Investigation On Macro-and Micro-mechanial And Critical State Behavior Of Granular Materials Under Varying Stress Paths

As a natural foundation material,sand is widely distributed in the world,and its mechanical behavior is complex,mainly manifested as heterogeneity,inhomogeneity,anisotropy,nonlinearity,pressure sensitivity,dilatancy,etc,and is also related to loading path and stress history.The classical constitutive theory is based on continuum mechanics and establishes mathematical descriptions of the mechanical response of soil at macro-scale.However,this method ignores the discrete properties of sand materials and cannot reflect the influence of its micro-scale structural properties on the mechanical response.Therefore,it is necessary to start from the micro-mechanism of sand,consider particle-scale characteristics(such as particle geometry,gradation curve,particle arrangement,etc.),and analyze its mechanical response and internal structure evolution mechanism under various stress paths.The Cambridge School proposed critical state soil theory,which greatly enriched the basic theory of soil mechanics and promoted the development of constitutive models based on dilatancy and critical state theories.However,this isotropic critical state theory lacks the description of the internal structure of the soil,and needs to introduce the corresponding physical mechanism through the empirical data,which has limitations in predicting the anisotropic mechanical behavior of the soil.In this paper,the method of discrete element method is used,based on the theoretical framework of anisotropic critical state,starting from the mico-mechanism of sand,to study the particle characteristics(particle shape and particle size distribution)and stress paths(true triaxial tests and multi-directional shear tests)on the mechanical behavior of sand,and discuss the macro-and micro characteristics of critical state.The main conclusions are as follows:(1)Through a series of conventional triaxial tests,the effects of particle shape and particle size distribution on the macro-and micro-mechanical behavior and critical state characteristics of sand were analyzed in detail.It is found that the particle shape has a significant effect on the strength,the critical state stress ratio and the critical state fabric norm.Coefficient of nonuniformity has little effect on critical state strength and critical state fabric norm,but the critical state line moves down with increasing coefficient of nonuniformity;the peak friction angle increases with increasing coefficient of nonuniformity.(2)A series of true triaxial simulations of equal effective normal stress were carried out.It is found that the critical state stress ratio decreases with the increase of the intermediate stress ratio;the critical state fabric norm increases with the increase of the intermediate stress ratio.The position of the critical state line is not affected by the triaxial shear mode;as the intermediate stress ratio changes from 0 to 1,the peak friction angle firstly increases and then decreases,and the peak value appears when the intermediate stress ratio is 0.5.(3)Radial monotonic loading and circular loading along yield criterions were carried out in πplane.Taking advantage of the constant volume at critical state,the neutral loading stress path were probed under the condition of the critical state,and find that the stress/fabric trajectory and the stress/fabric state from monotonic loading are consistent with each other,indicating the uniqueness of critical state,and proving that the three conditions about stress state,volume and fabric are sufficient and necessary conditions to maintain critical state.(4)Considering the relative magnitude of initial shear stress and cyclic shear stress,cyclic shearing tests were carried out along the linear,bi-linear,figure-8,and circular shear paths.Two main failure modes are found,residual strain accumulation failure and cyclic liquefaction.The effect of the initial shear stress on the liquefaction strength is related to both the relative magnitude of shear stress ratio and cyclic stress ratio and the shape of the stress path.The results also show that the combination of the figure-8 stress path and no stress reversal loading mode is the most dangerous case.Through the above research,the evolution law of anisotropic mechanical properties of sand under complex stress paths is clarified,and the influence mechanism of micro-properties such as particle morphology and grading characteristics on the mechanical responses of sand is revealed,which provides physical mechanism for introducing micro-parameters into constitutive models and lays a foundation for further improving the accuracy of numerical analysis of geotechnical engineering.