LANDSLIDES IN JAPAN(The Sixth Revision)

Japan Landslide Society
National Conference of Landslide Control
2002

Contents
PREFACE…………………………………………………………………………………………………………………………1
1. JAPAN AND ITS NATURE (LANDFORM, GEOLOGY AND CLIMATE) ……………………………………………2
2. HISTORY OF LANDSLIDE MITIGATION MEASURES IN JAPAN ……………………………………………………6
3. CHARACTERISTICS OF LANDSLIDES IN JAPAN ……………………………………………………………………11
4. LANDSLIDE INVESTIGATIONS AND PREDICTION …………………………………………………………………19
5. LANDSLIDE MITIGATION MEASURES ………………………………………………………………………………30
6. RECENT REPRESENTATIVE LANDSLIDES AND SLOPE FAILURES IN JAPAN …………………………………35
7. PROFESSIONAL ORGANIZATIONS ENGAGING IN LANDSLIDE STUDY AND MITIGATION ………………62
INDEX MAP OF RECENT REPRESENTATIVE LANDSLIDES AND SLOPE FAILURES ILLUSTRATEDIN THIS BOOK …………………………………………………………………………………………………………………64

Edited by

Chief, TSUNAKI Ryosuke
MATSUURA Sumio(Chief, Editorial Board of the Japan Landslide Society)
ENDO Hidenori

KANEKO Masanori

NAKAMURA Hiroyuki

FUJISAWA Kazunori

KOMATA Shinjuro

OKAMOTO Takashi

FUKUOKA Hiroshi

KURODA Seiichiro

OKUYAMA Takehiko

FUNAZAKI Masatsugu

MANTOKU Masaaki TAJIKA Jun

HIGAKI Daisuke

MARUI Hideaki

TAMURA Keiji

ISHII Yasuo

MORIWAKI Hiromu

YOSHIMATSU Hiroyuki

KAWANO Yasuro

NAGATSUKA Isamu

Published by
Japan Landslide Society
National Conference of Landslide Control
April, 2002

PREFACE
Landslide investigation and mitigation measures in Japan have advanced since the Maseguchi landslide that occurred in
Niigata Prefecture in 1947. Serious research in landslide and related phenomena began about 50 years ago. The Landslide
Prevention Law designed to prevent and mitigate landslides, the first of its kind in the world, was enacted in 1958. This
law is the foundation for the advancement in landslide control technology in Japan. Since that time, landslide investigation
methods and various mitigation measures have been developed and continuously improved. It appears that the techniques
for landslide mitigation, including investigation, analysis and design of mitigation measures, as well as the maintenance andadministration of the completed works, have been nearly completed. Based on such advancements, it is now possible to stabilize slopes that had been actively moving and to prevent further movement of landslides caused by highway and dam construction and housing developments.
The Japan Landslide Society and the active involvement of its members have contributed to the advancement of landslide
research and technical development of landslide mitigation measures. The society was established in 1963, and has growncontinuously to its current membership of 2500. The Society offers the opportunity for publication, presentation and exchange of ideas to researchers and engineers studying landslides and related fields.
It is my great pleasure to present Landslides in Japan, 6th Revision . This book reflects a milestone of the advancement and the development of landslide research and technical development in Japan. In this issue, the previously publishedfive issues are included in a CD-ROM so the reader can appreciate our advancement in landslide research and developmentof mitigation technology in the past 30 years. Descriptions of numerous landslides which occurred during that period arealso included.
In Japan, we are at a turning point in landslide research and development. In addition to the conventional landslide mitigation measures which focus on the stabilization of an active landslide, prediction of landslide hazard zones, the development of soft or non-structural measures involving landslide hazard avoidance, rock falls and large-scale failures which
will have a much larger social implication have been an important topic.
One of the objectives of the Landslides in Japan, 6th Revision is to introduce landslide phenomena in Japan to landslide researchers and engineers overseas, but we also hope this will facilitate and promote free exchange of research results
and technology for advancement in the field.
We sincerely hope that advancement in research efforts will aid in the reduction and avoidance of landslide related disasters worldwide.

Hiroyuki Nakamura
President, Japan Landslide Society

11．
JAPAN AND ITS NATURE (LANDFORM, GEOLOGY AND CLIMATE)
1.1 PHYSIOLOGY OF JAPANESE ARCHIPELAGO(COMPOSITION AND TOPOGRAPHIC DIVISION OF THE ISLAND ARC)
The Japanese archipelago is comprised of five main island arcs
extending approximately 3000 km in the north-south direction (Fig.
1-1), and encompasses a total area of about 378,000 square km.
Seventy-five percent of the total land area consists of mountainous
terrain. The five main island arcs are, from north to south, Kurile
Arc, Northeast Honshu Arc, Izu-Mariana Arc, Southwest Honshu
Arc and Ryukyu Arc. These arcs approximately represent plateboundaries of the North America Plate, Pacific Plate, Eurasian Plate,and Philippine Sea Plate. Because of this arc-plate relationship, Japan is located in an area of severe crustal movement, and is within
one of the world’s most seismically active regions. There are 86 active volcanoes in Japan, and represent approximately 10% of theworld’s active volcanoes. The deep-seated earthquakes around Japanare epicentered along the Kurile-Kamchatka Trench, Japan Trench,
Izu-Ogasawara Trench, Nankai Trough, and Ryukyu Trench, which
dip steeply into the continent along the Wadati-Benioff plane. Furthermore, a series of volcanic fronts are aligned between the
trenches and the continent. Typically the volcanic belts are locatedabout 100 to 200 km inward of the trenches (Fig. 1-2). Based on
the volcanic fronts, the five island arcs mentioned above are classified into volcanic inner arcs and non-volcanic outer arcs.
Based on the characteristic configuration and partial overlappingof the five island arcs, the Japanese archipelago is divided into 12
distinct physiographic zones (Fig. 1-2). Landslides occur frequently
within six of the zones : Zone A1, the main interior of Hokkaido(inner Kurile Island Arc) which exhibits medium to small scalelandslides ; Zone B1, the inner Northeast Honshu Arc (the representative landslide zone) ; Zone D1, the Northwest Kyushu Island
located within the inner Southwest Honshu Arc (the zone of highdensity landslide distribution) ; Zone D2, the outer Southwest Honshu Arc (Shikoku Mountains) and Zone DC1, the Central Western
Honshu (Chubu Mountains), represented by landslides of very slow
movement and very large-scale, rapidly moving failures ; and Zone
DC2, the Central Eastern Honshu (Kanto Mountains) which exhibitsa high density landslide distribution within low-lying mountains.

1.2 ENGINEERING GEOLOGIC STRUCTURE OF
THE JAPANESE ARCHIPELAGO
Based on the major geologic divisions and characteristics of theearth materials during slope movement, the Japanese archipelago isdivided into 15 engineering geologic divisions (Fig. 1-3). Furthermore, on the basis of plate tectonic research and assessment ofslope movement, the 15 engineering geologic divisions are further
subdivided into five distinct tectonic zones, discussed below.
I. Pre-Tertiary Accretionary Zone : This zone represents Mesozoic and Paleozoic sedimentary rocks (Division 4) and metamorphic
rocks (Division 3), ultrabasic rocks (Division 2), Cretaceous sedimentary rocks (Division 5), and Late Mesozoic to Early Miocene
age Flysch-type sedimentary rocks (Division 6). These divisions are
typically distributed along the extensional direction of the narrowisland arcs. The geologic divisions are juxtaposed against each otherfrom the oldest formations (on the continental side) to the youngest
formations (on the Pacific Ocean side). The contacts between thedivisions are separated by shear zones including thrust faults.

2

II. Plutonic Rock Zone : This zone consists of plutonic rocks ofmostly Cretaceous to Early Tertiary age (Division 1). However, a
few parts of this zone include Miocene and Quaternary age intrusive rocks. Plutonic rocks of pre-Tertiary age exhibit significant
weathering.
III. Tertiary Rock Mantle Zone : This zone represents the areasof highest landslide occurrence within the Japanese archipelago, andconsists mostly of Upper Tertiary (and some Lower Tertiary) semiconsolidated clastic materials (Division 8 and 10) and volcanic
rocks (Division 9) which overlie the Pre-Tertiary Rock Mantle Zone
and the Plutonic Rock Zone. Non-siliceous mudstones easily
weather or decay into clays due to increased water content andweathering. Alteration of the volcanic rocks changes the color to agreenish appearance, and thereafter they are called green tuff .
Tuffaceous mudstones contain abundant smectite clays, and contribute to one of the primary causative factors of landslides.
IV. Quaternary Volcanic Zone : Volcanoes consisting of lava,
welded tuff and scoria beds (Division 14) often form very steep
slopes. Furthermore, the volcanoes are often associated with hydrothermal alteration and volcano-induced earthquakes. The high reliefof the volcanic regions also attracts heavy precipitation. All of thesefactors contribute to slope instability. Cap rock conditions exist
when volcanic rocks overlie the clastic materials.
V. Quaternary Regional Pyroclastic Flow Zone : This zone
consists of large scale eruptions of Late Quaternary acidic, weldedand unwelded pyroclastic deposits (Division 13) that are distributed
throughout much of Japan.

1.3 CLIMATE OF JAPANThe Japanese archipelago is situated between North latitude 20
and 45°, facing the Pacific Ocean along the southeastern side andthe Sea of Japan and Eurasian continent along the northwestern side.
Due to the geographical position of Japan, the climate varies considerably.
During the winter months, continental cold air masses (highpressure zone) are formed in the Siberian region due to radiation
cooling. The cold seasonal winds that are generated from the cold
air masses move through the Sea of Japan and absorb large quantities of moisture during Japans’ winter months, which cause thedominating northwest winds. When the moist seasonal winds reachJapan, the cold air masses collide into the mountain regions. As the
air masses rise with increasing elevation, a large quantity of moisture is precipitated as snow along the slopes facing the Sea of Japan,
establishing one of the worlds’ famous snowy regions (Fig. 1-5).
In the spring, due to the low pressure zones moving west tonortheast, the cold-warm cycles are repeated and gradual warming
occurs. Numerous landslides have been triggered by the large quantity of snowmelt along the slopes facing the Sea of Japan. Cherry
blossoms and budding begins from the southernmost island andmoves progressively northward. In early June, the Northern PacificHigh Pressure Zones gradually move from the south, and the northern air masses move from the Sea of Okhotsk since early springsand collide above Japan, forming a stationary seasonal rain front.
Usually, this early summer stationary rain front (Baiu Front) lasts a
couple of months and intermittently drops large quantities of rain.
These rains often create sedimentation disasters.
In the summer months, Japan is a high temperature-high humidity region due to the Northern Pacific High Pressure Zones thatcover most of Japan. In the fall, typhoons that form in the low lati-

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A1 Interior regions of Hokkaido
A2 Outer regions of Hokkaido
B1 Inner Northeast Honshu Arc
B2 Outer Northeast Honshu Arc
C1 Inner Izu-Mariana Arc
C2 Outer Izu-Mariana Arc
D1 Inner Southwest Honshu Arc
D2 Outer Southwest Honshu Arc
DC1 Central Western Honshu
DC2 Central Eastern Honshu
E1 Inner Ryukyu Arc
E2 Outer Ryukyu Arc

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Fig. 1-2 Topographic divisions of Japan (Kaizuka1))

T

0

500km

Fig. 1-1 Trenches and island arcs around the Japanese Archipelago3Fig. 1-3 Major engineering geologic divisions of Japan

Fig. 1-4 Distribution of Designated Landslide Prevention Zones

4

Hokkaido region
1129.6mm/year
8.2℃/year
400 Sapporo
300
200
100
1

250
km

0

30
20

N
50°

10
0
12 -106

Tohoku region(Japan sea side)
1857.0mm/year
12.1℃/year

500

400 Sakata
300

Hokuriku region 200
2592.6mm/year 100
1
14.1℃/year

Tohoku region(Pacific Ocean Side)
1204.5mm/year
11.9℃/year

30
20
10
0

6

12

400 Sendai
300
200
100

-10

400 Kanazawa 30
20
300
10
200
0
100
-10
1
6
12

Chugoku region
1554.6mm/year
15.0℃/year
400 Hiroshima 30
20
300
200
10
0
100
-10
1
6
12

1

400 Tokyo
300
200
100
400 Nagoya

400 Osaka

400 Kouchi
300
200
100
1
400
300
200
100

Kumamoto 30
20
10
0
1

6

12

-10

Kyushu region
1967.7mm/year
16.2℃/year

6

6

30
20
10
0
12

-10

Sikoku region2582.4mm/year
16.4℃/year

30
20

300
200
100

10
0
1

6

12

30
20

300
200
100

-10

Kinki region
1318.0mm/year
16.3℃/year

1

1

6

12

6

-10

1954 9 26
1959 9 26
1982 8 21983 9 28
1990 9 19
1991 9 27
1993 9 3

40°

1761
5098
95
44
40
62
48

140° 145° 150° 150°E
N
50°
⑥

①

45°
②
⑤

③

40°

⑦

10
0
12

-10

30
20
10
0
12

① Toyamaru② Isewan③ No.10 in 1982
45° ④ No.10 in 1983
⑤ No.19 in 1990
⑥ No.19 in 1991
⑦ No.13 in 1993

30
20

④

35°

Tokai region
1534.9mm/year
15.1℃/year

30°

30°

25°

25°

20°

20°
③

⑦
400 Naha
300
200
100
1

6

30
20
10
0
12

35°

-10

Kanto region1405.3mm/year
15.6℃/year

10
0

115°
120° 125° 130° 135°
Name of
StrikeNumbertyphoonsdays
of deaths

-10

②
15°

⑤

④
①

0

500

⑥

15°
1000

km

Okinawa region
2036.8mm/year
22.4℃/year

Fig. 1-5 Regional temperature and rainfall records

Fig. 1-6 Courses of historic typhoons

tude regions of the Northern Pacific Ocean move northward and circle the western rims of the Northern Pacific air masses, and often
land in Japan (Fig. 1-6). These typhoons usually generate verystrong winds and very heavy rainfall, and cause frequent sedimentation disasters and flooding. The pressure distribution pattern in early
autumn is very similar to the stationary rain of early summer. Late
autumn (October-November) is generally clear and offers the beautyof the colored hills.
The annual precipitation in Tokyo (Pacific Ocean side) is 1405
mm. At Owase, however, the Kii Peninsula records 4002mm while
the Johetsu-shi (Sea of Japan side) records 2880mm (of which onehalf is snow).

downpours brought by typhoons at Kagoshima Prefecture in 1993,
and resulted 120 deaths ; torrential downpours in Fukushima andTochigi Prefectures in 1998 resulted in 22 deaths ; and torrential
downpours in Hiroshima Prefecture in 1999 resulted in 31 deaths.
Notable single disasters with casualties in the last few years include : a rapid slope failure in Okushiri Island, Hokkaido triggered
by Southwest Off Hokkaido Earthquake of 1993 (Volume : 1.5 x
105m3, 30 deaths) ; Nikawa landslide, Hyogo Prefecture triggered
by Southern Hyogo-Ken Earthquake of 1995 (Volume : 1.0 x 105m3,
34 deaths) ; rock fall at Toyohama Tunnel, Furubira-Cho, Hokkaido
in 1996 (Volume : 1.1 x 104m3, 20 deaths) ; debris flow in Gamahara Creek, Nagano Prefecture in 1996 (Volume : 3.9 x 104m3, 14
deaths) ; and a debris flow in Harihara River, Kagoshima Prefecture
in 1997 (Volume : 1.6 x 105m3, 21 deaths).
The population density of Japan is 340/km2 (based on the 2000
census population of 126.9 million). However, the population density of the flat and low lying areas and plateau regions of Japan is1358/km2 indicating the severity of land use in Japan. Landslides
generally occur along gently to moderately sloping ground which isalso important as these areas include residential and agricultural use.
Because of these conditions, an active effort must be developed toprotect the slopes from future landsliding and failures in Japan.

1.4 LANDSLIDE DISASTERS IN JAPANThe natural conditions discussed above are multiple affects thatcreate unique physical conditions susceptible to landsliding whichcannot be seen any other place in the world (Fig. 1-4). Since only
25% of Japans’ land area is flat and low lying with plateaus, theJapanese people have suffered numerous landslide disasters sinceancient times. For example, evidence of landslide failure has beenunearthed from the site (Oshimo Shell Mound, Asou-Machi, Ibaraki
Prefecture) of in the Middle to Late Jomon Period (3000-1000 BC).
Nihon Shoki (720 literature) recorded numerous landslides and failures associated with the mega-earthquake (along the Nankai
Trough) of November 29, 684.
Recent disasters include disasters associated with torrential

REFERENCE :1) Kaizuka, S., (1988) : Geographic divisions of Japan, Gekkan
Chikyu, Vol. 10, No. 2, pp. 99-101.

5

2．
HISTORY OF LANDSLIDE MITIGATION MEASURES IN JAPAN2.1 HISTORICAL PERSPECTIVES OF LANDSLIDEMITIGATION MEASURES IN JAPAN
2.1.1 PRE ENACTMENT OF LANDSLIDE PREVENTION LAW
In Japan, full-scale, modern landslide mitigation measures were
implemented after World War II, or in the 1950s. In this respect,discussion of the historical perspectives of landslide mitigation
measures in Japan is divided into two parts : pre-1950s and post1950s.
The historical records show that some civil engineering works
had been performed as landslide mitigation measures during the
Edo Period (1603-1868). According to Takano1), earth dams and achannel diversion were constructed in Tohkamachi-Shi, Niigata Prefecture as landslide mitigation measures. The measures included theconstruction of earth dams to stabilize a landslide that consisted oftwo 6.5m high, 50m long with a 5m crown earth dams ; and one 5
m high, 30m long with a 3m crown earth dam to stabilize a landslide. The channel diversion work included a 358m long tunnel,
1.82m high and 1.52m wide, to divert the flow from the toe of alandslide thereby stabilizing landslide movement. Both works werelarge-scale projects and the historical records indicate the works
were supported by the local administration.
In the Meiji Period (1868-1912), the government undertook
flood control works very seriously. In order to obtain advanced
European technology, the government invited European engineers.
One of the most prominent engineers was Johannes D’Rijeke, aDutch engineer who lived in Japan from 1873 to 1903. He strongly
urged the Meiji Government to consider the importance of sabo
works (erosion and sediment control works). In addition, he incorporated torrent works as a part of sabo works, and established thefoundation of modern sabo works in Japan.
Furthermore, the Meiji Government enacted the River Law in1896 and the Forest Law and the Sabo Law (Erosion and SedimentControl Law) in 1897, thereby establishing modern erosion andflood control works in Japan. Thus, the landslide mitigation works
were handled as a part of sabo works. During this period, most of
the landslide mitigation measures were limited to the works along
rivers and channels. Therefore, most of the works consisted of river
structures.
In the 1900s, full-scale landslide mitigation measures were implemented on some of the large scale landslides.
Several large landslides were induced along the right shore ofan open excavation of the Ohkouzu Diversion Channel during its
construction in 1916 and 1919 (Photo 2-1). The work was commissioned to reduce and prevent flooding along the lower reaches of
the Shinano River. Furthermore, after the completion of the diversion channel, a post-construction landslide in 1924 delayed thecompletion date from 1920 to 1927. The predominant control measure for these landslides was the removal of the active landslide
masses totaling about 2.88 x 107m3. In order to remove such a large
volume of earth, a locomotive was utilized, an evolutional approachin those days.
In 1931, a very large landslide occurred along the right shore of
the Yamato River at Kamenose District in Kashihara-Shi, Osaka
prefecture. This region consists of a narrow valley that connects the
Nara Basin and the Osaka Plain. A major National Railroad (KansaiLine) and a Highway (Route 25) run parallel to the Yamato Riverthat signifies the economic importance of the region. The horizontal
movement of the landslide reached a distance of 31 meters in 1931,
destroying the railroad tunnels. The landslide movement also up-

6

Photo 2-1 Ohkouzu diversion channel (landslide occurredon the right hand side of the photo) (courtesyof Ministry of Land Infrastructure and Transport)

Photo 2-2 Installation of interceptor underdrain at Chausuyama landslide (courtesy of Nagano Prefecture)

lifted a section of the Yamato River by 9 meters, and the subsequentdamming inundated upstream area. In order to maintain the river
flow and economic pipelines, about 1.87 x 106m3 of earth had been
removed by 1934.
According to Taniguchi 3) along with massive grading works,
surface drainage and interceptor underdrains were installed at the
Gamou landslide in Tottori Prefecture and the Chausuyama landslide (Photo 2-2) in Nagano Prefecture during that period.
In 1947, about 200ha of gently to moderately sloping terrain located along the foothills of Gongendake Mountain in Noudani-Mura,
Niigata Prefecture started to slide. The toe of the landslide reachedand deflected the Nou River (Photo 2-3). Damages included 85
homes that are completely destroyed ; 23 partially destroyed
homes ; and 50ha of farm land. Furthermore, extensive damage toinfrastructures such as roads, bridges and other community facilities
were reported. According to Taniguchi3) the investigation of thislandslide was an impetus in establishing the modern landslide investigation and research in Japan.
Following the landslide disasters described above, in 1951 thePhoto 2-3 Maseguchi landslide(courtesy of Niigata Prefecture)

Photo 2-4 Ishikurayama landslide(courtesy of Nagasaki Prefecture)
Ishikurayama landslide (Photo 2-4) and Ningyoishiyama landslide
occurred in Imari-Shi, Saga Prefecture. In 1953, severe damage wascaused by disastrous storms in northern Kyushu, and in Hakone
Town, Kanagawa Prefecture the Sohunzan landslide occurred. Thus,
the reality of frequent landslide disasters leads to the enactment of
the Landslide Prevention Law of 1958. Following the implementation of the law, landslide mitigation technology, including investigation methods and research, has gradually and systematically advanced in Japan.

