Seismic safety of Existing Dam
Large dams were some of the first structures to be systematically designed against earthquakes, starting in the 1930s. However, the seismic safety of older dams is unknown, as most were designed using seismic design criteria and methods of analysis that are considered obsolete today. Therefore, it is necessary to re-evaluate the seismic safety of existing dams, using current seismic design criteria and modern methods of dynamic analyses, and to rehabilitate deficient dams.
The seismic safety aspects of existing dams is an important issue, as most dam codes, regulations, recommendations, and guidelines are primarily concerned with the design of new dams. The design of a dam that was considered safe when it was commissioned may not remain safe forever. This fact may be contradictory to the general opinion of the owners and users of most dam structures. As earthquake engineering is still a relatively young discipline, design criteria, methods of analysis, design concepts, and so forth may be subject to changes, especially if a large dam that was designed according to the current state-of-practice should be damaged during an earthquake. Thus, there is a need for periodic checks of seismic design criteria and the earthquake safety of large dams (and other structures as well); that is, budgets for periodic seismic safety checks must be considered.
Again, the perception that a dam that was considered safe once will remain safe forever is a dangerous misconception. Therefore, several seismic safety assessments will be needed during the long service life of a dam.
To date, only an 18.5 m high embankment dam has failed during an earthquake, an event that occurred during the 2011 Tohoku earthquake in Japan, where eight people lost their lives in a flood wave caused by dam failure. This may give the impression that well-designed dams are safe against earthquakes. Nevertheless, it is necessary to reevaluate the seismic safety of existing dams based on current state-of-the-art practices and rehabilitate existing dams if necessary. As a prerequisite, the seismic hazard at dam sites must be reassessed to comply with the current seismic design criteria.
Through a comprehensive seismic safety review of large dams in California in the 1990s, it was found that 116 dams needed seismic improvements, including the control (lowering) of the reservoir level. In Switzerland, a seismic safety evaluation of all large dams under government control was carried out by the dam owners. The safety reports were submitted before the end of 2013. The average age of the dams was 65 years and most were designed against earthquakes using a seismic coefficient of 0.1 and a pseudo-static analysis method. For this seismic safety check, the government authorities allocated a period of 10 years; however, most reports were only completed shortly before the given deadline.
It is strongly recommended by ICOLD’s seismic committee that such seismic safety checks of older dams be carried out worldwide.