Effects of sample size on bender-based axial G~0 measurements

Bender elements are piezoelectric transducers frequently employed for the measurement of the small-strain shear modulus of soils. The measurement is based on transmission of a mechanical signal through a soil sample. A very common set-up involves transmission along the axis of a cylindrical sample, with source and receiver transducers mounted, for instance, in the end platens of a triaxial apparatus. Current test interpretation is generally based on the assumption of plane wave transmission between transducers. However, this model does not explain the heavily distorted transmission usually observed. The result is substantial measurement uncertainty. Although other phenomena do play a role, it is here proposed that a main culprit for signal distortions is sample-size effects due to lateral boundary reflections. To support this hypothesis, results from a series of numerical 3D simulations of the problem are analysed. Velocity estimates obtained from the simulated traces using plane-wave based time and frequency domain methods are compared with the known exact value. Errors in velocity determination are shown to be very important and directly related to lateral boundary influences. Comparison with some experimental data confirms the need to include sample-size effects in a renewed interpretative framework for header tests.