REDUCING THE UNCERTAINTY OF SITE CHARACTERIZATION STUDIES IN KARST SETTINGS THROUGH THE USE OF NEAR-SURFACE GEOPHYSICS

Characterization of subsurface soil and rock conditions is an integral part of most environmental and geotechnical engineering studies performed in support of linear construction and/or land development projects. Traditional approaches to subsurface field investigations have often been inadequate, relying upon the placement of exploratory borings or test trenches to obtain information on the natural setting. There are numerous pitfalls inherent to this discrete sampling approach that can result in an incomplete or erroneous understanding of subsurface conditions. The accuracy and effectiveness of such an approach is greatly dependent upon the assumption that subsurface conditions are uniform, and that regional trends are consistent throughout the extent of the project site. Much to the chagrin of project engineers and geologists tasked with site characterization activities, these assumptions are frequently invalid, resulting in the discovery of unexpected conditions during the construction phase of the project, which can contribute to significant project delays and cost overruns.

It has been our experience that subsurface conditions along the west side of the IH 35 corridor connecting the metropolitan areas of San Antonio and Austin generally consist of limestone terrain, containing strata exhibiting varying degrees of karst formation. Because karst features can have a significant impact on the performance of foundation systems and negatively affect or delay construction activities, it is generally prudent to evaluate the presence of significant karst features at proposed project sites in conjunction with the geotechnical engineering study, or other phases of project planning, prior to construction. Geophysical surveys can provide for rapid, accurate, cost-effective characterization of subsurface environments. With respect to karst evaluations, direct-current (DC) electrical resistivity imaging has proven to be particularly effective as this technique can delineate areas in the subsurface where large air-filled voids or zones of weathering (i.e., limestone dissolution) are present.