Discussion of “A procedure for the design of protective filters”Appears in Canadian Geotechnical Journal,<b>44</b>: 490–495.

According to the International Commission of Large Dams (ICOLD 1994), two fundamental functions are required for the filters in embankment dams and other hydraulic structures: (i) retention function, in which the filter must prevent the migration of base soil particles; and (ii) permeability function, in which the filter must accept the seepage flows from adjacent foundation or fill materials without the buildup of excess hydrostatic pressure. Therefore, the filter must be not only fine enough to prevent the erosion of the base soil but also coarse enough to be many times more pervious than the base soil. For this reason, two main criteria have been used separately in the design of granular filters: the retention criterion and the permeability criterion. Moreover, there are other conditions that the filter must meet: (i) it must not segregate, (ii) it must not change in gradation, (iii) it must not have apparent or real cohesion, (iv) it must be internally stable, and (v) it must have sufficient discharge capacity. Much research work has been done in relation to the retention criterion, mainly based on laboratory tests. These so-called empirical retention criteria usually compare the filter and base soil particle-size distributions (PSDs) (Sherard and Dunnigan 1989; Honjo and Veneziano 1989; Foster and Fell 2001), but other empirical retention criteria are based on a comparison between the filter permeability and the base soil particle sizes (Vaughan and Soares 1982; Delgado et al. 2006) because the permeability indirectly takes into account not only the whole PSD of the filter, but also other important characteristics such as compaction, particles shape, and porosity. Recently, important results have been found for retention criteria based on the fundamental physics of filtration. The so-called analytical retention criteria have been developed by Locke and Indraratna (2002), Lone et al. (2005), Indraratna et al. (2007), and others. For example, Indraratna et al. considered the constriction-size distribution (CSD) of the filter and also incorporated the effect of nonuniformity of the cohesionless base soil. Their analytical model has been verified by experimental data. In the case of permeability criterion, if the permeability of the base soil is known, for example from a permeameter test, the well-known theories of filtration indicate that the filter permeability should be at least 25 or even 100 times higher (ICOLD 1994). This criterion can be considered analytical, but sometimes the permeabilities of both the filter and base soil are estimated from their PSD (e.g., k 1⁄4 0:35ðD15Þ, Sherard et al. 1984), which is why the empirical permeability criterion D15f > 4D15b is normally used (ICOLD 1994; NRCS 1994). The authors have presented a procedure for the design of protective filters based on an analytical solution that takes into account factors like pore size, permeability, and factor of safety against soil boiling conditions. This analytical solution is offered as a single equation that would be able to substitute both the traditional retention criterion and the permeability criterion.

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