THE U.S. HIGHWAY 151 PROJECT: CHALLENGES OF BUILDING A MODERN SUPERHIGHWAY THROUGH AN OLD MINING DISTRICT

A five-year project to rebuild U.S. Highway 151 into a 4-lane highway through the Driftless Area of southwest Wisconsin was completed in 2005. The road followed a new alignment that required interchanges for bypassed communities and involved extensive rockcuts and multiple bridge structures. A major concern from the planning stage was that many of the proposed cuts, some exceeding 100 feet high, were within the historic Upper Mississippi Valley lead-zinc mining district.

Detailed section-scale maps showing zinc mine workings (1900 –1978 ) and areas of early shallow lead diggings (mid nineteenth century) only existed hand drafted on linen. These maps were scanned and georeferenced, and incorporated into a regional GIS that greatly aided identification of mined areas. Early concerns were that excavation would intersect workings, but it was soon found that altered and mineralized rock in areas marked by extensive lead diggings was the source of foundation and cut stability problems. The GIS allowed identification of potential problem areas in time to modify designs before the next construction season, resulting in considerable cost saving.

The completed cuts, the highest and longest in Wisconsin, provide an excellent cross section of the Early and Middle Ordovician rocks of the region. Examples of pitch-and- flat structures and veins associated with the lead-zinc mineralization are exposed, along with previously unknown examples of structures attributed to collapse of karst features within the Prairie du Chien Group. Collapse events apparently occurred over a significant time interval as the overlying St. Peter, Platteville, and Galena Formations were being deposited.