Water Power and Dam Construction. Issue January 2010

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DAM SAFETY

Thus far, the seismogenic faults of the four major RTE with magni-

tudes larger than 6.0 are all normal or strike slip faults [9][10].

Based on the above analysis and statistics of the available cases of

RTS, the general principles for estimating the relationship between an

earthquake and reservoir impounding can be summarized as follows:

UÊÊTo investigate if there is a correlation between the time of reservoir

impounding or reservoir water level variation, and the occurrence

of earthquakes. Normally, the frequency and intensity of small

earthquakes will increase signiﬁcantly when compared with the

background seismicity of the area before the construction of the

dam, and it is often correlated with the impounding or variation of

the reservoir water level. After the reservoir has reached its highest

level, or the main shock has occurred, with the adjustment of rock

mass stress in reservoir area, the activity of the earthquakes in the

reservoir area will gradually return to the background state.

UÊÊThe epicenters of reservoir-triggered earthquakes should be located

within the area affected by water inﬁltration along the fault zone.

The epicenter is normally thought to be located in the range of

5-10km away from the edge of the reservoir. The epicenters of non-

tectonic earthquakes are scattered in groups and correspond to the

locations of karst caves or mining tunnels. For the analysis and

assessment of reservoir-triggered (tectonic) earthquakes, besides the

distribution of epicenters and the variation tendency of intensity,

additional monitoring data should be collected. The occurrence of

reservoir earthquakes requires a special geological environment.

Either the reservoir area has the seismogenic structures that may

lead to tectonic earthquakes and also the hydrogeological condi-

tions to allow reservoir water to inﬁltrate deep rock stratum, or it

has fractured rock mass, karst caves and mining tunnels that may

lead to non-tectonic earthquake activity. It should be noted that

not all the faults are pervious – some old faults may be impervious.

Therefore the condition of faults also needs to be analyzed.

UÊÊWhen comparing the characteristics of tectonic (RTE) with sponta-

neous earthquakes, the focal depth of RTE is usually shallow. The

ground shaking attenuates rapidly with the distance from the seismic

source. The non-tectonic earthquakes occur generally as small earth-

quake swarms. The RTE normally has a foreshock-main shock-af-

tershock pattern. The four main cases of RTE with magnitudes larger

than 6.0 belong to this type. The magnitude ratio of the main shock

to the maximum aftershock is high. The b-value in the magnitude-

frequency relationship (log N = a - b M) is also high. The dominant

frequency of the ground acceleration and the ratio of the vertical

and horizontal earthquake components are all relatively higher. For

the same magnitude, the reservoir-triggered earthquake may have a

stronger surface shaking than a spontaneous earthquake [7, 8].

When assessing the possibility of RTS, you should ﬁrst determine the

seismogenic regions where earthquakes could occur. The judgment

should be based on the main impact factors, such as the activity,

state and magnitude of the faults in the reservoir area, the seepage

conditions for reservoir water inﬁltration into deep rock stratum,

dam height, reservoir storage capacity, rock properties and the devel-

opment of structure planes and karst phenomena in the reservoir

foundation, etc. The maximum magnitudes of the different reservoir

segments are mainly determined by referring to existing case histories

of reservoir earthquakes and to follow the principle of engineering

analogy. According to the main impact factors, the probability of dif-

ferent magnitude intervals can be determined from methods such as:

experienced discrimination, statistical hazard evaluation, grey cluster-

ing, fuzzy estimation, and artiﬁcial neural network, etc.

Since the occurrence of the strong reservoir-triggered earthquake

in the Hsinfengkiang reservoir in 1962, Chinese dam engineers have

given greater importance to the problems of RTS. Deciding whether an

incident is RTS is mainly achieved by comparing the seismic activity

monitored before and after reservoir impounding. Therefore, accord-

ing to the Seismic Design Code of Hydraulic Structures in China, for

reservoirs with a dam height of over 100m and a storage capacity of

over 50Mm

, if RTS events with intensity over VI (Chinese intensity

scale is very similar to the MMI scale) are estimated, a special seis-

mic monitoring network with high precision seismic sensors should

be installed before impounding to obtain the proper background data.

For both the Xiaolangdi and Zipingpu projects, a seismic monitoring

network was installed before reservoir impounding. A digital remote

monitoring network for observing RTS – the ﬁrst in China – was also

set up prior to impounding of the Three Gorges project. This system

includes 24 ﬁxed stations, three relay stations, a network center, eight

movable stations, and two non-remote stations.

