1 GEOPHYSICAL INVESTIGATION OF SEEPAGE

Internal erosion and the development of sinkholes and other piping features is one of the major reasons for embankment dam failures. Geophysical methods have the potential of detecting internal erosion and anomalous seepage at an early stage before the safety and integrity of the dam is at stake. Two non-invasive and cost effective geophysical methods (electrical resistivity and self-potential) were used to investigate the internal seepage at Crystal Lake Dam, Washington County, MO. Previous engineering drilling and grouting programs had been applied to the dam to minimize and control seepage through the body of the dam. However, due to lack of information about the problematic areas (seepage flow paths) through the body of the dam, grouting procedures were not as effective as hoped. In an effort to delineate seepage pathways prior to a second phase of grouting, geophysical data were acquired. Nine 2-D electrical resistivity profiles were acquired across the east and west side of the dam embankment. On the basis of the interpretation of these data, three zones of relatively uniform resistivity were identified and mapped. The first (uppermost) zone is characterized by moderate resistivity values (50-150 ohm-m). It represents unsaturated topsoil with thicknesses varying from 1 to 2.5 m. The second (intermediate depth) resistivity zone, with resistivity values ranging from 5 to 50 ohm-m and thicknes