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•S 2 8 AIR-TO-WATER PERMEABILITY RATIO 41-4 PERMEABILITY OF SOIL TO WATER 529
<br /> V= (k'a At)/2 [13] 41-4.2 Procedure
<br /> where V is in liters, k',, is in square microns (µ2), and At is in seconds.
<br /> For the purpose for which this measurement is made, i.e., to compare Follow the procedure of section 41-3.2 for preparing and packing
<br /> with water permeability as an index of soil structure, it is not necessary to samples. Following the air-permeability determination, remove the lid from
<br /> strive for a high degree of precision id measurement. It is well to keep in the soil-filled can, and place a brass-cylinder extension on top of the can.
<br /> mind that the scale of values for air to water permeability ratio is extremely Seal it in place with an elastic band or with wax. Place the soil sample on
<br /> large, ranging from 2 or 3 for very stable soils to as much as 50,000 for the rack, cover the soil surface with a disc of filter paper, and admit water
<br /> unstable soils.Errors associated with air-permeability determinations, which to the sample from the supply reservoir with the siphon. Adjust the water
<br /> mostly will be of the order of 10 to 20%, are therefore of little consequence level in the supply reservoir so that the height of the soil-plus-water column
<br /> for this use of the air-permeability values. If the air-permeability value is is approximately two times the soil-column length.
<br /> used for other purposes where a higher order of precision is required, then Record the temperature of the water, the time at which water is admitted
<br /> special attention should be given to sources of measurement error. In any to the container, and the time at which water first percolates through the
<br /> case, it would be desirable to check the method and equipment with an sample. Measure the volume of percolate obtained during a number of
<br /> independent method.This can be done by use of a stable permeable medium successive time intervals as dictated by the purpose of the determination.
<br /> such as a carbon or ceramic core, using water or other viscous fluid. If
<br /> water is used, care should be exercised to ensure complete saturation. Dis-
<br /> placement of air in the pores with CO., prior to wetting is a convenient and 41-4.3 Calculations
<br /> effective method of attaining saturation.
<br /> Air-permeability values for soils as determined by this method have Permeability of the soil to water is given by the equation
<br /> ranged from 0.10 to 18µ2. In one trial where the air-permeability determi-
<br /> nation was repeated on a given sample, the average deviation from the mean V. _ "► k = 'l VL 14
<br /> (duplicate runs on a large number of samples) was -�-1.25%, with a coeffi- P�,g P,�g A Ah At
<br /> [ ]
<br /> cient of variation of ±1.14%. Adding the variation from sampling and in which
<br /> packing, i.e., the reproducibility of air-permeability determinations on dif- V = intrinsic permeability with water, cm.2,
<br /> ferent samples of a given soil, the average deviation from the mean was k = hydraulic conductivity, cm. per sec.,
<br /> =9% with a coefficient of variation of =-9.3%. V = volume of percolate in time At, cm.',
<br /> "Gas slippage" is a factor involved in the Ilow of gases in porous media L = length of soil column, cm.,
<br /> and consequently is involved in air-permeability measurements. Errors Ah = difference in hydraulic head between the inflow and outflow ends of
<br /> of from 10 to 30% from this source have been reported for air-permeability the soil column, cm.,
<br /> measurements (Corey, 1957; American Petroleum Institute, 1956). Cor- A = cross sectional area of the soil column, cm.'-,
<br /> rection for this can be made by extrapolation of the air-permeability de- At = time interval for volume of percolate V to pass through the soil,sec.,
<br /> terminations at known pressures to a value at infinite pressure. For the n = viscosity of water at the recorded temperature, dyne sec. cm.-2
<br /> usual application of this method, a correction is not required; however, if (poises),
<br /> more precise values are desired, the correction should be made as described P. = density of water, g. per cm.a, and
<br /> by Corey (1957) and American Petroleum Institute (1956). g = acceleration of gravity, cm. sec.-2
<br /> Values for q, p,, and g can be obtained from the Handbook of Physics
<br /> 41-4 METHOD FOR PERMEABILITY OF SOIL TO WATER and Chemistry.
<br /> 41-4.1 Special Apparatus
<br /> 41-4.4 Comments
<br /> Prepare a soil container as described in section 41-3.1. For use in the
<br /> water-permeability measurement, arrange a constant-level water supply as The amount of water passed through the soil and the number of volume
<br /> illustrated in Fie. 41-2,part 3. measurements made will depend upon the purpose of the determination.
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