Experimental Study On Soil-Water Characteristic Curve Of Loess Using Filter Paper Method

The fundamental difference between unsaturated and saturated soil is the pore water pressure is negative in unsaturated soil.The difference between pore pressure and negative pore water pressure is defined as matric suction,which is used to reflect the stress characteristics of unsaturated soil.The relation curve between matric suction and soil water content is called soil-water characteristic curve,referred to as SWCC.SWCC is an important constitutive relation of unsaturated soil,which can be used to analyze and predict soil deformation,strength and permeability,so SWCC is the foundation of unsaturated soil research.There are many methods for testing matric suction,in which the filter paper method is the quickest,simple and large range.The samples in this research are taken from Shaanxi Jing yang L1 and L5 loess.On the basis of ASTM D5298-10,the paper studies the process of testing the matric suction by the filter-paper method,focuses on the analysis and improvement of the key steps such as the preparation and test conditions of the soil sample,the weighing of filter paper,the balance time,and compares the test results with other methods.1)The experimental used new seal method including the aluminum foil wrapping and the wax seal to replace the sealed tank which the ASTM D5298-10 recommended.The new seal method can solve the untight seal on sealing tank and inconvenient operation on soil sample,and makes better seal effect;2)Directly put the filter paper on the balance pallet to weight the filter paper instead of weighting those storage in aluminum box or storage bag.In this way the weighting precision can be improved;3)For the same kind of loess,the combined test of multiple ring cutters is easier to operate than one set of ring knives,and the error in operation process is reduced4)The balance time between soil sample and filter paper is less related to the dry density of soil sample while more related to the moisture content of soil.The shorter the equilibrium time,the longer the equilibrium time.For Jing yang L1 and L5 loess,the water content of soil sample is less than 5%,the recommended balance time is above 13 days,the water content of soil sample is between the number,the recommended balance time is more than 10 days,and the water content is more than 20%,the proposed balance time is above 5 days;5)SWCCs were fitted with three models,i.e.Gardner model,Van Genuchten model and Fredlund & Xing model,and the fitting degree of Fredlund & Xing model was the highest;6)Compared SWCC of loess that tested with filter paper method with the result of tensiometer,pressure plate,pressure film,dialysis method,the filter method test and the test result in this paper is more reliable and is available in whole matric suction range.