More recently, construction began on a microseismic monitoring

network of unprecedented scale in the reservoir areas of the four cas-

cade hydro power projects in the lower reaches of the Jinshajiang

River. This will include 62 ﬁxed stations and eight ﬂoating stations.

ZIPINGPU AND THREE GOR GES

Could the impounding of Zipingpu trigger the Wenchuan earthquake?

Zipingpu reservoir is located between the Beichuan-Yinxiu fault and

the Jiangyou-Guanxian fault of the Longmenshan fault zone. The

shortest distance from the southwestern branches of the reservoir

tail is about 4.5km away from the epicenter of the 12 May 2008

Wenchuan earthquake. The maximum water level of the Zipingpu

reservoir (elevation 875.4m asl) has not exceeded the natural water

level (elevation 877m asl) where the Min River crosses the Beichuan-

Yinxiu fault before reaching the northwestern branch of the reser-

voir tail, and the maximum reservoir level is even lower in the ﬂood

season when the Min River can reach elevation 884m asl. When the

Wenchuan earthquake occurred, the water level of the Zipingpu res-

ervoir was at elevation 826m asl, which is near the minimum operat-

ing water level of the reservoir. Furthermore, the Zipingpu reservoir

is located in the relatively stable geological region of the footwall of

V1.1

V1.2

V4.3

V1-4

V4.1

V4.2

V3.2

V1-2

V2.3

V2.2

V3.1

V2.1

V1-1

III-5

III-7

II-1

Shanghai

III

III-1

III-6

III-2

II-2

VII

I-2

South-China

seismic region

-2

Qinghai-Tibet

seismic region

III-4

Below, left: Figure 2 – The seismic zone in China and adjacent area; Right: Dam experts checking an earthen dam following the Wenchuan earthquake

32 JANUARY 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

DAM SAFETY

the seismogenic thrust faults. Therefore, the original hydrogeologi-

cal conditions of Beichuan-Yinxiu fault could not be affected by the

impounding of the Zipingpu reservoir.

Seismic activity of the reservoir area before and after impounding

In August 2004, 13 months before reservoir impounding, the

microseismic network with seven fixed stations was set up. On

27 September 2004, one of the two diversion tunnels was closed.

The reservoir water level rose from elevation 752m asl to elevation

775m asl. On 30 September 2005, the second diversion tunnel was

closed. Reservoir impounding started and the water level rose rap-

idly. In October 2006, the reservoir water level reached elevation

875.4m asl, 1.6m below normal storage level. The corresponding

reservoir volume was 900Mm

. In July 2006 and May 2007, the

water level was lowered to elevation 820m asl. Before the Wenchuan

earthquake (30 April 2008), the reservoir water level was at eleva-

tion 828.95m asl, and the reservoir volume was 300Mm

. From the

statistics of the annual seismic activity of the reservoir obtained from

the Department of Reservoir Earthquake Research of the Sichuan

Seismological Bureau, the frequency and intensity of seismic activity

after impounding of the Zipingpu reservoir was in the normal varia-

tion range of the seismic activity recorded before reservoir impound-

ing. The seismic activity remained almost unchanged before and after

reservoir impounding. The recorded seismicity also has no relation

with the reservoir water level variation. After reservoir impounding,

no signiﬁcantly increased seismicity was observed.

Recently, a case study of Zipingpu reservoir by Lei et al. was pub-

lished in Seismology and Geology in China and was quoted by Kerr

and Stone in a paper entitled ‘A Human Trigger for the Great Quake

of Sichuan?’ [2, 3]. In this study, an oversimpliﬁed two-dimensional

model is used with the assumption that the whole Zipingpu reservoir

is located on the fault. Actually, only the narrow portion of the Ming

River is located directly above the fault. Moreover, only a very small

change in the fault stresses has been calculated in this simple model,

which did not account for the large uncertainties in the properties of

the crustal materials and the water at hypocenter depth with high

temperature and pressure (>400˚C and 5kB). It is also mentioned that

an earthquake of M=8 can be triggered by small changes in stresses

and strength, which is difﬁcult to be veriﬁed as the initial stress state

and strength properties in the seismogenic fault of the Wenchuan

earthquake are not known. Furthermore, as shown in this paper,

even though the stress and strength changes induced both by the pore

pressure and added weight of the reservoir water are negligible at a

depth of 5-7km, the hypocenter of the Wenchuan earthquake has a

depth of about 15km.

