Numerical Study Of Seismic Response Of Pile Structure In Saturated Liquefiable Soil Foundation

As an widly concern in the design of most important structures,the prediction of seismic induced liquefaction and the corresponding prevention is always a frontier issue in the research of civil engineering earthquake engineering.As a historical and common used foundation type,it was also widly applied in the construction of tall buildings,bridges,large factory buildings for its high bearing capacity,easy construction,and high liquefaction resistacne characteristics.Especially along with the construction of offshore light pier,offshore wind farm,as well as various ocean platform structures,apart from the consideration of the dynamic coupled pile-soil interaction,the influence of rising of excess pore water pressure and even the occur of seismic liquefaction on the bearing capacity of soil foundation should be taken into accounted.Meanwhile,in view of the distinct liquefaction resistance of different type soils,therefore the mechanical property,liquefaction resistance of foundation soil should be precisely calculated,and adopt effective engineering measurements to reduce the risk of seismic liquefaction.As a multidisciplinary problem in civil engineering,there already has numerous researches on the issue of dynamic pile-soil interaction with liquefiable saturated soil foundation.In spite of the numerous past investigation on the liquefaction model of soil based on laboratory liquefaction test,there still lack of rigourous theoretic support,and also difficult to applied in real civil engineering projects.In order to resolve these issues,based on the reviews of the studies of dynamic pile-soil interaction,the seismic simulation of saturated free field and offshore pipe pier are carried out in both technical routes of simplified calculation and nonlinear FEM simulation methods.The first part of this paper contains the following sections:(1)Based on the Biot's equation of two-phase saturated soil medium,stiffness matrix of saturated soil layer is established,and then seismic simulation model of saturated layered soil field is established based on the earthquake analysis program of SHAKE91.Then the parametric study of seismic response are conducted under this framework,and at the same time,considering that in most time,the foundation layer confronted in practical projects are rearely ideal homogeneously distributed,expecially in offshore sedimentary geological environment,which displays more obviously,then the influence of inconsistant middle layer are studied in detail.(2)Owing to the existence of burden soil layer,particularly when the field layer exists inconsistent middle layer,the seismic wave incident from bedrock could be significantly affected for the selective filtering effect of soil layer.And thereby in the numerical seismic simulation program,the input earthquake wave could not add to structure foundation directly.In view of the distinct selective filtering effect of soil foundation with different layered soil,based on the seismic study of free field,the seismic response of light offshore pipe pier with the consideration of the dynamic coupled pile-soil interaction is systematically discussed in detail.The seismic research conducted in this paper could provide some useful reference for the construction of offshore pier structure in complex saturated layered soil foundation.The second part of this paper contains the following sections:(3)Along with the research of soil's nolinear characteristics,more and more studies have realize that soil turns into overconsolidated state under cyclic loading,and for the soil under overconsolidated state,which shows distinct compressibility,mechanical properties and liquefaction characteristics in its yield process,and then could significantly affect the seismic response of structure.Therefore,in order to study the mechanical properties of soil under cyclic loading,especially the cyclic undrained loading response with the consideration of the rising of excess pore water pressure,from Chapter 4,the stress-strain relationship,compressibility,rising of excess pore water pressure and the evolution of overconsolidation are well studied under the framework of subloading surface Cam-clay model.And besed on the study of overconsolidated soil,a dynamic nonlinear FEM model of pile-soil coupled system in two-phase saturated soil foundation is established,and then the seismic response of offshore light pipe pile pier under earthquake incident are carried out.(4)Based on the comprehensive study of overconsolidated soil,through the control of the evolution ratio of soil,the comparative study of liquefaction resistance of clayey and sandy soils are conducted in the unified theoretic framwork of subloading surface Cam-clay model,explains the liquefaction difference from a more rigorous theoretic view point of state variable.At the same time,considering the close correlation between soil density and the overconsolidation state of soil,then liquefaction resistance of compacted sandy soil after undrained cyclic compaction is evaluated,deepened the mechanism of seismic liquefaction resistacne of soil under different density condition.(5)Affected by the external and internal factors in its weathering process,the undisturbed natural soil forms the special porous structure formation process.Along with the develop of modern in-situ soil sampling technology,as well as the knowledge of the microscopic structure of natural soil,more and more studies indicate that there exist great mechanical and liquefaction characteritics difference between natural soil and remould soil after disturbance,then precisely simulate the natural structured soil,and evaluate the influence of soil structure on the practical civil engineering structures has great engineering significance.Based on the investigation of state variable of overconsolidation,with the introduction of soil state variable of structure,the mechanical and liquefaction properties of natural structured soil is well investigated under the framework of superloading surface Cam-clay model,and then comparative seismic response of pier structure with both natural and remold soil foundtions are also carried out.