Laserfiche WebLink
80 SOIL MOISTURE AND GROUND WATER <br /> Sl 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 Iining 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 ongin of deposition <br /> 1 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 /> I <br /> 3 Coarser samples show a greater decrease in permeability with an <br /> increase in stand u d deviation than do fine samples <br /> 4 Unimodal (one dominant size) samples have a greater permeability <br /> TABLE 45 Ranges of intrinsic permeabilates and hydraulic conductrv, <br /> ties for unconsolidated sediments <br /> Intrinsic Hydraulic ! <br /> Permeability Conductivity <br /> Material y l <br /> (dares (cm/sec; I <br /> Clay 10 6- 10-' l0 9-10- <br /> Silt, sandy silts, <br /> clayey sands, till10-' -70- 10 10 ' <br /> Silty sands, fine sands 10 '— 1 10 10 <br /> Well-sorted sands, <br /> Lglacial outwash l — 10� 7 0 ' —10 <br /> Well-sorted gravel 10 - i o, 10-''— 1 I <br /> i »JJ <br /> I <br /> f <br /> 1 <br />