Experimental research and numerical simulation on frost heave characteristics of silty saline soil
In cold and saline soil environments,water-salt wetting-drying cycles and salt-freeze-thaw cycles can cause damage and failure to rock and soil structures. The complex coupling of heat and mass transfer with salt-soil frost heaving characteristics presents significant challenges. Understanding the frost heaving and salt expan-sion characteristics of sulfate saline soil under freezing conditions is essential for engineering projects and under-ground structures in saline soil areas. This study investigates the water-salt migration and frost heaving character-istics of silty saline soil under unidirectional freezing in an open system through experimental and theoretical re-search. Saline soil is a multiphase continuous porous medium composed of solid,liquid,and gas phases. Changes in water-salt content in the soil occur with temperature variations,leading to water-ice phase changes and salt-crystal phase changes,resulting in frost and salt expansion,thus generating frost heave force. The varia-tion of frost heaving force in silty saline soil under unidirectional freezing conditions with changes in lower-end temperature,initial moisture content,salt content,and overlying load is examined,providing insights into the soil's structure and properties. We prepared silty saline soil samples containing sulfate and conducted tests using a self-developed frost heave experimental setup under unidirectional freezing conditions. Waterproof tempera-ture sensors and pressure sensors were used to monitor temperature and frost heave force,respectively. To delve deeper into the frost heave mechanism of silty saline soil,a three-dimensional equation for water,heat,and salt forces suitable for this soil type was established. The coupled equation was based on unsaturated soil seepage,heat conduction theory,and hydrodynamic models,following the laws of mass and energy conservation,consid-ering the effects of salt-water migration and phase change. The stress field equation was simplified from the salt expansion and frost heaving mechanisms. The PDE module of COMSOL Multiphysics software was used to cre-ate a coupled model of indoor unsaturated saline soil. In the numerical simulations,initial conditions such as soil moisture content,salt content,and temperature were set. Then,the distribution of temperature,moisture con-tent,salt content,and frost heaving force during the freezing process was iteratively calculated based on the model's equations. The numerical simulation results were compared with experimental data to validate the mod-el's effectiveness. From this study,several conclusions were drawn. Firstly,the temperature variation patterns of soil at different freezing temperatures were consistent,with lower freezing temperatures resulting in lower fi-nal and overall average temperatures,and a linear change in temperature stability values with depth. Secondly,lowering the lower-end temperature and increasing the initial moisture content promoted frost heaving force gen-eration,while increasing overlying load and soil salinity suppressed frost heaving force. This is because lower freezing temperatures lead to higher temperature gradients during the freezing process,dominantly promoting the increase in water migration,thus facilitating frost heave force generation. Simultaneously,the stress in the fro-zen soil is mainly induced by the frost heave of in-situ water and migrating water,so higher initial moisture con-tent results in greater frost heave force. An increase in salt content lowers the freezing temperature of the soil,prolonging the initiation of frost heaving. When the temperature drops below the crystallization temperature,the crystallization of ice and salt further affects the rate of water migration,inhibiting the generation of frost heave force. Under external loads,the reduction in pore size between soil particles inhibits water migration and hinders the expansion deformation caused by ice-water phase change,making it more difficult for the entire soil to under-go frost heave deformation and thus suppressing the generation of frost heave force. Lastly,the steady-state frost heaving force of silty saline soil exhibited a linear relationship with freezing temperature and a quadratic parabol-ic relationship with initial moisture content,overlying load,and salt content. This research provides a new meth-od for understanding the water-salt migration and frost heaving characteristics of silty saline soil during the freez-ing process. This method can contribute to a deeper understanding of the mechanisms of saline soil damage in cold regions and aid in the selection of appropriate preventive measures.
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