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Shanghai Is Sinking as Skyscrapers Make A Dent

Shanghai is sinking.

While the city isn't descending fast enough to warrant building an ark and rounding up the animals, it is sinking quickly enough to raise concerns among local officials worried about the effects the shift is having on local subway lines and skyscrapers.

The city government and local geologists vowed to tackle the issue over the next 20 years, with a stated goal of reducing annual land subsidence to 5 millimeters until 2020.

Currently, the city is sinking by 10 millimeters a year, caused by overpumping of underground water and the rapid construction of skyscrapers, geologists say. The problem is far less severe than during the 1960s, when Shanghai was sinking by more than 100 millimeters a year - a rate that would have put the city below sea level by 1999 if it had not been stopped.

"Despite the tremendous achievements the city has made to stop land subsidence, a lot more remains to be done over the next five years when hundreds of planned infrastructure projects are waiting to be imple-mented," said Wei Zixin, a researcher at the Shanghai Institute of Geological Survey.

Though Shanghai has made great efforts on the issue which troubles many cities as Bangkok and Mexico City, current situation is severe enough that it could limit future construction in the city.

The problem is most noticeable in Pudong's Lujiazui area, the financial hub that has seen its once-prominent farmland developed into towering skyscrapers over the last decade, said Wei at a regional young geologists forum held in Shanghai last week.

A survey conducted in April reported that land in the area sank 38 millimeters on average during last year, with the area around the 420.5-meter-high Jin Mao Tower descending 63 millimeters.

Because land subsidence isn't uniform around the city, Shanghai's Metro line tunnels have begun to deform, researchers said.

The city has given 35 million yuan (US$4.2 million) over the past five years to the Institute of Geological Survey to build a world-class monitoring system that can detect shifts in the city's foundation. Once subsidence is detected, the government can take action, such as pumping water into underwater reservoirs and prohibiting local construction, said Zhang Xianlin, of the Shanghai Bureau of Housing, Land and Resources Administration.

The system is comprised of 32 underground sensors installed around the city, a Global Positioning System that pinpoints the location of decline, software to analyze the detected data, and remote sensing through weather satellites.

"Our bureau will draw up a special regulation to confine local construction to reduce land subsidence," Zhang said.

The regulation, which is expected to be announced next June, requires a feasibility study for any construction project with a foundation deeper than 7 meters.

In order to raise public awareness of the issue, the government has turned one of its monitoring stations into a museum where local residents can watch geologist track shifts to the city's foundation.

Researchers at the new museum near the Bund say that the city has sunk 1.88 meters since local scientists began tracking the problem in 1921.

(eastday.com December 11, 2001)

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