Hydraulic Conductivity of GCL Overlap Area

Evaluation of Hydraulic Conductivity of Sodium Bentonite GCL Overlaps to Saline Solutions

Current regulation established by U.S. EPA requires the disposal of solid waste (e.g., municipal solid waste and coal combustion residuals) facilities to include a composite liner system that consists of a geomembrane layer overlying a minimum 0.6-m-thick clay layer. The compacted clay layer must be constructed to have minimum hydraulic conductivity of 10-9 m/s. A sustainable alternative is to use of geosynthetic clay liners (GCLs) in lieu of a conventional clay layer. This is of interest because GCLs have even lower hydraulic conductivity (i.e., < 10-10 m/s) than clay layer, take much less air space due to their thickness (0.005 to 0.01 m) and are much faster and easier to be placed in the field (compare to the construction of a clay layer). A GCL consists of one layer of sodium (Na) bentonite sandwiched between two layers of geotextile that can be woven or nonwoven. GCLs are produced in rolls and when they are installed in the field, these rolls are opened and placed on the ground. To cover large areas, these installed pieces are overlapped with each other.

Many studies have been performed to understand and evaluate the hydraulic conductivity of GCLs that come in contact with various leachates in the landfill (Daniel et al. 1997; Petrov and Rowe 1997; Shackelford et al. 2000; Ashmawy et al. 2002; Katsumi et al. 2008; Bradshaw and Benson 2014; Tian et al. 2016). However, these studies did not investigate the hydraulic conductivity of GCLs where they are overlapped.  If the hydraulic conductivity of the region where the GCL pieces are overlapped is higher than the hydraulic conductivity of the GCL itself, these areas will potentially control the bulk hydraulic conductivity of the GCL layer installed in the landfill liner system. Therefore, proper evaluation of this phenomenon is very important and is lacking in the literature.

The study conducted in GMU’s SGI research group focuses on evaluating the hydraulic conductivity of Na-bentonite GCLs that are overlapped and permeated with synthetic solutions that have characteristics of the leachate typically observed in landfills. Tests are being conducted under four effective stress conditions (20, 100, 250, 500 kPa) to simulate the placement of waste in landfills at different heights. The selected GCLs for the study consist of geotextiles where both sides of the GCL have nonwoven pieces and GCLs with nonwoven geotextile on one side and a woven geotextile on the other side. Two different overlap scenarios are evaluated where in one case the overlap region is filled with supplemental bentonite and in another case where the geotextile at the bottom of the GCL is cut (in lieu of placement of bentonite).