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Zipingpu dam tests rock fill design as quake shakes core

China's 156m tall Zipingpu dam, close to the epicentre of last week's 7.9 magnitude earthquake, became a focus for the emergency repair of 'considerable' damage this week.

Western dam engineers believe that the quake damaged joints and seals in the rock filled dam's concrete clad face.

"The joints and seals give the dam its watertight element," said Atkins director of dams and water resources Andy Hughes, former vice-president of International Commission on Large Dams (ICOLD).

"These watertight seals have been damaged." It is understood that the dam on the Min River, upstream from the devastated town of Dujianyan, has now been made safe thanks to the efforts of 2,000 soldiers who carried out emergency repairs.

These efforts were helped by the fact that Zipingpu dam held only one third of its capacity at the time of the earthquake, making it easier for engineers to inspect.

ICOLD committee on seismic aspects of dam design chairman Dr Martin Wieland said he believed that Zipingpu was safe, but needed continuous monitoring. "As considerable damage was observed it will be necessary to perform a thorough inspection of the dam before the start of the next approaching rainy season," added Wieland.

"In view of the numerous aftershocks and the observed damage it will be necessary to closely monitor the behaviour of the dam," said Wieland.

Hughes added: "Because accelerations due to the earthquake increase as you go up, it is most likely damage occurred in the upper part of the dam."

"To find the leak could be as simple as drawing down the level of the dam until the leak stops," he said.

"You could need to watch for a few weeks, as there could be progressive failures which could not be seen immediately." Hughes has experience of Chinese dams and described Zipingpu as a concrete faced rockfilled dam (CFRD), covered with concrete face panels and seals.

British Dams Society chairman Jonathan Hinks said that CFRDs were the most suited to earthquake- prone areas. "It is considered to be a safe form of construction in a seismic area," he said.

"You would need a very large flow of water through the rock fill for there to be a significant danger to the structure."

But Hinks said that despite broad approval of the design, no large dam of this type has ever experienced a significant earthquake.

"This [design] will need to be re-assessed in light of events," he said. Hinks also pointed out that many of the smaller dams in the region may not be as robust as Zipingpu, a thought echoed by Wieland.

"The seismic safety of these dams is a concern and it is expected that many of them will need repair and strengthening," said Wieland.

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