Therefore, it can be concluded that there is no observational evi-

dence or factual investigations that the Wenchuan earthquake was

triggered by the Zipingpu reservoir.

Could the Three Gorges reservoir impounding have triggered the

Wenchuan earthquake?

Firstly, the Longmenshan Fault and the reservoir of the Three Gorges

project are located in different seismic regions. They have no region-

al tectonic connection. The Longmenshan Fault is located in the

Longmenshan seismic zone of the Qinghai-Tibet seismic region, while

the Three Gorges reservoir area is located in the middle and lower

Yangtze River seismic zone of the South China seismic region.

Secondly, the distance from the epicenter of the Wenchuan earth-

quake to Chongqing city, where the tail of the Three Gorges reservoir

is located, is more than 300km, and the distance from the epicenter of

the Wenchuan earthquake to the dam site is over 700 m. In October

2001, almost two years before the ﬁrst stage reservoir impounding,

the digital remote reservoir earthquake monitoring system with 26

stations was set up. The monitoring data showed that the frequency

265

255

245

235

225

215

205

195

185

175

165

155

145

120

110

100

270

140

Main shock 1966.2

Impounding 1965.7

7 8 9 10 11 12 1 2 3 4 5 6

7 8 9 10 11 12 1 2 3 4 5

Elevation (m)

Number of recorded earthquakes

By 1 station

By 2 stations

By several stations

8000

6000

4000

2000

2220

2140

2060

1980

1900

1963 1964 1965 1966 1967

Inflow (m

)Water levels (ft)Earth quake

frequency

J J J J J J J J J

SSA O D M M M M M M N NS M MN N

JMMJSN JMMJSN JMMJSN JMMJSN JMMJSN JMMJSN JMMJSN JMMJSN JMMJSN JMMJSN JMMJSN JMMJSN

1959 1960 1961 1962 1963 1964 1965 1966 19671968 1969 1970 1971

Water level

480

470

460

450

440

430

420

410

400

Number of earthquakes

Elevation (m)

1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972

18000

16000

14000

12000

10000

8000

6000

4000

2000

110

100

(no./m)

E(Joules)

H(m)

6.1

5.3

Figure 3: The relation of reservoir earthquake and reservoir water level for the

four strong tectonic reservoir earthquake cases

Hsinfengkiang Dam, China (H=105m, M=6.1)

Kariba Dam, Zambia

(H=122m, M=6.0)

Kremasta Dam,

Greece

(H=120m, M=6.3)

Koyna Dam, India (H=103m, M=6.5)

WWW.WATERPOWERMAGAZINE.COM JANUARY 2010 33

DAM SAFETY

of small earthquakes had increased. However, site investigations

revealed that most of the earthquakes occurred at the location of

karst caves and in mining tunnel areas. The earthquakes were all

small shallow earthquakes with maximum magnitudes of 3.0 - 4.0.

The observations from the microseismic network have demonstrated

the non-tectonic nature of these reservoir earthquakes.

Moreover, there is a very thick impervious rock layer in the Three

Gorges reservoir area. Therefore there is no leakage problem in the

reservoir area and there is no possibility of the reservoir water inﬁl-

trating a fault zone at a distance of several hundred kilometers.

Therefore, it is obvious that there is no possibility the impounding of the

Three Gorges reservoir could have triggered the Wenchuan earthquake.

NO CHARACTERISTIC FEATURES OF RESERVOIR-

TRIGGERED EARTHQUAKES

As mentioned in the previous section, the main reservoir-triggered

earthquakes normally belong to the type with a distinct foreshock-

main shock-aftershock pattern. The four strongest reservoir-triggered

earthquakes with a magnitude larger than 6.0 belong to this type

(Figure 3) [6]. Their seismogenic faults are normal faults or strike slip

faults that are relatively easily triggered by reservoir water [9, 10].

The general situations of these four cases are as follows:

UÊÊ(i) Krematsa: From 1700 to 1965, the Kremasta area in Greece

had no signiﬁcant earthquakes. After reservoir impounding in July

1965, 740 earthquakes occurred in an area of 100km

, this was fol-

lowed by the main shock with a magnitude of 6.3. The seismogenic

fault was a normal fault.

