Experimental Research On The Frost Heave And Thaw Settlement Of The Remolded Saline Soil

The west of Jinlin province locates in salinized zone.The amount of sodium bicarbonate is so high that the saline soil in this area belongs to carbonate saline soil.Because of the special climatic and terrain conditions,and the higher mineralization of shallow groundwater,the salinization of soil is serious in the research area.The west of Jilin province is the typical seasonal frost region,the climatic conditions of hot summer and severe winter,makes kinds of constructions influenced by frost heave and salt expansion of the Saline Soil in this area.The frost heave diseases were common plaguing the engioneering works in this area,such as the soil at embankment slope toe or a water diversion and irrigation project often heaves and cracks under the action of carbonate and freezing.And the projects of salinized soil in this area were also influenced by frost heave and thaw settlement,which directly affect the stability of projects.For example,the freezing-thawing slump of roadbed slopes,the thawing settlement of subgrade body,in sufficient bearing capacity of the saline soil base under freezing-thawing.These diseases impact on the development of engineering construction and have become a key of success or failure insaline soil engineering.Threrfore the research on frost heave and thaw settlement of carbonate saline soil with different influencing factors has become the urgent problem.In this paper,the soil samples in one sampling point collected from Nong'an county of Jilin procince were taken as the research object.The basic physical and chemical properties of soil samples were analyzed.Understanding the basic characteristics of soil samples to determine the the magnitude of the water content,the compactness,the salt content and the temperature for the design of the frost heave and the freezing-thawing experiments.Then design the frost heave experiment of closed system based on the tempareture and groundwater recharge of the soil samples from various depths,to explore the frost heave characteristics of saline soil with different water content,the compactness,the salt content and the temperature.Then,two different kinds of mathematical methods were applied,including the back propagation neural network?BPNN?and support vector machine for regression?SVR?.The calculated results of BPNN and SVR were compared with the experimental data,and the analysis of sensitivity was carried out with various factors.And the freezing-thawing experiment was designed to analyse the characteristics of frost heave and thaw settlement of the Saline Soil.Finally,a coupled mathematical model of water,heat and salt was established and the simulation results were verified.The main researches in this paper were as follows:1.The physical and chemical properties of saline soil in Nong'an were analyzed.From the material point of view,the study area soil samples are mainly consisted of the silty clay,which the clay particles contents are greater than 30%.The main mineral components of soil samples are feldspar and quartz.There were also had smaller amounts of clay minerals such as illite and kaolinite.The natural water content of soil samples were relatively high,the plasticity indexes were greater than36.The soils below the 20 cm depth were highly liquid limit clay.The total soluble salt contents of different depths were 1%²%.According to soluble salt components,major cation was Na⁺,the anions were SO₄^2-,HCO₃^-and Cl^-,Na⁺content was very high.The organic matter and cation exchange capacity were relatively high,the pH values were 7.2⁸.0.The pores of soil samples were large pore based by mercury injection experiment,and the large pore percent content increased with the depth of soil layer increase.2.In the frost heave experiment of closed system,water content,compactness,salt content and temperature were selected variates of experiment.In the previous stage of dropping temperature,the variation of frost-heaving ratio was small above2.5?,and all the soil samples were in a contractive condition.The frost-heaving ratio changed very rapidly from-2.5?to-10?.Continue to dropping the tempareture,the frost-heaving ratio changed little.Lower water content contracts the soil samples and higher water content led to the samples frost heave.With the compactness increasing,the status of soil samples from contractive condition changed into the frost heave.And the lower salt content promoted frost heave while the higher salt content suppressed frost heave.3.Both BPNN and SVR prediction models can be established due to their powerful nonlinear mapping function.In this paper,two models were used to predict the frost-heaving ratio for the exprerience of closed system,and the precision of two models are contrasted and analyzed.The result showed the precisions of both two models are high,and the SVR model was superior to the BPNN model.The sensitivity analysis was carried out with four factors which influenced the frost-heaving ratio of saline soil.The result indicated that the importance of the four factors for the models were water content,temperature,compactness,salt content,respectively.4.In the freezing-thawing experiment of opened system,the maximum of frost-heaving ratio increased with the intial water content.Although the intial water content increased suppressed water transfering,the average water content of sample increased with the intial water content increasing.Increasing the compactness led to the dry density of soil samples increased and the porosity decreased,which was not good for the water migrating from bottom into the soil samples.Due to the frost heave in situ impacted the total frost-heaving hugely,the maximum of frost-heaving increased with the compactness increasing.With the salt content increasing,the frost heave decreased slightly.This is because the freezing point of soil solution decreased due to the increased salt content,the frost heave was suppressed of soil sample in a temperature range whne the tempareture dropping.With the tempareture of the cold end dropping,the maximum of frost-heaving increased.And the thaw settlement increased with the frost-heaving depth increasing when the soil samples thawed.The frost-heaving and the thaw settlement increased with the times of the freezing-thawing increasing.5.Because of the linearly dependent of time and tempareture in freezing-thawing experiment of opened system,using the correlation of time and tempareture to conduct the numerical simulation analysis for the change rule of water field and tempareture field,and get the predict results of frost heave and thaw settlement at different time.So in the chapter 6 of this paper,a coupled mathematical model of water,heat and salt was established by the the combination of moisture,heat,salt,basic migration equation,mass continuity equation,energy conservation equation and balance differential equation,to simulate each variable during the whole experiment.Using COMSOL Multiphyscis PDE equations in mathematical module interface to build physical fields are applied to solve the coupling mathematical model.Using COMSOL Multiphyscis PDE equations in mathematical module interface to build physical fields water,temperature and frost heave of soil sample in the freezing-thawing experiment of opened system are applied to solve the coupling mathematical model,and analyze the rules and reasons of water field,temperature field and displacement field.Finally,compare the predicted value with the test value of frost heave and thaw settlement in the freezing-thawing experiment,to verify the validity of the model.