京 都 大 学 防 災 研 究 所 年 報 第48号 C 平成17年4月
Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No. 48 C, 2005

Achievements of Aerial Prediction of Earthquake and Rain Induced Rapid andLong-Travelling Flow Phenomena (APERITIF) Project

Kyoji SASSA, Hiroshi FUKUOKA, Tatsuo SEKIGUCHI*, Hiromu MORIWAKI **, andHirotaka OCHIAI **** Geographical Survey Institute, Ministry of Land, Infrastructure and Transport** National Institute for Earth Science and Disaster Prevention*** Forestry and Forest Product Research Institute
Synopsis
Risk evaluation for earthquake induced rapid and long-travel landslides in densely populated
urban areas is currently the most important disaster mitigation task in landslide-threatened areas
throughout the world. The major research achievements of the Aerial Prediction of Earthquake andRain Induced Rapid and Long-Travelling Flow Phenomena (APERITIF) project which wascoordinated by Sassa, is introduced in this paper; (1) Detection of landslide prone slope using
airborne laser scanner, (2) Full-scale landslide flume test using artificial rainfall instrument, (3)Landslide triggering experiment on natural slope by artificial rainfall, (4) Landslide risk evaluationand hazard zoning in urban development areas. Test site of (4) is the Tama residential area near
Tokyo. A set of field and laboratory investigations including laser scanner, geological drilling andring shear tests showed that there was a risk of sliding surface liquefaction for both sites. A
geotechnical computer simulation (Rapid/LS) using the quantitative data obtained in the study
allowed urban landslide hazard zoning to be made, at individual street level.
Keywords: risk evaluation, hazard zoning, earthquake-induced landslides, fluidization, Airborne
laser scanning, full-scale model experiment, natural slope, pore-water pressure, sliding surface
formation, undrained ring-shear tests, numerical simulation1.

thus classified as earthquake disasters, meteorologicaldisasters or volcanic disasters. Nonetheless, the total
number of deaths in Japan owing to landslides during thepast 30 years, from 1967-1998, was 3,152, while thenumber owing to earthquakes was 6,254, including the
1995 Kobe earthquake. Taking an even longer period,
landslide disasters have caused a greater number ofdeaths than earthquakes, and indeed the number ofdeaths from landslides far exceeds that from earthquakes.
Though landslide disasters are not as sensational as
earthquake disasters, the former have the same level ofsocial significance.
Some landslides move slowly, on the order ofIntroduction

The protection of life and property from natural
disasters is indispensable in creating an environment forsociety that ensures safety and satisfaction for its people.
The national imperative towards safety and satisfaction isincreasing in countries suffering from natural disasters.
Landslide disasters are generally less recognized in thestatistics of the United Nations and many governments
than events such as earthquakes, volcanic eruption andmeteorological disasters, because most landslidedisasters often occur in association with earthquakes,
typhoons or hurricanes and volcanic activities and are1

Therefore, it is not always likened to the absolute valueof travel distance. Fig 2 presents what is the expectedtravel distance of landslides. The long or short travel
distance is not the function of travel distance itself, butrather the function of mobility. The index of mobility isexpressed by the apparent friction (H/L) and the apparent
friction angle (a) as shown in Fig. 2a. Fig. 2b presents thestatistics of H/L for the period of 1972 – 1995 forlandslides in Japan. In these statistics, most landslides(more that 97%) had an H/L value of more than 0.5 or anapparent friction angle of more than 26o. High values of
H/L indicate that the landslide masses moved onto a flatarea and subsequently lost energy, or were often blocked
by walls or houses. Less than 3% showed high mobility
in the debris slides and falls in Japan (locally called
Gake-kuzure, steep slope failures); 3% can be withinerror limits, or a negligible rate in political decisions.
However, this small number is still very important.

centimeters per year, whilst others move at meters perday, and still others move rapidly at velocities over 50
m/s: high velocities from which people cannot always
escape. Within this field of rapid landslides, some move
only a short distance, whereas others travel long
distances. As illustrated in Fig. 1, landslides can beclassified into four types according to velocity and travel
distance. The first type consists of landslides that are
rapid and short-moving, including most types of debris
slides and rock falls. In most countries these types oflandslides are the most frequent, although they areusually small in scale. This type of landslide impacts
people in the mountainous areas of the world, and theassociated risk is obvious, because of the steepness of theslopes and the proximity of dwellings to the steep slopes.
The second type consists of reactivated landslides thatmove slowly for a short distance. Often homes andfarmlands are constructed on such landslides. The third
type of landslide is slow and long-travel failures such as
earth flows. These latter two types do not threaten the
lives of people because it is often possible to evacuate
people from the impacted area quickly. The fourth typeare rapid and long-travel landslides which are clearly themost dangerous since the rapid motion does not allow forevacuation, and a great impact force is typicallygenerated which can destroy houses and disastrously
affect a large area. Often the danger is not obvious.
Houses that have been constructed away from steep
slopes, or often on gentle slopes can be destroyed by thisphenomenon. The Tsukidate landslide reported by
Fukuoka et al. (2004) is a typical example, as it moved
from a gentle slope of only 10o and traveled a long
distance at a speed of several meters per second.

Fig. 2a,b Index to express mobility and frequency ofdebris slides and falls in Japan. (a) Illustration of
apparent friction angle. (b) Statistical chart of life loss
(by the Ministry of Construction of Japan, 1997).
Fig. 1 Types of landslides in terms of velocities andtravel distances

In Fig. 3, some examples of recent major landslidedisasters are listed in terms of fatalities and the apparent
friction angles, mostly in Japan, but also including recent
large disasters in other countries. It should be understoodthat most of these had high mobility, with apparent

What is the definition of a long-travel landslide?
Long travel distance means that the travel distance isbeyond the normal expectations of most people.

2

friction angles of around 10o or even higher. However,
all of them had apparent friction angles of less than 20o,
which is below the 30o that is near the friction angle ofsandy materials mobilized during motion in the apparent
friction angle. Therefore, it is clear that large landslidedisasters have been caused by landslides of exceptionallyhigh mobility.

