A case study of the reliability of multi-electrode earth resistivity testing for geotechnical investigations in karst terrains

The thinly mantled karst areas of northeastern Pennsylvania have been associated with a number of structural failures caused by sinkholes. Geotechnical investigations in these areas have primarily relied upon intrusive probe tests, either borings or air-track drilling (Thomas and Roth, 1997). The silty clay soils overlying the carbonate bedrock have limited the use of non-intrusive ground penetrating radar and electromagnetic methods; however, multi-electrode earth resistivity testing may be a suitable method for locating subsurface features associated with sinkhole formation in these areas. The case study presented in this paper concerns a site in Northampton County where sinkholes have been occurring at an accelerated rate due to the influence of construction (Chen and Roth, 1997). The bedrock at the site is located between a meter and approximately 10 meters below the surface. The soil is primarily a silty clay. Seventy resistivity lines using a variety of electrode and line spacings were conducted at the site. Using the data obtained from the earth resistivity tests, 17 borings were located to assess the accuracy of the resistivity results. Comparisons between boring and resistivity results demonstrate that the resistivity tests provide good information concerning the depth to the irregular surface of the bedrock. However, use of the test to locate subsurface anomalies associated with sinkhole formation and to determine their size will require additional: research. The paper presents the results of the study and provides recommendations for geotechnical investigation in silty clay soils overlying shallow carbonate bedrock.