Volume 26, Issue 2 (June 2003)
Evaluation of Multi-Electrode Earth Resistivity Testing in Karst
Multi-electrode resistivity testing is quickly becoming a common tool for geotechnical site investigation in karst areas, particularly areas with clay soils. To study the effectiveness of this method, data from over 140 resistivity tests at two sites were collected and compared with the results from 51 borings. The tests were conducted to evaluate the effect of electrode arrangement and the presence of conductive utilities, to determine the repeatability of the results, and to evaluate the effectiveness of the method in determining depth to bedrock and the location and size of voids. The results demonstrated that multi-electrode resistivity can reliably map depth to bedrock with excellent repeatability, although there was some smoothing in areas where the true bedrock surface is highly irregular. Underground utilities presented the most serious problem when oriented parallel to the resistivity electrode spread. The multi-electrode resistivity method was not as effective at locating voids. The strongest indication was provided by comparison of the resistivity data from lines collected perpendicular and parallel to the geologic strike because the results tended to be very different when collected over a cavity. Recommendations for using resistivity testing as a site investigation tool are given.