HR: 0830h
AN: NS41B-04    [Abstracts]
TI: Comparison of static and dynamic bulk moduli in unconsolidated sands
AU: * Zimmer, M A
EM: mzimmer@nrlssc.navy.mil
AF: Naval Research Laboratory, 1005 Balch Blvd., Stennis Space Center, MS 39529-5004 United States
AB: The dynamic and static bulk moduli of reconstituted samples of three dry, natural sands and one glass bead sample were measured over a number of pressure cycles from 0 to 20 MPa. The dynamic modulus, K_dyn, was calculated from ultrasonic compressional- and shear-wave velocity measurements, while the static modulus, K_stat, was measured from the corrected volumetric strains between pressure steps. For a given sample, the static bulk modulus demonstrates a great deal of variation based on the loading history of the sample, while the dynamic modulus is only slightly sensitive to the loading history. The K_dyn to K_stat ratio on the normally consolidated, initial loading path varies from between 2 and 10 for the various samples, and decreases slightly with increasing pressure for a given sample as the dynamic modulus rises faster than the bulk modulus. On the first unloading step of any pressure cycle the dynamic and bulk moduli are approximately equal, while with continued unloading the K_dyn to K_stat ratio rises from approximately 1 at the initial unloading step to near 3 at zero pressure for each of the samples. The significant variability in the K_dyn to K_stat ratio with pressure history indicates that a robust prediction of the static bulk modulus from dynamic measurements made in situ would require information on the loading history of the sample and on the current effective pressure. The Preisach-Mayergoyz space analysis was adapted to account for the effects of the plastic strains, in addition to those of the elastic hysteresis, on the relationship between the static and dynamic bulk moduli. Inaccuracies in the volumetric strain measurements lead to a quantitative mismatch between the dynamic modulus predicted from this analysis and that observed in the data. Nevertheless, this analysis does demonstrate the degree to which both the strain magnitude dependence of the static modulus and the occurrence of plastic strain contribute to the difference between the static and dynamic bulk moduli on the loading portions of the pressure path. On unloading, there is no plastic strain in the samples, so the strain magnitude dependence of the static modulus is the sole cause of the difference in the static and dynamic bulk moduli.
DE: 5102 Acoustic properties
DE: 5120 Plasticity, diffusion, and creep
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly