| Expansive soil is a kind of special clay,which can absorb and expand water and shrink after losing water.There are a lot of expansive soil in Yanbian area of Jilin province,our country.Yanbian is located in the seasonal frozen area,which belongs to the temperate sub-humid continental climate.The temperature varies greatly.It is dry and windy in spring,hot and rainy in summer,cool and rainy in autumn,and long and cold in winter.Every year,with the change of season and climate,soil experiences the cycle of freeze-thaw,which leads to the destruction of local roadbed and slope,such as slope instability,landslide,road surface collapse,cracking and other engineering geological disasters.In order to deal with these engineering disasters,it is necessary to improve and harness the expansive soil.In order to further reduce the engineering problems of expansive soil in Yanbian area under three kinds of cyclic actions,it is necessary to improve the treatment.The common chemical solidifying agents at home and abroad are lime,fly ash and cement,among which cement can not only improve the strength of expansive soil effectively,but also cause less pollution to water quality,compared with other solidifying agents,it is more suitable for the related engineering of expansive soil.Therefore,it is of great significance to study the cement improvement of expansive soil in Yanbian area of Jilin province under dry-wet freeze-thaw cycle.Based on the National Natural Science Foundation of China(42207209)and the key research project of the Department of Education of Jilin province(JJKH20230345KJ),this paper studies the evolution mechanism and improvement of expansive soil under the action of freeze-thaw cycles in Yanbian region of Jilin province.Based on a series of physical tests of expansive soil in Yanbian area of Jilin province,the volume of cement-modified expansive soil was measured under freeze-thaw cycle,dry-wet cycle and dry-wet-freeze-thaw cycle,through triaxial test and direct shear test,mechanical properties such as shear strength under different conditions were obtained.BP neural network was used to predict the cohesion of expansive soil modified by six kinds of cement under freeze-thaw cycle,dry-wet cycle and dry-wet-freeze-thaw cycle.The main contents and results of this paper are as follows:(1)A series of physical tests were carried out on expansive soil in Yanbian area of Jilin province,including particle analysis test,critical moisture content test,compaction test and specific gravity test.The plastic limit,liquid limit,specific gravity,optimum moisture content,maximum dry density and particle size distribution of expansive soils in Yanbian area were obtained.The basic physical properties of expansive soil in Yanbian area are determined.(2)The cement improvement and freeze-thaw cycles of expansive soil in Yanbian area of Jilin province were studied.The mechanical strength and deformation of six kinds of cement-modified expansive soils were studied after 20 cycles of freezing and thawing,the volume change of 0% cement expansive soil is the biggest during the 20 cycles of freezing and thawing,and the average value of volume difference is also the largest during freezing and thawing,the average of the difference between the frozen and thawed volumes of the improved soils is decreasing,and the difference between the frozen and thawed volumes of the 6% and 8%cement-modified expansive soils is not significant.The cohesion and peak shear strength of six kinds of cement-modified expansive soils under different confining pressures of 50 k Pa,100 k Pa and 200 k Pa decreased with the increase of freeze-thaw cycles,during the first freeze-thaw cycle,its cohesion and shear peak decreased the most,the decreasing range of cohesion and shear peak values of six kinds of cement-modified expansive soils under the first 7 cycles of freeze-thaw cycles is larger than that from the 7th to 20 th cycles of freeze-thaw cycles.The mechanical properties and volume change are better when the cement content is 6%,8%,10%during freeze-thaw cycles for 20 times,considering economic factors,the optimum content of cement-modified expansive soil under freeze-thaw cycles is recommended to be 6%.(3)The mechanical strength of cement-modified expansive soil in Yanbian area was studied under the action of dry-wet cycle and dry-wet-freeze-thaw cycle.The shear strength and internal friction angle of expansive soil improved by cement under the action of 7 cycles of drying-wetting cycle and drying-wetting-freezing-thawing cycle show a decreasing trend under four different vertical loads,under the first cycle,the cohesion,shear strength and internal friction angle of six kinds of cement-modified expansive soils decreased the most.Through the comparison of cohesion of six kinds of cement-modified expansive soil under freeze-thaw cycle,dry-wet cycle and dry-wet-freeze-thaw cycle,it is found that the damage of freeze-thaw cycle to soil is the least,the next is dry-wet cycle,the dry-wet-freeze-thaw cycle is the most destructive to soil.When the cement content is more than or equal to 6%,the dry-wet cycle and dry-wet-freeze-thaw cycle show better mechanical indexes,considering the soil through dry-wet cycle and dry-wet-freeze-thaw cycle,the optimal cement content is 6%.(4)The cohesive force prediction model of cement-modified expansive soil in Yanbian area is studied.By means of BP neural network,the cohesion prediction model of six kinds of cement-modified expansive soils under freeze-thaw cycle,dry-wet cycle and dry-wet-freeze-thaw cycle is established,the result shows that the error is small and the precision of this method is high.It further explains the effect of cement improvement and the influence of dry-wet cycle,freeze-thaw cycle and dry-wet-freeze-thaw cycle on the cohesion of expansive soil.This method can be used to predict the cohesion of the same type of expansive soil under similar climate temperature conditions. |
