Wei Zixin and his colleagues at the Shanghai Geological Survey Institute and staff from the municipal land-management bureau worked round the clock when there was a cave-in at the construction site of a local cross-river subway tunnel early this month.

A magnificent view of Shanghai's fast-developing Pudong area from the Oriental Pearl Tower.[China Daily]

The cave-in took place along the M4 subway line now under construction and led to a wide area of land subsiding.

In a chain reaction, one building in the area collapsed and several others began to tilt. Part of a flood wall along the river also broke.

Although the city's land managers and geologists concluded that the incident was not linked to subsidence, it still highlighted the fact that they are at the forefront of a battle against land subsidence in the country's most urbanized metropolis. The fight is still arduous despite the fact that the rate has been kept in check to a certain extent.

Built on coastal sand and clay that lie 70 metres below the ground surface, Shanghai is suffering from creeping subsidence, like Los Angeles, Mexico City, New Orleans, Osaka and Venice.

Wei Zixin, chief engineer with the Shanghai Geological Survey Institute, who has been studying the area's geology for more than a dozen years, noted that the city's tendency to subside can be eased but it is almost impossible to reverse it.

The city sank 10.22 millimetres last year. Wei noted: "It is the slowest rate in the past decade."

Shanghai sank 10.94 millimetres in 2001 and 12.12 millimetres in 2000, according to the institute.

From 1990 to 2001, the city subsided at an average annual rate of around 16 millimetres.

Water extraction

Local experts have agreed that the overuse of underground water remains a main cause of the city's subsidence.

The Shanghai area was under sea water some 3 million years ago. As a result of movements of the Earth's crust, the area rose above the sea no more than 10,000 years ago.

Shanghai started exploiting its underground water in 1860 when international traders poured in and began to turn the small town into a metropolis. As the granite high-rise buildings of banks and business centres rose up to form the imposing riverside Bund district, the city's population grew to about 5 million in the 1940s.

As early as 1921, geologists discovered that Shanghai was sinking.

Since the 1920s, the city's 600-square-kilometre central area has sunk by 2 metres on average and even 3 metres in some areas.

Skyscrapers

As the most urbanized metropolis on the Chinese mainland, the land of Shanghai now shoulders more than 2,000 high-rise buildings of at least 100 metres in its 600 square kilometre central area.

Thus, in addition to the overuse of underground water, experts believe that the mushrooming of skyscrapers in central Shanghai has also contributed to the city's creeping subsidence.

Their belief is further enhanced by a two-year joint research project by the Shanghai Urban Planning and Design Research Institute and the Shanghai Geological Survey Institute.

Shen Guoping, director of the planning institute's research centre, said the two institutes jointly conducted the study in 1999 and 2000 by collecting data in Shanghai's bustling Xuhui District and the Lujiazui Financial Zone of Pudong District, which have the city's densest concentration of skyscrapers.

The study showed that the two areas did experience faster subsidence than the rest of the metropolis.

At a working conference in Shanghai earlier this year, Fang Dingke, a senior geology expert with the National School of Administration, estimated that the city's densely packed high-rise buildings might be responsible for 30 to 40 per cent of subsidence in the city centre.

Arduous battle

The cave-in earlier this month at a Shanghai subway construction site seriously affected buildings there.

Scientists and engineers began to tackle the city's subsidence some 40 years ago, when the city introduced controls on the extraction of water from wells.

In 1995, the city strengthened these measures by requiring each deep well in the city to have an official permit.

In 1996, the city government began to invest in a global positioning system (GPS) to monitor land subsidence in the city, covering an area of 700 square kilometres.

Wei Zixin said: "Such measures against the excessive exploitation of underground water are long-term and require the government's consistent support."

The Shanghai Municipal Land and Resources Administration Bureau, the designated official body dealing with the problem of land subsidence, has put forward specific goals: by 2005, annual subsidence is not to exceed 10 millimetres and, by 2010, the figure will be reduced to 5 millimetres.

To meet the goal, the bureau is leading a campaign against subsidence, even though the authorities must make sure that the deep-water resources do not become contaminated.

There are now controls on the amount of water drawn from underground, 80 per cent of which comes from the municipality's fourth and fifth aquicludes (bodies of rock of low permeability that may absorb water slowly but cannot transmit it in significant quantities) between 170 and 300 metres below the land surface. The annual limit is 100 million cubic metres, with most industries and construction projects relying on tap water that is purified from water drawn from the Huangpu and Yangtze rivers.

Zhang Xianlin, director of the bureau's Geo-Environment Division, said: "It is a big waste to use good underground water for industrial purposes while our people live on tap water.

"We are encouraging mineral water producers to make more use of the best water resources," he said.

With a total population of 14.86 million, Shanghai has an average of 24,806 people per square kilometre in the downtown area.

The city has more than 30 mineral-water producers. They consume about 600,000 cubic metres of underground water a year but are responsible for a very tiny percentage of the city's total subsidence.

As far as city planning is concerned, architects have voiced the opinion that city planners should consider Shanghai's subsidence when they design the city's new skyline.

Under the city's plans, Pudong's Lujiazui Financial Zone will be home to three skyscrapers above 400 metres tall. One of these is the 420-metre-tall building of Jinmao Tower , which was completed in 1998 and is currently China's tallest building. Another of the three will be the Shanghai World Financial Centre, which will be 492 metres tall and is scheduled to be completed in 2007. Land has been set aside for the third skyscraper but the building is still awaiting investors.

Zheng Shiling, vice-president of the Architectural Society of China, said the Shanghai authorities need to develop a long-term, overall plan when approving high-rise building projects, taking into full consideration the local geological conditions.

"Shanghai's skyscrapers, to some degree, are too dense and in a mess," said Zheng.

But Shen of the Shanghai Urban Planning Institute emphasized that certain areas of Shanghai are still "solid enough" to support a concentration of high-rise buildings.

"Geological status is always complicated and we need years of the further collection of scientific geological data around the city," said Shen.

To cope with the city's continuous subsidence problem, the Shanghai government must play a planning role in urban construction, he said.

As for the subsidence of individual buildings, Shen noted it is natural for some skyscrapers - such as Jinmao Tower - to sink a little in the first few years after construction. However, he would not provide detailed figures.

"Jinmao Tower is sinking but the degree is acceptable," said Shen.

Lin Yuanpei, a bridge engineer with the Shanghai Municipal Engineering Design Institute, told China Daily that it is permissible for large bridges across the Huangpu River to subside by 50 centimetres over 100 years'

"Such subsidence is acceptable and can be brought under control," Lin said.

Shen said that, to prevent individual skyscrapers from sinking, engineers are encouraged to make use of lighter building materials.

The main structure of Jinmao Tower, for example, is made of steel, which is much lighter than cement, Shen said.

Through all these measures, the city's land managers and city planners are remaining vigilant and working hard to further slow down the sinking of Shanghai.

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