The report of the panel to review the cause of Fujinuma-ike failure was published on January 25, 2012.
JCOLD translated its summary version into English. According to the Japanese legislation of river law, thedam is defined as the water storage structure which is on a regulated river and having the height of 15
meters or greater. Fujinuma-ike was on a non-regulated river and is treated as an irrigation pond, although
it is the dam by ICOLD definition.
February 6, 2012 (correction on February 28, 2012)

Tadahiko Sakamoto, Chairman, JCOLD--------------------------------------------------------------------------------------------------------------------------------Review of the Cause of Fujinuma-ike Failure--Summary Report-January 25, 2012
Panel to evaluate the seismic stability of dams and small ponds for agricultural purposeof Fukushima Prefecture
1. Introduction
Fujinuma-ike 1 (Location: Tamukaichi, Ebana, Sukagawa city) is an irrigation reservoir situated on the rightbranch of upper Sunoko River and it taps water from the main stream of Sunoko River by head race. Thereexists an auxiliary embankment dam on the right abutment saddle. The construction of Fujinuma dam,auxiliary dam and head race were started in April 1937, suspended during the World War Ⅱ

andcompleted in October, 1949. Fujinuma Dam is an earthfill dam with the height of 18.5 meter and the crest
length of 133.2 meter, while the auxiliary dam is of earth embankment type with the height of 10.5 meterand the crest length of 72.5 meter.
From 1977 to 1979, the spillway and surface protection work of Fujinuma Dam were repaired and from1984 to 1992, counter measures against leakage by grouting as well as upgrading of intakes were conducted.
The agricultural water taken from Fujinuma dam irrigates the land of 837 ha and the dam is managed andoperated by Ebanagawa Irrigation District.
Fujinuma Dam failed due to the 2011 Tohoku Earthquake (14: 46hrs. M=9.0) and large mount of released
water reached the downstream community causing the loss of seven lives and one missing. And due to theearthquake, some 750 of other small embankment dams and irrigation ponds were damaged in theFukushima Prefecture.
Therefore, the prefectural government set up "the panel to evaluate the seismic stability of dams and small
ponds for agricultural purpose" consisting of experts with relevant knowledge and experience on August 4,
2011. The panel also reviewed the cause of the failure of Fujinuma Dam which incurred especially severe
damage.
The first panel meeting was held on August 4 and 5, 2011 and up until January 25, 2012 on which day thefifth meeting was held, the panel had continuously made its efforts to investigate and find what had
happened to the Fujinuma dam and the auxiliary dam, seismic stability before the earthquake andmechanism of the failure due to earthquake shaking.
This report describes findings of the panel on the cause of the failure.
1"ike" means "pond" in Japanese.

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2.

Mapping the debris of Fujinuma main dam and auxiliary embankment after the quake

The mapping of debris of breached Fujinuma dam and damaged auxiliary dam confirmed the followingfindings.
(1) Most of upper part of embankment was washed away and most of middle and lower part of downstream
embankment was also washed away.
(2) From the movement of the structural elements, firstly, masonry riprap of upper part of embankment fellto the reservoir and subsequently the surface protection work from the middle to the right abutment moved
substantially to the reservoir (Figure 1).
(3) It was confirmed that slides occurred in the direction both of the reservoir and downstream judging fromthe distribution of main scarps, sliding surfaces and moved layers in Fujinuma Dam (Figure 2).
(4) In the auxiliary dam, the main slide with the width of 55 meter, length of 25 meter and depth of morethan 3 meter occurred and the secondary slide happened at its front (Figure 3).

Reservoir side

Left abutment

Right abutment

Downstream

Figure 1. Mapping of structural elements after failure
Notes: The dot colored blue is the parapet wall. The dot colored green is masonry riprap. The small squarecolored purple is the surface protection work. The rectangular colored yellow is concrete beam on the upper
upstream slope. Most structural elements moved to upstream.
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Reservoir side

Left abutment

Right abutment

Downstream

Figure 2. Possible slides estimated from geologic and topographic mapping
Notes: Encircled lines indicate the slide block.

.
Reservoir side

Left abutmentRight abutment

Downstream

Figure 3. Damage of structure and slides of auxiliary dam
Notes: The area colored blue is concrete parapet wall. The small square colored purple is concrete block forsurface protection work3

3.

