Comparison of Leachate Flow Through Compacted Clay Liners in Landfill Liner Systems

The purpose of this paper is to provide an approach for comparing the effectiveness of geosynthetic clay liners (GCLs) and compacted clay liners (CCLs) used in association with geomembranes to form composite liners. Comparing the effectiveness of these two types of composite liners is required in “equivalency demonstrations” intended to demonstrate that a geomembrane-GCL composite liner is equivalent to a conventional geomembrane-CCL liner prescribed by a regulation. In the first part of the paper, the contribution of the geomembrane is ignored and the paper presents analytical evaluations of advective leachate flow through GCLs, low-permeability soil layers (such as CCLs), and two-layer systems including a GCL and a low-permeability soil layer. The analyses presented explain why some calculations typically performed for landfill liner system design or for equivalency demonstrations lead to the paradoxical result that the advective flow of leachate is greater when a GCL is placed on a layer of low-permeability soil than when the GCL is placed on a layer of high-permeability soil. The second part of the paper presents an analytical method for comparing the effectiveness of a composite liner including a GCL and a composite liner including a CCL. This analytical method enables design engineers to compare the effectiveness of various composite liners without neglecting the beneficial effect of the geomembrane. A parametric study presented in the paper shows that neglecting the geomembrane liner in equivalency demonstrations (which is frequently done in the current state of practice) penalizes composite liners that incorporate a GCL.