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.

1.8.1 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.

1.9.1 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.