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20 million m
3
of water into the Madison River, forming a natural dam of 70 m. A
seiche swept the crest four times within a period of 10 to 20 minutes, causing scour
and a 1.8 m upstream settlement and a 1.2 m downstream. The dam did not fail: the
designer had planned a concrete core in this challenging seismic environment.
The 33.5 m Tokiwa Earth Dam was 700 m from the epicenter of the Kobe
earthquake (1995), which had a magnitude of 7.3. It was cracked only at the crest,
although the seismic center of Kobe recorded a maximum acceleration of 0.82 g.
Following the huge Tohoku underwater earthquake (M = 9.0) on March 11, 2011
almost all of the dams withstood minor to severe ground shaking and retained their
reservoirs with generally minor to moderate damage (Matsumoto, 2011). The
exception to this was the Fujinuma Dam, considered to be a pond-retaining structure
under the ministry of agriculture regulation. Its construction began in 1937 and was
halted during the Second World War. Following the war, the dam was completed in
1949. The failure of the dam resulted in the uncontrolled release of the entire
reservoir, which flowed downstream into a small village and killed eight people
(Matsumoto, 2011). This was the first time that fatalities were unfortunately caused
by the seismic failure of a large dam. The causes are under investigation; however,
once again poor compaction, poor construction design details, and liquefaction are
reported.
During the Chi-Chi earthquake (M = 7.3) in 1999, most dams settled no more
than 0.1% of their height. The conclusion can be drawn that a well-compacted fill
may undergo a maximum acceleration of 0.6 g without major damage.
In conclusion, only two agricultural large dams − the Chang Dam in India, after
the Bhuj earthquake (2001) and the Fujinuma Dam in Japan following the Tohoku
earthquake (2011) − failed and resulted in the release of the entire reservoir. Note
that the design of these two dams was poor and the lack of filter will, even without
an earthquake, lead to a high risk of failure by internal erosion.
We should keep in mind, in accordance with the Japanese data, that no large dam
with a height greater than 15 m built after 1959, according to modern design, has
failed. Those built with poor design features have been seriously affected by
earthquakes, however only two have failed. The two causes of the few seismic crest
settlements over 1 m are:
− loose backfill material;
− liquefaction of sandy materials.
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