Damage to ShihKang Dam Inflicted by Faulting in the Sept. 1999 Chi ...

Seismic Fault Induced Failures, 143-154, 2001 January
DAMAGE TO SHIHKANG DAM INFLICTED BY
FAULTING IN THE SEPTEMBER 1999 CHICHI
EARTHQUAKE
Yokito SUGIMURA 1 , Saburo MIURA 2 and Kazuo KONAGAI 3
1 Dr., Eng., Water Resources Development Public Corp.
(Kanda 936, Urawa, Saitama 338-0812, Japan, yokito_sugimurawater.go.jp)
2 Director, Vice Head of Engineering Division, INA Corp.
(Sekiguchi 44-10, Bunkyou-ku, Tokyo 112-8668, Japan, sb-miura@ina-eng.co.jp)
3 Dr., Eng., Professor, Institute of Industrial Science, University of Tokyo
(Komaba 4-6-1, Meguro-ku, Tokyo 153-8505, Japan, konagai@iis.u-tokyo.ac.jp)
One of the most spectacular aspects of the Sept. 21 ChiChi earthquake was the damage to structures
inflicted directly by faulting, and is posing us difficult problems about minimizing the fault-related
damage. Among the structures damaged, the damage to the ShihKang dam was extraordinary. Shih-Kang
Dam was built across the Ta-Chia river where the river’s fan-shaped plain begins to spread gradually
towards the East China Sea. This dam, 25 m tall and 357 m long with 18 gates lined up, has a total
concrete bulk of 141,300 m 3 . The vertical dislocation of the fault reaching 9m caused three spillways on
the rightmost side of the dam to be cracked up into large and small pieces. The detailed features of the
damage are described in this report.
Key Words :
Chi-Chi Earthquake, Dam, Fault-inflicted damage, Crack map
1. INTRODUCTION
1.1 Type area for the main text
It was an irony that a number of devastating
earthquakes took place in rapid succession in the
closing year of the International Decade for Natural
Disaster Reduction (IDNDR). Among them, Aug.
17, 1999 Kocaeli Earthquake in Turkey and Sept. 21,
1999 ChiChi Earthquake in Taiwan were
extraordinary. One of the most spectacular aspects
of these earthquakes was the damage to structures
inflicted directly by faulting, and is posing us
difficult problems about minimizing the
fault-related damage.
The Sept. 21, 1999 ChiChi earthquake in
Taiwan, which originated at a shallow depth of
about 11 km, produced spectacular reverse faulting
over nearly entire extent of the Chelungpu fault. As
of Oct. 20, the numbers of casualties, injured and
missing are 2,405, 10,718 and 79, respectively. The
Japan Society of Civil Engineers organized and
dispatched a reconnaissance team led by Prof.
Hamada, Waseda University. The authors, as the
Dam Investigation Team, surveyed four dams near
the fault, the dams include Shih-Kang (Da-Jia
River), Liyutan (JingShan Shi River, a small
branch emptying into Da-An River), Suei-Sheh
(Ru-Yue-Tan Lake, Juo-Suei river system) and
Chi-Chi (Juo-Suei River). Among them, the features
of damage to the Shih-Kang Dam, where the fault
has crossed the dam body, were extraordinary. This
report describes the features of damage to the
ShihKang dam based on the findings through the
investigation (Oct. 1-7, 1999) and two follow-ups
by Sugimura.
2. OVERVIEW OF THE EARTHQUAKE
2.1 Geological Features
The main island of Taiwan is spindle-shaped with a
high mountain ridge extending roughly in its
143

