Failure of the Fujinuma Dams during the 2011 Tohoku Earthquake

The Mw 9.0 Tohoku Earthquake resulted in failures of the two dams impounding Fujinuma Ike, in Fukushima Prefecture, Japan. After the event, several mechanisms were postulated regarding the failure modes of the dams. The authors made detailed observations at the site and performed numerical dynamic analyses whose principal aim was to determine the dams ’potential failure mechanisms. Our numerical analyses of the Fujinuma main dam predicted over 5 meters of lateral displacement of the downstream face due to seismic shaking, i.e., sliding, and a corresponding large drop in the crest elevation. This drop rendered the main dam vulnerable to overtopping which ultimately breached the dam. Additional numerical stability analyses also found that the Fujinuma saddle dam had a static factor of safety below unity under rapid drawdown conditions, and was therefore vulnerable to a rapid release of the reservoir water when the main dam was breached.