of technology to assess hazardous area, and (4) integrated
research for urban landslide zoning. Research topic (1)includes the development of dynamic loading ring shear
apparatus with a transparent shear box reported by Sassa
et al. (2004a). Topic (2) includes the airborne laser
scanner technology to measure micro-topography under
forests (Sekiguchi et al., 2004), while topic (3) includes
large-scale (similar to the real scale of landslides) flume
tests to reproduce landslides (Moriwaki et al., 2004), anda field experiment to reproduce a rapid and long-travel
flow phenomena by artificial rainfall onto a natural slopewith various monitoring equipment (Ochiai et al., 2004),
and topic (4) includes risk evaluation of rapid andlong-travel flow phenomena which may be triggered bysliding surface liquefaction and the trial urban landslide
zoning (Sassa et al., 2004c). This paper contains above
major research achievement from this project.
The development of reliable landslide risk evaluationand precise landslide zoning technology in urban areas
and locations of high societal value is currently apressing need. Much more research has yet to becompleted. However, the establishment of thistechnology is now possible because necessary key
technology is now in our hands. We can approach anddirectly observe the source area and the sliding surface oflandslides, and it is possible to measure the necessary
mechanical parameters, such that we can stabilize adangerous slope if the danger is identified with a highreliability, or we can avoid the landslide risk by
relocation of houses and facilities. The United Nations
World Conference on Disaster Reduction was organizedin Kobe, Japan, in January 2005, 10 years after the 1995
Kobe earthquake disaster, which included the Nikawa
rapid and long-travel landslide that killed 34 persons.
Authors believe the landslide research community is sure
to succeed in its steady progress, both in increasing thereliability of risk evaluation, and towards preparedness
for such dangerous landslides in the coming decade of2005 – 2015 across the world, under close collaboration
with the suporting organizations of ICL: four United
Nations bodies (UNESCO, WMO, FAO, UN/ISDR),IUGS (International Union of Geological Sciences), and
the Governments of Japan, USA and others. It will be an
important contribution to the United Nations
International Strategy for Disaster Reduction (UN/ISDR).
In the following sections, major achievements of
APERIF projects are introduced.

Fig. 3 Examples of recent catastrophic landslidedisasters in Japan and other countries
Both Fig. 2 and Fig. 3 visualize the significance of
research on rapid long-travel landslides with highmobility, although the physical occurrences are very
limited. A project called the Areal Prediction ofEarthquake and Rain-Induced Rapid and LongTravelling Flow Phenomena (APERITIF) was originallyproposed for a Special Coordinating Fund for Promoting
Science and Technology of the Ministry of Education,
Culture, Sports, Science and Technology of Japan(MEXT) under this understanding. It was adopted as aproject in the group category of Social Infrastructure fora 3-year period from 2001 to the end of March 2004. It
was named as “APERIF” (Aerial Prediction ofEarthquake and Rain Induced Flow Phenomena). After
March 2004, its achievements received rather highevaluation from the funding agency.
After that, this project was approved as one ofprojects of the International Programme on Landslides(IPL M101 = APERITIF) coordinated by theInternational Consortium on Landslides (ICL) when it
was established in 2002 for further worldwide
development of APERIF.
The APERITIF project consists of (1) researching themechanism of rapid and long-traveling flow phenomena,(2) development of micro-topographical survey
technology to extract dangerous slopes, (3) development3

2.

Airborne laser scanning uses an active sensor thatmeasures the distance from the sensor to the groundwhere the laser beam is reflected (Fig. 4). Aircraft
positions are calculated using a combination of Global
Positioning System (GPS) data, both on the aircraft andon the ground, aircraft acceleration and three-axial attitude
data measured by an Inertial Measurement Unit (IMU).
Furthermore, the direction data of the laser beams aremeasured by a sensor onboard the aircraft. These data are
combined to calculate the three-dimensional position (X,Y, Z) on the ground.
In this study, the measurement specifications were as
follows using the laser instrument, EnerQuest RAMS
system:laser wavelength:1064 nm;pulse rate:
24000 Hz;scanning frequency:
24 Hz;
flying altitude:
normally above 2600m;
flight speed:203.7 km/h;
swath width:
881 m.

Mapping of micro topography using
airborne laser scanning

In order to understand disastrous landslides it isimportanttoidentify
thegeometricalandgeomorphological conditions of slopes in which thelandslides have previously occurred. Photogrammetry has
frequently been used for such measurements. In addition,it has become possible to use a method called airborne
laser scanning for detailed three-dimensional
measurements.

Fig. 4 Measuring principle of airborne laser scanning
Airborne laser scanning has recently been used in termsof landslide identification (Aleotti et al., 2000; Hasegawa
and Okamatsu, 2001; Sato et al., 2002; Sekiguchi et al.,2003). It is expected that airborne laser scanning will
enhance not only high-precision mapping but, also
landform analysis (Ackermann, 1999; Wehr and Lohr,
1999; Masaharu et al., 2001). Using the acquired data,high-precision
topographic
maps,
inclination
classification maps and shading maps can be produced
more efficiently. Furthermore, through micro topographic
mapping, analysis for landslide simulation and unstableslope identification may be actively performed (Sekiguchi
et al., 2003). The purpose of this study is to show an
example of the micro topography mapping of hilly terrain
using airborne laser scanning data in the Tama Hills near
Tokyo, Japan. The study area was Tama Hills are locatednear the boundary between the western Kanto Mountains
and the Kanto Plain, southwest of Tokyo, Japan.
2.1. Data preparation(1) Airborne laser scanning

Fig. 5 Distribution of laser measurement points after
filtering. Red points show measurement points

Fig. 6 Results of the filtering processing (left: before;right: after)
The interval between measurement points was 2.5 m in
the track direction and 2.0 m in the cross direction. The
study area was covered by 10 flight courses with a course
interval of 400 m and a course overlap of 54.6 %. A 1 m

4

by 1 m grid was positioned over the study area and 91%
of the square metres contained at least one laser
measurement point.
(2) Filtering processing
In order to reveal only the ground surface, i.e. a Digital
Terrain Model (DTM), it was necessary to remove theground features from the DSM through various filtering
techniques. Laser measurement points in the study area
were approximated by quadratic polynomials, which gavea threshold value from 2 m to 9 m. Those measurement
points over the threshold value were eliminated from thesearch area. Furthermore, manual processing was
performed to obtain a smoother DTM. A contour map
was initially drawn, on a Triangulated Irregular Network(TIN) calculated by the DTM, and the contour map wassuperimposed on an orthogonal colour aerial photograph.
When the contour lines were dense such as on buildings
or trees, on the superimposed image, laser measurement
points were removed from the DTM. This manual
processing was repeated several times to produce a more
realistic DTM on the superimposed image (Fig. 5). The
results of the filtering processing are shown in Fig. 6.

