Geotechnical Engineering: Sinkholes, Subsidence, & Their Affects Author: Caryl Mangual, caryl@ucf.edu Introduction: Structural foundations are the premises to construct a quality building design. When constructing, in Florida, there is an added layer of uncertainty with the terrain. One specific challenge to be had is the terrain’s susceptibility to sinkholes and subsidence. It is important to know where it is possible to build a structure and how much load that land can uphold given these challenges. Let’s start with the understanding of what a sinkhole is and understand the differences between that and subsidence. In Florida, limestone is porous, and it allows the acidic water (or acidic rain) to percolate through their strata, dissolving some limestone and carrying it away in a solution (Florida Department of Environmental Protection). In turn, this creates extensive underground voids that lead into a drainage system in most of the carbonate rocks throughout the state. This then has an effect over the overlying sediments causing it to collapse into the ground cavities creating sinkholes. This is not the same thing as subsidence. Land subsidence can be summarized to a gradual settling or sudden sinking of the Earth's surface (Harris, 2024). Basically, compacting soil. The following will be two case studies which will allow us to explore these two ground dynamics and their consequences. Case Studies: Case 1: Summer Bay Resort in Clermont, Florida – Sinkhole On Monday, August 12th, 2013, tourists were rushed out of their condo resort due to the collapsing building structure in which they were residing. Thankfully, to the smart wit of the security officer, he was able to notice something was terribly wrong and got everyone out before the collapse happened. It took about 10-15 minutes for the sinkhole to swallow the building. So why does this happen? How are we able to know the likelihood of something like this happening to new building developments as well as existing structures? This all starts with understanding why Florida is more susceptible to sinkholes. As mentioned before, Florida is known for its porous limestone which creates karst landforms of caves, disappearing streams, springs, and underground drainage systems. “Karst is a generic term that refers to the characteristic terrain produced by erosional processes associated with the chemical weathering and dissolution of limestone or dolomite, the two most common carbonate rocks in Florida” (Florida Department of Environmental Protection). Many do not realize that several of Florida’s lakes and ponds began with a sinkhole formation. In order to mediate sinkholes, compaction grouting is used to fill the voids. In this process, pipes are drilled at various angles, then after the injection pipes are installed, the grout is injected under intense pressure filling voids and creates a strong layer beneath (Brown). Unfortunately, there is a lack of studies being done about sinkholes and other types of karst features in the state. If more studies are conducted, then Florida would have a clearer view of these karst landscapes that foster sinkholes. At one point, UCF got funding after a sinkhole opened in Winter Park in 1981, but it was defunded a decade later (Brinkmann, 2013). Figure 1 will give some insight as to what is happening in Florida’s landscape. Figure 1: Florida’s Karst Terrain Case 2: Champlain Towers South collapse in Miami (Surfside) – Structural On Thursday, June 24th, 2021, tragedy struct a 12-story beachfront condominium in Miami’s suburb of Surfside, Florida. Unfortunately, some were injured, and many lost their lives in the collapse of this building structure. Investigations have led to pinpoint that there was a long-term degradation of the reinforced concrete structural support located in the basement level of the parking garage, which happened to be directly under the pool deck (Wikipedia, 2021). Though this is the main cause of this collapse, along with insufficient reinforcing steel and corruption during construction, it has also brought light to the growing concern of subsidence. Land subsidence is the gradual settling or sudden sinking of the Earth’s surface. In simpler terms, soil compaction. Upon further research, there are spots along the coast that are lower than what they were years ago. It was estimated that Champlain Towers South building sank 2 millimeters from 1993 to 1997 (Harris, 2024). Good engineering practices have to consider the building load and its effects on the ground causing the soil to settle slowly over a period of time. Localized subsidence is a serious problem especially when one side of the building sinks faster or further than the other side of the building. This in turn causes a huge strain on the building’s foundation. When looking at the Champlain Towers South, since it’s already in a low-lying area, subsidence even by millimeters matter. The continued subsidence, combined with the rising seas, rising groundwater levels and rainstorms could have this south Florida region at serious risk. Because Miami Beach was built on reclaimed wetland, it will continue to be subjected to more subsidence (Tejedor, 2021). If it’s caught in time, then remediation can occur by engineers refortifying the structure. But a better approach would be to implement newer technologies that can alert detection of movement in areas. This way engineers or building inspections can understand that the building is showing signs of movement and be able to check right away the issues to prevent such catastrophic collapse. This figure shows a chart analyzing the subsidence occurrence in the Miami Beach area. Chart: InSAR Analysis Conclusion Whether it be due to sinkholes or subsidence, being proactive and increasing our knowledge of these terrains is necessary as the population growth continues to expand and construct in unique environments. Educating engineers and other construction specialties provides a particular insight into what is going on deep within the soil and will provide a better understanding of how the land may behave in the future. This way one can create solid foundations that are safe and sustainable for decades to come. References Brown, D. Foundation Professionals of Florida. Sinkhole Damages Buildings at Popular Resort in Clermont, FL. https://www.foundationprosfl.com/sinkhole-damages-buildings-atpopular-resort-in-clermont-fl/ Florida Department of Environmental Protection. About Florida Sinkholes. https://floridadep.gov/fgs/sinkholes Harris, A. March 29th, 2024. Sea levels are rising but is South Florida also sinking: What the research shows. https://www.miamiherald.com/news/local/environment/climatechange/article287215280.html Tejedor, C. June 29th, 2021. FIU Geophysicist Sheds Light on Miami Beach Subsidence Study. FIU News. https://news.fiu.edu/2021/fiu-geophysicist-sheds-light-on-miami-beach-landsubsidence-study Wikipedia. Surfside condominium collapse. https://en.wikipedia.org/wiki/Surfside_condominium_collapse
