Imaging the anatomy of earthen dams using ground penetrating radar

Ground penetrating radar (GPR) surveys were performed in support of seepage investigations of two earthen dams in Indiana and Michigan. Both surveys were initiated only when groundwater seeps emerged after initial remedial measures were implemented. High-resolution GPR images depicted the complex internal structures of the dams, and provided the bases for conceptual models of groundwater flow processes within the dams. GPR surveys of both dams included a series of longitudinal and lateral GPR profiles. In both surveys, vertical and lateral variations in the physical properties of the earth fill resulted in variable depths of radar penetration and resolution. In addition, unwanted but identifiable reflections resulted from overhead and underground objects. The reduction of their affects required slightly more advanced data processing procedures than was required over the remainder of the dams. At the first dam, standard data processing routines yielded GPR images of soil and fill boundaries within the embankment, the position of a geocomposite drain buried beneath the embankment, and seepage through the geocomposite into the overlying fill. A strong reflective body, representing a water-saturated zone, was observed near the location of the present-day seep. Additionally, the GPR images depicted soil boundaries that correlated with individual lifts placed during construction and zones of increased porosity. At the second dam, the GPR surveys imaged toe drains, lateral drains, and the interpreted presence of voids near an abutment wall. Additionally, the images suggested the presence of perched water near the seep area. Unfortunately, the phreatic surface could not be continuously mapped at either dam. The inability to image the groundwater surface is attributed to the low porosity and, consequently, large and gradual capillary zone present in both dams. However, groundwater flow characteristics were interpreted from the presence of signal amplitude anomalies and physical observations from seeps and piezometers. 12 pp., 11 figures.