2.1.2 POST ENACTMENT OF THE LANDSLIDE PREVENTION LAW
The landslide mitigation measures can be classified into two
major categories : (1) hard measures , involving the construction
and implementation of measures and structures ; and (2) soft
measures , involving land use restrictions and provisions for awarning and evacuation system. In Japan, the hard measures arefurther classified into two categories : 1) landslide control measures ; and 2) landslide restraint measures. The following sections
discuss the historical perspectives in the development of the various
landslide mitigation measures in Japan.
LANDSLIDE CONTROL MEASURES ： Landslide control
measures utilizing the river structures for sabo works have been implemented for a long time, as well as surface drainage control andinterceptor drain installation. Some of these cases have been described in the previous sections. In recent developments, the representative groundwater control works include : horizontal gravity
drains, drainage wells, and drainage tunnels.
Horizontal Gravity Drains : This method involves drilling aseries of near horizontal borings directed at the slip surface(s) andwater-bearing zones to reduce the pore water pressure along the slip
surface(s). Prior to developing this method around 1947, drainage
tunnels constructed by tunneling through the landslide mass to remove the groundwater were designed and implemented in the field.
However, such drainage tunnels had some disadvantages due tosafety and cost. The horizontal gravity drains were developed bychance at one of the mitigation sites in Niigata Prefecture in 1949.
Because it is safe and relatively inexpensive, the horizontal gravity
drain system replaced the drainage tunnels for groundwater control.
Since 1952, the horizontal gravity drains have become one of themost extensively used methods of landslide control.
Drainage Wells : This method involves excavating deep, 3 to
4 meter diameter wells, then drilling numerous radially-positioned
horizontal gravity drains at various elevations from the wall of thewell. By developing this method, it was possible to reduce thelength of the horizontal gravity drains and increase the number ofdrains, thereby significantly increasing the effectiveness of the dewatering process. The first drainage well was introduced in 1955 at
one of the mitigation sites in Niigata Prefecture. The earlier method
involved sinking (by gravity) 4 meter, outside diameter steel reinforced concrete cylinders. Since 1960, the liner plates method has
been developed such that the efficiency in installation of the wellhas been increased. In Japan, the drainage well is the most important groundwater control works today.
Drainage Tunnels : Similar to drainage wells, the liner plates
method is also utilized in drainage tunnel construction. As a fundamental rule, the drainage tunnels are bored below the slip surface
into a stable bedrock formation. Furthermore, NATM Method has
been used experimentally to improve its safety. The drainage tunnels have been used with increasing frequency as a landslide control
measure in large-scale landslides.
Other landslide control measures include grading works to improve the landslide stability by altering the topographic configuration of the site. There are two types of grading works : buttress fill
and earth removal.
Buttress Fill : This method is designed to add weight at thetoe portion of a landslide to resist the slide movement. This concept
has been used for a long time as evidenced by historical documents.
The documents describe the construction of sabo dams (checkdams) along the toe portion of a landslide to facilitate sedimentdeposition thereby increasing landslide stability. This method is currently employed both as a temporary emergency measure and as apermanent solution.

7

Photo 2-5 Grading work (Earth removal) at Sohunzan
landslide (courtesy of Kanagawa Prefecture)

Photo 2-7 Iwadonoyama landslide following the completion of mitigation measures (courtesy of Japan
Highway Public Corporation)

Photo 2-8 landslide at Ohdo reservoir following the completion of anchor installation (courtesy of Ministry of L. I. and T.)
Photo 2-6 Yui landslide (courtesy of Forestry Agency)
Earth Removal : In contrast to the buttress fill, earth removal
works have not been utilized as long. Earth removal has been an
important element of mitigation measures, and in fact very large
quantities of landslide debris have been removed from the Ohkouzu
Diversion Channel and the Kamenose landslide ; however, they are
different from the current concept of earth removal , which isbased on increasing the safety factor of the landslide. It has beensuggested that such earth removal works performed on the Ohkouzu
Diversion Channel and Kamenose landslide were merely the resultsof necessity and were implemented without a clear understanding of
the significance of the works. According to Taniguchi 3), the firstfull-scale earth removal work was performed in 1949 to remove
about 1.0 x 104m3 from the head portion of a landslide mass in Niigata Prefecture. Then, at Sohunzan landslide (Photo 2-5), about 1.1
x 105m3 of earth was removed in 1954. During 1961-1964, a largescale earth removal works including over 1.3 x 106m3 were performed at Yui landslide (Photo 2-6) in Shizuoka Prefecture.
LANDSLIDE RESTRAINT MEASURES： Typical landslide restraint works include : Steel Piles ; Large Diameter Cast-in-Place
Shafts ; and Anchors. The construction cost of restraint measures ismore than the control measures. However, in large projects such as
construction of a dam, a high degree of effectiveness of the groundwater control measures cannot be expected due to the nature of the
project - creation of a reservoir 2).
Pile : The first restraint works, steel piles, were installed in

81962 at a dam constructed by the Ministry of Construction. The
steel piles used in this job were 6.9mm thick, had an average lengthof 12.5m, a diameter of 318.5mm and were designed considering
only the shearing force. A bending moment was included in the design of the steel piles and the diameter of the piles were increased
to 458mm at a landslide caused by the dam reservoir in Shikoku region. Since then, the design technique has been improved and there
is now a clearer understanding of the function of the piles. Today,
steel piles are extensively used in Japan as a landslide restraint
measure.
Large diameter cast-in-place shafts : As the pressure of
rapid urbanization of land increased, the necessity of large-scalelandslide mitigation has also increased. With this national movement in the background, the first large-scale, large diameter cast-inplace shafts were installed in 1968 for the National Highway bypass
construction in Shizuoka Prefecture. This work included the installation of reinforced concrete structures that were 15m deep with anoutside diameter of 300cm, and 50cm thick walls, and completely
filled with reinforced concrete. The bottom 6m of the shafts wereembedded into the bedrock formation below the slip surface. A
landslide that occurred from the construction of the Chuo National
Highway at Iwadonoyama (Photo 2-7) temporary closed the highway. The landslide was 80m wide, 80m high and 15 to 20m deep.
Twenty five 2 to 3m diameter, cast-in-place shafts were installed as
landslide mitigation measure by the Japan Highway Public Corporation. Due to the high cost of installing such massive shafts, thismeasure is generally utilized only in very large and landslides ofhigh importance where implementation of other measures may not
be as effective. In some cases, the shaft diameters exceed 6 meters.
The anchor : The anchor is the most frequently utilized landslide restraint measure. The first application of the anchor as landslide restraint measure in Japan was the mitigation of a landslide induced by the relocation of an access road to a dam constructed by
the Ministry of Agriculture and Forestry in 1969. The anchor tendon consisted of twelve, 12.4mm PC steel bars with a design loadof 118t per bar. The length of the anchor tendon was 16m with ananchorage length of 6m. A total of 176 anchors were used for thislandslide. During the latter part of 1970s, these anchors have beenutilized extensively as a landslide mitigation measure for landslides
induced by reservoir construction (Photo 2-8). Since tensile force
can be added during construction, it will undoubtedly be a popular
landslide restraint measure in the future 2).
As demonstrated in the previous discussions, with the advancement of landslide mitigation technologies it is possible to control
slide movement of even very large landslides by applying appropriate advanced mitigation measures. However, it is clearly impossible
to mitigate and stabilize every landslide. Thus, the necessity of socalled soft measures , non-structural measures involving warning,
avoidance and evacuation, has been increasingly implemented sincethe 1970s.
Such soft measures are based on the development of warningand evacuation systems. Since the late 1980’s, heavy efforts havebeen invested in numerous instrumentations on some of the very active landslides to monitor sudden and unexpected movements. Thenplans for the evacuation of residences and traffic control have beendeveloped. The result is highly automated warning systems which
are able to relay the results of instrumentation observation in realtime in the latter part of 1980’s. The system automatically records
the quantity of rainfall, amount of ground movement, fluctuations ingroundwater level, and other parameters. When any or a combination of the measurements reaches a certain threshold, warning andevacuation signals are issued. A detailed discussion of this warning
system is presented in Chapter 4.
This system is indispensable for landslides where around theclock monitoring is required for safety, and for disaster sites that require constant watch for the safety of remaining residents and rescue workers.

2.2 SEDIMENTATIOIN DISASTER PREVENTION LAW
As previously discussed, the necessity of soft measures (nonstructural measures) as landslide mitigation has been increasing. In
particular, urban development has been expanding into hillside regions in recent years. Thus, it is important not only to rely on the
instrumentation of the automated landslide monitoring systems, but
it is also necessary to impose aggressive land use restrictions inhighly populated areas.
A large-scale sedimentation disaster associated with heavy
storms occurred in Hiroshima-Shi in June, 1999, claiming 24 deaths.
Since many of the victims were residents of recently completed
housing developments in hillside areas, attention to the reevaluationof land use policies and enforcement of stricter building codes andbuilding inspection processes for hillside developments was renewed.
Thus, a special task force was established by the Ministry of Construction (currently Ministry of Land, Infrastructures and Transport)that was directed towards drafting new legislation. The results of thedeliberations were summarized in the report Comprehensive Legislation for Sedimentation Disaster Prevention , and was consulted to
the River Inquiry and Planning Commission. Following reports fromthe Commission, the Ministry published A draft Legislation to
Promote Sedimentation Disaster Prevention in Affected Regions .
As a result, the legislation became law on May 8, 2000.
The following are the significant points of the legislation :1. The legislation specifically states that it is intended to promote the soft mitigation measures such as warning andevacuation systems, and not the conventional construction
emphasized landslide mitigation measures.
2. The legislation shall focus on the regions or areas affected bysuch disasters rather than the causative sites or source areas.
3. The legislation shall be comprehensive and be in conjunction
with the land development process and building permit process.
4. The legislation shall be designed to act as a catalyst between
the sincere effort of the administration to inform the public
and the eagerness of public to know.
The purpose of this legislation is to : (1) define and delineate
the areas susceptible to sedimentation disasters ; (2) establish warning and evacuation systems for the affected areas ; and (3) promote
mitigation measures to prevent sedimentation disasters by restricting
hillside development and imposing stricter building codes and inspection processes in areas susceptible to serious hazards. Thus, thislegislation is intended to promote soft mitigation measures to prevent sedimentation disasters. Furthermore, the objective of the legislation is to provide comprehensive sedimentation disaster prevention
measures with other existing related legislations such as the Sabo
Law, the Landslide Prevention Law and Law for Prevention of Disasters Due to Collapse of Steep Slopes.
A schematic outline of the legislation is presented in Fig. 2-1.

2.3 FRAMEWORK OF LANDSLIDE MITIGATIONMEASURES IN JAPANThe fundamental legislation that governs landslide mitigation
measures in Japan is the Landslide Prevention Law . As previously discussed, this law was enacted in 1958. The law consists of
55 chapters covering such topics as designating Landslide Threatened Area ; administrator ; Minister in charge ; restriction of activities within the district ; allocation of cost ; and penalties. The
landslide mitigation measures will be implemented by the governing
legislation. The summary outline is presented in the following paragraphs.
The Minister in charge is either the Minister of Construction(currently Minister of Land, Infrastructures and Transport), and theMinister of Agriculture, Forestry and Fishery. The designated Landslide Threatened Area are administered by either the Sabo Department, River Bureau of Ministry of Land, Infrastructures and Transport, or the Rural Development Bureau, Ministry of Agriculture,
Forestry and Fishery or Forestry Agency, Ministry of Agriculture,
Forestry and Fishery. The local administrator is appointed to be theMetropolitan and Prefectural Governor ; therefore, the governors
execute the landslide mitigation measures. However, in cases suchas extremely large projects or projects requiring a high level oftechnology, the Minister in charge may directly execute the works.
Thus, the landslide mitigation measures can be executed directly by
the Ministry of Land, Infrastructures and Transport ; Rural Development Bureau, Ministry of Agriculture, Forestry and Fishery ; andForestry Agency, Ministry of Agriculture, Forestry and Fishery (thethree agencies). The mitigation measures can also be executed as
subsidized projects by the corresponding department-agency of theMetropolitan and Prefectural governments to which the designateddistrict to belong.
Along with the projects administered by the three agencies(ministries and agencies), landslides induced by the construction of

9

SEDIMENTATION DISASTERS : RAPID SLOPE FAILURE, DEBRIS FLOWS, LANDSLIDES

Preparation of Guidelines for Sedimentation Disaster Prevention Measures（Minster of Land，
Infrastructure and Transport）
・Basis for Sedimentation disaster prevention measures
・Guidelines for baseline survey
・Criteria for designation of potential sedimentation areas
・Polices for relocation of structures within high potential sedimentation disaster areas

Implementation of baseline survey（by each prefecture）
・Investigation for designation of potential and high potential sedimentation disaster areas

Designation potential sedimentation disaster areas（by Governor of each prefecture）（Areas of potential sedimentation disaster）○Establish information，
warning and evacuation system○Inform residents regarding the warning and evacuation procedureWarning and evacuation system
・Disaster prevention plansfor each community（Emergency and Disaster
Management Law）

Designation of high potential sedimentation disaster areas（by Governor of each prefecture）（Areas where potential for damages to structure and bodily
insuries may exist）○Permit requirement for certain development
subjects : residential subdivision and some public facilities○Establishment of building regulations（Includes building contructed outside of the cities' sphereof influence）○Evacuation advice for inhabitants of structures subjected
to sedimentation disaster○Secure loans and financial aid for dislocated families

Building codes
・Establish building codes forstructures of human occupancy（Uniform Building Codes）

Aid for relocation
・Financial support by the
Housing Loan Corporation

Fig. 2-1 Outline of the Sedimentation Disaster Prevention Law
Table 2-1 Summary of Designated Landslide Threatened
Area
Ministry of Agriculture, Forestry
Ministry of Land, and Fisheries
Infrastructure andRural DevelopeTransport
Forestry Agency
ment Bureau

Table 2-2 Summary of landslide mitigation expendituresin fiscal year 2000
Ministry of Agriculture, Forestry and
Ministry of Land, Fisheries
Infrastructure andRural DevelopeTransport
Forestry Agency
ment Bureau

Total

Number of area

3,329

1,868

1,725

6,922

State Subsidy

37,505

14,06822,273

Total area (ha)114,023

107,061

97,927

319,011

Direct spending

8,421

5,892

6,471

Total

45,926

19,96028,744highways or dams or during the management of such works, and regardless of the nature of the project (direct or subsidized), the project administrator is responsible for the mitigation works. Furthermore, landslide disasters caused by severe storms, snowmelt or
earthquakes which require a high level of urgency, the law requiresthat the three agencies shall respond to the disaster recovery project.
Such emergency works can be administered as either direct or subsidized project.
Table 2-1 summarizes the Landslide Threatened Area administered by the three agencies as discussed above. As in the tableshows, about 0.8% of the total area of Japan (378,000 km2) is designated as a Landslide Threatened Area. The total expenditures oflandslide mitigation measures implemented by various agencies during the fiscal year 2000 is shown in Table 2-2.

10Unit : 1 Million Yen

REFERENCES :1) Takano, H., (1960) : Landslides and mitigation measures, Chikyu
Shuppan, pp. 182-190 (in Japanese).
2) Fujita, H., (1996) : Historical perspectives of landslide mitigation
technology and development, In Nakamura, S. ed., Developmentof landslide research and its future. Taimeido, pp. 342-356 (inJapanese).
3) Taniguchi, T., (1989) : Pioneering landslide investigation, Japan
Association of Landslide Control Techniques, 291p. (in Japanese).

3. CHARACTERISTICS OF LANDSLIDES IN JAPAN
3.1 LANDSLIDE CLASSIFICATION
Landslides are a type of slope movement that can be classified
by a combination of material compositions of the sliding mass andthe type of movement (Varnes1)). Cruden and Varnes2) further classified landslides based on three types of material compositions (rock,debris and earth) and five types of movement (topple, slide, spread,fall and flow) (Table 3-1). However, the type of landslide movement
may change from the initial stage of movement to the latter stages.
In order to express both types of movement, for example, a landslide that occurred as an earth slide and subsequently became anearth flow can be expressed as an earth slide−earth flow 1).
People have been living in the mountain region of Japan sinceancient times. Therefore, damages caused by the landslides have an
important influence on quality of life. The magnitude of landslide
damages depends on the size of the landslide mass and the rate of
movement. The Committee on Glossary of Geology, Japan Landslide Society, has proposed a new landslide classification system incorporating the damage aspect of the landslide along with six
basic types of movements : fall ; topple ; lateral spread ; slide ;flow ; and creep (Furuya and Kuroda3)). Creep includes slow continuous deformation of slip-surface clay and colluvium as well as
creeping of surficial materials, such as frost creep (solifluction) due
to the freeze-thaw cycle of the ground surface.
In Japan, slope movements have been classified into two basic
categories for a long time： jisuberi and houkai , and translate
into English as landslide and rapid slope failure or slope failure , respectively. As shown in Table 3-2, the rate of movement ofa landslide is generally slow (less than cm/minute) with a long duration (over 100 hours) and often repeated movement. The rate of
movement in rapid slope failures are very fast, and are often fasterthan 1m/sec. with a duration of movement of less than 1 hour. Further, there are differences in the magnitude of the event, location oforigin and slope inclination between the landslides and the rapid
slope failures. As described in the schematic diagram (Fig. 3-1), themoving mass in landslide often remains at or near the originating
zone, while in rapid slope failure the moving mass completely
vacates from the originating zone and is deposited on the lower
slopes. Consequently, economic damages are far greater in disasters
caused by landslides . In contrast, disasters caused by rapid slope
failures frequently threaten lives. Stabilization of moving masses is
the key element in mitigation measures for landslide , while
rapid slope failure emphasizes the slope protection of the originating zone and its surrounding areas. This book mainly focuses onlandslide .

3.2 DEVELOPMENTAL PROCESS OF LANDSLIDESAND LANDSLIDE MORPHOLOGY
The movement of a landslide develops various micro topographic features within a moving mass due to compressional andtensile stresses within the slide movement (Fig. 3-1). A bulged
slope caused by sagging and accompanied with a series of linear
depressions represents the stage prior to development of a slip surface, premonitory topographic expression of an eminent landslide orslope failure. The example in Fig. 3-2 shows several parallel, linear
depressions along the upper portions of the slope, and up hill-facing
scarps that indicate spreading toward the sides. Based on the landslide distribution maps developed from the interpretation of characteristic features associated with landslides and other slope forming
processes on topographic maps and aerial photographs, this indicates that very large portions of mountain regions in Japan are underlain by landslide deposits (Fig. 3-3). Many of the landslide morphologies show evidence of erosion, extensive development ofdrainage networks on the landslide features, which indicates an ancient origin. Currently active landslides are often found withinthese ancient landslide deposits.
The reasons for such a high frequency of landslide occurrencein Japan include : (1) Increased erosion potential caused by
mountain-building process. This is one of the most important factors
in landslide evolution, and can be attributed to crustal uplift of up
to 1500 m during the Quaternary Period. (2) The geographical location of the Japanese Archipelago produces an abundance of precipitation which accelerates toe slope erosion by rivers and provides
plentiful groundwater supplies in the hillsides. (3) Frequent large
magnitude earthquakes are also contributing factors. As discussed,
since the conditions for developing of unstable slopes exist in themountain terrains of Japan, the formation of the landslides constitutes the single most important factor in slope forming processes
during the Quaternary Period. Thus, these situations represent theimportance of landslide prevention in conservation of forest, farmland and settlements within these mountain regions and the construction of roads and dams.
As landslide movement continues, fracturing and accelerated
weathering of bedrock, and alterations in groundwater the flow regime advances. Physical properties and strength of materials composing the sliding mass will also change. Therefore, it is possible tointerpret the developmental process of a landslide from the landslide
morphology as the size and the type of movement of landslides
change. Micro topographic features of Yachi landslide located in the
Tohoku Region are presented in Fig. 3-4. The landslide area is underlain by hard Miocene mudstone and tuff. The upper slope consists of jagged scarps and large landslide blocks with numerous
cracks ; the middle portion of the slide consists of small folds andTable 3-1 Abbreviated classification of slope movement (Cruden and Varnes2))

Type of MaterialType of Movement
Fall
ToppleSlide
SpreadFlow

BedrockRock fall
Rock topple
Rock slideRock spread
Rock flow

Engineering Soils
Predominantly Coarse
Debris fall
Debris topple
Debris slide
Debris spread
Debris flow

Predominantly Fine
Earth fall
Earth topple
Earth slide
Earth spread
Earth flow

11

Table 3-2 Difference between landslides and rapid slope failures (Watari4))
Landslides

Rapid Slope Failures

Geologyoften influenced by geology and geologic
structure

little or no influenced by geology andgeologic structure

Soils

moves along slip surface(s) that consists of highly
plastic clay

usually involves topsoil, residual soil and (highly)
weathered bedrock

Topography

occur on gentle to moderate slopes of 5 to 30°
the upper slopes often have flat-plateau like
topography

usually occurs on slopes steeper than 30°

Nature ofmovement

continuous, recurrence (repeatitive occurrence)
duration of a single episode is generally longoccur suddenly
short duration

Rate ofmovement

generally slow to very slow
0.01 to 10mm/day(most common)very to extremely rapid
10 m/sec or fasterNature ofmoving mass
(blocks)

little disturbance within a sliding blockoften move while retaining the original shapeand characteristics

incoherent move as highly disturbed mass

Causes,triggering
mechanismgenerally influenced by exess groundwater,
elevated groundwater table

generally influenced by rainfall intensity

Sizesurface area is often large ranges
between 1 to 100 ha

surface area is generally small with an averagevolume of about 440m3

Warning
signs

often develop cracks, depressions, upheavals,
groundwater fluctuation, etc. prior to sliding

hardly any warning signs
almost always fail suddenly

Typicaloriginal
gradient

10 to 25°

35 to 60°

Crown cracks

Stable zone

Crown

Semi-stable zone
Head

Main scarp

Minor scarp(s)Linear depression(s)Sagpond(s)Zone of deflectionsge
RidSecondary slip surface(s)Main slip surface
Maicro shear planes

Lateral scarp
Lateral cracks

Lateral ridge
Lateral ridges
Lateral cracks
Longitudiral cracks

Toe of surface of rupture
Transverse cracksofne n
zo tio slipof ula ry
se m da )Ba cu n e
ac eco rfac(s su

Zone of accumulation

Compression ridges
Radeal cracks

e

Lo

t
ba

Toe
Tip

Fig. 3-1 Block diagram of idealized earth slide (Oyagi5))12

Up-hill facing scarps
Linear depression
Area of deformed slope

Fig. 3-2 Map showing unstable slope near Sanmoriyama at Iwate-Akita prefecture boundary(Japan Landslide Society, Tohhoku Branch6))
thrust faults that are oriented perpendicular to the movement direction ; and the lower slope facing the Naruse River is composed ofan undulated mass with a small-scale circular scarp. This can be attributed to the morphological transformation process that change
landslide configurations, caused by bedrock fracturing from theslide movement, and results in landslide morphology with uniquecharacteristics. In summary, the upper portion of the landslide isformed by a bedrock slide ; the middle portion of the landslide isdeveloped as a secondary landslide from the original bedrock slide.
Subsequently, rocks are fractured and weathered yet retain plastic
properties that form small scale folds and thrust faults. As downslope movement and fracturing and weathering progresses, thelower portion of the slide consists mostly of clayey materials anddevelops into a rotational landslide.
Depending on the rock types and geologic conditions, not alllandslides follow the development process discussed herein. However, it will be particularly useful, for the prediction of a potential
landslide location if an individual landslide topography and landslide phenomena can be understood by incorporating this as one aspect of the landslide morphological evolution process.