Fig. 11
Chelungpu fault
Figure 1 Tectonic Structure around Taiwan
(after Seno, 1999)
Figure 3 Faults in Taiwan
(Central Geological Survey, 1998)
Figure 2 Plate tectonic of Taiwan
(after Seno, 1999)
longitudinal north-south direction. The island is
located right upon the wedge of the Philippine Sea
(PS) plate, the wedge cutting deep into the Eurasian
(EU) plate. The northern side of the wedge subducts
along the Ryukyu Trench beneath the EU plate,
while the southwestern side of the wedge overrides
the EU plate (Fig. 1). The collision of the two plates
has led to the formation of the backbone ridge of the
island exceeding 3000m elevation with a number of
faults. The activated fault runs along the western
edge of the accretionary prism accumulated along
the subduction zone boundary (decollement) as
illustrated in Figure 2 (after T. Seno, 1999). The
earthquake therefore may be categorized as a
subduction zone earthquake. Figure 3 is taken from
“An Introduction to the Active Faults of Taiwan”,
Central Geological Survey (CGS), Ministry of
Economic Affairs, Taiwan. The CGS has confirmed
that 51 active and/or suspicious faults longer than 5
km exist, and are classified as follows:
(1) 1 st category active faults (9):
a) activated at least once in the past 10,000 years,
b) having caused some damage to existing
structures,
c) responsible for earthquake occurrences,
d) having deformed alluvial soil deposits, and
e) whose presences are clearly recognized from
surface geological configurations.
(2) 2 nd category active faults (15):
a) activated at least once in the past 10,000 years,
and
b) having deformed diluvial terraces.
(3) Suspects (27):
a) with ambiguous features in Quaternary
configurations,
b) with ambiguous features in laterite soils, and
c) looking alike but without any clear evidences.
The earthquake, with a magnitude of 7.3 (Central
Weather Bureau, CWB) took place at 1:47 AM,
local time on 21, Sept., 1999. The epicenter was
located at latitude 23.85°N and longitude 120.81°E.
Figure 4 shows the main shock and aftershocks.
The earthquake that originated at a shallow focal
depth of 6.99 km produced spectacular reverse
faulting. The activated fault seems to have appeared
exactly along the Chelungpu Fault, whose presence
had been already recognized and classified into the
2 nd category of active faults. Hence, the activated
fault, in this chapter hereafter, will be referred to as
the Chelungpu Fault. The activated fault, however,
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Ta-An River
Ta-Chia River
Tai-Chung
Figure 4 Aftershocks distribution
(http://wwweic.eri.u-tokyo.ac.jp/topics/taiwan/yosh
in2e.html)
appeared branching east off the recognized trace of
the Chelungpu in the vicinity of Feng-Yuan City,
and crossed the Shih-Kang area. When this area is
zoomed in on, it is found that this branch also shoots
further thinner separate branches out. Some details
of this area will be given later on. Figure 5 shows
dislocation vectors along the Chelungpu Fault
(Otsuki, 1999). The different diameters of gray
circles show the extent of vertical offsets, and
arrows represent net slips. The offsets, in general,
increase as we go north, and the maximum net offset
of 13.5m is reached at Point 10, the second largest
net offset of 11.1m at Point 12. Figure 6 shows
elevation increments measured over 7 km distance
in Chunshinhin town across the fault. The eastern
side of the fault seems to have been continuously
pushed up, the fact might be interpreted as an early
sign of the event.
2.2 Seismic ground motions
The Central Weather Bureau (CWB) is operating
a rapid earthquake information release system. The
motions of the main shock were recorded at more
than 500 observation stations. Figure 7 shows peak
values of three orthogonal components (NS, EW
and UD) of accelerations recorded at some
representative stations along both the Chelungpu
Fault and the Shuangtung Fault; the opinion was
divided on the conjecture that the latter fault might
have been activated. However, the rupture surface
inferred from aftershocks distribution does not seem
to support the opinion that the latter fault has
slipped. At TCU129 close to the epicenter, 983.0
cm/s 2 in EW, 610.7 cm/s 2 in EW and 335.0 cm/s 2 in
Cho-Shui
River
Chi-Chi
1 m
N
Epicenter
9m
11 m
1m
Net slipsVertical offsets
Figure 5 Dislocations along Chelungpu fault
(Otsuki and Yang, 2000)
Elevation Changes (mm)
Oct., '96 - Feb., '97
Feb., '97 - Feb., '98
Oct., '96 - Feb., '98
Chelungpu faut
Distance (Km)
Figure 6 Elevation increments measured over 7
km distance in Chunshinhin town
145