UÊÊ(ii) Koyna: The impounding of Koyna reservoir began in the rainy

season of 1962. In 1963, the earthquake activity increased. In 1964,

when the reservoir water reached 100m, an earthquake of magni-

tude 5 occurred at the dam site. In September 1967, there were two

earthquakes of magnitude 5. Three months later, the main shock of

magnitude 6.3 occurred. Many experts are of the opinion that the

seismogenic fault is a strike slip fault.

UÊÊ(iii) Kariba: The Kariba reservoir in Zambia has a storage capac-

ity of 153Bm

. The maximum water depth in the reservoir is over

80m. There were no seismic stations in the area before reservoir

impounding, and it was thought the area had no earthquake activ-

ity. After reservoir impounding in December 1958, earthquake

activity increased. In September 1963, there were six earthquakes

with a magnitude over 5.0. Then the main shock of magnitude

6.0 occurred. Investigations discovered that the area has the back-

ground of a normal fault with stress state in critical conditions.

UÊÊ(iv) Hsinfengkiang: The Hsinfengkiang reservoir was impounded in

October 1959. One month later, earthquakes with magnitudes of

2.0 - 3.0 occurred. After nine months, the frequency and intensity

of earthquakes increased substantially. In September 1961, when

the reservoir was nearly full, the activity of earthquakes became

more and more intense. The main shock occurred in March 1962.

The earthquake activity shows a typical foreshock-main shock-

aftershock pattern. The seismogenic fault is a strike slip fault.

The Wenchuan earthquake has no characteristics of the earth-

quakes above, which are assumed in literature as being reservoir-

triggered earthquakes.

For the four reservoir-triggered earthquakes, the b-value of fore-

shocks is 1.0-1.87. It is obviously higher than the b-value of the spon-

taneous earthquake in the respective area, which was in the range of

0.47 - 0.84 [7].

The main shock of the Wenchuan earthquake occurred on the dex-

tral obduction-type fault zone and the hypocenter depth was 15km.

The earthquake has a main shock-aftershock pattern.

CONCLUSIONS

Based on the review of the mechanisms for RTS, the worldwide

experience with dams experiencing RTS, and the discussion of the

Wenchuan earthquake, the following conclusions can be drawn:

UÊÊ(1) The cases of RTS are very few compared to the total number of

large reservoirs in the world.

UÊÊ(2) Two types of earthquakes associated with reservoirs must

be distinguished: (i) the non-tectonic earthquakes linked to the

karst caves, mine pit, and stress readjustment at the shallow

surface layer usually with magnitudes less than 3-4, and (ii)

tectonic earthquakes linked to nearby causative faults with

existing stresses close to failure and triggered by the water in

the reservoir with magnitudes not exceeding that of spontane-

ous earthquakes. The upper bound magnitude for RTS events

observed so far is 6.3. This kind of RTS is the main concern

for dam engineers.

UÊÊ(3) The mechanisms of RTS are not clear but the most common

perception is that the pore pressure of the reservoir water perme-

ated into the rock reducing its effective stresses and causing the

drop of shear resistance. As more commonly recognized, the effect

of added weight of impounding water on the stresses is negligible

in comparison with the stresses due to the weight of rock at the

location of the hypocenter.

UÊÊ(4) The impounding of the Zipingpu and Three Gorges reservoirs

did not create any conditions to trigger the devastating Wenchuan

earthquake. This earthquake also has no characteristic features of

reservoir-triggered earthquakes.

The authors are Chen Houquin, Xu Zeping and Li Ming,

China Institute of Water Resources and Hydropower

Research, Beijing, China

Acknowledgments

The authors are grateful to Dr. Martin Wieland, Chairman

of the Committee on Seismic Aspects of Dam Design of

ICOLD, for his constructive advice and comments.

This work is supported by the National Natural Science

Foundation of China (Grant No. 90510017)

IWP& DC

References

1 Fan Xiao, 2008, The Wenchuan Earthquake: A Fearsome Force within,

Chinese National Geography, Vol. 572, 36-52 (in Chinese)

2 R. A. Kerr, R. Stone, 2009, A Human Trigger for the Great Quake of

Sichuan? Science, News of the week, 323, January: 322.

3 Lei Xing-lin, M Sheng-li, Wen Xue-ze2008, Integrated analysis of stress

and regional seismicity by surface loading - A case study of Zipingpu

Reservoir, Seismology and Geology, 30(4) 1046-1063.

