Study on water and salt migration and deformation characteristics of coarse-grained sulfate saline soil
The thermo-mass transfer and salt heave and frost heave characteristics of saline soil under tempera-ture gradient are complex multi-physical field coupling problems.Understanding the salt heave and frost heave characteristics of sulfate saline soil under freezing conditions is crucial for soil salinization control,building dura-bility and underground structure in sulfate saline soil areas.In order to explore the water-heat-salt-mechanics cou-pling mechanism of coarse-grained sulfate saline soil under freezing conditions,this paper conducts theoretical and experimental research on the water and salt migration and deformation characteristics of coarse-grained sul-fate saline soil in a closed system during the freezing process.Saline soil is a multiphase continuous porous medi-um composed of solid,liquid and gas phases,and the water and salt content inside the soil will undergo phase change with the variation of temperature,which will lead to the change of the volume of the soil,thereby caus-ing the deformation of the soil.The analysis of unidirectional freezing of sulfate soils involves the coupling ef-fects of water and salt migration,heat transfer,ice-water phase transition,salt crystallization-dissolution,and soil deformation.Based on the thermo-elastic theory of unsaturated porous media and considering the phase tran-sition of pore water and salt,a multi-field coupling model for water-heat-salt-mechanical fields in coarse-grained saline soil under unidirectional freezing condition was established.Numerical simulations were carried out for temperature,water content,salt content and displacement distributions in coarse-grained saline soil under unidi-rectional freezing condition.In the numerical simulation,we first set the initial conditions of the soil,including the water content,salt content and temperature of the soil.Then,according to the equations of the model,we obtained the distributions of temperature,water content,salt content and displacement of the soil during the freezing process through iterative calculations.Indoor tests were conducted by preparing fine sand with sulfate as soil samples under unidirectional freezing conditions.Temperature and water content distribution during the freezing process were measured using temperature and humidity sensors,and deformation of the soil was mea-sured using displacement gauges.The results were compared with numerical results to verify the effectiveness of the theoretical model.The following conclusions are drawn from this paper.First of all,Through the compari-son of SWCC fitting models for clay and sand,it was found that due to the larger particle size of sand and the larger pore structure of soil,water and salt can penetrate and migrate more easily.In the unidirectional freezing test,the migration rate of water and salt is faster.Secondly,during unidirectional freezing,the axial deforma-tion characteristics of fine sand and clay are the same,showing a trend of first decreasing and then increasing,that is,the soil column will first undergo significant shrinkage deformation.Unlike clay,the duration of shrink-age deformation of coarse-grained saline soil is longer.Thirdly,the pressure exerted on the side walls of the soil column decreases as the height increases.This is due to the migration of water content from the warm end to the cold end,resulting in an increasing number of ice crystals at the cold end.The mass of the upper soil body in-creases,causing the lower soil body to be compressed.As a result,the porosity of the lower soil body decreases and its density increases,ultimately leading to a higher side wall pressure at the lower end compared to the upper end.At last,our research provides a new method for understanding the water and salt migration and deformation characteristics of coarse-grained sulfate saline soil during freezing.This method can not only be used for theoreti-cal research,but also provide reference for deeper understanding of the disease mechanism of saline soil in cold regions and selecting appropriate prevention measures.
coarse-grained saline soilmulti-field couplewater salt migrationdeformation characteristics
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