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rM <br /> 30 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 <br /> Unconsolidated coarse-grained sediments represent some of the most <br /> prolific producers of ground water Likewise, clays are often used for engineer- <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 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 /> 1. As the median grain size increases, so does permeability This is due E <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 permeawlity <br /> I� <br /> i <br /> TABLE 45 Ranges of intrinsic permeabilities and hydraulic conauctivi- <br /> ties for unconsolidated sediments <br /> Intrinsic Hydraulic <br /> Pe,meacibty Conductivity <br /> terial (darcys) <br /> (cin/scat) <br /> [4"D7 <br /> 10_6-1043 7Q 9-10-6 <br /> silts,sands, til! 1Q-3-ip-' 10-s-10-4 <br /> s, fine sands 1Q_z-1 <br /> d sands,utwash 1 -1021Q-s-10-= <br /> d gravel 10-10' <br /> IQ-Z-1 <br /> a <br /> t <br /> I <br />