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Landslides (2010) 7:351357DOI 10.1007/s10346-009-0178-zReceived: 23 July 2009Accepted: 24 September 2009Published online: 17 October 2009 Springer-Verlag 2009

Taro Uchimura . Ikuo Towhata . Trinh Thi Lan Anh . Jou Fukuda . Carlos J. B. Bautista .

Lin Wang . Ichiro Seko . Taro Uchida . Akira Matsuoka . Yosuke Ito . Yuichi Onda .

Sho Iwagami . Min-Seok Kim . Naoki Sakai

Simple monitoring method for precaution of landslideswatching tilting and water contents on slopes surface

Abstract A low-cost and simple monitoring method for early

warning of landslides is proposed. To detect abnormal deforma-

tion of a slope, this method employs a tilt sensor in place of an

extensometer on the slope surface. In order to examine the

relevance of measuring rotation angle on a slope surface by tilt

sensor, model tests were conducted, and rotation on the slope

surface was observed together with slide displacement along the

surface. The rotation data responded 30 min before failure in a

model test, which could be useful as a signal for early warning.

However, the behavior of rotation before failure varies from case to

case, and thus, criteria to issue warning should be defined more

carefully. For a model slope made of uniform loose sand,measurement of slide displacement along the slope surface is

sensitive to failure at the toe, while the measurement of rotation on

the slope surface is useful to detect the development of progressive

failure upward along the slope. Wireless sensor units with

microelectromechanical systems (MEMS) tilt sensor and volumet-

ric water content sensor were also examined on a real slope in

Kobe City, and a long-term monitoring was attempted. A simple

but possible way to define the criteria of judgment to issue warning

can be proposed based on combination of data obtained by the tilt

sensors and volumetric water content sensors.

Keywords Early warning . Monitoring . Rainfall induced

landslides . Tilt angle . Volumetric water contents

Introduction

There is a long history in prevention and mitigation of rainfall-

induced landslides. Typical measures to prevent slope failure are

retaining walls and ground anchors, which improve the factor of

safety against failure. These measures have been widely used

everywhere in the world and have been effective. However, they are

very expensive, resulting in a limited application only for large-

scale slopes. In reality, most landslides occur in small-scale slopes,

but in large numbers. It is very difficult to apply mechanical

reinforcement measures for these slopes with potential risk. So,

non-structural countermeasures have been conducted. Until now,

early warning systems have been based only on rainfall data.

The authors have proposed an early warning system for slope

disasters, as one of the more feasible countermeasures for small-

scale slope disasters (Towhata et al. 2005; Uchimura et al. 2008).

The system watches the behaviors of subsoil at a minimum number

of points on a slope with inexpensive and sophisticated sensors,

and the data are transferred through a wireless network. Thus, the

system is low-cost and simple enough so that the residents in

hazardous areas can use it to protect themselves from slope

disasters.

It is reported from model tests that gradual displacement and

high saturation ratio (80% to 90%) are observed at the toe of model

slopes before failure (Orense et al. 2003, 2004). Ochiai et al. (2004)

also reported gradual and accelerating displacement on a slope

surface observed before failure in an artificial rainfall-induced

landslide test conducted at Mt. Kaba-san, Tsukuba, Japan. Thus, it

is useful to monitor the displacement and the water contents on

the slope for early warning of landslides.

The system proposed herein watches the rotation and the

volumetric water contents near the slope surface. Microelectro-

mechanical system (MEMS) tilt sensors are used to detect

abnormal deformation on the slope surface, in place of extensom-

eter, which is commonly used for this purpose. The advantage of a

tilt sensor is that the long wire of an extensometer is not required,and therefore, the installation and maintenance are simple and

inexpensive. However, the measured tilting angle usually does not

indicate the displacement of the slope surface directly. Therefore,

the relevance of measuring rotation angle on slope surface is of

major concern in this study. Herein, some examples of measure-

ment of rotation on models and a real slope surface are described,

and its effectiveness is discussed.

