Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils
5.1 These test methods are used as an integral part of several engineering classification systems to characterize the fine-grained fractions of soils (see Practices D2487 and D3282) and to specify the fine-grained fraction of construction materials (see Specification D1241). The liquid limit, plastic limit, and plasticity index of soils are also used extensively, either individually or together, with other soil properties to correlate with engineering behavior such as compressibility, hydraulic conductivity (permeability), compactibility, shrink-swell, and shear strength. (See Section 6, Interferences.)
5.2 The liquid and plastic limits of a soil and its water content can be used to express its relative consistency or liquidity index. In addition, the plasticity index and the percentage finer than 2-μm particle size can be used to determine its activity number.
5.3 These methods are sometimes used to evaluate the weathering characteristics of clay-shale materials. When subjected to repeated wetting and drying cycles, the liquid limits of these materials tend to increase. The amount of increase is considered to be a measure of a shale's susceptibility to weathering.
5.4 The liquid limit of a soil containing substantial amounts of organic matter decreases dramatically when the soil is oven-dried before testing. Comparison of the liquid limit of a sample before and after oven-drying can therefore be used as a qualitative measure of organic matter content of a soil (see Practice D2487).
Note 1: The quality of the result produced by this standard 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, generally, are considered capable of competent and objective testing/sampling/inspection/etc. Users of this standard are cautioned that compliance with Practice D3740 does not in itself assure reliable results. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some of those factors.
1.1 These test methods cover the determination of the liquid limit, plastic limit, and the plasticity index of soils as defined in Section 3 on Terminology.
1.2 Two procedures for preparing test specimens are provided as follows: Specimen Preparation Procedure 1 (Wet Preparation), as described in 11.1 and Specimen Preparation Procedure 2 (Dry Preparation), as described in 11.2. The specimen preparation procedure to be used shall be specified by the requesting authority. If no specimen preparation procedure is specified, use the wet preparation procedure. (See Section 6, Interferences.)
1.3 Two methods for determining the liquid limit are provided as follows: Liquid Limit Method A (Multipoint Method), as described in Sections 12 and 13, and Liquid Limit Method B (One-Point Method), as described in Sections 14 and 15. The method to be used shall be specified by the requesting authority. If no method is specified, use Method A. (See Section 6, Interferences.)
1.4 The plastic limit test, Method for Plastic Limit, is performed on material prepared for the liquid limit test. When determining the plastic limit, two procedures for rolling portions of the test specimen are provided as follows: Plastic Limit Rolling Procedure 1 (Hand Rolling), and Plastic Limit Rolling Procedure 2 (Using the Rolling Device).
1.5 The liquid limit and plastic limit of soils (along with the shrinkage limit) are often collectively referred to as the Atterberg limits. These limits distinguished the boundaries of the several consistency states of plastic soils.
1.6 The plasticity index, Method for Plasticity Index, is calculated using results from liquid limit and plastic limit testing.
1.7 The methods described herein are performed only on that portion of a soil that passes the 425-μm (No. 40) sieve. Therefore, the relative contribution of this portion of the soil to the properties of the sample as a whole must be considered when using these methods to evaluate the properties of a soil.
1.8 The values stated in SI units are to be regarded as standard. The values given in parentheses after SI units are provided for information only and are not considered standard.
1.8.1 The converted inch-pound units use the gravitational system of units. In this system, the pound (lbf) represents a unit of force (weight), while the unit for mass is slugs. The converted slug unit is not given, unless dynamic (F = ma) calculations are involved.
1.8.2 It is common practice in the engineering/construction profession to concurrently use pounds to represent both a unit of mass (lbm) and of force (lbf). This implicitly combines two separate systems of units; that is, the absolute system and the gravitational system. It is scientifically undesirable to combine the use of two separate sets of inch-pound units within a single standard. As stated, this standard includes the gravitational system of inch-pound units and does not use/present the slug unit for mass. However, the use of balances or scales recording pounds of mass (lbm) or recording density in lbm/ft3 shall not be regarded as nonconformance with this standard.
1.8.3 The “alternative” sieve size designations listed in Specification E11 for the 425-µm and 2.00-mm sieves are given in parentheses following the “standard” designations, that is, 425-µm (No. 40) and 2.00-mm (No. 10).
1.8.4 The standard units for the resilience tester covered in Annex A1 are inch-pound, not SI. The SI values given in Fig. A1.1 are for information only.
1.9 All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice D6026.
1.9.1 For purposes of comparing a measured or calculated value(s) with specified limits, the measured or calculated value(s) shall be rounded to the nearest decimal or significant digits in the specified limits.
1.9.2 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 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.
1.10 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.11 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
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