Studies On The Behavior And Design Methods For Geosynthetic Encased Stone Columns Composite Ground Under Static And Dynamic Conditions

Geosynthetic encased stone column is made from stone column wrapped with geosynthetic encasement.The geosynthetic encased stone column could accelerate consolidation,reinforce foundation,densify the surrounding soil and so on,all these improve the ground for reducing settlement and liquefaction mitigation.However,the static design method,seismic response and seismic resistance design method are still unclear,which limit the use of the method.A static load test was conducted to reveal the deformation developed with load and the load transfer along the geosynthetic encased stone column shaft.Based on the tests results,the methods of analyzing the bearing capacity and settlement were proposed.A shaking table test was conducted to reveal the seismic response of the geosynthetic encased stone column composite foundation.Based on the results of the shaking table model test,the methods of analyzing bearing capacity in the seismic case were developed.The major new findings that can be drawn from the thesis are as follows:1.The characteristics of the stress and deformation of the geosynthetic encased stone column composite ground were revealed from the static loading test.The fully encased stone column and the partially encased stone column,compared to the stone column,has a smaller and more uniform radial deformation;and in the static loading test,the fully encased stone column transfers over half of the load acting on the pile head to the pile tip in the soft clay foundation.Besides,the fully encased stone column with higher stiffness of encasement fails because of punching into the upper gravel mat in the static case,while fails because of excess bulging with lower stiffness of encasement.2.The design methods of the geosynthetic encased stone column composite foundation in the static case were developed.The geosynthetic encased stone column refers to the fully encased stone column in the following text.When the geosynthetic encased stone column composite foundation fails from punching into the upper gravel mat,considering the pile-soil-mat interaction effect,the bearing capacity and settlement calculation method of the geosynthetic encased stone column composite foundation were proposed.While the settlement of the geosynthetic encased stone column composite foundation is mainly from radial bulging of the pile,the corresponding methods of analyzing bearing capacity and settlement were developed,with consideration of the stress and deformation equilibrium among the stone column,encasement,and surrounding soil in the vertical and radial direction.3.The seismic response of the geosynthetic encased stone column composite foundation is revealed by the shaking table test.The peak acceleration of the geosynthetic encased stone column composite foundation is higher than that of the stone column composite foundation under the same earthquake wave.Compared to the stone column composite foundation,the geosynthetic encased stone column composite foundation under the earthquake loading produces smaller cracks in the larger area;and the settlement of the geosynthetic encased stone column composite foundation is less than the stone column composite foundation.The geosynthetic encased stone column generally shows better seismic performance than the stone column.4.The calculation method of bearing capacity of geosynthetic encased stone columns composite foundation under earthquake loading were proposed.Based on the composite failure surface(a log-spiral radial shear surface and planar rupture surface)of the geosynthetic encased stone column composite foundation under earthquake,the seismic action was simplified to the inertial force acting on the structure and the inertial force acting in the soil.Then,the seismic bearing capacity factors related to cohesion,unit weight of the soil,and surcharge pressure were determined using limit equilibrium technique.The calculation methods of the bearing capacity of the geosynthetic encased stone column composite foundation in the seismic case were developed;and the validity of this proposed method was verified by the results of the shaking table model test and the available results.