Investigations, evaluation and review of the stability

The following investigations, evaluation and review of the stability were performed to identify the cause ofthe failure.
a. Field investigations and laboratory tests to recognize the zoning and properties of embankment materials
b. Investigations and evaluation of construction
c. Evaluation of the repair work
d. Evaluation of the safety of the dam by inspection before the earthquake
f. The stability evaluation based on the current design criteria
The results of investigations, evaluation and review follow;[Field investigations and laboratory tests to recognize the zoning and properties of embankment materials]
The results of field investigations including core boring revealed that Fujinuma dam consisted of threedifferent zones and in the middle and lower zones, construction on a layer-by-layer basis with the thicknessof 20 to 30 cm was conducted. The upper zone consisted of sand rich materials as a whole and lacks clear
compaction layer of placement.
[Investigations and evaluation of construction]
Hearings and evaluation of field investigations suggested that the dam was built by the normal construction
method and the state-of the-art at that time.
However, it is considered that the upper zone was constructed at the period of immediately after the war
when the construction environment was poor.
[Evaluation of the repair work]
It is considered that the repair work employed at Fujinuma Dam was selected from the normal methods ofconstruction and was effective to reduce the amount of leakage judging from the measurement of leakage
and phreatic surface.
[Evaluation of the safety of the dam by inspection before the earthquake]
Unusual behavior was not found by the ordinary and regular inspection of the owner of Fujinuma Dam.
Therefore, within the scope of ordinary and regular inspection, it is considered that the dam was stable.
[The stability evaluation based on the current design criteria]
According to the criteria based method of stability analysis, the factor of safety is 1.15 which is less than
the required value of 1.2 of the criteria when the seismic force is applied to the upstream direction. Thepanel regards that the dam was not in such condition as required any special measures.

4.

Mechanism of failure of the dam

Summing up the investigation results, slides are broadly classified to seven stages (Figure 4). Among theseslides, the upstream slide No.1 and No. 2 triggered the subsequent overflow and erosion and resulted in a
failure of the dam. The slide No.1 occurred in the upper embankment and it was recognized from theremnant of the structural elements in the reservoir and sliding surface was lost by wash-out. However, theearthquake deformation analysis (by modified Newmark procedure) considering the strength reduction due
to cyclic loading indicated the existence of this slide.
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There may be some possible processes of breach (initiation of overflow) such as the case of the slide No.1,
No.2 combined with the secondary downstream slide and the case of the downstream slide caused by thereduction of water tightness in the upper embankment and others.
Considering the loss of upper embankment in the large areas in a short period, it was possible that theseprocesses were combined resulting in a cause of overflow hence accelerated the velocity of erosion of thedam.

Reservoir side
石積み等の末端部へのす べ
すべり

Right abutment

石積み等のすべり
すべり
すべり

すべり

石積み等のすべり

Left abutment

す べ
すべり
すべり

すべり４

上部盛土内の下流側す
すべり４

すべり

すべり

すべり

Downstream

Figure 4. Integrated map of slides of Fujinuma main dam
Notes: The solid lines are existing slide bodies. The broken lines are estimated slide bodies which were lostby wash-out.

5. The cause of the failure of Fujinuma Dam
The panel considers that the primary cause of the failure of Fujinuma Dam is the nature of its upper
embankment and middle embankment and the triggering cause is the strong earthquake motion and its longduration.
In addition, the panel obtained the following findings from integrating the results of field investigations,laboratory tests and analytical procedures.
(1) The earthquake response analysis suggests that the peak acceleration at the dam crest is 442 cm/s2 andthe duration of the motion over 50 cm/s2 continued for 100 seconds which was an earthquake motion neverhad been experienced.
5(2) The compaction of the embankment was low compared to the compaction of the modern construction
method and the strength of the embankment is small in undrained condition during earthquakes. Especiallyin the upper embankment consisting of sand rich materials there was a saturated portion and it was proved
the strength loss occurs when it is subjected to the earthquake motion such as that of Tohoku Earthquake.
(3) The slide occurred in the auxiliary embankment consisting of similar material to upper embankment of
main dam and one of its failure causes is the nature of the soil of embankment, rich with sandy materials.
(4) In the slide occurred in the auxiliary dam, sliding surface was restricted to the embankment boundary ofthe different construction periods. In the main dam, the difference of the degree of compaction depending on
the different construction periods is possibly attributable to the occurrence of the sliding.

6.

Final remarks

In order to identify the cause of the failure of Fujinuma Dam the panel grasped the geotechnical propertiesof embankment by investigations of the remnant embankment and evaluated the construction and repair
works and examined the stability of the embankment before the earthquake.
The panel inferred the failure process based on the damage and movement of the structure and locations ofembankment slides from field investigations and estimated the cause of the failure by detailed analysis.
Through this verification, it has been proven that, among old earthfill dam and small irrigation ponds, even
if the dam was constructed by the normal method and the-state-of-the-art in that period, and, further, any
abnormality had not been detected by regular inspection, the possibility of incurring failure still exists if andwhen it is exposed to strong earthquake motion such as that of Tohoku Earthquake, depending upon its
construction material and degree of compaction.
The panel hopes the findings obtained from reviewing the cause of the failure of Fujinuma Dam willcontribute to upgrading the evaluation technology of seismic safety of dams and small irrigation ponds andis happy if they help to enhance the safe and secure society for the prefectural people.

List of members of the panel to evaluate the seismic stability of dams and small ponds for agricultural
purposePosition

NameOrganization

Chairman

Tadatsugu Tanaka

President, JARUS

MemberFumio Tatsuoka

Professor, Tokyo University of Science

MemberYoshiyuki Mohri

Director, National Agriculture and Food Research Organization-------------------------------------------------------------------------------------------Original report: http://wwwcms.pref.fukushima.jp/download/1/nosonkeikaku_kensyo_houkoku1.pdfTranslation by Japan Commission on Large Dams (N. Matsumoto)6