Although surface subsidence is not a new phenomenon, the rate of which happens has been accelerating at an astonishing rate.
On the morning of December 1, 2019, the road of the intersection between Guangzhou Road and Yudong West Road subsided, which was under construction of Line 11 Guangzhou subway’s Shahe Station.
At 5:30 pm on January 13, 2020, ground surfaces near the Qinghai Xining Red Cross Hospital bus stop suddenly collapsed, and a Line 17 bus that carried multiple passengers fell into the pit.
Subsidence, also called sinkhole or settlement, could happen slowly or instantaneously. It could be triggered by natural factors, such as natural sediment compaction, or by human activities, such as overexploitation of groundwater, gasoline, and natural gas. However, research suggests that subsidence caused by human activities is becoming more common.
According to data from the United States Geological Survey, more than 80% of known subsidences occur due to excessive pumping of groundwater. In fact, worldwide demand for groundwater is already at an all-time high.
In many regions, the rate of pumping groundwater exceeds the rate water is naturally replenished through natural processes, which caused the measurable groundwater level drop and significant subsidence of the overlaying soil.
At the moment, surface subsidence has become a common geological disaster all over the world, and China has to face the increasingly serious land subsidence issue as well.
Data shows that, as of December 2011, more than 50 cities in 19 provinces had subsidence incidents, with accumulated subsidence of more than 200 millimeters and total area of more than 79 thousand square kilometers. The Yangtze River Delta, the North China Plain, and the Fence And Weihe basins have been hit the hardest.
An Understated Question
A Spanish research team funded by the UNESCO developed a global surface subsidence model, which suggests that the increasing depletion of groundwater will put nearly a fifth of the world's population at risk by 2040.
Damages caused by surface subsidence has increased every year, like the previously mentioned casualties caused by sudden ground subsidence. In addition, land subsidence may impact the stability and functionality of buildings, causing severe financial loss.
Other than cities, infrastructures, such as slops, bridges, tunnel openings, reservoirs, also faces subsidence hazards.
Normally, ground subsidences are not easily detectable. Unlike sudden geological disasters, ground subsidence can happen quietly, and it will not be noticed that the ground below has gotten lower and lower until an abrupt change.
This is true for most major cities of the world, and for coastal areas that have overexploited underground resources, the threat is amplified several times. Jakarta sinks the fasted among them all.
We've written about the sinking and rising sea levels that forced Jakarta, Indonesia's capital, to move its capital, in the previous article "Disappearing Cities".
Most subsidence caused by human factors is irreversible, meaning that prevention is a better remedy. Based on the data available, scientists believe that stopping groundwater pumping can largely prevent land subsidence.
At the same time, we now have more precise tools to measure, quantify and even predict land subsidence than ever before. InSAR measurements, for example, can help us detect the risk of surface deformation by a fraction of a millimeter, which can help us mitigate its impact and plan more resilient responses to eliminate future problems.
The image is a monitoring of the belt area around the Second Ring Expressway in Chengdu by TerraQuanta, and it shows that within the monitoring region, different degrees of subsistence has occurred.
On September 30, 2021, a high level collapse occurred at the entrance of Sier tunnel in Heishui section of National Highway Line 347. TerraQuanta used normalized monitoring measures to catch abnormal changes and accurately locate the collapse area, and displayed different settlement rates in the monitoring area through different colors.
Note: with the impact of geological conditions, temperature and other factors, slight displacement of any building on the earth is a normal phenomenon.