Mining's underworld threatens Hwy. 169

DULUTH - There's an underworld on Minnesota's Iron Range, nearly forgotten and liable, now and then, to swallow chunks of earth above it.

Millions have driven Hwy. 169 past Chisholm without suspecting what lies 50 to 100 feet beneath: An intricate network of mine shafts and tunnels -- many miles and levels of them, laid out like city streets.

For historians and adventurers, the tunnels, remnants of a mining era that largely gave way to open-pit mining by 1950, is the stuff of dreams. But since 2007, they've been causing nightmares for highway engineers.

Two ominous things happened that year: First, a 20-foot-deep sinkhole appeared in the median of Hwy. 169. Then the 35W bridge in Minneapolis collapsed.

"After that, we had a greater awareness that if there's some kind of risk, we have to be proactive," said Duane Hill, assistant district engineer for the Minnesota Department of Transportation in northeastern Minnesota.

The DOT has spent nearly two years devising a plan to reinforce the highway in two areas deemed most vulnerable to sinkholes. The $4.9 million plan to build what would amount to "land bridges" of continuously reinforced concrete is first on the list of projects the district identified as candidates for the federal stimulus package being debated in Congress.

What lies beneath

Hundreds of the silent shafts and timbered tunnels remain on the Iron Range, testament to the young nation's need for steel and the sweat of thousands of immigrant miners. And the sinkholes they caused are nothing new.

Tom Lautigar, 60, of McKinley, said he knows of several that appeared in and around his little town since he was a boy.

"One was in somebody's back yard, next to their garage," said the miner and former city council member. "Another happened by a city playground."

Before leaving a property, mining companies often filled shafts and collapsed tunnels, also known as "drifts." But they inexplicably left many intact, and over decades rainwater sometimes washed dirt from the filled shafts into voids below, said Jason Richter, a DOT engineering geologist assigned to study the problem around Chisholm.

In some places, groundwater washed away the fill. But the Chisholm tunnels are above the water table and dry.

Richter said it's unclear why state officials felt comfortable routing Hwy. 169 over heavily tunneled areas in the 1960s. But almost immediately, trouble surfaced. In 1969, a car careened out of control after bouncing through a 3-foot-deep sinkhole that appeared in the westbound passing lane. The driver wasn't hurt, but his car ended up in the opposite lanes.

Another hole -- 27 feet wide and 4 or 5 feet deep -- appeared next to the highway in 1991. Crews filled both depressions with sand and gravel. Lautigar said that in McKinley, 6-foot fences were put up around old mine shaft openings with a history of sinking.

It's believed that the cylindrical, 10-foot-diameter depression that appeared on the median of Hwy. 169 in 2007 occurred because dirt collapsed into a cavern far below.

Richter said his task was made much easier by a mapping project being done by the state Department of Natural Resources. The agency scoured company and public archives for old maps of the underground mines on the central Iron Range and combined them on one computer-enhanced map and database.

Dale Cartwright, a DNR geologist, said they expected to map about 50 defunct mines. Instead, they documented 90 on the central Iron Range alone. The maps don't give the whole picture, though. Stories abound of companies that pushed tunnels past their allotted boundaries to exploit good ore. Such "pirate" drifts wouldn't show up on a company map.

Richter helped add to the picture through "electrical resistivity imaging," which measures gaps in resistance encountered by an electric current sent through the ground. And he had holes bored in many locations, to measure the depth of tunnels, the thickness of their roofs, and other indicators of vulnerability to collapse. Finally, he lowered a Chisholm Sewer Department camera down some bore holes, to see the tunnels for himself.

"It's something else," he said, "You know you're seeing something nobody has seen for 100 years."

Larry Oakes • 1-800-266-9648