Effect of Earthquake Induced Lateral Soil Movement On Pile Behavior

Effect of Earthquake Induced Lateral Soil Movement On Pile Behavior

K. Muthukkumaran (National Institute of Technology, Tiruchirappalli, India) and I.P. Subha (National Institute of Technology, Tiruchirappalli, India)
Copyright: © 2011 |Pages: 20
DOI: 10.4018/jgee.2011070106
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Abstract

One of the most common causes of ground failure during earthquakes is the liquefaction phenomenon, which produces severe damage to property. Although methods are available for seismic analysis of pile foundations, most of them consider soil to be an elastic material. Collapse of piled foundations in liquefiable areas has been observed in most recent strong earthquakes despite the fact that a large margin of safety is employed in their design. Lateral spreading of gently-sloping deposits of liquefiable sand is a cause of much damage in earthquakes, reportedly more than any other form of liquefaction-induced ground failure. The present investigation finds the effect of earthquake induced lateral soil movement on lateral pile capacity. Parametric study is carried out on the same model by changing the ground surface to different slopes on the top of the non liquefiable layer and by changing the length of the pile in the bottom layer of the non liquefiable layer. The paper focuses on the behaviour of pile under lateral soil movement due to earthquake. The bending moment and displacement behaviour of pile is studied in detail for different slope conditions.
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Introduction

The seismic waves generated due to earthquake cause the ground to vibrate and create severe natural disasters. Various types of ground deformations occur during vibration and such deformations leading to failure can be recognized as ground failures. The dramatic response of saturated loose sand deposit due to earthquake can be hazardous to constructed facilities in the seismic regions where such soil deposits exist. In liquefiable soils, earthquake excitation shall induce the development of excess pore pressure and when the excess pore pressure reaches the initial overburden effective stress, the soil is liquefied.

Liquefaction behaviour was observed during a number of earthquakes in the past. During Alaskan Earthquake (1964), liquefaction was the main cause of severe damage to 92 highway bridges, moderate to light damage to another 49 highway bridges, and moderate to severe damage to 75 railroad bridges. During Niigata Earthquake-Hamada (1964) liquefaction induced damage to the foundation piles of Yachiyo and Showa bridges and Niigata Family Court House building (NFCH) building and underground pipelines. During that same earthquake, girders of Showa Bridge toppled as the support structure and piles moved excessively due to liquefaction. During Kobe Earthquake (1995), liquefaction was the primary cause of damage to many pile supported or caisson supported bridges and structures. For example, Shin– Shukugawa Bridge was subjected to excessive pile foundation movement due to liquefaction. It has caused the collapse of the Daikai subway station. In the 1995 Kobe earthquake, massive liquefaction of reclaimed fills caused serious damage to numerous pile foundations of buildings, storage tanks and bridge piers.

The unprecedented level of damage to foundations of modern engineering structures stimulated a great number of research studies in an effort to improve the understanding of soil–pile interaction in liquefied soils and seismic performance of pile foundations. The effects of liquefaction on deep foundations are very damaging and costly. Lateral spreading especially occurs close to free boundaries such as rivers, the shore or quay walls, as here the water table is high and the boundary conditions favourable.

This implies that the bending moments or shear forces that are experienced by the piles exceed that predicted by those design method (or code of practice). All current design codes apparently provide a high margin of safety (partial safety factors on load, material stress), which would mean that the actual moment or shear force experienced by the pile is many times the predicted moment or shear. It may be concluded that design methods are not consistent with the physical mechanism that governs the failure. It is essentially a kinematic soil–structure interaction process involving large ground and permanent foundation deformations, with the deep foundation and superstructure responding pseudostatically to the lateral permanent displacement of the ground. Predicting the behaviour of a pile foundation in liquefying ground during an earthquake requires consideration of design motions, free-field site response, superstructure response, and soil-pile-superstructure interaction.

The foundation may be exposed to large lateral soil pressures, especially passive pressures from the nonliquefied shallow soil layer riding on top of the liquefied soil. The observed damage and cracking to piles is often concentrated at the upper and lower boundaries of the liquefied soil layer where there is a sudden change in soil properties. A significant damage tends to occur to piles when the lateral movement is forced by a strong nonliquefied shallow soil layer than when the foundation is free to move laterally and the forces acting on them are limited by the strength of the liquefied soil. Peak ground deformations can occur either during or toward the end of shaking, depending on the magnitude of transient ground movements during the lateral spreading process.

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