Evaluation of towed TEM potential for rapid characterization of levee foundations

Abstract

Assessing the geologic conditions of levee foundation soils is a challenging task owing to the extensive length of most levees and the heterogeneity of many alluvial deposits. Traditional investigation techniques (such as boring and cone penetrometer testing [CPT]) are expensive, invasive, and provide spatially-limited information. As a result, they are restricted to pre-identified problematic zones in the levee. To overcome these challenges, geophysical instruments capable of better spatial coverage are proposed for rapid geoelectrical characterization of levees. In this study, we presented a field-based application of the towed time-domain electromagnetic method (tTEM) system in characterizing the subsurface geology adjacent to a vulnerable levee that has experienced underseepage and repeated sand boil occurrences. The tTEM data was acquired along the landward side of the levee, and the results were compared with concurrent electrical resistivity tomography (ERT) profiles and pre-existing CPT and borehole data. The tTEM and ERT results reveal a coarse-grained sand substratum layer overlain by a fine-grained clay blanket of varying thickness. The results also show areas with a very thin or missing clay blanket and a higher degree of heterogeneity; these areas are prone to sand boil formation. A comparison of tTEM and ERT results suggests that the tTEM reliably mapped the clay layer blanket and sand substratum thickness across the study area. While ERT provides better resolution of the substratum heterogeneity, this case study demonstrates the potential of tTEM as a valuable tool to rapidly assess subsurface geology and identify problematic zones in levees over large areas, which could help in mitigating future levee failures.