ASTM D8296-19

5.1 Cyclic direct simple shear strength test results are used most often for evaluating the ability of a soil to resist shear stresses induced in a soil mass during earthquake loading, offshore storm loading, etc.

5.2 In this test, the cyclic strength is measured under constant volume conditions that are equivalent to undrained conditions; hence, the test is applicable to field conditions in which the soils have consolidated under one set of stresses, and then are subjected to changes in stress/strain without time for further drainage to take place.

5.3 The cyclic strength is a function of many factors including density, confining pressure, stress history, grain structure, specimen preparation procedure, frequency, and characteristics of the cyclic loading applied. Therefore, test factors shall be considered during evaluation of test results.

5.4 The state of stress within the direct simple shear specimen is not sufficiently defined nor uniform enough to allow rigorous interpretation of the results. Expressing the data in terms of the shear stress and vertical effective stress on the horizontal plane is useful for engineering purposes. Some effective stress parameters that could be derived from a cyclic direct simple shear test shall not be confused with corresponding parameters derived from other shear tests having better defined states of stress (that is, cyclic triaxial tests).

5.5 The values of settlement in saturated soil after cyclic loading can be assessed from the test results by allowing volume change at the end of the shearing to achieve same vertical effective stresses as at end of primary consolidation.

5.6 The data from the consolidation portion of this test are comparable to results obtained using Test Method D2435/D2435M provided that the more rigorous consolidation procedure of Test Method D2435/D2435M is followed.

1.1 This test method defines equipment specifications and testing procedures for the measurement of cyclic strength, number of cycles to liquefaction or cyclic properties (Modulus and Damping) of soils, after one-dimensional consolidation using a cyclic mode of loading.

1.2 The cyclic shearing can be applied using load control or displacement control. It shall be the responsibility of the agency requesting this test to specify the magnitude and frequency of the cyclic loading. Other loading histories may be used if required by the agency requesting the testing.

1.3 This test method is written specifically for devices that test cylindrical specimens enclosed in a wire-reinforced membrane or a soft membrane within a stack of rigid rings (this test method applies to Teflon coated rigid rings as well). Other types of shear devices are beyond the scope of this test method.

1.4 This test method can be used for testing cohesionless free draining soils or fine grained soils. However, this test method may be followed when testing most soil types if care is taken to ensure that any special considerations required for such soils are accounted for.

1.5 The shearing phase of this test is conducted under constant volume conditions. Since the lateral confinement system prevents radial specimen strains, the constant volume condition is accomplished by preventing specimen height change during shear. Shearing under constant volume can be performed on dry or saturated specimens. The constant volume condition is equivalent to the undrained condition for fully saturated specimens. Cyclic direct simple shear testing with truly undrained conditions (restricting pore water flow from and into the specimen) can be performed using some simple shear devices, but is beyond the scope of this standard.2

1.6 The cyclic strength of a soil is determined based on the number of cycles required to reach a limiting double amplitude shear strain or a single amplitude shear strain, while liquefaction is more commonly defined as 100 % change in vertical stress ratio (change in effective vertical stress during shearing divided by effective vertical stress at end of primary consolidation). The change in vertical stress ratio in constant volume shearing is equivalent to the excess pore pressure ratio (excess pore pressure during shearing divided by effective vertical stress at end of primary consolidation) under undrained conditions. The strain criterion is only applicable when performing load controlled tests; 100 % change in vertical stress ratio can be used for both, load and displacement control. For displacement control testing, the criterion to stop the test could be a specified number of cycles.

1.7 This test method is applicable to testing intact, reconstituted, or compacted specimens; however, it does not include specific guidance for preparing, reconstituting or compacting test specimens.

1.8 It shall be the responsibility of the agency requesting this test to specify the magnitude of the consolidation stress prior to shear and, if assigned, an unloading consolidation stage may be required for over-consolidating the specimen.

1.9 All recorded and calculated values shall conform to the guide for significant digits and rounding established in Practice D6026.

1.10 Units—The values stated in SI units are to be regarded as the standard. Reporting test results in units other than SI shall be regarded as conformance with this test method. In the engineering profession it is customary practice to use, interchangeably, units representing both mass and force, unless dynamic calculations (F=Ma) are involved. This implicitly combines two separate systems of units, that is, the absolute system and the gravimetric system. It is scientifically undesirable to combine two separate systems within a single standard. This test method has been written using SI units; however, inch-pound conversions are given in the gravimetric system, where the pound (lbf) represents a unit of force (weight). The use of balances or scales recording pounds of mass (lbm), or the recording of density in lb/ft³ shall not be regarded as nonconformance with this test method.

1.11 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.

1.12 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.