Design considerations for earth linings for seepage control
1982
Darcy's equation and unsaturated-flow theory are used to calculate flow through earth linings that are placed in ponds or channels to reduce seepage. The procedure utilizes the relation between unsaturated hydraulic conductivity and pressure head of the underlying material as calculated from the measured saturated hydraulic conductivity and the relation between water content and pressure head. The method enables the selection of the liner (thickness and hydraulic conductivity) that will keep seepage below a certain maximum limit. Since the hydraulic conductivity of clays is affected by the cationic composition and the salt concentration of the soil solution, the chemical composition of the liquid moving through the liner must be taken into account. This applies also to other chemicals, including solvents and other organic compounds that may be in the water. Travel times of water from the surface impoundment to the underlying ground water are calculated from the seepage rate and the corresponding water content in the vadose zone. Accumulation of solids (mine tailings, for example) can further reduce the seepage from the pond. Proper design of waste-water ponds also requires analysis of the response of the underlying ground water (mound buildup) and the movement of pollutants in the vadose zone and aquifer. In view of the high costs of earth liners, prediction of the seepage is necessary to make sure that the selected lining material, the thickness of the liner itself, and the method of construction will produce the desired results.
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