Many river dikes and embankments in the coastal region of Tohoku area were damaged due to strong ground motions of the 2011 Off the Pacific Coast of Tohoku Earthquake and subsequent attack by the tsunami. This paper analyzes the cause of damage to the Yoshihama river dike located in Ofunato city of Iwate prefecture based on past field investigations and numerical simulations. Analyses revealed that the dike body had low liquefaction resistance and the volume change after liquefaction was rather large. Due to main shock and aftershocks there is likelihood of liquefaction in dike body and the re-liquefaction possibilities are high even under small ground motions. Based on the series of analyses it could be confirmed that even though the dike body was covered with concrete panels, due to liquefaction induced settlement of the embankment, gaps developed between the embankment crown and the concrete covering. These gaps ultimately reduced the tsunami resistant characteristics of the dike, and thus resulting in the collapse of the dike.
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