Horizontal maximum acceleration (gal)
Soil-based ground
Foundation rock of dam site
Epicentral distance (km)
Figure 8 Attenuation of acceleration
ß
Figure 7 Observed peak values of orthogonal
components of accelerations (NS, EW, UD)
(Central Weather Bureau, Taiwan)
Figure 9 Ta-Chia River
UD directions were reached. When the dynamic
stability of dam is discussed, evaluation of the
attenuation of base rock motion with respect to the
distance is certainly a key issue. Since no
seismometer was installed in the Shih-Kang Dam,
the obtained accelerations will be of some help for
us to go on to further detailed discussions. Figure 8
shows the peak values of acceleration observed on
both rock outcrops and soil sediments together with
those observed in the 1995 Kobe Earthquake; the
accelerations are plotted with respect to the distance
from the nearest fault rupture plane. Among the
accelerations recorded in the ChiChi earthquake, the
peak value of 277 cm/s 2 was reached at the WuSheh
dam; the peak absolute value of the resultant vector
of the three orthogonal components. It is an
empirical finding, through long-term earthquake
observations, that the maximum value of a resultant
vector is often about 1.4 times as large as that of
either lateral component. Therefore, the peak value
of one horizontal acceleration component is
estimated to have reached about 194 cm/s 2 . The
outcrop accelerations observed in the ChiChi
earthquake, when plotted on this figure, seem to fall
in the possible range of scattering.
3 DAMAGE TO SHIHKANG DAM
3.1 General view
Shih-Kang Dam was built across the Ta-Chia river
where the river’s fan-shaped plain begins to spread
gradually towards the East China Sea (Figure 9).
The dam site lies over a shallow sandy and gravelly
soil deposit spreading over a laminated mass of mud
stone, silt stone and sand stone of the Pliocene
Epoch, Tertiary Period. The construction of the dam
began with the excavation of this shallow sandy and
gravelly soil deposit down to the underlying rock
surface, and was completed in 1977.
The Shih-Kang Dam, 25 m tall and 357 m long
with 18 gates lined up, has a total concrete bulk of
141,300 m 3 (Figure 10 “Plan”, “Elevation” and
“general spillway cross-section”). The reservoir
with a capacity of 3.38 million m 3 collects water
from a catchment area of 1,061 km 2 in the
Chung-Yang Mountains, and provides a steady
supply of water for irrigation, and etc. An intake
tunnel on the left abutment of the dam leads the
water through a diluvial terrace down to the
Feng-Yuan water-treatment plant.
146

2 sluices
Intake gate
Plan
18 spillways
Block
number
Sunken side
Sunken side
Most seriously damaged pier 2
Elevation
Original ground surface
shortly before
earthquake
Spillway
The construction of the dam was preceded by
some necessary geological investigations. The
contours in Figure 10 (above) describe the
configuration of the base rock of the Tertiary period
overlaid with sands, gravels and other suspended
matters that the Ta-Chia river has carried over
centuries. Figure 10 (middle) shows that the base
rock surface is quite shallow in the middle of the
Figure 10 Shih-Kang Dam
river bed, 3 to 4 m below the soil surface, and
reaches 10 to 13 m depths on both sides. These
sedimentary base rocks of the Tertiary period can be
recognized from their stratified feature. The strata of
these rocks run, in general, in about N40°E-40°S
direction, and meet the dam axis at an angle of 60
degrees. The strata planes have a dip of about 40
degrees down towards southwest. Hammer blows on
147

Figure 11 Shih-Kang area
these rocks do not create any sharp sounds,
suggesting that the compressive strengths of the
rocks are some 100 kgf/cm 2 or less. When the base
rocks were exposed after the excavation for
constructing this dam, no clear evidence of the
presence of a fault was found (Water Resources
Bureau, Ministry of Economic Affairs, Taiwan).
As has been mentioned, the activated fault
appeared branching east off the recognized trace of
the Chelungpu in the vicinity of Feng-Yuan City,
and crossed the Shih-Kang area. The complicated
features of this fault emerge as the Shih-Kang area
is zoomed in on. Figure 11 shows the offsets that
the authors found during their reconnaissance. In
this figure, line A forms a part of the northeast
extension of the Chelungpu Fault. Line A crossed
the Ta-Chia river near the Bei-Fon bridge, where the
southeast side of the fault rose up by about 6 m with
respect to the other side, causing a water fall to
appear (Photo 1). This line, however dies out as it
climbs the north mountainside, and another line, C,
appears abruptly a couple of hundred meters south
off line A. Line C crossed the northern end of the
Shih-Kang dam causing three spillways of the
Shih-Kang dam (No. 16-18) to be destroyed (Photo
2). The Water Resources Bureau, Ministry of
Economic Affairs, which maintains and operates
this dam, took quick action of surveying the
damaged dam site and its vicinity for its retrofitting,
and the elevations of a number of points were
measured. Among them, two points were put side by
side on the dam in such a way that line C runs
exactly through the two points, one on pier No. 15
Photo 1 Bei-Fong bridge and fault-created
Waterfall
Photo 2 Broken spillways of Shih-Kang dam
(Point a) and the other near gate No. 18 (Point b).
Table 1 compares the elevations of these two points
before and after the earthquake.
148

