Research On Static And Dynamic Loading Performance Of Geosynthetic-encased Stone Columns Improved Foundation

Geosynthetics encased stone column(GESC)method is a new foundation treatment method formed by vertically wrapping the ordinary stone column with geosynthetic encasement.The hoop effect of geosynthetic encasement can prevents the bulging deformation within the column top and improves the bearing capacity.At present,this technique has been successfully used in projects,such as roadbed,seawall and coal yard,and good reinforcement effect has been achieved.However,the theoretical research is at the primary state,especially the bearing deformation characteristics under dynamic load.The dissertation is supported through the National Science Foundation of China(Contract name “Research on the bearing-deformation mechanism and long term settlement calculation of geosynthetic encased stone column under traffic loads”,Contract No.51678231),the National Science Foundation of China(Contract name “Research on failure mechanism and stability analysis method of ‘geocell+geosynthetic encased stone column’ bidirectional reinforced composite subgrade”,Contract No.52078205),the Postgraduate Scientifc Research Innovation Project of Hunan Province(Contract name “Research on dynamic response of geosynthetic encased stone column reinforced composite subgrade under cyclic loads”,Contract No.QL20210108).Based on the literature,the static and dynamic loading performance of geosynthetic encased stone columns improved foundation are studied systematically through model test,theoretical analysis and numerical modeling.Firstly,the bearing,deformation mechanism and failure mode of the GESCs improved foundation under static loads are analyzed in this dissertation.The reinforcement mechanism of GESC is discussed by combining the function of the ordinary stone column(OSC)and geosynthetic encasement.The load transfer mechanism of GESC composite foundation is analyzed by the load distribution on the plane of pile top and load transfer in depth direction.The ultimate bearing capacity is studied by visualizing the whole process of GESC composite foundation from bearing to failure.The vertical deformation of pile,soil and the pile-soil coordinated deformation mechanism are discussed.Also,the transverse deformation mechanism of encased and non-encased column are analyzed.The failure mode of GESC composite foundation is closely related to its bearing and deformation characteristics,so the failure mode of OSC and GESC composite foundation is discussed based on the former research.Secondly,a self-designed assembled cylindrical mode tank is adopted,and a series of indoor model tests on GESCs improved foundation under cyclic loading are performed to reveal its working performance under cyclic loads.The stress distribution between pile and soil,accumulative settlement,excess pore water pressure and bulging deformation are monitored.The factors including reinforcement form,encasement length,geosynthetic stiffness,column diameter,cushion thickness and dynamic parameters(load amplitude and frequency)are studied.The correlations between the deformation characteristics and pile-soil load distribution under cyclic loads are discussed.The interaction of pile-geosynthetic-soil in GESCs improved foundation are studied.The evolution models of accumulation settlement,the accumulation and dissipation of pore water pressure,and bulging defromation of GESCs improved foundation are obtainedThirdly,based on the finite difference theory,a solution of GESCs composite foundation considering smear,clogging and arching effects for calculating the excess pore water pressure and consolidation settlement of composite foundation under static and dynamic loads is obtained.In this solution,the unit cell model is adopted,and the solution is solved by performing time domain analysis within the dispersed solution domain of the model.The cross section of the unit cell is divided into four areas,namely the unclogged column area,the clogged column area,the smear area and the undisturbed area.The smear effect is induced by the significant disturbance to the surrounding soil due to the construction of GESCs and clogging effect is induced by the blockage of the drainage path.The influence of smear and clogging effect on permeability coefficient and its influence range are discussed.The arching effect is considered in the solution.Based on the the limit analysis,the stress balance equation of vault element is solved,and the radial stress distribution at the pile plane is solved by combining with the vertical stress balance equation of unit cell surface.Finally,on the basis of the research results of the preceding chapters,the static and dynamic response of the GESC under uniaxial and triaxial conditions and the GESC improved ground under rigid and flexible loads are studied by combining the advantages of discrete element method and finite difference method.Based on the discrete element theory,the uniaxial and triaxial numerical analysis models of the GESC under cyclic loads are established.The relationship between the bearing deformation characteristics of the pile and the mesoscopic parameters of the particles assemble are obtained.Based on the discrete element and finite difference coupled numerical method,the numerical model of GESC improved foundation and GESCs supported embankments are established.The GESC is regarded as discrete medium,and the surrounding soil and embankment is regarded as continuous medium.The interaction and load transfer mechanism between the geosynthetic and stone aggregates can be revealed from the mesoscopic with a higher calculation efficiency.In order to evaluate the embankment system supported by GESCs,a three dimensional mechanical and hydraulic coupling numerical model is established to analyze the time-dependent response of the GESCs supported embankment,considering the drainage and consolidation process of the embankment system.The response of embankment system in construction filling stage,construction consolidation stage and service cycle loading stage is discussed.