A section of Highway 1/169 between Ely and Tower, Minnesota, was realigned to improve safety and traffic flow. Construction required extensive earthwork and exposed rock from the 2.5-billion-year-old Soudan iron formation. The folded and faulted stone contained sulfide minerals that, when exposed to air and water, can generate acid rock drainage (ARD) that may adversely affect runoff water quality.
The Minnesota Department of Transportation (MNDOT) developed an acid-generating rock mitigation plan and hired Barr’s qualified professional geologists to oversee plan execution.
Barr sourced neutralizing amendment materials, including limestone and slaked lime byproduct, from the local paper industry. The project called for specific dosing of ARD-generating materials with amendments, and construction of a large-scale, long-term covered repository.
The geologists were on site daily to assess the materials’ acid-generating potential, collect samples for geochemistry analysis, refine volume estimates, and implement mitigation measures as needed. Our work helped ensure that excavated material remained within the mitigation plan’s safety factors. Close collaboration also helped keep construction on schedule while enabling MNDOT to better manage risk and understand the onsite implications of implementing an ARD mitigation plan.
The project was completed in 2018, and long-term monitoring indicates chemical and geotechnical stability of the ARD rock repository. The original mitigation plan and design project won awards from the Association of General Contractors as well as from MnDOT's Office of Environmental Stewardship.