contour map made from the 2 m grid DTM produced themost factual results for this study area.
This contour map of the Hachioji area, called a laser
contour map in this study, is shown in Fig. 7. Fig. 8 is aphotogrammetric contour map. These two maps show thesame place at the same scale. An aerial photographic
interpretation of the micro topography of this area wasalso produced. It was found that the contour lines of thelaser contour map, such as the ridges, valleys andlandslides are very clear, and more realistic than thephotogrammetric contour map.
2.2. Micro topographic details from airborne laser
scanning data
As shown in Figs. 7 and 8, laser contour maps indicate
micro topographic characteristics clearly and precisely.
Laser contour maps are also more useful thanphotogrammetric contour maps to delineate landslides
and other micro topographic characteristics interpreted byaerial photographs. However, laser contour maps do notalways indicate actual landforms: a few errors wereconfirmed on valley bottoms and ridges. For example, it
was difficult to filter out bamboo grasses, because thegrasses are low in height (10-20 cm). However, trees
were successfully filtered out, such as conifer trees, whichhave a height of 10-20 metres.
3. Full-scale landslide flume experiment using a
rainfall simulator
Because it is very difficult to get on-site data
through field observations for either case of landslide
fluidization, field and model experiments have been
conducted with a focus on the relationship between
pore-water pressure and landslide failure process.
Iverson and his colleagues (Iverson and LaHusen 1989,
Iverson 1997, Iverson et al., 1997, 2000; Reid et al,1997) performed a series of full-scale landslide
experiments, and found that the pore pressure
responses during sliding are significantly dependent on
the initial soil porosity, and that rapid fluidizedlandsliding involves partial or complete liquefaction of
the mass by high pore-fluid pressure. Pore pressurechange induced by porosity change during landsliding
depends not only on initial porosity, but also on therelative time scales for soil deformation and porepressure diffusion. In addition, some modelexperiments covering “self-fluidization” have also beenFig. 7 (left) : 2 m grid laser contour map based on a 2 mgrid DTM, and Fig. 8 (right): 1:2,500 photogrammetricmap (after Tokyo City planning map, 1999).
(3) Contour map
A contour map of the study area was drawn on the
DTM. First, the TIN was calculated from the DTM. At
this stage, the DTM consists of randomly scattered laser
measurement points. Next, 1 m, 2 m and 5 m grid DTMs
were obtained from the TINs. The DTMs, here, consistedof regularly arranged points, namely grid data. Then,contour maps of 1 m, 2 m and 5 m contour intervals weredrawn on the respective grid DTMs. Finally, nine kinds ofcontour maps (three kinds of grid intervals by three kindsof contour intervals) were produced. In terms of smooth
contour lines, which express the micro topography andinterpret the landforms, it was found that a 2 m interval

5performed (Eckersley 1990, Moriwaki 1993, Spenceand Guymer 1997, Wang and Sassa 2003, Okura et al.
2002). In these experiments pore-water pressure
generation and failure processes were examined.

behavior of pore-water pressures before, during, andafter failure were examined from a comprehensive
standpoint to elucidate the fluidization process.
3.1 Full-scale model flume
The three-stage steel flume used in the landslide
experiment was 23 m long, 7.8 m high, 3 m wide, and1.6 m deep as shown in Fig.11. The main slope was a
10-m long section inclined 30 degrees, with a 6-m longsection inclined 10 degrees connected at the lower end,
and a 1 m long horizontal section connected at theupper end. A 6-m long horizontal extension, with thesame width and depth as the sloping flume, wasconnected to the flume at the downslope end of the
10-degree slope. One wall of the entire flume was clear
reinforced glass to enable direct observation of soil
deformation. The lower end of the flume was supported
by a retaining wall with a metal net for free dischargeof water. Steel bars of square 1.8 cm on a side addedtransversely roughness to the flume’s bed at 0.12-mintervals.
The rainfall simulator in the National ResearchInstitute for Earth Science and Disaster Prevention(NIED), Japan was used. It is 75 m long, 50 m wide,and 22 m high. This simulator is movable between fiveexperimental sites to efficiently conduct many
experiments all through the year. The simulator cansprinkle water in intensities of 15 to 200 mm/hr. The
height of the nozzles above the ground was 16 m, farenough for raindrop to reach the terminal velocity. In
this experiment, we sprinkled at a constant intensity of
100 mm/hr.

Fig. 9 Model configuration before experiment.

Fig. 10 Model configuration after experimentFig.11 Model slope and location of sensors.

3.2 Monitored items
We monitored the following items with certain
instruments listed below: surface displacement,piezometric level at the flume bed, pore-water pressure
within the soil, slip surface locations, and landslide
motion.
1) Surface displacement: extensometers with wireattachments were installed on the 30-degree slope.
2) Basal Piezometric levels: sealed diaphragm-type
pressure transducers fixed to the slope bed at aregular interval of 2 m. Transducers could measure
up to 3 m of pressure head.
3) Internal Pore-water pressures: three strain-gauge
type meters were installed at regular interval of 4 m
in the soil layer. These gauges were buried 30 cm, 60

Although these above-mentioned results soundreasonable and interesting, some were based on
small-sized flume tests. As well known, small-sized
experiments have problems with scale effects,
similarity relations, and the disruptive effects of sensors
and their cables. Therefore, a model scale as close as
possible to natural size is desirable in order torealistically reproduce a landslide phenomenon, and we
performed a full-scale rainfall experiment on 16
December 2003, targeting “self-fluidization” process.
We successfully initiateda landslide, anddocumented the slip surface formation (see Figs. 9 and
10). Data obtained on surface displacements and the6cm, and 90 cm deep, respectively, at each location.
4) Slip surface and deformation: colored-sand indiceswith a width of 6 cm were inserted verticallybetween the soil and the glass wall at a regularinterval of 1 m.
5) Landslide motion: high-speed video cameras anddigital cameras were used. Square markers with alength of 15 cm were attached regularly on the slope
surface.
Data from the sensors listed above were digitally
recorded using a personal computer. Some of thesensors were monitored using a pen-recorder.

Fig.13 Inferred water table just prior to slide.

3.3 Obtained Results
The soil layer began to move after sprinkled water
infiltrated to the base of the soil layer, forming asaturated zone. The soil deformation increased
gradually as the groundwater level rose. Then, a rapid
landslide triggered on the 30-degree slope section
occurred 154 minutes 27 second (9,267 seconds) after
the sprinkling started (Fig.10). Debris from this failure
flowed about 4 meters downward and stopped inapproximately 5 seconds. The maximum speed of thesliding mass was about 1.2 m/sec. Prior to the rapid
landslide, slope deformation lasted about 41 minutes.
(1) Behavior of water pressure and surface
displacement prior to the landslide(a) Water pressure
Fig. 12 shows changes in basal piezometric levels
leading up to rapid failure. All of piezometric levels
increased approximately linearly with time. The
piezometric level (P.L.) at G-9 where the horizontal
flume was connected to the 10-degree slope (Fig. 11)
responded first at about 6,300 seconds. It reachedalmost 110 cm of water head before the slope collapsed.
Response in the horizontal layer followed later (G-11).
The pizometric level. at the top end (G-1) started
slowly and remained the lowest until rapid failure.

