Water Retention And Permeability Characteristics Of Different Types Of Unsaturated Loess

Some common disasters occurred in loess area including landslide,collapse under wetting et al.,are all related to the seepage of water in loess.Due to the deep buried of groundwater in Loess Platue,the moisture in loess transfers in unsaturated form.Soil water characteristic curve（SWCC）and hydraulic conductivity curve（HCC）,which express the water retention and permeability characteristic respectively,reveal the rule of unsaturated hydraulic behavior as two essential parameters,which closely related to the pore characteristics.For these reasons,this paper did some research on these two paramaters of compacted and intact loess,including testing technique,micro structure and numerical simulation et al.,to analaze the hydraulic behavior of loess and it’s mechanistic.The paper has conducted the following study:（1）By combining the Washburn and Young-Laplace equations to determine matric suction,transferring the porosity during the MIP to water content at wetting and drying of loess,and taking the contact angle of loess as fitting parameter,a method of predicting the SWCC is proposed.The SWCC of several loess layers are derived according to this method.The method show good prediction with an appropriate contact angle.The contact angle of loess should be 70°for wetting and 50°for drying.The contact angle of paleosol should be60°for wetting.（2）A test method of HCC based on filter paper method is proposed.By combining filter paper with soil column,the hydraulic conductivity curve in the full suction range of 0-10⁵ k Pa can be obtained in approximately a week,which solves the problems of previous methods:small suction range,long time and expensive equipment.By using numerical simulation,the optimal condition of water supply rate,measurement point selection,suction value calculation,measurement time interval and soil sample quantity are proposed,which is helpful for the guidance and optimization of the actual experiment.（3）The pore character and hydraulic character of Yanan loess are explored under some laboratory tests on its pore distribution curve（PSD）,scanning electron microscope photo（SEM）,SWCC and HCC.Loess compacted under the same water content contains inter-granular pore and intra-granular pore,namly double pore structure,which corresponding to the bimodal SWCC and HCC which include four parts:saturated part,main transition part,sub-transition part,residual part,namedly bimodal form;the main transition part and the sub-transition part intersect at 70 k Pa,corresponding to the boundary of inter-granular pore and intra-granular pore;for compacted loess at defferent desity,the inter-granular pore are different,while intra-granular pore are same,the saturated part and the main transition part of the SWCC and the HCC are distinct from each other,while the sub-transition part and the residual part are identical.（4）The pore character and hydraulic behavior of Zhengning L₁-L₃ loess and S₁-S₃paleosol are explored under some laboratory tests on its PSD,SEM,SWCC and HCC.Due to the different sedimentary environment and clay content,the loess in Zhengning was an aggregate structure,and the paleosol was a matrix structure.Loess contains four kinds of pores:secondary pore（>32μm）,inter-granular pore（2～32μm）,inter-lamellar pore（0.15～2μm）,intra-lamellar pore（<0.15μm）（the inter-lamellar pores and intra-lamellar pores are all intra-granular pores）,which match the four parts of SWCC and HCC:saturated part,main transition part,sub-transition part and residual part.There are three types of pores in paleosols:secondary pore（>32μm）,inter-matrix pore（0.15～32μm）,intra-lamellar pores（<0.15μm）,which match the three parts of SWCC and HCC:saturated part,transition part and residual part.The main transition part and the sub-transition part of both compacted and intact loess intersect at 70 k Pa.The trace of every part of SWCC and HCC depend on the distribution of each type of pore:for loess,the more intra-granular pore of loess,the better the water retention ability and permeability in the transition part and residual part;the more the inter-granular pore,the better the water retention ability and permeability in the main transition part;the bigger the total porosity,the higher the saturation water content and saturated hydraulic conductivity.