Failure of the Fujinuma Dams During the March 11, 2011 Tohoku Earthquake

The magnitude 9.0 (Mw) Tohoku Earthquake resulted in the failures of the two dams that impounded the small Fujinuma Lake, in Fukushima Prefecture, Japan. Several teams visited the site shortly after the failures and postulated mechanisms explaining the failure modes of the dams. This paper presents the detailed observations made at the site by the authors and the numerical dynamic analyses performed afterwards. The principal aim of this research was to determine the most likely failure mechanisms of each of the dams. Evidence of large downstream lateral movements and bulging were observed on both abutments, below the Fujinuma main dam. Furthermore geomechanical numerical analyses predicted several yards of lateral displacement due to seismic shaking, i.e., a sliding failure with a corresponding large drop in the crest elevation. In our opinion, 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.