The often proposed alternative landfill liner system to Low Permeability Clay Layer or HDPE Lining Membrane consists of replacing the default design of compacted clay liner (CCL) with a Geosynthetic Clay Liner (GCL).
There are currently two major types of commercially available GCLs. One type consists of bentonite encased between two geotextile fabrics and the second type consists of bentonite glued to a HDPE geomembrane.
The type of clay typically used in GCLs is sodium bentonite. Sodium bentonite is the name given to the highly plastic clay mineral montmorillonite, with sodium as the primary exchangeable cation.
Bentonites used to fabricate GCLs are processed in an unhydrated state such that they appear to have a granular consistency. However, upon hydration with water, the bentonite swells to form a continuous clay layer.
GCLs are shipped in rolls typically 3.7 to 5.3 meters wide and 25 to 60 meters long. They are installed by unrolling to form panels. Adjacent panels are overlapped, and for some products, powdered bentonite is placed between the panels at overlaps.
Large-scale laboratory testing has shown that, when installed in accordance with the manufacturer’s specifications, GCL overlaps are self-sealing and do not create a preferential pathway for liquid flow.
GCLs have been used in liner systems and cover systems for landfills, surface impoundments, and tank farms, as well as in other structures. When used in landfills, GCLs are often substituted for the compacted low-permeability soil component of a composite liner. The function of the GCL in the composite liner is identical to that of a compacted soil liner, which is to provide a low-permeability barrier to liquid flow through any defect in the overlying geomembrane.
The GCL material is manufactured under strict quality control (QC) guidelines. The QC requirements include conducting index and performance testing on both the supplied materials and finished product at specified frequencies. After the material is approved at the manufacturing plant, care is taken to keep the rolls dry, not stack them too high, and keep them from damage during handling.
Prior to acceptance in the field, information concerning the manufacturer’s name, product name, lot and roll number, and length, width, and weight must be submitted to the on-site CQA representative, who will verify all records.
To analyze the leakage through a composite liner utilizing a GCL instead of a CCL, D’Arcy’s equation is utilized based upon an assumed design hydraulic head over the liner.
The leakage through a membrane liner has been found to be closely correlated with the hole defects. In a recent paper a defect size per acre of 1 cm2 was assumed, however assessments of defects and their likely frequency and size vary widely.