Sinking or soaring?

The name means "upon the sea" but Shanghai's close relationship with the sea and water can be turbulent. While much of the city looks upward with its skyscrapers soaring towards the heavens, some residents are concerned about the shifting sands the city has been built on. Are these huge edifices safe or is the ground trying to reclaim them?

Last Tuesday a construction site in an industrial zone in Songjiang district collapsed. The site, which was to be the home for two 18-story office buildings, was not occupied at the time and no one was hurt though gas pipes were broken and power was cut to the area for some time. A neighboring three-story factory collapsed at the same time and there were deep cracks on an adjacent road. The Songjiang district authority has launched an investigation into the accident.

Two weeks ago workers in Lujiazui in Pudong New Area found several major cracks in the ground near the construction site for the 632-meter Shanghai Tower which will be China's tallest building. Some of the cracks were wide enough to fit three fingers at a time. The cracks are thought to have been caused by subsidence as a result of the construction work.

Dramatic reports and pictures were posted rapidly on the Internet. The construction management, the Shanghai Construction Group, stated that the incident was normal and could be expected during construction. Subsidence on the site was being managed and controlled. Later the management told local media that the cracks had first appeared four months earlier. The company said similar cracks had appeared not far away two years before and these problems had been fixed, eastday.com reported.

Shaken and stirred

Land subsidence has been a major geological problem for Shanghai for more than 20 years. These recent events have shaken not just the buildings but many Shanghainese who have again begun to question how the level of construction of high-rise buildings and underground zones is affecting the land.

A Shanghai Tower construction company official surnamed Wu had said the company would submit daily monitoring figures of the site to authorities but the Shanghai Urban Construction and Communications Commission, the government body that oversees all construction work in Shanghai, had not received a report as of yesterday.

A senior official from the Geological Environment Department at the Ministry of Land and Resources, Tao Qingfa told CCTV earlier the cracks in the ground had been caused by pressures from work on the building's foundations.

Tao said that it was feasible to build more high-rise structures in areas like Lujiazui as long as appropriate techniques were used.

Jiang Mingjing,a professor from the Geotechnical Engineering Department at Tongji University, said the question of whether construction work could cause land subsidence depended on the ground soil and the construction techniques used.

"Whenever there is building, it is normal to have some subsidence at first, but as the ground soil grows more solid, the subsidence stops. In most cases, if you have carried out the proper preparations for the construction process, land subsidence will not be a major problem," Jiang told the Global Times.

Jiang is an expert on soft ground engineering and had more than eight years of field experience in major projects in Japan, England and Canada before he came to Shanghai. He was a member of the engineering team for the Kansai International Airport, a Japanese airport built on a sandy beach.

"It's like the way you will be disturbed by your neighbor's quarrels if you have thin walls in your home. If you carry out the proper protective measures beforehand, the impact on the soil will be minimal," Jiang explained. He said the specific explanation for the cracks around the Shanghai Tower site would have to wait until a complete analysis of the data there was available.

He did add that if construction on a new building began close to an older edifice it would certainly affect the older building. Comprehensive risk assessment and evaluation must be carried out before construction work begins.

Jiang said there could be a problem with different areas demanding different specifications for buildings and construction. Unequal forces from different building projects could cause problems and Lujiazui was the area most affected by subsidence.

Underground concerns

For some citizens it's not the high-rise buildings that are causing concerns for subsidence - it's the work underground spurred by the rapidly expanding subway network.

A month ago during the Chinese Spring Festival the Hailun Road metro station on Line 4 was closed for seven days for repairs after tracks in the area were found to be slightly out of kilter. The station was opened just eight years ago.

The Shanghai Shentong Metro Group which runs the metro, said some rail sections were raised up to 10.40 centimeters during the repair work. Shentong said the repairs were vital because uneven rail surfaces were a safety issue.

The Shanghai metro system has been dealing with subsidence since it began construction and operation. The soft soil the city is built on obviously does not have great bearing abilities and is likely to cause subsidence problems especially around the construction of highways, bridges and tunnels.

On July 1, 2003 during the construction of metro Line 4, subsidence at the Nanpu Bridge Station close to the Huangpu River caused a major collapse. A 60-meter section of the Huangpu flood prevention barrier was damaged and three six-story apartment buildings and a newly built pumping station had to be demolished. No one was injured but the incident caused more than 150 million yuan ($23.77 million) in damage, according to local media reports.

