Study On The Strength And Microscopic Charateristic Of Unsaturated Subgrade Soil In Seasonal Tundra

The unsaturated subgrade in seasonal permafrost area will undergo freezingthawing cycles repeatedly,and the freezing-thawing of soil is the main reason for subgrade to damage in highway engineering.Due to the alternating effect of freezing and melting on the unsaturated roadbed,the soil mass will be affected by frost heave.This effect will change the microstructure and macroscopic mechanical properties of the unsaturated soil.The damage to the structure of unsaturated subgrade soil led to the phenomenon of frost heaving and grouting occurred,resulting in the destruction of the road surface.In this paper,the freezing-thawing cycle tests were conducted on unsaturated silty clay in Zhangjiakou area under different freezing-thawing cycles and freezing negative temperatures.In the laboratory,a series of experiments were conducted on the soil-water characteristic curve,strength and microscopic mechanism of silty clay.The changes of strength and microcosmic mechanism of unsaturated subgrade silty clay under the influence of freezing-thawing cycle were summarized,and the relationship between microcosmic mechanism and strength under the influence of freezing-thawing was founded.The specific contents and conclusions are as follows:Firstly,the soil mechanics test of unsaturated subgrade soil was carried out.Through particle analysis test?compaction test and liquid plastic limit test,the optimum moisture content?maximum dry density and the type of unsaturated subgrade soil were obtained.The test scheme of unsaturated subgrade silty clay was determined,which provided the direction for the test.The soil-water characteristic curves of silty clay of unsaturated subgrade with different freezing-thawing cycles and negative freezing temperature were measured.Gardner model was used to fit the test data.The fitting degree was above 97%.According to the test results and fitting data,the moisture reduction rate of unsaturated subgrade soil first decreased and then increased with the number of freeze-thaw increases.The saturated moisture content of samples decreased after once freezingthawing cycle.The residual moisture content first increased and then decreased with the increase of cycle times.The saturation increases with the increase of negative freezing temperature,and the residual moisture content first increases and then decreases with the increase of negative freezing temperature.The strength of silty clay of unsaturated subgrade decreases first and then tends to be stable with the increase of freezing-thawing cycles.After the first freezing-thawing cycle,the strength decreases most obviously.The damage value of cohesive force is the greatest when the negative freezing temperature is-10?.The damage value of cumulative cohesion caused by matric suction increases linearly with the increase of freezing-thawing cycles and increases exponentially with the decrease of negative freezing temperature.Nitrogen adsorption method was used to measure the adsorption desorption curve of unsaturated subgrade soil,and the isotherms under the influence of different freezing-thawing cycles and negative freezing temperature were obtained.The isotherm of subgrade silty clay sample is the type IV isotherm H3 type hysteresis ring,and the soil pore structure is the slit hole formed by the lamellar example accumulation.Obtained by BET method after different external conditions affect the specific surface area of sample values,according to the nitrogen isothermal adsorption test to calculate the fractal dimension of unsaturated subgrade soil.The specific surface area and fractal dimension increased with the increase of freeze-thaw cycles and reached the peak at the third time.It decreases with the decrease of freezing temperature.The influence of matric suction on the cohesion of unsaturated soil after several freezingthawing cycles was explained from the microscopic point of view.