(April 20, 2010) Until the deadly Wenchuan earthquake in China in 2008, no large concrete face rockfill dam (CFRD) had ever been subject to a strong, ground shaking earthquake. But that changed on May 12, 2008 when the Zipingpu hydro dam, one of the largest CFRDs in China and sitting just 17 km from the earthquake epicenter, suffered higher than anticipated seismic forces, causing major damage to its concrete face and deflecting the giant structure 180 mm downstream. A leading scientist now says the earthquake should act as a wake up call for dam builders.
“[The] Zipingpu dam will be a milestone in the development of CFRDs. The lessons and experience from this project will provide an excellent reference for dam engineers in seismic areas of the world,” says Martin Wieland Chairman of the International Commission on Large Dams (ICOLD) Committee on Seismic Aspects of Dam Design, in the latest issue of International Water Power and Dam Construction.
While Dr. Wieland believes Zipingpu might be an excellent learning experience for dam builders, that will be of little comfort to a public who are now realizing that dam engineers are learning on the go.
“Quite a few dam engineers were of the opinion that CFRDs are inherently safe against earthquakes,” admits Dr. Wieland. “In the past the earthquake vulnerability of CFRDs was assumed to be lesser than that of other dam types,” he said.
Engineers may have been succumbing to sense of false confidence, suggests Dr. Wieland, as the number of CRFDs “is still relatively small, [and the] experience on the actual performance of these dams under strong earthquake loading is still very limited.”
“We have to realize that not all seismic problems pertaining to large dams have been solved,” Dr. Wieland warns, adding, for that reason, “every time there is another strong earthquake, upgrading the design criteria and design concepts may become necessary.”
Zipingpu appears to be the clarion call for tougher standards in dam design.
Completed in 2006, Zipingpu was thought to represent the latest CFRD technology in China: it was designed to withstand an intensity VIII (on the China Seismic Intensity Scale) earthquake. But the Wenchuan earthquake delivered a much higher level intensity of XI at the epicenter, and between IX and X at the dam site. The dam was also built assuming a peak ground acceleration of 0.26 g, when, in fact, the May 12 earthquake recorded peak accelerations over 2 g on the crest of the dam.
Now it appears the assumption that CFRDs were less vulnerable to earthquakes than other types of dams was too rosy. The Wenchuan earthquake reminded engineers that earthquakes could be stronger than anticipated and that they can present what Dr. Wieland calls “a multiple hazard, which, depending on the site, does not only include ground shaking but also fault movements, rockfalls and others.”
At the time of the earthquake, Zipingpu was drawn down and the reservoir was less than a third full. How the dam would have performed had it been full, is unknown. Dr. Wieland is now calling for thorough studies of the behaviour of Zipingpu for full reservoir condition.
Although CFRDs may be economically attractive alternatives to conventional earth-core rockfill dams, he says, “their seismic performance must be studied carefully taking into account all existing information.”
Dr. Wieland also warns that before more high CFRDs (currently the highest is Shuibuya in China, at 233 metres) are built based on existing models, “a more thorough understanding of the behaviour and safety of CFRDs under strong earthquake ground shaking is needed.”
Had Zipingpu suffered catastrophic dam failure, the outcome could have been unimaginable for the populations downstream. Even small dam collapses have lead to nightmare scenarios, resulting in a wall of water, mud, and debris rocketing down a river valley and wiping out everything in its path.
China is no stranger to this worst-case dam failure event, as Probe International’s 1998 book, “The River Dragon Has Come” detailed. When torrential rains caused the Banqiao and Shimantan dams to collapse in 1975 in China’s Henan province, a wall of water six meters high and 12 kilometers wide rushed into the river channel and surrounding valleys and plains and destroyed virtually everything in its path. An estimated 230,000 people died as they were washed away to their graves or in the aftermath from health epidemics and famine.
Dam engineers are likely relieved that none of their structures failed catastrophically in the Wenchuan earthquake. It now appears that they are heading back to their drawing boards to improve the design of CFRDs, at least. The Chinese public living downstream will no doubt look askance at this professional experimentation with their lives.
Read Dr. Wieland’s full article here.
Click here to see stunning photographs of the damage done to the Zipingpu dam in the wake of the May 12, 2008 earthquake.
Brady Yauch, Probe International, April 20, 2010
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