Permeability - University of Missouri Columbia Civil and...

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University of Missouri – Columbia Civil and Environmental Engineering PERMEABILITY TEST PURPOSE The purpose of this laboratory is to teach you how to measure the hydraulic conductivity of soils using constant and falling head tests. Although field permeability tests are generally to be preferred over laboratory tests (because field tests include the effects of joints, fissures, root holes, and other similar defects better than do laboratory tests), laboratory tests are still very useful for: - tests on compacted soils - studies of the effects of variables such as chemistry of the permeating fluid - studies of the effects of direction of fluid flow on hydraulic conductivity Laboratory tests also have the advantage of being far less expensive than field tests, which is probably the main reason why laboratory tests are still performed in relatively large numbers. BACKGROUND Terminology Analysis of water flow in soil is usually based on Darcy's (1856) law. The law has been written in many forms, depending on the discipline of the user and the date of usage. The form most commonly encountered is: A i k q - = (1) where q is the rate of flow (units of L 3 /T), i is the hydraulic gradient (the dimensionless ratio of rate of change of total head with respect to position along the path of flow), A is the cross- sectional area of flow (L 2 ), and k is a constant of proportionality (L/T) that is termed hydraulic conductivity in most disciplines but is often termed coefficient of permeability, or just permeability, by many civil engineers. In addition to being dependent on the properties of the soil, k is also dependent upon the properties of the permeating fluid. For example, motor oil will flow through a specimen of sand more slowly than water, all other factors being equal. To take into account the properties of the fluid, Darcy's law may be written in an alternative form: A i K q w - = m g (2)
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University of Missouri – Columbia Civil and Environmental Engineering Permeability Test p.2 where γ w (M/L 2 T 2 ) and μ (ML/T) are the unit weight and viscosity of the fluid, respectively, and K is a constant of proportionality (L 2 ) that is termed the coefficient of permeability in most disciplines but is sometimes termed intrinsic permeability. A third form of Darcy's law is: A dx dp k q - = m (3) where q is the rate of flow in units of cubic centimeters per second, μ is the viscosity of the fluid in units of centipoises, p is the pressure in the fluid in units of atmospheres, x is the position along the path of flow in units of centimeters, A is the cross-sectional area of flow in units of square centimeters, and k is the permeability in units of "darcies." For a permeant liquid of pure water at 20°C, the conversions are: 1 cm/sec = 1.02 x 10 -5 cm 2 = 1.04 x 10 3 darcy Because of the simplicity of Eq. 1 and the fact that geotechnical engineers are usually concerned with flow of a single fluid (water), most geotechnical engineers prefer to use Eq. 1. We will also
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This note was uploaded on 09/23/2011 for the course CIVIL ENGI CE 3400 taught by Professor Rosenblad during the Spring '11 term at Missouri (Mizzou).

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Permeability - University of Missouri Columbia Civil and...

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