Laboratory Experimental Study On Unsaturated Permeability Of Remolded Loess

Loess has often become a difficult problem in engineering construction and disaster prevention because of its characteristics of weak cementation,multiple voids and loose structure.Loess is widely distributed in the northwest of China,occupying as much as 6.3×10~5 km~2 of the land area,and about 60%of them are collapsible loess.Water plays an important role in the engineering geological characteristics of loess.At the same time,the groundwater level in the loess area is generally deep,and the surface soil is mostly in an unsaturated state.The study of unsaturated seepage of loess can help to solve the problems related to engineering construction in the loess area.This paper taken the Malan loess in Donggang area of Lanzhou City,Gansu Province as the research object,and analyzed the basic law of unsaturated seepage in loess by setting up one-dimensional soil column and simulating the rainfall in the natural environment.The unsaturated permeability function of different dry density loess was obtained by the instantaneous profile method using the saturated permeability coefficient and soil-water characteristic curve,and the experimental results were compared with the empirical results.After the soil column infiltration test,the stratified sampling was carried out to obtain the physical property parameters and soil-water characteristic curves of each layer of soil,and the permeability characteristics of the soil after seepage were analyzed with the depth.In the test,the suction value of the remolded soil sample was first tested by the filter paper method,and the reliability of the domestic filter paper test was proved by comparing with the test results of the pressure plate apparatus and Whatman’s No.42filter paper test.Through the analysis of the seepage data,it is found that the infiltration of water in the soil column is promoted in the form of a wetting front.The peak moisture content of the soil gradually increases with depth,reaching the maximum value at 70cm,and the stable moisture content is generally around 30%.By simulating the natural environment rainfall,it was found that when the soil moisture content is stable,the heavy rainfall or the heavy rainfall after evaporation can only make the soil moisture content change significantly within 40cm depth.The data of unsaturated permeability coefficient of soils were obtained by the instantaneous profile method,and compared with the empirical data obtained by VG model and CCG model.The results show:The unsaturated permeability coefficient of soil under the same water content decreases with the increase of the dry density;the initial dry and wet state of soil had some influence on the instantaneous profile method,and the initial wet soil column results were less discrete;During the infiltration process,the distribution of unsaturated permeability coefficient at different depths of the soil column was similar to that of the water content,the values of the permeability coefficient at different depths tended to be similar with the infiltration process.The empirical values of the unsaturated permeability coefficient calculated by the empirical equation were consistent with the measured values by the instantaneous profile method,but the results obtained by different SWCC fitting equations were different.The results show that the instantaneous profile method were closer to the empirical method by FX equation fitting SWCC when the water content was lower,and that by the VG equation fitting SWCC method were closer to those obtained by empirical method at high water content.After the infiltration test,the basic physical properties and SWCC of the soil at different depths were obtained by stratified sampling of the soil column,and the permeability coefficient function of the soil at different depths was obtained by empirical model.The experimental results showed that the physical parameters of the soil change with depth,which leads to the difference of the unsaturated permeability function of the soil with depth,and the curve of unsaturated permeability coefficient under 20%water content changed more obviously;Compared with the empirical value before seepage,the value of unsaturated permeability coefficient increased at the same water content after seepage,and the closer to the surface,the greater the value of permeability coefficient;the measured data of the instantaneous profile method was between the empirical value calculated by SWCC before and after seepage,and the measured data in the area with lower dry density of surface is closer to the empirical value calculated by SWCC after seepage.