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Vienne, Juillet 2018-------STUDY ON THE MECHANISM OF THE PECULIAR BEHAVIORS OF
ARATOZAWA DAM IN THE 2008 EARTHQUAKE(*)
Nario YASUDA & Norihisa MATSUMOTO
JAPAN DAM ENGINEERING CENTER
Masayoshi NARUOKA
Nuclear Power Eng. Department, ELECTRIC POWER DEVELOPMENT CO.,LTD
Zengyan CAO
Senior Engineer, JP BUSINESS SERVICE CORPORATION
JAPAN

1.

INTRODUCTION

Seismic design of embankment dams in Japan is done with the seismic
coefficient method using a constant design seismic coefficient stipulated
empirically for each region, based on the Cabinet Order concerning StructuralStandards for River Management Facilities, etc. and the Ministry of Land,
Infrastructure, Transportation and Tourism Ordinance for Structural Standard forRiver Administration Facilities. In the guideline for seismic design of embankment
dams (1991), the maximum ground seismic coefficient is stipulated as 0.15 evenin a strong seismic region, provided that a safety margin is ensured. But after The
1995 Kobe Earthquake, with the improvement of the earthquake monitoring
system, a number of earthquake records far stronger than the design seismic
coefficients of the dams have been obtained, for example, the 2000 Tottori
Earthquake, the 2004 Mid Niigata Earthquake, the 2008 Iwate-Miyagi Earthquake,and the 2011 Tohoku Earthquake. Examining the earthquake behaviors of the(*)

Étude sur le mécanisme des comportements particuliers du barraged’Aratozawa durant le tremblement de terre de 2008

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embankment dams subjected to strong earthquakes with numerical methods,
furthermore studying the deformation or damage mechanism would be extremely
beneficial for the seismic design of new dams and the seismic safety assessmentof existing dams.
Aratozawa dam is a 74.4 m high rockfill dam with central clay corecompleted in 1998. When the 2008 Iwate-Miyagi Earthquake struck, seismic
motion with the maximum acceleration 10.24 m/s2 was recorded in the dam
foundation. On the other hand, the maximum acceleration response in the stream
direction near the center of the dam crest was 5.25 m/s2. The acceleration
amplification ratio of the dam body was 0.51, which is far lower than thatrecorded previously in other rockfill dams. After the earthquake, a survey on thedam was made [1]. It was measured that the crest settled about 19.8 cm relative
to the original status about half year ago.
In this research, 3-D equivalent linear analysis is performed to simulate theearthquake behaviors of Aratozawa dam in order to clarify the generating
mechanism of aforementioned phenomenon. Based on the results of theearthquake response analysis, sliding stability analysis and cumulative damage
analysis were carried out to reason out the mechanism of the residual
deformation induced by the earthquake.

2.

2.1.

DAM SPECIFICATIONS AND EARTHQUAKE MONITORINGDAM SPECIFICATIONS

Table 1 shows the main features of Aratozawa dam. Fig. 1 shows the planview and the typical cross section of the dam body. The locations of theseismographs are also marked in the same figure. Seismographs were installedin the dam foundation (F), its middle of the center core (M), dam crest (T) and theright abutment (G) in the three directions (stream, cross stream and verticaldirections).

2.2.

RECORDED SEISMIC MOTIONS DURING THE 2008 IWATE-MIYAGIEARTHQUAKEOn June 14, 2008, the 2008 Iwate-Miyagi Earthquake (M7.2) occurred with
its epicenter only 16km away from Aratozawa dam. Fig. 6 shows the acceleration
records of the dam foundation (F), the middle of the center core (M), and the dam
crest (T). The maximum acceleration of 10.24 m/s2 was recorded in the stream
direction in the foundation bedrock. While the maximum accelerations in the報－24

same direction in the middle of the center core and the dam crest were 5.35 m/s2
and 5.25 m/s2 respectively. As the earthquake records in the dam foundation thisis the largest one recorded in Japan up to now.
By a survey [1] conducted on June 30 of the same year (i.e. 16 days after
the main shock), a settlement of about 19.8 cm at the dam crest and a residual
deformation of 4.3 cm toward the upstream side and 6.0 cm in the cross stream
direction were measured. It should be noticed that these displacements are theincrements since December 4, 2007 and include the deformation caused bymany aftershocks.
Table 1Main features of Aratozawa dam
Dam location
Dam type
Dam height
Crest length
Crest width
Crest elevation
Slope gradients
Dam volume
Basin area
Reservoir capacity
Design seismic
coefficient

