10th Asian Regional Conference of IAEG (2015)

Features of slope disasters on roads caused by the 2011 off the Pacific Coast of Tohoku
Earthquake
Ken-ichi ASAI(1), Koji MIYAMOTO(2) and Yasuhito SASAKI(1)(1) Public Works Research Institute, Japan
E-mail:asai@pwri.go.jp(2) Public Works Research Institute, Japan (Present: Oyo Corporation, Japan)
Abstract
In the 2011 off the Pacific Coast of Tohoku Earthquake, many slope disasters
occurred on roads. For the road slope disaster prevention in the future, we collected andanalyzed 168 cases of road slope disasters by this earthquake. In these 168 cases, failures ofembankment or road shoulder occurred most (46%), rock slope failures (large scalerockfalls) occurred to the second much (28%), and other cases were rockfalls, soil slope
failures (shallow landslides), deep-seated landslides etc. About the volume of collapsed
mass, 39% of the cases were within 100m3, 37% of the cases were 100m3 to 1,000m3, 24%
were over 1,000m3. Deep-seated landslide cases, particularly, contain large scale cases over
10,000m3 in volume. JMA seismic intensity near the each site was 5 upper and over in 99%of the cases, and 6 lower in 65% of the cases. Rock slope failures occurred in the area ofsedimentary and metamorphic rocks of Paleozoic to Mesozoic age, and 3/4 of these cases
occurred on the ridge slope. Deep-seated landslides occurred in the area of the Neogene
sedimentary and volcanic rocks, including many cases whose geological structure dip
concordant to the slope, and cases occurred in the part of old landslide topography. In thearea of granites, various type of disasters including rock slope failures, rockfalls, soil slope
failures etc. occurred. Many of failures of embankment or road shoulder occurred at the sitewhere water is likely to permeate such as embankment filling the valley. Moreover, past
deformations or disasters have been recorded in 10% of the cases in this study. It needs toinspect and examine countermeasures against these sites particularly.
Keywords: road, slope, disaster, earthquake
1.

Introduction
We could collect 168 cases of road slope disasters
occurred in Iwate, Miyagi, Fukushima and Ibaraki
Prefectures, northeast part of Honshu Island of Japan(Fig.1). These disasters were caused by the main
shock on 11th March 2011 and aftershocks including
aftershock on 11th April forming earthquake faults inFukushima Prefecture. Fig.2 shows the percentage ofthe disaster types. In these 168 cases, failures ofembankment or road shoulder occurred most (46%),
rock slope failures (large scale rock fall) occurred tothe second much (28%), and other cases wererockfalls, soil slope failures (shallow landslides),deep-seated landslides etc.
Fig.3 shows the distribution of the volume offailed mass. About the volume of failed mass, 39% of
the cases were within 100m3, 37% of the cases were
100m3 to 1,000m3, 24% were over 1,000m3.
Deep-seated landslide cases, particularly, containlarge scale cases over 10,000m3 in volume.

The 2011 off the Pacific Coast of Tohoku
Earthquake caused not only tsunami disasters and an
accident of the atomic power plant, but also manyslope disasters on roads in the Tohoku District ofJapan. These road slope disasters cut the road
network at various points, and obstructed humanactivity. It is important to reduce such road slope
disasters for improving reliability of road network.
Scientific approach is needed for the road slope
disaster prevention in the future, and analyzing pastdisasters is one of the bases for this approach.
Therefore, we collected and analyzed road slope
disaster cases by this earthquake to reveal features ofthe disasters and notice points for road slope disasterprevention (Miyamoto et.al., 2012, 2013 and 2014;
Asai et.al., 2014).
2.

General features of the disasters

1

10th Asian Regional Conference of IAEG (2015)

Fig. 3 Volume of failed mass
Fig. 1 Map of the study area
(red area in the map)

Fig. 4 JMA seismic intensity near the disastersite (Legend: same as Fig.2)

Fig. 2 Percentage of disaster typeFig.4 and Fig.5 show the relationship between theoccurrence of the disasters and seismic intensity.
JMA (Japan Meteorological Agency) seismic
intensity near the each disaster site was 5 upper andover in 99% of the cases, and 6 lower in 65% of thecases as shown in Fig.4. Maximum acceleration near
the each disaster site was 400 gal and over in 92% of
the cases as shown in Fig.5.
Fig.6 shows the relationship between theoccurrence of the disasters and slope shape. Fig.7
shows the relationship between the occurrence of thedisasters and basement geology. Rock slope failures
mainly occurred in the area of sedimentary andmetamorphic rocks of Paleozoic to Mesozoic age asshown in Fig.7, and 3/4 of these cases occurred onridge slopes as shown in Fig.6. 28% of the rock slope
failures occurred by the aftershock on 11th April in
the Mesozoic schist area in Fukushima Prefecture.

