Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Damslengths (frequencies) propagate at different velocities through nonhomogeneousmaterials. This variation in velocity is related to the shear wave velocity and thusshear modulus (Shaw, 2003).Different equipment is used depending on whether the SASW/MASW technique isto be used for geotechnical analysis of soil (to depths of 3 feet to 300 feet), or forstructural evaluation of concrete. The equipment generally consists of two or moregeophones, a hammer or other impact device for generating vibrations, and aseismograph or other data collection unit.Higher wave frequencies and close geophone spacing are used for shallowinvestigations, and lower frequencies with wider spacing are used for deeperinvestigations. The field data are later processed with specialized software, such asWinSASW, developed by the Geotechnical Engineering Center of the University ofTexas at Austin, or SurfSeis, developed by the Kansas Geological Survey.10.2 Self potentialSelf potential (sometimes referred to as streaming potential or SP) (figure 141),measures the electrical potentials (or voltages) that exist in the ground or within anembankment dam. Flowing water naturally generates these potentials as aconsequence of the separation of ions in the seepage water itself. SP is considered tobe the only geophysical method capable of direct detection of seepage (Corwin,2002). Other geophysical methods, such as resistivity, infer the existence of seepagebased on other measured parameters.Theoretically, it is possible to measure these potentials and predict seepageanomalies, such as along a conduit within the embankment dam, up to severalhundred feet deep. However, the technique is not widely applied, and few people orcontractors can expertly interpret the data. In addition, the measured potential(usually on the order of tens of millivolts) in any area can vary with other in-situparameters, and with man-induced voltages.Existing procedures were developed for the USACE’s Waterways ExperimentStation and published in 1989 (USACE, 1989), and also for the Canadian ElectricityAssociation Dam Safety Interest Group (Corwin, 2002). SP interpretation andmodeling computer programs are beginning to be developed along the lines ofexisting programs available in other geophysical disciplines, such as resistivity andseismic methods.Canadian Electricity Association Technology, Inc. has published a series of DOSprogram codes (Corwin, 2002) and the University of British Columbia has availablea modeling procedure that runs under Visual ModFlow, and models the SP response250