4 ICOLD Committee on Seismic Aspects of Dam Design, 2009, Reservoirs

and Seismicity - State of Knowledge, Paris: ICOLD Bulletin 137.

5 Standard of Ministry of Electric Power, PRC, 2001, Speciﬁcations for

seismic design of hydraulic structures (DL 5073-2000), China Electric Power

Press, Beijing, China,

6 Chen Houqun, Xu Zeping, Lee Min, 2008, Wenchuan Earthquake and

Seismic Safety of Large Dams, Journal of Hydraulic Engineering, 39(10):

1158-1167.

7 Yeh ShiqiangQu Zhu, 1996Cases and Analysis of Reservoir Earthquake,

Exploration, Planning and Design Institute of Water Conservation

Commission of Yellow River

8 Shen, H.-C., Chang, C.-H., Huang, L.-S., Li, T.-C., Yang, C.-Y., Wang, T.-C.,

1974, Earthquakes induced by reservoir impounding and their effect on

Hsinfengkiang dam, Scientia Sinica, 17(2):239-272, 1974.

9 Ma Zongi-jin, 2008, Wenchuan Earthquake and Damage Evaluation,

Material for the press conference of the State Council Information Ofﬁce,

September 4, 2008.

34 JANUARY 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

DAM SAFETY

T 242m high, the Sayano-Shushenskaya arch-gravity

dam on the Yenisei river in Siberia is one of the larg-

est dams in the world. The structure has a crest length

of 1074.4m, crest thickness of 25m and base thickness

of 105.7m. The maximum head is 220m, and the full and active

storage capacities of the reservoir are 30.71 and 14.71km

respec-

tively. The 189.6m long spillway has 11 low level openings and is

designed to pass 12870m

/sec with the reservoir level at the eleva-

tion 539m and 13090m

/sec at the surcharged level.

The region where the dam is located is characterized by a mildly

continental climate with a hot summer and cold winter. The mini-

mum recorded ambient temperature is -44˚C and the maximum

recorded ambient temperature is +40˚C.

The dam foundation is composed of igneous rock (orto-paraschist)

featuring practically uniform mechanical properties. Therefore,

in spite of structural discontinuities found in the rock mass which

houses the hydraulic structures, in design the rock mass was treated

as a quasi- homogeneous block composed of high strength rock. The

modulus of deformation of intact rock tends to vary in the rock mass

from 10 to 18 GPa depending on depth. The modulus of deformation

drops to 5-9 GPa in the inﬂuence zones of major tectonic faults.

After impoundment of the reservoir in 1989 it became evident that

the strain-stress state of the dam exceeds the values that were speci-

ﬁed at the design stage. Horizontal tensile cracks have developed on

the upstream face and disruption of the contact has been found in

the base of the upstream face. Seepage through the dam concrete and

foundation reached 520 and 546 l/sec respectively [1].

Personnel from the hydroelectric scheme, in cooperation with

specialists from the Lenhydroproject (Russia) institute and French

company Soletanche, worked out a new procedure for rehabilita-

tion, which involved applying epoxy grouts in the cracks in both the

dam and foundation during the period 1996-2003. As a result of

this repair, seepage through the dam and foundation has reduced by

99.5% and 78% respectively [1].

Implementation of the remedial measures has allowed for the

behaviour of the dam and its rock foundation to be examined. These

investigations discovered that:

U 1. Irreversible downstream movements of the dam went on till the

year 2005 and amounted to about 78% of the seasonal amplitude

of displacements, or 40% of the maximum crown displacement.

U 2. The process of adjustment of the dam-foundation system has

not ceased and buildup of residual deformations on the dam con-

tact with its rock abutment continues. In spite of rather low rates

of deformation increment with time, their presence indicates the

ongoing process of adaptation of the dam to its rock foundation.

It is likely that forces are increasingly being transmitted onto the

arches, leading to re-distribution of stresses in the dam body with

an increase in arch stresses and development of zones of loosening

or decompression near the upstream face of the dam, particularly

at low elevations on the abutments.

