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OCR for page 9
1
Introduction
The seismic safety of dams is an issue that has been receiving increasing
attention in many parts of the world during recent years. It has become a
major factor in the planning for new dams proposed to be built in seismic
regions and in the safety evaluations of existing dams in those areas. There
are two main reasons for this increasing level of concern: (1) The risk of a
major disaster due to dam collapse increases each year because the popula-
tion is increasing downstream of nearly all large dams. Consequently, large
population centers are now at risk in locations where only a few people may
have lived at the time when the dams were built. (2) Knowledge of the
complex nature of the earthquake motions that may attack a dam is increasing
rapidly as more earthquake records are obtained all over the world, and it is
becoming apparent that the seismic safety precautions taken in the past did
not fully recognize the hazard.
The NRC Panel on Earthquake Engineering for Concrete Dams was ap-
pointed to evaluate the present status of seismic safety evaluation of concrete
dams and to prepare a report summarizing its findings. Specific objectives
of the report were stated to be as follows:
1. To indicate to research workers the present state of knowledge of the
earthquake problem as it relates to concrete dams, and the areas where
knowledge is lacking and where further research is needed.
2. To bring the earthquake problems of concrete dams to the attention of
government agencies and other organizations that may initiate, direct, or
fund research and to provide them with information helpful in planning
such activities.
This evaluation has been limited to concrete dams because analysis proce-
dures and research needs for embankment dams are quite different.
9
OCR for page 10
10
THE HAZARD OF DAMS
Dams probably were among the earliest major structures to be created by
humans; the reservoirs retained by dams were key elements in water supply
systems that dated back at least 5,000 years. The first dams undoubtedly
were small earthen embankments that were designed by trial and error;
failures of these dams must have occurred frequently but with very little
consequence. However, with expanding needs for water and with confi-
dence derived from previous successful dam construction ventures, the sizes
of dams grew, leading to increasing potential for disaster. Understanding of
the changes in behavior associated with bigger dams tended to lag behind
the increasing boldness of the planners, with the result that failures contin-
ued to occur with increasingly tragic consequences. Ultimately, this history
of dam tragedies led to the formation of agencies responsible for dam safety.
A notable example of a dam failure that was responsible for imposition
of controls on dam safety was the collapse of the St. Francis Dam, located
about 45 mi north of Los Angeles, California. This concrete gravity dam,
205 ft high by 700 ft long, and impounding a 35,000-acre-foot reservoir, is
shown before and after the collapse in Figure 1-1. The collapse occurred
almost instantaneously just before midnight on 12 March 1928, creating a
surge of water 125 ft high and an estimated discharge rate of 500,000 cfs.
The resulting extensive property damage, and especially the loss of over
400 lives, clearly demonstrated the hazard presented by major dams and led
directly to the organization of an agency of the State of California responsible
for the safety of dams. This agency, now known as the Division of Safety of
Dams of the California Department of Water Resources, has jurisdictional
authority over nearly 1,200 dams. Its operation during the past 60 years has
served as a model for the organization of similar agencies in many other
states and in other countries.
The hazard posed by large dams has been demonstrated by failures of
many dams since 1928 dams of many types located all over the world.
Two recent examples of dam disasters occurred with concrete thin arch
structures Malpasset in France and Vaiont in Italy. The Malpasset Dam
failed in a sudden burst on the evening of 2 December 1959, creating a huge
wave that destroyed towns and villages downstream with tremendous loss
of life. As was the case with the St. Francis Dam, the failure occurred not
in the concrete structure but rather in a weak seam in the foundation rock
supporting the dam.
The situation at the Vaiont disaster was entirely different in that the dam
did not fail; it remained intact despite being severely overloaded when it
was overtopped on the night of 9 October 1963 by a great wave reaching
more than 300 ft over the crest. This wave was generated by a tremendous
landslide that dropped 350 million cubic yards of rock and earth into the
reservoir from a cliff high on the side of the valley. However, the overtopping
OCR for page 11
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OCR for page 12
12
had the same effect on property and people downstream as if the dam had
been breached; over 2,000 fatalities resulted.
