Two weeks ago, Bob Borg stepped into his front yard to discover a large hole, nearly 10-feet deep and approximately 10-feet wide, just a few steps away from his home. The earth eroded around a tall power pole, which was removed to eliminate the risk of it falling on the home at 105 West McKinney Street.
But there could be a bigger threat to the homes and city road nearby where the ground caved in, according to state officials.
Friday morning environmental engineers plan to dig into the sinkhole with an excavator, looking for an explanation of what caused it.
“There are several mines in the area,” said Mike Bourland, an environmental engineer with Iowa Department of Agriculture’s Mines and Minerals Bureau. “We don’t know the extent of the mines. We just know that they were located in Runnells.”
More than 200 closed-off underground mining locations have been located in Polk County alone, according to Iowa’s Mines and Minerals Bureau Chief Todd Coffelt. These locations involve veins of underground tunnels, which the state of Iowa required companies to document and map out for state records as of 1902.
But places like Runnells, where mining began in the 1880s, are problematic because there is no detailed map of the underground tunnels, Coffelt said.
The eastern Polk County community of about 500 people used to be a final coal stop for steam-paddled boats headed up the Des Moines River. This created a market for “mom and pop mines” within the community, Coffelt said.
“There’s no surveyed mine map known for that except for the general locations,” he said, noting the unpredictable risk of sinkholes. “There’s no way to predict when a subsidence event would take place.”
The phenomenon is nothing new for those living in the area, according to residents.
Runnells Mayor Ron Tate said in the last 20 years multiple sinkholes have appeared in a field on the east side of town, where two entrances to mines were located.
In the ’80s, a sinkhole nearly 25 feet in diameter caved in a portion of two residential roads, also on the east side of town, at the intersection of Person and Maple Street, Tate said. No homes were impacted in the incident.
“I knew there were mines in town, but I never really thought about it much until that one happened,” said Tate, adding that sinkholes still are not a major concern. “Not unless it starts happening quite often, then I’ll start worrying about it.”
State officials said Friday’s excavation should reveal whether the recent incident on the western part of downtown was the result of an abandoned mine, or some other feature like an old well or septic system. The cavity, which is currently fenced off, is located on the same block as City Hall and a U.S. Post Office building.
It the excavation reveals a mine shaft – identified by railroad timbers or other support structures beneath the soil – crews will fill the space with crushed limestone to minimize the risk of future sinkholes, said Bourland, who will oversee the investigation Friday.
“Anytime there is an abandoned underground mine there’s always some level of risk,” Bourland said.