Reply to the discussion by Delgado and Poyatos on “A procedure for the design of protective filters”Appears in Canadian Geotechnical Journal, 45: 437–439.

Abstract

The authors thank the discussers for their efforts in identifying a few issues related to the protective filter design methodologies and some of the aspects to be considered in the proposed methodology for calculating the D15f of the filter media. The authors agree with the discussers that the proposed methodology is based on the permeability criterion. As presented in the note, the developed solution is based on the permeability criteria only, as it is based on the Kozeny–Karman solution, and hence, the approach provides a value of (D15f)min. The calculated value of the D15f size of the filter media from the proposed analytical solution can be used in conjunction with the established empirical criterion. If the calculated value of the D15f size of the filter media falls outside the range of D15f, obtained empirically, the filter design can be revised to obtain a value of D15f in the range. It is established that the soil retention and permeability criteria are contradictory requirements with regard to the consideration of particle size D15f. There is a need to develop a solution based on the soil retention criterion as well, which will be able to provide a (D15f)max value. Therefore, based on the limit equilibrium of forces responsible for the soil retention such as hydrodynamic force, drag force, submerged weight of soil particles, and frictional force due to particle contact, the authors have conducted studies to develop a model to obtain a (D15f)max value, and the results are communicated for possible publication. As stated by the discussers, except for the soil retention and permeability criteria, other conditions that the filter must meet include the following: (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. As indicated by the discussers, ICOLD (1994) identifies two fundamental criteria, i.e., (i) soil retention and (ii) permeability, to be satisfied for the proper functioning of the filter media. In authors’ opinion, other conditions are related to post-construction behaviour and performance measurements of the filter media. A good quality control measurements will meet these requirements to ensure proper functioning of filter media. The following points address some of the other issues raised by the discussers with regard to the literature survey and the derivation of the proposed analytical solution: