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Engineers will test stability of interstate near sinkhole

Associated Press By Associated Press | 4 months ago

RAPID CITY, S.D. (AP) — The South Dakota Department of Transportation has hired an engineering company to study the safety of Interstate 90 near Black Hawk after a nearby sinkhole exposed an old mine.

The DOT is contracting with FMG Engineering of Rapid City to look for underground abnormalities. The study is expected to begin the last week of May.

A dozen families had to evacuate their homes after a massive sinkhole opened in a nearby subdivision. It exposed the fact the subdivision was built over the abandoned gypsum mine.

FMG Engineering will determine the stability of the interstate using an “electrical resistivity” method, said FMG project manager Alex Fisher.

“It’s a method by which electric current is introduced into the ground and variations in the resistive property of the soil are measured in two dimensions,” he said. What is measured “can indicate voids, either water-filled or air-filled voids.”

Fisher said his team will insert metal pins in the interstate median and on both sides of the highway and exit ramp. They then insert low-voltage electricity into the pins and measure how they communicate with each other, the Rapid City Journal reported.

Electricity failing to communicate between pins means there is an underground void since electricity can’t travel through air, Fisher said.