Solano‐Rojas and his colleagues found sinkholes in the area of an overpass near the Olivos station, which collapsed in 2021 while a Metro train was passing over it. “We did part of this analysis before 2021 and we detected that that area was having differential displacements,” says Solano-Rojas. “We thought, ‘Oh yeah, it looks like something might be happening here in the future.’ “We believe it is no coincidence that we found this.” Solano-Rojas was careful to say that the sinking’s possible contribution to the disaster would require a more thorough assessment, and official investigations have cited construction errors and don’t mention the sinking.
For this study, researchers looked at the Metro infrastructure on the surface, not the subway segments; basically, the parts of the system that they could visually verify. (The photo below shows the differential collapse of the columns supporting an overpass.) But by providing system operators with information about how fast their infrastructure could be sinking, it is hoped their work can inform interventions. Engineers can add material beneath train tracks, for example, to restore lost elevation. However, reinforcing the subway could be much more difficult. “We don’t have a concrete solution for this,” says Shirzaei. “In most cases, when that happens, it simply results in closing the project and trying to open a new lane.”
Courtesy of Darío Solano‐Rojas
This is not just the problem of Mexico City. Earlier this year, Shirzaei and his colleagues discovered that East Coast infrastructure is in serious trouble due to slower but steady subsidence. They estimated that 29,000 square kilometers of the Atlantic coast are exposed to subsidence of up to 0.08 inches per year, affecting up to 14 million people and 6 million properties. About 1,400 square miles are sinking up to 0.20 inches per year.
Researchers found that differential subsidence threatens not only railroads, but also all types of critical infrastructure, such as levees and airports. A metropolis like New York City has the added problem of weight pushing down on the ground, which itself leads to subsidence. The Bay Area is also sinking. On both coasts, subsidence is greatly exacerbating the problem of sea level rise: land goes down just as water rises.
Wherever it is happening in the world, people have to stop excessively extracting groundwater to stop the subsidence. Novel systems are already relieving pressure on aquifers. It is becoming cheaper to recycle toilet water into drinking water, for example. And more and more cities are deploying “sponge” infrastructure: lots of green spaces that allow rainwater to soak into the underlying aquifer, essentially re-inflating the land to prevent subsidence. Such efforts are increasingly urgent as climate change exacerbates droughts in many parts of the world, including Mexico City, putting increasing pressure on groundwater supplies.
With more and more satellite data, cities can better monitor sinking that they can’t immediately prevent. “I really feel that governments have the opportunity to use these types of studies to have a more structured action plan,” says Solano-Rojas.