ASTM D6539-00(2006)e2

This test method applies to the one-dimensional laminar (viscous) flow of air in porous materials such as soil.

Note 1—This test method deals with porous materials with both gaseous (air) and liquid (pore water) mobile fluids: The liquid phase is much less compressible, has a higher viscosity, and is much more tightly bound to the solid phase by chemical forces. The assumption of single-phase flow may still be presumed to be valid since the test gradient ensuring the conditions of laminar flow may be low enough that flow of the liquid phase is negligible.

The degree of saturation of the specimen shall be less than that which would produce significant internal transport of pore water or alter the continuity of air voids under the applied pneumatic gradients. The maximum permissible degree of saturation must be evaluated by an experienced analyst. In no instance shall the specimen be so saturated that pore water appears at the exit of the permeameter cell during the test.

This test method is based on the assumption that the rate of mass flow through the specimen is constant with time.

Note 2—When a specimen contains volatile materials this assumption is violated. The mass of gas flowing out will be greater than that flowing in, the pneumatic gradient is indeterminate and the test may become meaningless. Such specimens pose special problems and must be decontaminated before analysis in order to minimize health and safety concerns and to prevent contamination of the test apparatus.

The pneumatic permeability of porous materials may be strongly dependent on a variety of physical properties including the void ratio, the degree of saturation, percent and direction of compaction, and so forth. It is beyond the scope of this test method to elaborate these dependencies. Rather, this test method is intended to be a measurement technique for determining the pneumatic permeability under a certain set of laboratory conditions. It is the responsibility of the requestor to specify which soil parameters must be controlled to ensure a valid extension of the test results to field conditions.

It is assumed that Darcy's Law is valid. The validity of Darcy's law may be evaluated by plotting the volumetric flow through the specimen against the differential pressure drop across the specimen. If the individual test points lie within 25 % of a straight line passing through the origin, then Darcy's law may be taken as valid.

Note 3—Darcy's law is valid only when saturation does not change over time. Long measurement times associated with the use of bubble meters and manometers may indirectly be an uncontrolled source of variability when plotting flow versus pressure drop (see 8.2). The recommended use of digital electronic flow and pressure sensors leads to considerably reduced measurement times because the user can quickly determine by inspection when a steady state condition has been reached. At that point only a single reading needs to be taken for a reliable measurement. A rapid course of measurement will minimize dehydration of partially saturated specimens.

Note 4—Humidifying the test gas to minimize specimen dehydration is not recommended because: (1) there is no practical way to either measure or control the relative humidity of the test gas, either at the inlet or outlet of the specimen; (2) the calibration of the electronic flowmeter is for dry air only and would become unreliable in the presence of water vapor, especially in view of the potential for irreversible adsorption of moisture on the sensor elements; (3) there is a danger of permanent water condensation in the static transfer lines and other apparatus dead volumes; and (4) the test apparatus would become more complex and difficult to use.

This test method covers the use of two different types of permeameter cells, flexible wall and rigid wall, and two types of air flow regulation, mass flow control and pressure control.

A flexible wall permeameter is the preferred means for confining the test specimen in accordance with Test Methods D 5084, D 4525, and D 4767. This test method may be performed using a rigid wall permeameter and all reference to effective confining stress and the permeameter cell pressure system shall be disregarded.

For some specimens, the pneumatic permeability will be strongly dependent on the effective confining stress due to porosity reduction. Whenever possible, the requestor should specify the field overburden conditions at which this test method is to be performed. In some specimens, this stress will vary significantly with flow in an indeterminate way. All specimens should be evaluated for this effect by performing this test method at two or more different confining stress values when a flexible wall permeameter is used.

This test method is intended to support soil remediation operations such as: soil vapor extraction, air sparging, backfilling of soils in utility trenches, and similar engineering activities.

The correlation between results obtained with this test method and in situ field measurements has only been partially established. The small laboratory specimen used in this method may not be representative of the distributed condition on-site due to vadose zone fluctuations, changes in soil stratigraphy, and so forth. For this reason, laboratory test results should be applied to field situations with caution by qualified personnel.

Note 5—This test method is dependent on the competence of the personnel performing it and the suitability of the equipment and facilities used. Agencies which meet the criterion of Practice D 3740 are generally considered capable of competent and objective testing.