Aratozawa, Miyagi Prefecture
Rockfill dam with central clay core
74.4m (Lowest ground elevation: EL205.0m)
413.7m10.0mEL.279.4m
Upstream: 1:2.7 Downstream: 1:2.1
3
3,048,000 m
2
20.4 km3
13,850,000 m
0.15 (dam body), 0.18 (intake tower, bridge)
0.16 (spillway)●

seismographs

C.M.F

10m

181m

HWS.EL276.9

EL276.4(crest)

2.5m

155m
2.5m

T
M

G1,G2

3

(1) Plan view

F27.17m

3(2) Typical cross section

Fig. 1
Locations of seismographs at Aratozawa dam
Emplacements des sismographes au barrage d’Aratozawa

3.

REPRODUCTION ANALYSIS OF THE EARTHQUAKE BEHAVIORS

報－25

3.1.

ANALYSIS PROCEDURE

In order to investigate the mechanism of the acceleration decrement in thedam body during the mentioned earthquake, an earthquake response analysis iscarried out with the 3-D equivalent linearization method. The reference shear
strain and the maximum damping coefficient are identified by adjusting thesefactors for getting the analysis results most similar to the earthquake records. The
consistency between the analysis results and the earthquake records isevaluated, focusing on the values of the maximum acceleration response,
acceleration wave forms, Fourier spectra, and transfer functions between thepositions of the seismographs. The input earthquake motions are reproduced bypulling back the recorded seismic motions of the foundation to the bottom of theanalysis model (base of bedrock) by the 3-D transfer function method, which will
be described in the next section.
Fig. 2 shows the flow chart of analysis procedure. A specialized code forthe earthquake response analysis of dams, named “Universe” is used in theanalysis. As for the mechanism of the residual deformation of the dam due toearthquake, it will be mentioned in the next chapter.

3.2.

ANALYSIS CONDITIONS

3.2.1.

Analysis model

The 3-D analysis model shown in Fig. 3 is made based on the design
drawings of the dam body and detailed topographical data of the dam site. At theboundaries around and beneath the foundation viscous boundary condition [2] isapplied, which has much better energy absorption function than the conventional
one [3]. The initial effective stress of the dam body with the reservoir water levelof 270.1 m during the earthquake is calculated with the consideration of seepage
pressure. Because of the uncertainty concerning the effects of the excessive porewater pressure in the dam it is ignored in the earthquake response analysis. i.e.,
the analysis is done with the total stress method.

3.2.2.

Material properties

The equivalent linearization method is used to consider the effects of thenonlinear properties of the embankment materials during the earthquake. The
shear strain dependency of the shear modulus and the damping coefficient are
set with the hyperbolic model of Eq. [1] and Eq. [2].

報－26

Initial shear modulus andPoisson’s ratio based on
Sawada’s equation
Fundamental frequency
identification of the dam based on
the micro-earthquake records

Fundamental-frequencies
by eigenvalue analysis
Adjustment ofshear modulus

No

Consistent with each other?
Yes
Identification of initial shear modulus
for seismic reproduction analysis
Conversion of observed seismic
motion at the foundation (F) to input
earthquake motion at the base ofmodel by transfer function method
Responses by equivalent linear analysis
1) Peak accelerations, 2) Waveforms3) Spectra, 4) Transfer functions

Adjustment ofreference shear
strainNo

Earthquake records
1) Peak accelerations
2) Waveforms3) Spectra
4) Transfer functions

Is thefundamental frequencies byanalysis consistent with that byobservation?
Yes

Adjustment of max No& initial damping
ratio

Are the peak acceleration &waveform consistent?
Yes
OutputFig. 2
Flow chart of reproduction analysis
Diagramme d’analyse de reproductionG1=
G0 1 + γ γ r
h = hmax

[1]

