An assessment of veracity of filter criteria for earth dams

Abstract

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.

The importance of permeability in granular filter design and control

“…Finally, for the present study, this database has been updated with results from Vakili & Selamat (2014) and Vakili et al (2015). These researchers reported 46 NEF tests with dispersive soils.…”

“…The only authors that extensively have tested dispersive base soils are Vakili & Selamat (2014) and Vakili et al (2015), therefore it is interesting to analyse here their results. They employed twenty-one different samples of base soils, whose properties are given in Table 3, and thirteen different filters, whose properties are given in Table 4.…”

“…Vakili et al 46 Total size of data set for filter stability analysis 731 Note: ** For the lack of space, the list of references contained in table 3 could be seen in It is very important to compare the correlation between filter PSD and permeability obtained by Vakili & Selamat (2014), with equations 1 to 5. As can be seen in Figure 1, the filters tested by Vakili & Selamat (2014) and Vakili et al (2015), have very high permeability, even higher that the permeability that we have obtained without compaction of the filter. 0.12 0.17 2.00 4.0 6.73 x 10 -3 Note: C u = coefficient of uniformity of he filter.…”

“…In any case we have added these new results to our database. Figure 2 shows the results of the 348 NEF tests provided by external sources, 159 "standard" NEF tests from Delgado (2000), and the new 46 results with dispersive base soils obtained by Vakili & Selamat (2014) and Vakili et al (2015), all presented in the context of the well known and widely accepted soil group 1 & 2 filter design criteria by Sherard & Dunnigan (1989).…”

Editorial

“…This paper has been made open access and is free to download from the journal website. Vakili and Selamat (2014) look at the choice of filter materials for dam core protection and show how material assessment by standardised methods may lead to non-conservative material specification in different material types. Alternative more appropriate testing techniques are proposed.…”