3.3 LANDSLIDE TYPE AND CHARACTERISTICS
BASED ON THE ENGINEERING GEOLOGICZONESThis section describes the engineering geologic zones and particular landslide types often associated with each zone in Japanesearchipelago. This presentation is based on the previously discussed
landslide classification : type of movement, material composition
and rate of movement. Furthermore, the landslide types are classified with the incorporation of particular regional varieties andunique characteristics of bedrock formations in the region.
I Pre-Tertiary Accretionary Zone
Rock formations that compose this zone have been accumulated
along plate boundaries. The most recent accretion occurred during
the Miocene Period, and, piggy backed on top of the basement rockof that period. Formations within this zone were lithified throughdiagenesis and metamorphism. However, localized weak zones
caused by shearing and fracturing can often develop during tectonism and can be further intensified by weathering. Five landslide
types have been classified in this zone.
(1) Debris Creep / Slow Moving Debris Slide : This is a typeof movement that occurs in mountainous areas of high relief (over300m) where earth debris accumulates along the gentle slopes infront of steep slopes (30 to 40°). Debris creep is most common inthis zone. As shown in Fig. 3-5, the gentle slope represents the aggregate of several landslides. Many of the landslides are reactivations of older landslide deposits.
(2) Rapid Debris Slide : This failure is a rapid movement ofearth debris caused by heavy rain at the headwater zones of streams
and small canyons. Often, this is a source area for debris flows. The
age of the debris is considered to be younger than the debris described in (1) above(3) Rapid Rockslide : This is a rockslide involving more than 108
m3 in volume, and is caused by seismic activity or torrential rains.
Most of the rapid rockslides were suspected to have developed on
slopes subjected to bedrock deformation. The deformation was attributed to creeping induced by gravitational instability caused by
rapid uplift and erosion during the Quaternary Period. Fig. 3-6
shows an example of an overturned structure consisting of interbedded sandstone and slate by creeping followed by a mega cata-

13

EXPLANATION
Main scarp and Lateral Scarps

Outline and Boundary of Slide Body

Main scarp with fresh and uneroded crown

Side body with main scarp having a clearly defined orat least discernible outline.

Main scarp with partially eroded crown

Main scarp almost completely eroded but part of thelandslide mass.

Main scarp with heavily eroded crown

Although it is not completely separated from bedrock,it is assumed to be unstable.

Main scarp with common crown and facing opposite
directions

Questionable zones
Internal Structure
Minor (or secondary) scarps. Same as "Main Scarp andLateral Scarps" based on the level of erosion.
Boundaries of secondary slide block(s)Fig. 3-3 Landslide Map (Landslide Maps, Hirosaki, scale at 1/50,000 Hirosaki and Fukaura quadrangles 7) modified a section of Kawaharatai)

14

EXPLANATION
1. Ridge, small scarps near the ridge, mainscarp
2. Bedrock exposed by the slide
3. Minor or secondary scarp(s)4. Earth flows
5. Areas of micro-folding
6. Drainage and ponds
a. Scarp
b. Upper Block : bedrock sliding blocks
c. Middle Block : zone of compression
with folding
d. Lower Block : earth flow zone

Ohyanagi pond

Naruse River

Fig. 3-4 Micro-morphology of Yachi landslide (Miyagi et al.8))
Graphic representation of reliability
in landslide identification

definite

na
Mi

mi-

Da

io R

uncertain.

Youne Iandslide

Fig. 3-5 Landslide map from aerial photographic interpretation, Youne landslide
strophic failure.
(4) Slow Rockslide : For crystalline schist, the slip surface isroughly parallel to the plannar schistosity ; for non-metamorphic
rocks, the slip surface is sub-parallel to laminar-bedding planes andsmall low angle faults.
(5) Bedrock Creep : The characteristics of this landform developed in the mountains of high relief from bedrock creep and include scarps that face towards the ridge lines and are accompanied
by multiple minor (or secondary) scarps that are roughly parallel tothe ridge lines. This type also exhibits linear depressions near the
ridge line with bulges toward the lower slopes. For example, theprofile of the slope shows convex shape from the ridge to the drainage line. This represents the early stages of deformation of the landform and slope dynamics. However, it is suspected that distinctive
slip surface(s) have not yet been formed. There is high possibility
that this landform could develop into a rapid slope failure or landslide in future.

II Plutonic Zone
The type of rapid slope failures and landslides are similar within
the Plutonic Zone even though the intrusions occurred throughout
geologic time.
(1) Rapid Surface Slide : This involves failure of topsoil, residual soil and colluvium triggered by intense rainfall. It is most common in areas underlain by granitic rocks. Failed debris cascades
downstream while stripping surficial materials along the way,
thereby increasing its volume. Depending on the amount of water
available, this type of failure can often develop into debris flows.
(2) Rapid Debris Slide : Earth debris accumulated near the drainage head and knick line during the Pleistocene Epoch subsequently
failed by heavy rainfall. It can often develop into debris flows.
(3) Rapid Slide of Weathered Rock or Residual Soil : This
type of failure involves deeply and completely weathered bedrock
where the failure is generally trigged by intense rainfall. Failure surfaces are generally found within the weathered zone, and other failure surfaces are found along joint surfaces and dikes.
(4) Rapid Rockslide : Although it is rare occurrence, rapid rockslides could develop into a very large scale. This slide often occurs
along fault and fracture zones.
III Tertiary Rock Mantle Zone :The clastic materials observed within this zone are semiconsolidated, and mudstone could easily turn to clay by hydration
and weathering. Tuffaceous mudstones contain abundant smectitethat contributes and facilitates sliding. There are four types of landslides observed within this zone and are described below.
(1) Clayey Soil Creep / Slow Slide : This type of slide often
occurs in areas underlain by mudstone and colluvium along valleys
of low relief. Depending on the velocity, the sliding change into a
mud flow ． This type of slide is relatively small and ranges insize from 5-100m wide ; 100-500m long ; and 5-20m deep. The
cause of initial movement includes increased pore water pressure
from snowmelt in spring and intense rainfall.
(2) Rapid Slide of Semi-Consolidated Sediments : This is a

15

EXPLANATION
Creep zone

Knick line
Linear depression
Up-hill facing scarps
ScarpsShear andfracture zone
Creep zone
Sandstone

Fault

Slate
Inter-beded slate andlimestone
Tertiary volcanic rocks
Dip and strike of beddings

Linear depression

Keel-shaped depression

Knick line

Bedrock
Shear andfracture zone

Projection of A−A

Fig. 3-6 Geologic map and cross sections of the vicinity of Shichimenzan landslide (Chigira9))

Drainage wells

100m

Cast-in-place shafts
100m
Colluvium

Drainage tunnels

BasaltMostly shale
Mostly sandstone
Slip surface

Fig. 3-7 Cap rock structure of Washiodake landslide

160Mt. Mayuyama

Shimabara Bay

Fig. 3-8 Bird’s eye views and the contour maps of Mayuyama before and after the rockslide avalanche in 1792 (Inoue10))rapidly moving slide along very steep slopes triggered by excess
groundwater or earthquakes. The materials consist of clastic sediments such as gravels (conglomerate), sands (sandstone), and silts(siltstone) deposited during the Pleistocene and Upper Pliocene Epochs. In some cases the rapid slide can become mud flows.
(3) Slow Slide of Consolidated Sediments : The basic composition of the slope-forming materials includes the overlying hardcompetent cap rocks with underlying soft-incompetent formations(Fig. 3-7). The competent rocks include massive sandstone, siliceous shale, and massive volcanic rocks, while the incompetentrocks include mudstone and altered tuff. The maximum dimensionsof this type of slide could be on the order of : 4km wide ; 5kmlong ; and 100m deep.
(4) Bedrock Creep : This is the same type of slope deformation
defined in I, (3) above, and is also observed in the Green Tuff Zone.
IV Upper Tertiary-Quaternary Continental Volcanic Deposits Zone
This zone includes continental volcanic deposits of Upper Miocene to Holocene. There are four types of slope movement thathave been recognized within this volcanic zone.
(1) Rockslide Avalanche / Debris Avalanche : This type of
failure occurs only at composite volcanoes and lava domes of highrelief. As new volcanic activities centered within the deeper portionof the volcano that contain upward-migrating magma, earthquakes
and fluctuating groundwater levels could induce a catastrophic failure of the volcano and develop into rockslide avalanche/debris avalanche. This type of failure deposits large, scattered unbroken
blocks along foothills and the lower reaches of drainages. The volume of this type of failure is on the order of 108 to 1010 m3. Most of
the cases involve andesitic volcanoes, but there are cases that involve basaltic volcanoes. Fig. 3-8 shows bird’s eye views and topographic maps of hummocky avalanche deposits formed by a rock-slide avalanche that was triggered by a large magnitude earthquake
in 1792 at Mayuyama, Unzen Volcano, Nagasaki Prefecture. The
rockslide avalanche induced a massive Tsunami as the debris fell
into Shimabara Bay causing about 15,000 deaths.
(2) Rapid Clay Slide in Hydrothermal Alteration Zone : This
type of landslide occurs within zones that receive severe hydrothermal and fumarolic alteration located near the base of craters along
the foothills. Examples of this type of failure include Hakone Sohunzan of 1953, and a more recent example is a landslide triggeredby a small-scale phreatic eruption at Hachimantai in northern Tohoku Region in 1997.
(3) Slow Slide of Sedimentary Rocks Overlain by Volcanic
Cap Rocks : This is a type of landslide formed along the foothillsof volcanoes where the terrains are underlain by sedimentary rocks
that are in turn overlain by competent cap rocks of lava and pyroclastic rocks. As localized active down-cutting of channels proceeds,
the side slopes are over-steepened and became unstable. Thus, large
sections of the hillside piggy backing the volcanic rocks slide downslope. The scale of this failure type could be very large, measuring
up to 2 km wide and 4 km long. In some cases, lacustrine sediments that were deposited in older calderas beneath the volcanic
rocks contribute a triggering mechanism of landslides. Fig. 3-9shows abundant landslide topography around the foothills of an active volcano, Yakeyama Mountain. The investigation shows lacustrine sediments in an older caldera beneath the active volcano.
(4) Debris Flow : There are two types of debris flows : (1) The
first type involves recently deposited volcanic ash fall that is subjected to intense rainfall. The saturated ash could flow downstream,
erode the channels and cause numerous debris flows (Sakurajima
Mountain, Yakedake Mountain, and others) ; (2) The second type isassociated with high temperature pyroclastic flows that melt the existing snow, and flow down slope with the water from the melted
snow. The flowing body erodes and consumes materials along the17

Mt. Yakeyama

LegendAerial extent ofancient caldera

Other symbols aresame as Fig.3-3

Fig. 3-9 Landslides around the Quaternary volcanic rocks deposited on older caldera (Oyagi11))way, becoming a large scale mud flow-debris flow (Tokachidake
Mountain).
V Quaternary Regional Pyroclastic Flow Deposits Zone :The slope movements within this zone are represented by the
failure of unwelded pyroclastic flow deposits accompanied with intense rainfall. Furthermore, in this zone the failures are frequently
induced by seismic activity. However, it is considered that the majority of these rapid slope failures involve not only pyroclastic flow
deposits, but also include plentiful tephra and topsoil that underlies
by the pyroclastic flow deposits.

REFERENCES :1) Varnes, D.J. (1978) : Slope movement types and processes,
Schuster, R.L. and Krizek, R.J., eds. Landslides : Analysis andcontrol, Special Report 176, Transportation Research Board,
National Research Council, U.S.A., pp. 11-33.
2) Cruden, D.M. and Varnes, D.J. (1996) : Landslide types andprocess, Turner, A.K. and Schuster, R.L., eds. Landslides : Investigation and mitigation, Special Report 247, Transportation
Research Board, National Research Council, U.S.A., pp. 36-75.
3) Furuya, T. and Kuroda, K. (1993) : Landslide classification,
Proc. of Symposium, Problems regarding terminology of landslide geomorphology and geology, Japan Landslide Society, pp.
2-10 (in Japanese).

184) Watari, M. (1986) : Mechanism and mitigation measures ofslope disasters, Sankaido, 170p (in Japanese).
5) Oyagi, N. (1982) : Structure of landslide, Urban Kubota, No.
20, pp. 42-46 (in Japanese).
6) Japan Landslide Society, Tohoku Branch, (1992) : Landslide in
Tohoku Region, landslide distribution and morphology - a practical manual for engineers, Japan Landslide Society, pp.110 (inJapanese).
7) National Research Institute for Earth Science and Disaster Prevention, Science and Technology Agency (1985) : Landslide
Maps, Series 3, Hirosaki and Fukaura quadrangles.
8) Miyagi, T., Kobayashi, Y., and Tokunaga, H. (1996) : Landslide and development of morphology, ed. Nakamura, S., Development of landslide research and its future, Taimeido, pp. 26-41 (in Japanese).
9) Chigira, M. (1995) : Weathering and slope movement, Kinmiraisha, 204p. (in Japanese).
10) Inoue, K. (2000) : Bird’s eye views and contour maps of
Mayuyama after the rockslide avalanche, Nakamura, H. ,Tsuchiya, S. ,Inoue, K, and Ishikawa, Y. eds. Earthquake Sabo,
Kokonsyoin, 14drawings (in Japanese).
11) Oyagi, N. and Ikeda, H. (1998) : Landslide structure and regional perspective on Sumikawa landslide, Jour. of Japan Landslide Society, Vol. 35, No. 2, pp.1-10 (in Japanese with English
Abstract and figure captions).

4. LANDSLIDE INVESTIGATIONS AND PREDICTION
The flow chart shown in Fig. 4-1 describes the general investigation procedures in an attempt to understand the mechanism oforigination of disasters associated with slope movement and to predict the resulting deformation. Investigation items and investigation
methods are shown on Table 4-1.

4.1 PRELIMINARY INVESTIGATION
4.1.1 COLLECTION OF EXISTING DATA, DATA REVIEW
Landslides often occur at specific locations under certain topographic and geologic conditions. Therefore it is important to utilize
existing data (history of the problem, records of restoration work,and data review) in order to understand the topography, geology,
and properties of similar landslides. It is also important to understand their relationship with meteorologic factors, period of activity,
existence of any warning signs, groundwater conditions, chronologic topographic change or erosion by rivers, earthquakes, andother factors which may relate with the slope deformation surrounding the investigation site area prior to performing a detailed investigation.
4.1.2 TOPOGRAPHIC INVESTIGATION
It is necessary to identify any changes in the site topography.
That can be accomplished by recognizing ; (1) the overall topographic feature of the site ; (2) understanding the topographic characteristics of the site slopes ; and (3) estimating the regional geologic structure of the site. Such methods include comparing aerial
photographs of the site and vicinity taken before and after the sliding, and interpreting the topographic maps and aerial photographs.
In Japan, aerial photographs are taken every few years over theentire country at a scale between 1/10,000 to 1/40,000. These photographs are used to understand the chronologic and topographic
changes over the country. By utilizing aerial photographs, it is possible to interpret landslide phenomena and warning signs, geology
and geologic structure, topography and distribution of vegetation
type. For landslide investigations, it is useful to identify and interpret the distribution and continuity of knick lines, gentle slopes,
gullies and cracks in the photos to aid in preparing a photo interpretation map. The map can then be utilized during the field investigation.
The recent popularity of remote sensing using satellite imageries
has been particularly useful for analysis utilizing the Thermal Infrared Spectrum which is possible to estimate the distribution of slideareas and groundwater, and live vegetation. Remote sensing can beused for analysis of topographic characteristics and topographic
changes in terrain susceptible to landsliding.
4.1.3 FIELD INVESTIGATIONWith an approximate understanding of the overall topographic
feature and knowledge of the distinction(s) of movement and aerial
extent of the sliding block(s) (viewed from the opposite side), a detailed field investigation plan can be developed to delineate the aerial extent and a general direction of movement of the landslide zone,
assess the geology and geologic structure, estimate the cause(s) of
the sliding, and predict future movement. The field investigation
should not include just the actual landslide area, but should also extend to the surrounding areas where possible future sliding also exists. Additionally, the field investigation should include areas where
aerial photographic interpretation is difficult or unclear, and in areasthat could aid in the understanding of particular topographic fea-tures and characteristics.

4.2 DRAFTING A DETAILED INVESTIGATION PLAN
In order to properly examine the items listed below, a detailed
investigation which will satisfy the study objectives should beplanned by selecting appropriate investigation methods and instruments described in Table 4-1.
1. aerial extent of the slide, differentiation of moving blocks andidentification of the direction of movement
2. location and configuration of slip surface(s)3. nature of landslide block(s)4. possibility of further or future movement on slopes beyond
the existing slide
5. possibility of continual, future or accelerated sliding
6. distribution of groundwater
Survey control lines can be established on each moving blockon the ground where the slide mass is expected to be thickest andwhere the slope stability analysis and plan for mitigation measures
will be emphasized. As a general rule, the main survey control line
should be placed where the width of the slide exceeds 100 m withsubsidiary survey control lines established at approximately 50 m
intervals.
Exploratory borings should be drilled on the order of every 30
to 40 m. At least three borings should be drilled along the main
survey control line with one boring drilled at least 5 to 10 m below
the slip surface. During the early stage of the investigation, it is particularly important to have an accurate estimate of the configurationand location of the slip surface(s) so an adequate boring depth canbe achieved.

