Study On Static And Dynamic Mechanical Properties And Binary-Medium Constitutive Models For Frozen Moraine Soils

Moraine soils,a special geotechnical material,are widely distributed on the Tianshan Mountain,the Qinghai-Tibet Plateau and high altitude areas in China.In recent years,more engineering problems have progressively emerged.For example,the Lixiang Railway,the Taihe Iron Mine,the Kangding Airport,and the Sichuan-Tibet Highway have experienced a series of engineering problems related to moraine soils.Therefore,it is of great theoretical significance and practical value to study the mechanical properties of moraine soils in-depth and establish corresponding theoretical models.In this paper,mechanical properties of frozen moraine soils subjected to the static and dynamic triaxial loads respectively are studied by using a triaxial test system.Then,based on the deformation mechanism of frozen moraine soils and the homogenization theory,binary medium constitutive models under monotonic loading and cyclic loading respectively were established,and a strength criterion applied for describing strength characteristics of frozen moraine soils was also established based on a concept of binary medium.Finally,the proposed models for frozen moraine soils were preliminarily verified based on triaxial test results.The main research contents and conclusions in this paper are as follows:(1)Mechanical properties of frozen moraine soils subjected to static and dynamic triaxial loads respectively have been studied.A series of triaxial tests for frozen moraine soils were carried out under conventional triaxial and cyclic loading paths.Four temperatures,five confining pressures and three moisture contents were set in static conditions,and three loading frequencies,three dynamic load levels and three water contents were set in dynamic conditions.Tested results have shown that the stress-strain curves for frozen moraine soils behave strain softening under static loading,with an decrease of confining pressures,tested temperatures and water contents,the softening degree increases gradually,and the samples behave volumetric compaction first followed by dilatancy with an increase of cycle number.Under cyclic loading,the stress-strain curves for frozen moraine soils change from sparse to dense gradually,and the samples show volumetric compaction first followed by dilatancy with an increase of cycle number.The cumulative residual axial strain increases with an increase of cycle number,and the cumulative residual volume strain appears a volumetric compaction and then dilatancy during the loading process.The axial dynamic stiffness gradually increases with an increase of cycle number,and finally tends to a stable value.The damage variables were defined based on cumulative residual axial strains and cumulative residual volume strains respectively to describe damage properties of frozen moraine soils.As cycle number increases,the damage variable increases gradually.Besides,with an increase of cycle number,the damping ratio decreases gradually and the dynamic strength decreases linearly.(2)A binary medium model for frozen moraine soils subjected to static loading was proposed and verified preliminarily.In proposed model,frozen moraine soils are abstracted as a binary medium composed of bonded elements and frictional elements.It is assumed that bonded elements is an isotropic elastic-brittle material,and frictional elements is an nonlinear elastic material.Then,a damage evolution equation for bonded elements is established from a perspective of energy dissipation.Considering mechanical properties of frozen moraine soils,a binary medium constitutive model was proposed based on the homogenization theory.The reasonableness for this model was verified by conventional triaxial tests.The results show that the proposed model can well reflect the strain softening and shear dilatancy of frozen moraine soils.Besides,the sensitivity analyses of these parameters are done,from which we know the influences of their variations on mechanical properties of frozen moraine soils subjected to static loading.(3)A binary medium model for frozen moraine soils subjected to cyclic loading was proposed and verified preliminarily.The cyclic loading process is divided into two stages including loading and unloading process.The constitutive model in loading process and unloading process respectively is consistent with the static binary medium model,and the breakage ratio is also consistent with the static condition in loading process,but the parameter values in the model are different.This is because bonded elements will not be damaged in unloading stage,therefore,the breakage ratio in this stage is constant.The local strain coefficient gradually decreases in unloading process,which is related to the strain state.The comparison between predicted results and test results shows that the proposed model can well simulate the axial strain accumulation characteristics and shear dilatancy properties of frozen moraine soils subjected to cyclic loading.In addition,the influences of meso-structure parameters on the prediction results have been studied through a sensitivity analysis of these parameters.(4)A new strength criterion for frozen moraine soils is proposed based on a concept of binary medium model.Test results have shown that when the mean stress is less than the consolidation yield pressure defined as a boundary stress of the strength envelope,the strength envelope is nonlinear,while when the mean stress is greater than the consolidation yield pressure,the strength envelope becomes linear approximately and can be described by the Mohr-Coulomb strength criterion.In a meridian plane,strength of bonded elements increases slowly first and then decreases gradually,and strength of frictional elements increases slowly and tends to be stable gradually.Meanwhile,the shape function of the Mise strength criterion is used for bonded elements,and the shape function of the Mohr Coulomb strength criterion is used for frictional elements.Finally,a new strength criterion is proposed based on a concept of binary medium to describe the nonlinear strength behaviors of frozen moraine soils.The proposed strength criterion has taken heterogeneous characteristics of frozen moraine soils into account on the hypothesis of that a representative volume element(RVE)of frozen moraine soils is a binary medium consisting of some bonded elements and frictional elements.The new strength criterion presented in this paper can simulate the following characteristics of frozen moraine soils: the strength envelope on deviatoric plane and meridian plane is smooth;it does not pass through the origin of coordinate axis on the meridian plane;and it is axisymmetric with respect to principal stress coordinate axis on deviatoric plane.Compared with test results,the proposed strength criterion can accurately describe main strength characteristics of frozen moraine soils,and with decreasing mean normal stresses soil samples behave strength envelope nonlinearity transforming to strength envelope linearity slightly.Besides,the sensitivity analyses of these strength parameters are done,from which we know the influences of their variations on strength of frozen moraine soils.