so that the intensity of ground shaking was much lower than that used in the design of these dams. Nevertheless, it should be kept in mind that magnitude 6.0 earthquakes could occur in many parts of the world, although such events may be very rare. Large downstream and centreline tailings dams have survived strong ground motions associated with large earthquakes (e.g., in 2010 the El Maule Mw 8.8, and in 2015 the Illapel Mw 8.4 earthquakes in Chile). The above news is good news, but there have
been recent dam failures (both water storage and tailings dams) that suggests that most likely they were not designed against earthquakes using today’s seismic design and safety criteria, or they were not operated and maintained in accordance with good practice as otherwise they would have survived. It is the responsibility of all dam owners to ensure their dams satisfy today’s seismic safety criteria and they are properly operated and maintained. This means the safety of the dams must be reassessed periodically, especially after strong earthquakes, when the risk classification of a dam has changed or when there are new seismic design and safety criteria. The seismic safety re-evaluation of existing dams has been a long-term concern of the ICOLD Committee on the Seismic Aspects of Dam Design. Such evaluations have been carried out or are in progress in different countries but eventually such re-assessments must be carried out for all dams, to confirm that they comply with the current seismic safety criteria.
Also much is written and said about sustainability
of storage dams, but it must be kept in mind that dam safety, which is often governed by seismic and flood safety, is the prerequisite for any sustainable dam project. This fact has hardly been realised by many people.
Earthquake safety and performance
criteria The main objective of the earthquake-resistant design of any structure, including dams, is to protect the life of people and to protect them from injuries. This is a universally accepted principle. More specifically,
for storage dams the following seismic safety and performance criteria must be taken into account: 1. Retain the reservoir and protect people from the catastrophic release of water from the reservoir.
2. Control the reservoir level after an earthquake as a dam could be overtopped and destroyed if the inflowing water into the reservoir cannot be released through damaged spillways or low-level outlets.
3. Lower the reservoir level after an earthquake (1) for repair works, (2) for increasing the safety of a damaged dam or (3) when there are doubts about the safety of the dam.
The earthquake hazard is probably the most difficult hazard in the natural environment that a dam must be able to withstand, as strong earthquakes are unpredictable in terms of magnitude, time and location. There is no time for warning or taking precursory action, like lowering of the reservoir, as seismic waves can propagate with velocities of several km/sec, so that if an earthquake occurs only post-event rescue and evacuation is possible. For these reasons it is mandatory to ensure that dams are safe with regard to earthquakes. Earthquake safety implies that the dams are also safe against other types of actions. Quite a number of embankment dams and a few conventional concrete dams have experienced strong ground shaking, and so there is a basic understanding of their seismic performance. This is not the case for new types of dams such as RCC dams, concrete face rockfill dams, asphalt core rockfill dams, rock-filled concrete dams, hardfill dams, etc. as very limited information exists on their performance during strong earthquakes. The above list of topics shows that the earthquake safety is an important safety concern for most storage dams. The same is also true for nuclear power plant facilities.
2008 Wenchuan Earthquake The most important earthquake that has occurred
since the Montreal 2003 Congress is the Wenchuan earthquake in China. This magnitude Mw 7.9 earthquake of 12 May 2008, damaged over 1000
Above – Figure 3: Damage of the 156m high Zipingpu concrete faced rockfill dam caused by the magnitude 7.9 Wenchuan Earthquake in China on May 12, 2008: View of upstream face of the dam (left) and opening of downstream joint on dam crest (right)
Authors information
The authors are Martin Wieland, Chairman, ICOLD Committee on Seismic Aspects of Dam Design, Honorary Member of ICOLD, Dam
Consultant, Dietikon, Switzerland. mail: martin.wieland48@
gmail.com
Trevor Matuschka, Vice Chairman, ICOLD Committee on Seismic Aspects of Dam Design, Director, Engineering Geology Ltd,
Auckland, New Zealand. Email:
trevor.matuschka@
egl.co.nz
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