ANALYSIS OF HIGH HORIZONTAL STRESSES IN PRECAMBRIAN ROCKS OF NORTHERN MINNESOTA AND MICHIGAN AND THEIR EFFECT ON FOUNDATION CONSTRUCTION

Two recent bridge construction projects with rock socket foundations in Precambrian rock were completed in Northern Minnesota and the western portion of Michigan’s Upper Peninsula. The first project involved the relocation of US-53 near Virginia, MN over the abandoned open-pit Rouchleau Mine, while the second project involved the reconstruction of a bridge over in the Sturgeon River on US-2 near Iron Mountain, MI. The foundations for the US-53 relocation project were constructed in the lower cherty layer of the Biwabik Iron Formation, while the US-2 bridge foundation was erected directly into the Vulcan Iron Formation. The foundation rock for the US-53 and US-2 projects were estimated to have uniaxial compressive strengths of 400 and 300 MPa, respectively. A key parameter for the design of the rock sockets is the rock quality designations (RQD) obtained during the geotechnical investigation phase of the projects. The rock core, however, was highly fractured in both projects resulting in very low RQDs leading to rock socket depths of 20 m for the US-53 project and 6 m for the US-2 project. While it is highly possible that the rock formations were highly fractured in place, it is also possible that high horizontal stresses might have contributed to the rock core fracturing, also known as core dicing, during drilling operations. The presentation will discuss research into an analysis of high horizontal stresses in drilling brittle high strength rock. The results suggest but do not prove that high horizontal stresses might have contributed to the significant fracturing of the core in both projects leading to deeper rock socket construction than necessary.