Dispersive clays are those which normally deflocculate when exposed to water of low-salt concentration, the opposite of aggregated clays that would remain flocculated in the same soil-water system (3, 4, 7). Generally, dispersive clays are highly erosive, possibly subject to high shrink-swell potential, may have lower shear strength, and have lower permeability rates than aggregated clays.

Available data (1) indicates that the test method has about 85 % reliance in predicting dispersive performance (85 % of dispersive clays show more than 35 % dispersion).

Since this test method may not identify all dispersive clays, design decisions based solely on this test method may not be conservative. It is often run in conjunction with the crumb test (4, 7), the pinhole test given in Test Method D4647, or the analysis of the pore water extract (4, 7), or combination thereof, to identify possible dispersive clay behavior.

Note 1—Notwithstanding the statement on precision and bias contained in this test method; the precision of this test method is dependent on the competence of the personal performing it, and the suitability of the equipment and facilities used. Agencies that meet the criteria of Practice D3740 are generally considered capable of competent and objective testing. Users of this test method are cautioned that compliance with Practice D3740 does not in itself ensure reliable testing. Reliable testing depends on several factors; Practice D3740 provides a means of evaluating some of those factors.

1.1 This test method, when used in conjunction with a test performed by Test Method D422 on a duplicate soil sample, provides an indication of the natural dispersive characteristics of clay soils (1).

1.2 This test method is applicable only to soils with a plasticity index greater than 4 as determined in accordance with Test Method D4318 and more than 12 % of the soil fraction finer than 5-μm as determined in accordance with Test Method D422 (2).

1.3 This test method is similar to Test Method D422, except that this method covers the determination of percent of soil particles smaller than 5-μm in diameter in a soil-water suspension without mechanical agitation and to which no dispersing agent has been added.

1.4 The amount of particles smaller than 5-μm determined by this method compared with the total amount of particles smaller than 5-μm determined by Test Method D422 is a measure of the dispersive characteristics of the soil.

1.5 This test method may not identify all dispersive clay soils. Pinholes (Test Method D4647 and crumb tests, or both, (3-5)or the analysis of pore water extract (4-7) may be performed to help verify dispersion.

1.6 All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice D6026.

1.7 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.

1.8 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.