Study On Cracking Water Content And Slope Stability Of Expansive Soil

Expansive soil is a kind of special clay rich in hydrophilic minerals such as montmorillonite.Under the action of seasonal dry-wet alternation,expansive soil shows swelling and contraction,which is the cause of cracking.The cracking of expansive soil is the main cause of shallow sliding of most slopes under rainfall infiltration.Based on the fissure property of expansive soil,the law of fissure development:in expansive soil and the effect of fissure on soil permeability are studied through indoor evaporation test and rainfall infiltration test.Then,on the basis of the test,the stability of expansive soil slope with fissures under rainfall infiltration condition is studied by numerical method.The conclusions are as follows:(1)The results of indoor cracking tests showthat with the evaporation,the surface crack rate and average crack width of soil generally conform to the three-stage variation law.The cracking water content increases with the increase of plastic limit of soil,showing a good linear relationship.With the increase of soil sample thickness,the cracking water content gradually increases and eventually tends to be stable.The experimental results show that the factors causing cracking are not only the moisture content of the soil sample,but also the moisture gradient in the soil sample.The first two drying-wetting cycles have a great influence on the development of soil fissures,which shows the decrease of fissure rate and average width of fissures and the increase of the number of fissures.Then,with the increase of drying-wetting cycles,the development of fissures in soil tends to be stable.(2)The results of indoor rainfall infiltration test show that with the increase of the surface crack rate of expansive soil column,the time of runoff generation on the surface of expansive soil column is longer;the runoff rate increases sharply at first,then slowly,and finally tends to be stable.In the initial stage of rainfall infiltration,the infiltration rate of soil column is equal to the rainfall intensity due to the existence of cracks.With the generation of runoff,the infiltration rate declines sharply,and finally tends to be stable.The permeability coefficient of unsaturated expansive soil increases with the decrease of matrix suction.The larger the surface crack rate of expansive soil column,the smaller the initial unsaturated permeability coefficient and the larger the final stable permeability coefficient,the values of the stable permeability coefficient are between 10-5 m/s and 10-6 m/s.(3)Based on the laboratory test results,the rainfall infiltration and stability of expansive soil slope with fissures are simulated and analyzed by numerical method.The results show that with the increase of fissure depth and surface fissure rate,the influence of rainfall infiltration on slope is deepened,and the safety factor decreases.In the case of constant rainfall,rainfall patterns with low intensity and long duration can increase rainfall infiltration.When the rainfall intensity is less than the saturated permeability coefficient of soil,the strength ofsurface soil decreases faster with the increase of rainfall intensity,and the safety factor of slope is lower after the end of rainfall.Only when the depth of the main fissure exceeds the depth of atmospheric influence,the influence of the main fissure on seepage begins to appear.The position of the most dangerous slip surface of the slope is related to the depth of atmospheric influence,which indicates that the:instability of expansive soil slope may occur within the depth of atmospheric influence,and it is very likely to belong to shallow slip.