KENTWOOD, Mich. (WOOD) — They are big enough to swallow a car in some cases, and they’re appearing in local shopping center parking lots.
Twice over the last two weeks, large sinkholes have opened up along the Grand Rapids-Kentwood border.
The first happened on Sept. 6 when a sinkhole opened up at Centerpoint Mall. This one was deep enough to swallow the front portion of a vehicle.
The most recent sinkhole opened up Thursday in front of the 28th Street Michael’s store, not too far from Centerpoint Mall. By early evening crews were filling in the hole which looked to be three or four feet deep, opening up in the middle of the parking lot traffic lane.
Kentwood city engineer Jim Kirkwood said there are two causes of sinkholes.
The first is a failure of storm sewer lines, the pipes that carry runoff from streets and parking lots.
“Joints can pull apart. Then when it rains, sand can migrate down into the joints and over time the sinkhole forms as the sand disappears from underneath the asphalt,” Kirkwood said.
Those sinkholes happen gradually and usually come with some early warning, like a much smaller pothole.
In the case of the Centerpoint Mall sinkhole, it was caused by a busted water main, and those can open up with little or no warning.
“That’s just an act of God accident,” Kirkwood said. “You’ve got pressurize water and that will just wash out and flood all of the sand instantaneously.”
It’s not clear what caused Thursday’s sinkhole on 28th Street, but mud in and around the hole seems to indicate a water main break.
Both holes opened up on private property. Kirkwood said there is a reason you seldom see this kind of cave-in involving streets, sidewalks and other public right of ways.
On government projects, consultants work alongside construction crews.
“And they are inspecting the construction as the contractor builds it. All the specifications are there. So we’ll have rubberized O rings joints for all of the sewers,” Kirkwood said.
But the rules are different for private property. The city can require certain technical aspects of the line but not the material used or the way it’s put together.
And oversight is limited.
“With private, a lot of time the owner will hire an architect or engineer to design it. But then there’s nobody out there watching construction. And since it’s private, there’s no way to force them to have to spend the money for inspection,” Kirkwood said.
The U.S. Geological Survey has an entire section of the website devoted to explaining sinkholes. To learn more, click here.