Investigating The Yielding Behavior And Constitutive Modelling Of Natural Soft Clays

Many infrastructures in the largest cities in China and neighbouring belt road countries are built on natural soft clay deposits foundations.These soft clay deposits with soil structure were formed in the complex sedimentation and post-sedimentation process which proves to be a challenging task to normalize the relationship between the current state of stress,stress history,and its loading and unloading path.In engineering design,the determination of soil characteristics using state of the art equipment to introduce the constitutive models that can accurately describe the soft clay behaviour is a necessary step.Despite their engineering importance,advanced experimental investigation and precise constitutive model to describe the yielding behaviour of clayey soils are limited.Therefore,in this research study,systematic laboratory testing was performed on typical soft clay samples to investigate the yielding behaviour of soft soils and the elasto-plastic constitutive model was developed to accurately describe the natural behaviour of clayey soils.For this reason,high quality undisturbed clay samples were sampled from a construction site located in Lianyungang,China.The undisturbed soil samples were tested in automatic TKA automatic stress path triaxial apparatus.Following a serious of stress path tests,yielding behavior of soft clay was examined,and the effects of stress paths on the yielding behavior of soft clays were revealed in detail.The initial yield surface obtained from tests for undisturbed soils is remarkably different from the yield surface of the remoulded soils.The stress-strain curves of undisturbed soils have yield characteristics and the yield curve,an inclined ellipse,obtained in initial state was oriented along the K₀ line,describing the initial anisotropy of the fabric that was generated during soil deposition.The yield surface obtained in this study was compared with existing Nakano-model.It was found that model proposed by Nakano can well describe the initial yield characteristics.However,Nakano-model does not capture the rotation of yield surface corresponding to the process of soil structural gradual yielding.Therefore,a modified yield surface equation developed based on the stress path tests in this study which describes the initial yield characteristics and adequately captures the yield surface rotation due to the gradual yielding of structured soils.The elasto-plastic model with modified yield surface equation established proposed in this study consists of seven parameters(M,k,n,?_K0,p?_y,l^u,₀e ande_y),where M,k,nare the parameters of original Modified Cam-Clay model;p?_y characterizes the size of the initial yield surface of structure;?_{K 0}is stress ratio with respect to axis of the natural soil yield surface affected by the initial anisotropy after rotation;parametersl^u,p?_y,e_y are the compression characteristics of natural soil after initial yield.The validity of the model was verified by comparing the model predicted and stress path tests measured results published on Bothkennar Clay.