EVALUATING SINKHOLE DAMAGE TO A FLOOD CONTROL STRUCTURE IN SOUTHEASTERN MINNESOTA USING ELECTRICAL RESISTIVITY IMAGING

Southeastern Minnesota is situated in the tri-state Driftless Area which is a dissected Paleozoic plateau underlain by karstified Paleozoic carbonate rocks. The local geologic setting presents several unique challenges for geologists, hydrologists, and engineers seeking to implement best management practices, mitigate water quality problems, or develop flood control projects. The soil and unconsolidated sediment mantle often hide the evidence of sinkhole development until a catastrophic collapse expresses a developing void at the surface. A significant portion of rapid sinkhole development in our region occurs during wet periods, particularly in mid-spring, when soils thaw and can be liquefied and eroded away through dissolutionally enlarged fractures or conduits.

The discrete nature of sinkholes makes them difficult to detect during pre-construction site investigations, which have typically depended on soil borings for understanding the subsurface stratigraphy at the site. Soil borings provide information about the soil and unconsolidated sediment stratigraphy at the spot where they are collected—in a sense, they are “one-dimensional” data sets. In karst areas, significant lateral variation can occur over very small spatial scales. Electrical Resistivity Imaging (ERI) is a tool that can economically extend “one-dimensional” data from soil borings into two- or three-dimensional stratigraphic information. We used ERI to investigate the causes of sinkhole development behind a large earthen flood control structure near Caledonia, Minnesota. The structure was built in the 1960s underwent an initial phase of sinkhole development in the late 1980s . Additional sinkholes developed in spring 2008 during an extremely wet spring. By supplementing the extensive soil boring data for the site with four dipole-dipole ERI surveys we were able to identify the existence and trend of a small conduit and dissolutionally-enlarged fracture zone about 20 m below the surface that correlates with the position and trend of the sinkholes at the site. In addition to understanding failures in existing structures, ERI could be a valuable tool in conducting more robust pre-construction site investigations in the future.