Table 1 Elevations of Points a and b
(Water Resources Bureau, Ministry of Economic Affairs)
(1) (2) (1) – (2)
After EQ Before EQ
Point a 283.05 m 272.00 m 11.05 m
Point b 273.01 m 272.00 m 1.01 m
Table 2 Elevations of Points c and d
(1) (2)
After EQ Before EQ
(1) – (2)
Point c 262.36 m 256.24 m 6.12 m
Point d 258.85 m 256.24 m 2.61 m
From this table, it is clear that the south side of line
C has risen up by about 10 m with respect to the
north side. The entire picture of fault line C,
however, is not clear yet, because most of the line
lies under water. The roughly estimated line, thus, is
shown with the curved broken red line in Figure 11.
The following findings, however, allowed the
authors to draw the broken line.
(1) The reservoir bed rose up in its south half (left
side of the dam) slightly above the water level,
whereas no clear marks showing the varying water
level can be found on the right shore slope of the
reservoir.
(2) Just below the dam, the right riverbank subsided
over 20 to 30 meters distance, and the trees behind
the riverbank were deeply immersed in water that
must have flowed in there. The extension of this
depressed configuration seems to climb up the
mountainside.
(3) A remarkable vertical offset of the road appeared
on this extension.
In addition to faults A and C, another fault line, B,
appeared on the south side of the Da-Chia River. As
a whole, the north side of line B rose up by several
meters; that is to say, the entire area surrounded by
lines A, B and C was pushed up to that noticeable
extent. This line, B, crossed the intake tunnel of the
Shih-Kang Dam, shearing the tunnel completely.
Photo 3 shows the tunnel dug out after the
earthquake. The tunnel has an oval cross-section,
4.1 m and 3.8 m in upright and transverse diameters,
respectively. The roof line, which is clearly
recognized with the steel timbers meeting together,
was cut in half, showing the extent of the dislocation
which the tunnel experienced. At this site as well,
the Water Resources Bureau measured the
elevations of some points around the sheared tunnel.
Table 2 shows the comparison of the elevations of
two points (Points c and d) arranged side by side on
both sides of the fault.
Photo 3 Sheared intake tunnel
Photo 4 Cracked tunnel wall
Photo 5 Intake gate wall
149

Figure 12 Sketch of Shih-Kang dam from behind
The vertical offset here is found to be about 3.5 m
from the table, and judging from Photo 3, right
lateral slip seems to be about 3 m. All tunnel joints
cracked seriously (Photo 4). Large and small pieces
of concrete came off the tunnel wall, and the slack
of the roof cable shows that the tunnel experienced a
large axial force. The large axial force seemingly
punched out the intake gate wall by about 20 cm
(Photo 5).
Fault B was also responsible for damming up the
Shyh-Suoei-Keh Shi River, a small branch of the
Ta-Chia River. At this place, the north side of line B,
a sedimentary rock mass rose up by about 4 m,
dragging up a soft soil deposit of silt, sand and
boulders, and thus, causing houses on it to tilt to the
south. A ditch was immediately dug through this
sedimentary rock mass to let the dammed up water
flow into the Ta-Chia River.
3.2 Damage to the dam body
(1) Spillways immediately above the fault
As has been briefly mentioned above, the
Shih-Kang Dam suffered serious damage mostly
due to large dip-slip movement of fault line C. The
north part of the dam between spillway #18 and the
right abutment was cracked up along construction
joints into several huge blocks, and a considerable
amount of water leaked out through the cracks.
Figure 12, a sketch of the dam made from behind,
shows slightly tilted spillways #15 and #16, and
seriously slanting spillways #17 and #18, the latter
deeply immersed in the water stopped behind. These
dislocated spillways allow us to figure out the
possible shape of dislocation that happened
immediately beneath the dam. It is noted in this
sketch that spillways #16 and #17 slant to the left
and right, respectively, indicating that the left side
rock gradually rises towards the upper end of the
fault-created steep scarp. Judging from the
dislocated alignment of the gates, the right-lateral
dislocation of about 3 m is estimated to have
reached.
(2) Spillways and other structures off the fault
The dam is made up of 21 blocks whose numbers
are put in circles in Figure 10 (top). A pier sticks
upright in the middle of a block, and a pair of blocks
put side by side forms a spillway or a sluice. As is
illustrated in Figure 10 (bottom), total 18 spillways
are all equal in their cross-sections; 12.5m high and
36m wide spread concrete bulk, and thus are quite
stable against earthquakes. There are two sluices on
the left side of the dam
The cracks on the spillways and aprons were
closely mapped by Sugimura and Jyh 3) (Figure 14:
1/100 scale plan of the entire dam body, Figures
15-19: Cracks appearing on piers ) Needless to say,
cracks on such parts as those lying under water were
excluded from these figures. Those excluded
include the upstream slopes of spillways behind
gates, and spillways with their gates open. The
seriously and/or completely destroyed spillways
#16-18 were also excluded from this mapping.
Figure 13 shows the distribution of cracks over
the entire extent of the dam. Major cracks on
spillways were found between spillways #6 and #16,
namely on blocks - . In general, diagonal
cracks were found on most damaged spillways, and
transverse joints between blocks were opened in
such a way that the openings on the upstream side
were wider than those on the downstream side. The
entire dam body thus seems to have been slightly
warped towards upstream side. Rock masses on both
sides of a fault do not move as rigid bodies as was
illustrated in the sketch (Figure 13). Since the dam
was constructed directly upon the base sedimentary
rock mass, the pattern of cracks on the dam body
might indicate how the base rock was deformed
during the earthquake.
150

