Filters for Earth Dams

Between 1980 and 1985, the Soil Conservation Service, now known as the Natural Resources Conservation Service, performed an extensive study to determine appropriate gradation criteria for sand filters to be used for filter/drainage zones in embankment dams. The study was performed at the NRCS Soil Mechanics Laboratory in Lincoln, Nebraska with the assistance of the late James L. Sherard, eminent earth dam consultant. The study included a large number of tests simulating cracks or other anomalies in dams with the potential for developing concentrated leaks under high water pressure. Filters with varying gradations were placed downstream of a simulated core material containing simulated cracks to determine the gradation necessary to prevent movement of base materials through the filter and to provide a self-healing condition. A large variety of materials were used to simulate the base soil of the dam upstream of the filter/drainage zone. Specific testing was performed to verify the properties of the filter that determine its ability to prevent the base or protected soil from passing through it for use in designing filter gradations. These properties included the ratio of particle size at 15 percent passing of the filter to the particle size at 85 percent passing of the base soil, uniformity of the filter gradation, and other factors influencing segregation, permeability, and grading of the filter. Since the NRCS study, the US Army Corps of Engineers and the Bureau of Reclamation have revised their guidance for filter design. Their guidance generally conforms to the findings of the NRCS study. With further experience, both the USACE and Reclamation have modified their guidance and added important useful information about placement and construction techniques. 12 pp., 9 references.