By Kevin Wilcox
A research team has used satellite data to quantify subsidence rates and is now turning its attention to the threats that the sinking poses to critical infrastructure.
The portions of Beijing shown in orange and red are subsiding between 1.5 and 3.15 in. per year, while the areas shown in black are sinking as much as 4.3 in. Courtesy of Mi Chen, Roberto Tomás, Zhenhong Li, et al.
August 16, 2016—A team of researchers using the latest satellite imaging technology has found that portions of Beijing are sinking at a staggering rate—more than 100 mm per year, on average—driven mainly by aggressive groundwater extraction within a space constrained by active faults and exacerbating geotechnical conditions. It is believed that this subsidence has the potential to damage the city's critical infrastructure.
As the population of Beijing approaches 20 million, approximately 66 percent of the water used by residents is sourced from groundwater. The research team, which has examined subsidence in other cities in previous studies, focused on Beijing because it is well known that the city has been water-stressed for decades and suffering the effects of subsidence. The new research quantified the rate of the subsidence that has occurred between 2003 and 2011, and identified those locations in the city in which it is occurring, according to Zhenhong Li, Ph.D., a professor of imaging geodesy in the School of Civil Engineering and Geosciences at Newcastle University in the United Kingdom.
The geotechnical conditions beneath the city vary greatly. Quaternary deposits range from a single layer of gravel to complex layers of clay and sand with variable grain sizes. Also, these deposits vary from a few meters to several hundred meters thick. The hydrogeology is also complex. Aquifers formed at three distinct points in the Pleistocene era are found at depths ranging from less than 100 m to greater than 300 m.
"In this study, we found that the subsidence was correlated with a range of triggering factors, such as groundwater level and the locations of pumping wells, and conditioning factors such as accumulated soft soil thickness, the presence of active faults, and the different types of aquifers," said Li, who provided written answers to questions posed by
online. The team recently published its findings, " Imaging Land Subsidence Induced by Groundwater Extraction in Beijing (China) Using Satellite Radar Interferometry," in the journal
To quantify the subsidence rates, the team examined radar datasets comprising 41 images produced by satellites operated by the European Space Agency (ESA) and the German Aerospace Center (DLR). These satellites employ a microwave imaging system known as synthetic aperture radar (SAR), which can see through clouds, day or night. Interferometric SAR or InSAR, enables researchers to compare radar images of the same point on earth taken at different times and note such differences as subsidence.
The research reveals that the eastern zone of the high-profile Chaoyang District—home to many foreign embassies and a burgeoning central business district—is sinking at the fastest rate. The research also revealed that although many portions of the city and the surrounding area are sinking dramatically, other areas are not.
Li says that the subsidence issue is well understood in China, with some local governments, such as those in Jiangsu Province and Shanghai, implementing restrictions on groundwater use that have proven effective in lowering subsidence rates.
Of course, significant subsidence issues are not limited to Beijing or China. "Land subsidence is a worldwide, increasing problem," Li said. "It was reported in 1995 that there were more than 150 major cities with substantial subsidence in the world."
Portions of the Guadalentin Valley in Spain are experiencing subsidence of more than 100 mm per year, similar to Beijing, while portions of Mexico, the San Joaquin Valley in California, and Jakarta are sinking even faster, Li said.
Nevertheless, "China has 45 cities and municipalities where disastrous land subsidence has occurred or is occurring, affecting a total area of about 49,000 km
," Li explained.
Although subsidence in China is a recognized problem, the possible effects this subsidence will have on critical infrastructure is not well known, and is the topic of the team's upcoming research. "Damages are mainly associated to great distortions caused by differential settlements and not necessarily to high settlements," Li explained. "For example, if a building suffers a high uniform settlement, it probably does not suffer damages. However, if a part of a building settles and the other part is stable or presents a lower settlement, then the building suffers a distortion that causes internal stresses that can damage it. We are currently carrying out a detailed analysis of the impacts of subsidence on critical infrastructure in the Beijing plain."
Li said the team hopes to publish the results of its research into potential subsidence damage on infrastructure in the near future, and to provide some recommendations to local authorities to help them mitigate the hazards.