U 3. The study of the results of deformations on the contact between

the dam and its foundation (based on the readings of long-base

extensometers and deformations measured in the adits of the both

dam abutments) indicates that a considerable portion of the sec-

tion of each abutment on the upstream face stays in the zone of

decompression, while resulting forces exerted by the dam are being

transferred to the abutments by the remaining part of the section

which lies in the compression zone at the downstream face of the

Behaviour

management

The behaviour of Sayano-Shushenskaya dam after the fatal accident in the project’s

powerhouse in August last year is evaluated by N.I. Stefanenko, V.B. Zateev, L.S.

Permiakova, E.N. Reshetnikova and E.G. Gaziev

Above: Figure 1 – View of Sayano-Shushenskaya dam

WWW.WATERPOWERMAGAZINE.COM JANUARY 2010 35

DAM SAFETY

dam. The most stressed area of the dam interaction with the abut-

ments occurs in the middle third of the dam height.

U 4. Observations over the last few years point to the trend for stabi-

lization and decay of residual deformations which may relate to a

gradual increase in the modulus of deformation in the abutments

of the dam due to ongoing “consolidation” of the rock mass in the

dam abutments. The process of dam adjustment to its rock founda-

tion is often accompanied with re-distribution of stresses in the dam

body and its abutments, which in turn leads to development of new

conditions for interaction of the “dam-foundation” system.

U 5. Rates of compression deformation buildup can be regarded as one

of the control diagnostic parameters indicating the state of the dam and

degree of its safety. Stabilization of the abutment deformation on the

contact with the rates of deformation buildup dropping off to zero will

point to onset of the normal conditions of the dam function.

THE EVENTS OF 17 AUGUST 2009

On 17 August 2009 the seismometers installed in the Sayano-

Shushenskaya dam registered dynamic impact on the dam, caused

by the tragic accident in the powerhouse. Response of the dam section

33 at elevation 344m (the point closest to the accident location) along

the ﬂow has had the following peculiarities:

U 17.7 sec from the beginning of registration before the occurrence of

dynamic impact, there were background vibrations of the dam from

the generating units operating under the normal conditions.

U During the next 32.5 sec, damping low frequency vibrations of the

dam with a duration of 4.5 sec and maximum amplitude of 125

µm were recorded. These were caused by the rotating parts of Unit

2 being forced from their housing and subsequently falling to the

ﬂoor.

U Over the next 74 sec polychromic vibrations occurred ranging in

duration from 0.05 to 0.18 sec and with a maximum amplitude of

up to 120 µm. This was caused by the devastating impact of the

water ﬂow from the Unit 2 turbine chamber on the powerhouse

structure.

U Up to the end of record, vibrations with durations of 0.07 to 0.15

sec increasing to a maximum amplitude of up to 424 µm were reg-

istered, caused by the impact of Unit 7 and Unit 9 running at the

runway speed.

Turbine 2 with its shaft mounting the rotor was thrown out vertically

from its housing, causing extensive damage and ﬂooding to the pow-

erhouse. All 10 units were shut down, the spillway was opened and

the entire Yenisei ﬂow was diverted to the spillway dam.

The cast-in place reinforced concrete structural ﬂoors in the area of

Units 2, 7 and 9 (elevation 327m), the columns under the ﬂoors of these

units, and the walls of the generator pits have been completely destroyed.

The columns supporting the ﬂoors below elevation 327m were broken

or sustained damaged. The enclosed structure of the power house has

been completely destroyed in the area of Unit 2, 3 and 4.

The heaviest damage was inﬂicted to the powerhouse at elevations

327m and 320m. Elevation 315 of the powerhouse sustained less

damage, with the condition of the load bearing structures considered

satisfactory. The equipment that suffered the most damage were the

turbine-generator units 2, 7 and 9 (Figures 4 and 5).

After the water was pumped out, the condition of the joints between

the turbine bays was investigated. Based on the readings of geodetic

instruments (three-axial joint meters and hydrolevels) the condition

of joints at elevation 306.2m corresponds to the pre-accident state.

BEHAVIOUR OF THE DAM FOLLOWING THE ACCIDENT

Immediately after the accident the structural behaviour surveillance

unit of the Sayano-Shushenskaya hydro power station began detailed

examinations of the dam, producing daily reports.

Over the period 11-18 August 2009, the reservoir level rose by

22cm. The mean daily ambient temperature dropped from 21.60C to

11.1˚C. The dam has sustained considerable dynamic impact caused

by the accident in the powerhouse. Because of the shutdown of all

generating units, all 11 spillway gates were opened to release water

Above: Figure 2 – Layout of Sayano-Shushenskaya dam

Below: Figure 3 – Section through Sayano-Shushenskaya dam and powerhouse

36 JANUARY 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

DAM SAFETY

from the reservoir starting from 13:00 hours on 17 August.

