By News@TheU
A new study by scientists at the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science and collaborators has revealed that 35 buildings along the Miami Beach to Sunny Isles Beach coastline are experiencing subsidence, a process where the ground sinks or settles.
The analysis, which examined buildings from 2016 to 2023, found subsidence ranging from two to eight centimeters, with the most significant effects in Sunny Isles Beach and, to a lesser extent, Surfside. The study also found subsidence at two buildings in Miami Beach and one in Bal Harbour.
The study published Dec. 13 in the open-access journal Earth and Space Science, of the American Geophysical Union, employed a satellite-based technique known as Interferometric Synthetic Aperture Radar (InSAR). By combining 222 SAR images from the European Sentinel-1 satellites, the research team created a surface displacement time series. The technique utilizes “persistent radar scatterers” as reference points for measurement. These scatterers include fixed elements on a structure such as building balconies, rooftop air conditioning units, and boardwalks, which reflect the radar signal back to the satellite antenna. Satellites flying at 700 kilometers above Earth can measure millimeter-scale displacements.
While it is well known that newly built high-rises show settlement by up to several tens of centimeters during and immediately after construction, this study showed that subsidence can persist for many years.
“We found that subsidence in most high-rises slows down over time, but in some cases, it continues at a steady rate,” explained the study’s senior author Falk Amelung, a professor of geophysics in the Department of Marine Geosciences at the Rosenstiel School “This suggests that subsidence could persist for an extended period.”
“The discovery of the extent of subsidence hotspots along the South Florida coastline was unexpected,” said Farzaneh Aziz Zanjani, the study’s lead author, a former post-doctoral researcher and alumna of the Rosenstiel School. “The study underscores the need for ongoing monitoring and a deeper understanding of the long-term implications for these structures.”
Researchers from other universities validated the findings by leveraging independent data from the German Space Agency’s TerraSAR-X satellite and alternative data analysis methods.
What causes subsidence?
Miami’s subsurface is composed of limestone interspersed with layers of sand. The researchers suggest that much of the observed subsidence — where the ground sinks or settles — stems from the shifting of grains in the sandy layers, a process triggered by the weight of the high-rise buildings and the vibrations caused during foundation construction. In addition, the researchers propose that the ongoing subsidence may also be influenced by daily tidal flows and the cracking of limestone beneath the surface. While builders make efforts to minimize construction vibrations, these factors contribute to a gradual, continuous sinking of the land over time.
“There are likely additional factors, beyond the weight of the new structure, causing the grain re-arrangements,” said study co-author Khaled Sobhan, a geotechnical engineering professor at Florida Atlantic University. “These may include groundwater flow from pumping for underground construction and daily tidal flow.”
“The subsidence is probably a combination of several mechanisms. These findings raise additional questions which require further investigation.” said study co-author Gregor Eberli, the Robert N. Ginsburg Endowed Chair of Geosciences, director of the Comparative Sedimentology Laboratory and a professor in the Department of Marine Geosciences.
Preliminary data suggests there is subsidence also along the Broward and Palm Beach coast, warranting further investigation. The subsidence in South Florida may be linked to the region’s geologic history, where along-shore currents brought quartz sands from the Appalachians, forming sand layers within the limestone believed to be responsible for the observed sinking.
Monitoring structural stability
The team also analyzed data for Champlain Towers South in Surfside, which tragically collapsed in 2021, taking the lives of 98 people. However, no displacement signals were detected before the collapse, indicating that settlement was not the cause of collapse. According to the scientists, the absence of settlement is surprising, especially considering the strong vibrations from the construction of the nearby Eighty-Seven Park Tower, which has subsided by several centimeters since its completion. The team suggests this may be due to the area’s lower concentration of sandy layers.
“This study is a game changer,” said study co-author Antonio Nanni, professor of civil and architectural engineering at the University of Miami College of Engineering. “We now have a satellite-based method to monitor the ground stability of coastal high-rises which can contribute to safety monitoring in this area. While the findings may not always provide definitive answers, higher-resolution satellites are available for further analysis. Residents and authorities should be informed about any subsidence occurring of their buildings and the rate at which it occurs.” said Nanni. The scientists underscore the need for transparency. “Establishing a public database, similar to those in Europe, would enable residents, housing associations, and authorities to monitor their buildings for signs of subsidence, inspect for cracks, and take proactive measures.” added Nanni.
A key research priority for the scientists is determining whether differential settlement — where different parts of a structure sink at varying rates — is occurring and potentially causing long-term damage. While tentative evidence of this type of settlement by 1-2 centimeters has been found, further investigation is required because structures can also exhibit a tiny tilt or rotation. To better understand the causes of subsidence, the team aims to secure additional research funding to drill for sediment cores and test the sands and limestone in the lab to replicate the observed processes. In addition, expose samples to freshwater to investigate potential limestone dissolution from stormwater and whether heavier buildings could be at greater risk.
“Our hope is that officials in Florida will recognize the value for continued research on this issue across the state and explore how this novel technology can benefit coastal residents.” said Amelung.
The study titled “InSAR Observations of Construction‐Induced Coastal Subsidence on Miami’s Barrier Islands, Florida” was published on December 13, 2024, in the journal Earth and Space Science. The study’s authors include: Farzaneh Aziz Zanjani, Falk Amelung, and Gregor Eberli from the University of Miami Rosenstiel School; Antonio Nanni and Esber Andiroglu from the University of Miami College of Engineering; Khaled Sobhan from Florida Atlantic University; Amin Tavakkoliestahbanati and Pietro Milillo from the University of Houston; Mahmud Haghighi and Andreas Piter from the University of Hannover in Germany; Mahdi Motagh from the GFZ German Research Centre for Geosciences; and Sara Mirzaee from the California Institute of Technology.
Funding for the study was provided by a grant from the University of Miami Laboratory for Integrated Knowledge (ULINK).
This piece was originally published at https://news.miami.edu/rosenstiel/stories/2024/12/new-study-finds-subsidence-hotspots-impacting-high-rise-buildings-in-south-florida.html.
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