Shanghai tackles issue of sinking city

The local legislature in Shanghai on Tuesday started to discuss a draft regulation on preventing land subsidence, which aims to better supervise high-speed rail, underground transportation and other major municipal construction projects in the city.

Gan Zhongze, a lawmaker who has been participating in the draft, said that the city has sunk by 29 cm in the past 40 years. "The speed of sinking has slowed down, especially since 2005, but it remains a major hazard for the city," he said.

Feng Jingming, director of the Shanghai Planning and Land Resources Bureau, said a diminishing water table, combined with a growing number of skyscrapers, is causing Shanghai to sink. The average altitude of Shanghai is only 4 meters above sea level.

The draft regulation stipulates that a monitoring network to measure land subsidence for Shanghai's major municipal projects should join the city's monitoring network for ground setting, and that those major construction organizations should update their day-to-day monitoring data to the Shanghai Planning and Land Resources Administration for better prevention of subsidence.

It also requires construction companies to evaluate land before digging pits of 7 to 15 meters deep, which is considered a deep-foundation ditch. A third party is also needed for evaluation and reporting to the planning and land authority of Shanghai if the foundation is deeper than 15 meters.

Violators could face a fine up to 500,000 yuan ($80,200), according to the draft.

Land subsidence in Shanghai became a real hazard in the 1950s when the city's groundwater resources were extensively exploited for cooling during the summer by Shanghai's newly developed industrial sector.

Li Lin, assistant chief engineer from Shanghai Tunnel Engineering Co, who's currently in charge of a cross-river tunnel that is under construction, said they've been keeping an eye on the land-sinking problem during the progress because "such an issue is fatal and essential for the risk control of the project".

"Generally speaking, land subsidence is brought by two reasons - the subsidence of groundwater level or construction activities," said Li, who graduated from Tongji University's department of geotechnical engineering.

"We've got professional industry techniques to prevent sinking for the second cause, but it's good to hear our legislative body start to take action to prevent it by passing a regulation."

It has been estimated that every millimeter of subsidence costs Shanghai as much as $20 million for restoration and maintenance. For Shi Yishao, a professor from the department of surveying and land information engineering in Tongji University in Shanghai, prevention is always better than the cure.

"That's why I'm in favor of this regulation," said Shi. "However, concerns are that imposing the fine (as punishment) is not enough."

According to Shi, Shanghai attempted to take control over its dense high-rise construction by imposing a fine to limit the number of it as early as 2003, but it didn't work out as "the rich developers seem don't care about such little economic loss (from fine)". Thus, he's calling for harsher punishment.

There are about 65 buildings higher than 200 meters in Shanghai now, while Tokyo has 45, according to Emporis, one of the world's leading providers of building statistics.

Earlier this year, wide public concern was triggered when a road crack that was 10 meters long emerged in Shanghai's Lujiazui area, where dozens of skyscrapers, each taller than 100 meters, are located. Officials later concluded that the crack was "caused by a usual settlement of the foundation ditch, which is in a controlled and safe state".

"The crack was caused by the increased pressure of foundation, but it didn't come from one high-rise or two, it came from the area where the 101-story Shanghai World Finance Center and 88-story Jin Mao Tower sit," said Chen Junhong, from Guangzhou Institute of Geography's environmental research center.

Hongqiao area, Pudong's Sanlin and Zhangjiang areas have had the most severe land subsidence, according to documents from the Shanghai People's Congress. 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, which cuts through the city.

shiyingying@chinadaily.com.cn