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Sea rise under scrutiny in condo collapse: Corrosion likely, but no sign of sinkhole

A photo posted July 20 on Twitter by Florida Sen. Jason Pizzo, D-North Miami Beach, shows the Champlain Towers South site cleared of all rubble from the tower’s collapse. The emptied-out foundation, free of prior supports, has a Surfside engineer worried about a cave-in that could cause Collins Avenue to collapse, too. COURTESY JASON PIZZO


Scraped clean of tons of rubble late last month, the bare garage floor of Champlain Towers South appears to rule out at least one early suspect in its catastrophic collapse.
There were no telltale signs of a sinkhole.

The garage floor, the building’s lowest level, remains in one piece with no craters or potholes suggesting unseen geological forces were at work. The “sinkhole” a doomed resident saw opening from her balcony in a final phone conversation was likely not erosion beneath the building but the implosion of the concrete pool deck above the garage floor — the seeming trigger event of a massive and still unresolved structural failure.

Jennifer Huergo, a spokesperson for the National Institute of Standards and Technology, the federal agency investigating the cause of the collapse, agreed she did not see an obvious sinkhole in a photo provided by the Miami Herald, but a formal engineering assessment has not been completed and results will not be released for some time.

“The slab appears to be intact and there is no obvious sinkhole,” Huergo said. “That said… our experts will be looking at every aspect, above and below ground, for potential triggers.”
Those aspects will include the effects of rising seas on the 40-year-old building, which remain on the long list of contributing factors for structural engineers and other experts trying to piece together the causes of the unprecedented disaster. If increasing tidal flooding didn’t undermine the building, it may have pushed corrosive brine into the parking garage.

Even now, time-lapses of the empty garage floor show flooding that rises and falls with the tide throughout the day. And a former maintenance manager described to Herald news partner CBS4 regular pumping of salt water from the garage — conditions experts say would likely exacerbate the effects of rust on reinforcing steel in concrete slabs and columns.

“We’re chasing like 50 different things and trying to understand them one at a time,” said Allyn Kilsheimer, the independent structural engineer that the town of Surfside hired to investigate the collapse.
Kilsheimer himself isn’t ready to rule anything out, including sea-rise impacts and possible undetected voids or sinkholes under the building. Among the many things he wants to assess are the effects of tides and full moons on the underground water table. He will drill a hole near the north building (a few blocks from the south building) and insert a device called a piezometer, which measures groundwater levels. That, he said, is a challenge in an underground garage on a barrier island — one that also underlines the looming threat of sea-rise to South Florida.
“We can’t drill through the basement of the garage because it could create a geyser at high tide,” he said.

A RISING THREAT

No matter what, if any, role rising seas played in the collapse, there is no escaping the rising risks to Surfside and other coastal communities up and down the Florida coast. Low-lying garages in South Florida have flooded for years, some famously so. Recall the 2016 photo of an octopus finding its way up a drainpipe into a Miami Beach condo’s garage.

The two feet of sea level rise expected by 2060 will swamp septic tanks, homes, parks and roads. And as waters keep rising, it will eventually render some places permanently uninhabitable.
In a meeting just last year, the town heard from consultants that it faces structural threats from sea rise. The most pressing risk is not along the higher elevations of the beach where Champlain Towers was but on the low-lying side adjacent to north Biscayne Bay.

Streets in the single-family home part of town nearer to the Intracoastal Waterway flood so deeply after heavy rains that the water has been reported ruining cars and soaking into houses, garages and crawl spaces. Residents even floated the idea of installing “No Wake” signs to protect their homes.

The meeting was the public debut of a flood model the city commissioned from Atkins Engineering through a partnership with the American Flood Coalition. Caroline Resor, a strategy associate with the flood coalition at the time, told residents that sea-rise-driven flooding could occur as often as 100 days per year by 2030. She said the more extreme of those floods could cause $400,000 to $2 million in damage to homes.

Atkins Engineering made a map of flooding hot spots in Surfside, which consultants presented to the city in a November 2020 meeting on sea level rise impacts. ATKINS ENGINEERING

“That is obviously a risk that is increasing,” she said. “Other parts of town are far less at risk from tidal flooding.”

