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August 19, 2009

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Metro officials increase subsidence surveillance

THE Shanghai Metro authorities are stepping up efforts to monitor and prevent possible land subsidence during massive underground network construction.

Measures such as using a surveillance-camera system that can detect subsidence during construction and operation have been taken to curb possible threats to underground safety.

Other methods involving grouting and extra tunnel supports would be used if subsidence was detected, officials said yesterday.

Liu Jianhang, a tunnel expert, said yesterday that the city faced great challenges as the Metro network had been gradually established while the city's geology was very "complex."

"The task of digging a Metro tunnel is like punching a hole in a piece of tofu since the earth here is relatively loose," he said.

The city is on a subway-building fast track, with five new lines and 116 Metro stations now under construction, while eight lines are already in use.

Metro management officials said they checked the tunnels twice a year and supervised more than 200 safety programs.

"The subsidence has slowed year on year since Line 1 began running in 1995," said Wang Rulu, an engineer with the Metro operator.

Normally subsidence should be controlled within 10 millimeters according to a technical manual released by the Metro authority in 2006. Wang said that so far levels were within the safe range.

A safer way to see less subsidence would be to dig tunnels deeper underground, said Bao Zhicheng with Shanghai Geotechnical Engineering and Geology Institute.

"But this practice costs more," he said.


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