12-1.5.3

shear-strain amplitude, e.g., less than 10-2 percent, shear modulus data may be

obtained from laboratory tests and usually involves applying some type of high-

frequency forced vibration to a cylindrical sample of soil and measuring an appropriate

response. Some types of tests allow the intensity level of the forced vibration to be

varied, thus yielding moduli at different shear strains.

High strain-level excitation, e.g., 0.01 to 1.0 percent, may be achieved by

low-frequency, cyclic loading triaxial compression tests on soil samples. The modulus,

damping and strain level for a particular test are calculated directly from the sample

response data. The usual assumption for calculating the modulus and damping from

forced cyclic loading tests on laboratory samples is that at any cyclic strain amplitude

the soil behaves as a linear elastic, viscous, damped material. A typical set of results

may take the form of a hysteresis loop as shown in Figure 12-9. Either shear or normal

stress cyclic excitation may be use. The shear modulus is calculated from the slope of

the peak-to-peak secant line. The damping is computed from the area of the hysteresis

loop, and the strain level is taken as the single-amplitude (one-half the peak-to-peak

amplitude or origin to peak value) cyclic strain for the condition during that cycle of the

test. Note that the equations for modulus and damping shown in Figure 12-9 assume

the soil behaves as an equivalent elastic viscous, dampened material, which is linear

12-17

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