A structural collapse of a building is usually due to multiple causes occurring simultaneously or in sequence. Subsidence, the lowering of a building, either from the ground movement, building movement, or both, can start a failure, or in the worst case, like an earthquake, be the primary cause.
The Champlain Towers South building that collapsed June 24 was indeed subsiding. The rate of subsidence, 1.9 mm per year, was mentioned in a 2020 study Local Land Subsidence in Miami Beach (FL) and Norfolk (VA) and its Contribution to Flooding Hazard in Coastal Communities Along the U.S. Atlantic Coast by Simone Fiaschi of the University of Dublin and Shimon Wdowinski, professor at Florida International University. A couple of millimeters a year may not seem like much, especially considering the whole of Mexico City is sinking at a rate of 40 centimeters every year. But millimeters add up to inches over the years, says Wdowinski below. The Champlain Towers South building was 40 years old.
Dr. Wdoniski is interviewed after the collapse by Florida International University’s Chrystian Tejedor. Following is a transcript of the interview.
[Wdowinski] Our study, done with my students (who have long since graduated), looked at two communities that have been subjected to coastal flooding in two coastal communities, Miami Beach and Norfolk, Virginia. We wanted to evaluate the contribution of flooding to land subsidence. We used satellite data, which is radar from space, synthetic aperture radar, that is being collected daily and we analyzed that for this area of the Atlantic coast. There weren't many observations at the time so we relied on data that was acquired from 1993 to 1999. We used data from InSAR (Interferometric Synthetic Aperture Radar) and analyzed it to see how things change over time. We analyzed images over Miami Beach [see Figure 1]. You'll notice that there are pockets of subsidence. These are places where the surface is moving down with respect to the surrounding surface. Not everywhere is moving the same way so there are pockets of subsidence both in Miami Beach and in Norfolk, Virginia. We concluded that in these areas, the impact of sea level rise on coastal flooding is higher than in other states.
[FIU] How much subsidence is taking place?
[Wdowinski] In Miami Beach, the subsidence rate was between one and three millimeters a year, which is very small. But when we think about the accumulation over time, then it will be a few inches over decades and that was our concern. So, the rates were 1, 2 or 3 millimeters per year in Miami Beach and in Norfolk, it was between 1 and 5 millimeters per year.
[FIU] Were there any concerns raised from the study?
[Wdowinski] The focus of this study was land subsidence[i], which usually occurs over a wide area. Now this technology, InSAR, has also been used to monitor buildings. In many cases, we see buildings that have some cracks or they move. They also show some subsidence. We've seen that in another study that detected sinkholes in Central Western Florida. There were some buildings that had moved and when we went to check over there, we saw there were some cracks. It's pretty common in Florida and other places around the world. This technology can detect movement of buildings and in most cases, these buildings just move and there is no catastrophic collapse, like in the case here in Surfside, which was very unfortunate. We saw several pockets of subsidence. Most of them were in the western part of the city where we expected to see subsidence. It was unusual, this pocket we saw in Surfside, which was in the eastern side of the city, known to be the stable side. Over there, we do not expect to see subsidence. We didn't pay too much attention to that. It was reported because that's what the data show and we said that it occurred in this condominium building that is 12 stories high. We just reported that because the focus of the study was about land subsidence and we were not trying to analyze building damage or anything like that. It was a byproduct of the analysis of the movement that happened in the 1990s. We didn't pay too much attention to that, we just reported it in the paper. [FIU]
[FIU] How common is land subsidence?
[Wdowinski] What we measure with this technology is movement of buildings. What returns the signal is a building. It can be because of the land itself that the foundation is moving downward. Like we see in many other places. In the case of the land itself moving downwards, usually it's a broader signal. It’s not very localized. In places where the building itself is like moving, due to cracks and things like that, it’s a more localized signal. I've studied land subsidence over broad areas like in Mexico City, where most of the city is subsiding and very high rate of, maybe 40 centimeters year which is very large. There we studied the impact of this very light subsidence on infrastructure on the Metro station, scene of the accident that happened two months ago. That is in an area that we identified as a place likely to have problems because of the subsidence there. But in other places where we don't have an entire area subsiding, we can have very localized subsidence. This is like our detection of sinkholes in western Florida, where the area is much more localized and the area that has the subsidence there is in much smaller pockets, usually a 10 yard or 20 yard square, about the size of the house. Individual houses are moving. That movement can be induced simply to movement in the subsurface. In some cases, the movement can be due to the building not built properly and it can have cracks, because of a problem in the building itself. We can detect this movement with this technology.
