| With the implementation and "China’s Western Development Policy" strategy,"the Belt and Road" initiative,"a country with strong transportation network" and other policies,a large number of infrastructure will emerge in China’s western regions,and the Hexi Corridor is an important transportation hub through the west,which is widely distributed with salinized soil.In addition,the special climatic characteristics lead to the migration and redistribution of water,steam and salt in saline soil,the deformation of frost heave,salt expansion,thaw collapse and dissolution sedimentation,which seriously affect the durability and safe operation of buildings and structures.Scholars have made a preliminary exploration on the coupling effect of water,heat,salt and force in saline soil,and some achievements have been made.However,the existing four-field coupling model of saline soil is still imperfect,which is unable to accurately predict and scientifically evaluate the frost heaving and salt expanded deformation of seasonal frozen soil projects,which has become the bottleneck problem restricting the construction and safe operation of saline soil projects.Therefore,the paper aims to carry out research on water-heat-salt-steam migration and salt expansion and frost heave deformation mechanism of sulphate saline soil in Hexi Corridor to provide scientific basis and theoretical support for project construction and safe operation in this area.In this paper,experimental,theoretical and numerical studies are carried out on the water-heat-salt-steam migration and the deformation mechanism of salt frost heave in unsaturated saline soil.The water-heat-salt-force coupling mechanism of unsaturated saline soil is analyzed,and the water-heat-salt-force coupling model of unsaturated saline soil is established.The calculated temperature,water content,salt content,frost heave and salt expansion displacement are consistent with the measured results.The results show that the model can reflect the water-heat-salt-force coupling process in the freezing and freeze-thaw cycles of unsaturated saline soils.Finally,the water-heat-salt-force coupling model is applied to unsaturated saline soil roadbed engineering,and the heat and mass transfer behavior and the law of frost heave and salt expansion deformation are analyzed and predicted.The main research results are as follows:(1)In different seasons,the saline soil in Hexi Corridor was sampled at multiple points and at different depths at the same point.The physical and chemical properties and structural characteristics of the saline soil were analyzed through laboratory tests,and the influences of salt content,water content and temperature on the mechanical indexes of the saline soil were explored,and the influencing mechanism was revealed.The results show that the saline soil in the Hexi Corridor is a medium saline soil,which is a typical sulfate saline soil and sulfite saline soil.It belongs to the low liquid limit silty clay with poor gradation.The pores are unevenly distributed,mainly small pores,and salt crystals are adsorbed on the surface of soil particles.The pore solution concentration of saline soil will change the thickness of double electric layer,which will affect the distance between particles and the canopy effect of saline soil.Temperature and water content directly affect the ice content in saline soil,and ice crystal plays a role of "reinforcing" in soil grain skeleton.(2)Through unidirectional freezing and freeze-thaw cycle tests of unsaturated saline soil,the law of heat-mass transfer and salt frost heave deformation in saline soil is clarified.The latent heat of crystallization,crystallization impedance and migration behavior of water in gaseous phase were revealed.The formation process of crystalline salt and the structure damage mechanism under freeze-thaw cycle were investigated by means of scanning electron microscope.The results show that the temperature fluctuation in saline soil under unidirectional freezing is caused by crystallization latent heat.The top of the saline soil sample was sealed,and the top moisture content showed a maximum value,showing an obvious "canopy effect",which proved the migration behavior of gaseous water.Repeated freezing and thawing cycle under the action of saline soil through a series "freeze-condensation-crystallization and melting-dissolved" pore structure are increased after the procedure,and formed through the cracks,by freezing and thawing soil structure in front of the layered structure into flocculent structure.(3)Based on the Darcy law of unsaturated soil,law of conservation of energy,law of conservation of mass and theory of soil mechanics,the latent heat of phase change,crystal consumption,crystal impedance,self-purification behavior of ice crystals,migration behavior of gaseous water and the effect of mechanical parameters on salt content,water content and temperature are comprehensively considered.The coupling model of water,heat,salt and force in unsaturated sulphate saline soil was established.Heat transfer,water-steam-salt migration redistribution,crystal phase transformation behavior and the deformation mechanism of salt expansion and frost heave in unsaturated saline soil are analyzed theoretically.(4)COMSOL Multiphyscis was used to simulate the coupling model established in this paper and compare and analyze the results of unidirectional freezing and freeze-thaw cycles of unsaturated saline soil.The effectiveness of the coupling model was verified.The changes of the temperature field,water field,salt field and displacement field of soil in the freezing and freeze-thaw processes were mainly studied.The formation process of freezing front,ice crystal and salt crystal is described,which avoids the limitation of measurement technique.The results show that the temperature gradient can aggravate the water-vapor-salt migration behavior,and the solute potential can inhibit the water-salt migration.Unidirectional freezing results in layered distribution of water and salt content in space.The salt crystals produced in the pores of saline soil consist of supersaturated crystals and self-purifying crystals of ice crystals.Under freeze-thaw cycle,liquid water,ice crystals,salt concentration,salt crystals and the deformation of salt expansion and frost heave all show a periodic increasing law with temperature.It can be seen from the analysis that the water-vapor-salt migration provides the material basis for the freeze-thaw cycle process under the condition of no pressure recharge.(5)The water-heat-salt-force coupling model proposed in this paper is applied to the actual saline soil subgrade engineering to analyze and predict the heat-mass migration and frost heaving and salt expanded deformation of saline soil subgrade under the seasonal temperature variation and sunny-shady slope effect.The results show that the temperature of saline soil subgrade in Hexi Corridor reaches the lowest in January.The maximum freezing depth in February was 1.35 m on the shady slope.The freezing period of the shady slope is 2months longer than that of the sunny slope.During the freezing period,the monthly deformation of the saline soil subgrade presents an asymmetrical distribution,and the frost heaving and salt expanded displacement on the shady slope is > the frost heaving and salt expanded displacement on the sunny slope > the frost heaving and salt expanded displacement on the subgrade surface.In February,the frost heaving and salt expanded deformation of the shady slope of the saline soil subgrade was the largest,which was31.72 mm.In March,the difference of transverse displacement between sunny-shady slope of saline soil subgrade was 18.49 mm.The uneven frost heaving and salt expanded deformation of the sunny-shady slopes of the saline soil subgrade leads to longitudinal cracks in the pavement. |
PDF Full Text Request