2889
A design verification method for pile foundations used in combination with solidified
improved columns
Une méthode de vérification de la conception des pieux en combinant avec des colonnes de sol
améliorés
Tomisawa K.
International member, Civil Engineering Research Institute for Cold Region
Miura S.
International member, Hokkaido University Graduate School of Engineering
ABSTRACT: In this study, research and development were conducted on a method of forming composite ground around piles mainly
with solidified improved columns and reflecting the increased shear strength in pile design to support progress with a new foundation
type for application in soft ground. The use of this approach, which is referred to as the composite ground pile foundation method, in
line with site conditions is expected to reduce construction costs and improve the earthquake resistance of foundations. To
systematize the technique, the study examined a design verification method based on the results of a large-scale model experiment.
RÉSUMÉ : Dans cette étude, une méthode de l’amélioration du sol autour des pieux à l’aide des colonnes solidifiées améliorées a été
développée. L’effet de l’augmentation de la résistance au cisaillement dans la conception des pieux a été étudié pour un nouveau
système de fondation dans les sols mous. L'utilisation de cette approche, appelé la méthode fondation composite, avec les conditions
du site devrait permettre de réduire les coûts de construction et d'améliorer la résistance au séisme des fondations. Afin de
systématiser la technique, l'étude a porté sur une méthode de vérification de la conception basée sur les résultats d'une modèle
expérimentale à grande échelle.
1 INTRODUCTION
To support progress with a new foundation type for soft ground,
research and development were conducted concerning a rational
design method (Tomisawa
et al.
2005, Tomisawa
et al.
2007) in
which composite ground consisting mainly of solidified
improved columns is formed around piles and shear strength
enhanced by such ground improvement is reflected in the form
of horizontal resistance and bearing capacity. Although the use
of this approach (tentatively referred to as the composite ground
pile foundation method) in line with site conditions is expected
to reduce construction costs, partial improvement to the support
mechanism of piles alone is not enough; it is also necessary to
ensure the seismic performance of foundations (Japan Road
Association 2002) and to fully systematize the design and
construction involved in the method.
Accordingly, this study closely investigated past research
results (Tomisawa
et al.
2005, Tomisawa
et al.
2010) on the
static and dynamic mechanical behavior of pile foundations
used in combination with solidified improved columns, and
examined the concept of a design verification method for
composite ground pile foundations in line with existing design
approaches (Japan Road Association 2002, Architectural
Association of Japan 2001) based on the results of a large-scale
model experiment.
2 DESIGN CONCEPTS
The concept of the composite ground pile foundation is based
on reducing the foundation size to cut construction costs and
improve seismic performance. Figure 1 shows a comparison of
pile foundation specifications with the conventional method and
composite ground foundation. For very soft ground, the number
of piles is increased to ensure safety against the permissible
horizontal displacement, and especially against the reaction
force of the superstructure (Japan Road Association 2002).
More than ten rows of cast-in-place piles measuring 1,000 mm
in diameter are needed for preliminary design at an actual site,
as shown in the figure.
Conversely, if a composite ground pile foundation is used for
the same site, construction costs (including expenses incurred
for ground improvement) can be cut by approximately 30%
because only two rows of piles are required and the substructure
and building frame sizes are reduced considerably.
The key points in the design of composite ground pile
foundation are listed at the bottom of Fig. 1. In the new basic
design method, the horizontal subgrade reaction/bearing
capacity of piles is converted into the modulus of ground
deformation E based on the increased shear strength, and the
necessary range of ground improvement (i.e., the range of
horizontal resistance of the piles) is set as a three-dimensional
quadrangle that includes an inverted cone raised to the gradient
of the passive slip surface
θ
= (45° +
/2) (
: angle of soil shear
resistance) from the depth of the characteristic length of piles
1/
β
. In other words, the elastic subgrade reaction design method
for piles is applied with the improvement strength of solidified
improved columns set relatively low (equivalent to
q
u
= 200
kN/m
2
). The validity of this method of handling solidified
improved columns around piles as a reaction mass has been
confirmed statically in horizontal/vertical loading and centrifuge
model tests involving actual piles at several sites (Tomisawa
et
al.
2009). Centrifugal excitation testing and dynamic nonlinear
finite element analysis have also verified seismic performance
improvement effects, such as the reduction of pile foundation
deformation by almost half compared to the conventional design
method for unimproved ground, against Level 1 (acceleration:
approx. 150 gal) and Level 2 (acceleration: approx. 750 gal)
earthquake motion (Tomisawa
et al.
2008).
However, for pile foundations used in combination with
solidified improved columns to satisfy the performance
requirements of the current design method (Japan Road
Association 2002, Architectural Association of Japan 2001), it
is considered necessary to develop a design verification