[FIU] Why was the western side of the island more at risk?
[Wdowinski] The city of Miami Beach is built on a barrier Island, which has a rocky foundation on the Eastern side. The Western side of the city is where wetlands were reclaimed and they built houses on these reclaimed wetlands. Reclaimed land tends to subside in a process called soil consolidation. Since the city was built about 100 years ago we expected to see some movement as the soil settled and the buildings on it also settled. We have a project now looking at some of the historic data, looking to see what causes that and we can measure that progress. When we have these soil conditions we expect to see subsidence in the Eastern side of the city. Where the building is built over bedrock, it’s less likely to have movement of the building or the ground beneath the building. That’s why we didn't expect to see any movement along the Eastern side of the city. We were a little bit surprised but we know it can happen so didn’t pay it too much attention.
[FIU] Tell us about the correlation between subsidence and the immediate threat.
[Wdowinski] We measure subsidence by the movement of a building. The building can move due to the land or it can move due to cracks within the buildings and upgrades. Buildings have cracks and they move, not necessarily just in Miami. I am familiar with many studies that report on cracks in the building that are moving. That doesn't mean that they will collapse. There was something from the engineering point of view that caused it to collapse. Still we recorded that we have some movement in the building, whether it's because of the ground underneath or because of some failure that formed in the building in the 1990s. We didn't have data to look after that but we're working on that and that will be our priority now. What we reported in the paper is movement of the ground or the building on the on the ground between 1993 and 1999. And there were more than 20 years afterwards, nothing happened so it doesn't really always happen.
[FIU] Can we use subsidence as an early warning sign for building safety?
[Wdowinski] When we measure subsidence or we see movement of the buildings, it's worth checking why it happened. We cannot say what is the reason for that but we can say there was a movement that caused it and was detected by this technology, by satellite images. It's worth doing it. I know from our other projects in Western Florida that people who have houses don't want to know there is sinkhole under them because it can lower the value of their property. I think it's important to implement new technologies that can allow detection of movement and then provide the engineers or people who are in charge to say this is building shows movement. You need to check right away to avoid catastrophic collapse. We cannot do it from the satellite images. We just say there was movement over here. It's worth checking. Please do that.
[FIU] Anything else you want to add?
[Wdowinski] I would like to say something about the background [see Figure 2]. What we see there is the image of a GPS stations that monitors subsidence here in Miami-Dade’ Haulover Park. It’s part of a project in which we monitor subsidence. We use two technologies. One is GPS and the other is InSAR. With both we try to see what happened to the ground. Our focus is to see how we can help understand the hazard induced by sea level rise, subsidence and the impact of the coastal flooding hazard.
We take many measurements. Now [see Figure 4] we are seeing the study of Miami and the Miami Beach area using the data acquired by satellites ERS 1 and 2.
We used a total of 24 acquisitions between 1993 and 1999 in this area but we focus only on this study in this area of Miami Beach and this is what we see here [Figure 5]. These are the velocities we measured after looking at them. We see these pockets I mentioned in yellow. They are the pockets of subsidence. There is a very concentrated pocket in Surfside [here Wdowinski indicates the area of the collapsed Champlain Towers South building] but we can see over in the western part of the city, different pockets where we see subsidence. On the right of Figure 5 is what we call the uncertainty, how uncertain we are, what is the level of certainty that we have. We measure between a minus four to plus two millimeters per year of subsidence in those pockets with an uncertainty is between zero and one millimeter per year.
And when we look at these different points [See Figure 1], we see subsidence here [Wdowinski is pointing to topmost circled yellow spot in Figure 1, the area of collapsed Champlain Towers South building] of about two millimeters per year as we reported in the paper. This is the building. Again, we see subsidence in the 1990s mostly in the western part of the city but this is the only pocket in the eastern part, this one concentrated pocket. We noticed today that this was the building that actually collapsed. We were very shocked about it.
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[i] The building is not identified by name or address, only as a “12-story high condominium building.” 12 stories is the maximum for buildings in Surfside.