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    Constant-Flow and Constant-Gradient Permeability Tests on Sand-Bentonite-Fly Ash Mixtures

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    Flexible-wall permeability (hydraulic conductivity) tests are conducted on compacted specimens consisting of 50 percent Class F fly ash with a 50 percent soil admixture (sand with 0, 6.25, 12.5, 18.75, and 25 percent bentonite). All test specimens are back-pressured before permeation with distilled water. Three specimens from each sand-bentonite-fly ash mixture are tested under high (72 or 73) and low (6.1) constant-gradient conditions as well as under constant-flow conditions for a total of 15 tests. Induced hydraulic gradients from the constant-flow tests range from 3.5 to 41. A decrease in permeability with an increase in bentonite content is evident regardless of the test method. However, the permeability is minimized at a bentonite content of 18.75 percent regardless of the test method. The overall lowest permeability value is 1.6 × 10-9 m/s. An increase in hydraulic gradient of 11.9X resulted in a 3.1X to 6.2X decrease in measured permeability values for all test specimens. A significant portion of the difference in measured permeability values is attributed to differences in applied or induced hydraulic gradients regardless of the test method.


    permeability, hydraulic conductivity, fly ash, soil admixture, constant gradient, flow pump, flexible wall permeameter, laboratory testing

    Author Information:

    Shackelford, CD
    Associate Professor, Colorado State University, Fort Collins, CO

    Glade, MJ
    Environmental Engineer, Engineering-Science, Inc., Denver, CO

    Committee/Subcommittee: D18.04

    DOI: 10.1520/STP23906S