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80 <br /> 501E MOISTURE AND GROUND WATER { <br /> SI system of units hydraulic conductivity is in meters per day. A derived unit of <br /> gallons per day per square foot was used in the past. It can be converted to feet <br /> Per day by dividing by 7.48. <br /> 4.3.3 PERMEABILITY OF SEDIMENTS <br /> ;:- Unconsolidated coarse-grained sediments represent some of the most <br /> Prolific producers of ground water. Likewise, clays are often used for engineer- <br /> jing purposes, sucb as lining solid-waste disposal sites, because of their ex- <br /> tremely low intrinsic permeability. There is obviously a wide-ranging continuum ; <br /> of permeability values for unconsolidated sediments (Table 4.5). <br /> The intrinsic permeability is a function of the size of the pore opening. <br /> The smaller the size of the sediment grains, the larger the surface area the water <br /> contacts (Figure 4.13). This increases the frictional resistance to flow,;which <br /> reduces the intrinsic permeability. For well-sorted sediments, the intrinsic , <br /> permeability is proportional to the grain size of the sediment (25). <br /> For sand-sized alluvial deposits, several factors relating intrinsic perme- <br /> ability to grain size have been noted (26). These observations would hold true � <br /> for all sedimentary deposits, regardless of origin of deposition. <br /> L <br /> 1. As the median in size increases so does permeability. This is due <br /> t� � P Y a <br /> to larger pore openings. ; <br /> 2. Permeability will decrease for a given median diameter as the stan- <br /> dard deviation of particle size increases. The increase in standard <br /> deviation indicates a more poorly sorted sample, so that the finer <br /> material can fill the voids between larger fragments. <br /> 3• Coarser samples show a greater decrease in Permeability with an <br /> k increase in standard deviation than do fine samples. <br /> 4. Unimodal (one dominant size) samples have a greater permeability <br /> - TABLE 4.5 Ranges of intrinsic permeabilities and hydraulic conductivi-; <br /> ties for unconsolidated sediments <br /> intrinsic Hydraulic _ <br /> Permeability Conductivity <br /> Material (darcys) (cm/sec) <br /> Clay 10-6-10-3 10_9—10-6 � 1 <br /> Silt, sandy silts, <br /> clayey sands, till 10-3--10-' 10-6-10-4 <br /> Silty sands, fine,sands 10_1-1 10-5-10-3 <br /> - Well-sorted sands, ' <br /> glacial outwash 1 —101 10-3_10- <br /> 10—103 3 0_Z—1 <br /> W611-sorted grave! L <br /> ! _ 1 <br /> 1 <br /> si <br />