Study On The Static And Cyclic Behavior And Constitutive Model Of Q₂ Loess Under Principal Stress Rotation

Along with the implementation of Western Developments and“the Belt and Road Initiatives”,a large number of traffic infrastructure systems are being constructed on the loess.Under traffic loads,both the magnitude and the direction of the principal stresses in the subsoil may vary.Because of the principal stress rotation,the strength of the loess will decreases,and that induce to the irreversible deformations.Consequently,it is significant to investigate the strength and deformation behaviors of loess under the principal stress rotation and to propose the strength criterion and constitutive model for this kind of loess.In this paper,a series of stress-controlled monotonic and cyclic hollow cylinder tests were conducted.Based on the monotonic test,an anisotropic unified nonlinear strength criterion was proposed to consider the principal stress rotation,then,a 3-D anisotropic constitutive model was proposed to predict the stress-strain cures.The results will provide some experimental and theoretical basis for the study of subgrade soil considering the principal stress rotation.The main work and research conclusions are as follow:（1）A series of monotonic tests were conducted to discuss the influence of strength,pore pressure and non coaxial characteristics on Q₂ remolded loess and intact loess under a fixed principal stress angle with different inclinations of the major principal stressα（0°、15°、30°、45°、60°、75°、90°）and different intermediate principal stress ratios b（0、0.5、1）.According to the experimental results,therefore,the generalized shear strain of 6.5%was proposed as the failure strain for the remolded loess.Compared with remolded loess,the failure generalized shear strain of the intact loess is smaller,the magnitude is about 2%.Both kinds of the loess have obvious strength anisotropy and non coaxial.（2）A series of the pure rotation tests were conducted to discuss the influence of the evolutions of pore pressure and pore pressure on the saturated sample and the constant water content（the optimal water content）under the different deviatoric stresses and intermediate principal stress ratios.Based on the test data,the non-coaxial angles of remolded loess were calculated under different conditions,and the variation law of non-coaxial angles with the rotation angle of principal stress under different stress paths is obtained.According to the results,the excess pore pressure was accumulated owing to the rotation of principal stress direction,even though the deviatoric stress was maintained constant during the pure rotation test.The intermediate principal stress parameter b and the magnitude of deviatoric stress had a significant effect on the response of excess pore pressure.The rate of excess pore pressure generation was increased with increasing b,and the larger the deviatoric stress,the faster the pore pressure buildup.The strain envelope was kinematic in position and tended to stabilise with the cyclic stiffness stability at lower deviatoric stress levels.However,with the cyclic stiffness degradation under higher deviatoric stress,the strain envelope in the deviatoric strain space developed towards failure in an open spiral manner.Apparent non-coaxiality was observed during pure principal stress rotation.The non-coaxiality showed segmentation characteristics.Shear stress–strain stiffness and normal differential stress–strain curves were observed.The difference between them indicated incremental anisotropy,which was an internal mechanism of non-coaxiality.With cyclic stiffness stability at lower deviatoric stress levels,the non-coaxiality tended to increase.Conversely,with cyclic stiffness degradation at a higher deviatoric stress level,the noncoaxiality tended to weaken.（3）The―heart shaped‖stress path is brought out to study the pore water pressure and strain accumulation behaviors of remolded loess with different water contents,different cyclic deviatoric stresses and different shear stresses.The test results show that the evolutions of the pore pressure and generalized shear strain are highly dependent on the principal stress rotation（PSR）.In addition,both the magnitude of the cyclic deviatoric stress and shear stress rule strain accumulation as well as the water content significantly contributes to the differences.When the number of cycles is low,the cumulative effect of axial strain is obvious,the plastic strain develops rapidly,and then the strain keeps stable,the resilient strain increases with the increase of torsional shear stress.The existence of torsional shear stress leads to more strain accumulation and accelerates the failure of the specimen.（4）Based on the test results,the effect of the stress ratio on the plastic strain was obtained,based on this,the evolution of the strain can be categorized into stable and destructive types.Finally,a nonlinear empirical formula was proposed for the permanent generalized shear strain considering the effects of PSR and can be used for future feasibility studies on remolded loess subjected to traffic loading.（5）Based on Lade criterion and Mises criterion,a new unified nonlinear strength criterion（UNS criterion）is proposed by introducing linear coefficient of the shape function,for the sake of versatility,to capture the complex strength behaviors of frictional materials in geotechnical field.The correctness of the strength criterion is verified by strength data of soil materials and rock materials in the references.Considering the cross-anisotropy of the geomaterials,based the fabric evolution law of granular materials,a new anisotropic nonlinear strength criterion（AUNS）is proposed to accurately characterize the strength anisotropy based on UNS criterion.By comparing the predictions to the test results this paper,the new criterion is able to provide excellent traits of the strength anisotropy for various geomaterials.（6）Based on the UNS criterion and critical state theory,a 3-D isotropic constitutive model is proposed to describe the effects of intermediate principal stress and the strength nonlinearity of the geomaterials.Based on the AUNS criterion,a 3-D anisotropic constitutive model is proposed to describe the effects of intermediate principal stress and the strength anisotropy of the geomaterials.By comparing the predictions to the test results this paper,the new constitutive model is able to provide excellent traits of the strength anisotropy.