4.3 DETAILED INVESTIGATION
4.3.1 INVESTIGATION OF SURFACE DEFORMATION
The investigation of surface deformation is conducted to define
the boundaries of the landslide, size, level of activity and direction(s)of the movement, and to determine individual moving blocks of themain slide. The presence of scarps and transverse cracks are usefulfor determining whether the potential for future activity exists.
Examples of instrumentation used for the surface deformation
investigation include extensometers, ground tiltmeters, movement
determination by survey methods including transverse survey, grid
survey, laser survey from the opposite bank, movement determination by aerial photographs, and G.P.S. (Fig. 4-2).
(1) Simple Method to Measure Movements
One of the simplest method to determine landslide movement is
to drive stakes across a tension crack along the direction of slide
movement (Fig. 4-3). Then attach horizontal board to the stakes,
and saw through the board. Any movement across the tension crack
can be determined by measuring the space between the sawed portion of the board.
(2) Extensometer
The extensometer is used to measure relative movement bycomparing the extension of two points. The extensometers are generally installed across the main scarp, at transverse cracks and transverse ridges near the toe or front portion of the slide and parallel tothe suspected slide direction (Fig. 4-4). Measurements should be accurate to within 0.2 mm, and the magnitude of the movement anddaily rainfall data should be recorded to establish the relationshipbetween the measurable movement and the precipitation

19

Preliminary investigation
Collection of existing
data, data review

Topographic
investigation

Field
investigation

Drafting a detailed
investigation plan

Investigation of
surface deformation

Investigation ofgeologic structure

Geotechnicalinvestigation

Investigation ofgroundwater

Investigation ofslip surface

Analysis oflandslide mechanism

Analysis model
Stability analysis(critical equilibrium analysis)

Slopeassessment,
Examinationof safety
factorNO

OK
Design of
mitigation works

Analysis model
Detailed safety analysis(Numerical analysis,
FEM, DEM, others)

Construction

SurveyInvestigation offacility's effects

Monitoring of
surface deformation

Monitoring ofgroundwatar

Yes

Monitoring ofdischarge

ProblemMonitoring ofinternal stress

Yes

No

Completion

Fig. 4-1 Flow chart for landslide investigation and analysis

20

○

◎

○

○

○

slope height, location ofsafety objective△

formational process of slope○

landslide topography (areal extact, moving blocks, direction of movement)

◎

◎

◎

knick line (yes or no)

◎

○

○

micro topography of slope△

rock type and rock quality△△

Geology andgeologic structure

◎

◎

◎

○

△

△

△

○

◎

◎

△

◎

◎

◎

◎

○

○

◎

◎

○

○

◎

◎

○

◎

◎

◎
○

density of rock formation

distribution of faults andfracture zones
distribution and depths ofslip surface

○

△

△

inclination of slip surface

quality of slide mass, composition of slip surface materials
mechanical coefficient ofslip surface
deformation conditions ofslip surface

deformation of structures△◎◎continuity◎○

filling◎◎△△

Estimation of pore-water pressure

existence and conditions ofscarpssecondary failures and rockfalls
conditions of toe area andupliftexistence of extremely relax
beds and rocks

composition ofrocks and hardness◎

Deformation condition of groundsurface
Conditions of groundwater andsprings

history of past disasters

Notes

△

degree of weathering andlevel of relaxation

Existing conditions
and warning signs

Groundwater
tests
Investigation ofslip surface
Investigation ofland deformation

Topographyslope inclination◎

Soil tests○

slope form

Geophysicalexploration

Investigation items

Test adit

Invenstigation methods

Interpretation ofaeral photographs
Literature
research
Topographic
surveyTopographic
investigation
Surficial geologic
investigation
Subsurface exploration by borings

Table 4-1 Investigation items and investigation methods

○

◎

○

○

△

◎

△

○

◎

△

○

○

◎
△
◎

◎ ： very effective investigation method○ ： effective investigation method△ ： depending on the situation, may be effective method

21

700

15

70
60

500

50

400

40

groundwater level BV-1-3

300

30

200

20

100

10
0

0
25

26

27

28

29

June

30

1

2
July

16
Precipitation (mm)extensometer(AS-1)

600

17
18
19
20
21
22
23

Groundwater level (GL.-m)

Minor scarp

Displacement by extensometer (cm)Main survey control line

Main scarp

24
25

3

1991

Fig. 4-5 Example of measurement by extensometer

0

10

20m

Explanation
Groundwater measurement
Borings Subsurface strain measurement
Inclinometer
Geophysical logging
Extensometer
Simple deformation detection method (Boards)
Tiltmeter
Transverse survey stakes
May be possible to eliminate under certain condition

Fig. 4-2 Example of instrumentation

Saw cut

Fig. 4-6 Orbit of G.P.S. satellites

Crack
Stable groundLandslide moving bodyFig. 4-3 Simple deformation detection method (Board)

Extensometer
Wooden or P.V.C. pipe

Stable ground
Crack

rate (Fig. 4-4 and 5).
(3) Laser Survey
A control point is established along the opposite bank on stableground, and survey stakes are positioned within the slide. It is mosteffective where the movement is large. Recently, non-prism opticaldistance meter has been developed which does not require a specifictarget, and is used for monitoring on very steep slopes.
(4) G.P.S.
G.P.S. (Global Positioning System) is the state of the art technology that uses signals from satellites to determine the threedimensional positioning of the slide. G.P.S. has been used in recent
landslide investigations where a high degree of success has been reported (Fig. 4-6).
(5) Other Methods
The ground tiltmeter is useful for determining the deformation
at the head and toe portions and sometimes along the flanks of thelandslide, or to assess the possibility of future deformation. A level
type tiltmeter is most conventional.
Recently, slope deformation detection systems using fiber optics
has been tried. This system adopts the property of reduction in the
optic medium within the fiber optics as it bends. It is possible to record the amount of deformation as well as the location of the deformation.

Invar wire

Landslide moving body
Stake

Fig. 4-4 Simplified diagram of extensometer installation

22

4.3.2 INVESTIGATION OF GEOLOGIC STRUCTURE
In most cases, the investigation of geologic structure relies on
exploratory borings ; however, in cases where the bedrock distribution is ambiguous or a better understanding of the regional geologic
structure is needed, then a geophysical exploration (seismic survey,
electrical survey and radioactive survey) is combined with the boring data.

1) Borings
The majority of the borings drilled are larger than 66 mm in diameter. Core samples are recovered from the borings and are stored
in core boxes. Boring logs should be prepared along with photographs of the core samples (Photo 4-1). The boring logs shall include such information as : geologic and soil description ; color ;hardness ; lithologic description ; degree of weathering ; alterations and fractures ; strike and dip of bedding and joints ; boring
conditions ; initial and stabilized groundwater levels ; and rate ofcore recovery.
Geologic assessments based on the boring data obtained fromthe drilling site should include a discussion regarding the differentiation of moving earth blocks, semi-moving earth, and stableground. Clays within the slip surface generally have a high moisture
content, are highly sticky and plastic and are often associated withabrasion scars and slickensides. During drilling, squeezed earth
could occur near slip surface.
Furthermore, using the data from the borings the following information must be assessed or determined.
1. Evaluation of slip surface
2. Groundwater level measurements
3. Groundwater logging

4. Groundwater tracer tests
5. Standard penetration tests, Horizontal loading tests, in-situ
tests such as in-situ permeability tests
6. Sampling for soil tests
7. Various geophysical logging.
2) Geophysical Surveys
Geophysical surveys (seismic survey, electric survey and radioactive survey) are conducted to understand the approximate geologic conditions of the slide itself and the surrounding area. P-wave
refraction surveys are the most common seismic survey. Other
methods, such as S-wave and P-wave shallow refraction, are seldom
used. Electric survey is the specific resistance method and is applied
to determine the distribution of aquifer(s) and to understand thegeologic structure. These surveys include the development of thegeotomography method.
4.3.3 EVALUATION OF SLIP SURFACE
Determining the slip surface for actively moving landslides utilize the fact that the rates of movement differ significantly along theslip surface. Depending on the requirements for surveying accuracy
and magnitude of movement, the appropriate instrumentation shallbe selected from the following representative instruments : 1. Pipestrain gauge ; 2. Inclinometer ; and 3. Multi-layer movement meter.
(1) Pipe Strain Gauge
P.V.C. pipes with strain gauges are inserted into the boreholes,
and the movement is estimated by the change in the strain as the P.
V.C. pipe bends (Fig. 4-7, 8). The accuracy of the strain gauge increases as the intervals of the gauge narrows, however, it is acceptable to widen the space as much as 1 m for investigations involvingvery thick slide materials and when it is difficult to handle the survey extension wires. Two of the lowest strain gauges must be anchored into the bedrock below the slip surface so that data fromwithin the intact formation can be obtained.
(2) Inclinometer
A grooved casing is inserted into the borehole extending into
the bedrock formation, and an adequate quality of grout should beplaced into the borehole to assure an intimate contact with the borehole. By lowering a probe equipped with a tilt sensor, deformationin the casing can be detected and movement of a landslide can be
determined (Fig. 4-9). An accurate measurement is possible where
the deformation of a landslide is relatively small. As a landslide
movement increases, the borehole and casing will bend making insertion of the probe difficult or the casing will exceed the tilt detection limit of the instrument.

Lead wires

Waterproof film
Strain gauge

PVC pipe
Direction markPhoto 4-1 Photos of core samples

Fig. 4-7 Pipe strain gauge

236Strain(10- )0 20 40 60

Gravelly soil

BV-3
Boring log
0
Surficial soil
3
6
9
12
15
18
21
Slip surface
24
27
30
33
36
39
Slate

(24m)

Stran(10- 4)

60

B3(24m)40
20

Base line

0
-20

Date

18 20 22 24 26 28 30 1Month

4 6 8 10 12 14 16 18 20 22 24 26 28 30 1

Jul.

Aug.

4
Sep.

Fig. 4-8 Example of measurement by pipe strain gauge

Straight type potentiometer

Counter weight

Stainless steel wire

Plumb line

500sinθ(mm)θ

Bed rock

Inclinometer

a

urf

ps

Sli

ce

Guidepipe

Fig. 4-9 Sensor of insertion type inclinometer(3) Multi-Layer Movement Meter
Several wires are anchored at various depths within a borehole
and the attached wires extend up to the ground surface. The magnitude of the displacement of each wire segment can be measured directly using a ruler or potentiometer (Fig. 4-10). It is possible to install 20 to 30 wires per borehole. This method is not suitable forlandslides with small displacement. This instrument is most effective where the slide movement is so large that some of the other instruments cannot be used. Applying the same principle, a verticalextensometer can be constructed by fixing a wire on the bedrock at
the bottom of the borehole.
(4) Pressure-Sensitive Cable
The pressure-sensitive cable consists of an inner wire and outershield wires that are separated by conductive flexible material and a

24

Fig. 4-10 Multi-layer movement meter
mesh-like insulator. When the cable is compressed and squeezed by
the movement at the slip surface, the inner wire and the outer shieldwires short-circuit. The electric pulses sent from the ground surface
through the cable is reflected at the short-circuited location. By
measuring the arrival time of the reflected pulse, the depth of theslip surface can be determined.
(5) Other Methods
Other methods to evaluate the slip surface include : slip surface
detection probe ; creep wells ; and sounding penetration test.
4.3.4 GROUNDWATER INVESTIGATION
Investigation of groundwater, which is a driving force of sliding,includes determining groundwater level, pore water pressure,groundwater logging, groundwater tracing test, pumping test, watercoefficient of permeability can be determined.
(5) Geothermal Investigation
This procedure utilizes ground temperature measurements
throughout the study area, including ground temperatures near the
groundwater veins. By measuring the temperature differences at non-groundwater areas and near groundwater veins, it is possible to isolate the groundwater veins where the temperature difference between the two is largest. By conducting the geothermal investigationin summer months or winter months where near surface groundtemperature is influenced by air temperature, good results have beenobtained for the isolation of relatively shallow groundwater.
(6) Other Methods
Following the injection of tracers such as soluble dye or inorganic chemicals (NaCl) into a borehole, water samples are collectedchronologically from other boreholes and water discharging points(springs, ponds, and creeks). From such data, groundwater flow direction(s) and permeability can be estimated. Groundwater tracer

quality analysis, electric survey, geothermal survey, and geophysicallogging (electric logging and radioactive logging). Based on the results of the above measurements and tests, groundwater control
works can be planned and designed.
(1) GroundWater Level Observation
As a general rule, groundwater levels should be measured in all
the boreholes. In some of the more important boreholes, continuous
rainfall data is kept by an automatic recorder to determine the correlation between the slide movement, rainfall and groundwater levels, and collects data on the groundwater distribution and movement
regime.
(2) Pore Water Pressure
Groundwater levels in boreholes will often reflect seepage fromhighly fractured formations or indicate water levels of a predominant aquifer. Therefore, for stability analysis it is best to measure
the pore water pressure along the slip surface. Sometimes it is difficult to accurately estimate the depth of the slip surface. In suchcases it is desirable to install piezometers in the beds with low
seepage or low shear strength. Standard piezometers used in landslide investigations must be durable, and be the open piezometerwater level type (Fig. 4-11).
(3) Groundwater Logging
Locations of groundwater flow and flow directions can be determined by measuring the increasing specific resistance of groundwater in flow over time. The measurements will be continued lowering specific resistance of groundwater by injecting a salt solution
into the borehole. There should be at least two borings for groundwater logging at the head portion of the landslide where abundant
ground water is expected. The measurement results should be recorded along with the boring logs, and the relationship between thelocation of groundwater flow and stratigraphic positions, and magnitude and variation of specific resistance of groundwater should be
discussed. Furthermore, the results of the analysis should be recorded along with the geologic cross sections in order to understandthe overall groundwater flow (Fig. 4-12).
(4) Drawdown Test
In order to estimate the yield and to calculate the coefficient of
permeability, water within a borehole is pumped to certain levels
after raising the boring casing every 2 to 3 m. A time-recovery
curve can then be plotted using Jacob’s and other formulas, and theDepth Boring log(m)SymbolName
0.0

Piezometer
installation

ter

dwa

oun

gr
Free

Siliceous sand
Bentonite

Piezometer

ater

ndw

rou
ed g

fin

Con

Water level gauge
PackerBentonite

BentonitemortarPackerPressure headFig. 4-11 Observation method of pore-water pressure

Electric specific resistance (Ωm)
0Surficial soil

100

50

Water level

150

200

8.20m

Recent lava

8.20

Measure water depth
NaCI concentration

20.15m
1.0%

)

10.0

15.0

Recent lava(

Ⅲ

Groundwater
flow

Open piezometerwater level type

Explanation
After 10 minutes

After 120 minutes

After 30 minutes

After 180 minutes

After 60 minutes

Groundwater
flow

20.0

Tuff

Groundwaterflow

Not measurable(due to sliding)Fig. 4-12 Example of groundwater logging

25

Peak strength of overconsolidated clay

tests and water quality tests are effective in examining the distribution of the groundwater regime and flow direction(s) where the subject landslide is very large and the groundwater system is expected
to be complicated.

Shear stressσ' constat

Rate of movement detected bylaser survey(mm/day)

4.3.5 GEOTECHNICAL INVESTIGATION (SOIL-ROCK MECHANIC TESTS)
In order to conduct slope stability analyses and to design appropriate control measures for landslides, physical properties such as
the strength of slip surface, location and depth of the slip surface
and stable ground areas must be determined. The following tests are
generally performed : physical tests, Standard Penetration Tests,
soil mechanic tests (unconfined compression, tri-axial compression,box shear, ring shear, and in-situ shear (along the slip surface)) (Fig.
4-13). In order to obtain the earth reaction coefficient for the designof the landslide restraint measures, there is a current tendency to

Movement detecter by laser survey(mm)

300

200

100

×
Peak strength of normal consolidated clay0

Displacement(a)Fig. 4-13 Shear properties of normal and over consolidated clay

10

400

Residual strength

Movement detected by laser survey

9
8
7
6
5
4
3

Rate of movement detected bylaser survey

2
1
0

0
400

120m

200

0

200

150

100

Daily rainfall working rainn (mm)Elevation of groundwater level in a borehole

200

600

so

ten

Ex

Tiltmeter
83m

C

400

Buttress fill

300

66m

15

B

10

ent

m
displace
Bed rock eterm5rLoad cell of ancho

70

50

D

No. of anchors driven

500

400
300
200
100

200

55m

C

A

60

40

250

800

Amount of discharge(I/mm)

Bedrock displacement
meter(mm/m)

100

Load cell of anchor( t )No. of anchors driven

400

Tiltmeter(second)

300

)
(m
ss
ne
ck
hi
lt
fil

600

es
ltr
Bu

Extensometer(mm)

D

r

te
me

Amount of discharge

Groundwater level

150

100

Working rain

50

Month
Year
Mitigation
works

1

2

3

Anchor works

4

6
7
1988
Drainage borings
5

8

9

Buttress fill

10

11

12

1

2

3

4

Anchor works

Fig. 4-14 Examples of landslide movements registered by various instruments

26

piles and drainage wells. Determination of the effectivenessof construction.
(1) Semi-Automatic Monitoring System
Semi-automatic systems manually collect data from the datalogger and data-center established at the site on a periodic basis.
Data can be retrieved directly from the hard drive of the computeror the computer disc can be replaced.
(2) Full-Automatic Monitoring System
Full-automatic monitoring systems permit remote control in realtime and rapid graphic data processing and display. It is possible tostore long term data accurately and effectively and would provide
early warning signs of slide activity, thereby reducing landslide hazards (Fig. 4-14). Furthermore, recent developments in the informationalized construction systems at construction sites using the realtime facilitates safety control during construction. In recent years,
landslide monitoring systems utilizing IT technology along with theexisting full-automatic monitoring system and GIS has been developed (Fig. 4-15, 16).

conduct more horizontal loading tests and plate loading tests to determine the modules of deformation. Furthermore, the intensity anddegree of alteration of the slip surface clays are evaluated by X-ray
diffraction methods. The results have also been applied to analyze
the origin of the slip surface.
4.3.6 AUTOMATED MONITORING SYSTEM
In the past, measurements of slope deformation have been performed manually. More recently, automatic monitoring systems using data loggers and computers are being used. The instrument setup in the field has been designed for easy installation, and is weatherproof, durable, maintenance-friendly and economical.
There are three main advantages in using the automated survey
system :1. Surveillance of the conditions of landslide : Issuance andcancellation of landslide watches and warning announcements
based on the velocity of the movement, piezometric pressuresand variations in the rainfall amounts. Prediction and forecasting of the landslide failure.
2. Understanding of the conditions of the landslide deformation :Chronological measurements of movement velocity. Determination of the slip surface depth. Determination of the relationship between the slope deformation and factors of slide
occurrence (pore water pressure against the slip surface, critical pore water pressure related to the time of sliding, rainfall
and snowmelt).
3. Effectiveness in determining landslide mitigation measures :Measurement of the amount of earth movement and porewater pressure. Measurement of the earth pressure affected by

4.4 PREDICTION OF LANDSLIDE
4.4.1 LANDSLIDE DISTRIBUTION MAP
Most of the new landslides are reactivated old failures in landslide terrain, and unless there are special causes, it is extremely rarethat non-landslide terrain fails. Those topographic characteristics canbe interpreted from aerial photographs and topographic maps, andcan be verified through field reconnaissance.
Furthermore, bedrock landslides and weathered bedrock landslides with past movement at the time of sliding is small, andSurface extensometers

CircuitRain gauge
Piezometers
Multi-layer
movement meter

Local offices

Groundacceleration
sensor

sur

fac

e

Bedrock tiltmeter

Data
display
controlsystemControl center

PPP
Connection
RouterDataloggerData base system

Printer

HUB

Fi rewall

Modem tooutsideinternet
and web
serverOutside provider

RouterMail transsition

slip

I NTERNET

Urgent messages toaffiliated organization(telephone,fax,e-mail)Prefectural offices
PPP
Connection

Static monitoringdata recording andtransmission system

RouterData display
control system
Local offices

Ground acceleration
data recording andtransmission system

PPP
Connection

RouterData display
control system

Research institutes
Network camera
CCD camera server

HUB

PPP
Router Connection

Data display
control system

PPP
Connection

RouterData display Work stations orcontrol system internal network

Regional network

Fig. 4-15 Automated monitoring system using IT Technology

27

Fig. 4-16 Automated monitoring system combining with IT Technology and GIS System

Fig. 4-17 Landslide map database available
on website(http: //lsweb1.ess.bosai.go.jp)
sheared bedrock and topographic features related to the early stagesof sliding that were subjected to creep deformation in the deeper
portions often do not exhibit clear landslide topographic characteristics. Because of these reasons, double ridge topography associatedwith mountain deformation, parting ridges, breaks-in-slope, knicklines, distribution of old and deep scarps, bulging at the tip of ridge
lines, discrepancy in the geologic distribution following the investigation, geologic structure, degree of shearing, degree of creep andother factors must be considered when evaluating landslide topography.
Landslide distribution maps with the above descriptions are generally limited to small areas, however, recent regional maps cover-28

ing the entire country of Japan have been published (Fig. 4-17).
4.4.2 LANDSLIDE PREDICTION
Now it is possible to predict the timing of a slope failure by interpreting the rate of deflection measured by extensometers placed
across selected tension cracks of a slope. Failure predictions rely on
extensometers placed across scarps, and affected areas are considered ”off-limits” when the rate of movement exceeds 2 to 4mm/
hour. Based on the change in the rate of movement, the following
methods are commonly used to predict the timing of landslide
failune.

(mm)
2

500

0
0

5

10

15

20

Time on July 26

8

4

tr=

18

:00

17

:30

2

0

:0

17

1
16
:30

00
16:

Allowance time till failure tr-t hr

6

1000

1

0
0

5

10

15

20

Time on July 26

Fig. 4-18 Landslide prediction using Graphic
solution of tertiary creep curve(Saito1))0.5600Minutes/mm

1000

1/speed

Failure time : 17:00

Cumulative movement

1500

1500

Amount of movement (mm)Fig. 4-19 Landslide prediction using
Semi-logarithm (Saito1))

Fig. 4-20 Landslide prediction using
inverse number of velocity(Fukuzono2))(1) Saito1) Method
This method focuses on the third-stage creep of the strain velocity during the final stage of landslide failure. Graphic solution (Fig.
4-18) and semi-log solution (Fig. 4-19) have been proposed, andutilizes the inverse relationship between the strain velocity and theresidual time of the failure. This method successfully predicted thefailure of the Takabayama landslide along the National Railroad in
Niigata prefecture. This was the first occurrence in the history of
the world that the specific time of a failure was ever predicted.
(2) Fukuzono2) Method
Based on large scale landslide simulation experiments with artificial rainfall, the surface displacement and the acceleration in failing masses show a straight line on a full logarithmic graph. Using
the inverse curve relationship of the velocity, the inverse prediction
method” was proposed (Fig. 4-20). This method can be intuitively
understood by not only the landslide specialists, but also the lay
persons, and could be applied to predict the time of slope failures.
REFERENCES :1) Saito, M. (1968) : Prediction of the time of slope
failure by
third-stage creep, Journal of Japan Landslide Society, Vol. 4,
No. 3, pp. 2-8 (in Japanese).
2) Fukuzono, T. (1985) : A method to predict the time of slope failure caused by rainfall using the inverse number of velocity ofsurface displacement, Journal of Japan Landslide Society, Vol. 22,
No. 2, pp. 8-13 (in Japanese with English Abstract and captions).

