Coupled Hydro-thermo-deformation Model Of Expansive Soil During Frost Heave

China has the third largest frozen soil area in the world,and the seasonally frozen areas reaching 4.46×10~6 km~2 with frost depth exceeding 0.5 m have important impact on the project.Most of them distribute in the Northeast China,most areas of Inner Mongolia and parts of the Western China.In the meantime,China is one of the countries with the expansive soils distributed widely.It contains more than 20provinces,cities and autonomous regions.Nowadays,China has entered a new era of accelerating the development of high-speed rail transit.Engineering practices and disease investigations show that there is a large area of residual Deluvial expansive soil in the seasonally frozen soil region of China.These areas are rich in groundwater with a shallow level.Because of its significant swelling and shrinkage characteristics,frost heave characteristics,expansive soil in seasonally frozen soil region may cause engineering diseases.Therefore,this study takes the deep residual deluvial expansive soil along the Jilin-Tumen-Hunchun high-speed railway and Harbin-Jiamusi high-speed railway project in seasonally frozen soil region as a study object.Main content,method,and results are as follows.According to the frost heave test characteristics of expansive soil,an open-system frost heave,thaw settlement,swelling and shrinkage tests equipment for expansive soil is developed.The optimization design and reliability evaluation of the freezing chamber,temperature control system,water supply and drainage system,unidirectional loading system and data acquisition system are carried out.A kind of annular freezing chamber with tensile length is proposed,and a high sensitivity Mariotte bottle based on hydrostatic pressure monitoring is developed.Then,suggestions of the test methods and standardized procedures for the frost heave and swelling-shrinkage test of expansive soil are proposed.Focusing on the basic characteristics and differences between north and south expansive soils,a series of tests on substances,basic properties,water retention characteristics,thermos-physical properties,swelling-shrinkage properties are carried out.Comparing with the southern expansive soils,it shows that montmorillonite with poor crystallinity is the main clay mineral of expansive soil in deep seasonally frozen soil region,which the content of clay particles less than 2?m is less than 20%.The liquid plastic limit index is generally lower than that in South China,and the residual water content is relatively higher.The relationship of pore water content,conductivity and suction of Yanji expansive soil is deduced.The model of thermal conductivity of Yanji expansive soil under frozen and unfrozen conditions is proposed.The applicability of Briaud model in swelling deformation prediction of Yanji expansive soil is verified.The engineering geological differences between north and south expansive soils are compared.It shows that the expansive soil in Yanji basin in seasonally frozen soil region is mainly composed of medium and weak expansive soil,which was formed earlier than that of southern expansive soil.It has experienced complicated weathering,erosion and sedimentation in history.Effect of extreme freeze-thaw and dry-wet cycles aggravates the weathering and decomposition of original rock.In the light of the pore-water thermo-effect characteristics of freezing expansive soil,the freezing temperature tests of expansive soil are carried out.It shows that the pore water freezing point of expansive soil varies greatly with the initial water content,and is relatively lower with a higher saturation.Based on the concept of water activity,the method for predicting pore-water freezing point of expansive soil is proposed.Meanwhile,the differential scanning calorimetry tests of expansive soil are carried out.It shows that the heterogeneous nucleation occurs at the ice-water interface during pore-water crystallization,and the growth type of pore-water crystal is spherical crystal three-dimensional growth.Then,a crystallization kinetics model is proposed to describe the freezing process of pore water in expansive soil.In the light of the frost heave characteristics of expansive soil,frost heave tests of expansive soil under complex boundary conditions are carried out.The time history of temperature field,frost heave deformation and freezing front variation,water migration and cyrostructures,state path of expansive soil under freezing state are analyzed.The results show that the temperature time history of freezing expansive soil includes fast cooling stage,supercooling stage,slow cooling stage and stable cooling stage.The freezing front propagation is significantly influenced by the pore water content and the water redistribution area is relatively large.It is found that the wet expansion and dry shrinkage characteristics of expansive soil have a certain influence on its frost heave deformation characteristics.It can be clear that the frost heave deformation of expansive soil mainly includes clay mineral expansion and migration frost heave in a larger area.However,the in-situ frost heave and frost heave caused by ice lens growth are not significant.According to the state path,the water content variation in melting stage is larger than that in freezing stage,which will cause short-term and large swelling effect of clay minerals.The coupling mechanism of frost heave,swelling and shrinkage of expansive soil considering the dynamic phase change zone is proposed.Based on the coupled hydro-thermo-deformation analysis method of frozen soil,considering the effect of pore-water state on the freezing point,and the prediction method for swelling deformation of expansive soil,a coupled hydro-thermo-deformation frost heave model of expansive soil,named FH?ex?Model,is established.In this model,uncoupled algorithm is used to calculate the phase transition process.Crank-Nicolson scheme is used for spatial discretization,and forward difference method is used for time discretization.Then,the influence of wet expansion and dry shrinkage characteristics on frost heave characteristics of expansive soil can be described.The frost heave component and swelling-shrinkage deformation component are decoupled from the freezing process of unsaturated expansive soil.The research achievements further deepen the understanding of coupled hydro-thermo-deformation frost heave mechanism of expansive soil.It is of guiding significance for engineering disease analysis of expansive soil site under multi-factor coupling effect in seasonally frozen soil region.