SGER: Forensic Analysis of a Final Cover Relying on a Clay Barrier

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

Final covers that rely on a compacted clay barrier layer as the primary impediment to percolation are commonly used in the US for waste containment and isolation of contaminated materials. The cover generally consists of a layer of compacted clay 450-900 mm thick overlain by an erosion layer that is 150 mm to 900 mm thick. The saturated hydraulic conductivity of the clay generally is required to be less than 10 -7 cm/s, which corresponds to a maximum percolation rate of 32 mm/yr under conditions of continuous gravity drainage. Thousands of these covers have been constructed in the US. However, there is growing concern that these covers are unlikely to perform as intended over the long-term due to the destructive effects of desiccation and frost action. Field data from several sites suggest that these processes are resulting in percolation rates that are much higher than intended (hundreds of mm per yr), but detailed investigations have not yet been conducted to show that the high percolation rates are definitively tied to morphological changes in the clay barrier layer. In the proposed study, a forensic investigation of the cover will be conducted, the result being a unique and novel data set describing the effects of weathering on the fabric, hydrologic properties, and field performance of a clay barrier in a cover. A series of field and laboratory tests will be conducted. Field hydraulic conductivity tests that permeate different volumes of soil will be conducted directly on the barrier layer and large undisturbed samples of the barrier layer will be retrieved for laboratory hydraulic conductivity testing. The pathways conducting flow will be marked with dye added to the permeant water used for the hydraulic conductivity tests. After permeation, transects will be excavated in the cover to study the aperture, length, and spacing of the dye-stained cracks. A similar examination will be conducted after permeating the laboratory specimens. The study has economic, environmental, and societal impacts. Economic and environmental impacts will be realized by providing a thorough evaluation of the effectiveness of the most common remedy used in Superfund projects. The results of this study may show that other cover technologies are likely to be more cost effective and environmentally protective over the long-term than covers relying on a compacted clay barrier. Societal impacts will be realized through the diversity efforts of the PI in undergraduate and graduate studies.