From 2007 to September 2009, the strain-stress state of the dam

was governed by variations in hydrostatic loads (WL) and temperature

effects. No abnormal displacements have been detected in the crown

cantilever of the dam (section 33) following the accident (Figure 6).

The accident took place with HWL at elevation 537m. The post-

accident period from 17 August to 10 September is characterized by

an inconsiderable rise of the reservoir level to the maximum elevation

537.68m (22 August) and its subsequent drop to elevation 536.9m

(10 September). In the post-accident period, readings were taken at

closer intervals. In particular, eight cycles of measurements have been

carried on the plumblines during this period.

From 18-24 September radial displacements of the dam have

increased from 0.3mm at elevation 344m to 1.2-2mm at the crest.

The maximum increase in displacements (2mm at elevation 542m)

was recorded in central section 33.

No irreversible components of downstream displacements of

the dam have been detected over the period from 2007 to 2009.

Maximum displacement recorded at the crest of crown section 33

reached 132.6mm (2008).

In the zones of the dam for which quantiﬁed safety criteria are

speciﬁed, maximum compression stress remained below its critical

values and worked out to be: 11.9 MPa (section 33, elevation 504m)

and 9.7 MPa (section 33 elevation 504m) in the arch direction at the

upstream and downstream faces respectively and 12.8 MPa (section

45, elevation 322m) in the cantilever direction. The given stresses

have been computed considering initial creep during the ﬁrst year.

Consideration of long-term creep gives a reduction in measured

stresses of 20-25% by the clusters of strain meters.

In the zone of compression stresses in each abutment there devel-

oped zones in which long-base deformation meters continue to register

residual deformations. Out of total number of 45 deformation meters

available in the left abutment, 21 instruments (46%) continue to record

an increase in compression deformations, in the right bank abutment

the residual compression deformations are still being recorded by 13

instruments (26%) out of 50 available deformation meters.

In sections 18 and 25 in the dam foundation (94-105m from the

upstream face), a daily increase in compression was recorded by ﬁve

gages out of the six vibrating wire instruments installed during con-

struction, and which are still in working condition at present. The

maximum value of compression deformation increment recorded over

the last nine years worked out to be 1×10

-5

relative units per year.

Consolidation of the rock foundation goes on under the downstream

wedge of separate dam sections. In recent years compression deforma-

tions in the foundation of central section 33 continue to increase at a

low but constant rate of about 1.6×10

-5

relative units per year. .Over

the period from 1990 to 2009, the total deformation under the down-

stream face of section 18 worked out to be 9.2× 10

-5

relative units

with the tendency for decay displayed over the recent years. Decay of

deformations, decrease in their rates with time take place gradually at

a number of deformation meters which are being monitored.

Despite the small deformation values obtained, their presence does

suggest that the process of the dam adjustment to its rock foundation

still goes on.

In addition, seasonal variations in the reservoir stages cause con-

solidation of the bank abutments and displacement of the dam sec-

tions towards the banks, which entails an additional inclination of the

dam in the downstream direction.

The pattern of deformation diagrams for the right bank rock abut-

ment is much calmer than that for the left bank, which is most likely

due to more intensive jointing of the left bank.

The total seepage ﬂow through the foundation and banks at WL

= 537.44m (as on 20 August 2009) worked out to be 80.1 l/sec

which is 5 l/sec higher than the seepage ﬂow before the accident at

the hydro power plant. No new ﬁssures, new sources of seepage, or

local increase in the seepage ﬂows through the impounding structures

have been detected.

From 13-20 August, the seepage ﬂow through the impounding

structures slightly increased, from 14.3 to 14.9 l/sec. In practically all

monitored zones of the impoundment, the seepage ﬂow has slightly

exceeded the values recorded in 2007 but remained below the values

recorded in 2006.

Uplift pressure under the dam base has not exceeded the speciﬁed

values, i.e. efﬁciency of the grout curtain is secured. When measuring

the seepage pressure in the foundation of the river channel portion

of the dam (sections 16-17) on the 20 August 2009, a small rise of

the piezometric levels was recorded under monoliths III and IV of the

Above: Figure 4 – Damaged turbine and generator of Unit2; Right: Figure 5 –

Damage to generator 7 due to shortcircuit on ﬂooding of powerhouse

WWW.WATERPOWERMAGAZINE.COM JANUARY 2010 37

DAM SAFETY

dam. This did not negatively affect the condition of the grout curtain.

