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The core of an earth dam is normally made of clayey base soils and protected by a sandy filter. Designing the filter can be very challenging considering the various criteria established today. This study commences with a review of three prominent criteria referred to in designing filters by the indirect method. No-erosion filter tests are then carried out involving 11 fine-grained, base soil samples with different properties; these are a highly dispersive soil, a soil with a medium to high degree of dispersion, a soil with a low-plasticity silt (ML) classification and a soil with a broad gradation curve. These tests represent the direct method. A comparison of results arising from the two methods shows that designing filters the indirect way, by following established criteria alone, is not always safe. Hence, it is proposed that such designs should be experimentally verified using no-erosion filter tests. Furthermore, instead of following the prevalent criteria in the indirect design method, the perfect filtering method for highly dispersive soils and the reduced particle size distribution method for broadly graded soils are proposed to be used, with merits given for each method. Finally, the paper recommends certain filter criteria from the literature for base soils with a moderate degree of dispersion and an ML classification respectively.

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