29

5. LANDSLIDE MITIGATION MEASURES
Landslide mitigation measures are conducted in order to stop or
reduce the landslide movement so that the resulting damages can be
minimized. Depending on the situations, the mitigation measures
can be divided into two types ; emergency measures and permanentmeasures. Further, the mitigation measures are classified into two
categories ; landslide control measures and landslide restraint measures. Although it is not a direct mitigation measure, a slope monitoring system has been adopted in order to reduce the landslide
damages. For an emergency measures, dewatering is accomplishedby employing surface drainage improvements, interceptor underdrains, and buttress fill.
The landslide control measures involve modifications of thenatural conditions of landslides such as topography, groundwater,and other conditions that indirectly control the landslide movement.
The restraint measures, on the other hand, rely on stopping thelandslide movement by directly adding a resisting force to the sliding movement.
Specific measures included in the landslide control measuresand landslide restraint measures are listed in Fig. 5-1.
Because of the abundance of rainfall, the landslide control
measures in Japan are concentrated on groundwater and surface
water control as they are the main causes of landslides. For naturally occurring landslides, groundwater is controlled by installingdrainage wells and drainage borings ; surface drainage improvement will be implemented for surface drainage control. In particular,
the utilization of control measures with emphasis on dewateringlandslides whose movement tends to accelerate during the intenserain or snowmelt period, and where abundant groundwater and rainfall are present. The control measures are often employed during the
emergency mitigation work. Landslide restraint measures should be
utilized on landslides that contain little groundwater or where landslide movement cannot be controlled using dewatering procedures
alone, or where construction related measures such as buttress fills
and dams are anticipated to be submerge under water. Anchors,piles and large diameter cast-in-place shafts should be utilized as apermanent solution for slope protection and preservation. Retaining
walls and slope restoration works can be incorporated with otherlandslide mitigation measures.

clude two major categories : drainage collection and drainage channel works. The drainage collection is designed to collect surface
flow by installing corrugated half pipes or lined V-ditches along theslopes, and then connecting them to the drainage channels. The
drainage channels are designed to remove the collected water fromthe landslide zone as quickly as possible, and are constructed fromthe same materials as the drainage collection works. The surface
drainage control measures are often combined with the subsurface
control measures (Fig. 5-2, 5-3, Photo 5-1).
5.1.2 GROUNDWATER CONTROL MEASURESThe purpose of the groundwater control measures is to remove
the groundwater within the landslide mass and to prevent the inflow
of groundwater into the landslide mass from outside sources. The
groundwater control measures include shallow and deep groundwater control measures.
(1) Interceptor Underdrains And Interceptor Trench Drains
These systems are most useful to remove shallow groundwaterfrom up to 3 m below the ground surface. The interceptor un-

Fig. 5-2 Arrangement of ditches and interceptor drains

5.1 LANDSLIDE CONTROL MEASURES
5.1.1 SURFACE DRAINAGE CONTROL MEASURESThe surface drainage control measures are implemented to control the movement of landslides accompanied by infiltration of rainwater and spring flows. The surface drainage control measures in-Landslide
ControlMeasuresLandslide
Restraint
MeasuresFig. 5-3 Example of a drainage ditch with interceptor drain

Surface Drainage Control
Groundwater Control
Shallow Groundwater Control(Interceptor Under Drains, Horizontal Gravity Drains, Interceptor Trench Drains)
Deep Groundwater Control(Horizontal Gravity Drains, Drainage Wells, Drainage Tunnels)
Earth Removal
Buttress Fill
River Structures
Piles
Large Diameter Cast-in-Place Shafts
Anchors
Retaining Walls
Fig. 5-1 Landslide mitigation measures

30

Photo 5-1 Surface drainage ditches

Photo 5-2 Interceptor trench drains

Photo 5-3 Horizontal gravity drains installation
derdrains contain impervious sheets at the bottom of the trench, andgravels are wrapped with filter fabric and the drains are connected
at ground sils or catch basins (Photo 5-2)
Structurally, the interceptor trench drain is a combination of theinterceptor underdrain and surface drainage control, and are commonly used (Fig. 5-3).
(2) Horizontal Gravity Drains
In order to remove groundwater, 30 to 50 m-long horizontal
gravity drains are installed. The pipes could be either perforated P.V.
C. (polyvinyl chloride) or steel construction, and are typically
drilled at an angle of 5 to 10°from the horizontal line (Photo 5-3,
5-4).
(3) Drainage Wells
Drainage wells with a diameter of 3.5 to 4.0 m are excavatedwithin areas of concentrated groundwater. A series of radiallypositioned horizontal gravity drains are drilled at various elevations
and collect the groundwater into the drainage wells. They are con-structed of either steel or reinforced concrete segments, and concrete is used at the well bottoms and the upper portion of the well(Fig. 5-4, Photo 5-5, 5-6).
(4) Drainage Tunnels
The primary purpose of the drainage tunnels (which are constructed below the slip surface within a stable bedrock formation) is
to remove collected water from within the landslide mass by interconnecting the drainage wells. The series of gravity drains drilledfrom the tunnel tends to increase the effectiveness of the drain system. This is the most effective and reliable drainage measure where
numerous groundwater veins exist within the landslide mass. Furthermore, this work is most effective in maintaining the existing facilities.
Generally, the diameter of the tunnel is between 1.8 and 2.5 m,
and the drainage channel is constructed along the invert (Fig. 5-5,
Photo 5-7).

31

Photo 5-4 Outlet of horizontal gravity drain pipes
Horizontal
gravity drains

Drainage well
Drainage boring

Horizontal
gravity drains

Photo 5-5 Drainage well using steel segments
Drainage boring
Drainage well

LandslideRiver

Fig. 5-4 Drainage well, horizontal gravity drains, drainage
boring

Photo 5-6 Drainage well using reinforced concrete segments

32

5.1.3 EARTH REMOVALThis is one of the methods where the most reliable results canbe expected, and generally applies to small to medium sized landslides. Except for special cases, the earth removal is focused on thehead portion of the slide (Photo 5-8).
5.1.4 BUTTRESS FILL
The buttress fill is placed at the lower portions of the landslidein order to counterweigh the landslide mass. This method is often

employed as an emergency measure. It is most effective if the soils
generated by the soil removal works are used (Photo 5-9).
5.1.5 RIVER STRUCTURES
Degradation and channel bank erosion reduces earth stability
and often tends to induce slide activity. In such cases, sabo dams,
ground sils, revetments and other river structures can be constructedto prevent further erosion (Photo 5-10).

Photo 5-7 Inside of a drainage tunnel

Fig. 5-5 Cross section of a drainage tunnel

Photo 5-9 Buttress fill

Photo 5-8 Earth removal

Photo 5-10 Sabo dam (check dam)33

5.2 LANDSLIDE RESTRAINT MEASURES
5.2.1 PILES
Piles are driven into the pre-drilled shafts in order to tie themoving landslide blocks and the stable ground together. The piles
are designed to resist shearing and bending stress, and generally
consist of thick walled steel construction. The interior of the piles
are filled with concrete (Photo 5-11).
5.2.2 LARGE DIAMETER CAST-IN-PLACE SHAFTS
The large diameter, cast-in-place shafts function similar to thoseof the piles and are also designed to tie the moving landslide andthe stable ground together. However, this measure involves muchlarger diameters. Construction is similar to that of drainage wells,
and generally consists of shafts with diameters of 1.5 to 6.5 m, andare then filled with reinforced concrete. Compared to the piles, thelarge diameter cast-in-place type are much more resistant to bending
stresses (Photo 5-12).

through the slide mass and into stable bedrock, and are connectedto thrust blocks located on the ground surface. The thrust blocks areanchored with a tendon that counteracts the driving forces of thelandslide to restrain the slide movement. The advantage is that large
restraint forces can be obtained from a relatively small cross sectional tendon (Photo 5-13).
5.2.4 RETAINING WALLS
Retaining walls are constructed to prevent smaller sized and secondary landslides that often occur along the toe portion of thelarger landslides. Because of the large-scale earth movement andnumerous springs that are expected in landslide terrain, more flexible crib walls are common instead of conventional reinforced concrete retaining walls (Photo 5-14).

5.2.3 ANCHORS
The anchors utilize the tensile force of anchor bodies embedded

Photo 5-12 Installation of large diameter cast-in-place shafts

Photo 5-11 Pile driving

Photo 5-13 Completed anchor installation

34

Photo 5-14 Crib walls

6．RECENT REPRESENTATIVE LANDSLIDES AND SLOPEFAILURES IN JAPANThis chapter introduces 27 representative landslide and slope failure cases that occurred recently in Japan. Most of the cases are landslideor jisuberi , and slide as defined by the committee on glossary of geomorphology and geology, the Japan Landslide Society. The latitudes
and longitudes described in this chapter are Japanese designations, and not the international designation.

6.1 JOZANKEI USUBETSU-GAWA LANDSLIDE(1) Location : Jozankei, Minami-Ku, Sapporo-shi, Hokkaido ;Ishikari office of Okujozankei National Forest (42°54’ N, 141°07’
E)(2) Size : Length : 700±m ; Width : 100±m (Max width 270m) ; Area : 7ha ; Thickness : 10 - 30m ; Volume : 2.0 x 105 m3.
This landslide is divided into three blocks : the Upper ; Middle ; and Lower Blocks. The width of the Upper Block is about
250m wide, and the Middle Block is about 100m wide. The Lower
Block is a fan-shaped depositional zone.
(3) History and Damages : This landslide occurred on May 15,
2000. The Middle Block of the landslide, consisting of about 2.0 x
105 m3, became a mudflow and the sediments dammed the Usubetsu-Gawa River. The flow continued for about 300m downstream.
(4) Geology, Mode of Movement : The geology of this landslide includes Miocene tuff breccia and tuff which are strongly altered by hydrothermal alteration and silicification. The ridge area isunderlain by Pliocene andesite and pyroclastic rocks that are loose.

There was an abundant snow accumulation in the year 2000 ; furthermore, prolonged and heavy rain during the snowmelt periodcaused excess surface water infiltration into the hydrothermally altered, weak, clay rich tuff breccia and tuff. This resulted in the failure of the lower portion of the Upper Block and the movement continued to the lower portion of the Middle Block. The continuous
heavy rain critically increased the water content of the landslide
mass and triggered the mud flow.
(5) Mitigation Measures : Mitigation measures were performed
as a direct project by the Forestry Agency. Since the triggering
mechanism was the increased groundwater level in the Upper andMiddle Blocks, and some of the landslide mass remained. The mitigation measures included groundwater control and pile installation.
For the Lower Block, which is a depositional zone of the sliding
mass, soil retaining structures and piles were installed. Sediments
deposited by the mud flow were removed, and river structures suchas sabo dams and revetments were constructed along the Usubetsu
River.

Lower Block

Middle Block

Upper Block

Original ground surface

Colluvium
Usubetsu-Gawa
Tuff breccia, Tuff
River
Tuff breccia
Mudstone, Tuffaceous mudstone
Slip surface

100m
50
100m 50

0Fig. 6-1 Cross section of Jozankei Usubetsu-gawa landslide

Photo 6-1 Jozankei Usubetsu-gawa landslide (courtesy of F. Agency)35

failed faces and debris, it is suspected that the initial fall took place
after separation and dropped almost vertically, making striation
marks on the parting faces and then landing at the gently slopingbase of the cliff. The block then toppled and slid further down theslope. It is further suspected that multiple factors were involved
over a long period in the formation and deterioration of the parting
faces, including : (1) weathering of the bedrock by groundwater ;(2) its weight (gravity) ; (3) freezing and thawing ; and (4) earthquakes. Heavy rainfall immediately proceeding the failure could
have affected the cause of the fall.
(5) Mitigation Measures : As part of the investigation for the recovery works of the damaged highway, a slope stability analysis
was performed on the rock face, including the failed face, for a distance of about 1.7km. The results of the investigation revealed that
there are numerous locations, including the subject site, that show ahigh possibility of further rock falls. In order to avoid future failures, a new tunnel has been constructed. The construction of the
1806m tunnel began in November 1997 and was completed on
April 8, 1999.

6.2 ROCK FALL AT DAINI SHIRAITO TUNNEL(1) Location : Shimamaki-Mura, Shimamaki-Gun, Hokkaido ; National Highway, Daini Shiraito Tunnel south portal (42°39’ N,
139°52’ E)(2) Size : Initial Fall : Max. Height : 130±m ; Max. Width : 70
±m ; Max. Thickness : 20±m ; Volume : 4.2 x 104m3 ;Second Fall : Max. Height : 80±m ; Max. Width : 30±m ; Max.
Thickness : 20±m ; Volume : 1.4 x 104m3.
(3) History and Damages : On August 25, 1997 at about 14 : 30,
the wall at the south portal of the Daini Shiraito Tunnel locatedalong the National Highway failed. The fall damaged about 114m
of spread-out and 12m of retaining wall, and covered a section of
the highway about 120m wide. The debris reached the ocean which
is located about 80m down slope. No one was injured and no otherdamages occurred other than that to the highway structures. The
second fall took place three days after the initial failure.
(4) Geology, Mode of Movement : A 200 to 300m high cliff
covers the site area which is composed of Pliocene inter-bedded tuff
and water-cooled cataclastic rocks. Based on observations of theConglomerate, Tuff breccia
Andesitic water-cooled cataclastic rocks
Pumice tuff

50m
Ground surface following the initial fall
25
Original ground surface
50m

25

0

Fig. 6-2 Cross section of rock fall at Daini Shiraito Tunnel

Photo 6-2 Initial fall of rock fall at Daini Shiraito tunnel(courtesy of Ministry of L. I. and T)

36

Photo 6-3 Second fall of rock fall at Daini Shiraito tunnel(courtesy of Ministry of L. I. and T)

6.3 OIRASE LANDSLIDE(1) Location :Shiribe-Yama National Forest, Towadako-Cho,
Kamikita-Gun, Aomori (40°31’ N, 140°
58’ E)(2) Size : Length : 160m ; Width : 150m ; Area : 2.4ha ; Thickness : 20m ; Volume : 2.0 x 105m3(3) History and Damages : The landslide occurred on March
10, 1999. Earth materials mixed with large blocks of rock formationof about 2.0 x 105m3 slid down slope and closed the National Highway. The debris also buried and closed the scenic Oirase River.
(4) Geology, Mode of movement : The geology of the sitearea consists of, from youngest to oldest : volcanic ash and pumice
falls of the Towada Volcano ; tuff and welded tuff ; and Lower
Quaternary, moderately hard siltstone, sandstone, and conglomerate.
The slip surface is located at the contact between sandy siltstone
and conglomerate (some within the mudstone which immediately
overlies the siltstone). The welded tuff, although closed, shows welldeveloped vertical columnar joints facilitating rain water andgroundwater infiltration, and is suspected to be the cause of theslide. Further, an anticline axis near the main scarp contained a se-ries of parallel joints that developed due to the tensile stress, providing conditions for easy groundwater infiltration. The lower sedimentary formation is comprised of siltstone with well developed
lamina plane, and acts as an aquiclude. Due to the presence of anticline, the site area is in a dip-slope condition, which is a structural
factor that also contributed to the sliding. Furthermore, it was determined that the immediate cause was accelerated snowmelt due toincreasing temperatures in early March of that year resulting ingroundwater infiltration into the bedding planes.
(5) Mitigation Measures : The mitigation measures include threelevels of anchors, three drainage wells, earth embankments utilizing
on-site materials to prevent future failures from the near verticalmain scarp, and various surface drainage improvements.
Since the landslide is located within the Towada Hachimantai
National Park, all of the anchor blocks have been buried for aesthetic reasons. Circulation mad from borings had been treated prior
to discharge into the scenic Oirase River. Following field studies regarding the effect of noise caused by the construction works tobirds and other wild life, construction was not performed during
breeding periods.
Anticlinewn

Cro
Original ground surface
Colluvium-landslide deposits
Earth removal
Welded tuff
Pumice,Volcanic ash,Tuff
Anchors
Inter-bedded siltstone and mudstone
Pyroclastic flow deposit
Siltstone
Slip surface
Oirase River

Earth embankments
Drainage wells

50m

25
Horizontal gravity drains

50m

25

0

Fig. 6-3 Cross section of Oirase landslide

Photo 6-4 Oirase landslide (courtesy of F. Agency)

37

6.4 YAMADATE LANDSLIDE(1) Location : Yamadate, Tawara, Esashi-Shi, Iwate(39°11’ N, 141°14’ E)(2) Size : Length : 235m ; Width : 100m ; Area : 2.35ha ;Thickness : 15m ; Volume : 3.5 x 105m3
This landslide is divided into two blocks : Upper Block andLower Block. It appears that each block influences the other.
(3) History and Damages : There are no historical records forthis landslide. The first landslide occurred suddenly on September
13, 1998, the second slide occurred on September 28, 1998, and thethird landslide occurred on January 7, 1999. The first landslide buried and closed the main Prefectural Highway for a distance of 120
m and buried about two-third of the Ide River. The second landslide
involved the head-ward enlargement of the first for 25m. The thirdlandslide continued to enlarge head-ward for an additional 60m andcompletely blocked the Ide River. In all of the other landslides, themovement finished within several minutes.
(4) Geology, Mode of Movement : The site geology includes
Upper Tertiary volcanic rocks consisting of tuff breccia dipping 15
to 20°toward the Ide River forming a dip-slope condition. The tuff
breccia includes small cobbles of several centimeters to large boulders of several meters in diameter with thin tuff beds void of large
particles. Advanced weathering in the tuff beds is evident, and tuff
often turns into clay. The slip surface was found in one of the tuff
beds.
It is suspected that the triggering mechanism of the first landslide is attributed to : (1) heavy rainfall between August 27 to 31
with total precipitation of 250mm ; and (2) Magnitude 6.1, Iwate
Mountain Southwest Earthquake of September 3, 1998. It is also
suspected that the third landslide occurred due to rapid snowmelt by
off season heavy rain. In all cases, it is evident that the triggering
mechanism was an unusual and rapid groundwater increase along
the geological zones of weakness.
(5) Mitigation Measures : The mitigation measures were implemented as highway damages recovery works and erosion and sediment control works, including : (1) earth removal at the head area ;(2) dewatering by drainage wells ; (3) piles ; and (4) surface waterdrainage improvement.
The emergency recovery works conducted to maintain the flowin the river immediately following the third landslide were performed by unmanned equipment operated remotely and under close
watch aided by monitoring of extensometers and pipe strain gauges.

Photo 6-5 Yamadate landslide area prior to sliding(courtesy of Iwate Prefecture)

Photo 6-6 Yamadate landslide following the first landslide(Sep. 13, 1998) (courtesy of Iwate Prefecture)

Photo 6-7 Yamadate landslide following the second landslide(Jan. 7, 1999) (courtesy of Iwate Prefecture)
Landslide mass
Tuff
Tuff breccia
Slip surface

Earth removal

Groundwater table
Original ground surface
Drainage wells
Ground surface
following the completion
mitigation measures

Crib wall
Piles

100m

50
Upper Block

Lower Block

Ide River

100m

Fig. 6-4 Cross section of Yamadate landslide

38

50

0

6.5 SUMIKAWA LANDSLIDE(1) Location : Kumazawa National Forest, Hachimantai, Kazunoshi, Akita (39°59’ N, 140°48’ E)(2) Size : Length : 800m ; Width : 400m ; Area : 19ha ; Thickness : 60m ; Volume : 6.0 x 106m3
Based on the landslide morphology and distribution of major
scarps, the landslide is divided into at least three blocks.
(3) History and Damages : Signs of landslide movement such as
cloudy drinking water supplies and cracks on the road, appeared
during the snowmelt period, around May 4, 1997. Following rainfall
in excess of 100mm between May 7 and 9, major landslide movement occurred on May 10 around the 2 : 30 hour, destroying a bathhouse at a Sumikawa hot spring resort which is located within theslide blocks. On the following day, May 11, around 8 : 00 further
movement completely destroyed nine residence halls. By then thelandslide debris had turned into a debris flow and completely destroyed seven structures at another hot spring resort (Akagawa hot
spring resort) located 1.2km down stream. The debris flow continued and buried a bridge of the National Highway. Fortunately, the51 guests and employees of the resort were warned in advance andevacuated prior to the arrival of the debris flow.
(4) Geology, Mode of Movement : The geology of the sitearea consists of, from the youngest to the oldest : Quaternary andesitic lava ; Lower Quaternary altered clayey tuff ; Lower

Quaternary-Upper Tertiary altered and hard white tuff. The slip surface developed within the altered clayey tuff, and boring samples atthe slip surface and the exposed slip surface at the toe area exhibited slickensides. The slip-surface clay at the upper portion of thelandslide exhibits a greenish color, while the lower portion exhibits
a whitish color. Geothermal exploration near the Sumikawa Hot
Spring indicates the existence of montmorillonite and chlorite atshallow depths. Therefore, it is suspected that the slip surface materials may contain the minerals above.
(5) Mitigation Measures : Since the landslide occurred within
the National Forest, the Tohoku Regional Forestry Office was responsible for the majority of the mitigation works. Emergency mitigation measures were executed in a timely manner immediately following the landslide, and included construction of temporary drainage improvements to divert the flow, temporary surface drainage
works and installation of drainage borings. As the site is situatedwithin a geothermal region and the potential for toxic gases typically associated with volcanic activity exists, construction of drainage wells and drainage tunnels, which requires prolonged activities
underground, were determined to be unfeasible for safety reasons.
Thus, permanent measures for this landslide have emphasized theinstallation of piles, earth removal and placement of buttress fill.
Furthermore, in order to prevent further discharge of accumulated
landslide debris, construction of check dams and channel improvements were performed for the rivers affected by the debris flow.