γ
+ h0
γ + γr[2]

where G and G0 represent the equivalent shear modulus and its initial value. h ,h0 , and hmax represent the equivalent damping coefficient, its initial value and its
maximum value respectively. γ represents the effective shear strain (2/3 of its
maximum value), and γ r represents the reference shear strain. While in the

報－27

reproduction analysis, γ r is adjusted as one of the identification parameters as
mentioned in Fig. 2.
The initial shear modulus G0 of the embankment materials in the analysis isset based on the Sawada’s equation [4]. But it is modified by eigenvalue analysis
in order that the fundamental frequency got from the analysis conforms to that
estimated according to the transfer function obtained from the past earthquake
records. It is assumed that the initial damping coefficient of the embankment
materials at the small strain level is uniformly 5%. In order to improve thereproducibility of earthquake responses, the maximum damping coefficient is
modified.
Furthermore, the material damping is evaluated as Rayleigh’s type
damping with the following formula.

[C ]e = α [M ]e + β [K ]e[3]

where [C ]e , [M ]e and [K ]e represent damping, mass and stiffness matrices ofeach finite element respectively. α and β are parameters defined by the following
formulae.

Viscous boundary

Viscous Boundary

Fig. 3
Analytical Model
Modèle analytique
α = 1.4 ⋅ h ⋅ ω1

[4]

β = 0.6 h ω1

where ω1 represents the fundamental circular frequency of the dam body.
The shear modulus of the spillway concrete is assumed to be 12,500
N/mm2, which is the general value of the dam concrete. The shear modulus of
the foundation bedrock is set to be 5,500 N/mm2, obtained by conversion fromthe shear wave velocity 1,440 m/s of the CM class rock (Index of rock property inJapan). The Poisson’s ratios of the embankment materials are set based on the
Sawada’s equation.

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The densities of the embankment materials used for the analysis are shownin Table 2, which are the material test results for the quality control of the dam
construction.The Poisson’s ratio of the spillway is 0.2 as concrete material, andthat of the foundation bedrock is set to be 0.25.
Table 2
Densities of the embankment materials, spillway and foundation bedrock3

Material category
Core
FilterTransition
Upstream rockDownstream rock
Spillway
Foundation bedrock

3.2.3.

Inner
Outer
Inner
Outer

Density(g/cm )Wet
Saturated
2.04
2.10
2.34
2.43
2.24
2.33
2.15
2.29
2.15
2.32
2.18
2.32
2.13
2.30
2.40
2.60

Input earthquake motion

In the analysis, the input earthquake motion is prepared by pulling back theearthquake records of the foundation (Point F in Fig. 4) to the bottom surface of
the analysis model using a 3-D wave transfer function method. The core of themethod is how to find the transfer function of the foundation model. The image ofthe method is shown in Fig. 4, and it can be described in the followings.

F

Earthquake record

Spectrum FF[T]
Spectrum FB

B
Input motion

Fig. 4
Reproduction of input earthquake motion
Reproduction de mouvement de tremblement de terre d’entrée
Step 1:To get the responses of the foundation (Point F)
3-D earthquake response analysis is performed with the dam – reservoir –foundation model where the water level is set at that during the earthquake(270.1m). In the analysis, the earthquake records of the foundation (Point F) areused as the input earthquake motion to the bottom boundary, but the 3 directional

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components are inputted separately. Hence, 3 sets of response results are
obtained and named as,

{
{
{

}
}
}

 R xx , Ryx , Rzx When WFx is inputted R xy , Ryy , Rzy When WFy is inputtedR , R , Ryzzz When WFz is inputted
 xz

[5]

where Rij (i , j = x , y , z ) indicates the response of Point F, and WFi (i = x , y , z )indicates the earthquake motions recorded at Point F.
Step 2:To get the input earthquake motions of bottom boundary (Point B)
The Fourier spectra FWFi (i = x , y , z ) of the input earthquake motions WFx ,WFy , WFz and those of the responses at Point F FRij (i , j = x , y , z ) are obtained,
and then the transfer functions Tij (i , j = x , y , z ) of the foundation can be got withthe following equations.
Txx =

Txy =

Txz =

FR xx

FWFx
FR xy
FWFy

FR xz

FWFz

, Tyx =

, Tyy =

, Tyz =

FR yx
FWFx
FR yy
FWFy

FR yz
FWFz

, Tzx =

, Tzy =

, Tzz =

FR zx

FWFx
FR zy
FWFy

[6]

FR zz

FWFz

Then, the Fourier spectra FWBi (i = x , y , z ) of the input earthquake motion at
the bottom boundary of the foundation are got with the Eq. [7].

