Hydraulic Conductivity of Geosynthetic Clay Liners Exhumed from Landfill Final Covers with Composite Barriers

摘要

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 years 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 sampling locations. Hydraulic conductivity was measured using two dilute solutions commonly used as permeant water: standard water (SW, 0.01M CaCl_2 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 ≤3 × 10~(10) m/s. Higher hydraulic conductivities and sensitivity to permeant water did not correspond directly to the amount of cation exchange. Exhumed GCLs with higher gravimetric higher water contents (>50%) 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. These GCLs with higher water contents were placed on subgrade having water content in excess of optimum water content (standard Proctor). 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 (≤5 × 10~(-11) m/s), even if the native Na is ultimately replaced by divalent cations. Subgrade with water content ≥ optimum water content is recommended.