strain law in the process of frozen soil thawing.In this paper,a small-scale indoor soil freeze-thaw experiment platform was built,and the quartz powder was used as the experimental sample to study the mechanism of water-heat transfer and settlement characteristics in the process of frozen soil thawing.The results show that there are two phase change fronts in the process of soil thawing,of which the upper phase change front has fast migration speed and long migration path,which is the main reason for the temperature change in the soil.The rate of soil thawing settlement first increases and then decreases,which is related to the temperature change,and there is a lag effect with the temperature change.With the decrease of soil particle size and the increase of thawing temperature at the top of the sample,the displacement and speed of soil thawing settlement increase significantly.Based on the heat and mass transfer differential equation and Darcy’s percolation law,a coupled thermal and mass field model of frozen soil thawing is established.The numerical solution is carried out through the PDE module in COMSOL multiphysics to study the characteristics of water and heat transfer in the process of frozen soil melting without water source.The results show that the migration velocity of the phase change front is mainly affected by the thawing temperature,and has a nonlinear relationship with the temperature increase.With the increase of the thawing temperature,the migration velocity increases more and more slowly;The migration rate of pore water is mainly affected by the soil particle size.The larger particle size,the more obvious the gravity effect,and the faster migration rate.Based on the established thermal-mass coupling model and one-dimensional large strain thawing consolidation theory,a thermal water mechanical three field coupling model for frozen soil thawing is established.The numerical solution is carried out through the PDE module in COMSOL multiphysics to study the characteristics of water-heat transfer in the process of frozen soil thawing with water source.The results show that the existence of segregation ice slows down the migration of the two phase change fronts,especially hinders the migration of the lower front;the increase of excess pore pressure caused by the sudden change of soil mechanical properties due to phase change is the main driving force of water migration,and the migration speed of pore water is much faster than that under the condition of without water source;water seepage from the bottom and surface of soil is the direct cause of thawing settlement. |