FW  Txx Txy Txz  FW 
 Fx    Bx FWFy  = Tyx Tyy Tyz  FWBy 
F
 


 WFz  Tzx Tyz Tzz  FWBz [7]

By inverse Fourier transformation of the spectra FWBX , FWBY and FWBZ , theinput earthquake motions WB1 (WBx1 , WBy1 , WBz1 ) at the bottom of the foundation
model are reproduced. And the superscript 1 of WB1 indicates the first estimation.
Step 3: To improve the accuracy of the input earthquake motions
In order to improve the accuracy of the estimated input earthquake motion,
above procedure is repeated because of the nonlinearity of the embankment
materials. In the response analysis of Step 1 of the first estimation, the dam bodyis treated as linear material. Therefore, the response analysis of Step 1 isperformed again with the equivalent linearization method and by inputting the

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earthquake WBi1 (i = x , y , z ) , instead of WFi (i = x , y , z ) . Then the Step 2 is repeatedwith a more accurate of the transfer functions Tij (i , j = x , y , z ) , and a more accurate
estimation of the input earthquake motion WB2 is got and used in the earthquake
behavior reproduction analysis.

3.3.

REPRODUCTION ANALYSIS RESULTS

3.3.1. Identified reference shear strain and the maximum damping
coefficient
The reproducibility of the earthquake behavior is examined by comparing
the reproduction analysis results and the earthquake records, focusing on thevalues of the maximum acceleration responses, response wave forms, Fourier
spectra and the acceleration transfer functions of the dam body.
The reference shear strain and the maximum damping coefficient of theembankment materials are repeatedly modified for getting higher reproducibility.
As a result, in the case shown in Fig. 5 and Table 3, the earthquake response of
the dam body is thought to be consistent generally with the earthquake records.
Therefore, it is assumed that the material properties shown in Table 3 reflect thestate of the actual dam. And in general, these identified physical values conformto the results of the past laboratory material tests [5], [6], [7].
②
⑥

⑥
⑤

④

① ④

Numbers indicate thematerials listed in Table 3⑤③

Fig. 5Zoning of dam body

Zonage du corps du barrage
Table 3
Identified reference shear strain and the maximum damping coefficient
Category① Core (middle&lower)② Core (surface)③ Filter④ Transition⑤ Rock (inner)⑥ Rock (outer)

The max. damping
coefficient hmax
20%
30%
30%
23%

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Reference shear strainγr
3.0×10-4

4.0×10-4

3.3.2.

Acceleration response

Table 4 summarizes the values of the maximum acceleration and therelative displacement response of the reproduction analysis and the earthquake
records. The fundamental frequency of the dam got by the analysis andearthquake record is also listed in the table. Figs. 6(1) to 6(3) shows theacceleration response histories of each position of the seismographs. Fig. 7, as
an example, compares the Fourier spectra and the transfer functions of theacceleration responses at the dam foundation and the crest in the stream
direction.
Table 4The maximum responses of the dam body
Item

Position
Dam crest(T)Max. acceleration
2(m/s )

Middle of thecenter core(M)Foundation(F)Direction

RecordAnalysis

Stream

+5.10

-5.25

+6.80

-5.44

Cross stream

+4.55

-4.37

+6.14

-4.72

Vertical

+4.88

-6.22

+4.48

-4.72

Stream

+4.02

-5.35

+3.27

-3.04

Cross stream

+4.78

-3.80

+2.72

-2.58

Vertical

+4.46

-4.70

+3.08

-4.2

Stream+10.24

-6.73

+9.93

-5.46

Cross stream

+8.99

-7.13

+8.68

-6.24

Vertical

+6.91

-6.14

+5.61

-5.24

Fundamental frequency (stream direction (Hz))
Relative displacement of the crest to the foundation in
stream direction (cm)1.47
+11.13

