where n is the porosity of the soil. Note that v is always larger than q.

Intrinsic Permeability

The hydraulic conductivity K is a material constant, and it depends not only on the type of soil but also on the type of fluid (dynamic viscosity /) percolating through it. The hydraulic conductivity K is expressed as

where k is called the intrinsic permeability and is now a property of the soil only. Many attempts have been made to express k by such parameters as average pore diameter, porosity, and effective soil grain size. The most familiar equation is that of Kozeny-Carmen k = Cd2


n = porosity d = the effective pore diameter

C = a constant to account for irregularities in the geometry of pore space

Another equation by Hazen states k = CD2 = C1D20


D = the average grain diameter

D10 = the effective diameter of the grains retained

Values of hydraulic conductivity can be obtained from empirical formulas, laboratory experiments, or field tests. Table 9.2.1 gives the typical values for various aquifer materials.

Validity of Darcy's Law

Darcy's law is restricted to a specific discharge less than a certain critical value and is valid only within a laminar

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