Fig.14 Change of pore-water pressure (middle part of
30 degree slope). Kp-4 is 30cm deep, kp-5 is 60cm,
and kp-6 is 90cm respectively.
Fig. 13 portrays the inferred water table just prior to
the rapid slide (blue-colored zone), based on
piezometric levels. The highest piezometric level. wasrecorded at G-9. The piezometric level at thehorizontal part decreased gradually toward the end of
the flume due to free discharge. The lowest piezometriclevel was recorded at the upper end of the 30-degreeslope. The data indicate that the entire soil layer
contained a saturated zone just before failure.
The behavior of pore-water pressure in the soil prior
to slope collapse is shown in Fig.14. Pore-waterpressure gauges kp-4, kp-5 and kp-6 were buried 30, 60,
and 90 cm deep, respectively. Kp-6 began to riseapproximately at 8,000 seconds, and kp-5 increased
later. At failure both gauges reacted dramatically. The
shallowest gauge of kp-4 changed to a negative state at
an intermediate stage while the deepest one shifted to a
positive state. As for the decreasing pore-water pressure
shown by kp-4, it might be due to the possible
settlement of superficial soil layer during rainfall,
which would lead to a downward movement tendency
for the soil layer, and then result in the increment ofsuction to resist this downward movement tendency.
Other factors, such as temperature variation, watercontent, etc., may also affect the response of pore-waterpressure gauge. However, because we did not measure

Fig.12 Change of piezometric level before the rapid
slide.

7

experiment equaled to about 0.005 (assuming that thelength of the source area was 10 m), and was slightly
short of the proposed failure stage values. Precursor of
surface displacement lasted for 41 minutes before rapid
failure. The velocity curve of surface displacement until
the rapid landslide can be approximately classified into
three stages; (A) asymptotic increase (about
6,500-7,700 seconds from sprinkling), (B) exponential
increase (about 7,700-8,900 seconds), and (C) linear
increase (about 8,900-9,276 seconds).
(2) Failure process and slip surface formation
The rapid movement during failure occurred over 5
seconds (9,267-9,272 seconds after the start ofsprinkling), following the long precursory period of
deformation down slope. The maximum velocity of thesliding mass was about 1.2 m/sec. The velocity of thesliding mass at failure was radically different from that
before failure.
Fig. 17 illustrates the progression of the landslide for 5
seconds on the basis of deformation of white andyellow indices recorded by digital cameras. The top
sketch in Fig. 17 is a side view just before the rapid
slide. The bottom sketch of Fig. 17 is after the slide
stopped. The other sketches are during movement.
Overall, the main scarp was located at the top of the
30-degree slope and the toe was in the middle part of
the horizontal layer.

these items during the test, it is difficult to make aconclusive explanation here. As for the difference in the
response time between kp-5 and kp-6, it may be due tothe fact that the base of the soil layer was first saturated.
(b) Surface displacement
Figs. 15 and 16 show the change in cumulative
surface displacement and the velocity before the rapid
slide. Velocity analysis was based on data for each30-second period. The surface displacement meter(extensometer) on the upper part of slope, D-1, began
to respond at 6,780 seconds, while that on the lower
part of slope, D-5, showed the most displacement until
8500 seconds, when it was surpassed by D-1 and D-3.
From then on, all the sensors moved downward at anaccelerating speed. The slope collapsed at 9,267
seconds when the surface displacements at D-1 andD-3 were about 4.7cm.

Fig.15 Change of cumulative surface displacement
before the rapid slide.

Fig.16 Change of velocity of surface displacement
before slide (computed using each 30 seconds of data).
Fig.17 Illustration of soil deformation during the rapid
slide. The top figure is before slide. The bottom figure
is after the landslide stopped.

Moriwaki (2001) proposed an Index of critical strain,
defined as the ratio of final cumulative displacement to
the length of the source area, and four stages based on
this index, a precursor stage (<0.003), a warning stage(0.003-0.006), a failure stage (0.006-0.02), and aperfect failure stage (>0.02). The critical strain in this(3) Rapid fluctuation in piezometric levels andpore-water pressure at failure
Piezometric levels (measured by fixed sensors),8

pore-water pressures in the soil mass rapidly increased
throughout the slope as soon as rapid failure occurred.
After failure pressures at some gauges decreased
although most of the pressure gauges retained
high-pressure states. All pressures reached the first peakwithin 1-3 sec after the start of sliding, and theincrement in pore-water pressures was remarkable.

pore-water pressures (measured by unfixed sensors)rose immediately during the failure. After the rapid rise,pressures varied from place to place throughout the soil.
Some pressures in the upper part of the slope declined,
whereas some in the lower part of the slope gradually
increased.
(a) Piezometric levels
Fig. 18 shows changes in piezometric levels (P.L.)
during the rapid slide (9,267-9,272 seconds after thestart). All P.L. values throughout the slope andhorizontal section increased rapidly.
Small oscillations (1-2.5 Hz) in waveform wereobserved at sensors G-1, G-3 and G-5 in the 30-degreeslope, where the soil layer slid down in parallel to theslope. The other sensors showed no remarkable cyclic
fluctuations. Iverson and LaHusen (1989) reported ageneration of a 1 Hz fluctuation in a 10-m long slope
failure experiment. Moriwaki (1993) found in his
experiment that a rapid increase of P.L. occurred during
the slide. He showed the peak pressure head of the P.L.
in the soil layer was approximately two times as large
as the head just prior to sliding.
From the response of these piezometors and theshear displacement shown in Fig. 18, it could beconcluded also that high pore-water pressures weregenerated during failure, i.e., were the result of failure.
This supports the findings in earlier works (i.e., Iverson
et al 1997, 2000; Eckersley 1990; Wang and Sassa
2003, among others). Furthermore, it is made clear thathigh pore-water pressure could also develop outside ofthe landslide mass, due to the compression induced by
the landsliding mass, as shown by G-11.

4. Fluidized landslide experiment on a natural slope
by artificial rainfall
A rainfall-induced landslide experiment on a naturalslope was conducted, which had more complex andheterogeneous characteristics than the indoor models,in an attempt to investigate the dynamic movements ofthe soil surface, the formation of the sliding surface,and hydrological characteristics, based on the results of
the indoor flume testing. Landslide experiments onnatural slopes by sprinkling or water supplying fromtrenches have been reported four times in Japan and
U.S.A. The respective conditions for each experimentare shown in Table 1.
The experimental slope was 30 m long and 5 m wide,and mainly covered by weathered disintegrated granite
sand. Soil-surface movements were monitored by using
stereo photogrammetry (5 stereo pairs of CCD video
cameras). Hence white-coloured targets were placed on
the experimental slopes and the movements of thesetargets were traced by image analysis. To detect theformation of the sliding surface, soil-strain probes wereinserted into the soil to 2 m depth at deepest.
Tensiometers were used to measure changes inpore-water pressures within the soil.

Fig.18 Fluctuation of piezometric levels during theslide (D-3; surface displacement).

Fig. 19 View of the experiment site at Mt. Kaba-san.

