Shear Strength Test And Preliminary Study On Retrieving Soil-Water Characteristic Curve For Unsaturated Soils

Effective stress and soil-water characteristic curve are important subjects in unsaturated soil research.Verifying the validity of effective stress and measuring soil-water characteristic curve quickly and simply are the problems that many researchers need to solve.In view of these two problems,the paper studies the shear strength,soil water characteristic curve and unified effective stress.The direct shear tests of silicon micro-powder and Guiping red clay with constant water content were carried out,and the shear strength and saturated soil shear strength indexes at different saturation were obtained;Secondly,the mathematical model of soil water characteristics(MVG,Modified VG)based on the VG model(Soil-water characteristic curve model proposed by Vangenuchten)and adapted to the full suction range was improved,and the soil water characteristic curve of silicon micro-powder and Guiping red clay was obtained.The shear strength of soil is determined by the effective stress equation,based on the shear strength test data and soil-water characteristic curve data which is also validated by collating a large number of literatures;Finally,it is discussed that the soil-water characteristic curve can be retrieved from shear strength and effective force equation.We can draw the following conclusions:(1)Compared with other strength tests,the constant water content direct shear test of unsaturated soil has the advantages of short matrix suction(moisture)balance time,simple test process,cheap equipment and wide range of applications.The internal friction angle of silicon micro-powder increases with the decrease of saturation,in direct shear tests with different saturation.When the saturation is higher than 50%,the shear strength increases slowly with the decrease of saturation.When the saturation is less than 50%,the shear strength increases slowly at first and then decreases with the decrease of saturation.However,the internal friction angle and shear strength of Guiping red clay are less affected by saturation.(2)MVG model can fit the test points of soil-water characteristics in the full suction range.The critical value of residual saturation(constant)is used in boundary effect zone and transition zone,and model fitting is used in residual stage zone.The distribution of residual saturation with suction is verified by experimental data and literature data.(3)Effective stress-soil skeleton stress,determines the shear strength of soil.Direct shear test with constant water content and soil-water characteristic curve data of silicon micro-powder and Guiping red clay are processed by effective stress equation,and the results are consistent with those of effective stress prediction.It is verified strength in the full suction range for literature data on various types of soils and different test methods.The soil-water characteristic curve can be quickly retrieved from the direct shear test with constant water content,and the validity of inversion of soil-water characteristic curve from effective stress equation is proved by literature data.