22.An engineered landfill may also be sited on land where the unconsolidated deposits do not meet the impermeability requirements of section 20, provided that the disposal areas have a double liner system on the bottom and sideslopes that is composed of
(1) a lower composite liner consisting of
(a) a layer of clayey materials at least 60 cm thick after compaction
— that consists of at least 50% by weight of particles 0.08 mm or finer in diameter and at least 25% by weight of particles 0.005 mm or finer in diameter;
— with a constant hydraulic conductivity of 1 x 10-7 cm/s or less throughout its thickness;
(b) a geomembrane at least 1.5 mm thick placed over the layer of clayey materials; and
(2) an upper liner consisting of a second geomembrane at least 1.5 mm thick.
The geomembranes must be of the high-density polyethylene (HDPE) type or have equivalent properties; they must be installed with an inclination of at least 2% to allow leachate to flow by gravity towards the drains.
An alternative double liner system may also be used in the case referred to in the first paragraph if it is at least as effective as the system required by the first paragraph.