ASTM D3999/D3999M-26

1.1 These test methods cover the determination of the modulus and damping properties of soils in either intact or reconstituted states by either load- or displacement-controlled cyclic triaxial techniques. The standard is focused on determining these properties for soils in isotropically-consolidated, undrained conditions.

1.2 The cyclic triaxial properties of initially saturated or unsaturated soil specimens are evaluated relative to a number of factors including: strain level, density, number of cycles, material type, and effective stress.

1.3 These test methods are applicable to both fine-grained and coarse-grained soils as defined by the unified soil classification system or by Classification D2487. Test specimens may be intact or reconstituted in the laboratory, as determined by the requesting agency.

1.4 Two test methods are provided for using a cyclic loader to determine the secant Young's modulus (E) and axial damping coefficient (D_axial) for a soil specimen. The term axial damping coefficient is used in this standard, as the damping coefficient determined from a cyclic triaxial test is expected to be different from the damping coefficient obtained from a cyclic torsional shear test. The test method selected depends on the application under investigation. The test methods are as follows:

• 1.4.1 Test Method A—This test method requires the application of a constant cyclic load amplitude to the test specimen. It is used for determining the secant Young's modulus and axial damping coefficient under a constant cyclic load amplitude condition.
• 1.4.2 Test Method B—This test method requires the application of a constant cyclic displacement amplitude to the test specimen. It is used for determining the secant Young's modulus and axial damping coefficient under a constant cyclic displacement amplitude condition.

1.5 Limitations—There are certain limitations inherent in using cyclic triaxial tests to simulate the stress and strain conditions of a soil element in the field during an earthquake, with several summarized in the following sections. With due consideration for the factors affecting test results, carefully conducted cyclic triaxial tests can provide data on the cyclic behavior of soils with a degree of accuracy adequate for meaningful evaluations of modulus and axial damping coefficient below a axial strain level of 0.5 %. The calculations used in this standard make the assumption that the soil behaves such that the load-displacement hysteresis loops close within a set closure error that will be discussed in 9.4.6.

• 1.5.1 Nonuniform stress conditions within the test specimen are imposed by the specimen end platens.
• 1.5.2 A 90° change in the direction of the major principal stress occurs during the two halves of the loading cycle on isotropically confined specimens.
• 1.5.3 The maximum cyclic axial stress that can be applied to a saturated specimen is controlled by the stress conditions at the end of confining stress application and the pore-water pressures generated during undrained compression. For an isotropically confined specimen tested in cyclic compression, the maximum cyclic axial stress that can be applied to the specimen is equal to the effective confining pressure. Since cohesionless soils cannot resist tension, cyclic axial stresses greater than this value tend to lift the top platen from the soil specimen. Also, as the pore-water pressure increases during tests performed on isotropically confined specimens, the effective confining pressure is reduced, contributing to the tendency of the specimen to neck during the extension portion of the load cycle, invalidating test results beyond that point.
• 1.5.4 While it is advised that the best possible intact specimens be obtained for cyclic testing, it is sometimes necessary to reconstitute soil specimens. It has been shown that different methods of reconstituting specimens to the same density may result in significantly different cyclic behavior. Also, intact specimens will almost always be stronger and stiffer than reconstituted specimens of the same density.
• 1.5.5 The interaction between the specimen, membrane, and confining fluid has an influence on cyclic behavior. Membrane compliance effects cannot be readily accounted for in the test procedure or in interpretation of test results. Changes in pore-water pressure can cause changes in membrane penetration in specimens of cohesionless soils. These changes can significantly influence the test results.

1.6 Units—The values stated in either SI units or inch-pound units [presented in brackets] are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in non-conformance with the standard.

1.6.1 It is common practice in the engineering profession to concurrently use pounds to represent both a unit of mass (lbm) and a 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. This standard has been written using the gravitational system of units when dealing with the inch-pound system. In this system, the pound (lbf) represents a unit of force (weight). However, the use of balances or scales recording pounds of mass (lbm) or the recording of density in lbm/ft³ shall not be regarded as a nonconformance with this standard.

1.7 All observed and calculated values shall conform to the guide for significant digits and rounding established in Practice D6026. The procedures in Practice D6026 that are used to specify how data are collected, recorded, and calculated are regarded as the industry standard. In addition, they are representative of the significant digits that should generally be retained. The procedures do not consider material variation, purpose for obtaining the data, special purpose studies, or any considerations for the objectives of the user. Increasing or reducing the significant digits of reported data to be commensurate with these considerations is common practice. Consideration of the significant digits to be used in analysis methods for engineering design is beyond the scope of this standard.

1.7.1 The method used to specify how data are collected, calculated, or recorded in this standard is not directly related to the accuracy to which the data can be applied in design or other uses, or both. How one applies the results obtained using this standard is beyond its scope.

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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.9 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.

5.1 The cyclic triaxial test permits determination of the secant modulus and axial damping coefficient for cyclic axial loading of a prismatic soil specimen in isotropically-consolidated, undrained conditions. The secant modulus and axial damping coefficient from this test may be different from those obtained from a torsional shear type of test on the same material.

5.2 The secant modulus and axial damping coefficient are important parameters used in dynamic performance evaluation of both natural and engineered structures under dynamic or cyclic loads such as caused by earthquakes, ocean waves, or blasts. These parameters can be used in dynamic response analyses including, finite elements, finite difference, and linear or non-linear analytical methods.

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 facility used. Agencies that meet the criteria of Practice D3740 are generally considered capable of competent and objective testing/sampling/inspection. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some, but not all of those factors.