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November 21, 2012

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High-rises, subways cause city subsidence

SHANGHAI'S land has subsided by 29 centimeters in the past 40 years, becoming a major geological threat for a city with an average altitude of only 4 meters above sea level, city officials said yesterday.

Subsidence takes place mainly because groundwater is pumped out, often for pit construction of the foundations of high-rises and the city's Metro lines, Feng Jingming, director of the Shanghai Planning and Land Resources Bureau, told the city's lawmakers.

The lawmakers are considering the city's first land subsidence prevention regulation, which includes a fine for violators of up to 500,000 yuan (US$80,200).

"The city's land sunk by 1.69 meters between 1921 and 1965. ... The sinking rate has slowed down since 2005, but it remains a major hazard for the city," said Gan Zhongze, a lawmaker on the measure's drafting panel.

The Hongqiao area in Changning District, and the Sanlin and Zhangjiang areas in the Pudong New Area have had the most severe land subsidence.

Some areas have sunk over 3 centimeters per year, according to Shanghai People's Congress, the legislative body.

The city now has a total of 162 square kilometers of land in both the downtown and outskirts areas with subsidence of over 60 centimeters.

The most seriously affected area has sunk about 3 meters in the past five years, becoming even lower than the level of the Huangpu River, according to the congress.

Shanghai has become like a reversed bowl cover - low in the middle and higher in surrounding areas, said Tang Yiqun, a professor with the geological environment research center of Tongji University.

A road crack that was 10 meters long emerged in the Lujiazui area, the city's financial zone, earlier this year.

The incident made headlines and stirred concerns among local residents, prompting talk about whether the high-rise construction fever should be cooled.

Officials later concluded that the crack was the "result of the foundation ditch construction" of the Shanghai Tower project, which will be China's highest skyscraper, at 632 meters, upon completion by 2014.

Construction companies must evaluate land before digging foundation pits of 7 to 15 meter deep. Also, they must ask a third party to do the evaluation and report to the planning and land authority if the foundation is deeper than 15 meters, according to the draft of the regulation issued yesterday.

Construction companies will be fined from 50,000 yuan to 500,000 yuan for violations.

The planning and land authority has also installed more than 1,000 facilities to monitor the subsidence of the land across the city, Feng said.

The city has restricted the construction of large foundation pits for new building projects in an effort to reduce land subsidence hazards.

Projects that involve building deep and large foundation pits inside the Outer Ring Road will be closely studied by the watchdogs for their potential influence on the underground environment.

Also, new approvals of such projects will be restricted, government officials have said.


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