| Heilongjiang Province,as a typical deep seasonal frozen soil area in China,is located in the northernmost and easternmost part of China.The climate is extremely cold and changeable,which leads to the complex and frequent changes of soil moisture and temperature.The agricultural channels,roads and bridges,roadbed and other basic projects are affected by frost damage,which has a negative impact on the development of agriculture,water conservancy and many other aspects.In order to study the frost heave characteristics of soil in the cold season,this paper takes Heilongjiang water conservancy science and Technology Experimental Research Center as the experimental area,and conducts in-situ monitoring on the dynamic change process of soil freezing thawing from winter 2018 to summer 2019,so as to obtain the nature of important soil parameters,such as regional temperature change,heat flux conduction,soil water,heat change process and the lag effect of soil temperature response temperature characteristics of change.Take the soil samples in the field and conduct the closed system freezing test on the indoor remolded samples,study and analyze the laws of soil freezing characteristics and physical characteristics changes,and discuss the mechanism of soil frost heave failure,the change of water migration and the influence degree of different influencing factors on the physical characteristics of soil.The main research results are as follows:(1)Based on the in-situ measurement results,the dynamic balance of water and heat in the process of soil natural freeze-thaw cycle,the time lag change of soil temperature response to air temperature,and the change rules of soil,temperature,water and other important parameters are revealed: the overall air temperature in the test area has a sinusoidal change trend;the vertical contour change of soil temperature conforms to the typical process of "one-way freezing,two-way melting",from November From the beginning of the year to April of the next year,the freezing and thawing cycle process was completed,with the maximum freezing depth of 135 cm;the time lag of soil temperature response to temperature change occurred,the vertical depth increased(0?200 cm),the maximum lag correlation R value increased,the corresponding correlation coefficient decreased,gradually approached linear change,and the lag time of spring thawing period was relatively longer than that of freezing period;the change process of water field was divided into rapid cooling In the four stages of constant negative temperature,warming and melting,and continuous warming,the change trend of unfrozen water content in surface soil is relatively most obvious,and the change trend of increasing soil depth is gradually weakened,there is a lag effect;in the period of surface heat flux,the positive and negative values alternate,that is,the direction of heat transfer is opposite,the amplitude of heat flux disappears,and the soil gradually enters the frozen state.(2)In the closed system,the frost heave response characteristics of remolded silty clay samples under the influence of different factors such as moisture content,dry density and freezing temperature are studied.The freezing process is controlled and simulated by the soil frost heave tester,and the single factor analysis of soil frost heave variation under different boundary conditions is carried out.The variation characteristics are as follows: the change of soil temperature field can be divided into rapid There are three stages of slow decline and steady change.The farther away from the top,the longer the time for the temperature to enter the steady stage;the lower the temperature of the cold end,the shorter the time for the internal temperature field to enter the steady stage;the development and change of the freezing front are divided into three sections: rapid,slow and stable freezing.The depth of soil freezing increases with the decrease of the temperature of the cold end,and the time for entering the stable stage is shortened;different under the conditions of freezing temperature,moisture content and dry density,the temperature gradient shows three changes: growth,decline and tend to be stable.The lower the freezing temperature is,the shorter the time to reach the peak value is,and the larger the temperature gradient peak value is,the less the influence of moisture and density factors is;the lower the cold end temperature of the soil is,the less the water accumulation phenomenon is,and the moisture content in the frozen edge area changes compared with the initial one The lowest freezing temperature,the smaller initial dry density,the higher initial moisture content and the earlier frost heaving phenomenon appear.The maximum frost heaving rate of soil is negatively related to freezing temperature and positively related to moisture content and dry density.(3)In order to prepare the remolded soil samples,the boundary conditions of test control are selected: moisture content,dry density and temperature.The transient test method is used to measure the thermal physical property index of soil mass.The thermal conductivity and thermal diffusion coefficient are obtained through the direct test of portable thermal property analyzer.The single factor test analysis of the change of the thermal conductivity of soil mass under different influence factors is carried out.The characteristics of the change of the thermal physical property parameters are as follows: there is no special change of the thermal conductivity under the normal temperature,and the temperature is controlled to 0 ?,and then the temperature is reduced to 0 ? there is a sudden increase,and then the change tends to be gentle;the thermal conductivity of soil is positively related to the change of dry density and moisture content;the thermal diffusion coefficient of soil is positively related to the change of dry density and moisture content,but when the soil moisture content is higher and tends to be saturated,the increase of thermal diffusion coefficient tends to be stable or even reduced.(4)Based on the analysis of two different physical characteristics of soil,the optimal combination of test indexes(within the test range)is obtained by the analysis results,and the primary and secondary degree of the influence of selected boundary conditions on frost heave and thermal conductivity is determined.In the closed system,the influence degree of each factor in the orthogonal analysis of frost heave: moisture content > dry density > freezing temperature;the influence degree of each factor in the orthogonal analysis of thermal conductivity: moisture content > dry density > temperature.(5)By combining theoretical analysis with experimental data and applying mathematical statistical analysis software to carry out multiple linear regression analysis,a prediction model of frost heave and heat conduction with comprehensive influencing factors(temperature,moisture content and dry density)is constructed,and variance analysis and model verification are carried out.The standardized regression coefficient of frost heave prediction model shows that the contribution level of each factor: moisture content > dry density > freezing temperature,which is mutually verified with the results of orthogonal analysis;the standardized regression coefficient of thermal conductivity prediction model shows that the contribution level of each factor: moisture content > dry density > temperature,which verifies the results of orthogonal analysis.Moreover,the fitting degree between the test value and the predicted value of the model is high,and the model has strong practicability,which can make the basic prediction in time for the actual project. |
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