In-Service Hydraulic Properties of Two Landfill Final Covers in Northern California

Hydraulic properties of cover soils in two final cover test sections located in northern California were evaluated after the covers were in service for 6.5 yr. One test section represented a conventional cover with a composite barrier (geomembrane over compacted clay) and the other a store-and-release cover. Field hydraulic conductivity tests were performed and large intact samples were collected from the barrier and storage layers for laboratory analysis. Laboratory tests on large-scale specimens yielded similar saturated hydraulic conductivity as the field tests, whereas tests on small-scale specimens yielded consistently lower saturated hydraulic conductivity. Field hydraulic conductivity of the storage layer in the store-and-release cover (geometric mean = 6.8 x 10^–7 m/s) was 10 times larger than the hydraulic conductivity of the clay barrier in the conventional cover (geometric mean = 7.0 x 10^–8 m/s). The soil water characteristic curves (SWCCs) were also sensitive to the size of the specimen tested. Large test specimens (254-mm diameter) had lower air entry suction (higher α), a more gradually varying SWCC (lower n), and higher saturated water content compared to conventional 73-mm specimens. These findings indicate that large-scale tests should be conducted in the field or laboratory when evaluating the in-service hydraulic properties of final covers.