As a result, it may be concluded that the dam-foundation system

of the Sayano-Shushenskyaya hydroelectric scheme is in the normal

working condition.

HANDLING THE YENISSEI FLOWS

During the winter season of 2009-2010 one of the biggest challenges

to be addressed is the handling of the Yenissei ﬂows through the spill-

way with 10 generating units shut down. This is challenging as, his-

torically, the project’s spillway has never operated in winter. During

this period water ﬂow used to pass through the operating turbines.

The release of water ﬂow through the 161m high spillway at a total

head of 210-220m always causes formation of dense water mist, with

some water spilling over the side walls of the spillway chutes (Figure

7). The operation of the spillway in winter will bring up a number

of other problems including: icing up of the gate guides, affecting

their handling; icing up of chute walls; a drop in downstream face

temperature because of the penstock dewatering, which may lead to

an increase in the downstream inclination of the dam (with operating

units, water ﬂowing through the dam conduits has the temperature

not lower than +20C); and drop of temperature in the powerhouse

due to ingress of cold air through the emptied penstocks.

At the full supply level of 539m, the discharge capacity of the

service spillway permits handling the maximum spring ﬂood ﬂow of

0.01%, provided that at least one unit is running. However, compu-

tational studies and practical experience conﬁrm that in passing the

ﬂows exceeding 5000m

/sec, the stilling basin does not possess the

required strength and its prolonged operation may lead to partial

destruction of the ﬂoor slabs.

The most efﬁcient solution is to set the spillway gates to open, which

will permit drafting of the reservoir to dead storage level of 500m by

1 May 2010 (to accommodate the spring ﬂood inﬂow) ensuring mini-

mum environmental ﬂow release for riparian water supply and for the

Maina re-regulating dam, with due account of restrictions imposed on

maximum winter ﬂows in the vicinity of the town of Minusinsk. The

scheme has been worked out for operation of the Sayano-Shushekaya

spillway dam from November 2009 to May 2010 based on the han-

dling of average year ﬂows. Under such a scheme of spillway opera-

tion, the Sayano-Shushenskaya reservoir is to be drafted from elevation

536m to the dead storage level of 500m.

In October 2009, the ﬂow of 1700m

/sec was being passed.

When the inﬂow to the reservoir drops to 1000m

/sec, the gate

opening will be reduced to 1.2m (half of the ﬁrst step). These

measures will ensure release of 1100m

/sec and, on drawdown of

the reservoir level, ﬂow release will drop to 800-900m

/sec which

would provide ﬂow for operation of two generating units at the

Maina re-regulating dam.

Stefanenko N.I. - Head of Structural behaviour

surveillance unit at Sayano-Shushenskaya HPS;

Zateev V.B., Ph D (Tch Sc) - Specialist of I-category

of Structural behaviour surveillance unit at Sayano-

Shushenskaya HPS;

Permiakova L.S., Ph D (Tch Sc) - Specialist of I-category

of Structural behaviour surveillance unit at Sayano-

Shushenskaya HPS;

Reshetnikova E.N., - Head engineer of Structural

behaviour surveillance unit at Sayano-Shushenskaya HPS;

Gaziev E.G., Dr. (Tch Sc) - Chief specialist of

Geodynamic Research Center of Power Industry,

Moscow.

IWP& DC

References

1. Bryzgalov V.I. “Rehabilitation of the Sayano-Shushenskaya HPS

dam-foundation system”. Proceedings of the International Congress on

Conservation and Rehabilitation of Dams, Madrid, Spain 11-13 November

2002, pp695-697.

2. Bryzgalov V.I., Gaziev E.G.. “ Behaviour of Sayano-Shushenskaya Dam

before and after Rehabilitation”, Vestnik (Proceedings) of Krasnoyarsk State

Architectural and Civil Academy, 6, Krasnoyarsk, 2003, pp 77-86.

Above: Figure 6 – Diagram of dam crest displacements in central section 33

over last 10 years

Below: Figure 7 – Operation of spillway with 11 spillway bays opened at 0.25

of full gate (1 step of opening)

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