In 2020 it was the 30th anniversary of the magnitude 7.5 Manjil earthquake in Iran (June 20, 1990) and the magnitude 7.7 Philippine earthquake in the Philippines (July 16, 1990). During the Manjil earthquake, the Sefid Rud buttress dam was damaged and up to now it is the concrete dam that has experienced the strongest ground shaking. During the Philippine earthquake, the Ambuklao, Binga, Magat and Pantabangan rockfill dams experienced very strong ground shaking and the crest of the Ambuklao dam settled by more than 2 m.
These are important dams, but many of you may never have heard of them as they are not well documented in the literature. It was also the 10th anniversary of the magnitude 8.8 Maule earthquake in Chile (Feb. 27, 2010), in which several large dams suffered various amounts of deformation. Furthermore, in a few weeks it will be the 50th anniversary of the magnitude 6.5 San Fernando earthquake in California (Feb. 9, 1971) in which the Lower San Fernando hydraulic fill dam was severely damaged and the Pacoima arch dam was subjected to very strong ground shaking.
The San Fernando earthquake is of particular interest to dam engineers, as it was the initiating event for using modern earthquake engineering concepts in the dam industry. In particular, it was realized that the damage observed in embankment dams could not be predicted by the pseudo-static analysis method used in the seismic analysis of dams. Therefore, it was concluded that this method is obsolete and should no longer be used. It is also the 20th anniversary of the magnitude 7.7 Bhuj earthquake in India (Jan. 26, 2001), in which many earth dams were damaged and 245 had to be strengthened after the earthquake.
The observed damage to these dams has shown that they were damaged although they were designed to resist earthquakes using the pseudo-static analysis method and, therefore, should not have been damaged. These strong earthquakes have also shown that dams are not inherently safe against earthquakes as some people may believe, especially when dams have survived for more than 50 or 100 years without any earthquake damage, which is the normal case for most dams.
There have been quite a few developments in the field of earthquake resistant-design and the seismic safety assessment of dams within recent decades. As most existing dams built more than about 30 years ago involved seismic analysis, design and safety concepts that are outdated today and since all dams should satisfy the same minimum safety criteria, there has been an increasing need for the seismic safety evaluation of existing dams and to upgrade the dams with seismic safety deficiencies. This has been my main concern, when I took over the chairmanship of the ICOLD Committee on Seismic Aspects of Dam Design in 1999.
Safety evaluation is a long-term task and it is obvious that due to the very long life-span of well-maintained dams, the safety of these dams has to be checked periodically, i.e. around every 30 years, when new information on the seismic hazard becomes available or new seismic design and safety criteria are introduced, etc. Therefore, I encourage you to carry out such seismic safety evaluations of your existing dams. It must be done, the sooner the better.
Experience has shown that such safety evaluations are only carried out when required by the authorities.
Some of my thoughts may be repetitious, but important tasks must be repeated.
Dr. Martin Wieland
Chairman Committee on Seismic Aspects of Dam Design (International Commission on Large Dams)
c/o AFRY Switzerland Ltd.
E-mail: [email protected]