Behavior of rotation on model slope

In 2006, the authors developed prototypes of sensor unit with a tilt

sensor and a volumetric water content sensor, and tested them on a

1-m-high model sandy slope under artificial heavy rainfall,

conducted by Public Works Research Institute (PWRI), Tsukuba,

Japan (Fig. 1). The model slope had a gradient of H=2: V=1, and

was made of a compacted sandy material (Dmax=4.57 mm, D50=

0.17 mm, Fc=14.3%, Gs=2.69, d=1.37 g/cm3, Dr=80%), and its

initial water content was 19%. After filling water on the back side of

the slope to reproduce the situation of river dike with a high water

level, an artificial continuous rainfall of 15 mm/h was produced.

Two sensor units, equipped with a MEMS tilt sensor and a

volumetric water contents sensor, were installed on the slope as

shown in Fig. 1. The installation procedure was simple, just

embedding the units and the attached water content sensor on the

slope at a depth of 20 cm, taking less than 30 min for each unit.

Figure 2 shows the data of the rotation and water content at

each sensor unit. The slope failure was progressive starting from

the toe, and the lower part with sensor unit 2 failed at around 2 h

after starting rainfall. The rotation showed an abnormal change

30 min before failure. Such behavior could be used as a signal for

early warning. In contrast, the upper part around sensor unit 1

failed after 3 h of rainfall, but the development of rotation was not

as clear as in the lower part. The behavior of rotation before failure

is case-by-case, and thus, criteria to issue warnings should be

defined carefully.

On the other hand, the records of volumetric water contents are

presented in Fig. 3. As the void ratio is e=0.935, the volumetric

water content will be 0.48 if the soil is fully saturated. The

Technical Note

Landslides 7 (2010) 351


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1m

1m 2m2m

0.8m

2:1 1.5m

0.5m

Sensor

unit 1

Sensor

unit 2

+

Sealing

S

Volumetric water

content sensorsensorMEMS tilt

200mm

Radio module

ensor unit 1(higher)

Sensor unit 2 (lower)

Fig. 1 Arrangement of slope model

and sensor units for the test 2006

9540 9600 9660 9720 9780-50

-40

-30

-20

-10

0

10

20

30

40

50

Elapsed time (min)

Rotation(mm

/m)

(Positivefordownwa

rdtilting)

Sensor unit 1(Upper position)

Sensor unit 2(Lower position)

10:27:00

Sensors startedto move9:15:00

Failure at lower part10:54:00

Rainfall startedTime = 9:00:00

Failure athigher part

12:10:00

10:45:00

Fig. 2 Behaviors of inclination for the

test 2006

Technical Note

Landslides 7 (2010)352


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measured water contents increased after starting rainfall, but they

did not indicate nearly saturated condition before the failure. Thus,

it was found to be difficult to detect precursor of failure by only

monitoring water content.

Comparison between rotation and displacement

In 2008, the authors conducted a slope model test with monitoring

the behaviors of the slope surface for the rotation and the slide

displacement, which is measured along the slope (Figs. 4 and 5).The test was conducted at a test site of National Research Institute

for Earth Science and Disaster Prevention, Tsukuba, Japan. Figure 5

shows the side view of the model, and the locations of sensors for

rotation and slide displacement. In the test presented herein (case

4), a 6-m-long and 0.5-m-deep slope was constructed in a sand box

with a width of 1.5 m and a slope angle of 30 deg. The material was

loose sandy soil (Gs=2.63, d=1.45 g/cm3, Dr= 65%). Artificial

rainfall of 80 mm/h was produced continuously, and the

deformation and the water contents in the slope were monitored.

Each MEMS tilt sensor was placed on a small, T-shaped peg,

which was pushed into the slope surface over around 3 cm of depth

to measure the rotation. The sensor for slide displacement

consisted of a phosphor bronze strip with strain gages at thecenter, and an L-shaped target plate was placed on the slope

surface. As one end on the strip was connected on a stationary

beam, the slide displacement of the L-shaped target along the

surface caused bending of the strip, and the strain gages sense this

bending deformation. These sensors were installed at three points

on the slope surface, 30, 140, and 220 cm (L30, L140, and L220,

respectively, in Fig. 5) from the lower end of the slope. Volumetric

water content sensors were also embedded at a depth of 25 cm at

the same points.

