The longest crack in the dam that is still under construction is 31.5 metres in length, one millimetre wide, and six metres at the deepest point.

Nguyen Kim Toi, head of the project management, said the six-metre deep crack is not big compared to the size of the dam, which is one kilometre long, 90 metres at the widest point and 130 metres at the highest points.

Toi added that the crack, which has been reported to higher authorities, has been covered with a steel sheet.

"The cracks are not dangerous," he said. "Cracks are common in dam projects worldwide."

The vertical fractures were caused by heat shock, which occurs when the dam's inner concrete layers are hotter than the outside ones, Toi said citing the project consultant's explanation.

He said the project's designers also said the cracks weren't a problem.

As the cracks occurred on the part at the lower section of the Da River, the management head said "There's little concern about water absorbing into the dam wall."

A senior official from the Ministry of Construction told Thanh Nien that the ministry has requested construction experts to study the impact of the cracks on the dam's quality and safety.

Dang Hung Son, chief engineer of the project, said the fractures run perpendicular to the water line.

Son said measures have been taken to isolate the fractures, adding that such cracking was likely in mass concrete pours.

Construction of the 42 trillion dong (US$2.4 billion) Son La hydropower plant started in 2005.

As one of Southeast Asia's biggest hydropower facilities, the plant is expected to produce about 9.4 billion kilowatts of electricity per year.

Thanh Nien News - February 14, 2009