In-Situ Characterization of the Near-Surface Small Strain Damping Ratio at the Garner Valley Downhole Array Through Surface Waves Analysis

The quantification of attenuation properties of soils has great relevance in geotechnical earthquake engineering. The small strain damping ratio is generally obtained from either direct laboratory tests on small samples or generic empirical relationships. Alternatively, some promising techniques for extracting in-situ small strain damping ratio rely on the analysis of surface wave data. This paper presents a subset of results from a massive dynamic site characterization study at the Garner Valley Downhole Array, wherein in-situ damping ratio profiles have been extracted from several multichannel analysis of surface waves (MASW) datasets. Waveforms generated from both a sledgehammer and a dynamic shaker were recorded, allowing for comparisons between the damping estimates obtained from both types of sources. Dispersion data and attenuation curves were derived from the waveforms using several approaches presented in the literature, and one new approach developed by the authors. This paper documents the inter-method differences and similarities across approaches in terms of uncertainties in the wavefield attenuation, together with the impacts on the amplification of the soil deposit. This study contributes towards better, in-situ characterization of the attenuation properties of soil deposits, enhancing the accuracy of ground models used in dynamic analyses. It is expected that progress in this area will lead to greater reliability of predicted ground motion amplification.