1.47
-1.76

+7.72

-11.58As shown in Table 4 and Fig. 6, although there is some discrepancy in thevalues of the maximum acceleration at each position between the analysis results
and the earthquake records, the acceleration response waveforms generally
conform each other. Good reproducibility is obtained throughout the entire time
histories of the acceleration responses at the foundation and the middle of thecentral core in particular. In those of the crest, during the first 4 seconds from the
beginning both the recorded waves and those got from the analysis show almost
identical phases, or else the peaks of the recorded waves appear slightly fasterthan those of the analysis. While in the later histories, to the contrary, theresponses based on the analysis appear to transmit more quickly than therecorded ones. As the reason for this, it is thought that the stiffness of theembankment material in analysis is a constant value throughout the whole time
history (equivalent linearization method). In contrast, in the actual dam, it isthought that due to the major motion of the earthquake, the increment of theshear strain response induced the decrement of the embankment material’s
stiffness. Accordingly, the seismic motion transmission velocity gets down. From
Fig. 7, it can be found that, in the low frequency domain up to about 2 Hz, both
the Fourier spectra and the transfer functions obtained from the earthquake
records and analysis results conform closely each other. As the frequency

報－32becomes higher, the difference between records and analysis becomes large
gradually. It is presumed that the material properties, i.e., the shear modulus anddamping coefficient evaluated according to the maximum shear strain are proper
for the low frequency domain. Consequently, the same material property brings
relative large error to the response of the higher frequency components.

3.4.

DISPLACEMENT RESPONSE

Fig. 8 shows the response history of the relative displacement of the dam
crest to the foundation in the stream direction. The relative displacement obtainedby integrating the earthquake records is also shown in the same figure. The
maximum relative displacement in the stream direction is about 11 cm (halfamplitude). It is clear that relatively large displacement occurred in the dam bodyduring the earthquake. This large relative displacement is mainly the contributionof the vibration component of a period of 1 second or longer between the time of
3 seconds and 4 seconds of the acceleration records. With the displacement
response, it can be presumed that large shear strain occurred in the dam body(average shear strain is about 1.48x10-3 (relative displacement/dam height)).

Fig. 6(1)Time histories of acceleration response, Crest (T)Diagramme de temps de réponse d’accélération, créte (T)報－33

Fig. 6 (2)Time histories of acceleration response, Middle of core (M)Diagramme de temps de réponse d’accélération, milleu du noyan (M)Fig. 6 (3)Time histories of acceleration response, Foundation (F)Diagramme de temps de réponse d’accélération, fondation (F)報－34

(1) Fourier spectra(2) Transfer function
Fig. 7
Fourier spectra and transfer function
Spectre de Fourier et fonction de transfert

Fig. 8
Displacement in the stream direction of the dam crest relative to the foundation
Déplacement dans la direction du flux de la crête du barrage par rapport à lafondation

3.5.

INVESTIGATION ON THE MECHANISM OF PECULIAR ACCELERATIONRESPONSE

Fig. 9 shows the distribution of the maximum acceleration response in thestream direction obtained by reproduction analysis. The greatest feature of thisfigure is that the acceleration response is decaying sharply through the rocksurface. In addition, the amplification of the acceleration is very light near the
crest and the slope surfaces. Near the contact surface with foundation rock, thecontrast of the stiffness of the embankment materials and the foundation rock
was large. It is presumed that a large phase difference occurred between thesetwo kinds of media due to the major motion of the earthquake. As a result, large
shear strain occurred at the bottom of the rock zone (see Fig. 10). Consequently,
the shear modulus of the embankment materials greatly decreased and their報－35

damping became larger. It is presumed that the acceleration response at the
middle of the core zone was drastically attenuated due to such reasons.
Near the crest and in the slope surface layers, shear strain increased dueto the major motions of the earthquake (see Fig. 10). The embankment materialsof these parts became loose (as a result, the stiffness decreased and thedamping increased). Therefore, it is presumed that the amplification of theacceleration response to the comparatively high frequency components of theearthquake became difficult. The almost complete lack of high frequency
component after the major motions in the recorded acceleration response at thedam crest as shown in Fig. 6 also supports this speculation. It is inferred thatthese factors caused the phenomenon of the reversal relationship of theacceleration response at the dam crest and the foundation of the dam during theearthquake.
(m/s2)(1) cross section(2)Axial section

Fig. 9Maximum acceleration response distribution in the stream direction
Distribution de réponse d’accélération maximale dans la direction du flux
(×10-4)

Fig. 10Distribution of the maximum shear strain
Distribution du cisaillement maximum

4.