(b) Pore-water pressures
As was the case with piezometric levels, all the

The purpose of this experiment is to produce
hopefully a fluidized landside on a natural slope by

9were specially prepared. To measure saturation
conditions within the soil, tensiometers with porous
ceramic cups were set into the slope.

artificial rain fall. The test site was selected by two
conditions; 1) to secure complete safety during theexperiment, 2) to have a possibility producing a
fluidized landslide. Then, a natural slope in the Koido
National Forest at Mt. Kaba-san, Yamato village, 25
km north of Tsukuba-city, Ibaraki Prefecture, Japan was
selected for the controlled experiment on landslide andpossible fluidization in cooperation with the Forestry
Agency of Japan.
The selected portion of hillslope (Fig. 19) was 30
metres long, with an average gradient of 33 degrees(maximum 35 degrees). The soil was 1 to 3 metresdeep. A 5-m-wide experimental slope was isolated fromits surroundings by driving thin steel plates about
1-m-deep into the soil. These plates prevented lateral
diffusion of infiltrated rain water and cut the lateral treeroot network that imparts resistance within the soil
layer. The surface of the slope was covered by straw
matting to prevent surface erosion and promote rainfall
infiltration. Surface material on the slope consisted offine weathered disintegrated granite sand, called“Masa” in Japan. Loamy soil blanketed the upper
portion of the regolith to a depth of about 1 m; this soil
mainly originated from tephra of Mt. Fuji, Mt. Akagi,
and other volcanoes located west of Mt. Kaba-san.
Artificial rain at the rate of 78 mm/h was applied tothe slope segment during the experiment by way of arainfall simulator. The simulator consisted of aframework of steel pipes with 24 sprinkling nozzles
arranged 2 m above the soil surface. Water for
sprinkling was pumped from a dam constructed in asmall creek at the base of the slope into 80 water
storage tanks (1 m3 for each) on the neighboring
hillslope prior to the experiment.
To obtain information on the formation of thesliding surface, four soil-strain probes were inserted.
The diameter of the probes was only 10mm and theyFig. 20 Views of the debris flow resulting from thelandslide fluidization between five seconds and sevenseconds after failure.
4.1 Overview of the triggered landslide movement
On 12 November 2003, artificial rainfall was given
to the slope at a rainfall intensity of 78 mm/h for fourhours and a half until sunset. No slope movement wasobserved. The second experiment was conducted on 14
November 2003. Artificial rainfall started from 9:13 ata rainfall intensity of 78 mm/h, the slope deformation
was detected from around 15:00, then a clear
movement was observed beginning at 16:03. The

Table 1 Examples of landslide experiments in natural slopes.
References

Location

VolumeWuL

Oka 1972

Ikuta,
Kawasaki,Japan

500 m3

Yagi, Yatabe, & Enoki, 1985

Matsuyama,Japan

10 u 25 m

Trench cut at upperand both sides

Rainfall simulator (Sprayedon upper half slope)

Yamaguchi, Nishio, Kawabe,
Shibano, & Iida, 1989

Yui,
Shizuoka,Japan

10 u 30 m

Upper trench cut
Lower open cut

Supplied from upper trenchHarp, Wells & Samiento, 1990

Utah,
USA

1.6 u 1.3 m
2.0 u 2.2 m
3.2 u 4.0 m

Upper trench cut
Lower open cut

Supplied from upper trench

Excavation etc.

Water supply

Sensors

Sprayed from fire hose

10

Fluidization
Yes

Extensometer
Piezometer
Strain meter
Extensometer
Piezometer
Inclinometer
Extensometer
Piezometer

Yes

No

No

Fig. 22b. For the strain probe at P4 (Fig. 22b), positive
values of strain were observed at a depth of 70 cm fromabout 20 minutes after sprinkling began, and negative
strains at a depth of 60 cm. We interpret from the paired
positive and negative values that the sliding surface
formed between 60 cm and 70 cm depth. At failure(410 minutes), the accumulated strain at a depth of
70-cm was larger than 0.04, which was the capacity ofthe strain gauge. Similar results were obtained at P3(Fig. 22a). Positive values of strain were observed at
120 cm depth from about 300 minutes and negativevalues at 110 cm depth, indicating a sliding surface
between 110 cm and 120 cm depth. The depth ofsliding at P3 was twice as deep as at P4.

initiated landslide was a type of an expected fluidizedlandslide, the landslide mass rapidly moved andtravelled long.
The cover of the tensiometer started to incline
downslope at 24,627.5 sec (410m 27.5s) after
sprinkling commenced. We interpret this as indicatingthat slope failure initiated at 24,627.5 sec.

Fig. 21 The landslide deposit one day after theexperiment.
Images at 24,632.5 sec (5 sec after failure) and
24,634.5 sec (7 sec after failure) are shown in Figs. 20a
and b. Fig. 20 presents the movement of liquefied
landslide mass. The failed landslide mass had entered
the stream and was about to collide with theconfronting slope (Fig. 20a). After collision, thefluidized landslide turned to the right (Fig. 20b),changed into a debris flow, and travelled downstream
for 10 seconds on about 10 degree gradient, as much as
30 m. It took 17 seconds from the initiation of thelandslide to the end of deposition. Fig. 21 shows thelandslide deposit one day after the experiment. The
straw matting, the cover of the tensiometers, and thewhite-coloured targets were conveyed to the toe of thefluidized landslide.

Fig. 22 Soil-strain accumulation curves. (a) at P3 and(b) at P4.
Changes in pore water pressure including suction
monitored by the tensionmeters are presented in Fig. 23.
The tensiometers were placed at depths of 50, 100, 150,
200, 250, and 290 cm. All tensiometers showednegative pore pressures at the start of sprinkling,
indicating that the soils at all depths were unsaturated
or partly saturated. When the wetting front passed, thetensiometers showed increases in pore pressure insequence of the depths. At 410 min, when the failure
took place, all of the tensiometers showed positive porepressures. The pore pressure of the deepest tensiometer(290 cm) rapidly increased its values from about 290

4.2 Observation of sliding-surface formation
process
When a sliding surface forms, soil above the sliding
surface tends to move downslope, whereas soil below itremains stable. Fig. 22 shows the accumulation of soil
strain during the experiment. The results fromsoil-strain probe P3 is shown in Fig 22a, and for P4 in

11

of 1995 Nikawa landslide (34 fatalities, Fig. 24)
triggered by the Hyogoken-Nambu earthquake in
Nishinomiya city between Osaka and Kobe; the othersite corresponds to slopes inside or adjacent to a large
scale residential development in the Tama hills near
Tokyo. Locations of the investigated sites are shown in
Fig.25. Achievements in the study on 1995 Nikawa
landslide site was reported in Sassa et al. (2004). In
this section, recent achievement of the assessment of
rapid and long-travel landslides which will be triggered
by future huge earthquake in a densely populated
residential town near Tokyo.

min. This almost coincided with the time when thestrain gauge at 110 cm depth in the strain probe at P3
started to show strain (Fig. 22a). Hence, it can be
deduced that general slope instability increased from
290 min, before final failure at 410 min.

