5.1 Cyclic triaxial test results are used for evaluating the ability of soil to resist the shear stresses induced in a soil mass due to earthquakes or other cyclic loadings.

5.1.1 Cyclic triaxial tests may be performed at different values of effective confining pressure on isotropically or anisotropically consolidated specimens to provide data required for estimating the cyclic response of the soil.

5.1.2 Cyclic triaxial tests may be performed at a single effective confining pressure, usually equal to 100 kPa [14.5 lbf/in.²], or alternate pressures to compare cyclic response results for a particular soil type with that of other soils (2).

5.2 The cyclic triaxial test is a commonly used technique for determining cyclic soil response.

5.3 Cyclic response depends upon many factors, including density, effective confining pressure, applied cyclic stress, stress history, soil fabric, age of soil deposit, specimen reconstitution method, and the frequency, uniformity, and shape of the cyclic waveform. Thus, close attention must be given to testing details and equipment.

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.

1.1 This test method describes performing load-controlled cyclic undrained triaxial tests to evaluate the cyclic response of saturated soils in either intact or reconstituted states.

1.2 The cyclic response of the soil is evaluated relative to a number of factors, including the magnitude and offset of applied cyclic stress, the development of cyclic and average axial strains, the number of cycles of stress application, the development of excess pore-water pressure, state of effective stress, and the soil fabric. A comprehensive review of factors affecting cyclic triaxial test results is contained in the literature (1).2

1.3 The cyclic triaxial tests described in this standard are conducted under undrained conditions to simulate essentially undrained field conditions during an earthquake or other cyclic loading.

1.4 Cyclic triaxial tests are destructive. Failure may be defined based on the number of stress cycles required to reach a limiting axial strain or 100 % pore pressure ratio. See Section 3 for Terminology.

1.5 This test method is generally applicable for testing coarser-grained, free-draining soils of relatively high permeability. When testing well-graded materials, silts, or clays, pore-water pressures monitored at the specimen ends may not represent pore-water pressure values throughout the specimen. However, this test method may be followed when testing most soil types if care is taken to ensure that problem soils receive special consideration when tested and when test results are evaluated.

1.6 Further applications of the cyclic triaxial test and the interpretation of its results in determining soil’s cyclic behavior and classifying them is the subject of ongoing research.

1.7 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.

1.7.1 Nonuniform stress conditions within the test specimen are imposed by the specimen end platens. This can cause a redistribution of the void ratio within the specimen during the test.

1.7.2 A 90° change in the direction of the major principal stress occurs during the two halves of a symmetric loading cycle on isotropically consolidated specimens.

1.7.3 The maximum cyclic stress that can be applied to the specimen is controlled by the stress conditions at the end of consolidation and the pore-water pressures generated during testing. As the pore-water pressure increases during tests performed on isotropically consolidated specimens, the effective confining pressure is reduced, contributing to the tendency of the specimen to neck during the unloading portion of the load cycle, possibly invalidating test results beyond that point.

1.7.4 The isotropic consolidation and equal axial cyclic compression and extension stresses applied to the specimen are not a direct representation of the stress state and cyclic shear stresses acting on the specimen under in-situ conditions.

1.7.5 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 responses due to the differences in initial soil fabric imparted by different specimen reconstitution methods. Therefore, the appropriate selection of a specimen reconstitution method is a crucial step in the design of a laboratory program. Such selection must account for the condition of the real natural or man-made deposits being modeled and the real engineering application of interest.

1.7.6 The interaction between the specimen, membrane, and confining fluid influences cyclic behavior. Membrane compliance effects cannot be readily accounted for in the test procedure or interpretation of test results. Changes in porewater pressure can cause changes in membrane penetration in specimens of coarser-grained cohesionless soils. These changes can significantly influence the test results.

1.7.7 Since the area of a triaxial test specimen is not constant across the specimen height due to end restrictions, it can change throughout a test and is different at large strains when the maximum and minimum cyclic load is applied. The maximum, minimum, and mean cyclic stresses will then be offset from the values applied at the start of cyclic loading. This is different from the symmetric horizontal cyclic stress in the simple shear case of level ground shaking.

1.8 Units—The values stated in either SI units or inch-pound units 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. Reporting of test results in units other than SI shall not be regarded as nonconformance with this standard.

1.8.1 The gravitational system of inch-pound units is used when dealing with inch-pound units. In the system, the pound (lbf) represents a unit of force (weight), while the unit for mass is slugs. The slug unit is not given unless dynamic (F = ma) calculations are involved.

1.8.2 It is common practice in the engineering/construction profession in the United States to concurrently use pounds to represent both a unit of mass (lbm) and force (lbf). This practice implicitly combines two separate systems of units; the absolute and the gravitational systems. 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 of mass. However, the use of balances and 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 terms density and unit weight are often used interchangeably. Density is mass per unit volume, whereas unit weight is force per unit volume. In this standard, density is given only in SI units. After the density has been determined, the unit weight is calculated in SI or inch-pound units, or both.

1.9 All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice D6026 unless superseded by this test method.

1.9.1 The procedures used to specify how data are collected/recorded or calculated in the 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, the 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.