100m

Sumikawa River

50

100m

50

0

Colluvium
Andesitic lava flows
Tuff
Tuff breccia
Slip surface
SpringFig. 6-5 Cross section of Sumikawa landlside

Photo 6-8 Sumikawa landslide (courtesy of F. Agency)

Photo 6-9 Deposition of debris and sediments by debris
flow caused by Sumikawa landslide (courtesyof F. Agency)

39

6.6 HIRANE LANDSLIDE(1) Location : Hirane, Tsunokawa, Tozawa-Mura, Mogami-Gun,
Yamagata (38°39’ N, 140°10’ E)(2) Size : Length : 200 - 1000m ; width : 350 - 800m ; Area :301.9ha ; Thickness : 15 - 45m ; Volume : 1.55 x 107m3
This landslide is divided into four blocks : the North ; the Central ; the Southeast ; and the South Blocks.
(3) History and Damages : Since the initial movement in 1899,
movements repeated over a dozen times, and the movement was almost always during the snowmelt period. Most of the damages occurred on farm land and roadways ; however, landslides that occurred within the North Block in 1944 and 1974 involved damages
to some residential structures. The head portion of the Southeast
Block clearly indicates evidence of old scarps and displays hummocky topography.
(4) Geology, Mode of Movement : The site geology consists of
Pliocene sandstone, mudstone and conglomerate which is unconformably overlain by Quaternary pyroclastic flow deposits of about

10,000 years old. This region receives heavy snowfall, and the average maximum snow accumulation between 1982 and 1998 was 338
cm. It is suspected that the triggering mechanism of the movement
was increased pore water pressure along the slip surface within theweathered Pliocene sedimentary rocks as snowmelt water percolated
through the unwelded pyroclastic flow deposits in April and May.
(5) Mitigation Measures : This landslide was designated as a
Landslide Threatened Area in 1962, and mitigation works were administered by the Yamagata Prefectural Government. Since 1972,
the National Government took over as a direct project. The typicalmitigation measures include : drainage wells ; drainage tunnels ;horizontal gravity drains ; and surface drainage improvements.
Drainage tunnels were constructed to remove groundwater at theSoutheast Block, the largest of the four blocks which has a clear
graben topography. The effect of the drainage tunnels is very evident as the groundwater table near the drainage tunnels has dropped
5 to 10m. At the North Block, a number of experimental underdrains were installed between 1999 to 2001 and show promising
results.
Loose pyroclastic flow deposits
Highly weathered volcanic rocks
Slightly to moderately weathered
volcanic rocks
Water level -1978

Cross-section
Southeast Block

H.W.L of 2001
Slip surface
Drainage tunnels

Drainage wells
Hanoki
River

200m

100

200m

Fig. 6-6 Cross section of Hirane landslide

North block
Center block

Southeast block

South block

Photo 6-10 Hirane landslide (courtesy of Minstry of L. I. and T.)

40

100

0

6.7 DOZANGAWA LANDSLIDE(1) Location : Minamiyama, Okura-Mura, Mogami-Gun, Yamagata(38°38’ N, 140°12’ E)(2) Size : Length : 1200m ; Width : 1100m ; Area : 130ha ;Thickness : 110m ; Volume : 1.3 x 108m3(3) History and Damages : In recent years, landslide movements
were observed in June 1966 and April 1981. The most recent largescale movement occurred in May 1996. The affected area involves
130ha, and a portion of the National highway which traverses near
the head of the landslide was destroyed. Numerous cracks and grabens have developed within the forest and farm land, disrupting thelife of the residents and the local economy. Further, the 1996 landslide destroyed previously installed anchors and drainage wells. As
the toe of the landslide mass reached and fell into the Dozan River,dangers of debris flow were seriously considered for the residents
located in down stream areas. Fortunately, the movement slowed
down in July and reduced the possibility of the debris flow.
(4) Geology, Mode of Movement : The topography of the sitearea includes volcanic, hilly terrain underlain by thick layers of pyroclastic flow deposits. Along the Dozan and Furumizu rivers, there
is advanced erosion and dissection forming very steep cliffs. The
site area is underlain by Upper Tertiary black mudstone and tuffaceous sandstone that is overlain by thick pyroclastic flow deposits
which tend to lose cementation with increases in moisture content
and then develops into loose rock mantle. Subsurface explorations
including borings revealed that the upper beds of the Upper Tertiary
sediments are incoherently mixed and highly sheared. From this information, it is clear that a slip surface exists within these beds, andas the slide movement continues, the moving masses are repeatedly
deformed and sheared to the extent of turning the slide masses completely over. It is suspected that groundwater infiltrated into theslide mass through relatively porous pyroclastic flow deposits, promoting the weathering process and adding to its load thereby reducing the strength of the rock. With the presence of groundwater,
shearing was initiated along the bedding planes and the landslide
began. A landslide moving toward a lower elevation as the Dohzan
River removed its support caused the development of a graben at
the head portion the landslide. This condition produced instability
to the main scarp and contributed to the development of retrogressive landslide.
(5) Mitigation Measures : The Yamagata Prefectural government
administered the landslide mitigation works in the early stage of thework ; however, because of the magnitude of the mitigation measures involved, the Tohoku Regional Forestry Office of the Forestry
Agency took over as a direct project since 1992. Immediately following the 1996 landslide, numerous pieces of monitoring equipment were installed to prevent the disasters associated with debris
flows. Because of the enormous thickness of the landslide mass, themain thrust of the mitigation measures were centered on the
groundwater control including drainage tunnels, drainage wells,
drainage borings. In order to enhance the effectiveness, the abovemeasures were combined and inter-connected 3-dimensionally. Dueto the extra time required to complete the 3-dimenssional drainage
networks, temporary emergency pumping has been employed tosupplement the dewatering process until the proposed permanent
works are completed.

Subsidence Zone

Subsidence Zone
Furumizugawa River

Dozangawa River
200m
Unwelded pyroclastic flow deposits
Tuffaceous sandstone
Sandy mudstone(Furukuchi formation)
Slip surface

100

200m

100

0

Fig. 6-7 Cross section of Dozangawa landslide

Photo 6-11 Dozangawa landslide (courtesy of F. Agency)

41

6.8 YUZURIHARA LANDSLIDE(1) Location : Yuzurihara, Onishi-Cho, Tano-Gun, Gunma (36°08’ N, 139°02’ E)(2) Size : Length : 600m ; Width : 1700m ; Area : 100ha ;Thickness : 40 - 50m ; Volume : 2.0 x 107m3
This landslide is divided into three blocks : Kayakabu Upper ;Kayakabu Lower ; and Shimokubo Blocks.
(3) History and Damages： Record show that there were somemovements prior to 1940. The records also indicate that residenceswere evacuated. In recent years, following the concentrated heavy
rain brought by a typhoon in October, 1991, and in August, 1992,
the landslide was reactivated and developed numerous cracks andscarps causing wide spread damages in residential structures, highways and other infrastructures.

(4) Geology, Mode of Movement : The site area is underlain
by Mesozoic Era crystalline schist belonging to the Sambagawa
Metamorphic Belt. The crystalline schist has well developed planar
shistocity and joint faces. These structural surfaces form a dip-slope
condition. Furthermore, numerous faults and shear zones exist
throughout the belt. Such structural conditions with abundant rainfall promote deep weathering that facilitates the development of slip
surface.
(5) Mitigation Measures : The Gunma Prefectural Government
administered the mitigation works as a subsidy project until 1995.
Since then, the National Government took over as a direct landslide
mitigation project. The mitigation measures performed in the kayakabu Upper and Kayakabu Lower Blocks include : drainage tunnels,
horizontal gravity drains, drainage wells and surface drainage control. In the future, it is planned to implement control works in the
Shimokubo Block.

Landslide mass
Bedrock
Slip surface
A

A'

Large diareter
Cast-in-place shafts
Drainage well

Drainage wells
National
Highway

Anchor withgrating cribs

100m

Kanna River
Drainage
Borings

Road

Drainage Tunnels

50

100m

50

Fig. 6-8 Cross section of Yuzurihara landslide

Shimokubo Block

Kayakabu
Upper BlockA'
Kayakabu
Lower BlockA

Photo 6-12 Yuzurihara landslide (courtesy of Ministry of L. I. and T.)

4206.9 IYOGATAKE LANDSLIDE(1) Location : Arakawa-Higurinaka, Tomiyama-Cho, Awa-Gun,
Chiba (35°06’ N, 139°55’ E)(2) Size : Length : 300m ; Width : 100m ; Area : 3.5ha ; Thickness : 34.5m ; Volume : 1.5 x 105m3
Based on the morphology, this landslide is divided into four
blocks : Upper ; Middle ; Lower ; and Channel Blocks.
(3) History and Damages : The landslide was induced by intense rain in excess of 330mm/day during Typhoon No. 17 in September 22, 1996. The failed landslide mass turned into a debris
flow. The flow dissected forest roads, scouring the channel banks
and beds for 80m. Although no injuries or damages were sustained
from this flow, the possibility of damages by subsequent debris
flows to the down stream areas was very high.
(4) Geology, Mode of Movement : The geology of the sitearea consists of Upper Tertiary inter-bedded sandstone and mudstone. The fracturing and shearing from a complicated geologic
structure promoted accelerated alteration and weathering in the Upper Tertiary sedimentary rocks. It is speculated that the accelerated
geologic process probably promoted the conditions susceptible tothe landsliding.
The lower section of the Upper Block which had been subjectedto past movement failed, and this failure may be the immediate
cause of the landslide. The Middle Block had been stable due to theprevious mitigation measures ; however, the intense rain by the typhoon caused an increase in the pore water pressure. Furthermore,sliding of landslide masses from the Upper Block on to this block
added weight, thereby reducing the stability of the Middle Block.
The combined factors contributed to the movement of the Middle
Block.
(5) Mitigation Measures : Considering the landslide morphology,
a design safety factor of 1.10 was established. Combined mitigation
measures of control measures and restraint measures have been
adopted.
The improvement of the safety factor was attempted by earth removal from the head portion of the Upper Block and supplemented
by dewatering. For the Middle Block, an increase in the safety factor was accomplished by installing four drainage wells supplemented by three rows of steel piles. Mitigation measures in theLower Block included earth retaining structures, sabo dams, andground sils to prevent further movement of earth materials.

Earth removal
Anchors

Landslide mass
Bedrock

Drainage well
50m
Piles

Slip surface
Groundwater table

25

50m

25

0

Fig. 6-9 Cross section of Iyogatake landslide

Photo 6-13 Iyogatake landslide(courtesy of Chiba Prefecture)

Photo 6-14 Iyogatake landslide following the completionof mitigation measures (courtesy of Chiba
Prefecture)43

6.10 OHRISAWA LANDSLIDE(1) Location : Sawajiri, Kouzushima-Mura, Tokyo(34°12’ N, 139°08’ E)(2) Size : Length : 70m ; Width : 40m ; Area : 0.3ha (Landslide
Threatened Area : 22.92ha) ; Volume : 2.8 x 104m3(3) History and Damages : An earthquake of Magnitude 6.4 occurred on July 1, 2000, and total rainfall of 291mm was recorded
between the 7 and 8 of July with a maximum rainfall intensity of
52mm/hour. Furthermore, on July 9, another earthquake of Magnitude 6.0 with a series of after shocks induced failure at the toe portion of the landslide with cracks and scarps along the head area.
Part of the debris derived from the failure caused serious damage tothe resort hotels.
(4) Geology, Mode of Movement : The region is a large Landslide Threatened Area with relief of 10 to 278m from the beach to
the ridge line. The terrain is divided into two zones : (1) the upper
steep terrain with an average slope of 35°along the highway ; and(2) lower gentle slopes with average gradients of 10°to 20°. The
landslide blocks which make up the upper slopes show a little disturbance by the slide movement. The results of the subsurface exploration with borings indicate the materials consist mainly ofweathered volcanic ash and volcanic cobbles with an N-value of up
to 30. The lower slopes below the highway are far more disturbed
than the blocks of upper slopes. The boring logs indicate that thematerials mostly consist of colluvium-landslide deposits with an Nvalue of up to 10. The borings also reveal that the site area is underlain by multiple layers of paleosols inter-bedded with colluviumlandslide deposits.
(5) Mitigation Measures : Since the site required immediate stabilization of the affected area, landslide control measures have beenused. Landslide control and restraint measures have been utilized
for selected economically and socially important landslide blocks.
Considering the prevention of the failures in front of the affected
blocks, restraint measures, including combined anchor-grating crib,were implemented.

A

A'

Anchors

Colluvium
Pyroclastic flow deposits
Slip surface
Groundwater table

Grating cribs

20m
10
20m 100

Fig. 6-10 Cross section of Ohrisawa landslide

A'A

Photo 6-15 Ohrisawa landslide (courtesy of Tokyo Metropolitan Government)

44

6.11 SORAKUMASHINDEN LANDSLIDE(1) Location : Sennoh, Noh-Machi, Nishikubiki-Gun, Niigata (37°06’ N, 138°02’ E)(2) Size : Length : 260m ; Width : 280m ; Area : 7.28ha ; Volume : 2.9 x 106±m3(3) History and Damages : The landslide occurred on March 17,
1997. Damages include 430m of roadway, 430m of water main, 0.3
ha of farm land, and 680m of channels.
(4) Geology, Mode of Movement : The site area is underlain
by Pliocene gray mudstone inter-bedded with thin, occasional, fineto medium-grained sandstone beds. The upper portion of the formation consists mainly of inter-bedded mudstone and medium-grainedsandstone. The overall structure of the mudstone includes advanced
fragmentation and softening, and often exhibits onion-skin weathering. Such weathered mudstone is soft and friable.
The geologic structure in the region is controlled by a syncline
located east of the landslide, and at the site the mudstone beds aredipping about 20°to the east. Reflecting such geologic structure,
the site area represents cuesta topography, and more landslides oc-

cur on the left side of the river due to the dip-slope structure.
This region is underlain by numerous old landslides, and thesubject landslide is a reactivation of a much larger, older landslide.
It is suspected that heavy rainfall followed by rapid snowmelt inlate November to early December of 1996 supplied an enormous
amount of groundwater, thereby reactivating a part of an old landslide block.
Based on the height of the main scarp and size of the graben in
the mid portion of the landslide, it is estimated that the vertical displacement is on the order of 10 to 15m. The horizontal displacement is estimated to be a maximum of 30m where sediments were
produced by the movement of toe. This indicates that this landslide
has an unusually large vertical displacement compared with otherlandslides that occurred on these Tertiary sedimentary rocks.
(5) Mitigation Measures : The mitigation measures focused on
dewatering as the excess water within the landslide mass was theimmediate triggering mechanism of this landslide. The mitigation
measures included : a drainage well, horizontal gravity drains, surface drainage improvements and erosion prevention in order to enhance the overall stability of the landslide.

Drainage wells
Horizontal gravity drains Crack

Fault
Landslide mass
Bedrock
Slip surface
Groundwater table
50m 2550m
250Fig. 6-11 Cross section of Sorakumasinden landslide

Photo 6-16 Sorakumashinden landslide prior to the sliding (Mar. 13, 1997) (courtesy of Niigata Prefecture)

Photo 6-17 Sorakumashinden landslide following thesliding (Mar. 17, 1997) (courtesy of Niigata
Prefecture)45

6.12 KINYUBA LANDSLIDE(1) Location : Kamifunakura, Yasuzuka-Cho, Higashikubiki-Gun,
Niigata (37°05’ N, 138°30 E)(2) Size : May 6, 1996 landslide : Length : 350m ; Width : 150
m ; Area : 5.3ha ; Thickness : 17m ; Volume : 9.0 x 105m3
August 12, 1996 landslide : Length : 250m ; Width : 130m ;Area : 3.3ha ; Thickness : 17m ; Volume : 5.5 x 105m3(3) History and Damages : The May 6, 1996 landslide damaged
a 150m section of the roadway. The landslide mass had very highwater content and was very fluid. The fluid landslide mass traveled
down 900m along the Funakura River, filling the river with debris,and buried three sabo dams and destroyed one bridge. The August
12, 1996 landslide moved 80m in response to the instability createdby the May 6 landslide. An additional 200m section of the roadway
was also destroyed.
(4) Geology, Mode of Movement : The site area is underlain
by Upper Tertiary black mudstone that is overlain by 10 to 20m ofthick clayey colluvium with mudstone fragments. The contact between the bedrock and colluvium is clear at the upper and the central portions of the landslide, however, the contact is not well defined at the lower portion. It is suspected that the slip surface exists
near the contact between the bedrock and the colluvium. The
groundwater table is located between the bedrock and the slip surface. The groundwater in the lower half of the landslide mass isconfined. The structural geology of the site area is monocline dipping to the northwest at 20 to 30°, forming a dip-slope structure.
The triggering mechanism of the May 6 landslide is attributed
to be the increase in pore water pressure due to excess water infiltration into the landslide mass from snowmelt water. As discussed,
the water content of the landslide mass was extremely high andvery fluid due to the abundant and continuous supply of water fromthe snow pack. On the other hand, the August 12 landslide was triggered by the 20 to 30m scarp formed by the May 6 landslide. This
created a very unstable condition for the up slope area which finally
failed.
(5) Mitigation Measures : Mitigation measures for the May 6
landslide include ground sils and earth retaining structures to prevent further flow of the colluvium ; surface drainage improvements
to prevent surface water from entering into the landslide blocks ;and cast-in-place grating cribs to prevent weathering of the exposed
bedrock. Drainage wells, horizontal gravity drains and piles were
installed for the unstable area above the main scarp. Mitigation
measures for the August 12 landslide are focused on dewatering
around the slip surface with horizontal gravity drains and surface
drainage improvements. The large quantity of sediments-debris flow
into the river was removed as a Disaster Related Emergency Erosion and Sediment Control Project and a series of ground sils wereconstructed.

Aug. 12, 1996
landslide
Horizontal gravity drains

Drainage well
Piles

Original ground surface
Check dams

Retaining wall

100m
Landslide mass
Bedrock

50

Slip surface
100m

50

0

Fig. 6-12 Cross section of Kinyuba landslide

Photo 6-18 Kinyuba landslide (May 6, 1996) (courtesy of
Niigata Prefecture)46

Photo 6-19 Kinyuba landslide following the second landslide (Aug. 12, 1996) (courtesy of Niigata Prefecture)6.13 KUWANOKIDAIRA LANDSLIDE(1) Location : Ohmogidaira, Yasuzuka-Cho, Higashikubiki-Gun,
Niigata(37°02’ N, 138°29’ E)(2) Size : Length : 430m ; Width : 80m ; Area : 3.42ha ; Thickness : 10m ; Volume : 1.3 x 105m3
This slide is divided into two blocks : Upper Block and Lower
Block.
(3) History and Damages : This is a partial reactivation of an
existing very large older landslide. Previous mitigation measures forthe older landslide include pile driving in 1986 at the lower portionof the landslide following the completion of a major forest road. On
April 29, 1999, due to a rapid snowmelt caused by warm temperature and rainfall reactivated a portion of a large landslide. The slide
movement damaged sections of the forest road and previously installed piles. The slide debris dammed the Oguro River, destroying
the existing dam and flooding upstream areas.
(4) Geology, Mode of Movement : The site area is underlain
by Upper Tertiary sandy mudstone including inter-bedded black
mudstone and sandstone, ryolitic tuff, blocky black mudstone andblocky siltstone. Because of the age, the sandy mudstone is not welllithified and easily weathers and becomes clay. The geologic structure of the site area is complicated as an east-northeast to westsouthwest trending Takaba fault runs near the site. Factors contributing to the partial reactivation of the older landslide include : (1)The site is underlain by soft clayey soil ; (2) This is one of themost famous regions for snow and up to 5m of snow fall ; (3)Higher mountains continue upslope providing topographic conditions for concentrated groundwater accumulation ; and (4) Recently,
unused rice paddy fields have increased in the region. The neglectedclay liner of the paddy fields permits the infiltration of surface
water into the existing landslide mass, developing cracks and small
scarps. As discussed, this failure represents the reactivation of larger
and older landslide (Length : 400m ; Width : 250m).
(5) Mitigation Measures : The mitigation measures for the Upper Block include : two drainage wells ; 60 piles ; and horizontal
gravity drains at six locations. The mitigation measures for the
Lower Block include : revetments, check dams, seven earth retaining structures, earth removal, interceptor underdrains, and channel
improvements. The exposed slopes are protected and stabilized bylog grating cribs and terracing with logs and re-vegetated with theaide of straw mats.

Colluvium-landslide deposits
Weathered mudstone
Mudstone
Slip surface

Horizontal gravity drains
Drainage well
Piles

Horizontal gravity drains
50m

25

50m

25

0

Fig. 6-13 Cross section of Kuwanokidaira landslide

Photo 6-20 Kuwanokidaira landslide(courtesy of F. Agency)

Photo 6-21 Kuwanokidaira landslide following the completion of mitigation measures (courtesy of F.
Agency)47

the older rocks. Block A, the most active part of the landslide, isunderlain entirely by the welded tuff with a maximum thickness of
60m. Block D, which includes Block A and Block B, does not appear to be active at this time. However, based on the geologic investigation it is suspected that the slip surface is located within the Jurassic marine sedimentary rocks.
(5) Mitigation Measures : Since the maintenance of the Matsukawa River is a direct erosion and sediment control project of the
National Government, buttress fill was added onto the existing revetment in 1997 by the National Government. Drainage tunnels,
drainage wells and horizontal gravity drains were added upslope as
landslide mitigation measures. The 455m drainage tunnel was constructed by NATM method, and with a system of drainage wells
discharging several hundred liter per minute.
Hakuba-Mura (meaning village of white horses) on which thelandslide is located was the site of the 1998 Nagano Winter Olympic Games. This area offers various winter sport events such as the
ski jump and alpine skiing, and is visited by many skiers and tourists year around. The landslide area is located opposite the Happouone Ski Resort that was the site of downhill competitions during the
Olympics. This region is commonly called Donguri-Mura or Acorn
Village with numerous hotels and condominiums.

