1499
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
1
Study on long-term subsidence of soft clay due to 2007 Niigata Prefecture Chuetsu-
Oki Earthquake
Étude sur affaissement à long terme en raison d'argile molle à 2007 la préfecture de Niigata
Chuetsu-Oki Tremblement de terre
K. Isobe & S. Ohtsuka
Nagaoka University of Technology
ABSTRACT: In the 2007 Niigata Prefecture Chuetsu-Oki Earthquake, ground liquefaction was particularly prominent at the foot of
sand dunes and at old river channels in the area affected. Although extensive damage to clayey ground was not seen immediately after
the tremor, long-term subsidence has been observed in such ground in the Shinbashi district of Kashiwazaki City. Against such a
background, the present study was conducted to investigate the mechanism of this long-term subsidence through ground
investigations in the form of a boring survey and indoor element tests on soil samples. The results showed that the samples were very
soft and relatively highly struc
tured. In the study’s second part, ground subsidence was simulated
via elasto-plastic analysis of soil
coupled with classical consolidation theory by using FEM. In the simulation, the Cyclic Mobility model developed by Zhang et al.
(2007), which incorporates the concepts of subloading and superloading (as described by Hashiguchi and Ueno (1977) and Asaoka et
al. (2002)) was used as the constitutive model with parameters determined in line with the element test results. Based on the outcomes
of the simulation, the post-quake behavior of soft clay and the related mechanism were elucidated, and the progress of subsidence was
predicted.
RÉSUMÉ : Lors du Tremblement de terre au Chuetsu-Oki (Préfecture de Niigata) à 2007, la liquéfaction des sols a été
particulièrement importante au pied des dunes de sable et aux canaux des rivières anciennes dans la zone affectée. Bien que des dégâts
significatifs du terrain argileuse n’aient pas été vus tout de suite après le tremblement de terre, le tassement à long terme
a été observé
dans le quartier de Shinbashi, à Kashiwazaki. Dans un tel contexte, l’étude actuelle a été effectuée pour étudier les mécanis
mes de ce
tassement à long terme à traverse d’une étude par carottage et des tests au laboratoire sur des échantillons
de sol. Les résultats ont
montré que les échantillons étaient très doux et relativement très structuré. Dans la deuxième partie de cette étude, l'affaissement du
sol a été modélisé par le couplage de sol-eau et des analyses élasto-
plastique à l’aide de l’éléme
nt finis. Dans la simulation, le modèle
de mobilité cyclique développé par Zhang et al. (2007), qui intègre les concepts de sub-charge et super-charge (comme étant décrit
par Hashiguchi et Ueno (1977) et Asaoka et al. (2002)) a été utilisé comme loi de comportement en utilisant des paramètres
déterminés par des essais expérimentaux. En se basant sur des résultats de la modélisation, le comportement post-sismique de l'argile
molle et les mécanismes associés ont été élucidés, et l’évolution du tassement dans
le temps a été aussi prédit.
KEYWORDS: earthquake, soft clay, long-term ground subsidence, over consolidation, soil structure, soil-water coupling, FE analysis
1 INTRODUCTION
The Niigata Prefecture Chuetsu-Oki Earthquake that hit Japan
on July 16th, 2007, killed 11 people and damaged more than
6,000 residences. The quake registered 6.8 on the magnitude
scale of the Japan Meteorological Agency (JMA), and a
Kyoshin Net (K-net) accelerograph measured a maximum
ground acceleration of 813 Gal. The results of a ground
subsidence survey conducted by Niigata Prefecture officials in
the Shimbashi district of Kashiwazaki City indicated that long-
term ground subsidence has continued there since the quake
occurred.
To investigate the cause of earthquake-related long-term
subsidence in soft clay ground, boring investigation involving
standard penetration tests (SPTs) was conducted in this study in
the vicinity of a site where ground subsidence was observed.
Static and cyclic CU tri-axial tests were also carried out to
clarify the mechanical characteristics and liquefaction strength
of Kashiwazaki clay. Subsequently, soil-water coupling elasto-
plastic finite element analysis was performed with soil
parameters determined from the above element test results to
simulate the observed subsidence. In this study, the cause of
long-term subsidence in soft clay was discussed and future
subsidence was mathematically predicted based on the results of
simulation.
2 OVERVIEW OF GEOGRAPHICAL CONDITIONS IN
KASHIWAZAKI CITY
Figure 1 (a) shows a map of the city highlighting cumulative
ground subsidence both before 2009 and after the earthquake,
and Figure 1 (b) shows a terrain classification map of the area.
The zone in which ground subsidence was observed before the
quake is mainly located on land consisting of soft alluvial clay,
while that in which it developed after the quake is located near
the sea and is mostly covered by the above-mentioned area.
Stratified subsidence and ground water levels were measured by
Niigata Prefecture officials in the Shinbashi district (marked as
B in Figure 1 (b)) from 1989 to 2011, and Figure 2 shows
temporal changes in ground water level and cumulative
subsidence to a depth of 23 m. It can be seen from the figure
that subsidence progressed rapidly after the quake of 2007 and
continued for over three years. However, this was not caused by
consolidation of soft clay due to changes in the ground water
level because no drastic changes in water level were observed in
the area. To pinpoint the cause, boring investigation involving
standard penetration tests (SPTs) was carried out in the vicinity
of a site where ground subsidence was observed, and the area’s
soft clay was sampled. Figure 3 shows geological columns of
the area based on the results of the autho
rs’ boring investigation
and information from the Hokuriku Geological System
database. Land in the Shinbashi district consists mainly of very
Chuetsu-Oki Earthquake
l’affai sement à long term d’argile molle dû au trembl
ement de terre de la
préfecture de Niigata Chuetsu-Oki en 2007
Isobe K., Ohtsuka S.
Nagaoka University of Technology
