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' 80 SOIL 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 n <br /> Unconsolidated coarse-granted sediments represent some of the most -F <br /> prolific producers of ground water Likewise, clays are often used for engineer- t <br />' ing purposes, such 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 openuig <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 gram size of the sediment (25) <br /> For sand-sized alluvial deposits, several factors relating intrinsic perme- <br /> ability to gr= size have been noted (26) These observations would hold true <br />' for all sedimentary deposits, regardless of origin of deposition <br /> I. As the median grain size increases, so does permeability This is due <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 /> increase in standard deviation than do fine samples <br />'. 4. Unimodal (one dominant size) samples have a greater permeability <br /> TABLE 45 Ranges of tntnnsic permeabilittes and hydraulic conductivi- <br /> ties for unconsolidated sediments <br /> intrinsic Hydraulic <br />' Permeability Conductivity <br /> Material (darcys) (cm/sec) <br /> Clay 10-1-10-3 10-9-10-6 <br />' Silt, sandy silts, <br /> clayey sands, till 1 o-3--1 o-' 10_6-10--4 <br /> Silty sands, fine sands 10_2-1 9 10-5-10-3 <br /> Well-sorted sands, <br />' glacial outwash 1- 102 10-3_10-1 <br /> Wel!-sorted gravel 10--103 10'2-] 3 <br />