Figure 13 Crack map of Shih-Kang dam
Diagonal cracks on blocks , , , , ,
, and are reaching piers, and the crack on
Block is further extending diagonally up
through Pier 10 (See Photo 6) The largest crack
openings of 20 mm, 8mm and 40 mm are reached
on Block , Pier 10 and the block joint between
and , respectively. (See Figure 13).
Cracks along horizontal construction joints were
found on four piers #9, #10, #15 (Fig. 14), #16 and
#17. No major cracks were found on blocks more
than 200 m away from fault C. In this less-damaged
part also, Piers #1-#5 were found cracked. The
cracks on Piers #1-#5, however, have some different
features from those found on the piers closer to fault
C. Some small fragments of concrete came off a
jagged horizontal crack on the sluice-side wall of
Pier #2 (Fig. 15), and the rupture surface inferred
from the cracks seems to run diagonally down
through this pier to the other (spillway-side) wall of
the pier. It is noted here that the concrete bulk of the
sluice is 2.5m higher than the next spill way.
Upper construction joints of Piers #3 and #4 were
cracked as illustrated in Figure 16. A crack that
developed diagonally up through Pier #1 is
illustrated in Figure 17, whereas the crack found on
Pier #5 seems to have developed upright from the
similar position.
Simply supported RC bridges span all spillways
and sluices. They all came off their bearings as
shown in Photo. 8.
The cracks on the dam body caused 6 gates (5 for
spillways and one for sluice) to be inoperable. Photo
11 shows a buckled gate plate.
151

Figure 14 Cracks on Pier 14 Figure 15 Cracks on Pier 2
Figure 16 Cracks on Pier 3 Figure 17 Cracks on Pier 1
Photo 6 Cracks extending diagonally up through
Pier 10
Photo 7 Cracks on Pier 2
152

4 SUMMARY
The rupture of the Chelungpu Fault, which was
originated in the vicinity of Chi-Chi town has
traveled north across rivers flowing from the
Taiwan’s central mountain ridge into the East China
Sea. This fault rupture changed its path quite
abruptly towards the east in the vicinity of
Feng-Yuan City, and ran across the Shih-Kang Dam
constructed at the lower reach of the Da-Chia River.
The features of damage to the Shih-Kang Dam were
thus extraordinary. The fault’s vertical dislocation,
reaching about 10 m at the northern end of the dam,
caused three spillways near the right abutment to be
severely destroyed. Another fault branch that
appeared on the south side of the Da-Chia River has
crossed the intake tunnel of the Shih-Kang Dam,
shearing the tunnel completely. Compared with the
features which the fault dislocation was the most
responsible for, the other dams near the fault were
slightly damaged due to an intense shake. The
accelerations obtained from Liyutan Dam for
example will be of some help for further detail
discussions.
ACKNOWLEDGMENT: The authors
acknowledge gratefully kind helps for their
reconnaissance offered by Dr. Lai Jihn-Sung,
Professor Gwo-Fong Lin, Professor Lee Tim-Hau,
Hydraulic Research Institute, National Taiwan
University, Professor Huang Ben Hung-Din,
Department of Agricultural Engineering, National
Taiwan University, and Professor Wang
Ching-Ming, Graduate Institute of Environmental
Education, National Taiwan Normal University.
They have taken all the trouble in arranging for the
authors’ trip, and took the authors to the damaged
dam sites. The authors are indebted to Mr. Yeh
Chwen-Song, Deputy Director, Central Water
Resources Bureau, Ministry of Economic Affairs,
and Professor Chen Cheng-Hsing, President of the
Taiwan Geotechnical Society and also a Professor at
the National Taiwan University, who have provided
the authors with important data of the damaged
dams and some pieces of information regarding
soil-related damage. The authors are also grateful to
Professor Steven Kramer, University of
Washington, and Dr. Leslie Harder, Department of
Water Resources, California, who, as members of
US delegates, provided important information and
constructive comments for future collaborations.
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