Research On The Constitutive Relation For Unsaturated Sandy Silt In The Flooded Area Of Yellow River

In the last few decades,the saturated soil mechanics has been used widely as a classical theory to solve the soil mechanics problems in constructional engineering.Nevertheless,the main condition of soils is unsaturated in earth’s surface even in the soft layer.The saturated soil should have been deemed as a special state of the unsaturated soil.So it is not scientific obviously that the saturated soil mechanics was applied to all soil conditions.China’s famous scholars cheng-gang bao has pointed out that more than half of the countries in the world are suffering or experienced engineering geological disasters caused by unsaturated soil.However,the studies on character of unsaturated soil started very late and theoretical research is rare.It is quite clear that the researches on elementary physical properties and mechanics theories of the unsaturated soil are of great theoretical and practical significance.Based on the soil water characteristic curve(SWCC)and load-collapse(LC)equation gained from the experimental studies on the unsaturated sandy silt which widely exists in the flooded area of Yellow River,a constitutive model of unsaturated sandy silt has been established by the means of introducing a yield surface equation derived from thermodynamics and continuum mechanics in the BBM.Compared with the BBM,the new model have a more solid theoretical basis,what’s more,it can reflect the geotechnical behavior of sandy silt in the flooded area of Yellow River.Consolidation-undrained shear tests have been conducted by the way of controlling water content in the specimen to control the suction on the basis of SWCC.Comparing experimental data with theoretical calculation results,a conclusion can be drawn that the constitutive have a good simulation effect for the stress-strain relationship of sandy silt in the flooded area of Yellow River.The content of the research embraces three principal sections:(1)Researching on soil characteristics.The soil water characteristic curve(SWCC)of unsaturated sandy silt in the flooded area of Yellow River has been measured with filter paper method.And conduct the consolidation tests and shear tests on soil samples with different moisture content on the undrained condition by the GDS triaxial instrument test system.(2)Establishing theoretical constitutive model.First of all,the expression form of effective stress principle of unsaturated soil is came up on the basis of thermodynamics and continuum mechanics theory,and then the equation of unsaturated soil yield line is deduced.The load-collapse(LC)equation can be obtained according to the results of consolidation tests.Finally a new constitutive model for unsaturated soil is established using hardening equation same to BBM model in form and non associated flow rule.(3)Contrastive analysis of the theoretical calculation results and shear tests data.Calculate the stress-strain relationship of unsaturated soil sample according to the new constitutive model with the condition of constant suction and compare with undrained shear tests data to verify the applicability of the new constitutive model.It is found that the theoretical model can not accurately reflect the strain softening tendency of sandy silt with low saturation,but the prediction effect is quite good for the shear curve of soil samples with medium and high degree of saturation.