Soil-Pile-Structure Interaction Of Offshore Structures During An Earthquake

ABSTRACT Fundamental characteristics of soil-pile-structure interaction under seismic loading conditions were studied with mathematical solutions developed for an elastic soil with hysteretic damping and with a finite element method that included heterogeneous and nonlinear soil response. These studies quantified the influence of loading condition, pile-soil flexibility ratio, pile slenderness ratio, soil damping, soil Poisson's ratio, exciting frequency, and nonlinear stress-strain behavior of the soil on the dynamic lateral load-deflection relationships of piles. A simplified approach was presented to construct lateral load-deflection relationships of piles embedded in heterogeneous and nonlinear soil layers, and the results compared favorably with those from the finite element method. The simplified approach provides a rational and economic means to evaluate the soil-pile-structure interaction under seismic loading. INTRODUCTION Pile-supported structures have been subject to failure and major damage during severe earthquakes. For example, a number of pile foundations for bridge structures suffered damage during the Niigata and Alaska Earthquakes of 1964 (Fukuoka, 1966; Ross, Seed, and Migliaceico, 1969). Although liquefaction was the major cause of this damage, the buckling failure of the steel pipe pier of the Showa bridge during the Niigata Earthquake was reportedly due to the resonance of the bridge as a result of spil-pile-structure interaction. Unexpected resonance of the system can occur during seismic events if the resonance frequency of the system approaches the predominant frequency of the ground motions. This can occur when the free field ground deformations and soil-pile interaction reduce the stiffness of the soil-pile springs as a result of nonlinearity and degradation of the soil stiffness with cyclic loading. Thus, the dynamic response analysis of soil-pile-structure systems must represent correctly the dynamic characteristics of the surrounding soils. Many analytic models have been proposed to analyze the soil-pile-structure interaction; these models can be categorized as:foundation spring method;beam-on-Winkler foundation method;three-dimensional elastic method; andfinite element method. Most of the widely used analytic methods of the soil-pile-structure interaction, however, are based either on empiricism or on approximate soil-pile springs determined from integrating the Mindlin's static linearly elastic solution over a finite pile length or using a plane strain assumption in which an infinitely long pile is included in an elastic space. Current knowledge of the soil-pile-structure interaction under seismic loading condition needs development because characteristics of lateral load deflection relations of piles under seismic loading, fundamental behavior of the soil-pile-structure system, and the effects of soil nonlinearity on the soil-pile-structure response are not well understood. Increasing exploration activity for hydrocarbon reserves in earthquake-prone offshore areas, such as the Gulf of Alaska, the East China Sea, and offshore California, have emphasized the need for an improved understanding of the response of pile-supported structures to earthquake loading.