Last Thursday, 24th June, 55 apartments in Champlain Towers, Miami, fell to the ground. Within 12 seconds, only dust remained [1]. Over 150 remain missing whilst 11 have been reported dead as search and rescue teams search the rubble with canine units [2].
Estimates indicate that, in any given year, the probability that a reinforced concrete or steel building in a technologically advanced country, like the United States, will fail ranging from one in a million to one in a hundred trillion. The probability of death from a structural failure is approximately one in ten million per year – that’s 25 per year in the US [3]. So, what allowed this single building collapse to take such a percentage of the average?
Structural engineer Allyn Kilsheimer is looking into the reasons for the collapse and believes his investigation could last longer than a few months. He will use a “meticulous, computer-assisted process of elimination to attempt to identify the cause(s)” [2].
Video footage taken by a surveillance camera shows that the building fell in stages. First, the south-facing centre of the building crumbled; followed by the north face; and eventually, the east face.
The pool had been identified as an area of major concern in the building’s 2018 structural report. This is where the slump began. The pool caved into the underground car park which had previously shown cracks in columns, beams, and walls, and water damage [1]. Failure to establish adequate waterproofing was sure to further concrete deterioration.
Concrete damage permits the exploitation of faults by water. Hydraulic erosion, pressure, and thermal expansion increases concrete damage [4]. Steel rudders within beams are exposed to rust activating flaking and concrete breaks away.
The disaster has called attention to the danger of construction in areas under pressure of sea-level rise. The integrity of buildings is under question as salty water advances, weakening foundations. Salt intrusion affects varieties of rocks differently; less resistant rocks with low tensile strength undergo much stress from crystal growth and thus, more damage occurs. Rocks with a lower porosity will have a higher tensile strength and be more resistant to damage by crystallisation [5].
Much of the Biscayne Aquifer, Miami, is formed of highly porous and permeable limestone – hence, Miami’s geology is particularly vulnerable to the persistent weathering of mother nature [6]. Climate change and sea-level rise will also impact coastal aquifers by changing the quantity of water and its salt level [7]. Therefore, the groundwater system will be off-balance, and shallow water tables are likely to impact more types of infrastructure like septic systems and roads.
Prior to construction, and to now find the root cause, samples of soil should be analysed for rock and salt types present. This would help confirm whether any concrete stress was caused by crystal expansion or chemical hydration rather than thermal expansion. Evidence of cracking further up the building’s concrete structure would be due to the capillary transport and evaporation of saltwater intrusion [5].
Left: Champlain Towers, Right: After 24th June’s Collapse
Sources
[1] BBC, Visual Journalism Team, "Miami building collapse: What happened, and how quickly?," BBC News, London, 2021.
[2] C. Devine, "As engineers hunt for answers in the Surfside building collpase signs point to the building's lower reaches," CNN, 29 June 2021. [Online]. Available: https://edition.cnn.com/2021/06/28/us/surfside-condo-collapse-cause/index.html. [Accessed 29 June 2021].
[3] H. Petroski, To Engineer Is Human, New York City: St Martin's Press, 1985.
[4] GGNC Construction, "How Standing Water Damages Cement Foundations," GGNC Construction, 19 February 2014. [Online]. Available: https://gg-nc.com/water-damages-foundations/. [Accessed 29 June 2021].
[5] S. Bates, Critical Salt Weathering, Oxford: Geoverse, pp. 9-10.
[6] USGS, "Miami Limestone," U.S. Department of the Interior, Washington D.C, 1967.
[7] W. A. K. Alek K. Manda, "Saline intrustion: Adaptation strategies to address rising water tables in coastal environments under future climate and sea-level rise scenarios," 2019.
[Online]. Available: https://www.sciencedirect.com/topics/earth-and-planetary-sciences/saline-intrusion [Accessed 30 June 2021].
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