Research On Constitutive Model Of Structural Reinforced Soil And Differential Settlement Mechanism Of Subgrade In Highway Expansion

There are great differences in the structural property of soil in new and old subgrade respectively in highway extension projects due to the different load history,consolidation time and reorganization process,which results in different mechanical behaviors of subgrade soil and differential settlement in highway extension projects.The differential settlement will not only lead to the premature pavement disease,which reduces the comfort and cost-effectiveness or even endanger the driving safety,but also affect the overall stability and shorten the normal service life of the highway.Application of geogrid in subgrade soil reinforcement can effectively alleviate differential settlement of the subgrade in highway extension project,but the geogrid material in highway expansion project has obvious creep characteristics,which leads to an obvious post-construction deformation of geogrid reinforced soil subgrade and reduces differential settlement control effect.The following aspects of research work are carried out in view of the problems above:(1)A new elastic-plastic boundary surface model is proposed based on the elastic-plastic boundary surface model of soil and the attenuation law of structural factors.The model shows that a greater confining pressure results in a higher soil strength,and the confining pressure leads to deformation of the soil pore,thus significantly affecting the structural strength characteristics of soil.In cyclic loading,a greater cyclic stress ratio results in a more serious structural damage and a faster decrease of the soil strength results in a greater strain when the cyclic stability state is reached.The structural attenuation factor shows a sinusoidal attenuation law with the increase of loading times.(2)A viscoelastic-plastic critical state constitutive model of reinforced soil is derived based on the coordinated deformation between geogrid and soil in reinforced soil,the Modified Cambridge Constitutive(MCC)model of soil and the viscoelastic model of the geogrid material.The result shows that the yield trajectory of the model in the p-q plane is still an elliptic trajectory equation.However,the long axis of the ellipse is no longer located on the p paxis,but rotates around the center point and has an angle with the p axis.When there is no friction between reinforcement and soil,the reinforcement ratio is 0 or the stiffness of reinforcement is 0,the model degenerates into MCC model of soil.(3)Conventional triaxial tests of reinforced soil specimens were carried out and the determination method of parameters in the model and the relationship between parameters of MCC model and soil were expounded.The method of calculating the parameters of viscoelastic-plastic constitutive model of reinforced soil material by soil parameters,reinforcement parameters and reinforcement ratio in practical engineering is put forward.The influence of the fabric tensor representing the anisotropy of reinforced soil material on the shear strength parameter M of MCC material is discussed.(4)Settlement of old subgrade caused by layered filling of new subgrade in highway extension project is calculated numerically based on the structural soil boundary surface model.The numerical results are verified and model parameters are back-analyzed by the site data of subgrade settlement in a highway extension project.Cumulative settlement of subgrade,as well as differential settlement between new and old subgrade,is calculated and predicted at different stages after highway expansion construction based on the viscoelastic-plastic constitutive model of reinforced subgrade soil.(5)The mechanical properties of the pavement splicing structure were tested and the influence of stiffness and position of pavement splice on numerical calculation of cumulative settlement and differential settlement between new and old subgrade after construction is discussed.Finally,some suggestions for deformation control of reinforced subgrade-pavement system in highway expansion are given in terms of pavement material selection,reserved post-construction settlement and overlap width of new pavement according to the calculation results.The paper has 74 figures,23 tables,145 references.