In Situ Measurements of Damping Ratio Using Surface Waves

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9402358 Rix An important soil parameter that controls the amplification of seismic waves as they propagate from bedrock to the surface is the hysteretic damping ratio of the soil. Damping is usually measured on soil samples tested in the laboratory, but the effect of the disturbance on the soil properties when the sample is removed from the ground is uncertain. Moreover, the frequencies used in the laboratory tests are usually much higher than those of interest in earthquake engineering. An alternative approach is to use field tests to measure the damping ratio in situ. However, despite the advantages of in situ tests, there have been relatively few field attenuation measurements reported. One of the objectives of this project is to develop and implement an insitu test procedure for damping measurements using surface waves. Surface wave measurements offer several advantages compared to crosshole and downhole techniques, one being that they are non-invasive. Another objective is to understand better the causes of the frequently-observed differences in the damping values between laboratory and in situ measurements. The project involves three tasks: 1. To acquire surface wave attenuation data at sites within the Savannah River Site in Aiken, South Carolina, using several different types of input motion (swept-sine, random, and transient) to determine which one provides, for example, the best signal-to-noise ratio. Attenuation data will also be recorded at several different source-receiver combinations to evaluate the influence of near-field waves. 2. To develop and evaluate algorithms for: (i) estimating particle motion amplitude spectra, (ii) calculating attenuation parameters from observed surface wave amplitudes, and (iii) inverting attenuation data to obtain damping ratio. Throughout this task, emphasis will be placed on quantifying the uncertainties involved in each step, with the aim of establishing not only the most likely value of damping , but also the possible range of values. 3. Once appropriate field procedures and analysis methods have been developed as part of Tasks 1 and 2, and in order to compare surface wave damping values with values obtained from in situ borehole methods and laboratory tests, to perform surface wave measurements of attenuation at the Treasure Island National Geotechnical Experimentation Site in San Francisco Bay, and at one or more sites in the Los Angeles area affected by the Northridge earthquake. ***