A study is presented of potential errors in, and methods of interpreting, the results of cantilever-type, piezoceramic bender element tests for measuring the shear wave velocity of laboratory soil specimens. Interpretations based on the first direct arrival in the output signal are often masked by near-field effects and may be difficult to define reliably. Interpretations based on characteristic points or cross-correlation between the input and output signals are shown to be theoretically incorrect in most cases because of: (1) the effects of wave interference at the boundaries; (2) the phase lag between the physical wave forms and the measured electrical signals; and (3) non-one-dimensional wave travel and near-field effects. Interpretations based on the second arrival in the output signal are theoretically subject to errors from non-one-dimensional wave travel and near-field effects. Differences in Vs values obtained by the different interpretation methods are illustrated analytically and experimentally.
A new simple shear apparatus is described which can apply bi-directional simple shear cyclic and monotonic loading. Cylindrical specimens are placed in a pressure chamber so that confining pressure and back pressure can be applied. Bi-directional shear loads are applied to the specimen's base, which is mounted on a series of two horizontal rolling tables which can be independently loaded in two orthogonal directions while preventing cap and base platen “rocking.” A computer-automated feedback-loop-controlled pneumatic servovalve system provides excellent control of loads and displacements. Displacement control during cyclic tests on softening specimens is improved by the inclusion of a parallel stiffness. Procedures and test results are presented for three different undrained cyclic simple shear loading conditions and for a strain-controlled monotonic loading condition.
Analyses of liquefaction-induced lateral spreading during earthquakes often contain simplifying assumptions about the stratigraphy of a site (i.e., selecting representative cross sections and properties), which can be a major source of uncertainty in the estimated deformations. This paper examines the potential for liquefaction-induced lateral spreading in a deposit of interbedded sand and clay using nonlinear deformation analyses. The soil is modeled as a binary mix with varying percentages of potentially liquefiable sand and non-liquefiable clay. The location of each soil type is represented through different realizations of interbedded stratigraphy generated using a probabilistic geologic model and a correlation structure consistent with natural fluvial deposition. The procedures used to generate these realizations and perform the dynamic numerical simulations are described. Different distributions of sand and clay are examined to determine how the percentage of liquefiable material influences the magnitude of surface deformations. The undrained strength of the clay is varied to further explore how this affects the relationship between deformation and sand percentage.
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