Critical Filters for Impervious Soils

Final results of an extensive laboratory research program on “critical” downstream filters in embankment dams with impervious sections are presented. The investigation was carried out at the Soil Mechanics Laboratory, Midwest National Technical Center, Soil Conservation Service, United States Department of Agriculture, Lincoln, Nebraska. A new laboratory test, called the “no erosion filter” (NEF) test, was found to have substantial advantages over the slot and slurry tests used in the earlier part of the research program for silts and clays. It gave essentially the same results for silty and clayey sands with low fines content as was obtained with conventional filter tests on uniform sands. A wide range of different fine silts and clays and clayey and silty sands of different geologic origins were tested. The results of the investigation confirm conclusively that sand filters containing appropriate quantities of fine sand will reliably control and seal concentrated leaks through the impervious sections of embankment dams. The investigations show that for most fine silts and clays, a downstream sand filter with $D_{\mathrm{l5}}⩽0.5\mathrm{mm}$ is conservative and that broadly graded soils, such as those from glacial moraines, need a relatively fine filter.