GEOPHYSICAL EXAMINATION OF OHIO RIVER LEVEES FOR ENHANCED FLOOD PERFORMANCE EVALUATION

Most of the Nation's levee systems were built more than 50 years ago and have since been modified by subsequent housing, commercial and agricultural development. As such, the uncertainty in the existing condition of the levees points to the potential for increased risk to public health and safety and to property damage during flood events. Advanced geophysical techniques offer a relatively rapid and cost-effective means to examine levee soil and foundation characteristics that could result in levee instability and failure during high water conditions. This presentation will focus on a case study of one levee system along the Ohio River with more than five lineal miles (26,800 feet) of levee embankment and foundation soils to evaluate. Geophysical survey techniques supplemented with previous widely-spaced boring information were used to confirm the likely embankment material types, the thickness and quality of an upper natural clay blanket layer, and the character of a deeper underlying pervious foundation aquifer down to bedrock. The survey techniques included the completion of both electromagnetic (EM) terrain conductivity on the levee itself and two-dimensional electrical resistivity imaging (2D-ERI) through the foundation soils. The results, calibrated against previous soil boring and geotechnical engineering testing data, helped to identify the location of potentially higher permeability preferential seepage zones for further evaluation and analysis by direct soil sampling and testing, field hydraulic testing and long-term water level monitoring.