Fig. 23 Changes in soil-water pressure in tensiometers.
Fig. 25 Location map of study areas

Fig. 26 Air photo of the densely populated developedarea near Tokyo (A: Hino site; B: Hachioji site).
5.1 Field study for the development area near Tokyo
During the 1923 Kanto earthquake, 142,800 persons
were killed. There have been no other large earthquakes
since. However, it is not possible to predict how long
this situation will continue. The return period of big
earthquakes in the Tokyo area is estimated to be about
69 years (Kawasumi 1970). Large-scale urban
development after 1923 in and near Tokyo has been
extensive. To assess landslide vulnerability of sandy
slopes that are widespread in the Tama hills (aresidential area of Tokyo), a large-scale experiment
was conducted in November 1971 as a joint project of

Fig. 24 Air photo of the densely populated developedarea near Tokyo (A: Hino site; B: Hachioji site).
5. Landslide risk evaluation and hazard zoning for
rapid and long-travel landslides in urban
development areas
The APERITIF project comprises landslide risk
evaluation and urban hazard zoning for rapid andlong-travel landslides, conducted in two selected sites
in Japan. One is the upper slope connecting to the site

12

colour contour lines of the 1940 map for part of infilledvalley. Fig.27b presents its longitudinal section, andFig.27c shows its cross section. Those two sections
indicate that a valley 15-17 m deep was filled over
300m to develop the flat residential area. Fig. 28 shows
the topography and geological sections of site B. Fig.29
presents photos of the Site B in the Hachioji city.

the research institutes of several ministries of theGovernment of Japan. In this in-situ experiment, sixboreholes were conducted, 10 extensometers and other
instruments were installed on the field, where thepreparatory artificial rainfall events were repeated since
May 1971, and finally a landslide was triggered (Oka,1972). However, the landslide that was triggered was
far more severe than had been anticipated, since it wasmuch faster and travelled much further than had been
allowed for. 15 people, including technical observers
and journalists, were killed and more than ten persons
were injured. At that time, rapid and long-travel
landslides could neither be understood nor predicted.
They are still top targets of landslide research; they are
the research rationale of APERITIF. In another view,
this event demonstrated the risk of rapid andlong-travel landslides in this area by a real event.

Fig. 28 Map of the Hachioji site. (a) Present
topographic map, (b) location of the infilled valley, (c)Topographic ground surface map obtained by removing
the effects of forests and houses by the laser scannertechnology, and (d) geological cross section along A-Ain (c).

Fig. 27 Topographic map of the infilled valley andlocations of sections in the Hino area. Red contours
show ground levels in 1940 prior to infilling. (a)Topographic map, (b) Longitudinal section of 2-2', (c)Cross section of 1-1’

Fig.29 (a): a general view of the developed residential
area. The red arrow indicates the location of the
extrusive ridge, (b): front view of the ridge, (c): sideview of the ridge, (d) : ground water indicating that asaturated zone exists below the ridge because bedding
planes in the area are almost flat.

(1) General description of the area
Fig. 26 shows a study area in the hilly Tama area near
Tokyo. It is located at the border of Hino and Hachioji
cities. The area is covered by an almost flat
sedimentary sandy layer (Kazusa group soil), which
was formed during the Pliocene and Pleistocene. Twosites, A and B, were selected in this area. Site A is alarge infilled valley within the residential area. Site B isan projecting ridge toward the residential area. Those
areas are shown in the rectangular boxes in Fig.26.
Fig.27a shows a topographical map including the red

Sedimentary sandy layers are distributed throughalmost horizontal bedding, so this saturated sandy layer
should extend below the extrusive ridge. To investigate
this situation, four drillings were planned. However,
due to non-cooperation of a development company inthis area, two drillings could not be conducted along
the central section. So they were drilled some distance13

apart at a higher elevation. As a result, the geologicalsection presented in Fig. 28d was obtained. the blue
colour part indicates an unsaturated silty layer.
Probably this layer cannot experience rapid andlong-travel landslides. However, if the saturated sandy
layer will be liquefied or semi-liquefied during
earthquake loading, there is a high possibility oflandslides along the sliding surfaces because mostresisting parts in these expected landslides are parts
passing inside the saturated sandy layer.

(2) Experiments for landslide risk evaluation during
earthquakes
Two boreholes (A and B) were drilled at the Hinosite (Fig 27). Samples were taken from the filledmaterials in the infilled valley. An undrained cyclic
shear stress loading test was performed to assess theacceleration required to cause failure. An initial normal
stress conditions to reproduce the potential sliding
surface of 15 m deep in a 12-13 degrees slope wereapplied. The initial ground water table was 2.5 m fromthe surface. Therefore, an initial pore-water pressure ofabout 120 kPa (corresponding to the water table above
the sliding surface) was imposed. A control signal toproduce the required cyclic shear stress by increasing
its value step by step up to around 600 kPa ensured theoccurrence of failure. As seen in Figs.30b, c, themobilized maximum shear stress was about 115 kPa(which corresponds to 220 gal) in shear stressincrement to cause failure from the following relation(see Fig. 31).
af

( 'W f / m )('W f / W 0 ) u g sin T(1)Here, m: mass of a unit column between the groundsurface and the potential sliding surface, T : angle ofthe potential sliding surface, W 0 : Initial shear stressdue to self weight of the column acting on the sliding
surface, 'W f : seismic shear stress increment required
to cause failure (= ma f ) , a f : seismic acceleration tocause the above seismic shear stressFig. 30 Undrained cyclic loading test on a sample takenin the Hino area (BD = 0.96). (a) Control signal fornormal stress and shear stress, (b) Time series data, (c)Stress path. Note that values of 39 and 30 degrees inFig. 30c were approximate estimated in reference toFig.32 and Fig.33 because pore pressure was notaccurately measured except the value in the steadystate.

The steady state shear resistance reached 16.3 kPa.
The apparent friction angle was only 3.5 degrees. It is atypical sliding surface liquefaction. Rapid andlong-travel landslides should result from such stresscondition.
Then, a naturally drained cyclic loading test using
the same sample, control signal testing procedure withthe undrained test (Fig.30) was performed. The test
results are shown in Figs.32a, b. These are similar to
the undrained test, although the excess pore pressure
could not be monitored because the pore pressuretransducer inlet is located 2 mm above the shear
surface. Excess pore pressure was generated within theshear zone (there, grains are crushed and causing adecrease in permeability). The influence of open valve
drainage is much great. However, the shear resistancedecreased until 29kPa and shear displacement
accelerated. It is apparently the sliding surface

Fig. 31 Shear stress increment due to seismic
acceleration. Here, 'W f ma f

14

Fig.28b provides a further problem. The case ofundrained loading was analyzed and illustrated by
Sassa et al. (1997) and by Sassa et al. (2004) which is
introduced in Fig.34. The landslide mass causes a static
stress increment due to the increase of self weight( 'W ) and provides a dynamic stress increment Fd
when it is moving. It is expressed by using the dynamic
coefficient ( kd )liquefaction phenomena. The steady state shear
resistance was around 1.8 times greater than in the
undrained test. The difference can be explained by porepressure dissipation. The real soil response should fallin between these extremes.

