Abandoned mine likely cause of Des Moines' south-side sinkhole

Abandoned coal mines likely contributed to a sinkhole that swallowed a 40-foot tree and the front yard of a home on Des Moines’ south side early Wednesday morning.

The incident has prompted an investigation by the Iowa Mines and Minerals Bureau and has forced to the surface some history buried beneath the city's soil.

“This whole area is nothing but one big mine underneath here,” said Melanie Bower, whose mother lives near the sinkhole that formed in the 4100 block of Southwest Fifth Street. “We’re a little concerned with that.”

Historical mining maps show the South Des Moines Coal Co. operated a 203-acre mine beneath where the south-side neighborhood now sits. It was one of more than 200 mines that operated in the Des Moines area during the late 19th century and early 20th century.

“If you’re in the Des Moines area, there’s a good chance there’s a coal mine beneath you,” said Calvin Wolter, a Geographic Information Systems analyst with the Iowa Department of Natural Resources.

Despite the widespread prevalence of abandoned coal mines, officials with the DNR say they seldom create sinkholes. The last sinkhole related to a coal mine in the metro was reported in 2013.

The 30-foot wide and 25-foot deep void that formed Wednesday was located just yards away from a mine shaft sealed long ago, according to a historical map of the mine drawn nearly 100 years ago.

“We’re definitely aware of that shaft right there and looking into that,” Susan Kozak, bureau chief with the Iowa Mines and Minerals Bureau, said Wednesday afternoon.

“They were all closed so long ago that I really don’t know how (the entry shafts) were filled in at the time or how big they were.”

Investigators estimate the mine shaft dropped 200-plus vertical feet, based on similar mines in the area, according to Jonathan Gano, Des Moines’ director of public works.

'It just dropped vertically'

The ground began caving in around 1 a.m. Wednesday.

A neighbor called police to report that his home was shaking and he heard noises that sounded like someone filling a Dumpster, according to Brian O'Keefe, spokesman for the Des Moines Fire Department.

The man drove around his neighborhood to investigate. When he returned home, he noticed a 40-foot tree had disappeared from his neighbor’s front yard.

“It just dropped vertically,” O’Keefe said.

The incident temporarily cut off power to the home, which was was restored Wednesday while crews secured a perimeter around the hole.

“We’re fairly confident that it’s stable enough that it’s not an imminent threat to life and property,” Gano said Wednesday night.

Soil was still falling from the steep edges of the sinkhole, but "it’s not growing appreciatively in diameter. It's just the edges falling in a bit,” he said.

The city advised homeowner Darrell Zeliadt, whose yard was eaten by the sinkhole, to stay out of his home for the night. Zeliadt could not be reached for comment.

Public works crews planned to return Thursday morning with a crane to start removing the tree.

“The trunk is buried in the soil, so we can’t just pull it out,” Gano said. “We’ll need to cut it and take it out in pieces. The buried trunk will have to stay.”

Crews will likely plug the hole with concrete and then fill it with dirt, he said.

Be aware, but don't panic

According to the Mining and Minerals Bureau, two or three mine-related sinkholes typically occur each year in Iowa.

This is the first one in the Des Moines area since 2013, when a 10-foot wide and 10-foot deep hole caved in a homeowner’s yard in Runnells.

Thousands of other naturally occurring sinkholes have been documented in the state, but most of those are isolated to northeast Iowa and are caused by dissolving limestone bedrock, according to state geologists.

Ryan Clark, a geologist with the Iowa Geological Survey, said collapsing mines are typically the culprit with sinkholes in central Iowa.

“If the underground mines did not have sufficient rock above them, then (the ground) is relying on those old support beams,” he said.

“As those structures deteriorate with time, they’re probably going to fail at some point.”

When shafts are deep or covered by firm bedrock, cave-ins typically go unnoticed above ground.  But problems can arise with shafts closer to the surface or in areas where bedrock has dissolved.

The mine beneath the latest sinkhole spans from Southwest Ninth Street to beyond South Union Street and from Thornton Avenue to McKinley Avenue.

“This whole south side, they say, was all coal mines,” said Chuck Baker while surveying the hole outside his neighbor’s yard.

He said stories about earlier sinkholes have circulated through the community.

Bower, whose mother is another neighbor, said her family began researching the area's mining history after noticing basement wet spots and areas where the "floor was giving” in her mother’s house.

“We just looked it up the other day because we’re concerned about it. Then this hole opens up today,” she said.

Clark, the geologist, said there's no reason for residents to panic, but abandoned mines are important for residents and developers to consider.

“It’s something that I think people should be well aware of. Unfortunately, it’s not until situations like this that people think maybe we should check this out,” he said.

6,000 abandoned mines in Iowa

The Iowa Geological Survey estimates that as many as 6,000 underground coal mines are scattered across 38 counties and could potentially affect 80,000 acres of Iowa land.

Many of these mines are located in central Iowa.

The Iowa DNR maintains an online map that marks all of Iowa’s identified coal mines. It’s available online at programs.iowadnr.gov/maps/coalmines/#.

Mines in Des Moines area

The former mine in the area of the sinkhole. A black dot near the center of the map identifies an "air shaft" just yards away from where the sinkhole appeared in the front yard of a residence in the 4100 block of Southwest Fifth Street.

A historic map of the Des Moines area, provided by the Iowa Department of Natural Resources, shows dozens of coal mines (indicated by the color markings) throughout the metro.