A Structured UH Model

Soil structure exists in all the soils in the nature.The existing of soil structure defers the deformations which should occure in low stress and this part of deformations rapidly occur in higher stress when soil structure happens to collapse.This is very bad to the practical engineering.Consequently,it is necessary to extend the constitutive models of soil to reflect soil structure effects.In order to get a understanding of typical behaviors of structured soil,experimental investigations on artificial structured soil are presented.A simple and practical method to prepare artificial structured soil sample named cemented mud pouring is discussed.By the GDS standard stress path triaxial test apparatus,a series of tests on artificial structured soil including isotropic compression tests,one-dimensional compression tests,stress path correlation tests and triaxial extension tests are taken.The test results give strong support to the formulation of the structured UH model presented in this thesis.The UH model,by the one-dimensional modeling idea and the concise unified hardening parameter formulation,is qualified to continuously and smoothly reflect the strain hardening/softening and positive/negative stress dilatancy with the same model parameters as that of the Cam-clay model.The transformed stress method is then applied to reasonably reflect the convex triangle shape of yield curve for geomaterials in the plan of the three-dimentional stress space.By deep dissecting,the core of the UH model describing constitutive relationship of overconsolidated soils is the evolution of the inner variable.Therefore,in this thesis,soil structure effect is attempt to be introduced to the inner variable,then the UH model is extended to a structured UH model considering soil structure effects.A moving normal compression line(MNCL)is introduced in this thesis.Instead of the NCL in the UH model,MNCL is applied in the structured UH model to determine the value of the inner variable.This is the core idea of the structured UH model.After formulating the evolution law of the MNCL evolving with soil structure decay,the structured UH model in ~ stress space is obtained.Applying the transformed stress method,the structured UH model is reasonably applied to three-dimensional stress states.The structured UH model has 2 more new model parameters than the UH model: the initial soil structure potential Δ and the soil structure decay rate.The 2 new model parameters can be determined stably in order by trial method.The structured UH model is qualified in: Reflecting the tangent compression index to be decreasing firstly and increasing then and tend to the compression index of the normally consolidated soil finally in isotropic compression;Reflecting reasonably the increasing of shear strength and volume shrinking in drained triaxial shearing;Reflecting reasonably the stress path correlation of soil structure decay;Reflecting strain softening with positive pore water pressure increment in undrained shearing at higher confining pressure.By rotating the yield surface for a proper angle,the structured UH model is extended to consider the initial K0 condition.By the comparisons of the model simulations and the test data from the author and the other scholars,the reasonability of the structured UH model is verified.The formulated structured UH model,with a relatively concise mathematical expression,can reasonably describe the behaviours of isotropic compression,constant stress ratio compression,drained / undrained triaxial compression / extension,stress path correlation,one-dimensional compression and shearing with initial K0 state of structured soils.