Could a sinkhole have caused the Miami apartment collapse?

The 12-storey Champlain Towers, located in Surfside, Florida, collapsed on Thursday morning.

A search and rescue operation continues with more than 140 people still unaccounted for. So far, 24 people have been reported dead.

University of Leeds engineering geologist Bill Murphy said that when he first saw the aerial images and the fact that the building “really came down just on one side”, his first reaction was that it looked like a sinkhole.

Sinkholes can occur in carbonate rocks such as chalk and limestone, which are prevalent in Miami.

“There are plenty of examples of sinkholes in the Miami area so it’s not impossible,” Murphy said.

“The ground conditions I would expect at this site would be limestone underlying engineered fill. There have also been reports of a leaking swimming pool and structural distress, which wouldn’t be a surprise if there was a sinkhole locally. The collapse of the overburden into a sinkhole could have been triggered by seepage or vibration.”

However Murphy emphasised that it is too early in the investigation to be able to decisively determine the cause.

Meanwhile, Wilgeo director Phil Wilson added said given the “anecdotal evidence” of sinkhole challenges and the ground conditions in Florida, when he saw the collapse he also considered sinkholes to be a possible explanation.

“I know nothing of the site,” he emphasised, “but the other thing that got my suspicion was reported problems around the swimming pool - a combination of a source of water (a swimming pool) and a rock that’s prone to solution collapse.

“This is pure speculation on my behalf whether the two are linked and I’m sure the investigators will have that on their agenda of things to look at. The fact that it was a dramatic collapse also suggests it was a sinkhole but again I’m sure the investigators on the ground will be alert to these problems.”

Uncertainty

However geotechnical specialist Clive Edmonds urged caution.

“It’s difficult to be certain [regarding the cause],” he said. “A sinkhole a possible factor but I wouldn’t necessarily say it’s the main line of enquiry at this stage. But I’m sure at some point once we have more data it will clarify whether unstable ground beneath the property has played a role.”

Edmonds said more information is needed regarding whether the building was constructed on stable limestone, and emphasised that the type, depth and diameter of the piles is also unknown.

He added: “It looks like the area is underlaid by limestone but, having said that, the ground level is virtually at sea level so it’s fully saturated and any voids in the limestone would obviously contain water.”

When water is sitting in a limestone aquifer the water will tend to give some hydraulic support, Edmonds said, but if the limestone is filled with air with water washing through from the surface it can cause erosion.

Ground investigations

Built in 1981, the apartment block was due its standard 40 year review. According to officials, it was undergoing its "recertification" process and required repairs.

Wilson said it would be useful to know what site investigations were undertaken prior to construction.

“You wouldn’t be able to see the sinkhole from the surface,” he said. “I would expect the people retained to look at the problem to look at what site investigation was done for that building given that it’s a known problem in the area.”

Murphy added that site investigation practices have changed since the building was constructed.

“There would probably have been a site investigation [before the building was built] but boreholes are not the best way to identify voids. Geophysical methods such as resistivity or microgravity surveying give much better detection of sinkholes,” he said.

“You’re not necessarily dealing with something that forms after construction – a lot of sinkholes exist before construction and the void migrates afterwards.”

Edmonds added: “Given that the building is 40 years old it’s hard to know what piling technology at the time would have been or how rigorously they would have checked things. If we were doing this now with piles we would probably insist on some drilling to check that you’ve got no voids beneath the piles and that they have stable support.”

Subsidence

Experts who studied the apartment complex last year warned that it was unstable, with one study from a Florida International University researcher finding that the building had been sinking at a rate of 2mm per year in the 1990s. There was a “localised area of subsidence” around the building, researchers have said.

According to Murphy, areas where the formation of sinkholes has occurred “tend to be very localised” – sometimes measuring a couple of metres although there are examples in Florida that are “much bigger”.

“So with having an area where there has been ground movement, if there was a sinkhole it is perhaps no great surprise,” he said.

However Edmonds said there could be a number of explanations for the movement.

"The amount of movement being measured shows there is a settlement ongoing there," he said.

"It is not massively dramatic in some ways but it does show a trend. But this could also fit with a settlement over compressible materials, so it doesn’t necessarily guarantee that it may be related to sinkhole development. You can’t rule it out but it’s not definitive.

“We need some bore holes and a lot more data but that’s not going to come for some time.”

John Pistorino, a Miami structural engineer who evaluated the Florida International University bridge collapse, has been hired to investigate the cause of the collapse.

Other possible explanations have included issues with the ground floor pool deck area, ground movement from nearby construction work, salt impact on the building and structural issues.

An engineering report from 2018 has revealed that the building had “major structural damage” and needed to be extensively repaired.

Like what you've read? To receive New Civil Engineer's daily and weekly newsletters click here.