Research On Rheological Properties And Constitutive Theory Of Saturated Soft Clay

Soft clay layers are widely distributed in coastal areas and inland areas along lakes and rivers.These soft clays have high water content,low bearing capacity and strong rheology.With the rapid development of regional economy,more and more large buildings(structures)are built on the soft clay foundation.These soft soil projects may have problems such as excessive post construction settlement,uneven settlement and insufficient bearing capacity due to the strong rheological properties of soil,resulting in engineering accidents.In this paper,the rheological properties of saturated soft clay were analyzed and studied by three methods: mechanical properties study,theoretical model study and numerical application study.The main works are concluded as follows:(1)Experimental study on rheological properties of saturated soft clay was carried out.The triaxial constant rate of strain test,undrained creep test and over consolidation shear creep test of Hangzhou soft clay were carried out using GDS stress path triaxial apparatus.The effects of strain rate effect,long-term creep characteristics and creep characteristics under over consolidation status of Hangzhou soft clay were systematically studied.The single-stage CRS test was carried out under the confining pressure of 200 k Pa.The results show that the shear strength of Hangzhou soft clay is 91.5 k Pa,98.5 k Pa and 115.5 k Pa respectively at the loading rates of 2%/h,4.5%/h and 10%/h,which indicates that the loading rate will affect the shear strength of soft clay.The greater the loading rate,the higher the strength.The results of multi-stage CRS test also prove this conclusion.The results of long-term creep test show that the greater the stress level is,the more obvious the creep characteristics of soft clay are.The results of over consolidation creep test show that the larger the over consolidation ratio of soft clay,the smaller the creep coefficient,and there is an exponential relationship between them,which indicates that the post construction settlement of soft clay can be effectively reduced by surcharge preloading method.(2)One-dimensional rheological constitutive models of soft clay were established.According to the nonlinear mechanical properties of soft clay,the fractional derivative theory was introduced into Caputo differential operator,and then the fractional Merchant rheological constitutive model was obtained;In addition,according to the over consolidation creep characteristics of Hangzhou and Huzhou soft clay,considering the functional relationship between shear creep coefficient and over consolidation ratio,an over consolidation shear rheological constitutive model was established under the theoretical framework of elastic viscoplastic model.It is proved that the two models can well reflect the nonlinear creep characteristics of soil,and the newly established over consolidation shear rheological constitutive model can well reflect the influence of over consolidation ratio on the creep characteristics of soft clay.(3)Three-dimensional elastic-viscoplastic constitutive model of soft clay was established.Based on Perzyna’s overstress theory,two different elastic-viscoplastic constitutive models were established in the form of double yield surface and single continuous yield surface.The new model can consider compression rheology reflecting shear-contraction mechanism,and shear rheology reflecting shear-dilatancy mechanism.In the process of establishing the two models,Yin Graham’s equivalent time method was adopted and extended to convert the real time into the strain rate.The established model is a rate dependent constitutive model,which can reflect the influence of stress history and stress path.The correctness of the model was verified by comparing with the experimental results of long-term creep tests and CRS tests.(4)The one-dimensional rheological consolidation of single-layer foundation was analyzed.The fractional Merchant model was introduced to establish the governing equation of the onedimensional rheological consolidation of single-layer foundation,considering the influence of rheological properties and soil self-weight.The analytical solution of the governing equation in Laplace domain was obtained by Laplace transform,and the rheological consolidation solutions of stress and settlement in time domain were obtained by Laplace numerical inversion method(AFT method).The rationality of the consolidation equation was verified by degradation verification and experimental verification.The consolidation process with and without considering the self-weight stresses was analyzed.The effects of fractional order α,viscous coefficient η and elastic modulus ratio R on the development of consolidation settlement were discussed.(5)The rheological consolidation theory of multi-layered foundation under complex boundary and load conditions was analyzed.The one-dimensional rheological consolidation theory of single-layered foundation was extended to multi-layered foundation.Arbitrary loads were introduced to extend the loading conditions;continuous drainage boundary conditions were introduced to extend the drainage boundary conditions.Subsequently,the rheological consolidation control equations of multi-layered foundation under complex conditions were established.The semi-analytical solutions of stress and settlement of consolidation equation were obtained by Laplace transform and numerical inversion,and thus the consolidation system of multi-layered soil foundation was verified.The rheological consolidation process under instantaneous load,single-staged construction load and cyclic load was analyzed,and the effects of model parameters,such as boundary parameter b and viscosity parameter θ,on soil settlement and degree of consolidation were obtained.(6)Development and application of rheological constitutive model through UMAT interface was completed.Based on ABAQUS finite element software,the finite element development of fractional derivative Merchant model was realized by UMAT subroutine interface through Fortran code.Then the finite element subroutine was imported into ABAQUS to simulate the triaxial creep test under different stress levels,The rationality of UMAT subroutine was verified by comparing the experiments results with the finite element calculation results;Then the finite element subroutine was applied to the engineering case to establish a one-dimensional fluid-structure interaction consolidation project for soft clay and a three-dimensional embankment surcharge engineering of soft clay to study the creep characteristics of soft soil engineering under long-term loading.