Figure 6 shows the time histories of the rotation, the slide

displacement, and the saturation ratio calculated from the

volumetric water contents. The slope failed progressively from

the toe. The rainfall continued until the failure reached the point of

L140, and then, some amount of water was injected from the

bottom of the sand box to induce failure in the higher part of the

slope.

The lower part of the slope, L30, failed after the saturation ratio

reached 90%. The values of the slide displacement responded first,

and the rotation angle followed. At the same time, the slide

displacement started at the higher part of the slope (L140 and L220),

showing that the entire of slope became unstable when the toe failed.

After that, the slide displacement at L140 and L220 increased at the

same rate until the failure front reached respective points.

In contrast, the rotation on the slope surface at L140 did not

respond before the failure front reached its vicinity. The rotation at

L220 did not respond before the water injection process because

the failure front did not reach there.

9540 9600 9660 9720 97800.0

0.1

0.2

0.3

0.4

0.5

Failure atlower part10:54:00

Failure athigher part12:10:00

Sensor unit 1(Upper position)

Sensor unit 2(Lower position)

Rainfall started9:00:00

Elapsed time (min)

Volumetricwa

tercontents

(mm

3/

mm

3)

Fig. 3 Behaviors of water contents for

the test 2006

Front ofprogressive failure

Fig. 4 Front view of slope model test in 2008



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Thus, it can be concluded that, for a slope with uniform loose

sand, measurement of slide displacement along the slope surface is

sensitive to the initial failure at the toe, while the measurement of

rotation on the slope surface is useful to detect the development of

progressive failure toward the upper part of the slope.

Monitoring of rotation on real slope

The authors developed wireless sensor units with a MEMS tilt

sensor and a volumetric water content sensor, and installed them

on a steep slope in Rokko Mountain, Kobe City, in Japan (Fig. 7).

The slope has 40 to 50 degrees of gradient with weathered granite,

which is a typical soil in this area, on the slope surface. At each

monitored point, a steel rod was hit into the slope by around

30 cm, which is the thickness of unstable soil layer on the slope

surface. Then, a wireless sensor unit was attached at the top of the

rod. The volumetric water content sensor was installed at around

20 cm of depth under the slope surface.

The system is designed to be wireless. The sensor units measure

the rotation angle of therod andthe volumetric water contentsin the

soil every 10 min and transfer the data to a main unit, which is also

placed near the slope, by radio communication. The main unit

collects the data from all the sensor units and sends themto a WWW

server through a cell phone network. Thus, the data can be browsed

anywhere and anytime on the Internet. Each sensor unit is powered

with four AA alkaline batteries, and functioned well in the site for

duration of more than 1 year, although thedata were lost accidentally

due to errors in Internet communication for some duration.

Figure 8 shows typical observation of the rotation, which is

positive for down-slope tilting, and the water content obtained by

sensor units B and D for around 8 months. There were several

events of heavy rain in this region, but the slope did not move

during this period. The recorded values of rotation fluctuated

within a range of 10 mm/m probably due to the effects of

temperature and other factors on the sensor, which is much smaller

than what was observed in the model tests mentioned above. The

water content data show clear response at every rainfall event, and

decays gradually afterward.

Although high water contents due to heavy rainfall can cause

instability of the slope, it is difficult to evaluate the probability of

slope failure quantitatively based on the values of water content.

However, it is easy to know whether it is raining or not, by

observing the change in water content values. Thus, a simple but

possible way to define the criteria of judgment to issue warning

is:

(a) The record is judged to be abnormal if the sensor unit tilts more

than a threshold value, which is prescribed in advance

according to the data in normal conditions.

(b) The above judgment is ignored to avoid false alarms if no

rainfall event is observed in the data of the volumetric water

contents for the previous duration of a few days.

Conclusions

A low-cost, simple monitoring method for precaution of rainfall-

induced landslides is proposed, which uses tilt sensors on the slope

surface to detect abnormal deformation.

In the first model test with tilt sensors, the rotation on the

slope surface showed abnormal behaviors 30 min before

Rotation Tilt sensorMEMS

600cm

110cm

80cm

30cm

30deg40cm

Stationary

beam

Slide disp.bronze strip

Phosphor

Strain gages

Fig. 5 Arrangement of displacement,

tilting, and volumetric water content

transducers

Technical Note



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failure. Such behavior could be used as a signal for early

warning. However, the behavior of rotation before failure is

case-by-case, and thus, criteria to issue warning should be

defined carefully.

