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January 13, 2012

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Subsidence closes Metro station for Spring Festival

A Shanghai Metro station is to be closed during the Spring Festival because of uneven subsidence.

Metro Line 4's Hailun Road station will be off-limits to allow checks and repair work to be carried out, officials with the city's construction and transport department said yesterday.

Details of when the station will close and when it will reopen have not been released but would be announced soon, Metro officials said.

No similar problems have been detected at other stations on the city's booming underground network, said Huang Rong, director of the department.

But he said the authorities were "keeping a close eye on some aged lines like Metro Line 1 that reported overwhelming passenger turnouts."

The city government said that, together with Metro operator Shanghai Shentong Group, it was supervising all 11 lines in service and three lines under construction.

Subsidence has long been a problem for Shanghai because of the city's rapid development and relatively loose earth structure.

Increased risks

Xu Liping, an engineer with Shanghai Geotechnical Investigations & Design Institute, said the risks increased as builders dug deeper to develop underground spaces.

"Over the years, the subway and other vehicle tunnels were mostly built between 10 to 15 meters below the ground. As suitable space is used up, urban planners have to seek a deeper section underground between 15 to 30 meters," Xu said.

Developing deeper underground spaces increases the risk of structural failure and future structures required improved daily monitoring and more sophisticated engineering methods, Xu said. She added: "Even subsidence is usually not harmful but uneven subsidence is. It causes the structures to tilt and eventually sustain damage."

She cited one of the causes of uneven subsidence of a subway tunnel.

"If the pit of a building next to the subway is not properly built, that might cause the tunnel structure to subside in an uneven manner," she said.

Fast and frequent construction projects were usually seen at the same site within years of each other, during which time the land was still in an unstable state.

Structural safety

At Line 4's Hailun Road station, workers built a transit stop on the new Line 10 in 2010 before the World Expo.

"To improve structural safety of underground tunnels, local legislators had already enacted a regulation saying that any project built within 50 meters of an existing subway should be strictly controlled and monitored," Xu said.

"But standards are standards. In cases when they are not strictly followed, there could still be risks," she said.

Wei Zixin, a researcher with Shanghai Geological Investigation Institute, said one of the effective ways of curbing subsidence was to pump back groundwater drained during construction, and the city had been doing this.

The amount of groundwater drained had been decreased to 25.52 million cubic meters a year over the past five years, down from a previous more than 74 million cubic meters, said Wei in a report to the city government last year.

The water pumped back has been increasing each year. It was 17.55 million cubic meters in 2010 and is still on the rise.

"The tendency of continued subsidence along Metro lines has been curbed," Wei said in the conclusion to his report.


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