GRANITE FALLS — The underground movement that has ripped open a nearly three-foot fissure on state Highway 67 near the Upper Sioux Agency State Park in Yellow Medicine County is beginning to slow, according to Cody Brand, a soils engineer with the Minnesota Department of Transportation's District 8.
“It is continuing to grow at the surface, (but) it is slowing down in the ground,” Brand told members of the District 8 Area Transportation Partnership at its Oct. 11 meeting at the Granite Falls truck station.
MnDOT is now in the process of developing a plan to address the slope failure that has caused the fissure, and closed the highway since April 4. But at the earliest, work to fix the problem won’t likely get underway until next summer.
Any solution will likely be expensive, according to Brand. No cost analysis has been done, but rough estimates for addressing the problem start at $3 million and climb upward to $15 million.
Three options are under consideration. The first will require installing pilings to a depth of 90 feet below the road surface. The option also requires work to stabilize the bank of the Yellow Medicine River, located about 300 to 400 feet from the problem area. There will also be a need for some type of drainage system to reduce the amount of water affecting the bluff on which the road is built.
Other options include building a bridge to span the roughly 1,500-foot problem area, or rerouting the road itself. The road slices through the Upper Sioux Agency State Park at that location. Moving the road to a location away from park lands is also problematic, as there are few crossings over the Yellow Medicine River in that area, Brand said.
The fissure that tore open the pavement is technically a slope failure. Two things are occurring.
The high water flows in the Yellow Medicine River are eroding its bank below the road. As the bank material washes into the river, the shelves of the upper bluff slough toward the river.
At the same time, underground water movement is essentially allowing one geologic layer of material to move on top of another. MnDOT has four inclinometers drilled at the site, and they continue to measure movement at the layer boundary. It is occurring roughly 80 to 90 feet below the surface of the road.
Adding to MnDOT’s challenges, there is a separate fault area that is moving the Yellow Medicine River bridge abutment near the site as well.
Interestingly, Brand said MnDOT had been conducting a risk assessment of its roadways using technology that allows it to identify areas where slope failures could occur. Prior to the fissure showing up, MnDOT’s central office had identified the Highway 67 site as the area of highest risk in District 8.
The central office notified the Willmar MnDOT office and arrangements were made for an April 9 visit to the site. The fissure beat the engineers. It opened up April 4, starting with a 6-inch drop in the pavement that has now grown to 3 feet.
Brand said the movement is likely to slow to a stop during the winter. It could begin anew next spring and summer if we continue to see above normal precipitation levels, he warned.