(a) If a filter layer is included, it must be designed to prevent the migration of fine soil particles into a coarser grained material, and to allow water or gases to freely enter a drainage structure (e.g., pipe or drainage blanket) without clogging.

1. (1) Graded cohesionless soil filters. Granular soil material used as a filter must have no more than 5 percent by weight passing the No. 200 sieve and no soil particles larger than 3 inches in any dimension.

2. (2) Geosynthetic filters. Geosynthetic filter material must demonstrate adequate permeability, soil particle retention, resistance to clogging, and chemical and physical resistance to adjacent materials so that it is not adversely affected by waste placement, overlying material or leachate generated at the landfill. Geosynthetic filter openings must be sized in accordance with the following criteria:

   1. (i) permeability criteria: k_f > 10k_s where:

      1. (a) k_f is the geosynthetic filter permeability; and
      2. (b) k_s is the overlying soil permeability;

   2. (ii) retention criteria: O₉₅ of the geosynthetic filter < 3d₈₅, where:

      1. (a) O₉₅ is the apparent opening size of the geosynthetic filter at which 95 percent of the soil particles will be retained; and
      2. (b) d₈₅ is the soil particle size at which 85 percent of the particles are finer.

3. (3) Clogging potential of soil or geosynthetic filter system must be assessed using a long-term permeameter test method or other methods acceptable to the department.

   (c) Construction requirements.

   Geosynthetic filters must not be damaged during installation and must be installed in a manner that does not reduce their ability to function as designed.

   (d) Certification requirements.

   For geosynthetic filters, the project engineer must assess the polymer type and density, ultraviolet stability, mechanical properties, weight, tensile strength, permittivity, apparent opening size, and puncture strength. The project engineer must certify that post-construction care procedures were implemented to protect the soil or geosynthetic filter from the intrusion of fines related to water-borne and wind-borne sediments.

(b) Materials required.