Today the city monitors subway subsidence at more than 200,000 positions throughout the 11-line network, according to Wang Rulu, the deputy-general manager of the metro's operating company.

Metro stations are usually between 16 and 23 meters below the surface. The way the foundations for the stations are excavated and built is the key to the success of the building.

The Shentong website has actually responded to fears that large-scale subway construction work could cause land subsidence. It assures readers that although construction work would draw water from underground sources to ensure the safety of the operation, the water is quickly replaced before it can have an effect on the ground's stability. The website says the metro operations are safe and free from any threat of major land subsidence incidents.

But the Shanghai Institute of Geological Survey, a government supported institute responsible for monitoring Shanghai land subsidences has not officially concurred. Wang Hanmei, the head of the institute's research department, said authorities had not yet approved a response.

Problems at home

Subway construction has been a problem for many Shanghai residents. Li Jisheng lives on the first floor in a residential compound on Yishan Road near the Guilin Road Station on Line 9 in Xuhui district. He told the Global Times that despite repeated attempts since 2008 to lobby authorities, urging them to fix the problems that began affecting the building close to the Guilin Road Station when construction began, they had received no help at all to repair the eroding foundations beneath their homes.

"Our building has sunk something like 15 centimeters over the past two years," Li said. "Whenever it rains a lot, my home floods and sometimes the water levels reach up to 10 centimeters.

"My furniture is soaked and sometimes I lose power to my home," Li said. He has hung a protest banner outside his home saying: "Our building is leaning, falling and splitting apart. It is not safe."

Similar problems have been experienced by residents near the Jiangsu Road Station on Line 11 in Changning district. Construction of the station began in 2007. "The walls are starting to crack, and my microwave and washing machine have stopped working properly due to the uneven floor," complained Xu Jinyan, a resident living near the Jiangsu Road Station.

The metro authorities have acknowledged that rapid expansion of the subway system may have contributed to some of the problems described by the residents living in old residential compounds near Guilin Road Station. This is why they have offered renovation grants to affected residents, Zhang Yueming, a project manager of the Line 9 metro, told the Global Times.

Residents like Li and Xu have refused to accept compensation from the metro authority until the problems with subsidence in their homes has been solved.

A piece of land at the intersection of Guilin Road and Yishan Road in front of Li's building has been vacant for some time now. It was to have been a Guilin Road Station entrance but residents believe its construction could cause more subsidence problems and so the land remains empty.

Going down in history

Land subsidence in Shanghai was reported in the 1920s when the city was regarded as having a serious subsidence problem. One of the major reasons for subsidence was the excessive use of underground water sources.

Shanghai started tapping into underground water sources in 1860 and up until 1966 tapping the underground water table of the city was common with factories using large amounts and unregulated wells being sunk across the city. From 1921 to 1965 this helped cause the city to drop 1.69 meters, an average 38 millimeters every year, when the city was only 4 meters above sea level, according to a government report from the Shanghai Archives Office.

In 1963 when the extraction of underground water approached a peak 203 million cubic meters for the year, the subsidence levels also reached a record high of 110 millimeters, the report said. In 1966 the Shanghai municipal government imposed restrictions to reduce the misuse of underground water sources and began replacing the water.

In 2010 Shanghai pumped a historic low of 19.71 million cubic meters of water from underground sources and replaced a record 18.93 million cubic meters, according to the Shanghai Water Authority.

This has affected the rate of subsidence in the city. From 1966 to 2000, the city only sank 218 millimeters. Over the past decade, the rate of subsidence has dropped to within 7 millimeters each year.　

"If we hadn't acted early, Shanghai might have sunk into the sea by now," said Liu Shouqi, a former secretary-general of the Shanghai Geological Society.

Thirty years ago Liu was invited by Thailand to study the subsidence problems in Bangkok. Bangkok today is sinking between 15 millimeters and 53 millimeters a year. A large area of downtown Bangkok is already below sea level. Like Shanghai, Liu said one of the major causes of subsidence in Bangkok was the excessive use of underground waters.

But Bangkok did not move in time. Now every annual flood season sees the city struggling to cope.

Tongji University's Professor Jiang suggested authorities improve their supervision of subsidence by investing more money, improving planning and analysis, upgrading construction work and monitoring. "As long as officials pay enough attention and are willing to ensure the safety of buildings and subways, I am very optimistic about Shanghai's future," he said.