Abstract
The small-strain (elastic) shear stiffness of soil is an important parameter in geotechnics. It is required as an input parameter to predict deformations and to carry out site response analysis to predict levels of shaking during earthquakes. Bender element testing is often used in experimental soil mechanics to determine the shear (S-) wave velocity in a given soil and hence the shear stiffness. In a bender element test a small perturbation is input at a point source and the propagation of the perturbation through the system is measured at a single measurement point. The mechanics and dynamics of the system response are non-trivial, complicating interpretation of the measured signal. This paper presents the results of a series of discrete element method (DEM) simulations of bender element tests on a simple, idealised granular material. DEM simulations provide the opportunity to study the mechanics of this testing approach in detail. The DEM model is shown to be capable of capturing features of the system response that had previously been identified in continuum-type analyses of the system. The propagation of the wave through the sample can be monitored at the particle-scale in the DEM simulation. In particular, the particle velocity data indicated the migration of a central S-wave accompanied by P-waves moving along the sides of the sample. The elastic stiffness of the system was compared with the stiffness calculated using different approaches to interpreting bender element test data. An approach based upon direct decomposition of the signal using a fast-Fourier transform yielded the most accurate results.
Highlights
The bender element test is the most commonly used dynamic test in experimental soil mechanics
A parametric study in which the discrete element method (DEM) model parameters were systematically varied was carried out to assess the influence of these parameters on the wave propagation velocity and to compare the various options available for travel time determination
The frequency of the input signal was adjusted as different frequencies are often used in physical bender element tests
Summary
The bender element test is the most commonly used dynamic test in experimental soil mechanics This test is used to measure small-strain, or “elastic” stiffness of soil. When a bender element is moved, a voltage is generated During a bender element test a voltage is sent through the transmitter bender element; this produces movement, which in turn generates a seismic (stress) wave that propagates through the sample. The induced movement causes a voltage to be produced and creates an electrical signal that can be read on an oscilloscope During this process it is the seismic wave that propagates through the sample and generates the motion of the soil particles.
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