Significance and Use
5.1 All water contained in a saturated soil cannot be removed by gravity drainage alone. The amount of water retained after gravity drainage is usually expressed as the water holding capacity or specific retention of the soil. These values may be influenced by elapsed time, the particle-size distribution and the plasticity of the soil. In most cases, as the plasticity increases so does the moisture equivalent value.
5.2 The centrifuge moisture equivalent is determined by applying a centrifugal force great enough to reduce the capillary fringe zone sufficiently so that it can be ignored without introducing error. The centrifical force is maintained sufficiently low as not to withdraw a large proportion of the water that is held securely above the capillary fringe (see Note 1).
5.3 It has been determined that for at least medium-textured soils (sandy to silty particle-size distribution) the centrifuge moisture equivalent approximates the water holding capacity and when combined with the bulk density can be used to calculate an approximate specific retention and specific yield. These properties when combined with porosity can be used to estimate aquifer storage coefficient.
NOTE 1: If a soil will hold water 100 mm by capillarity acting against gravity, the soil will theoretically be able to hold the water only 0.1 mm against a centrifugal force that is 1000 times greater than the force of gravity.
NOTE 2: The statements on precision and bias contained in this test method; the precision of this test method is dependent on the competence of the personnel 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 assure reliable testing. Reliable testing depends on many factors; Practice D3740 provides a means of evaluating some of these factors.
Scope
1.1 This test method covers the determination of the moisture equivalent of soil in the laboratory by means of a centrifuge technique.
1.2 This test method is limited to specimens of coarse-grained sandy soils having a maximum particle size of less than 2.00 mm and with fines of low plasticity. Soils having a unified soil classification, based upon procedures outlined in Practice D2488 such as SP, SW, SC-SM, or SM are considered acceptable for the test method.
1.2.1 For soils that are predominantly fine-grained, coarse-grained soils with medium to high plasticity, intact specimens or soils being tested at a specific density or unit weight refer to Test Methods D6836.
1.3 This test method is intended to be performed in a constant temperature environment. Variations in temperature exceeding the range outlined in 8.7 may influence the test data.
1.4 Units—The values stated in SI units are to be regarded as the standard except for sieve designations, which also include the “alternative” system in accordance with E11.
1.5 All recorded and calculated values shall conform to the guide for significant digits and rounding established in Practice D6026.
1.6 The procedures used to specify how data are collected/recorded and 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 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 commensurate with these considerations. It is beyond the scope of these test methods to consider significant digits used in analysis methods for engineering design.
1.7 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.
