Project Report

The properties of stiffness and material damping are used to describe soil behavior under dynamic loadings, and they are strongly influenced by strain levels. In practice it is common to measure these properties in the laboratory at very low levels of strain (≈10-3%) and fit an empirical curve to the measured data that allows practitioners to extrapolate the properties to higher levels of strain; which are often more relevant for the design at hand. The bender element test is a quick, low cost test for measuring the stiffness of soil specimens, but as of yet an efficient methodology has not been proposed to estimate the soil’s internal damping from the test data. This research investigated the ability of utilizing bender element tests to provide an estimate of material damping. If such a methodology can be found to accurately quantify damping, then the robustness of the bender element test would be enhanced for academic and professional practice. Essentially engineers would be able to extract two different material properties of soil with a laboratory test that previously only provided one material property. Four (4) different kaolin specimens were prepared using slurry consolidation techniques, saturated, and consolidated to a range of confining pressures for bender element testing. Data from the bender elements tests was obtained and filtered to remove unwanted signal noise. Initially, shear wave velocity was measured from the test data and related to material stiffness to ensure the recorded data was reasonable. Next a simplified technique to estimate material damping was applied to the measured data. The simplified technique is based on the decay of free vibrations in the specimen. Measured values of material damping were compared to published data for similar soils. The comparison indicated that the measured values were 2 – 3 times greater than the expected values for material damping in similar soils. It is believed that the inherent damping of the testing equipment influenced the measured data. At this point the ability of bender elements to estimate material damping of soil specimens on the assumption of free-vibration decay is questionable but feasible. Results indicate that the damping of the testing system needs to be quantified prior to applying the simplified technique to soils. Future experimental work should consider synthetic, calibration specimens with exact material properties so that the influence of the surrounding equipment can be removed from the recorded data.

National Science Foundation (NSF)
Office of International and Integrative Activities (IIA)
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Carter Kimsey
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Rocco Nicholas T
United States
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