Fig. 5 Maximum acceleration near the disastersite (Legend: same as Fig.3)
Deep-seated landslides mainly occurred in thearea of the Neogene sedimentary and volcanic rocks
as shown in Fig.7. In many of these cases, geologicalstructure dip concordant to the slope. Some of thecases occurred in the part of old landslide
topography.
In the area of granites, various type of disasters
including rock slope failures, rockfalls, soil slope
failures etc. occurred as shown in Fig.7. Many joints

210th Asian Regional Conference of IAEG (2015)

generally exist in the granites, and opening of joints
effects the occurrence of rock slope failures androckfalls. And granites are often weathered severely

to become soil what is called ‘masa’ in Japan, which
effects the occurrence of soil slope failures.
Fig.8 shows the percentage of the cases in whichpast deformations or disasters have been recorded ornot. Past deformations or disasters have beenrecorded in 10% of the cases in this study. In othercases, records of deformations or disasters areunknown. (It does not always mean that nodeformations or disasters have been exist.)3.

Typical cases of main disaster types

3.1 Rock slope failure
Photos 1 and 2 show a case of rock slope failure
on a ridge slope, about 600m3 in volume, caused by
the aftershock on 11th April in Fukushima Prefecture.
Basement geology is the Mesozoic schist. Open jointsof reverse dip exist as shown in Photo 2, which

Fig. 8 Percentage of cases whichpast deformations or disasters havebeen recorded or not

Fig. 6 Slope form at the disaster site
(Legend: same as Fig.3)

Fig. 7 Basement geology at the disaster site310th Asian Regional Conference of IAEG (2015)

suggests toppling movement of the slope before thefailure.
Photos 3 and 4 show another case of rock slope
failure on a ridge slope, about 3,000m3 in volume,caused by the same aftershock. Basement geology isthe Mesozoic granodiorite. Joints of dip slope exist as
shown in Photo 4, which effected the occurrence ofthe failure.

road.
3.4 Deep-seated landslide
Photo 11, 12 and 13 show a case of deep-seatedlandslide, about 775,000m3 in volume, caused by theaftershock on 11th April in Fukushima Prefecture.
Basement geology is the Miocene sandstone andsiltstone. Its slip surface dips 10 degree, which isconcordant to the dip of the bedding plane. No oldlandslide topography was recognized in this site.
Photo 14 and 15 show another case of deep-seatedlandslide, about 65,000m3 in volume, caused by thesame aftershock. Basement geology is the Mesozoicschist, but landslide mass consists mainly ofweathered soil as shown in Photo 15. This landslide
occurred in the side part of old landslide topography
as shown in Fig.9.

3.2 Rockfall
Photo 5 and 6 show a case of rockfall caused by
the main shock. Basement geology is the Paleozoicslate. Several rocks, a maximum diameter of 1m, fell
from a loosen rock slope, broke the rock fence andreached the road.
Photo 7 and 8 show another case of rockfall.
Basement geology is the Mesozoic granodioraite.
One of the corestones, a maximum diameter of 0.6m,remained from weathering fell down to the road.
3.3 Soil slope failure (shallow landslide)
Photo 9 and 10 show a case of soil slope failure
on a cut slope, about 1,100m3 in volume and 1.5m inthickness, caused by the main shock. Basementgeology is the Mesozoic granite. Surface soil ‘masa’formed by weathering of granite fell down to the

3.5 Failure of embankment or road shoulder
Photo 16 and 17 show a case of embankment
failure, about 900m3 in volume, caused by theaftershock on 11th April in Fukushima Prefecture.
This embankment was constructed filling a valley,
maximum height of 20m. In the case of theembankment filling the valley like this, it is thoughtthat ground water is likely to gather, resulting in the

Photo 1 A case of rock slope failure in theMesozoic schist area

Photo 3 A case of rock slope failure in theMesozoic granodaioraite area

Photo 2 Open joints in the rock slope at the failureshown in Photo 1 (white broken lines)

Photo 4 Joints of dip slope in the rock slope at thefailure shown in Photo 3 (white broken lines)4

10th Asian Regional Conference of IAEG (2015)

Photo 8 Source of the rockfall shown in Photo 7
Photo 5 A case of rockfall in thePaleozoic slate area

Photo 9 A case of soil slope failure on a cut
slope in the Mesozoic granite area

Photo 6 Source of the rockfall shown in Photo 5

Photo 10 Upper part of the failed slope
shown in Photo 9

Photo 7 A case of rockfall of a corestone
remained from weathering in the Mesozoicgranodiorite area

Photo 11 A case of deep-seated landslidein the Miocene sedimentary rock area5

10th Asian Regional Conference of IAEG (2015)

