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

Horizontal maximum acceleration (gal)
Soil-based ground
Foundation rock of dam site
Epicentral distance (km)
Figure 8 Attenuation of acceleration
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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.
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