Significance and Use
5.1 The initial shear modulus (Gmax) of a soil specimen under particular stress and time conditions is an important parameter in small-strain dynamic analyses such as those to predict soil behavior or soil-structure interaction during earthquakes, explosions, and machine or traffic vibrations. Gmax can be equally important for small-strain cyclic situations such as those caused by wind or wave loading. Small-strain Gmax is also vital for non-linear analyses of large strain situations, where the larger strain soil stiffness results could come from torsional shear tests, for example. Shear wave velocity and Gmax can be used to compare different soil specimens in a laboratory testing program, and also for comparing laboratory and field measurements of these parameters.
5.2 Torsional resonant column tests (Test Method ) are often used to determine properties of a soil specimen at small shear strains up to and possibly slightly beyond 0.01%. Resonant column test results can include Gmax versus time, shear modulus versus strain, damping ratio versus time and damping ratio versus strain. Bender element tests can only provide the first of these, Gmax versus time. The strain level in bender element tests is small (constant Gmax strain levels), but the strain magnitude is not known and the strain is not constant along the shear wave travel path due to material and geometric damping. Bender elements can therefore not be used to evaluate shear modulus versus strain and do not provide information about damping ratio. However, bender elements can be incorporated in a variety of different laboratory testing devices, allowing the measurement of small-strain and large-strain stiffness on the same specimen at the particular conditions of the test and possibly eliminating the need for additional resonant column tests.
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 are generally considered capable of competent and objective testing/sampling/inspection/etc. Users of this standard are cautioned that compliance with Practice does not in itself assure reliable results. Reliable results depend on many factors; Practice provides a means of evaluating some of those factors.
1.1 This test method covers the laboratory use of piezo-ceramic bender elements to determine the shear wave velocity in soil specimens. A shear wave is generated at one boundary of a soil specimen and then received at an opposite boundary. The shear wave travel time is measured, which over a known travel distance yields the shear wave velocity. From this shear wave velocity and the density of the soil specimen the initial shear modulus (Gmax) can be determined, which is the result of primary interest from bender element tests.
1.2 This shear wave velocity determination involves very small strains and is non-destructive to a test specimen. As such, bender element shear wave velocity determinations can be made at any time and any number of times during a laboratory test.
1.3 This test method describes the use of bender elements in a triaxial type test (for example, Test Methods , , , or ), but a similar procedure may be used for other laboratory applications, like in Direct Simple Shear (Test Method ) or oedometer tests (for example, Test Methods and ). Shear wave velocity can also be determined in unconfined soil specimens held together by matrix suction.
1.4 Shear wave velocity can be determined in different directions in a triaxial test, for example vertically and horizontally. Shear waves generated to determine shear wave velocity can also be polarized in different directions, for example a horizontally propagating shear wave with either vertical or horizontal polarization. This test method describes the use of bender elements mounted in the top platen and base pedestal of a triaxial test specimen to measure shear wave velocity in the vertical direction. With additional bender elements mounted on opposite sides of a triaxial specimen, a similar procedure may be used to determine horizontal shear wave velocity.
1.5 A variety of different interpretation methods to evaluate the shear wave travel time in a soil specimen have been proposed and used. This test method only describes two of these, Start to Start and Peak to Peak using a single sine wave signal sent to the transmitter bender element. Other interpretation methods producing similar results may also be used.
1.6 Bender element measurements may not work very well in some situations, like in extremely stiff soils where the generated shear wave amplitude may be exceedingly small.
1.7 This test method does not cover the determination of compressional wave velocity in soil specimens. This measurement requires a different type of piezo-ceramic element configuration, and such determinations are generally not useful in saturated soft soil specimens as the earliest identifiable compressional wave arrival at the receiver end of a saturated specimen will likely have been transmitted through the (relatively incompressible) specimen pore water rather than the (compressible) soil skeleton.
1.8 Units—The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.9 All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice , unless superseded by this test method.
1.9.1 The procedures used to specify how data are collected/recorded and 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, 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 these test methods to consider significant digits used in analysis methods for engineering data.
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.