The tiny town hasn’t ignored the threat and actually has a brief history of pioneering climate change policy. In 2019, Surfside created a fund developers paid into designed to help buy out future residents when their homes became unlivable. But it was immediately overturned by the new mayor, Charles Burkett, and a slate of commissioners. At a meeting in November, Burkett gave a presentation on his preferred solution to avoiding the floods — elevating single-family homes with federal grant money.

There was no discussion on what to do with seaside condominiums.

SEA RISE A FIRST SUSPECT

Nevertheless, the effects of sea rise almost instantly emerged as a suspect in the immediate aftermath of the collapse — fueled by both the “sinkhole” report and national news coverage of a 2020 Florida International University survey of the coast that showed that the condo had slowly been sinking. Though the FIU professor himself downplayed the connection, noting it was a small shift at one point in time, the specter of sea rise taking down entire buildings made headlines for weeks.

The sinkhole theory was serious enough that a trade publication, New Civil Engineer, produced an inconclusive report about it in early July.
Investigators have since dismissed most of those theories.

Was the building built on an old inlet that was slowly reclaiming the land? No, that seemed like a misread of an old and poorly written map. Was it built on wetland that was slowly settling? No, old development maps show that Champlain Towers South went up on an original section of the island, and wasn’t filled in with mowed-over mangroves and dirt like the western side.

But the corrosive effect of routine flooding at the building’s base remains an unanswered question — and very much part of the forensic analysis.

Water, especially saltwater, can worm its way into concrete, creating pockets of air that eat away at the adhesive between the components of cement, eventually reaching (and destroying) the reinforcing steel rebar embedded within.

Rubble and debris of the Champlain Towers South condo seen in Surfside, Florida on Tuesday, July 6, 2021.

The rubble shown is from the front portion of the condo towers, which was demolished 11 days after the back part of the tower collapsed with people inside. MATIAS J. OCNER MOCNER@MIAMIHERALD.COM

Corrosion from salt spray, storm surge and tidal flooding is an old foe in coastal areas, often the biggest and most expensive repair faced by aging buildings. Buildings constructed in the 1980s — before tougher codes and improved construction techniques — are particularly vulnerable. One pool maintenance worker told the Miami Herald the garage had standing water in it 36 hours before the building collapsed, and he was told that the underground pool maintenance room ran through a new water pump every two years to keep the room dry.

Still, many experts, including Dawn Lehman, a professor of civil and environmental engineering at the University of Washington retained by the Herald as a consultant, stress that corrosion alone is unlikely the reason the building failed. Buildings, she said, are designed to have structural redundancies.

“We know corrosion is an issue for many buildings on the coast and it must be addressed, but we do not expect it to lead to full building collapse without other serious structural deficiencies,” she said
But flooding episodes also have become more common as sea levels have risen, including in the Champlain Towers underground garage, which sits about 10 feet below ground in a part of the barrier island where the water table is only about two feet below the surface. In the 40 years since the towers were constructed, sea levels have risen about eight inches at Virginia Key, the nearest official tidal gauge.

WATER FROM BELOW

That tidal rise can have significant impacts in a place like South Florida, which is built on porous limestone that allows water to flow freely below the surface. It’s why sea walls are ineffective at holding rising tides at bay.
The top layer of that water is a small lens of freshwater that floats on top of the denser saltwater pushing in from below. In the case of Miami Beach, the lens gets shallower near the edges, which means saltwater is closer to the surface. But just how close is hard to know.

And that could be important in the Champlain Towers probe because of potential impacts on piles — the concrete columns shoved deep underground that help hold up buildings. It remains unclear how deep the piles under Champlain Towers were. It’s not recorded in the original plans the city of Surfside released to the public, and the town’s consulting engineer Kilsheimer said it would require sonic testing at the site to know for sure.

