An Anisotropic Bounding Surface Model For Structured Saturated Loess

Loess is a kind of sediment formed under the action of dry climate and wind in the Quaternary period.It is a kind of special soil which has the characteristics of uniform texture,large space,weak cementation and columnar joint development.The particle size of loess is between clay and fine sand,and the particle composition is mainly silt,which is light yellow or brown.It is mainly distributed in Gansu,Shaanxi,Ningxia and Shanxi,and has the characteristics of wide area,large thickness,complete formation and complex landform.For the purpose of theoretical research and practical engineering,this paper takes the experimental and theoretical research as the main research object.Based on the dynamic secant modulus of various soil,the undisturbed saturated loess of the engineering site of intersections of East Third Ring Road and the Subway Line 3 in Xi’an City,the undisturbed loess of Shibei village in the northeast of the Liupanshan Airport in NingXia as the research object.In this paper,the static and cyclic loading tests of undisturbed saturated loess samples under the condition of isotropic consolidation and K0 consolidation are carried out.According to the structure and anisotropy of loess proposed a single side bounding surface plasticity model base on isotropic generalized Cam-clay model,or “RSA-MCC” for short.The stress path,stress-strain relationship and pore water pressure were simulated by RSA-MCC model,the main research contents and results are as follows:（1）The relationship between G and λ was systematic studied,the G is essentially the secant modulus under the present stress state,and it is a deformation theory of plasticity and may describe the stiffness of geomaterials.Because there is a conversion relationship between the elastic modulus E and the shear modulus G,this paper is no longer strictly differentiated,both using G_sec and as the dynamic secant modulus for short,unless otherwise stated.The shear modulus G and secant shear modulus G_sec of soil at the elastic stage are the same.However,the λ is a measure of wave energy dissipation,it is a physical quantity that describes the cementation and compactness of geomaterials.Firstly,the relationship between 1/ G_sec and dynamic shear strain γ was analysis by linear regression method,the optimized parameters a and b representing the intercept and slope of this regression line.Then,the linear regression analysis of the G_sec with the previous linear relationship and λ also was discovered a good linear relationship by the least square method.However,the all literature show that the relationship between G_sec and λ is nonlinear at large deformation stage.The results show that this linear relationship can overcome the shortcomings of the nonlinear relationship found in the large deformation stage and can predict λ in the hysteresis loop that is not closed case.According to the results of centrifuge model test,it is proved that G_sec is more reliable than λ,which provides a simple and feasible method for selecting the correctly G_sec for further evaluation of seismic safety at engineering sites.Finally,a hyperbolic model was constructed by two optimization parameters a and b,the error between the predicted value and the test value is less than 12%.（2）In the framework of critical state soil mechanics and isotropic generalized Camclay model（GCC）,the basic theory and experiment of soil plastic mechanics were introduced in detail.A new bounding plasticity model（RSA-MCC）is proposed for the study of the undisturbed saturated loess based on the GCC,the bounding surface is used to characterize the degree of preconsolidation of the soil,and the new RSA-MCC model has no yield surface.Three kinds of hardening parameters are used in the undisturbed saturated loess,which are consolidation pressure pc,structural parameters r and anisotropic parameters α,The structural parameters are obtained by using Rouainia et al.（2000）,while the anisotropic parameters are selected by Dafalias（2013）proposed for the typical rotational hardening parameters.The mapping center of RSA-MCC model adopt the origin of stress during the initial loading process,the mapping center moved to the starting point of the unloading stress when unloading and reverse loading.The total plastic modulus is determined by the plastic modulus on the bounding surface and interpolation function between the true stress and virtual stress,this modulus formulation can ensure the plastic modulus can continuously change in the bounding surface.The plastic strain,hardening and softening of the soil also may continuously change in the bounding surface.（3）In this paper,the structure properties of undisturbed and remolded saturated loess are studied under the condition of isotropic and K0 consolidation.RSA-MCC model is used to study the drainage behavior of undisturbed saturated loess under K0 consolidation state,because this kind of state of consolidation to soil structural disturbance is small.The drained and undrained shear tests was carried out to the undisturbed saturated loess of the engineering site of intersections of East Third Ring Road and the Subway Line 3 in Xi’an City,and the two-way cyclic shear loading tests was carried out to the undisturbed saturated loess of Shibei village in the northeast of the Liupanshan Airport in NingXia.The RSA-MCC model is used to simulate the undrained shrear tests and the two-way cyclic shear loading test under K0 consolidation state.The simulation results of stress-strain relationship and pore water pressure are given,the results shown that the new proposed RSA-MCC model is reasonable.