Pneumatic Permeability (Air permeability)
ASTM D6539
Scope and Use
     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.  This test method covers laboratory determination of the Coefficient of permeability for the flow of air (pneumatic permeability) through partially saturated porous materials. This test method may be used with undisturbed or compacted coarse grained soils, silts, or lean cohesive soils that have a low degree of saturation and that have pneumatic Permeability between 0.001 square micrometers (1.01milli- darcy) and 100 square micrometers (101darcy).

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

     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.  Cooper Testing Labs can perform this test at multiple effective confining stresses if requested.

Definitions
     Pneumatic (Air) permeability - The capacity of a porous medium to conduct gas in the presence of a gas (air) pressure gradient measured as the ratio of volumetric flow through a specimen to the resultant pressure drop across it.  This is also commonly known as pneumatic conductivity or air permeability.  As a sample becomes more saturated, the air permeability will decrease because the amount of air-filled pore spaces available for flow decreases.

     Effective permeability - A measure of the ability of a single fluid to flow through a rock when the pore spaces of the rock are not completely filled or saturated with that fluid.

     Intrinsic permeability - While air permeability or hydraulic conductivity are a function of  both the porous medium and the fluid flowing through it, the intrinsic permeability is only a function of the medium itself and is independent of the permeating fluid.  The intrinsic permeability is a function of the size of the pore spaces through which the fluid moves.

Sample Requirements
Sample Type:
Undisturbed (can also be run on a remolded sample). 
Container: Brass or steel liner (shelby tube); pitcher tube. 
Sample Size:
2", 2.5", 3",  4" and 6”. Two-inch diameter samples and smaller can be tested but are not recommended due to typical sampling disturbance seen in these small diameter samples.  See our newsletter article regarding sample diameter and disturbance for more information. Testing costs for four and six inch diameter samples are higher than the standard cost.  See our fee schedule for more information. 
Sample Height: The minimum sample height should be greater than 2 to 2.5 times the diameter.  More sample is required if other tests will be performed. 
Sample Storage: The sample should be sealed to prevent the loss of moisture and be kept cool, but not frozen, and in a vertical position. The sample should be delivered to the lab as soon as practical after sampling. 
Sample Handling: It is critical to handle with care. The permeability test can be very sensitive to sample disturbance. 
Turnaround Time:
For effective permeability (Native State – In-situ moisture content), 5 business days, depending on quantity and workload.  For intrinsic permeability (Specific permeability – air dry state) approximately 10 business days depending on the length of time required to air-dry the specimen while avoiding cracking. 

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