That testing, of the concrete caps on the piles and the dirt around them, can determine what condition the piles are in, as well as if there are any voids or sinkholes underneath the building. Underground issues certainly aren’t unheard of in South Florida and the underlying limestone geology is pocked with holes. There are anecdotal stories of individual buildings in Miami with secret underground canals undermining their foundations, or old Miami Beach hotels that look perfect on the surface but had crumbling foundations underground.

But sinkholes capable of swallowing cars, homes and buildings are far more common in Central Florida, and can even occur in mainland Miami, but they’re rare on barrier islands, according to Florida’s sinkhole report database.
John Pistorino, a longtime Miami-Dade structural engineer hired by attorney Stuart Grossman for a lawsuit representing the survivors and victims of the collapse, said in his 50-year career investigating building failures in South Florida he couldn’t recall a time where sinkholes were a major issue.

“In my experience with the soil conditions along the coast where all these high rises are, sinkholes are not a concern. The bigger concern would be storm surge, scour… which would undermine shallow foundations but not deep foundations like these buildings have,” he said. “But there’s a first for everything.”

HUNTING FOR CORROSION

With corrosion the more likely contributing factor, investigators are looking for clues in the now exposed garage, where regular flooding occurred, and the remains of the pool deck and ground floor above it.
At least six engineering experts told the Herald it appeared as if the initial failure that triggered the building collapse happened in that concrete slab that supported both the outdoor pool deck and the first floor of the building itself. Why it failed remains the big question — did the slab simply crack at a weak spot or did the failure occur in the connection to the garage floor columns supporting it from below?

Previous engineering reports revealed the slab had serious deterioration of the concrete and reinforcing steel and was in line for multimillion-dollar repairs. Video obtained by the Miami Herald from minutes before the collapse also showed what appeared to be concrete rubble assumed to have fallen from the pool deck slab to the floor of the underground garage below.

Herald consultant Lehman, after examining dozens of photos, videos, documents and all available design plans for the building, considers the slab itself a leading suspect.

That’s where regular garage flooding comes in. Lehman said it’s possible that could have kept the garage humid and salty, a bad environment for most metals. She said her examination of reports on concrete cores and other repairs indicates that the amount of concrete cover over the reinforcing steel may have been insufficient for that type of exposure to the elements.
She noted that photographs and prior repairs show the reinforcement in the pool deck had corroded and was not replaced.

“The cause of that corrosion is probably in the garage,” she said. “Because they designed that bottom cover as if it were an interior slab, but the exposure of the garage to regular flooding is deteriorating the ceiling of the garage, the bottom of the pool deck.”

Lawrence Kahn, a professor emeritus of structural engineering at Georgia Tech, said the rainwater collecting on the pool deck, which a consulting engineer noted had a design flaw that retained water, could be a more likely culprit.
“If there hadn’t been seawater inundation that reached the bottom of the slab then it’s more likely that it came from the top,” he said. “The humidity carries a lot of salt with it, but generally, it’d be much slower to absorb from the bottom.”

But he said it is also possible that rust could have spread from the garage floor up through steel in the columns into the pool deck above.

Planned materials testing may help piece together the puzzle and how tidal flooding and rust fit in.

Kahn said investigators can tap columns with a geologist pick and listen to the results. A clear ringing is a sign the column is structurally sound, while a dull thud hints that it’s cracked or deteriorated.
“That’s the first sign that something’s wrong,” he said.

Large sections of concrete are transported from the debris field of the 12-story oceanfront condo, Champlain Towers South in Surfside on Wednesday, July 7, 2021. Al Diaz ADIAZ@MIAMIHERALD.COM

For a more thorough examination, investigators are likely to drill holes every inch or so and test the dust for chloride levels, an indicator of salt, said Paul Danforth, principal engineer and vice president of Universal Engineering Sciences. The map they form with their findings helps engineers see where saltwater and air may have infiltrated, exposing the steel in the core of the concrete.

For now, any onsite testing underway is by NIST. Surfside’s investigator, Kilsheimer, is also eager to begin his tests to determine what exactly caused this catastrophic collapse.
“I’m not ruling out anything, because what you can see with your eyeballs means nothing,” he said. “What you have to do is get in there and do the sampling and the testing at the site.”


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