Exterior deformation is one of the most important items to measure when managing the safety of embankment dams. Exterior deformation is usually measured by a conventional surveying method using targets installed on the crest and the slopes of the embankment dams. It is difficult, however, to measure the deformation by such a method immediately after an earthquake or other emergency. A displacement monitoring system using GPS (Global Positioning System) was developed to continuously monitor the deformation of dams. The system can provide three-dimensional displacements at many points simultaneously with high accuracy at the millimeter level. The GPS measurement system has already been installed at many embankment dam sites in Japan to ensure their appropriate safety management in light of this system's excellent advantages.
Ishibuchi Dam is a concrete-face rockfill dam; it is one of the oldest rockfill dams in Japan. After the large settlement caused by the Iwate-Miyagi Nairiku Earthquake of 2008, the above GPS displacement monitoring system was installed to continuously monitor the deformation behavior of Ishibuchi Dam. On March 11, 2011, the Great East Japan Earthquake struck near the northeast coast of Honshu Island. This paper describes the results of the deformation monitoring of Ishibuchi Dam before, during and after the Great East Japan Earthquake.