Attenuation Mechanisms and Measurements

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ABSTRACT: CMS-9616013, Glenn Rix, Georgia Tech The response of particulate materials such as soils to dynamic loads is characterized by the velocity and attenuation spectra over a range of frequencies. Prior research on the near-surface characterization of soil deposits has emphasized the study of velocity, due in part to difficulties in measuring attenuation in the laboratory and field. However attenuation data can be uniquely used to more fully characterize the response of near-surface soils to dynamic loads. The particulate nature of soil and the attenuation due to geometric spreading and wave-phenomena add to the complexity of evaluating the loss mechanisms for material attenuation in soil. The objectives of this research program are to enhance the understanding of loss mechanisms in near-surface soils and to develop robust assessment procedures. The problem is approached from two extreme scales: the clarification of underlying physical processes at the microscale and the development of adequate methods for field (i.e., macroscale) measurement. These are initial steps toward developing laboratory and field measurement procedures which reflect a detailed understanding of the loss mechanisms. Research tasks include the study of energy losses in double layers (micro-level simulations and experimental studies), reassessment of hysteretic losses at low strain in light of new phenomenological hypotheses (experimental studies), and development of active and passive field testing procedures (analytical, model, and field studies). Specialized devices, new laboratory and field measurement procedures, and unique simulation software and algorithms are used. An improved understanding of loss mechanisms and the development of new methods of assessment will enhance the ability to characterize near-surface materials (e.g., aging, chemical effects, saturation), and improve the wave-based monitoring of geoprocesses (e.g., inversion of state of stress in situ, assess ment of the spatial evolution of liquefaction, and response of systems to dynamic excitation). The improved understanding of material loss mechanisms and advances in field assessment methods will also benefit site characterization for earthquake hazard mitigation.