Photo 12 Depression zone at the top of thelandslide shown in Photo 11

Fig.9 Landslide Map by National ResearchInstitute for Earth Science and Disaster
Prevention, Japan
Black line: Old landslide topography
Red line (added by the authors): Newlandslide shown in Photo 14

Photo 13 Uplifted road at the toe of thelandslide shown in Photo 11

Photo 16 A case of failure of embankment
constructed filling a valley

Photo 14 A case of deep-seated landslidein the Mesozoic schist area

Catchment
area
ValleyPhoto 15 Outcrop in the lower part of thelandslide shown in Photo 14

Photo 17 Aerial photograph of the embankment
in Photo 16, taken 36 years before the disasterby Geographical Survey Institute (present:Geospatial Information Authority of Japan).
White broken line with arrow: Valley
Yellow broken line: Catchment area(Broken lines were added by the authors)

610th Asian Regional Conference of IAEG (2015)high water content in the embankment and high riskof collapse.
Photo 18, 19 and 20 show another case ofembankment failure, about 9,800m3 in volume,caused by the main shock. This embankment was apart of a pastureland near the road, which collapsed
and filled the road. This pastureland looks like agentle slope now. However, as shown in Photo 20, anaerial photograph taken 35 years before the disasterrevealed that the pastureland was constructed byfilling the valleys, and the failed embankment was
just situated in one of the filled valleys.

4.

Notice points
prevention

forroadslope

disasterFor road slope disaster prevention, it needs toinspect and examine countermeasures against thesesites particularly. Notice points for the disasterprevention are suggested as follows.
1) The ridge slope which is loosen and joints or
cracks have been opened is easy to be deformed or
fail by earthquake.
2) Embankment slopes filling up the valley are weak
points against disaster, which are likely to be
deformed or fail by earthquake.
3) For finding the hazardous site of deep-seatedlandslide by the earthquake, especially the sitewhich does not show landslide topography and thegeological structure of which dip concordant to theslope, regional investigation of topography andgeological structure is important.
4) The embankment filling up the valley on the slope
near the road cannot always be found only by the3.6 Other cases
Photo 21 shows a case of the collapse of masonrywall on the embankment slope near the road. Manystones were scattered on the road. Many of masonrywalls constructed in old age, and have a steep dip.
They are likely to fail or deform by the strong
earthquake.

Valley（shallow）

Valley（deep）

Photo 18 A case of failure ofembankment which filled a road
Photo 20 Aerial photograph around the site in
Photo 18, taken 35 years before the disaster, byGeographical Survey Institute (present:
Geospatial Information Authority of Japan).
This photo shows the topography before fillingthe valley. The embankment filling the deep
valley collapsed, and the embankment filling theshallow valley did not collapsed but deformed.
(Broken lines were added by the authors)

Photo 19 Upper part of embankment
failure shown in Photo 18

Photo 21 A case of failure of masonry wall near the road710th Asian Regional Conference of IAEG (2015)

investigation of present geography. It needs toinvestigate history of topographic change bydevelopment using past aerial photograph, etc.
5) Many of masonry walls are likely to fail or deformby the earthquake because they constructed in old
age and have a steep dip. It needs to find these sites
as hazardous sites against disaster.
6) The disaster is easy to occur repeatedly at the sitewhere the past disaster history exists and the causeof the disaster remains even now.
5.

meeting of Japan Society of Engineering Geology,
pp. 171-172 (in Japanese).
National Research Institute for Earth Science andDisaster Prevention (2004): Landslide Map ofTAKANUKI, Technical Note of National ResearchInstitute for Earth Science and Disaster Prevention,
No.247 (in Japanese).

Conclusions

We collected and analyzed road slope disastercases by the 2011 off the Pacific Coast of Tohoku
Earthquake for the road slope disaster prevention inthe future. We revealed the features of these disasters
and suggested notice points for the disasterprevention as mentioned above. It needs to inspectand examine countermeasures against these sites
particularly.
Acknowledgements
The authors are grateful to the staffs of Iwate,
Miyagi, Fukushima and Ibaraki Prefectures, whooffered various materials concerning the road slope
disasters caused by the 2011 earthquake. All of thephotos in this paper except for Photo 17 and 20 were
offered by them.
References
Asai, K., Miyamoto, K. and Sasaki, Y. (2014):
Features of slope disasters on roads occurred in
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Coast of Tohoku Earthquake, Proceedings of 2014
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pp. 169-170 (in Japanese).
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disasters on roads occurred in Fukushima Prefecturecase by the 2011 off the Pacific Coast of Tohoku
Earthquake and its induced earthquake, Proceedingsof 2012 meeting of Japan Society of EngineeringGeology, pp. 143-144 (in Japanese).
Miyamoto, K., Asai, K. and Sasaki, Y. (2013): Slope
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Characteristics of slope disasters on roads occurredin Iwate Prefecture case by the 2011 off the Pacific
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