Susceptibility of Dispersive Clay at Grenada Dam, Mississippi, to Piping and Rainfall Erosion

摘要

Past performance of Grenada Dam indicates piping of embankment and foundation soils through joints of the collector pipe for the toe drainage system (prior to its replacement with an open paved ditch in 1961) occurred. Also, rainfall erosion tunnels developed on the downstream slope of the dam, primarily in the valley section between sta 105+00 and 145+00, soon after completion of the main embankment in 1949 and continued to develop at a decreasing rate to the present. Based upon laboratory tests conducted on undisturbed soil samples and reservoir water samples obtained in 1973 and 1976, the embankment soil is nondispersive at the surface and dispersive below a depth of about 6 ft. Limited data obtained below the embankment indicate the foundation soil is dispersive at the surface and nondispersive to dispersive with depth. The Australian method of analysis, using the exchangeable sodium percentage of the soil, total ionic concentration of the reservoir (eroding) water, and predominate clay mineral of the soil, indicates both the embankment and foundation soils would be potentially susceptible to dispersive clay piping if Grenada Dam contained cracks or sandy lenses traversing the width of the dam where the reservoir water would have a path of rapid access across the dam. Since piping failure through the embankment or foundation has not occurred, either the dam is free from cracks or sandy lenses traversing the width of the dam, or if cracks are present, the soil is able to swell and seal the flow channels.