PhD thesis advisor: Professor Malcolm Bolton, the University of Cambridge (U.K)
PhD committee:
1) Professor Ross Boulanger of U.C.Davis
2) Dr. Kenichi Soga of the University of Cambridge (U.K).
Download thesis. Available by the Geoengineer website upon request of the author.
Abstract:
Large magnitude earthquakes are low-probability but high-risk events. While these events cannot be
accurately predicted or prevented, understanding the behaviour of structures under such events
enables engineers to design safe earthquake-resistant structures.
This thesis describes the results of an investigation into the failure mechanism of piled foundations,
which are a particular type of deep foundation for heavily loaded structures e.g. high rise buildings,
bridges, ports, flyovers where these occur in areas of potential seismic liquefaction. Detailed dynamic
centrifuge testing, in-depth study of field case records and analytical studies form the basis of this
investigation.
Collapse of piled foundations in liquefiable soils is still observed after strong earthquakes despite the
fact that a large factor of safety (against bending due to lateral inertia loads) is employed in their
design. This thesis critically reviews the current design methods and the underlying mechanism behind
them. The current method of pile design under earthquake loading is based on a bending mechanism
where inertia and slope movement (lateral spreading of soil) induce bending moments in the pile. This
thesis aims to show that this hypothesis of pile failure is inconsistent with some of the observed mode
of failures. The well-known case study of the Showa Bridge is used to illustrate that although the
design of the piles in the bridge satisfies the latest Japanese Code of Practice (JRA, 1996), the bridge
actually failed during the 1964 Niigata earthquake.
A theory of pile failure, based on buckling instability is proposed in this thesis. The main postulate of
this theory is that if piles are too slender they require lateral support from the surrounding soil if they
are to avoid buckling instability. During earthquake-induced liquefaction, the soil surrounding the pile
loses effective confining stress and can no longer offer sufficient support to the pile. A slender pile may
then buckle sideways in the direction of least elastic bending stiffness pushing aside the initially
liquefied soil, and eventually rupturing under the increased bending moment and shear force. Lateral
loading due to slope movement, inertia or out-of-straightness increases lateral deflections, which in
turn induces plasticity in the pile and reduces the buckling load, promoting more rapid collapse. These
lateral loads are, however, secondary to the basic requirements that piles in liquefiable soil must be
checked against Euler's buckling. This theory has been formulated based on a study of fourteen case
histories of pile foundation performance and verified using dynamic centrifuge tests. Analytical studies
also support this theory of pile failure. A hypothesis of post-buckling pile-soil interaction is also
developed.
Centrifuge tests were designed in level ground to avoid the effects of lateral spreading and the main
aim was to study the effect of axial load as soil liquefies. The failure mode observed in the tests was
similar to those observed in the field in laterally spreading soil. It is concluded in this thesis that it is not
necessary to invoke lateral spreading of the soil to cause a pile to collapse. The pile may even collapse
before lateral spreading starts. The key parameter identified to distinguish whether buckling is a likely
failure mechanism is the slenderness ratio of the pile in the liquefiable region. The critical value of this
parameter is approximately 50.
In summary, it has been shown that the current codes of practice for pile design omit considerations
necessary to avoid buckling of fully embedded piles in liquefiable soils. These codes should be modified
to address buckling. Many of the structures designed based on the current codes of practice may be
unsafe and may need retrofitting. Therefore, a design method is proposed taking into consideration the
buckling effect.
Keywords: Pile failure, Buckling instability, Liquefaction, Case histories, Centrifuge tests, Slenderness
ratio, Lateral spreading.
PhD committee:
1) Professor Ross Boulanger of U.C.Davis
2) Dr. Kenichi Soga of the University of Cambridge (U.K).
Download thesis. Available by the Geoengineer website upon request of the author.
Abstract:
Large magnitude earthquakes are low-probability but high-risk events. While these events cannot be
accurately predicted or prevented, understanding the behaviour of structures under such events
enables engineers to design safe earthquake-resistant structures.
This thesis describes the results of an investigation into the failure mechanism of piled foundations,
which are a particular type of deep foundation for heavily loaded structures e.g. high rise buildings,
bridges, ports, flyovers where these occur in areas of potential seismic liquefaction. Detailed dynamic
centrifuge testing, in-depth study of field case records and analytical studies form the basis of this
investigation.
Collapse of piled foundations in liquefiable soils is still observed after strong earthquakes despite the
fact that a large factor of safety (against bending due to lateral inertia loads) is employed in their
design. This thesis critically reviews the current design methods and the underlying mechanism behind
them. The current method of pile design under earthquake loading is based on a bending mechanism
where inertia and slope movement (lateral spreading of soil) induce bending moments in the pile. This
thesis aims to show that this hypothesis of pile failure is inconsistent with some of the observed mode
of failures. The well-known case study of the Showa Bridge is used to illustrate that although the
design of the piles in the bridge satisfies the latest Japanese Code of Practice (JRA, 1996), the bridge
actually failed during the 1964 Niigata earthquake.
A theory of pile failure, based on buckling instability is proposed in this thesis. The main postulate of
this theory is that if piles are too slender they require lateral support from the surrounding soil if they
are to avoid buckling instability. During earthquake-induced liquefaction, the soil surrounding the pile
loses effective confining stress and can no longer offer sufficient support to the pile. A slender pile may
then buckle sideways in the direction of least elastic bending stiffness pushing aside the initially
liquefied soil, and eventually rupturing under the increased bending moment and shear force. Lateral
loading due to slope movement, inertia or out-of-straightness increases lateral deflections, which in
turn induces plasticity in the pile and reduces the buckling load, promoting more rapid collapse. These
lateral loads are, however, secondary to the basic requirements that piles in liquefiable soil must be
checked against Euler's buckling. This theory has been formulated based on a study of fourteen case
histories of pile foundation performance and verified using dynamic centrifuge tests. Analytical studies
also support this theory of pile failure. A hypothesis of post-buckling pile-soil interaction is also
developed.
Centrifuge tests were designed in level ground to avoid the effects of lateral spreading and the main
aim was to study the effect of axial load as soil liquefies. The failure mode observed in the tests was
similar to those observed in the field in laterally spreading soil. It is concluded in this thesis that it is not
necessary to invoke lateral spreading of the soil to cause a pile to collapse. The pile may even collapse
before lateral spreading starts. The key parameter identified to distinguish whether buckling is a likely
failure mechanism is the slenderness ratio of the pile in the liquefiable region. The critical value of this
parameter is approximately 50.
In summary, it has been shown that the current codes of practice for pile design omit considerations
necessary to avoid buckling of fully embedded piles in liquefiable soils. These codes should be modified
to address buckling. Many of the structures designed based on the current codes of practice may be
unsafe and may need retrofitting. Therefore, a design method is proposed taking into consideration the
buckling effect.
Keywords: Pile failure, Buckling instability, Liquefaction, Case histories, Centrifuge tests, Slenderness
ratio, Lateral spreading.
Pile Instability during earthquake liquefaction,
PhD thesis of Subhamoy Bhattacharya,
Cambridge University, 2003.
PhD thesis of Subhamoy Bhattacharya,
Cambridge University, 2003.
Copyright © 2004-2007 Dimitris Zekkos. All rights reserved.
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| A paper published titled "PILED STRUCTURES STILL COLLAPSE IN LIQUEFIABLE SOILS! – THE MISSING CONSIDERATIONS IN PILE DESIGN" from this thesis won the 2005 EAST OF ENGLAND REGION - YOUNG MEMBERS PAPERS COMPETITION. Read the paper. |
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