ANALYSIS OF THE RESIDUAL DEFORMATIONAs aforementioned, after the 2008 Iwate-Miyagi Earthquake the maximum
settlement of 19.8 cm was measured at the crest of Aratozawa dam. This fieldsurvey was conducted 16 days after the main shock, so the measured residual
deformation included the displacement by many aftershocks. Furthermore, themeasured value of 19.8 cm is an increment from December 4 of 2007, so it is報－36

impossible to accurately confirm the quantity of residual deformation caused byonly the main shock.
However, it is assumed that a high percentage of the measured residual
deformation of the dam body was caused by the main shock of the 2008 IwateMiyagi Earthquake. In this study, even if it is qualitative, the mechanism of theresidual deformation is investigated.

4.1.

ANALYSIS METHOD

The residual deformation of a rockfill dam induced by an earthquake isconsidered to include two kinds of components. One is that caused by sliding
behavior and the other is that caused by shaking subsidence of the embankment
materials during the earthquake (cumulative damage). For this study, theearthquake response of the dam body obtained by the reproduction analysis
described in the previous sections is used
(1) to calculate sliding deformation and(2) to analyze cumulative damage.
The mechanism of residual deformation of the dam body is investigated bychecking which kind the measured residual deformation is.

4.2.

SLIDING CALCULATIONThe sliding calculation of the typical cross-section of the dam body as
shown in Fig. 5 is carried out with the Watanabe and Baba method [8].
In order for the sliding arcs to cover the entire dam body, they are divided
into a case of sliding on the upstream side of the dam and a case of sliding on
the downstream side of the dam (total of 1,235 arcs). The sliding arcs on the
upstream side are further divided into those with both ends passing through only
the upstream surface (called the “U1 Group”), those with the upper end passing
through the crest and the lower end through the upstream surface (called the “U2Group”), and those with the upper end passing through the downstream surface
and the lower end through the upstream surface, i.e., cutting the core zone
(called the “U3 Group”). The sliding arcs on the downstream surface are similarly
divided into “D1 Group”, “D2 Group”, and “D3 Group”. As an example, Fig. 11(1)shows the arcs for the case of sliding on the upstream side through the crest (inother words, U2 Group).
Of the sliding arc groups, the arcs with the smallest sliding safety factors
are abstracted and summarized in Table 5. Fig. 11(2) shows the locations of thecorresponding sliding arcs. Focusing on the arcs with sliding safety factors less報－37

than 1.0, the sliding displacements are calculated as shown on Table 5. The
minimum sliding safety factor is 0.839 on the Arc No.1, a limited part of the
upstream surface. But its sliding displacement is almost zero (0.0006 cm). This
result suggests that sliding behavior did not occur during the 2008 Iwate–Miyagi
Earthquake at Aratozawa dam, and the residual deformation measured after theearthquake was not caused by sliding.

(1) setting the arc passing through the dam
crest and upstream side (U2 Group)(2) Arcs with smallest safety factors ineach arc group

Fig. 11
Sliding calculation
Calcul du glissement

Table 5
Sliding calculation results
Sliding direction
Upstream side

Downstream
side

4.3.

Arc Group

Arc No.

Safety factor

Sliding displacement (cm)

U1

1

0.839

0.0006

U2

2

2.027

－

U3

3

2.395

－

D1

4

2.581

－

D2

5

2.090

－

D3

6

2.135

－

CUMULATIVE DAMAGE ANALYSIS

It has been pointed out that subjected to intensive seismic motions, theresidual deformation of a rockfill dam may be caused by the shaking subsidenceof the embankment materials [9]. Such cumulative deformation can be simply
evaluated by assuming that it was caused by a decline of stiffness, performing
static dead-weight analysis using the shear moduli of the dam body before andafter the earthquake. Here, using the initial shear moduli of the dam body andthose obtained by the equivalent linear analysis in Section 3, the deformation
calculation based on dead-weight analysis is performed. The difference between

