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Collapse behavior of slope due to change in pore water pressure
Effondrement d'une pente à cause d’une variation de la pression interstitielle « + Corr BOULON »
Nakata Y., Kajiwara T., Yoshimoto N.
Yamaguchi University ,Ube, Japan
ABSTRACT: In 2010, a slope failure occurred due to heavy rain following an elevation in ground water level. Although a significant
rainfall was experienced at the site in the previous year, there was no deformation with a similar elevation. In the past several
collapses were recorded at the site due to heavy rain. Thus tests were performed using a plane strain compression apparatus to
understand the deformation behavior of sand subjected to cyclic loadings in pore water pressure which were simulated the observed
data on the collapsed slope. Rainfall records and data obtained from the observation well and extensometer were also shown. Finally
lessons learned from the data were outlined.
RÉSUMÉ : En 2010, un glissement de terrain eu lieu après une élévation du niveau de la nappe phréatique à cause de fortes pluies.
L'année précédente, bien que des précipitations légèrement plus fortes aient été enregistrées sur le site, il n'y eu pas de mouvement de
terrain avec un niveau similaire de la nappe. Dans le passé, plusieurs effondrements ont été enregistrés à cause de fortes pluies. Des
essais ont été effectués en utilisant un appareil de compression en déformation plane pour comprendre le comportement en
déformation d’un sable soumis à des pressions interstitielles cycliques qui expliquent les données observées sur la pente effondrée en
2009-2011. Les relevés pluviométriques et les données obtenues dans un puits d'observation et à partir d’extensomètres sont
également indiqués. Enfin on souligne les leçons tirées de l’ensemble de ces données .
KEYWORDS: slope stability, heavy rain, ground water level, pore water pressure, monitoring system, .
1 INTRODUCTION
Slope failures induced by heavy rainfall are one of the most
destructive natural hazards and these have claimed untold
numbers of lives and millions of dollars in infrastructure losses
every year in many parts of the world. The rapid progress of
global warming is already leading to changes in climate and
related environmental problems. In line with global trends,
temperatures in Japan are rising. At the same time active
seasonal weather fronts carrying heavy clouds are resulting in
increasingly heavy local downpours. As a result of these local
downpours of torrential rain there is a national increase in
geotechnical failures including slope failure and debris flows.
These notable tendencies due to climate change have been
increasingly recognized since the beginning of the 21st century.
In 2010, a slope failure due to heavy rain occurred at Hagi
city in Yamaguchi Prefecture, which has been known as an
unstable landslide area. In 2009, rainfall of significant intensity
was experienced at the site, but there was no evident the
deformation at that time. Additionally in the past, several
collapses occurred at the site due to heavy rain. This means that
several cycles of rise-and-fall of the ground water level have
occurred at the site. The cycle could be linked to the cyclic
changes of pore water pressure.
This paper firstly focuses on the cyclic change of pore water
pressure. So far, there has been little research work done on
cyclic loading of pore water pressure using triaxial compression
apparatus (Ohtsuka & Miyata, 2001, and Orense et al., 2004).
Therefore, in order to understand the deformation behavior of
soil subjected to cyclic loading of pore water pressure, an
experimental research using plane strain compression apparatus
was carried out for sand specimens and the results are
presented. Subsequently, the observed data on the collapsed
slope in 2009-2011 is explained. Especially rainfall records and
the data obtained from the observation well and extensometer
are shown. Finally, the lessons learned from the data are
described.
2
DEFORMATION BEHAVIOR SUBJECTED TO
CYCLIC CHANGE OF PORE WATER PRESSURE
2.1 Plane strain compression test
In order to simulate the cyclic rise-and-fall of ground water
level, experimental tests were performed by applying the cyclic
change of pore water pressure in the specimen. In past
researchers (Ohtsuka & Miyata, 2001, and Orence et al., 2004),
tests were carried out using triaxial compression test apparatus
applying monotonic loading of the pore water pressure. Thus,
plane strain compression tests were adopted to reproduce stress
condition closer to the site.
Each specimen was approximately 60mm wide, 160mm high
and 80mm deep. Axial loading was applied through an electric
motor from the top pedestal of the specimen. Confining and
pore water pressures were applied by E/P transducers. All the
external forces were controlled by a developed computer
program. Grease was painted on the front and back planes of
specimen in the direction of zero strain to minimize the friction.
Note that the intermediate principal stress could not be
measured in this study. Photos of the front plane of the
specimen in the direction of zero strain were taken during the
shearing process to observe local deformations.
The material used was Toyoura sand which has a higher
permeability to propagate the quick change in pore water
pressure and to keep the homogeneity of the effective stress
condition. The material can be considered to show the relatively
small compression deformation component. The specimens
were prepared by air pluviation method with a constant falling
height of 125mm. The specimen was formed in 5 layers with
the falling height kept constant for each layer. As a result, the
