Testing Investigation Of Static And Dynamic Properties And Constitutive Modeling For Frozen Silty Clay

As the foundations of constructions and the subject of geotechnical structures in cold regions,frozen soil is always subjected not only to constant static loading,but also to cyclic loading caused by train and vehicle.However,as a multi component phase system,frozen soils are consisting of solid mineral particles,ice crystals,unfrozen water and gaseous inclusions,whose mechanical properties are very sensitive to temperature,water content,external loading and its duration.Therefore,to deeply investigate the mechanical behavior and its variation of frozen soils has important theoretical and practical significance for design,construction and operation maintenance of frozen ground engineering.For this point,a series of static and dynamic triaxial compression tests are carried out to study the deformation and strength behaviors,dynamic characteristics and creep law of frozen soils.A strength criterion,an anisotropic elastoplastic constitutive model and an unsteady fractional dynamic creep model of frozen soils are developed through the experiment results.The main studies and results in this paper are as follows:(1)In order to study the effect of initial water state on deformation and strength behaviors of Qinghai-Tibet Plateau silty clay,a series of triaxial compression tests are conducted under different initial water contents and different confining pressures at-6?.The results show that the frozen silty clay presents strain softening and strain hardening successively under low initial water content with increasing the confining pressure,but the frozen silty clay mainly presents strain softening when the initial water content is more than16%.Meanwhile,the relationship between initial tangent modulus and confining pressure changes from linear increase to parabola form,the tendency of the initial tangent modulus increases at first then decreases is more and more obvious with the initial water content increasing.In addition,the relationship between strength and confining pressure of frozen silty clay shows three forms:the first one is strength increases nonlinearly with the increase of confining pressure when the initial water content is 12.5%,the second one is strength increases at first then decreases with the increase of confining pressure when the initial water content is 14.0%and 16.0%,the last one is strength remains basically unchanged with the increase of confining pressure when the initial water content is 18.0%and 20.0%.(2)In order to study the deformation and triaxial strength behaviors of frozen loess,a series of triaxial compression tests are conducted under confining pressure varying from 1 to15 MPa at-6°C.The results show that the shape of stress-strain curves presents strain softening and strain hardening successively,the initial tangent modulus increase at first then decrease with the increase of confining pressure.Furthermore,the triaxial strength of frozen loess increases at first then decreases with the increase of confining pressure.(3)In order to describe the nonlinearity between strength and confining pressure of frozen soils,a nonlinear Mohr strength criterion is developed based on the envelope theory in?-?plane,the strength criterion can reflect the nonlinear variation of strength increases at first then decreases with the increase of confining pressure for not only frozen silty clay but also frozen loess.(4)According to the strength characteristics of frozen silty clay under different initial water content,the expression of modified effective hydrostatic pressure is adopted to represent the nonlinear curve equation of the strength criterion on the p-q plane.Then a shape function g(_??)in?plane combined with the Lade-Duncan strength criterion is proposed,and a strength criterion under complex stress conditions which can reflect effectively the influences of hydrostatic pressure,initial water content and pressure melting is established.(5)The elastic parameters of the frozen silty clay are obtained through a series of triaxial shear loading and unloading tests and isotropic pressure loading and unloading tests.Based on the results of triaxial shear test and according to the elastoplastic constitutive theory,a yield function considered the modified effective hydrostatic pressure and a plastic potential function with non-associated flow rule are proposed.Meanwhile,considering the initial anisotropy of frozen silty clay and the stress-induced particle rotation under shear loading,the double hardening law contains isotropic hardening and rotational hardening are developed,and then an anisotropic elastoplastic constitutive model for frozen silty clay is established.Finally,a calculation program based on the proposed model is formulated to check the validity of the proposed constitutive model.The results show that the model is preferable to describe the stress-strain relationship of frozen silty clay.(6)In order to study the dynamic characteristics of Qinghai-Tibet Plateau silty clay,a series of dynamic triaxial compression tests under the conditions of multi-stage loading and constant amplitude stress loading are conducted.The results of multi-stage loading dynamic triaxial compression tests show that the dynamic elastic modulus decreases nonlinearly with the dynamic strain amplitude increasing,the damping ratio decreases at first then increases with the increase of dynamic strain amplitude,and the area of hysteretic curve increases nonlinearly with the increase of dynamic strain amplitude.Compared with the H-D model,the M-D model can simulate the phenomenon that the backbone curve presents strain softening under high dynamic strain amplitude of the frozen silty clay.In addition,the results of constant amplitude stress loading dynamic triaxial compression tests show that the relationship between accumulated plastic strain and vibration number presents three type curves which are steady type,critical type and failure type,and then the critical dynamic stress of frozen silty clay under different confining pressures are obtained.Furthermore,it is found from the test results that the dynamic strength of the frozen silty clay decreases with the increase of the failure vibration,and the corresponding amplitude of the decrease is gradually receded with the confining pressure increasing.(7)According to the characteristics in the three phases of the dynamic creep curve of frozen silty clay,based on the fractional calculus operators theory,the Abel dashpot is introduced into the dynamic creep model,meanwhile,considering the unsteady character of the creep parameters when the dynamic stress becomes lager than the critical dynamic stress,then a unsteady fractional order dynamic creep model of frozen silty clay is established.Furthermore,the validity and rationality of the model are verified,and the results show that the proposed model can simulate the three phases of the dynamic creep curve,the simulated curves are in good agreement with the experimental results.