In the second model test, which compared the rotation and

the slide displacement on a slope surface, for a slope with

uniform loose sand, measurement of slide displacement along

the slope surface was sensitive to the initiation of failure at the

toe, while the measurement of rotation on the slope surface was

found to be useful to detect the propagation of progressive

failure upward.

Wireless sensor units with a MEMS tilt sensor and a

volumetric water content sensor were developed by the authors

and installed on a real slope in Kobe City, and long-term

monitoring was attempted. A stable data of rotation within some

range of fluctuation was obtained continuously. It is difficult to

evaluate the probability of slope failure quantitatively based on

the values of water content. Conversely, it is easy to detect

rainfall events from the water content data. Thus, a simple but

possible way to define the criteria of judgment to issue warning

can be proposed based on combination of the tilt sensor and

volumetric water content sensor.

0 1800 3600 5400 7200

0

20

40

60

80

Time (sec)

Failure near L30

L140

L220L30

0 1800 3600 5400 7200

-600

-400

-200

0

200

400

600

Rainfall start (80mm/h)

Water injectedat bottom of box

Rotation(mm/m)

Slidedisp.

(mm

)

Failurenear L140

L30 L140

L220

0 1800 3600 5400 7200

0

5

10

15

20Failure near L30

Slope surface slidedtogether for L140 and L220

Zoom up

Time (sec)

L140

L220L30

0 1800 3600 5400 7200

-40

-20

0

20

40

60

80

100

Rotationstart at L140

Zoom up

Rotation(mm/m)

Slidedisp.

(mm)

L30

L140

L220

Rainfall start (80mm/h)Failurenear L140

Water injectedat bottom of box

0 1800 3600 5400 72000

20

40

60

80

100

Saturationratio(%

)

Time (sec)

L140

L220

L30

Fig. 6 Behaviors of displacement,

tilting angle, and volumetric water

content



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Rotation

on slope

Wireless data

transmission

Unsta

blelay

erofsl

ope

Intactb

aselay

erofsl

ope

Steel rod is inserted on slope surface.

Tilt sensor

Volumetric watercontent sensor

It is in contact with the base layerif the unstable layer is thin.

Fig. 7 Wireless unit with tilt and

water content sensors on a slope

(2008, Kobe)

-50

-40

-30

-20

-10

0

10

20

One monthSensor units

were remounted

Rota

tion(mm

/m)

(Positivefordownwardtilting)

Data lost

Unit B

Unit B

Unit D

Unit D

Data lost

0 720 1440 2160 2880 3600 4320 50400.0

0.2

0.4

Time (hours)

Volumetricwater

content(m

3/

m3)

Unit B

Unit D

Data lost

Time = 0 at 2008/4/17 00:00:00

Data lostUnit B

Unit D

Unit B

Unit D

Fig. 8 Typical data obtained by the

sensor units (B & D) on slope (2008,

Kobe)

Technical Note



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Acknowledgements

The authors appreciate corporation by Mr. K. Furumoto, Mr. H.

Mori, and Ms. Y. Saito of the Public Works Research Institute,

Tsukuba, Japan, who conducted the slope model tests. A part of

this research is supported by Grants-in-Aid for Scientific Research

of Japan Society for the Promotion of Science and Construction

Technology Research and Development Subsidy Program of

Ministry of Land, Infrastructure, Transport, and Tourism of Japan.

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T. Uchimura ()) . I. Towhata . T. T. Lan Anh . J. Fukuda . C. J. B. Bautista

Department of Civil Engineering, University of Tokyo,

Tokyo, Japan

e-mail: [email protected]

L. Wang . I. SekoChuo Kaihatsu Corporation,

Tokyo, Japan

T. Uchida . A. Matsuoka . Y. Ito

Public Works Research Institute,

Tsukuba, Japan

Y. Onda . S. Iwagami . M.-S. Kim

Department of Integrative Environmental Sciences, University of Tsukuba,

Tsukuba, Japan

N. Sakai

National Research Institute for Earth Science and Disaster Prevention,

Tsukuba, Japan