報－38

the deformation results after and before the earthquake is regarded as residual
deformation caused by the earthquake. In short.
[8]

U r = Ua − U bwhere, U r is the residual deformation caused by the earthquake. U a and U b are
the deformation by dead-weight analysis after and before the earthquake
respectively.
Fig.12(1) shows the calculated residual deformation of the dam body. The
contours show the quantity of settlement on the downstream side. Fig. 12(2)shows the settlement of the dam in the axial section. The maximum settlement is
6.81 cm which occurred near the right abutment. At the crest of the largest crosssection of the dam body, the settlement is about 4.26 cm. For the reason
mentioned above, the calculation results of the residual deformation cannot be
quantitatively compared with the values measured before and after theearthquake. However, based on this calculation result it can be inferred that
subjected to the main shock, shaking settlement of the embankment materials
caused the residual deformation of the dam body.
Fig. 13 compares the calculated settlement strain with the measured one
along the center of the core zone. Both the calculation and measurement show
small strain below the elevation of 245 m. Above this elevation, the settlement
strain fluctuated wildly and the value is relatively large. For this reason, it isinferred that residual deformation of the dam body after the earthquake wasprimarily due to the settlement at the higher elevation of the dam body down to adepth of 30 m from the crest. Similarly to the aforementioned residual
deformation, the measured settlement strain is also the increment since
December 4 of 2007, preventing a quantitative comparison. But, clearly, thedistribution tendency of the calculated settlement strain and the measurement
results generally conform even in the depth direction.
From the above, it is inferred that the measured residual deformation of thedam body was caused not by sliding behavior of the dam body, but by shaking
settlement of the embankment materials.
(cm)

Settlement:4.26cm(1) downstream side(2) axis section

Fig. 12Residual deformation
Déformation résiduelle

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6.81cmFig. 13Comparison on settlement strain (period: Dec.4, 2007 to June 17, 2008)
Comparaison sur la contrainte de règlement (période: du 4 décembre 2007 au 17
au juin 2008)

5.

CONCLUSION

From above study, following conclusions can be drawn.
The mechanism that caused the peculiar damping phenomenon(amplification ratio: 0.5) of the acceleration response in Aratozawa dam during
the 2008 Iwate-Miyagi Earthquake is examined. Subjected to the major motionsof the earthquake, large shear strain occurred near the rock contact surface.
Such shear strain response reduced the stiffness, at the same time, increased
the damping of the embankment materials. This caused the seismic motions toattenuate immediately near the rock contact surface. Furthermore, near the crestand the slope surface layers, the major motions of the earthquake caused theembankment materials to loosen. This variation restrained the response to thehigh frequency components of the earthquake motions. Therefore, theamplification of the acceleration at the crest and the slope surface is very small. It
is drawn that these two points caused the reversal of the magnitude of theacceleration response at the crest and the foundation bedrock during thisearthquake.
The generating mechanism of residual deformation of the dam bodycaused by the 2008 Iwate-Miyagi Earthquake is investigated by sliding calculationand the cumulative damage analysis. It is confirmed that sliding phenomenon did
not occur during this earthquake. It is also inferred that the residual deformation
after the earthquake was caused mainly by shaking settlement of theembankment materials. The settlement strain is large to the depth of about 30m
from the crest, and it is deduced that the measured residual deformation wascaused primarily by the settlement of the upper part of the dam body.

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ACKNOWLEDGEMENTS
The earthquake records provided by Japan Commission on Large Dams
(JCOLD) for this research are gratefully acknowledged. The authors wish toacknowledge the Miyagi Prefecture for preparing the materials of Aratozawa dam.
This research was also guided by advice given by Dr. Sasaki of the River
Department of the National Institute for Land and Infrastructure Management,
and by Mr. Sato of the Hydraulic Engineering Research Group of the Public
Works Research Institute. And Mr. Sato and Mr. Asaka of the Social &Environmental Engg. Dept., JP Business Service Corporation provided generous
advice and assistance with the reproduction analysis of the earthquake behavior.
The authors are deeply grateful to them for their contribution.

