NRCS Filter Design Criteria - A Step by Step Approach

An extensive research program on sand and gravel filters was conducted in the Soil Conservation Service (SCS) (now the Natural Resources Conservation Service), Soil Mechanics Laboratory at Lincoln, Nebraska, in about 1980-1985. The research was made under the direction of Lorn Dunnigan, PE. head of the laboratory (now retired), James Talbot, PE. National Soil Engineer for SCS (retired, now in private practice with GE] Consultants. Inc.. Raleigh, North Carolina), and James L, Sherard, Ph.D., PE, noted private consultant on earth darn design and construction (now deceased). Dr. Sherard’s association with SCS arose from an earlier joint study on dispersive clay soils in dams, His work with SCS on these studies was done under a cooperative agreement at his own expense. Dr. Sherard devoted a large amount of time and effort to this research, which contributed greatly to its success. Results of this research program were published in several papers listed in the bibliography. In 1986, SCS published a revised Soil Mechanics Note No. 1, “Guide for Determining the Gradation of Sand and Gravel Filters,” which contained criteria based on the results of the research. Criteria and examples in this first revision document sometimes gave wide gradation limits for filters which were not well defined and unsuitable for proper construction control according to feedback from field engineers. Other SCS engineers, including Danny McCook, co-author of this paper. developed additional criteria to restrict the width of filter limits and provide more uniformly graded material and prevent gap-graded or skip-graded filters from being used. Design examples for the major base soil types and a step-by-step approach were included in the final SCS document entitled “Gradation Design of Sand and Gravel Filters” (Reference 1 of the bibliography). This paper presents a condensed version of the SCS reference which contains more examples and background than is permitted by the restricted length of this article. Copies of the SCS reference may be obtained from Danny McCook, NRCS, PO BOX 6567, Fort Worth. TX 76115. 10 pp., 4 references.