【国外标准】 Standard Test Method for Measurement of the Permeability of Unsaturated Porous Materials by Flowing Air (Withdrawn 2022)

4.1 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.4.2 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 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.4.3 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 gradient cannot be determined 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.4.4 The permeability of porous materials may be strongly dependent on a variety of physical properties including the void ratio, the degree of saturation, and percent and direction of compaction. It is beyond the scope of this test method to elaborate upon these dependencies. Rather, this test method is intended to be a measurement technique for determining the permeability under a certain set of laboratory conditions. It is the responsibility of the test requestor to specify which soil parameters must be controlled to ensure a valid extension of the test results to field conditions.4.5 Calculation of the permeability using Darcy’s law requires laminar flow conditions through the soil specimen. The conditions for laminar flow shall be evaluated by plotting the volumetric flow rate of air through the specimen against the pressure drop across the specimen. If the individual test points lie within 25 % of a straight line passing through the origin, then laminar flow conditions are present and Darcy's law may be used to calculate the permeability.Note 3—The permeability calculated using this standard is valid only when the degree of saturation does not change over time. Long measurement times associated with the use of bubble meters and manometers may indirectly lead to variability when measuring flow versus pressure drop (see 8.2) due to evaporation. 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 unsaturated 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 typical digital flowmeters are generally 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.4.6 This test method covers the use of two different types of permeameter cells (flexible and rigid wall permeameters) and two types of air flow regulation (mass flow control and pressure control).4.7 A flexible wall permeameter is the preferred means for confining the test specimen in accordance with Test Methods D5084, D4525, and D4767. This test method may still be performed using a rigid wall permeameter, but all reference to effective confining stress and the permeameter cell pressure system shall then be disregarded.4.8 For some specimens, the permeability will be strongly dependent on the effective confining stress due to porosity reduction. Whenever possible, the requestor shall 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 shall 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.4.9 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.4.10 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, caution should be used by qualified personnel when applying laboratory test results to field situations.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 D3740 are generally considered capable of competent and objective testing.1.1 This test method covers laboratory determination of the coefficient of permeability for the flow of air through unsaturated porous materials.1.2 This test method may be used with intact or compacted coarse grained soils, silts, or lean cohesive soils that have a low degree of saturation and that have permeability between 1.0 × 10-15 m2 (1.01 millidarcy) and 1.0 × 10-10 m2 (101 darcy).1.3 The values stated in SI units are to be regarded as standard.1.3.1 By tradition in U.S. practice, the permeability of porous media is reported in units of darcy, although the SI unit for permeability is m2.1.4 All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice D6026.1.4.1 For the purpose of comparing a measured or calculated value with specified limits, the measured or calculated value shall be rounded to the same precision as the specified limits.1.4.2 The procedures used to specify how data are collected/recorded or calculated, in this standard are regarded as the industry standard. In addition, they are representative of the significant digits that generally should be retained. The procedures used do not consider material variation, purpose for obtaining the data, special purpose studies, or any considerations for the user's objectives; and it is common practice to increase or reduce significant digits of reported data to be commensurate with these considerations. It is beyond the scope of this standard to consider significant digits used in analysis methods for engineering design.1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.