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This method describes a procedure for the determination of the permeability of water through granular (cohesionless) soils in a steady-state condition.

The determination of the coefficient of permeability, k, as defined in this method, was developed under the assumptions of the validity of Darcy's Law, which states that the coefficient of permeability is the ratio of the flow rate to the hydraulic gradient.

This test procedure is intended for use with disturbed granular soils containing not more than 10 percent passing the 75-µm (No. 200) sieve. Soil materials with significant amounts of material passing the 75-µm (No. 200) sieve may be subject to consolidation effects that may influence the outcome of the test results. Permeability of soils not meeting the above criteria should be performed by use of a flexible wall permeameter test procedure, such as ASTM D5084.

Note 1-Soils containing less than 14 percent passing the 75-µm (No. 200) sieve may be tested if it is determined that the liquid limit does not exceed 25 and plasticity index does not exceed 5, and the hydrometer analysis from AASHTO T 88 reveals the majority of the 75-µm (No. 200) particles are silt. The height of the sample after the permeability test should be obtained to ensure no consolidation effects have occurred during testing. A moisture content using T 265 should be performed from the sample after the permeability test to ensure it has been adequately saturated with a Degree of Saturation in the range of 98 percent to 100 percent.

Two methods of test, Method A and Method B, are provided. The method used is dependent on the permeameter used to complete testing.

The quality of the results produced by this standard are dependent on the competence of the personnel performing the procedure and the capability, calibration, and maintenance of the equipment used. Agencies that meet the criteria of R 18 are generally considered capable of competent and objective testing/sampling/inspection/etc. Users of this standard are cautioned that compliance with R 18 alone does not completely assure reliable results. Reliable results depend on many factors; following the suggestions of R 18 or some similar acceptable guideline provides a means of evaluating and controlling some of those factors.