6.14 KURASHITA LANDSLIDE(1) Location : Hakuba-Mura, Kitaazumi-Gun, Nagano(36°43’ N, 137°51’ E)(2) Size : Length : 800m ; Width : 800m ; Landslide Threatened
Area : 78.64ha ; Thickness : 60m ; Volume : 3.2 x 107m3 This
landslide is divided into five blocks : Block A through Block E.
(3) History and Damages : Some deformation including cracks
were observed on revetments along the Matsukawa River in 1995.
Further examination of up slope areas revealed landslide related deformation on roads and retaining walls.
(4) Geology, Mode of Movement : The site is located adjacentto the Fossa Magna (Itoigawa-Shizuoka Tectonic Line), a major tectonic boundary in Japan. Therefore, the region has been subjected
to repeated episodes of tectonic activity since the Permian Period,and exhibits a very complicated geologic structure. The landslidearea is underlain by Jurassic shallow marine deposits, and quartz
andesite co-exists with the Lower Tertiary welded tuff. Marine deposits composed of Permian to Triassic shale, sandstone, and conglomerate deposited on top of the basement rock of serpentine melange. Following deposition, continuous tectonic activities, including
faulting, mixed the sedimentary rocks and the serpentine melange
together. The Lower Tertiary welded tuff unconformablly overlies

Matsukawa River100m
50100m 50
Colluvium
Welded tuff
Sandy shale
Quartz magnesite
Serpentinite-peridotite
Slip surface

Fig. 6-14 Cross section of Kurashita landslide

E
D
C

B

A

Photo 6-22 Kurashita landslide (courtesy of Nagano Prefecture)

4806.15 SHIOMOTO LANDSLIDE(1) Location : Takefusa, Shinshu Shin-Machi, Kamiminochi-Gun,
Nagano (36°33’ N, 138°01’ E)(2) Size : Length : 360m ; Width : 250m ; Area ; 6.05ha ;Thickness : 43m ; Volume : 2.0 x 106m3
This is a partial reactivation of a larger and older landslide.
There are shallower secondary landslide blocks within the reactivated landslide.
(3) History and Damages : The historical records indicate there
was a large-scale movement during the Edo Period (1603 - 1867).
The records also show that there was wide spread damage to structures and the landslide mass extended to the river. In recent years,
as the landslide moves continuously, damage occurs to roads, farmland (mostly on rice paddy fields) and structures by cracking andother deformations associated with the landslide movement.
(4) Geology, Mode of Movement : The site area is situated at
the northern Fossa Magna region, and the geology consists mostlyof Tertiary sedimentary rocks and volcanic rocks. In general, thesedimentary rocks are soft and weak, and in some areas it had been
deformed and fractured by localized tectonic activity such as faulting and folding. Such geologic conditions contributed to the reactivation of the landslide.

This region is located in the central portion of Honshu and has
been subjected to very rapid up-lifting (about 0.5mm/year), resulting in accelerated erosion. This is one of the factors contributing to
the formation of numerous landslides in this region.
As discussed, the landslide area is underlain mainly by Pliocene
sandstone and mudstone that is overlain by Holocene colluvium.
The upper portion of the landslide is underlain by volcanic rocks
such as andesite and tuff breccia. These volcanic rocks can hold anenormous amount of groundwater, which can be released later tovarious landslide blocks, thereby contributing to landslide formation.
(5) Mitigation Measures : The mitigation measures had been
implemented by the Nagano Prefectural Government. However, due
to the magnitude of the mitigation works and the consorted effort
and high level of technology required, the National Government
took the project over as a direct project in 1992. Since the affectedareas are in agricultural and is farming communities, the Ministryof Agriculture, Forestry and Fishery administers the mitigation
works. The mitigation measures are focused on lowering the porewater pressure in large areas by installation of drainage tunnels,
drainage wells, and horizontal gravity drains. Furthermore, in orderto stabilize the shallower secondary landslide blocks, drainage wells,horizontal gravity drains, drainage channels, and earth retaining
structures have been constructed.

Colluvium
Volcanic rocks
Mudstone
Sandstone
Conglomerate
Slip surface
Springs

200m

100
Drainage tunnel
200m

100

0

Fig. 6-15 Cross section of Shiomoto landslide

Photo 6-23 Shiomoto landslide (courtesy of Ministry of A. F. and F.)49

6.16 KOSHIO LANDSLIDE(1) Location : Kashio, Ohoshika-Mura, Shimoina-Gun, Nagano(35°37’ N, 138°05’ E)(2) Size : Length : 800m ; Width : 250m (Max. Width : 350m) ;Landslide Threatened Area : 149.12ha ; Thickness : 20 - 75m ;Volume : over 6.0 x 107m3(3) History and Damages : Historical records indicate there had
been small-scale landslide movements in the past. However, activity
has increased in recent years. The accelerated movement occurredfollowing heavy rains in June 1992. The immediate damages included destruction of one check dam and deformation of 8 othercheck dams, and numerous tension cracks that developed along theslope. Further movement of the slide mass damaged residential
structures and National Highways, and buried a section of the Kashio River which created the potential for a large scale debris flowon the central area of Ohoshika Village located down stream of theslide area.
(4) Geology, Mode of Movement : The Median Tectonic Line
traverses along the west side of the landslide. A fragmented (discontinuous) syncline axis extends on the upper slopes where the bedding planes dip into the slope. The site area is underlain by highpressure metamorphic rock while the lower hillside are underlain bypelitic schist, basic schist, and siliceous schist of the Sanbagawa
Metamorphic Belt with intrusive bodies of serpentine. The upper
slopes are underlain by greenstone, peridotite and serpentine of the
Mikabu Metamorphic Belt. As a whole, the schistose rocks are severely fractured and create favorable conditions for argillization.
The currently active landslides consist of multiple slide blocks, witha maximum depth of 75m. The maximum depth of the Upper Blockand the Lower Block are 30m and 50m, respectively.
(5) Mitigation Measures : As a dewatering procedure, a total of
45 drainage wells, two drainage tunnels and horizontal gravity
drains at six locations had been constructed by 2001. The movement of the Lower Block gradually slowed down. The shallow
blocks on the upper slopes, even though the rate of movement was
reduced compared to the earlier stages, still show uneven cumulative movements. Thus, mitigation measures focused on groundwater
control such as drainage wells and drainage tunnels have been implemented.

Photo 6-24 Koshio landslide (courtesy of F. Agency)

Drainage wells

Anchors

Koshiozawa
River

Drainage tunnels

Colluvium
Weathered rock
Greenstone
Serpentinite
Siliceous schist
Green schist
Black schist
Slip surface
200m

Fig. 6-16 Cross section of Koshio landslide

50

200m

100

100

0

6.17 NYUYA LANDSLIDE(1) Location: Nyuya, Ohshika-Mura, Shimoina-Gun, Nagano (35°34’ N, 138°04’ E)(2)Size : Length : 1000m (Max.) ; Width : 1200±m (Max.) ;Area : 138±ha ; Thickness : 30 - 40m(3) History and Damages : The sliding history includes movements in 1961, 1982, 1983, and 1992. Heavy rains caused by typhoons in 1982 and 1983 produced damages such as tilting andcracking in residential structures, roadways, and retaining walls. The
movement in the toe of the landslide was prominent during thesnowmelt period in 1992.
(4) Geology, Mode of Movement : As the site area is situatednext to the Median Tectonic Line, the underlying rocks have been
intensely sheared, fractured, and highly metamorphosed. Such con-ditions promote deep weathering and accelerated the argillization
process. Thus, this region represents a highly concentrated cluster offracture zone- related landslides and rapid slope failures. The structure of the landslide includes about 30m of greenstone and related
rocks that form the sliding mass underlain by the intact bedrock
formation composed of inter-bedded black and green schist. A slip
surface consisting of serpentine or talc schist exists near the contactzone.
(5) Mitigation Measures : The early stage of the mitigation
works were conducted by the Nagano Prefectural Government.
Comprehensive mitigation measures have been conducted by the
National Government since 1988. The mitigation measures include
drainage wells, horizontal gravity drains, and installation of anchors
and piles.

A

A'

Drainage wells

Colluvium-landslide deposit

50m

Highly weathered rocks
25

Slighty to moderately weathered rocks
Slip surface

50m 250Groundwater table
Fig. 6-17 Cross section of Nyuya landslide

AA'

Photo 6-25 Nyuya landslide (courtesy of Ministry of L. I. and T.)51

6.18 JINNOSUKEDANI LANDSLIDE( 1 ) Location : Shiramine , Shiramine-Mura , Ishikawa-Gun ,Ishikawa (36°07’ N, 136°45’ E)(2) Size : Length : 1500m ; Width : 1050m ; Landslide Threatened Area : 502.5ha ; Thickness : 50 - 100m ; Volume : 7.0 x 107
m3
This landslide is located along the foothills of the Hakusan
Mountain between 1600 and 2200m in elevation. The slide is divided into three major blocks and several minor blocks.
(3) History and Damages : It is not clear when the initial movement took place ; however, it is speculated that the movement was
triggered by a large magnitude earthquake in the past. Damages tothe existing drainage tunnel and sabo dams were reported in the
1960s. The landslide is actively moving as evidenced by the cracks
on sabo dams and their wings.
(4) Geology, Mode of Movement : The site area is underlain
by Mesozoic Era shale and sandstone, and these sedimentary rocks
are irregularly overlain by Tertiary volcanic eruptive rocks consist-ing of lava, volcanic ashes, and pyroclastic flow deposits. The contact between the lava rocks and the sedimentary rocks forms themain scarp and developed into several large-scale landslide blocks.
The slip-surface is suspected to be a clay layer originated within thesandstone formation that had been fractured and sheared by thecontinuous movement of the landslide blocks.
(5) Mitigation Measures : Signs of movement detected on asabo dam in the 1950s were the impetus for the mitigation works.
This landslide has been assigned as a direct project of the National
Government and mitigation has been concentrated on dewatering
works. Based on the assumption that the stabilization of the smaller
lower landslide blocks could eventually lead to the stabilization of
the upper major landslide blocks, effort has been devoted to themitigation measures on the small landslide blocks. With installationof drainage tunnels, horizontal gravity drains and drainage borings.
the movement stopped by 1972 ; however, the movement renewedaround 1975. The work resumed in 1981, and many drainage wells
have been installed since 1995.

A

A'

Drainage wells
100m

50

Colluvium

100m

50

Shale, Sandstone

Drainage Tunnel

Slip surface
Groundwater table

Fig. 6-18 Cross section of Jinnosukedani landslide

A'A

Photo 6-26 Jinnosukedani landslide (courtesy of Ministry of L. I. and T.)

5206.19 HANSAI LANDSLIDE(1) Location : Hansai, Kikukawa-Cho, Ogasa-Gun, Shizuoka (34°45’ N, 138°05’ E)(2) Size : Length : 170±m ; Width : 100±m ; Thickness : 20±m ; Volume : 2.2 x 105m3(3) History and Damages : Repeated visits of typhoons between
September 15 and 22, 1998 resulted in record breaking rainfall.
Further, another heavy rain on September 24 triggered an earlymorning landslide. One home was destroyed without any injury.
However, with the possibility of further movement, in particular,
failure at the toe area, it was feared that homes located at the lower
slopes would be destroyed. Affected residences were ordered toevacuate, and residents remained at the emergency shelter for nine
and a half months.
(4) Geology, Mode of Movement : This region is underlain by
Pliocene inter-bedded sandstone and siltstone, and the beds dip 5 to
8°to the northwest. A joint set almost perpendicular to the bedding
creates potentially weak zones within the bedrock. Furthermore, thebedrock is not well lithified and the expansive nature of the formation promotes stress release and weathering. By comparing detailed
topographic maps of the site, before and after the event, it is determined that the subject landslide moved about 20m along the dip, inapproximately the same direction. The main scarp and the northeastern lateral scarp formed near vertical walls. The sliding blocks
are intact without breaking and slid along the bedding plane. This isclassified as a translational or block slide.
(5) Mitigation Measures : Comprehensive mitigation measures
were necessary for this landslide because : (1) it was induced byheavy rains ; (2) it is relatively large ; and (3) it was a potential
danger to residential structures and river. The selected landslide
control measures to increase the safety factor include : earth removal at the head area ; buttress fill at the toe area ; drainage
wells ; and horizontal gravity drains. Further, in order to maintainthe design safety factor, steel piles were driven at selected locations.
At the scarp area, the inclination of the slopes were reduced by cutting, followed by hydromulching for erosion control, and the gradedfaces were supported by anchors.

Colluvium
Highly weathered rocks

Crib wall

moderately weathered rocks

Earth removal

Slip surface

Anchors

Drainage wells
Buttress fill
Piles

20m
10
20m 100

Fig. 6-19 Cross section of Hansai landslide

Photo 6-27 Hansai landslide (courtesy of Shizuoka
Prefecture)

Photo 6-28 Hansai landslide following the completion of
mitigation measures (courtesy of Shizuoka
Prefecture)53

6.20 SUGI LANDSLIDE(1) Location : Sugi 1-Chome, Hirakata-Shi, Osaka(34°48’ N, 135°43’ E)(2) Size : Length : 70m ; Width : 130m ; Area : 0.9±ha ;Thickness : 10m ; Volume : 1.2 x 104m3(3) History and Damages : This landslide occurred on October
19, 1996. This region is located within a northeast to southwest
trending ridge, and the landslide is situated along the northwestfacing slope that forms a scarp and grabens. Damages include thedestruction of a couple of residential structures and relocation of 11
families of 55 people.
(4) Geology and Mode of Movement : The basement rock of
the site area consists of granitic rocks of the Paleozoic Era that isoverlain by the Plio-Pleistocene inter-bedded clays, sands, and gravels. The geologic structure includes a gentle dip-slope condition.
The landslide debris consists entirely of the Plio-Pleistocene deposits. The sliding was initiated by weakened upper clayey beds with agroundwater table that rose due to rainfall.
(5) Mitigation Measures : The Osaka prefectural Government
administered the mitigation measures as emergency work following
authorization by the Ministry of Land, Infrastructure and Transport.
The landslide mitigation measures included : piles, horizontal gravity drains, and earth removal.
Following the completion of the mitigation measures, the city of
Hirakata converted the site into a regional park that is in harmony
with the surrounding environment.

A

A'

Sand and Gravel
Clays
Sands
Slip surface

Profile immediately following
the landslide
Profile following
the earth removal Earth removal
Piles

20m

10

20m
Fig. 6-20 Cross section of Sugi landslide

A'A

Photo 6-29 Sugi landslide (courtesy of Osaka Prefecture)

54

10

0

6.21 KAMENOSE LANDSLIDE(1) Location : Karindoubata, Kashihara-Shi, Osaka(34°35’ N, 135°39’ E)(2) Size : Length : 1100±m (Max.) ; Width : 900±m (Max.) ;Thickness : Max. over 80m, Average 30 - 40m ; Landslide Threatened Area : 94.29ha ; Volume : 1.76 x 107m3
The landslide is composed of two large blocks ; Touge Blockand Shimizudani Block, with each block influencing and interfering
each other as they move.
(3) History and Damages : 1931-1932 : Two parallel northeast
to southwest trending cracks of 1.5cm wide were discovered in a
farm field at the northeast corner of Touge Block. Cracks and deformation were observed at the west portal of the Kamenose Tunnel,
one of the main lines (Kansai Line) of the National Railroad. A reservoir for rice paddy developed leakage and eventually dried up.
The western half of Touge Block moved first, which induced movement in the eastern half of the block, affecting a total area of about
32ha. The total horizontal movement during that period was recorded to be 31 to 33m. The maximum horizontal movement wasrecorded to be about 51cm/day, and is a direct response from heavy
rains. Eventually, the slide movement destroyed the Kamenose Tunnel, and the railroad trucks and the tunnel had to be relocated onto
the left bank of the Yamato River. The slide movement uplifted the
National Highway (Route 25) for about 16m and the Yamato Riverbed for over 9m, causing the areas of upstream to flood. Other
damages included the destruction of 25 homes in Touge Block.
1967 : A crack running through the middle of the eastern secLandslide masstion of Shimizudani Block was discovered. Later, the movement extended into the previously moving Touge Block, with a total affected area of about 50ha. The horizontal movement was recorded
to be about 26m with the maximum enlargement of the crack about
25cm/day. The National Highway was uplifted about 1.3m.
(4) Geology, Mode of Movement : The site geology consists of,from the oldest to the youngest : granitic basement rocks ; Miocene volcanic and sedimentary rocks ; and Plio-Pleistocene clays,
sands and gravels. The slip surface within Touge Block was foundin a thin tuffaceous conglomerate that is sandwiched between andesitic lava flows. In Shimizudani Block, the slip surface was foundin a highly argillizated bed between two members of Miocene rocks.
(5) Mitigation Measures : In the earlier stages of the mitigation
work, the Osaka Metropolitan Government administered the works.
According to the Landslide Prevention Law of 1959, the Kamenose
landslide was designated as a Landslide Threatened Area. Because
the landslide is extremely large and mitigation measures required
advanced technology, the works became a direct project for the National Government. Earth removal work was performed as one ofseveral permanent measures for Shimizudani Block. Following the1967 movement, as an emergency dewatering measure, groundwaterdrainage works were implemented along with the installation ofsteel pilings at selected locations. Since 1970, a series of large diameter cast-in-place shafts were installed at both blocks. The main
mitigation measure in Touge Block since 1986 consists of the installation of Large diameter cast-in-place shafts measuring 6.5m indiameter with depths ranging from 80 to 100m.

Touge Block

Upper andesite lava

Shimizudani Block

Lower andesite lava
Old andesite lava
Conglomerate
Tuffaceous conglomerate

200m

YamatoRiver

Granitic rocks
Slip surface

100

200m 1000Fig. 6-21 Cross section of Kamenose landslide

Shimizudani Block

Touge Block

Photo 6-30 Kamenose landslide (courtesy of Ministry of L. I. and T.)55

6.22 LANDSLIDES IN KITAKOBE REGION(1) Location : Kobe-Shi ; Miki-Shi ; Yokawa-Cho, Minou-Gun,
Hyogo (34°50’ N, 135°10’ E)(2) Size : This region includes 38 Designated Landslide Threatened Area with numerous landslides of varying size. Larger landslides measure up to : Length : 600m ; Width : 300m ; Depth :45m.
(3) History and Damages : Historical records show a large-scalelandslide movement in 1897. A landslide occurring in September
1942 destroyed eight homes. Since then, numerous slide movements
have destroyed residential structures, roadways, and farmland.
(4) Geology, Mode of Movement : The site area is locatedalong the north side of Rokko Mountain at an elevation of between
100 and 250m. The site is underlain by Upper Tertiary sedimentaryrocks. Active tectonism in the region uplifted Rokko-San resulting
in the majority of the beds dipping towards the west. The site area
has been subjected to other severe forms of deformation including
folding, faulting and flexing, and is one of the primary causes of
sliding.
Geology of the site area consists of highly weathered mudstone,
and fine grained tuff that shows a high level of argillization (tomontmorillonite). The slip surfaces developed due to infiltration ofrain water and groundwater.
(5) Mitigation Measures : In order to remove the groundwater(the primary cause of the sliding), drainage wells, and horizontal
gravity drains were installed. Surface drainage improvements andpiles works have also been implemented. Further, in conjunction
with the farmland restoration projects, large scale buttress fills havebeen placed to improve stability in some of the landslides.

Photo 6-31 Landslides in Kitakobe Region (courtesy of Ministry of A. F. and F.)

56

6.23 ZENTOKU LANDSLIDES(1) Location : Zentoku, Nishiiyayama-Son, Miyoshi-Gun,
Tokushima (33°52’ N, 133°50’ E)(2) Size : Length : Max. 2000m ; Width : Max. 900m ; Landslide
Threatened Area : 221±ha
This landslide is divided into seven large blocks, Z1 Blockthrough Z6 Block and Imakubo Block.
(3) History and Damages : The landslide movement occurred at
the eastern section (Z6 Block) in the early Meiji Period (1860s).
Further movements in 1945, 1949, and 1954 caused serious damages to roadways. In 1965, movement caused damages to residential
structures and roadways. A lower portion of Z6 Block failed (110mlong ; 50m wide ; and 15m thick) in 1984. Z2 Block started tomove in 1987, causing further damages to the roadways. Z1 Blockdisplayed numerous cracks in 1992.
(4) Geology, Mode of Movement : The site area is comprisedof the Sanbagawa crystalline schist belt and is sandwiched between
the Median Tectonic Line and the Mikabu Tectonic Line, and consists predominantly of argillaceous schist. These crystalline rocks
exhibit well developed structural zones of discontinuity such as planar schistocity, joint surfaces, and fault planes promoting accelerated weathering and softening of the rocks. The entire Zentoku

Large diametercast-in-place shafts

landslide is in a dip-slope condition.
The Zentoku landslide was formed due to unfavorable geologic
conditions and complicated geologic structures which are typical oflandslides in fracture zones. Furthermore, heavy rainfalls increased
the groundwater table that induced the movements. This landslide isunique because there are two types of movements within one landslide. There are relatively shallow landslides with the slip surfaces
located in colluvium, and deep landslides where the slip surfaces
are located in the weathered bedrock formation. These landslides
are inter-mingling with each other, developing very complicatedmovement patterns.
(5) Mitigation Measures : It is speculated that the Ansei Earthquake of 1854 may have triggered the Zentoku landslide, since active movement continued. The landslide was named as a DesinatedLandslide Threatened Area in 1959. Since 1952, the Tokushima Prefectural Government has implemented mitigation measures such as
horizontal gravity drains, drainage wells, and surface drainage improvements. However, in 1982 the Ministry of Construction took it
over as a direct project. Comprehensive mitigation measures started
in 1992, including groundwater control measures and installationsof large diameter cast-in-place shafts and anchors for Z1 Block. Recently, mitigation measures have been implemented for Z6 Blockand Imakubo Block.

