Demolition of the last cofferdam protecting the just-completed main wall of the Three Gorges Dam, the world's largest, started at 4:00 on Tuesday afternoon in the middle reaches of the Yangtze River, China's longest, with explosives enough to topple down 400 10-storey buildings.

The operation took about 12 seconds, causing nearly 190,000 cubic meters of concrete fragments from the upper-30-meter section of the cofferdam to tumble into the river.

Li Yong'an, general manager of the China Yangtze River Three Gorges Project Development Corp., announced the success of the demolition.

Wang Xu'guang, a demolition expert and an academician with the Chinese Academy of Sciences, "I was a bit nervous before the explosion. I'm so pleased to see the success of the operation, which indicated a big step forward in the development of China's demolition business."

S. Thomson, an Australian explosion expert invited by the China Yangtze River Three Gorges Project Development Corp., said the cofferdam demolition was "the toughest blast in the world and a long blast with little charges."

The removal of the cofferdam means the main wall of the Three Gorges Project will formally begin its role in flood control, which will be two years ahead of schedule, according to Li.

The world's largest dam in Three Gorges area, central China's Hubei Province, was completed on May 20, signifying a milestone accomplishment of major structure of the mammoth Three Gorges water control project that aims to tame the flood-prone Yangtze River.

Measures were taken to keep the main dam safe and sound against the demolition, said Yu Ying, a technical expert with the Gezhouba Dam Group, operator of the demolition.

For instance, Yu said, the demolition was operated by segments and made fragments of the cofferdam tumble upstream like building blocks.

Two arrays of "bubble curtains", resembling air-filled bags for the protection of drivers and passengers in car accidents, were set to absorb tremendous shock waves and surge pressure from the blast, so as to reduce their energy by 50 to 70 percent before they arrive at the main dam, Yu explained.

According to Yu, 192 tons of dynamite was used in the demolition, including 152 tons of high-effective emulsified explosives.

The toxic-chemical-free explosives combusted after the blast, with no solid residues left, and were seen as environmentally-friendly, Yu added.

Three hours before the demolition, fishes in the area, including rare species, were dispelled from the explosion site.

"The explosion and the reservoir's water storage will spark off no geological disasters," said Li Yong'an.

To evaluate the demolition, scientists measured the shock waves, surge pressure, ground swelling, vibration, dynamic strain, sound waves and noise the explosion produced.

They will also follow up possible influence of the explosion on the main dam and the adjacent buildings.

The cofferdam, with its southern end joining the main dam, was measured 580 meters long, 140 meters at the highest point and eight meters wide on the top.

The cofferdam began to store water in 2003 and has since safeguarded the southern section of the main dam and ensured the construction of the power station on the southern bank of the Yangtze River.

It, together with the northern section of the main dam, helped the reservoir to store water to the level of 135 meters.

While the upper-30-meter section of the cofferdam has been demolished, the under-water part, 110 meters in height, is retained as a protective structure for the main dam.

A water control project at the Three Gorges area was first envisaged in 1918 by Sun Yat-sen, the forerunner of China's democratic revolution. It had undergone protracted debates and researches for around half a century before it was finally approved in 1992 by the National People's Congress, China's top legislature.

The project is schemed to function in flood control, to generate clean energy and to benefit shipping, with flood control as its major effect.

Upon completion in 2008, one year ahead of the schedule, the project will help the middle and lower reaches of the Yangtze River valley to withstand deluge that can be seen every 100 years.

The project recruited 26,000 people, including professionals and specialists from 50-odd countries and regions, at the peak of its construction. On the basis of absorbing foreign advanced technologies, the project has set 100-plus world records.

The project, the world's largest hydroelectric project, boasts an electricity generating capacity of 22.4 million kilowatt, an equivalent of the energy produced by a coal mine with an annual production of 50 million tons or by an oil field with an annual production of 50 million tons of crude oil.

The project's main dam, which as 2,309 meters long and 185 meters high above sea level, involves concrete placement of 16 million cubic meters, smashing the 12.57-million-cu-m record set by the Itaipu Hydropower Station on the border of Brazil and Paraguay.

The last cofferdam protecting the just-completed main wall of Three Gorges Dam on the Yangtze River is demolished in Yichang, central China's Hubei Province, June 6, 2006. The removal of the cofferdam means the main dam of the Three Gorges Project will formally begin its role in flood control, two years ahead of the schedule. (Xinhua Photo)

The last cofferdam protecting the just-completed main wall of Three Gorges Dam on the Yangtze River is demolished in Yichang, central China's Hubei Province, June 6, 2006.  (Xinhua Photo)
 

Editor: Yan