Bridge Over Rubbled Water

Minnesota Department of Transportation determines 1,150-ft. long, 220-ft. high structure will be used for highway relocation over Rouchleau Pit, an abandoned open-pit mine near Virginia, MN.

The water-filled Rouchleau Pit near Virginia, MN, is in the spotlight because of a proposed highway plan that relocates US Highway 53 over the mine pit which has now filled with water from groundwater seepage and rainfall. The pit is 250 ft. deep in some places. Tests show the bottom of the Rouchleau Pit, which was back-filled with mine waste, contains rocks that vary in size from small pebbles to the size of Volkswagens. The rubble ranges from 40 to 140 ft. thick. 

IDEA Drilling, in Virginia, MN, used an 8140LS Sonic, working from a barge on the water’s surface, to collect core samples of the rubble. Because the bottom of the pit is neither uniform nor stable, the tests were needed to help determine if the rubble is stable enough to hold the massive bridge foundations needed to support the four-lane expressway.

“This tailings pit contained some of the hardest drilling I have ever encountered,” said Joel Christy, Geoprobe^® Sonic Operator. “They used 6-in. conductor casing and ran 10-ft. core runs using SDT45 (sonic dual tube) to collect the samples,” Christy reported after a site visit. “I was concerned about the toll the tough formation was going to take on our tools, but the IDEA drillers, who had experience in this material, said the tool system had exceeded their expectations. Even the onsite geologist was impressed with our equipment!”

The highway has to be rerouted because of a codicil in a 1960 agreement between owners of the mineral rights under the highway and the state. The deal allowed the state to build the highway, but also gave the owners the right to remove the highway with seven years’ advance notice. In 2010, Cliffs Natural Resources and holders of the mineral rights, decided to expand the Thunderbird Mine that provides taconite iron ore. If the taconite plant is to remain open, it needs access to the ore that now lies beneath the highway.