As U.S. Highway 169 winds its way south and west of Chisholm, it passes over a series of underground tunnels burrowed decades ago by miners in pursuit of iron ore.
The 40-plus-year-old road has stood the test of time so far, but the possibility of a collapse nags at the nerves of Duane Hill.
"The question is: What do we do to make sure we don't have a catastrophic failure?" asked the assistant district engineer for the Minnesota Department of Transportation.
A hole that opened in the highway's median dramatically brought the issue to the fore in 2007. Now, Hill is part of a team studying what's underneath in order to determine the risk of failure and prevent it. For weeks, he and his MnDOT colleagues have been poring over maps, ground boring samples and data from underground electrical resistance tests -- studies that he estimates cost the state between $400,000 and $500,000.
"We've got lots of information, and now we're trying to determine the best form of mitigation," he said.
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The team is most concerned about the honeycomb of tunnels running beneath two one-mile sections of the highway -- one on the southern edge of Chisholm and another just west of town. They expect to have an action plan this spring.
Helping out is a $300,000 study of now-defunct underground mines by the state Department of Natural Resources. Using old mine maps and records, researchers have developed three-dimensional models of abandoned shafts and corridors in the mid-section of the Mesabi Iron Range.
Many of those documents date to the late 1800s and early 1900s, and they range in quality of detail, said Dale Cartwright, a DNR geologist and information technology specialist.
"We found lots of maps, and we were able to assemble a pretty good picture of underground mining in the area, but we don't know if we have a complete picture yet," he said.
Cartwright said the model relies on documentation rather than fieldwork, and he said that there probably are holes in the written record of early Iron Range mining -- especially so in the event of any unauthorized mining activities.
Hill said that many of the shallower mine corridors were purposefully collapsed after ore was extracted, but many remnants of the deeper forays into areas of bedrock remain.
One idea involves piping a grout mixture into the old mine shafts and tunnels under the roadway. But Hill said it would be expensive and challenging, as the
boulder-strewn soils beneath the highway can make it nearly impossible to drill an intended direction.
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A more promising solution would be to rebuild the roadway on a solid, monolithic steel-reinforced concrete slab -- a structure that could bridge a section of ground in the event of a collapsed mine corridor. Hill said that such a land bridge could be equipped with sensors to alert authorities of any shifting of earth beneath the road.
Funding could come from the federal economic stimulus bill. MnDOT has put $4.96 million for the repair on its wish list out of the total federal aid. With it would come about 111 jobs.
"We don't want to raise people's expectations unduly, but these are the types of projects that could be ready to go and that seem to fit the criteria of the economic stimulus package," said Kevin Gutknecht, MnDOT's director of communications.