The failure of Teton Dam, built in Idaho by the Bureau of Reclamation?
demonstrated that embankment dams also present a threat to populations
downstream, even when they are built by modern construction techniques.
The failure occurred on 5 June 1976, when the water level was 3.3 ft below
the spillway during the first filling of the reservoir. It was initiated by
leakage in the contact zone between the impervious core material and the
foundation rock; the leak quickly led to erosion of the embankment and
caused total release of the reservoir within a few hours. The resulting flood
produced property damage estimated at over one-half billion dollars, but
fortunately flood warnings were issued early enough that only 14 lives were
lost.
None of the aforementioned catastrophes was related to earthquake activ-
ity, but several other events during the same time span have demonstrated
the additional risk that applies to dams located in seismic regions. Two
significant examples of earthquake damage to concrete dams were incidents
observed at Hsinfengkiang Dam in China and at Koyna Dam in India. These
are discussed more fully in subsequent chapters of this report, but are men-
tioned here to emphasize that earthquakes truly are a major hazard to concrete
dams. Hsinfengkiang Dam is located in Kwangdong Province; it is 1,444 ft
long and 344 ft high and was completed in 1959. As originally designed, it
had 19 buttress blocks in the central segment, with gravity sections located
on either side. The original design was intended to resist ground shaking of
only intensity 6 on the Modified Mercalli Scale, but when numerous earth-
quakes were experienced in the vicinity during and shortly after the filling
of the reservoir, the dam was strengthened to withstand earthquakes up to
intensity 8. On 19 March 1962 the dam was severely shaken by ground
motions considerably above intensity 8, caused by an earthquake of magni-
tude 6.1 located very close to the dam. No instrumentation had been installed
in time to record this event, but strong-motion seismographs were installed
in the dam to record the effects of aftershocks, and a magnitude 4.5 aftershock
produced a peak horizontal acceleration of 0.54 g recorded at the dam crest.
During the even more intense motion that occurred in the main shock, the
dam developed horizontal cracks at a change of section in the nonoverflow
buttress blocks on each side of the spillway, but no leakage occurred. Sub-
sequent analysis demonstrated that cracking was to be expected at this location
on the structure during an earthquake of this intensity, and the dam was
then strengthened so that it could withstand even more intense ground mo-
tions.
Koyna Dam is a straight gravity structure built of rubble concrete in
southwestern India. It is 2,800 ft long and 338 ft high at the tallest block; it
has an unconventional change of slope on the downstream face resulting
OCR for page 13
13
from changes introduced while construction was in progress. Originally the
dam was intended to be built in two stages, but while the first stage of
construction was in progress it was decided to build the dam to its final
height in one stage. This change of plans necessitated modification of the
dam cross section. On 11 December 1967 a magnitude 6.5 earthquake
occurred, with its epicenter at a distance of 8 mi but producing a fault trace
that passed as close as 2 mi from the dam. Instrumentation within the dam
recorded peak horizontal accelerations of 0.63 and 0.49 g in the cross-
channel and the stream directions, respectively, and horizontal cracks caused
by these motions appeared on both the upstream and downstream faces of
the tallest nonoverflow blocks. They were located at the elevation where
the slope of the downstream face changed, and subsequent dynamic finite
element analysis demonstrated that such cracking was to be expected from
an earthquake of the observed intensity. However, it is noteworthy that the
reservoir was not released by this damage, even though minor leakage was
observed. Thus, the cracking did not cause dam failure, and there was no
flooding disaster downstream, although 180 people in the vicinity were
killed by the earthquake ground vibration effect. Subsequently, the dam
was strengthened by the addition of buttresses on the downstream face of
the nonoverflow blocks.
A notable example of earthquake damage to an embankment dam occurred
at Van Norman Dam near Los Angeles, California, during the February
1971 San Fernando earthquake. This hydraulic fill structure was built in a
series of stages starting in 1912, with the last stage completed in 1940. It
retained the lower San Fernando reservoir, and by 1971 tens of thousands of
people were living immediately downstream from it. During the earthquake
a large slice of the upstream embankment—extending up to include the dam
crest slid into the reservoir. Only the fact that the reservoir had been
lowered significantly due to concerns about seismic safety (it was 35 It
below the dam crest at the time of the earthquake) prevented the dam from
being overtopped, with disastrous consequences to the population downstream.
