Study On Creep Constitutive Model Of Warm Frozen Sand Soil On Qinghai-Tibet Railway

Subsidence of permafrost subgrade,as the crucial factor of engineering projects construction and resource development in permafrost regions,has caused great attention on the research of the permafrost engineering.Frozen soil exhibits creep behavior,which is owed primarily to the micro cracks and presence of ice within soil.Since the operation of Qinghai-Tibet Railway over the decade,the ballast thickness,induced by frequently supplementing ballast and elevating ballast,has reached up to 1.7 m.Few effective measures have been taken,and the stability of subgrade and the traffic security is being challenged by permafrost soil.Therefore,it is necessary to investigate the creep properties of frozen soil and provide the forecasting methods of the train-induced subsidence of the permafrost subgrade.For that reason,this paper made the Qinghai-Tibet Railway as a project,firstly,developing a series of tri-axial creep tests of frozen sand and investigating the creep properties of frozen sand under different negative temperature,dry density and grain size distribution conditions,then,proposing two creep constitutive models for frozen sand soil,finally,the new creep models were verified based on the test results and field monitoring data.The main research contents and results are as follows:(1)The experiment method of freezing-consolidation-creep was adopted based on the deformation characteristics of permafrost subgrade of Qinghai-Tibet Railway,this paper studied negative temperature,dry density and grain size distribution,how to effect the creep characteristics of frozen sand.The tested results show that the rheological properties of frozen sand are significantly affected by the temperature,the creep deformation of frozen sand is obvious even under a small load when the temperature is higher than-1.5 ?;When accelerating creep stage happening,the corresponding critical deformation value would be affected by the compaction degree of frozen sand;the creep deformation decreases first and increases then with the increase of the fine particle content of frozen sand.(2)Based on the Nishihara model,considering the coupling influences of time and stress on the model elements,two different non-linear creep constitutive models for frozen sand were proposed.Method 1: The coefficient of viscosity in viscoelastic part was modified considering the effect of time and stress,and the viscoplastic element is improved by introducing the damage variable;Method 2: Considering the effect of time and stress on the viscoelastic element,the elastic modulus of viscoelastic elements is modified to be a function of stress,and the viscoplastic element is improved by introducing the damage variable,two different non-linear creep constitutive models for frozen sand were proposed.(3)The new creep models were verified based on the test results.It is found that the creep curves predicted by these models agree well with the corresponding experimental results from low stress to high stress,and these proposed models can describe the unsteady,steady and accelerating creep characteristics accurately under different temperatures and stress levels.This study can provide a new choice for the creep prediction for frozen sand,and provide a theoretical basis for establishing prediction method of subgrade settlement of Qinghai-Tibet Railway,at the same time it can accumulate data for permafrost mechanics research.