Fd

kd u 'W

Fig. 32 Naturally drained cyclic loading test on asample taken in the Hino area (BD = 0.97). (a) Time
series data, (b) Stress path.
Fig. 33 presents the result of a test on a sample takenfrom sands in the Hachioji area. The control signal for
cyclic loading is the same as in previous case. However,
the initial condition is different. Initial normal stressand shear stress corresponding to a depth of 20 m in a
10 degrees slope without initial pore water pressure
were reproduced. The slope is in the projecting ridge,
therefore a high ground water level is not expected inordinary period without rainfall, while the bottom ofridge (sandy layer) is expected to be saturated. In thesimulating experiment failure took place for a shear
stress around 151 kPa, corresponding to a criticalseismic acceleration of 270 gal. The steady state shear
resistance was 35 kPa, and the apparent friction angle
was 4.5 degrees. This value is low enough to suggest
the possibility that long-travel landslides will be apotential hazard in this area.
If a landslide fails from the slope onto the residentialarea, then the loading is undrained. The ground watertable is shallow (as suggested by photo of Fig.29d) andthe infilled valley in front of this part of slope shown in

Fig. 33 Undrained cyclic loading test of the sample
using the drilling core in the Hachioji area (BD = 0.99).
(a) Control signal for normal stress and shear stress, (b)Time series data, (c) Stress path.
Using the same sample as in previous tests, anundrained dynamic loading test was conducted. The
normal stress was 380 kPa, corresponding to a depth of
20 m, and the initial normal stress corresponding thedepth of infilled valley (or the depth of the saturated
layer) corresponds to around 2 m deep. A dynamic
coefficient of 0.9 was assumed. The test resultspresented in Fig. 35 shows that the mobilized steadystate shear resistance was 5 kPa. Based on those values,
a computer simulation based on the geotechnicalapproach (Sassa, 1988) was conducted to predict the15hazard area.

“softness” or “potential for lateral spreading” of themoving mass. According to the parameter, the lateralpressure acting on a soil column is close to 1.0 if thematerial appears to be liquid. Liquid spreads laterally
without limitation on the flat plane. In contrast, if thematerial is completely rigid (like metal or a rock mass),
the lateral pressure ratio K = 0. A hard rock can standon the plane and never spread laterally. Soils are in
between. It will be reasonable to assume that the value
inside a moving soil mass will be between 0 and 1,
though the lateral pressure ratio in the static soil masscan be more than 1.0 under the over consolidated state.
The value was expressed using the mobilized
apparent friction angle inside the soil mass ( Iia ) as anextension of Jaky’s equation of the lateral pressure ratioat rest in the first time loading (Lamb & Whitman
1979) to the lateral pressure within a moving soil mass
(Sassa, 1988).

Fig. 34 Model of the landslide triggered debris flow
(Sassa et al. 1997).

K 1 sin Iia
the mobilized apparent friction angle inside the soilmass( Iia ) is estimated
where tan Iia {c  (V  u ) tan Ii } / V
tan Iia is the mobilized friction angle during themoving soil mass which must be between the peakfriction angle and the friction angle mobilized duringmotion. Usually the value will be the friction angle
mobilized during motion ( Im ), namely the effective
friction angle obtained in the ring shear test
reproducing landslide motion or those in Sassa et al.
(2004) and cohesion will be zero. However, if the soil
mass moves with an undisturbed state except thesliding surface like the Takarazuka landslide, Japan
during the Kobe earthquake in 1995 (Sassa 1996, Sassa
2000), the tan Ii can be between I p and Im , andcohesion is not always zero. Therefore, Iia take agreater value, then K can be smaller.
This value was assumed higher for the Hino site
(0.65 – 0.80) because the ground water table was highand the landslide mass was almost saturated based on
approximate estimation; Ii = 30 degrees, c = 0,
(V  u ) / V =0.58 (15 m in depth of soils and 12.5 m inwater depth above the sliding surface) leads to thecalculation of Iia as 18.5 degree. Then, it gives thevalue for K = 0.68. The value will be getting greater
when mobilization during motion will increase porewater pressure. Then, it was assumed to be in the range

Fig. 35 Test to simulate the undrained loading on thefilling in valley in the Hachioji area. (a) Control signal,(b) Time series data of normal stress, shear resistance,and pore-water pressure.
5.2 Computer Simulation in investigated areas
A geotechnical landslide simulation model was
proposed by Sassa 1988. It was improved to acomputer code for general use during APERITIF
project especially in the data input and three
dimensional presentation of output. This new computercode for rapid landslides (Rapid/LS) was applied toreproduce the two cases using the measured values ofthe steady state shear resistance obtained throughundrained and naturally drained ring shear tests. In
addition to the steady state shear resistance, anotherparameter for geotechnical simulation, the lateralpressure ratio K (= V h / V v ) was introduced to express

16

condition). The hazard area is much greater in this case.
However, since the source area includes many houses,
the assessed risk is quite large even for the naturally
drained case.
The second case concerns the Hachioji site, which is
at risk due to a landslide located in an adjacent steep
slope. As parameters 35 kPa for the steady state shear
resistance is expected and 5 kPa in the undrained
loading area. The rapid loading test under naturally
drained condition has not yet been conducted. The
value must be higher. Using the ratio of steady state
shear resistances between undrained state and naturally
drained case on the Hino area (29/16=1.8), the value
can be around 10 kPa.

of 0.65-0.80 in this simulation.
Similar consideration was made for the Hachioji site,then, it was assumed relatively lower (0.50 – 0.65)
because the landslide mass was normally less saturated.
For both cases, a higher K value was given in the lower
parts of travel area because the landslide mass should
be more mixed and lose its original structure, namely
should become soft.

Fig. 36 Results of simulation results for Hino area
(shear strength at steady state: 16 kPa). (a) original
topography, (b) and (c) during motion, (d) depositedareas corresponding to the steady-state-shear strengthof 16 and 29 kPa, respectively. Lateral pressure ratio
k=0.65-0.80. The three dimensional view was made by
the Geographical Survey Institute of Japan.
Fig. 37 Simulation results for Hachioji area (shear
strength at steady state: 16 kPa). (a) original topography,(b) and (c) during motion, (d) deposited area.
(Three-dimensional view was made by the
Geographical Survey Institute of Japan using the laser
scanner data for this APERITIF project. Lateralpressure ratio k=0.50-0.65.)

