Properties of Barrier Components in a Composite Cover after 14 Years of Service and Differential Settlement

A case study is presented describing the effects of age (14 years) and differential settlement (≈0.3 m vertical over ≈0.4 m horizontal along a horizontal distance of ≈10 m) on the engineering properties of a soil barrier layer, a geosynthetic clay liner (GCL), and a geomembrane within a composite cover. Samples of the soil barrier layer had hydraulic conductivity below the design requirement of $5.0\times {10}^{-7}\mathrm{m}/\mathrm{s}$, except in areas that were cracked because of differential settlement. Tests showed that the geomembrane exceeded design specifications for tensile yield strength ($\ge 22.9\mathrm{kN}/\mathrm{m}$) and elongation at tensile yield ($\ge 13.0\%$), and current standard specifications for oxidative induction time ($\ge 100\min$) and stress crack resistance ($\ge 500\mathrm{h}$). Geomembrane seams also exceeded design specifications for peel strength ($\ge 15.9\mathrm{kN}/\mathrm{m}$) and shear strength ($\ge 22.9\mathrm{kN}/\mathrm{m}$). Geosynthetic clay liner samples showed a reduction in swell index relative to the as-built condition (from 27.9 to $21.0–24.5\mathrm{mL}/2\mathrm{g}$) because of cation exchange. However, all GCL samples had hydraulic conductivity below the design requirement of $4\times {10}^{-11}\mathrm{m}/\mathrm{s}$.