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(Russian, fil’tratsiia), the movement of a liquid—such as water or petroleum—or a gas—such as air or natural gas—through a porous medium in the ground beneath the surface of the earth. Seepage also means the percolation of water through soil or concrete, for example, through the body of an earth or concrete dam. In Russian, the term fil’tratsiia is often used along with the term fil’trovante (filtration) to describe similar processes that occur under industrial or laboratory conditions (seeFILTRATION).
The flow rate of a seeping liquid or gas is usually given by the relation Q = kShw/L, where k is the empirical coefficient of seepage, S is the total cross-sectional area of the seepage and includes the cross sections of both pores and solid particles, and hw is the head loss over the length L of the seepage path. The seepage velocity is given by Darcy’s law: W = kl, where hw/L = I is the pressure gradient, which is also called the hydraulic gradient and indicates the pressure drop per unit length of the seepage path. The seepage velocity is less than the actual flow rate of the liquid or gas in the pores because the fluid moves only through the portion of the cross-sectional area S that is occupied by the pores. Darcy’s law is valid for a laminar flow in the pores of a filter medium; seepage is a laminar flow in most cases, for example, in sandy and clay soils or in concrete.
Seepage in coarse-grained materials, such as the rock used in rock-fill dams, is a turbulent flow. For such a flow, the seepage velocity is given by other relations, for example, W = k’ Im where k’ and m are seepage characteristics of the material. Here, k’ is analogous to the coefficient of seepage, and m ranges from 1 to ½.
REFERENCESAravin, V. I., and S. N. Numerov. Teoriia dvizheniia zhidkostei i gazov v nedeformiruemoiporistoisrede. Moscow, 1953.
Polubarinova-Kochina, P. Ia. Teoriia dvizheniia gruntovykh vod. Moscow, 1952.
Shchelkachev, V. N., and B. B. Lapuk. Podzemnaiagidravlika. Moscow-Leningrad, 1949.
Bogomolov, G. V. Gidrogeologiia s osnovami inzhenernoi geologii, 2nd ed. Moscow, 1966.