Assessment of Karst Activity at Construction Sites using the Electrical Resistivity Method (Greene and Jefferson Counties, Missouri)

Two-dimensional (2-D) electrical resistivity data were acquired across and in proximity to active sinkholes at two Missouri Department of Transportation (MoDOT) highway-construction sites. Construction site #1 is located in Greene County, Missouri; site #2 is located in Jefferson County, Missouri. Two relatively dense grids ("1A" and "1B") of electrical resistivity data were acquired at Greene County site #1 as part of a geotechnical investigation of a proposed interchange. Several active sinkholes with exposed throat diameters on the order of 1.5 ft had been discovered during a routine visual inspection of the site, and the District Geologist decided that a geophysical investigation was warranted prior to routine boring. the primary objective of the electrical resistivity study was to determine if substantive air-filled karstic cavities were present in the subsurface. Secondary objectives were to estimate depth to bedrock and identify anomalous subsurface conditions that might compromise the integrity of the proposed intersection or complicate construction. the electrical resistivity data proved to be of significant utility. Substantive air-filled voids were not imaged on the resistivity profiles. a depth to bedrock structure contour map was generated for the site, and prominent clay-filled and apparently inactive karstic cavities were identified beneath segments of proposed new roadway. a relatively dense grid of electrical resistivity data was also acquired at Jefferson County site #2. These electrical resistivity data were acquired because a previously unmapped, oval-shaped, clay-filled, sinkhole (approximately 50 ft x 35 ft x 15 ft; length x width x height) near the centerline of the northbound lanes effectively "emptied" overnight after overlying soil had been stripped by earth moving equipment. the supervising geologist authorized the acquisition of the geophysical data in order to image the subsurface immediately beneath and adjacent to the sinkhole. the primary concern was that the sinkhole was underlain by a large air-filled cavity. the interpretation of the resistivity data indicated the reactivated sinkhole was not underlain by a substantive cavity. Rather, the soil in the sinkhole appears to have "flowed" into the subsurface through solution-widened system of joints that do not underlie the sinkhole and probably do not pose a significant risk in terms of catastrophic collapse under load.