REFERENCES[1] SHIMAMOTO K., SATO N., OHMACHI T., KAWASAKI H., IWAI S. “Reporton survey of damage to dams by the 2008 Iwate – Miyagi Earthquake”, 2009
(in Japanese)[2] MIURA F., OKINAKA H. Dynamic Analysis Method for 3-D Soil-StructureInteraction Systems with the Viscous Boundary Based on the Principle ofVirtual Work, Proc. of JSCE, No.404/I-11, April, 1989 (in Japanese)[3] LYSMER, J., KUHLEMEYER, R. L. Finite dynamic model for infinite media,
Proc. ASCE, EM 4, pp.859-877, 1969[4] SAWADA, Y., TAKAHASHI, T. Study on the material properties and theearthquake behaviors of rockfill dams, Proc. of the 4th Japan Earthquake
Engineering Symposium, pp.695-702, 1975
[5] YASUDA M., OGATA N., SHIMADA, M. Dynamic deformation properties ofcore materials according to vibration triaxial testing, Proc. of the 14th
Conference on Soil Engineering Research, 1980
[6] BABA, K., WATANABE, H. On a Consideration for an Earthquake-resistant
Design Method for Rockfill Dams, 13th Congress on Large Dams, New Delhi,
Q51, R15, pp1049-1074, 1979.
[7] MATSUMOTO T., YASUDA N., SAKAINO N. Dynamic property testing ofcoarse-grain materials using a large cyclic triaxial tester, Civil Engineering
Research Documents of the Ministry of Construction, No. 2146, 1984
[8] WATANABE, H., BABA, K. A study on sliding stability evaluation method
based on dynamic analysis of fill dams, Large Dams, No.97, pp.25-38, 1981
(in Japanese)[9] Committee on the strengthening technology of Murayama dam and reservoir.
Murayama Reservoir Maintenance Committee Report., 2002

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SUMMARY
During the 2008 Iwate-Miyagi Earthquake (M7.2) of June 14, 2008, seismic
motions with the maximum acceleration of 10.24 cm/s2 in the stream direction
were recorded at the foundation bedrock of Aratozawa dam, a rockfill dam
located approximately 16km from the epicenter. However, the maximum
response acceleration in the same direction near the center of the dam crest was
5.25 m/s2, and the acceleration amplification ratio of the dam body was far lower
than that normally considered for a rockfill dam. Furthermore, it was measured
that the crest settled down 19.8 cm after the earthquake. In this study the
dynamical properties of the embankment materials have been identified with thereproduction analysis of the past earthquakes, and the recorded behaviors of thedam body during the mentioned strong earthquake have been simulated. The
generating mechanism of the peculiar earthquake behavior has been
investigated based on the results of earthquake response analysis. Furthermore,in order to understand the deformation mechanism, sliding stability analysis andcumulative damage analysis are performed. According to the results, the residual
deformation of the dam body after the strong earthquake is inferred to be causedby the shaking settlement of the embankment materials.

RÉSUMÉ
Lors du tremblement de terre d’Iwate-Miyagi (M7.2) du 14 juin 2008, desmouvements sismiques avec une accélération maximale de 10.24 m/s2 dans lesens du courant ont été enregistrés sur les fondations du barrage d'Aratozawa,
un barrage en enrochement situé à environ 16 km de l'épicentre. Cependant,l'accélération de la réponse maximale dans la même direction près du centre dela crête du barrage était de 5.25 m/s2, et le rapport d'amplification del'accélération du corps du barrage était bien inférieur à celui normalement
considéré pour un barrage en enrochement. De plus, il a été mesuré que la crête
se stabilisait à 19,8 cm suite au tremblement de terre. Dans cette étude, lespropriétés dynamiques des matériaux de remblai ont été identifiées grâce à
l'analyse de reproduction des tremblements de terre passés, et lescomportements du corps du barrage enregistrés lors du grand séisme ont été
simulés. Le mécanisme de génération des comportements sismiques particuliersa été étudié en fonction des résultats de l'analyse de réponse aux tremblements
de terre. En conséquence, afin de comprendre le mécanisme de déformation,
une analyse de stabilité de glissement et une analyse des dégâts cumulatifs ont
été effectuées. Selon les résultats, la déformation résiduelle du corps du barrageaprès le grand tremblement de terre est supposée être causée par la secoussedes matériaux du remblai.
KEYWORDS; finite element method, rockfill dam, seismic resistance

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