PERMEABILITY TESTING OF SOILS WITH LOW HYDRAULIC CONDUCTIVITY
Buettner, Walter George
Soils are often used to line landfills, lagoons, and tailing ponds to control the seepage of water, industrial waste products and hazardous fluids. These soil liners are required to have a low hydraulic conductivity and to remain effective for the life of the project. Permeability testing of soils with a low hydraulic conductivity is an important step in producing a soil liner that meets these requirements. A literature review indicated that the triaxial permeameter was the most suitable permeameter for permeability testing of soils with a low hydraulic conductivity. As a result two special triaxial permeameters were developed which could accurately measure hydraulic conductivity. A special feature of these permeameters was that hydraulic conductivity was measured in both flow in and flow out conditions which also allowed for the determination of volume change. All soils tested in the triaxial permeameters were saturated prior to testing by means of a combination of vacuum and backpressure. As well, flow rates were corrected for leakage from the triaxial permeability systems to produce more accurate determinations of hydraulic conductivity. The triaxial permeameters that were developed could accurately measure hydraulic conductivity as low as 2xlO-11 cm/sec. With the triaxial permeameters described above, the permeability characteristics of various compacted mixtures of sand-bentonite were studied. When the sand-bentonite mixtures were modeled under conditions typical of those on a soil liner, hydraulic conductivity generally decreased with time and with increasing gradient. This was the result of consolidation and the movement of clay particles within the voids of the sample. When the sand-bentonite was permeated with a saturated brine solution, chemical osmosis and the reduction in the thickness of the diffuse double layer of the bentonite caused an increase in hydraulic conductivity. Lastly, it was found that the minimum hydraulic conductivity of a compacted mixture of sand-bentonite was achieved when the mixture was compacted at a moisture content in the range of 1.5 to 2.0 percent wet of optimum.