Hydraulic Conductivity Of Geosynthetic Clay Liners Exhumed From Landfill Final Covers With Composite Barriers

by Craig H. Benson

Abstract:

Geosynthetic clay liners (GCLs) were exhumed from composite barriers (i.e., geomembrane over GCL) in final covers at four sites after 4.7 to 6.7 yr to evaluate the in-service condition. Monovalent bound cations were replaced by divalent cations in all GCLs, with near complete exchange at two-thirds of the sites. Hydraulic conductivity was measured using two dilute solutions commonly used as permeant water: standard water (SW, 0.01 M CaCl2 solution) and Type II deionized water (DW). Hydraulic conductivities to SW varied over four orders of magnitude, whereas identical specimens (i.e., from same sample) had hydraulic conductivities to DW consistently ≤ 3x10-10 m/s. Higher hydraulic conductivities and sensitivity to permeant water did not correspond directly to the amount of cation exchange. GCLs exhumed at higher water contents (> 50%) placed on subgrade with total cations (soluble) per mass (TCM) ≤ 0.8 cmol+/kg exhibited a gel structure indicative of osmotic hydration and had lower hydraulic conductivities to both SW and DW, regardless of the amount of sodium (Na) replaced by divalent cations. Based on the findings in this study, conditions that promote rapid hydration and osmotic swell in a GCL are recommended to ensure that a GCL in a composite barrier maintains low hydraulic conductivity (≤ 5x10-11 m/s) even if the native Na is ultimately replaced by divalent cations. Subgrade with water content ≥ 10% and TCM ≤ 0.8 cmol+/kg is recommended.

Key words: geosynthetic clay liner, landfill, final cover, hydraulic conductivity, cation exchange, hydration, osmotic swell, crystalline swell, preferential flow

Wisconsin Distinguished Professor, Geological Engineering, University of Wisconsin, Madison, WI 53706 USA, chbenson@wisc.edu

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