Publication Details

This article was originally published as Locke, M and Indraratna, B, Filtration of Broadly Graded Soils – The Reduced PSD Method, Geotechnique, Institute of Civil Engineers, 52(4), 285-287.


Granular filters are used in earth structures, such as embankment dams, to protect fine soils from erosion due to seepage forces. Successful filtration requires that the filter voids are fine enough to capture some of the coarse fraction of the base soil. These retained particles are then able to capture progressively finer base soil particles, and eventually a filter interface forms that is able to prevent any further erosion. This process is called self-filtration. Lafleur et al. (1989) examined self-filtration in cohesionless, broadly graded base soils. It was found that the extent of mass loss before selffiltration occurs was greater in broadly graded materials: hence a finer filter was required to reduce this mass loss. Filters for cohesive base soils are commonly designed using the Sherard & Dunnigan (1985) design criteria. While these criteria have been developed from extensive laboratory data, they may not be applicable to all fine base soils, particularly broadly graded materials. In this paper, a series of filtration tests on various base soils are described. Data from the current study, and the published results of laboratory tests from several sources are compared to examine the filtration of broadly graded base soils. Based on this analysis, a new design procedure is proposed for filters to protect fine base soils, which determines the ability of the coarse fraction of the base soil to retain the fine fraction (i.e. a self-filtering base soil).

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