2637
Large-scale Piled Raft with Grid-Form Deep Mixing Walls on Soft Ground
Comportement en vraie grandeur d’une fondation mixte radier-pieux établie dans un sol meuble
amélioré par quadrillage de mélange profond de sol
Yamashita K., Wakai S., Hamada J.
Research & Development Institute, Takenaka Corporation, Chiba, Japan
ABSTRACT: This paper offers a case history of a large-scale piled raft supporting a twelve-story building founded on liquefiable
sand underlain by soft cohesive soil in Tokyo. The building, 55.7 m in height above the ground surface and measuring 120 m by 100
m in plan, is a steel-framed structure with a base isolation system of laminated rubber bearings. An assessment of a potential of
liquefaction during earthquakes indicated that the loose clayey sand between depths of 5 and 15 m had a potential of liquefaction
during earthquakes with the peak horizontal ground acceleration of 3.0 m/s
2
. Therefore, a piled raft combined with grid-form deep
cement mixing walls was employed to cope with the liquefiable sand and also to reduce settlements of the soft cohesive soil below the
sand. To confirm the validity of the foundation design, field measurements were carried out on the foundation settlements, the axial
loads of the piles, the contact pressures between raft and soil and the pore-water pressure beneath the raft from the beginning of
construction to 11 months after the end of construction.
RÉSUMÉ : Cet article présente une étude de cas en vraie grandeur d'une fondation mixte radier-pieux d’un bâtiment à douze niveaux
construit à Tokyo. Cette fondation est établie dans une couche de sable liquéfiable reposant sur une couche de sol cohérent et meuble.
Le bâtiment, qui fait 120 m par 100 m dans le plan et 55.7 m en hauteur au-dessus du sol, a une structure métallique en portiques. Il
est isolé à sa base par un système d’appareils d'appui en élastomère fretté. Le potentiel de liquéfaction estimé du sable argileux entre 5
et 15 m de profondeur serait atteint sous une acceleration horizontale maximum de 3.0 m/s
2
. Pour pallier à ce phénomène et reduire le
tassement de la couche sous-jacente de sol cohérent et meuble, une fondation mixte radier-pieux a été adoptée en combinaison avec
l’amélioration, en forme de quadrillage, de la couche de sable par mélange profond. L’article discute le comportement de cette
fondation sur la base d’une série de mesures sur site, qui se sont poursuivies depuis le début jusqu’ à onze mois après la fin de la
construction du bâtiment. Le dimensionnement de cette fondation est estimé convenable considérant les mesures de tassement, des
forces axiales sur pieux, des contraintes sur le sol et de la pression de l’eau interstitielle sous le radier.
KEYWORDS: piled raft foundation, deep cement mixing wall, soft ground, field measurements, settlement, load sharing
1 INTRODUCTION
In recent years there has been an increasing recognition that the
use of piles to reduce raft settlements can lead to considerable
economy without compromising the safety and performance of
the foundation (Poulos, 2001). Detailed investigations of many
high-rise buildings founded on piled rafts in Germany have
been carried out (Katzenbach et al. 2000). Piled raft foundations
have been used for many buildings in Japan and the settlement
and the load sharing between raft and piles have been carefully
investigated for the selected buildings (Yamashita et al. 2011a;
Yamashita et al. 2011b). It has become necessary to develop
more reliable seismic design methods for piled rafts,
particularly in highly seismic areas such as Japan.
This paper offers a case history of a large-scale piled raft
supporting a twelve-story building founded on liquefiable sand
underlain by soft cohesive soil in Tokyo. To cope with the
liquefiable sand and also to reduce settlements of the soft
cohesive soil below the loose sand, piled raft foundation
combined with grid-form deep cement mixing walls was
employed. To confirm the validity of the foundation design,
field measurements were carried out on the foundation
settlements, the axial loads of the piles, the contact pressures
between the raft and soil and the pore-water pressure beneath
the raft from the beginning of construction to 11 months after
the end of construction. During the construction period, the
2011 off the Pacific coast of Tohoku Earthquake struck the site
of the building. The effects of the earthquake on the settlement
and the load sharing between the raft and the piles are also
discussed.
2 BUILDING AND SOIL CONDITIONS
The twelve-story office building is located in Tokyo, 0.3 km
southeast from the twelve-story residential building (Yamashita
et al., 2011b). Figure 1 shows a schematic view of the building
and the foundation with a soil profile. The building, 55.7 m in
height above the ground surface and measuring 120 m by 100 m
in plan, is a steel-framed structure with a base isolation system
of laminated rubber bearings. The foundation levels were
between depths of 3.6 and 7.2 m.
The subsoil consists of an alluvial stratum to a depth of 44 m
below the ground surface, underlain by a diluvial very dense
sand. The ground water table appears approximately 3 m below
the ground surface. The soil profile down to a depth of 15 m is
made of fill which consists of loose clayey sand, sandy clay and
rubble. Between the depths of 15 to 44 m, there lie very soft to
medium silty clay which is slightly overconsolidated with an
OCR of 1.3 or higher. The shear wave velocities derived from a
P-S logging were 150 m/s at the foundation levels and 290 m/s
in the dense sand below the depth of 44 m.
3 FOUNDATION DESIGN