Drainage wells

Road

100m

Road

Anchors

Colluvium
Terrace deposit
Highly weathered rockModerately weathered rocks
Slightly weathered rock50

Slip surface

Iya River

100m

50

0

Fig. 6-22 Cross section of Z1 Block of Zentoku landslide

Z6
Z1
Z2

Z3

Z4

Z5

Imakubo

Photo 6-32 Zentoku landslides (courtesy of Ministry of L.
I. and T.)

Photo 6-33 Zentoku landslides, Z1 Block following thecompletion of mitigation measures (courtesyof Ministry of L. I. and T.)57

6.24 KASHIO LANDSLIDE( 1 ) Location : Kashio , Higashiiyayama-Son , Miyoshi-Gun ,Tokushima (33°50’ N, 133°53’ E)(2) Size : Length : 1100m ; Area : 50ha ; Thickness : 20 - 50m ;Volume : 1.5 x 107m3
The landslide is divided into three blocks : B and C Blocks(upper blocks) and A Block (lower block) displaying a continuous
landslide morphology.
(3) History and Damages : There is no historical record of thelandslide ; however, the slide is thought to have originated in prehistoric time. The movement is almost imperceptible and chronic,
about 5cm/year, showing some deformation on residential structuresand settlement in roadways. In 1993 and 1994, increased movementof up to 30cm/year caused partial destruction of a house, roadway
settlements, pushed out revetments, and pressure cracking on embankments.

(4) Geology, Mode of Movement : The site is located within
the Sanbagawa crystalline schist belt that is sandwiched between theMedian Tectonic Line and the Mikabu Tectonic Line. The rocks in
the Sanbagawa belt in this area include pelitic schist, basic schistand siliceous schist. Due to large scale tectonic activity, the rocks
are highly fractured and weathered. The slip surface is recognizedwithin highly weathered pelitic schist which is stratigraphically overthe basic schist. With response to heavy rainfall (150mm/day) andan increase in water table by 2 to 3 m, the slide movement is recorded to be 2 to 3cm/year (prior to the implementation of the control measures).
(5) Mitigation Measures : Since 1995 integrated groundwater
control measure involving four drainage tunnels and 36 drainage
wells have been constructed. At the same time, surface drainage improvements were implemented. Future mitigation measures included
installation of large diameter cast-in-place shafts and anchors as restraint measures.

Landslide mass
Bedrock
Slip surface
Groundwater table

Drainage wells

200m

100
Drainage tunnels
200m

100

0

Fig. 6-23 Cross section of Kashio landslide

C

B

A

Photo 6-34 Kashio landslide (courtesy of F. Agency)

58

Photo 6-35 Surface deformation of Kashio landslide(courtesy of F. Agency)

6.25 MARUYAMA DISTRICT LANDSLIDE(1) Location : Iyomishima-Shi, Ehime(33°58’ N, 133°33’ E)(2) Size : Length : 250m ; Width : 160m ; Thickness : 30m(3) History and Damages : This landslide developed during
grading operations for the proposed freeway construction. The cut
involved a 220m long, maximum height of 40m and the proposed
removal of about 2.0 x 105m3 of earth. Because of local geologic
conditions and the magnitude of the proposed grading, Landslide
had been anticipated and the works proceeded with close monitoring for any signs of slide. According to the design, piles had been
installed as mitigation measures for the anticipated landslide in
1981. As grading work progressed, cracks were discovered in theupper cut slope, and the monitoring equipment also revealed signsof movement. It was determined that the size of the landslide wasmuch larger than previously anticipated. Consequently, the grading
work was suspended. Following a supplemental investigation, additional mitigation measures were implemented. The grading workand the mitigation measures were completed by March of 1984.
Original ground surface(4) Geology, Mode of Movement : The site area lies within anarrow belt sandwiched between the Median Tectonic Line to thesouth and the Ikeda fault to the north. The northern side of the Median Tectonic Line is underlain by Cretaceous sandstone and granite,while the southern side is underlain by metamorphic rocks of the
Sanbagawa Belt. The landslide is located within a hilly terrain
about 130m in elevation, with the region displaying large-scalelandslide morphology.
Reddish topsoil is developed over highly weathered and fractured pelitic schist. Furthermore, clay beds, with the appearance ofan ancient slip surfaces which may have developed from large bedrock landslides, were observed in several locations.
(5) Mitigation Measures : As a mitigation measure for the anticipated landslide, a total of 66, 50.8cm diameter piles were driven
into the cut face. Following the realization of the actual size of theslide, a 245m long drainage tunnel was installed in order to lower
the groundwater level. Furthermore, as a landslide restraint measure,a total of 190 anchors were installed and an additional 122, 50.8cm
diameter piles were driven into the cut faces.

Anchors

Piles

Road

50m

Drainage tunnel

25
Landslide mass
Bedrock
Slip surface

50m

25

0

Fig. 6-24 Cross section of Maruyama district landslide

Photo 6-36 Maruyama district landslide (courtesy of Japan Highway Public Corporation)

596.26 HARABUN LANDSLIDE(1) Location : Harabun-Cho, Sasebo-Shi, Nagasaki(32°13 N, 129°43’ E)(2) Size : Length : 70m ; Width : 50m ; Area : 0.3ha ; Thickness : 7m (max 10m) ; Volume : 1.7 x 103m3(3) History and Damages : This landslide occurred on July 7,
1997 at the 14 : 00 hour and was triggered by intense rainfall generated by a stationary rain front which develops annually in the earlysummer in Japan. Continuous rain further enlarged the landslide
head-ward, contributing to further damages. The reported damages
include : complete destruction of three homes ; partial destructionof three homes ; damaged roadways ; and disruption to the lifelines. Since the landslide occurred within a heavily populated area,
51 families totaling 146 residents living down slope of the landslidewere evacuated and had to stay in emergency shelters for twomonths.
(4) Geology, Mode of Movement : The site area is underlain
by Upper Tertiary mudstone and sandstone and is overlain by recent
basaltic caprock. The site area is also underlain by many ancientlandslide deposits. The landslide is located at the base of a steep
slope along an alluvial plain. The moving mass consists of clayey
colluvium derived from volcanic rocks, and the slip surface is located near the boundary between the sedimentary bedrock formationand the colluvium. As discussed, the slide was triggered by the intense rain associated with the early summer stationary rain front
with total rainfall of 410mm. The maximum movement was recorded to be 174mm per hour between July 10 and 13, 1997.
Movement of over 10mm per hour continued for about one month.
Movement in the head area reached a maximum of 5m in twomonths.
(5) Mitigation Measures : Emergency Measures : As the landslide occurred within a heavily populated area, secondary failures
and further down slope movement of the slide debris was an eminent concern for the officials. For the emergency landslide mitigation measures, horizontal gravity drains and temporary earth retaining structures to capture future debris movement were constructedat the base of the slope.
Permanent Measures : As landslide control measures, horizontal gravity drains and surface drainage improvements were implemented. As landslide restraint measures, piles and anchors were also
installed.
A number of structures were partially destroyed within the landslide block, which created potential danger. As the typhoon of July
25 approached, in order to prevent further damage by the disintegration of the damaged homes, they were removed. This landslide
occurred as the result of the pressure for urban development onto
the steep slope. If the plateau area had left undeveloped, the damages to the structures constructed on the plateau would not have occurred. In this respect, this landslide could be classified as an urbanization induced disaster.

Photo 6-37 Harabun landslide immediately following thesliding (courtesy of Nagasaki Prefecture)

Photo 6-38 Harabun landslide causing damages to residential structures (courtesy of Nagasaki Prefecture)

Photo 6-39 Harabun landslide following the completionof mitigation measures (courtesy of Nagasaki
Prefecture)

Colluvium

Weathered mudstone, Sandstone
Mudstone, SandstoneRoad

Profile following
mitigation measures
Profile following the sliding
Anchors

Road
Horizontal gravity drains

5

Piles
10m

Fig. 6-25 Cross section of Harabun landslide

60

10m

5

0

6.27 CHUJUN LANDSLIDE(1) Location : Chujun, Kitanakagusuku-son, Nakagami-Gun, Okinawa (26°18’ N, 127°47’ E)(2) Size : Length : 150m ; Width : 120m ; Thickness : 18m ;Volume : 2.2 x 105m3 (for the main block)This landslide is divided into four blocks : the largest Main
Block ; a Small Block located below the Main Block ; Side BlockI and Side Block-II located upslope of the Main Block.
(3) History and Damages : A total of 220mm of rain fell between October 4 and 5, 1998. The slide movement increased on October 5 and reached a total cumulative distance of 10m. The damages caused by the movement included ; destruction of one home,partial destruction of two homes and 90m of roadway, and nearcomplete destruction of a park facility. The toe of the slide reached
the community center and partially damaged the building. Fortunately, no one was injured.
(4) Geology, Mode of Movement : The site area is underlain

by Upper Tertiary mudstone and sandstone. This mudstone is highly
susceptible to weathering and easily turns into clay. Okinawa regions underlain by this mudstone are prone to landsliding. It is determined that the landslide was triggered by a rapid increase in porewater pressure from the heavy rain and infiltrated into this highly
weathered and weak mudstone formation. The morphology of thelandslide is an amphitheatre-shaped crown and tangue-shaped toewith a keel-shaped slip surface.
(5) Mitigation Measures : The mitigation measures have been
implemented as an emergency work. Since the increased groundwater table was the cause of sliding, groundwater control measures are
the focus of the mitigation measures in conjunction with landslide
restraint measures for each block. For the Main Block, 50.8cm diameter steel piles were driven ; for the Small Block, 50cm diameter
steel piles with anchors were installed ; for Side Blocks-I and II,
anchors were installed. Groundwater control measures included numerous horizontal gravity drains positioned in such a way as tomaximize the efficiency of surface drainage.

Clayey soil
Grating cribwith spraying

Moderately weathered rocks
Slightly weathered rocks
Slip surface

Original ground surface

Location of
the roadfollowing the slide

Original locationof the road

Anchors
house

Piles

Community
center

50m

25

50m

25

0

Fig. 6-26 Cross section of Chujun landslide

Photo 6-40 Chujun landslide (courtesy of Okinawa Prefecture)61

7. PROFESSIONAL ORGANIZATIONS ENGAGING IN LANDSLIDE STUDY AND MITIGATION
7.1 JAPAN LANDSLIDE SOCIETYThis academic Society was established in 1964, with a current
membership of about 2,500 composed mostly of scientists and engineers working in the fields of geology, geomorphology, geophysics,
civil engineering, sabo engineering (erosion control engineering),
forestry, agricultural engineering, and other fields concerning landslides and related phenomena. Many of the members are affiliatedwith universities, research institutes, public agencies, consulting
firms and other private sectors.
The journal JISUBERI” (Landslide) is published quarterly withover 147 published issues by the end of 2001. The journal contains
predominantly research papers printed mostly in Japanese with English subtitles, abstracts and captions.
Annual meetings attract about 1,200 participants and over 100
papers are presented. In addition, the Society and its sections sponsor symposia and workshops in various cities annually throughout
Japan.
As for international activities, the International Division was established and has organized and sponsored numerous international
events. The society organized the first International Symposium on
Landslide (ISSMGE-TC11 ISL) in 1972 in Kyoto. The symposium
is held every four years. The 2nd International symposium was organized in Tokyo in 1977, with the 3rd symposium held in New
Delhi in 1980. The last symposium was held in UK in 2000. The
Society also joined and organized the Landslide Field Conference”
with American colleagues in 1979 and 1983 (in U.S.A.), and in1980 and 1985 (in Japan) to exchange experiences, and the conference developed into an organization of International Conference andField Workshop on Landslides (ICFL) that is supported by theUnited Nations, the U. S. National Research Council and other organizations. Over 200 professionals participated in the 4th International Conference held in 1985 in Tokyo following the field workshops throughout Japan, and 31 papers were published in proceedings. This success resulted in the 5th ICFL in Australia and New
Zealand, 1987 ; the 6th ICFL in Switzerland, Austria and Italy,
1990 ; and the 7th ICFL in Czech and Slovak Republics, 1993 ; the
8th ICFL in Spain, 1996 ; and 9th ICFL in England, 1999. The Society has jointly organized many International meetings on landslides
with Asian countries.
The annual International Newsletter Landslide News” has been
published in English by the Japan Landslide Society to promote theexchange and opinions on landslides on a worldwide basis since1987. The latest issue (No. 13) was published in June 2000. Eachissue is about 40 pages and contains Recent Landslides/Research”,
IGCP-425 : Landslide Hazard Assessment and Cultural Heritage”,
Landslide Research Organization”, and News/Reports of Meetings,
Information.” International editorial board review and select papers
for publication. About 2,000 copies of Landslide News” are distributed free of charge to overseas landslide researchers, government
officials, and consulting firms. The Landslide News” and its Japanese translation are also distributed to the members of Japan Landslide Society. The first ten issues of Landslide News” were consolidated into a single volume titled Landslides of the World” andpublished at Kyoto University Press in 2000. All issues of theLandslide News” can be reviewed at the Homepage of the society.
If anyone wishes to inquire or learn more about the society, or to beon the Landslide News” mailing list, requests should be directed to:62

Japan Landslide Society
President :Hiroyuki Nakamura
Address:
Kaga Bldg., 5-30-7, Shinbashi, Minato-ku, Tokyo
105-0004 Japan
Phone :
81-3-3432-1878
Fax :81-3-5408-5250
E-mail :office@landslide-soc.org
Home Page : http : //japan.landslide-soc.org/index-e.html

7.2 NATIONAL CONFERENCE OF LANDSLIDE CONTROLThe Conference is composed of the governors of the 47 Prefectures of Japan. The purpose of this conference is to contribute thetechnical improvement of landslide mitigation measures for engineers working for the local governments through annual workshops
and presentations on investigation and research.
Chairman : Governor of Niigata Prefecture
Address : c/o Sabo Section, Department of Public Works,
Niigata Prefectural Government
4-1, Shinko-cho, Niigata 950-8570 Japan
Phone :
81-25-285-5511

7.3 FOREST CONSERVATION ENGINEERING RESEARCH ASSOCIATION
The association was established in 1956, and its members include forest conservation engineers, academics, public and privateorganizations and industries engaging in forest conservation and related disciplines. The purpose of the association is to advance theforest conservation technology and to develop and increase businessactivities through research and development in forest conservation
technology and exchange of technical information.
The chairman of the association is the Director of Forest Conservation Department of the Forestry Agency, and consists of eightchapters with 7500 members. The association publishes the monthly
journal CHISAN” (forest Conservation) and holds annual meetings
for the presentation of research papers. In addition, each chapter
sponsors various activities.
President : Director, Conservation Division, Private Forest
Department, Forestry Agency
Address : 1-2-1 Kasumigaseki, Chiyoda-Ku, Tokyo,
100-8952 Japan
Phone :
81-3-3502-8111

7.4 JAPAN ASSOCIATION OF LANDSLIDE CONTROL TECHNIQUESThe Association was organized in 1974 by consultants and contractors engaged in landslide mitigation works and the manufacturers and dealers of materials, machines and instruments related to themitigation works. The Association publishes a technical journal
Landslide Control Techniques”, sponsors lecture meetings, and isincreasing their efforts of mitigating landslide techniques in order tostudy new control methods and develop new instruments.

President :Address :Shigenobu Sakano
Kaga Bldg., 5-30-7, Shinbashi, Minato-Ku, Tokyo
105-0004 Japan
Phone :
81-3-3438-0493
Fax :81-3-3438-0803
Home Page : http : //www.jisuberi-kyokai.or.jp/

7.5 UNIVERSITIES AND COLLEGESAcademic studies of landslides are being conducted by researchers in the universities and colleges individually or jointly.

7.6 PUBLIC AGENCIES
1) Ministry of Agriculture, Forestry and Fisheries ;Forestry Agency, Ministry of A. F. and F :Home Page : http : //www.maff.go.jp/eindex.html
conducts investigations and implements the landslide mitigation measures related to the conservation of farm and forest land.
2) Ministry of Land, Infrastructure and Transport :Home Page : http : //www.mlit.go.jp/english/index.html
The Ministry overseas the National Institute for Land andInfrastructure Management, Geographic Survey Institute andPublic works research Institute, and conducts investigations andimplements landslide mitigation measures on slope failures andlandslides especially along rivers, highways, dams and residential areas.
3) Japanese Highway Public Corporation :Home Page : http : //www.jhnet.go.jp/english_new/index.html
conducts investigations and implements landslide mitigation
measures along freeways.
4) Water Resources Development Public Corporation :Home Page: http://www.water.go.jp/html/index.html (in Japanese)
conducts investigations and implements landslide mitigation
measures on landslides caused by construction of dams.
5) Local Governments :conducts investigations and implements landslide mitigation
measures on landslides under the jurisdiction of their respective
local governments.

11) Fukuda Geological InstituteHome Page : http : //www.fgi.or.jp/index-e.html
12) Central Research Institute of Electric Power Industry
Home Page : http : //criepi.denken.or.jp/eng/

7.8 OTHER PROFESSIONAL ORGANIZATIONS
1) Japan Society of Erosion Control Engineering
Home Page : http : //www.jsece.or.jp/indexe.html
2) Japanese Geotechnical SocietyHome Page : http : //www.jiban.or.jp/
3) Japan Society of Civil Engineers
Home Page : http : //www.jsce.or.jp/e/index.html
4) Geological Society of Japan
Home Page : http : //wwwsoc.nii.ac.jp/gsj/index-e.html
5) Japan Society of Engineering Geology
Home Page : http : //wwwsoc.nii.ac.jp/jseg/ (in Japanese)
6) Japanese Forestry SocietyHome Page: http://wwwsoc.nii.ac.jp/jfs/index.html (in Japanese)
http : //wwwsoc.nii.ac.jp/jfs/jfr.html (Journal of Forest Research)
7) Association of Japanese GeographersHome Page : http : //wwwsoc.nii.ac.jp/ajg/english/xindexe.html
8) Japanese Society of Irrigation, Drainage and Reclamation Engineering
Home Page :http : //www.jsidre.or.jp/english/brochure/eibunpanf.htm
9) Japanese Geomorphological Union
Home Page : http : //wwwsoc.nii.ac.jp/jgu/homee.html
10) Landslide Research Council of Japan
Home Page :http : //landslide.dpri.kyoto-u.ac.jp/j-LRCJ.htm (in Japanese)
11) International Consortium on LandslideHome Page :http : //landslide.dpri.kyoto-u.ac.jp/ICL-e.htm

7.7 RESEARCH INSTITUTES
1) National Institute for Land and Infrastructure Management
Home Page : http : //www.nilim.go.jp/english/eindex.htm
2) Geographical Survey InstituteHome Page : http : //www.gsi.go.jp/ENGLISH/
3) National Research Institute for Earth Science and Disaster Prevention
Home Page : http : //www.bosai.go.jp/index.html
http : //lsweb1.ess.bosai.go.jp/ (landslide maps)
4) National Institute of Rural Engineering
Home Page : http : //www.nkk.affrc.go.jp/index-e.htm5) Forestry and Forest Products Research InstituteHome Page : http : //www.ffpri.affrc.go.jp/
6) Public Works Research InstituteHome Page : http : //www.pwri.go.jp/english/eindex.htm
7) Civil Engineering Research Institute of Hokkaido
Home Page : http : //www.ceri.go.jp/index_e.html
8) Geological Survey of Hokkaido
Home Page : http : //www.gsh.pref.hokkaido.jp/e_index.html
9) Railway Technical Research InstituteHome Page : http : //www.rtri.or.jp/index.html
10) Sabo Technical Center (STC)
Home Page : http : //www.stc.or.jp/ (in Japanese)63

INDEX MAP OF RECENT LANDSLIDES ANDSLOPE FAILURES ILLUSTRATED IN THIS BOOK

(6.1)Jozankei Usubetsu-Gawa
Hokkaido(6.2)Daini Shiraito Tunnel

Sapporo(6.3)Oirase(6.5)Sumikawa(6.6)Hirane(6.4)Yamadate(6.7)Dozangawa(6.11)Sorakumashinden (6.13)Kuwanokidaira(6.12)(6.14)Kurashita
Kinyuba
Niigata(6.15)Shiomoto(6.18)Jinnosukedani

Sendai(6.8)Yuzurihara(6.9)Iyogatake(6.16)Koshio(6.17)Nyuya

u
sh

n

Ho

Tokyo

(6.22)Kitakobe
NagoyaOsaka(6.26)
HarabunFukuoka(6.19)Hansai

Hiroshima
TakamatsuuShikok(6.24)Kashio
Kyushu(6.20)Sugi(6.21)Kamenose(6.10)Ohrisawa(6.23)Zentoku(6.25)Maruyama(6.27)Chujun

64