How mud buildup in an ancient cave may have caused the giant sinkhole in Alton park

A build up of mud and sediment in an “ancient cave” that gave way may be the cause of a massive sinkhole in an Alton Park, Mayor David Goins said Tuesday.

Goins said that’s based on preliminary findings by the U.S. Mine Safety and Health Administration, but officials of the company that operates a limestone mine under the collapsed area said they are unaware of that theory and the investigation into the cause is continuing.

“We’re not seeing it now as we drill,” said Mike Clarke, CEO of New Frontier Materials of Maryland Heights. “So, we want to keep going.”

A recent inspection of the mine under the park by MSHA in early May found no major problems, Goins said.

The timeline to determine the exact cause of the sinkhole — and the necessary repairs — are still being fleshed out.

Leaders from the city of Alton and New Frontier Materials said Tuesday they are working to determine those details, but it will take time.

“We clearly understand the urgency necessary to get this done,” said Clarke. “That said, we can and must prioritize safety over all else.”

New Frontier Materials continues investigating drilling to figure out what caused the 100-foot-wide sinkhole to form and if any areas near the collapse are also at risk, Goins said.

MSHA, a U.S. Department of Labor agency, is assisting and advising the investigation. Drilling started near the access road into the park and is moving closer to the sinkhole.

The sinkhole has roughly doubled in diameter since the collapse, according to Justin Kleinschmidt, an engineer from the firm Sheppard, Morgan & Schwaab, who’s working with the city.

“It’s not getting any bigger underground, is what we’re being told,” Kleinschmidt said. “But it’s sloughing off the sides. We expected that to happen. It’s going to continue to do that until it gets to a natural point where everything is stable on those edges.”

After the investigation is completed, the hole will be filled. It is not yet clear who will pay for restoration of the soccer field where the sinkhole currently stands, Goins said.

“That would be an ongoing discussion between the legal counsels on both sides,” he said.

Mine operations stopped after the collapse on June 26. A handful of employees have not been working. There won’t be permanent layoffs, but New Frontier is still trying to determine what the immediate future looks like for those employees, Clarke said.

Since late June, recreational activities for a variety of youth and adult sports leagues have been suspended. Michael Haynes, the city’s parks and recreation director, said many have been able to find an alternative location in a nearby town.

Haynes said it’s not a question of “if“ the park reopens, but “when.”

When it does, Haynes said it’s reasonable to assume it may take a while to build up the public’s confidence. As of now, the city is staffing the park all day to keep the public out while the investigation ensues.

Sam Nowak, co-founder of the geomechanics and mining consultant firm OptiRock Group , said the city and company should publish a map of the underground facility.

“Those maps of the underground areas should be widely available for people to see and understand the risks of what they’re building on top of it,” Nowak said.

However, Clarke said that’s not something New Frontier Materials “wants to send out to the public” because those maps contain competitive information. The company does share the maps with the city’s engineers, he said.

It’s not uncommon for underground mines to be built underneath roadways, buildings or other public structures, Kleinschmidt and Nowak said.

Bluff City, the Alton mine and quarry, first opened in the early 1950s, according to the company. Gordon F. Moore Park was built in the mid-80s.

In all, 55 employees hold positions at the Alton facility — 28 of which work underground — according to federal records from the last quarter of 2023.

Over the last five years, federal inspectors from MSHA have regularly inspected the mine every couple of months.

Bluff City sits below the average of peer mines for “significant and substantial” violations, according to federal data. Since the start of 2019, that rate stands 1.64, while the national average is 2.78.

Timeline of events

• 4:30 a.m. — New Frontier Materials first detects mine subsidence.
• 7:30 a.m. — Company employees reported signs of the subsidence to senior management. The park is cleared of people.
• 8:30 a.m. — Company management performed an on-site inspection underground.
• 9 a.m. — Company contacted MSHA to report the incident and seek assistance.
• 9:18 a.m. — The subsidence escalated, and the sinkhole formed.