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Deformation and water seepage observed in a natural slope during failure process
by artificial heavy rainfall
Déformation du sol et infiltration d'eau observes le long d'une pente naturelle pendant le processus
de glissement du a de fortes pluies artificielles
Uchimura T., Gizachew G.
Department of Civil Engineering, University of Tokyo, Tokyo, Japan
Wang L., Nishie S., Seko I.
Chuo Kaihatsu Corporation, Tokyo, Japan
ABSTRACT: Monitoring and early warning is one of the most effective ways toward reduction of disasters induced by landslides due
to heavy rainfall. The authors has developed a simple and low-cost early warning system which measures tilting angles on the surface
and/or in the ground of a slope, as well as volumetric water contents in the ground. Unlike conventional sensing devices,
measurement with tilt sensors is simple and easy, but the translation of the obtained data of tilting angle is still under consideration.
The authors conducted an artificial rainfall test on a natural slope of weathered and loose andesite deposit in order to observe its
prefailure behaviours. The tilt angles and volumetric water contents were measured during the deformation and failure process. The
tilt sensors showed tilting rates between 0.1 and 0.5 degree / hour before failure. In addition, there was a unique relation between the
deformation and the water content, which is independent of the time history of the artificial rainfall. Simple shear tests were also
conducted to observe the prefailure deformation of an unsaturated soil. These observations will give us ideas on the warning criteria
for slope failure based on monitoring of tilting angle and water contents.
RÉSUMÉ : Donner l'alerte au plus tôt est sûrement la solution efficace pour réduire l’ampleur des désastres induits par les glissements
de terrain dus à des chutes massives de pluie. Un système d’alerte simple et à bas-coût qui mesure les angles d’inclinaison de la
surface du sol et/ou dans la terre, ainsi que le volume d'eau absorbé est proposé. Contrairement aux appareils conventionnels, la
mesure avec les sondes de l'inclinaison est simple et facile, mais la traduction des données obtenues est encore à l'étude. Un test de
chute de pluie artificielle sur une pente naturelle d'andésite durcie et dégagée pour observer les comportements des pré-fractures a été
réalisé. L’orientation de l'inclinaison ainsi que le volume d'eau ont été mesurés pendant la déformation de la surface. Les sondes ont
révélé des pentes comprises entre 0,1 et 0,5 degré/heure avant glissement. De plus, une relation unique indépendante de la durée de la
chute de pluie artificielle est établie entre la déformation du sol et le contenu d’eau. Des tests de cisaillement ont aussi été effectués
pour observer la déformation des pré-fractures d’un sol non saturé. Ces observations nous donnent des informations sur les critères
d'alerte pour prévenir un glissement de terrain.
KEYWORDS: landslide, slope failure, monitoring, early warning.
1 INTRODUCTION
There is a long history in prevention and mitigation of rainfall
and/or scouring-induced landslides. Mechanical counter
measures to prevent slope failure, like retaining walls and
ground anchors, have been widely used. However, they are
expensive and it is not realistic to apply such mechanical
measures for all of these slopes with potential risk, because
most of landslide occurs in small scale, but a large number of
slopes. Therefore, careful monitoring of slope behaviors and
consequent early warning is reasonable as alternatives.
The authors have proposed and developed an early warning
system for slope failures, as one of feasible countermeasures
(Figure 1) (Uchimura, et. al. 2010 & 2011a). The system
consists of minimum number of low-cost sensors on a slope,
and the data is transmitted through wireless network. Thus, the
system is low-cost and simple so that the residents in risk areas
can handle it to protect themselves from slope disasters.
There are several publications which report incremental
deformation of slopes before failure due to heavy rainfall
(Ochiai, et. al. 2004, Orense, et. al. 2003 & 2004). These
behaviors can be used as criteria of early warning for slopes
disasters. Extensometers and borehole inclinometers are
conventional and widely used devices to detect such
displacements of slopes. But, these devices require high skills
and equipments for installation and operation, resulting in a
respectable cost.
Therefore, the proposed early warning system uses tilt
sensors to detect abnormal deformation of slopes as shown in
Figure 1. The surface tilt sensor is installed with a rod inserted
into the slope surface for a depth of 0.5 or more. It has a MEMS
tilt meter (nominal resolution = 0.04 mm/m) to measures the
tilting angle in the surface layer of the slope. The obtained tilt
angle is equivalent to shear deformation of the surface layer,
which can be translated to the surface displacement if the tip of
rod reaches the intact base layer.
Surface tilt sensor
Miniature ground inclinometer
Receiver
(Gateway)
Internet
Realtime(every10min.)
Tiltresolution:0.003°
Battry lifetime>1–3yr.
Numberofunits
extended according
to slopeconditions
0. 5 m
0. 2 m
Di spl acement , dKx
dKx = 0.5 m・si n(dX1)
+ 0.5 m・si n(dX2)
(rel ati ve t o bottom)
0.5 m
Ti l t angl e, dX1
Ti l t angl e, dX2
Bottomsegment
as ref erence
Intact base layer
Unstable layer
Volumetric
water content
sensor
Tilt sensor &
Temperature
sensor
Tilting
angle
Steel rod in contact withbase layer,
or, inserted for depthof 0.5 - 1m
if theunstable layer is thick.
Wireless data
transmittion
Figure 1. Tilt sensor units and wireless monitoring system
The miniature ground inclinometer consists of stainless steel
pipe segments with a length of 50cm or 1m and outer diameter
of 25 mm. The segments are connected to each other by a