The first case concerns the Hino residential site, apart of which was constructed in the infilled valley. Fig.
36 presents three-dimensional visual output of theresult; Fig. 36a shows the initial state, Fig. 36b shows aintermediate stage during motion; Fig. 36c reports the
final deposition state; and Fig. 36d presents theassessed hazard area superimposed to the three
dimensional view of the area obtained by the laser
scanner technology (Japanese Geographical Survey
Institute). In the Fig. 36d, the larger area corresponds tothe data obtained by the undrained ring shear test (using
16 kPa as the steady state shear resistance), and thesmaller area corresponds to the naturally drained ringshear test (using 29 kPa as the steady state shear
resistance). The scale of the landslide is rather large forlandslides in urban areas, because the head of thelandslide in the source area and the toe of landslide
after deposition will be 480 m (under the undrained

Further works are necessary to obtain a reliablehazard zoning for this site, however, a computer
simulation was applined using those values ofparameters and the detailed topography obtained by theairbone laser scanner as a reference.
Fig. 37 shows the results for the case of 20 m deep
landslide mass onto the infilled valley in the Hachojiarea; Fig. 37a presents the initial state before landslidein which some steps as possible head scarps are visible;
Fig. 37b and Fig. 37c show an intermediate state duringmotion; Fig. 37d is the final situation after deposition in17

which the border of deposited landslide mass isdelineated. The landslide mass covered a considerablylarge area along the infilled valley.

conducted in undrained conditions and in naturally
drained conditions by opening the drainage valve
on the top of ring shear box. Results from thenaturally drained test for the silty gravel layer of
the Nikawa landslide and the sandy fill material of
the Hino site are consistent with sliding surfaceliquefaction and rapid post-failure motion.
However, the naturally drained test for the sandy
layer of the Nikawa landslide did not produce
parameters consistent with sliding surfaceliquefaction, and the shear displacement whichstarted during the main shock terminated at lessthan 7cm, probably because the pore pressure
dissipation rate from the shear zone is great
enough comparing with pore pressure generation
rate in the shear zone. This suggests that thelandslide risk is affected by both of pore pressure
generation in the shear zone and pore pressure
dissipation from the shear zone. The drainage pathin the ring shear test is shorter than the real state,
therefore, the real state in the slope will be
between both of the undrained state and thenaturally drained case.
(6) The geotechnical computer simulation code
(Rapid/LS) using geotechnical data obtained by
the ring shear tests and detailed topographical dateobtained by laser scanner presented urban hazard
zoning for rapid landslides in two densely
populated residential sites near Tokyo. The
parameters will be different for the expected
landslide movement mode depending on whether
the sliding surface will be formed inside thelandslide mass or inside the ground layer overwhich the landslide mass moves. For the Hachiojisite, the test data obtained from the ring shear test
to simulate the undrained loading of landslide
mass onto the infilled valley by assuming that thesliding surface will be formed inside the groundwas used in the traveling area. Reliable hazard
zoning needs careful examination for the expected
landslide movement mode. However, the method
proposed here seems a reliable and convincing
approach of urban hazard zoning based on thegeotechnical parameters at street-level scale forthese two sites.

6. Conclusion
The following conclusions can be drawn:(1) Airborne laser scanning was carried out in theTama Hills to produce laser contour maps. It was
found that laser contour maps depicted landforms,
such as the break in slopes and landslides, more
precisely than photogrammetric contour maps.
(2) A full-scale landslide experiment was conducted toclarify the failure process of a landslide triggeredby rainfall, using a loose sandy soil. A complete
shear zone (sliding surface) was formed within thesoil layer, and the displaced soil mass on the upper
slope section moved as a unit, while the lower
parts were compressed.When a slope consists ofloose soil with high water content, the pore-waterpressures can increase remarkably during failureand simultaneously decrease the shear strength of
the slope. In this experiment, a large saturation
zone and a rapid increase of subsurface waterpressures throughout the soil layer during failure
were observed.
(3) An experiment to induce a fluidized landslide by
artificial rainfall was conducted on a natural slope.
It first slid, then fluidized, and changed into adebris flow. Formation of the sliding surface was
detected by soil-strain probes. The tensiometer
showed a rapid increase in pore-water pressure
after about 290 min from the start of sprinkling.
This almost coincided with the time when thestrain was first observed on the sliding surface.
(4) Landslide risk evaluation for rapid and long-travel
landslides triggered by earthquakes was conductedusing data obtained by the ring shear tests on
samples from drilling cores, together withgeological and topographical investigation of thesites. The results of investigation for three selectedsites in the Nikawa area and the Tama area near
Tokyo presented above show that all three sites
could be subject to sliding surface liquefaction
with resulting rapid and long travel earthquake
induced landslides.
(5) Ring shear tests to reproduce stresses and shear
displacement during and after earthquakes were18

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Acknowledgments
This research was conducted as a part of the Areal
Prediction of Earthquake and Rain Induced Rapid andLong-Traveling Flow Phenomena (APERITIF) projectof the International Programme on Landslides (IPL
M101), which was funded by the Special Coordinating
Fund for Promoting Science and Technology of the
Ministry of Education, Culture, Sports, Science andTechnology (MEXT) in 2001-2004.
This APERITIF project was also supported by allgroup members of this project including members of
University of Tokyo (Ikuo Towhata, Kazuo Konagai),the Japan Landslide Society (Takahiko Furuya, Chigira
Masahiro), Niigata University (Hideaki Marui), andthis research was benefited from the advice of Prof.
Osamu Sato, Prof. Akira, Iwamatsu and Dr. Norio
Oyagi of the project management committee.
The authors deeply appreciate the cooperation of theoffice of Disaster Prevention Research of MEXT for its
cooperation with the implementation of this project.
The authors also appreciate the members of ResearchCentre on Landslides of Disaster Prevention ResearchInstitute of Kyoto University for their cooperation.
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地震豪雨時の高速長距離土砂流動現象の解明(APERITIF)プロジェクトの成果
佐々恭二・福岡 浩・関口辰夫*・森脇 寛**・落合博貴****

国土交通省国土地理院
独立行政法人防災科学技術研究所***
独立行政法人森林総合研究所**

要旨
都市域の住宅密集地における地震時高速長距離地すべりの危険度評価は世界中の地すべり危険地帯のどこにおいても現在もっとも重要な災害軽減における課題となっている。本報では佐々が代表を務める地震豪
雨時の高速長距離土砂流動現象の解明( APERI F) プロジェクトのうち、重要な成果をとりあげ紹介する。具体的には、（１）空中レーザースキャンによる地すべり危険地形の抽出、（２）人工降雨装置を用いた実規模
斜面模型による実験、（３）自然斜面における人工降雨による地すべり発生実験、（４）都市化域における地すべり危険度評価と災害危険地区予測、の４課題について紹介する。（４）の試験地は東京近郊の多摩地区の住宅密集地２カ所に設定され、現地調査、地質ボーリング、レーザースキャン型高度計による測量、リングせん断試験など室内実験の結果をあわせたところ、両試験地においてすべり面液状化の可能性があることを示した。本研究において得られた定量的なデータを用いた地盤工学的地すべり運動シミュレーション( RAPI D/ LS) により、都市域における地すべり危険値予測を街区単位で実現することができた。
キーワード: 危険度評価, ハザードゾーニング, 地震時地すべり, 流動化、航空レーザースキャナー, フル
スケール土槽試験, 自然斜面, 間隙水圧，すべり面形成，非排水リングせん断試験, 数値シミュレーション

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