This near disaster was a major factor leading Congress to promulgate the
National Program of Inspection of Dams in 1972. The subsequent nonseismic
failures of Teton Dam in 1976 and Kelley Barnes Dam in Georgia in 1977
caused additional concern for dam safety on the part of Congress and resulted
in the appropriation of funds to implement the inspection program.
SEISMIC SAFETY OF CONCRETE DAMS
The dam inspection program was the first formal step toward improve-
ment of dam safety in the United States. It provided for talking a census of
all dams in the country and for identifying those for which detailed studies
would be needed to determine whether they met the required level of safety.
OCR for page 14
14
Possible failures due to all types of causes were considered for both con-
crete and embankment dams. For dams located in seismic regions it generally
was concluded that further investigations of seismic safety were needed,
due to advances in knowledge of the earthquake input as well as to better
understanding of the dynamic response mechanisms. Accordingly, major
reevaluations have been made of the seismic safety of many existing dams
during the past decade. In addition, performance predictions have been
made for the relatively small number of new dams built or proposed to be
built in seismic regions in this country during this time.
In order to put the question of the seismic safety of concrete dams in
proper perspective, it must be noted that there is no record of any concrete
dam ever having been damaged due to earthquake excitation to the extent
that the reservoir was released. This perfect record stands even though more
than 100 concrete dams of all types have been subjected to measurable
shaking due to earthquakes in many parts of the world. But this record does
not justify complacency, because in very few of these numerous examples
were measurements actually made of the intensity of shaking, and the severity
of the tests generally is not known. In one of the few cases where severe
shaking was recorded at Pacoima Dam, a concrete arch in the epicentral
region of the 1971 San Fernando, California, earthquake the earthquake
forces were greatly reduced because the reservoir surface at the time was at
less than half the design water level. Hence, it is not possible to draw
definitive conclusions concerning seismic safety from this history of perfor-
mance. Consequently, even though the earthquake safety record of concrete
dams is excellent, much more research must be done if the details of their
seismic behavior are to be understood and predicted reliably.
Until two or three decades ago, the only consideration given to earth-
quakes in the design of concrete dams was to apply a static horizontal force
specified as a fraction of the weight of the structure to represent the seismic
design loading. In later stages of its use this equivalent static load design
procedure often included an additional weight to represent the inertial resistance
of the water behind the dam. In these analyses a gravity dam was modeled
as a simple cantilever column of varying cross section, but with the development
of digital computers and the finite element method of analysis, it became
possible to model the geometric configuration of the dam in a realistic way.
By using three-dimensional finite elements, even arch dams could be mod-
eled reliably. Subsequently, the analysis procedure was extended to treat the
true dynamic character of the earthquake motions and the dynamic interac-
tion of the dam with the reservoir water and foundation rock; then the
aforementioned use of equivalent static loads was applied only in preliminary
analyses. Present studies that take advantage of these advances in analytical
procedures give greatly improved estimates of the seismic safety of existing
or proposed concrete dams. However, uncertainties still remain in nearly
OCR for page 15
15
every aspect of the analyses, and an intensive continuing research effort is
needed to reach the point where full confidence in the predictions of seis-
mic safety will be attained.
SCOPE OF THE REPORT
This report describes the present state of the art of earthquake response
analysis for concrete dams and identifies major areas where additional research
is needed. The subject matter that follows is divided into five chapters,
each dealing with a different aspect of the analysis procedure. It is impor-
tant to note that the critical issue of defining the design earthquake (i.e., the
ground motions expected to act at the dam site) is not considered in this
report. It is assumed that geologists and seismologists will have established
the magnitude and epicentral position of the greatest earthquake that must
be considered in the seismic performance evaluation and that this informa-
tion will have been applied, together with knowledge of the foundation rock
properties, to obtain an estimate of the most intense free-field ground motions
that may be expected in the vicinity of the dam. Thus, this report deals only
with procedures for evaluating the response of the concrete dam to these
specified earthquake motions. Urgent research needs that have been identified
in the various subject areas are listed and described at the end of each
chapter.
Representative terms from entire chapter:
seismic safety
