Small Strain Elasto-Plastic Model For Over-Consolidated And Structural Clay And Its Application

The mechanical properties of clay are important factors to determine the deformation and stability of foundation and its upper(internal)structures.In the history of the development of soil mechanics,mastering the mechanical properties of clay and constructing the corresponding constitutive model are the key problems to improve the accuracy of the numerical calculation of soft soil engineering.In this paper,laboratory tests and theoretical analysis were carried out to establish a three-dimensional elastic-plastic constitutive model of clay that can uniformly describe the characteristics of over-consolidation,structure and small strain stiffness,and the constitutive model was applied to the finite element numerical analysis of foundation pit engineering to verify its applicability.The main research contents and achievements of this paper are as follows:(1)Model description of influence of overconsolidation of soft clay under three-dimensional stress state.Based on the relationship between the stress ratio and the dilatancy ratio of normally consolidated remolded clay in the true triaxial test,a modified plastic potential equation is established in the transformation stress space which can consider the dependence of the stress path of strength and volume strain.The difference of void ratio between over-consolidated state and normally consolidated state is regarded as over-consolidated state variable,the influence of over-consolidation on the mechanical properties of soft clay under three-dimensional stress state is described.Compared with the experimental results,the model can describe not only the strain softening and dilatancy characteristics of the overconsolidated soil,but also the stress dependence of the strength and volume deformation of the overconsolidated soil.(2)Model description of influence of structure of soft clay under three-dimensional stress state..The true triaxial test is used to obtain the strength and deformation rules of Shanghai 4th layer structural soft clay and remolded soil under different stress states.Based on the the concept of superloading yield surface,the difference of void ratio between structure state and remolded state is regarded as structure state variable,the influence of structure on the mechanical properties of soft clay under three-dimensional stress state is described.Compared with the experimental results,the results show that the model can not only describe the strain softening and volume compression characteristics of structural soil,but also describe the influence of principal stress on the strength and volume deformation of structural soil.(3)The proposal of the overconsolidation and structural soil constitutive model considering small strain stiffness.By using a triaxial instrument with LVDT sensor,the stress-strain relationship of Shanghai shallow natural clay(2-6)in a large strain range and the nonlinear attenuation law of secant shear stiffness in a small strain range were obtained.In order to describe the influence of natural clay overconsolidation and structure in theory,the overconsolidation and structural state variables were introduced on the normal consolidation yield surface,and the shear modulus in the small strain range was modified according to the test results in the small strain range.The results show that the model can not only describe the overconsolidation and structural influence of soil,but also reflect the attenuation law of soil stiffness in the small strain range.(4)Application of finite element numerical calculation of constitutive model.The constitutive model is applied to the numerical analysis software Gleaves.By comparing the numerical solution of the finite element program with the theoretical solution of the constitutive model,the reliability of the numerical calculation results after the constitutive model is embedded in the numerical analysis software is verified.Then,the finite element model of a foundation pit project was established based on the soil model parameters determined by the test results of soil layer mechanical properties in Shanghai,and the lateral deformation of the retaining structure and the ground settlement behind the wall caused by foundation pit excavation were simulated and analyzed to verify the applicability and effectiveness of the constitutive model.The importance of the new constitutive model to improve the precision of the numerical analysis of foundation pit engineering is revealed.