| Anisotropy is an important characteristic of geotechnical engineering materials,and anisotropic mechanical characteristics have always been an important and difficult problem in disaster prevention and mitigation engineering.In this paper,the undisturbed Q3 loess in Bailuyuan,Xi ’an,Shaanxi Province was taken as the research object,and the slope loess was sampled in different physical directions,and the microstructure and pore characteristics of undisturbed loess in different sedimentary directions were analyzed by scanning electron microscope and mercury injection test.The strength and deformation characteristics of undisturbed loess in different stress spatial domain are analyzed by true triaxial consolidation drainage test.Then,based on the results of microstructure test and macro-mechanical test,the numerical model of structural loess sample unit is established by discrete element PFC software,and the anisotropic micro-parameters are obtained.Based on the analysis of the relationship between the spatial domain strength change of octahedral principal stress,spatial slip surface and stress state,the anisotropic parameters which comprehensively reflect the stress state and material characteristics are established.Finally,based on the spatial slip surface theory,the orthogonal anisotropic strength criterion of structural loess is established.Through the research of this paper,the following conclusions are drawn:(1)The microscopic test results show that the three microstructure characteristics,namely,the arrangement characteristics dominated by layered arrangement,the pore characteristics dominated by macropore overhead structure and the cementation characteristics dominated by aggregate cementation,have regular changes in different directions,which indicates that the anisotropy of loess is closely related to the contact mode and pore distribution.(2)The macroscopic test results show that the strain of the specimen is hardening on the whole.The strength of the undisturbed loess increases with the increase of the intermediate principal stress,and the ratio of the intermediate principal stress can reach 55%in the second domain.Under higher confining pressure,the strength change caused by the change of stress space domain will be smaller,and the influence of confining pressure can reach 45%in the second domain.In domain Ⅰ and domain Ⅲ,the difference between large and small principal stresses is less affected by the increase of confining pressure and the ratio of medium and principal stresses,while in domain Ⅱ,the difference between large and small principal stresses is more affected by the increase of confining pressure,and the maximum influence can reach 24%in different spatial domains.(3)Among the contact parameters of PFC numerical simulation,the contact effective modulus has the greatest influence on the stress-strain curve,which affects the elastic stage,peak strength and residual strain.The bond normal stiffness and bond tangential stiffness mainly control the peak strength and residual strain of the stress-strain curve,while the contact stiffness ratio and friction coefficient mainly affect the peak strength.After calibrating the contact parameters,a meso-parameter I which can reflect the anisotropy is put forward according to the change of contact force and porosity when the numerical simulation results are broken.(4)Considering the assumption that the strength angle of anisotropic soil varies with the stress state and material characteristics in the stress domain,and introducing me so-parameters to define an anisotropic comprehensive parameter which comprehensively reflects the stress conditions and material characteristics,based on the theory of spatial sliding surface,the material failure depends on the maximum shear stress ratio,and an expression of orthogonal anisotropic strength criterion(AN-SMP)considering the change of spatial sliding surface of geotechnical materials is proposed.(5)By comparing the AN-SMP criterion with the existing common strength criteria in terms of spatial failure plane and plane shape,and the error analysis results of each criterion on undisturbed loess,it is shown that the AN-SMP strength criterion based on spatial plane change and considering multi-scale structure can better reflect the strength characteristics of materials,especially for predicting the strength of stress domain when the principal stress axis deflects. |
