Study On The Mechanism Of Frost Heave And Salt Expansion Of Sulphate Saline Soil And Electrochemical Prevention Technology

Saline soil is widely distributed in the seasonal frozen regions of western China where the ambient temperature changes intensively.The change of ambient temperature induces the migration and phase change of water and salt,leading to severe frost heave and salt expansion which have caused serious damages to infrastructures,agriculture and ecotope in seasonal frozen regions.The frost heave and salt expansion of saline soil is an extremely complex physical and chemical process,and is affected by various factors such as soil texture,water and salt content,climate and human activities.Among the various ions contained in the natural salty soil,sodium sulfate expands significantly in volume after crystallization.Qinghai-Tibet Plateau is an indispensable part of “the Belt and Road”,climate changing dramatically,temperature signally differing between day and night,and the annual evaporation more than rainfall.Driving by the capillary force or soil water potential,the groundwater migrates to upper soil layers,and simultaneously drags sodium and sulphate ions for accumulation and crystallization,which causes serious damages to the existing roads and railways.Moreover,with the further advance of "the Belt and Road" initiative,an increasing numbers of infrastructures will be subjected to frost heave and salt expansion.In this context,how to reduce the damage of frost heave and salt expansion,serving the development of infrastructure and economy along “the Belt and Road”,is an urgent problem needs to be solved.Although researchers have made efforts in laboratory and theoretical studies,some inadequacies still exist in this field.For instance,salt alters the thermal properties of soil,such as thermal conductivity,affecting the frozen depth and frost heave amount.While,previous thermal conductivity models often regarded the pore solution as water,ignoring the influence of salt on soil thermal conductivity,which did not agree with practical situation.Furthermore,previous work focused on the saline soil expansion caused by water and salt crystallization,ignored the influence of crysuction and thermal contraction,and neglected the soil shrink during cooling process.In addition,although the conventional preventive measures can inhibit the frost heave and salt expansion to some extent,they are not adequate to treat existing subgrade,and the treatment depth and range are unsatisfactory because of the low soil permeability.Therefore,it is necessary to propose a highly efficient in-situ preventiontechnology.Based on the analyses mentioned above,the representative sodium sulfate saline soil originated from the Qinghai-Tibet Plateau was selected for laboratory test.By combining thermodynamics and crystallization kinetics,the soil thermal conductivity,freezing temperature and water-salt crystallization process were investigated.Further,the mechanism of shrink and expansion deformation of saline soil was revealed.Finally,the electrochemical prevention technology was proposed.The main studies and results can be summarized as follows:(1)Considering the influence of salt on soil's thermal behavior,the thermal conductivity models for sodium sulphate saline soil were established.Furtherly,by studying the variation of thermal conductivity with different temperature and freeze-thaw cycles,the effect of salt content,freeze-thaw cycles and natural temperature on soil's thermal conductivity were assessed.(2)One-side freezing experiments of sulfate saline soil with different salt contents and temperature boundary conditions were conducted to explore the characteristics of deformation in complex conditions.In addition,the frozen depth,frost heave amount,water and salt distribution,dry density and frozen morphology were also described,studying the influence of hydro-thermal-salt interaction on the deformation of frozen sulfate saline soil,and further expounding the mechanism that salt content alters the frozen depth and frost heave amount by changing the soil thermal properties.(3)A theoretical formula for calculating the freezing temperature of sodium sulfate saline soil was derived,which combined thermodynamics with Pitzer ionic model and considers the influence of pore structure,simultaneously.Afterwards,the dynamic processes of phase change of water and salt were described using classical nucleation theory and Arrhenius equation.The crystal amount and crystal pressure during freezing process were calculated subsequently.Considering the influence of water pressure,crystal pressure,crystal suction and thermal effect,the calculation formula of saline soil deformation was established,which can describe the thermal shrink and expansion deformation of saline soil.(4)An electroosmotic drainage model was established,and was also confirmed by laboratory testing results.The contents of sodium and sulphate ions in drainage and treatedsoil were tested to verify the migration of anion and cation under electroosmotic force,which provided a convincing evidence for electrochemical prevention.Combining the testing results and the advantages and disadvantages of electroosmosis,the treatment of electroosmosis incorporated with calcium chloride solution were proposed.The preventing mechanism was investigated in terms of drainage,mechanical strength and microstructure,verifying a relatively better treating measure.