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ISOTROPIC SOILI -eDrainIimpervious LayerMidpointBetween Dia insDraindlmpervious LayerMidpointBetween DrainsFigure 1-7, Streamlines and equipotentials
"Hydraulic conductivity" of a soil is a numerical value for permeability. Itis equal to the proportionality factor K in the Darcy equation. The Darcyequation is an expression of effective velocity of flow as a function of hydraulicgradient and the transmission properties of the soil and water. It was found thateffective velocity is proportional to hydraulic gradient, all other things beingequal :Lwhere v = effective flow velocity, dimensions -T(Effective flow velocity is the velocity with respect to the total areaof the porous medium--not the void area alone. It may be defined asthe quantity of flow per unit of time divided by the total area of theporous medium producing that quantity of flow.)LK = a factor, dimensions -Ti = hydraulic gradient, dimensionlessThus, hydraulic conductivity is the effective velocity of flow when the hydraulicgradient is unity. In drainage design, it is convenient to express v and K ininches per hour. Darcy's Law is valid for flow velocities in almost any naturaldrainage situation.Hydraulic conductivity depends on properties both of the soil and of the transmittedwater. A high value is associated with high porosity, coarse open texture,and highly developed structure. Soils do not vary greatly in porosity, but afew large pores are more effective in contributing to high conductivity thanmany small pores. Fine-textured soils-may depend almost entirely on the structuralpores for their conductivity. The quality of the water transmitted, particularlythe salinity and alkalinity, may have a marked effect on hydraulicconductivity.Soil within a drainage-problem area seldom has uniform permeability. This variationis exhibited in two important characteristics: the soil may be nonhomogeneousdue to stratification, barriers, or other distinct masses; or it may have a higherpermeability in one direction than another, even though homogeneous. Soils withthis latter quality are called "anisotropic" (see section on "Paths of Flowt').Rate of flowThe rate of flow (Q) passing a given cross-sectional area of saturated soil (A)is the product of the area and the effective velocity of flow through the section(v) :Combining this expression with the Darcy Lawwe have the expression
Page 5 and 6: Classification of drainage methodsT
Page 7 and 8: occur where the soil is either slow
Page 9 and 10: apidly permeable deposits may serve
Page 12 and 13: Bernoulli's theorem is used to comp
Page 14 and 15: --Pressure Headat Point "P" =Elevat
Page 16 and 17: Figure 1-3, Equipotenrial surfaces-
Page 18 and 19: Figure 1-5, Flow direction in isotr
Page 22 and 23: This equation may be used to estima
Page 24 and 25: conditions if the curvature of the
Page 26 and 27: ModelsSand or soil is sometimes pla
Page 28 and 29: Drainage coefficients for pumping p
Page 30 and 31: This equation neglects the curvilin
Page 32 and 33: References(1) DARCY, H.1856. Les Fo

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