(a) After a landfill ceases to accept waste as specified in section 363-10.3 of this Part, a final cover consisting of a composite barrier must be installed. The project engineer must consider the projected service life of the final cover system, settlement, erosion, and seepage forces in the overall stability of the final cover system.

1. (1) The composite barrier layer must consist of a GCL and a separate geomembrane.

   1. (i) GCL. The GCL must be specified by the project engineer upon demonstrating both physical and chemical stability of the bentonite used in the GCL. The GCL component of the composite cover must meet the requirements of section 363-6.7(b)(1) and (c) of this Subpart.

      1. (a) On slopes equal to or greater than 25 percent and for side slope terraces on those slopes, the GCL component of the composite barrier may be eliminated.

   2. (ii) Geomembrane barrier layer of composite cover. The barrier layer must be constructed to limit precipitation migration into the landfill.

      1. (a) The geomembrane material must be chemically and physically resistant to materials it contacts, and be able to accommodate the expected forces and stresses such as those caused by settlement of waste.

      2. (b) A geomembrane comprised of linear low-density polyethylene polymer must have a nominal thickness of 40 mils or thicker. A geomembrane comprised of HDPE must have a nominal thickness of 60 mils or thicker.

         (b) Construction requirements.

2. (1) GCL. GCLs must be constructed in accordance with section 363-6.7(b)(1) of this Subpart.

3. (2) Geomembrane barrier layer. Geomembrane barrier layers must be constructed in accordance with the requirements of section 363-6.8(b) of this Subpart with the following exceptions:

   1. (i) the geomembrane must be placed between a 4 percent minimum slope and a 33 percent maximum slope; and

   2. (ii) where GCL is used, the geomembrane must be placed in direct and uniform contact with the underlying GCL.

      (c) Certification requirements.

      Certification for the installation of a composite barrier layer must be conducted in accordance with the same conditions found in section 363-6.8(c) of this Subpart, except for the electrical resistivity testing provisions of section 363-6.8(c)(3)(vii) of this Subpart.