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RC Crews Backfilling Sink Hole

Rapid City crews are backfilling a large sink hole area in the west side of town that began eroding Thursday morning.

Dale Tech is the public works director for Rapid City. He says the city has consulted with geotechnical engineers, who believe the cause is natural.

He says sink holes like this in Rapid City are very rare.

“I’ve heard of anecdotal evidence of one that occurred about 40 years ago in this part of town—a little further west of here,” Tech says. “There hasn’t been anything in recent times. The geology of this area is kind of prone to that, if anything’s prone to that in Rapid. It’s called a Spearfish Formation, so it’s a red clay, then it has gypsum that’s naturally occurring in that formation. That gypsum, overtime, will dissolve underground and leave voids.”

Tech says crews will work through the weekend and into next week to fill the sink hole. They’re backfilling with crushed limestone and then eventually dirt.

Officials say the sink hole is roughly 25 feet wide, 30 feet long and 30 feet deep.