Study On Dynamic Properties And Safety Evaluation For Liquefiable Ground Reinforced By Granular Columns

Granular columns are widely used in liquefiable ground to improve its liquefaction resistance.The seismic response and liquefaction mitigation effect of sites reinforced by granular columns have received continuous attention in recent decades.However,there are few studies on the seismic instability analysis and safety evaluation for granular column reinforced liquefiable ground.This thesis proposes a three-dimensional(3D)plasiticity model for coarse grained soil based on an existing plasticity model for large post-liquefaction deformation of sand,developes a numerical simulation scheme for analyzing the seismic response of granular column improved liquefiable ground,and proposes a method for dynamic stability evaluation.Seismic performance and liquefication mitigation effect of granular column reinforced liquefiable ground under different working conditions is studied and practical analysis and dynamic safety evaluation are also carried out.New achievements have been made in the following aspects:1.A three-dimensional nonlinear anisotropy strength criterion of coarse grained soil is proposed,a plastic cyclic constitutive model of coarse grained soil is developed based on the proposed nonlinear strength criterion,and the plasticity model for large post-liquefaction deformation of sand and the plasctitiy model for coarse grained soil are implemented in the FLAC^3D code,during which the problem of calculation instability in liquefaction of zero effective stress is resolved.2.A numerical scheme for 3D elastoplastic fluid-solid coupling dynamic analysis of granular column improved liquefiable ground is established.The validation of the constitutive model,numerical integration algorithm,and program is preliminarily verified by satisfactorily simulating element tests of sand and coarse grained soil,and centrifuge shaking table model tests of saturated sand ground and saturated sand ground reinforced by granular columns.3.The seismic response patterns of liquefiable ground reinforced by granular columns with different granular column stiffness,drainage characteristics,and densification degrees of surrounding soil are revealed,including:a)the strain and stress distributions of granular columns and surrouding soil change continuely during shaking;b)the vertical effective stress ratio may increase to 3-6 while the shear strain ratio may decrease to 1/7-1/10;c)the influence region of each column is about 2.5-3 times of its diameter,being larger in deeper depth;d)the drainage and densification effects of columns can limit the accumulation of the excess pore water pressure;and e)the stiffness effect of columns can reduce the peak acceleration at ground surface.Granular columns can increase the shear stress ratio peak in the adjacent soil,and can significantly alleviate the lateral spread of slopes and the development of water film in liquefiable soil with low-permeability interlayers.4.Seismic response,liquefaction mitigation effect,and post-earthquake settlement of the liquefiable ground reinforced by granular columns with a superstructure based on an actual engineering project are investigated via numerical simulations,with input from the field data and estimated model material parameters.5.A strength reduction dynamic analysis method for liquefiable ground is preliminarily proposed,and a comprehensive evaluation criterion of dynamic instability is developed including the abruptness of displacement,the relative velocity divergence of the control point,and the run-through of plastic zone and liquefying zone.Adopting the method,the dynamic stability and safety evaluation of embankments on granular columns reinforced liquefiable ground considering the seepage-shaking coupling is developed.It is found that post-earthquake velocity inertia and liquefaction extension are the main reason of embankment dynamic instability.