| Laterite is widely distributed in Jiangxi Province and is widely used as engineering filler for slope and subgrade.The main problem with laterite roadbeds is the severe shrinkage and cracking that can occur on the surface during construction.On the one hand,cracks will form weak areas on the surface of subgrade soil,which will reduce the strength of soil;On the other hand,it will provide channels for water flow,accelerate water infiltration and affect the hydraulic characteristics of soil.Therefore,cracks will largely affect the permeability of the roadbed,the stability of the slope,and easily cause the roadbed collapse,even the slope instability and other engineering problems.Some achievements have been made in the research on the crack development and hydraulic characteristics of laterite,but most of them are in Guizhou,Guangxi and Yunnan.Due to the different formation conditions and properties of laterite,there are few studies on the development of dry shrinkage cracks in Jiangxi laterite and the influence of cracks on hydraulic characteristics.Taking the red soil in southern Jiangxi as the research object,this paper explores the development law of dry shrinkage cracks during drying process,and obtains the change law of quantitative analysis indexes of cracks through image processing software;The soil-water characteristic curve(swcc)of laterite with and without cracks was obtained by dynamic multi-step overflow test with pressure plate instrument;The saturated permeability coefficient of samples with and without cracks is measured by variable head test,and the unsaturated permeability coefficient of samples is calculated by dynamic multi-step overflow method and swcc prediction method;The quantitative analysis indexes of fractures are analyzed by fuzzy comprehensive evaluation method,and it is found that the total length and average width of cracks have the greatest impact on hydraulic characteristics;Multi-factor regression analysis was carried out by SPSS software,and swcc prediction model,saturated permeability coefficient prediction model and unsaturated permeability coefficient prediction model considering cracks were obtained;Based on the soil column test,the influence of cracks on the advance speed of soil wetting front is analyzed.The calculation data of the prediction model considering cracks are substituted into the finite element software for simulation,and the simulation results are compared with the soil column test results.The main contents and conclusions are as follows:(1)The development law of dry shrinkage crack under different dry density and different initial water content is studied,and the influence of dry and wet cycles on crack development is also studied;Surface crack rate,total crack length,average crack width and fractal dimension are used as crack quantitative analysis indexes.These indexes decrease with the increase of dry density and increase with the increase of initial water content.(2)After the dry shrinkage cracks occur in the laterite,the cracks will further develop under the dry-wet cycle,and all indicators will reach the maximum value after the third dry-wet cycle,while the cracks will heal at the fourth time,and the quantitative analysis indicators of each crack will not change at the fifth dry-wet cycle.(3)Based on the pressure plate instrument,the dynamic multi-step overflow method was used to test the red soil in southern Jiangxi,and swcc was obtained,and different fitting models were used to fit the test data.The results show that the v-G model has high applicability for red soil in Jiangxi.(4)The influence of cracks on swcc is analyzed.The results show that cracks can significantly increase the air intake value of the soil and change the slope of swcc.The air intake value of soil mass will decrease with the increase of each index value of cracks;At the same time,the larger the quantitative index value of the crack,the larger the slope of the transition section of swcc.(5)Based on the variable-head method,the influence of cracks on the saturated permeability coefficient is explored.The results show that cracks increase the saturated permeability coefficient of soil by 2-3 times,but this effect will be affected by dry density,and is negatively correlated with the latter;At the same time,based on the dynamic multi-step overflow method and swcc prediction method of the pressure plate instrument,the unsaturated permeability coefficient under the crack state is calculated,and the unsaturated permeability coefficient obtained by three different methods is compared.The three methods have the following size relationship: CCG model>dynamic multi-step overflow method>VGM model.(6)Based on the fuzzy comprehensive evaluation method,the influence of four quantitative analysis indexes of cracks on the hydraulic characteristics of soil is analyzed.The analysis shows that the total crack length and average crack width have more influence than the other two indexes,so swcc modified v-G model and saturated permeability coefficient model that can consider the total crack length and average crack width are established;At the same time,the regression analysis of the unsaturated permeability coefficient predicted by the three methods is carried out,and the modified dynamic multi-step overflow method,the modified CCG model and the modified VGM model that can consider the crack are proposed.(7)Based on the soil column test,the seepage of cracks is studied.By placing matrix suction sensors at different heights the degree of increase in the rate of advance of the wetting front was compared between the cleft group and the control group.Soil column seepage tests on unsaturated laterite were simulated,and the crack-related information from the crack groups in the soil column tests was substituted into three modified models to calculate the permeability coefficients and used as the cracked soil parameters.The simulation results were compared with the test results,and it was found that the predicted results of the dynamic multi-step overflow method matched the actual ones the best,followed by the modified CCG model and finally the modified VGM model. |
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