Determination of the material damping ratio in the soil from SASW tests using the half-power bandwidth method

This paper presents a novel technique for the determination of the material damping ratio in shallow soil layers. It is based on the spectral analysis of surface waves (SASW) test. The technique is an alternative to existing methods, where the damping ratio is determined from the spatial decay of the Rayleigh wave. These methods rely on the knowledge of the geometric damping, and may lead to incorrect results if the geometric damping is calculated based on an inaccurate shear wave velocity profile. The existing methods may also lead to incorrect results when higher modes contribute to the wavefield in the soil. In the proposed technique, the wavefield is transformed to the frequency-wavenumber domain. The resulting frequency-wavenumber spectrum exhibits a peak that corresponds to the fundamental Rayleigh wave. The dispersion curve is derived from the peak's position, whereas the attenuation curve is derived from its width, using the half-power bandwidth method. Due to the use of the appropriate wavenumber transformation, this method does not require the calculation of the geometric damping. In addition, the occurrence of higher Rayleigh modes does not affect the attenuation curve associated with the fundamental Rayleigh wave, as higher modes appear as separate peaks in the frequency-wavenumber spectrum that do not interfere with the peak corresponding to the fundamental Rayleigh wave (except at the osculation points). Three benchmark problems are considered to validate the outlined technique; the results are compared with those obtained using existing methods. All methods perform well when applied to a regular soil profile, where the stiffness of the soil increases with depth. For soil profiles with a soft layer trapped between two stiffer layers, or where the soil properties vary smoothly with depth, the proposed technique yields more accurate results than the existing methods. The practical applicability of the new method is finally illustrated using experimental data collected from a test site in Belgium