Difference between current and voltage measurements in resonant-column testing

Abstract

The resonant-column test is one of the most common procedures for dynamic characterization of soils (American Society for Testing and Materials standard). The test procedures require the measurement of the current flowing through the driving coils. In many cases, however, the input voltage has been measured and the results analyzed as a traditional current measurement. In this paper, the difference between current and voltage measurements is clarified; specifically, a new transfer function is presented for the analysis of data when the input voltage is measured. The new model is evaluated by testing three aluminum probes. Wave velocities computed from voltage and current measurements are similar; however, damping ratios are significantly overestimated if voltage-based measurements are analyzed with the standard procedures because the motion of the magnets induces an opposite voltage in the coils. Results from voltage and current measurements agree only if the new transfer function is used for the analysis of voltage data. Exploratory results for a dry-sand specimen, tested under different shear-strain levels (maximum shear strain γmax< 10–3), demonstrate the effect of nonlinear behaviour on current and voltage measurements. Current-based measurements are recommended because they do not require the evaluation of the complex impedance of the driving coils. Furthermore, current-based measurements are more precise, especially for the characterization of specimens that have low resonant frequencies.Key words: resonant-column testing, damping ratio, attenuation, mechanical waves, shear modulus, wave velocity.