Study On Bearing Mechanism And Test Of Encased Stone Column Composite Foundation Under Traffic Load

Encased stone column has the functions of displacement,compaction and drainage consolidation on weak foundation.The bearing capacity of granular pile mainly depends on the confining force of pile,and the radial binding for encased column body is provided by geogrid sleeve and soil.Compared with ordinary stone column,encased stone column can greatly improve the composite foundation bearing capacity and reduce the settlement.Meanwhile,encased stone column has been widely used in the reinforcement of soft foundation,because it can use local materials and reduce cost,besides,it can be simplely constructed and widely applicated.However,the existing research work on the composite foundation of encased stone column mainly focus on the characteristics of the composite foundation under static load,and less attention is paid to the research under traffic load.Therefore,based on the existing research results of encased stone column composite foundation,model test,numerical simulation and theoretical analysis methods are used to research the load transfer mechanism and cumulative settlement law of encased stone column composite foundation under traffic load.This dissertation is supported by the National Natural Science Foundation of China project.Firstly,based on the existing research results,the form of traffic flow,the size and pattern of traffic loads in this dissertation are proposed,and then the load transfer mechanism and bearing mechanism of encased stone column composite foundation under the traffic loads are deeply discussed.And the bearing capacity calculation methods and calculation formulas of single encased stone column and encased stone column composite foundation are mentioned.On the premise of fully considering the influence factors,the large heavy test has been taken through gravel soils of the sampling site.So the heavy value of the gravel soils is determined.In order to control the component of pile material,the laboratory test of particle analysis has been processed.According to the particle analysis of test results,the gravel soils are remodeled by homemade test box which is suitable for the big granular soil direct shear test.The strength index of pile material is determined by direct shear test.And final strength value of the gravel soils is determined by in-situ dynamic penetration test and statistical analysis of test parameters.At last,large scale model test has been taken to systematically study the bearing mechanism and stress characteristics of encased stone column composite foundation under cyclic load with different cyclic load ratios.And the cumulative settlement at the top of the composite foundation is measured and analyzed.Secondly,based on the finite difference numerical analysis method,the dynamic characteristics of encased stone column under traffic load are investigated.The effects on the behavior of the encased stone columns by the gravel density,pile encased length,pile length and pile diameter are evaluated.The results of numerical calculation are compared with those of laboratory model test,so the rationality of the numerical results is verified.And the dynamic characteristics and load transfer mechanism of current highway encased stone columns composite foundation under traffic load are studied.The effects of different thicknesses of asphalt,car speeds,traffics and pile spacings on the dynamic displacement and dynamic stress of pavement embankment foundation structure are analyzed.And the load energy transfer mechanism in current highway encased stone columns composite foundation structure is revealed.Then,based on the stress and deformation characteristics of encased stone column composite foundation under cyclic loading and unloading,the main loading characteristics and key influencing factors of soil around the pile under cyclic loading are studied and summarized,according to the existing research results and laboratory model test results.Based on the ideal plastic model,the exponential function is used to describe the weakening characteristics of normal stress with the number of cyclic loading.The cyclic weakening model of pile-soil interface and the hysteresis model of cyclic loading at pile bottom are established,and the formulas for calculating the settlement and pile side resistance of composite foundation with the effect of cyclic loading are derived.Then the analysis model of encased stone column composite foundation under vertical cyclic loading with considering the cumulative deformation is established.The relationship between the amplitude of cyclic load and the degree of cyclic effect of soil is established by weakening influence factors,and then the variation rules of the weakening and cumulative deformation of bearing capacity under vertical cyclic load are obtained.Finally,according to the researches of this dissertation,the designed calculation method of encased stone column foundation under traffic load is proposed.And the feasibility analysis of the treatment scheme of the encased stone column composite foundation is carried out from many aspects by the engineering example,which reflects the advantages of the treatment of the encased stone column composite foundation.