The determination of the damping parameter of soils with the sasw method

Abstract eng:
This paper presents a technique for the determination of the material damping ratio of 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 are based on the hypothesis that the response of the soil in the SASW test is due to a single mode surface wave. Therefore, the resulting attenuation curve can become erroneous when higher modes contribute to the soil’s response. Moreover, in these methods, the estimate of the attenuation curve is based on an estimate of the geometric spreading function. The latter is computed using the shear wave velocity of the soil, which is determined by inversion of the experimental dispersion curve. Errors in the experimental dispersion curve and in the inversion procedure lead to an erroneous geometric spreading factor and, consequently, experimental attenuation curve. In the proposed technique, the f -k method is used to determine the experimental dispersion and attenuation curves and, subsequently, the soil profile. The dispersion curve is derived from the peak’s position, while the attenuation curve is derived from its width, using the half-power bandwidth method. In this method there is no need for calculating the geometric damping. 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 f -k spectrum and do not interfere with the peak corresponding to the fundamental Rayleigh wave. Synthetic examples are used to validate the outlined technique; the results are compared with those obtained using two state-of-the-art methods. It is demonstrated that all three methods work well when applied to a regular soil profile, where the stiffness of the soil increases with depth. In case of soil profiles with soft layers trapped between stiffer layers, or profiles including a lot of thin layers that are approximated by a few thick layers, the proposed technique leads to accurate results.

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