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On the use of shear wave velocity to characterize changes in fines content of a silty sand Campbell, Jessica E.


Seismic waves are commonly used to characterize the mechanical properties of soils, either through in-situ techniques or laboratory measurement. Several factors are known to influence shear wave velocity, including stress, void ratio and soil fabric. In the case of internally unstable soils, where a seepage-induced fines loss occurs over time, the phenomenon may be well-suited to geophysical monitoring. This laboratory program investigates the relation between shear wave velocity and fines content of a silty sand, and examines how that relation varies with seepage flow. Specimens of varying fines content (Sf = 0, 13, 18, 23, 28 and 33 %) were reconstituted as a slurry in a rigid-walled permeameter outfitted with bender elements. Shear wave velocities of the specimens were found to be influenced primarily by stress, and, to a lesser degree, changes in fines content and hydraulic gradient. The change due to fines content is small (15 rn/s at σm’= 12 kPa and 34 m/s at σm’ = 200 kPa) but the trend is consistent across the range of mean effective stresses examined in testing. Shear wave velocity was found to diminish with increasing fines up to a threshold value of approximately 23 %. At, and beyond, the threshold fines content, a subtle increase in shear wave velocity is observed; the relation becomes more pronounced with increase in stress. The influence of hydraulic gradient on shear wave velocity is solely due to the change in confining stress imposed by the downward seepage force. The findings of the study and comparison to triaxial test data for similar gradations, show that a subtle change in shear wave velocity occurs with change in fines content, however, shear wave velocity is more sensitive to change in mean effective stress. Comparison of field and laboratory data indicates differences in shear wave velocity between the moisttamped/ compacted materials and the slurry-deposited materials, and a convergence of shear wave velocities with increasing mean effective stress. Change in fines content in the field caused by possible fines migration may be detected using seismic surveying techniques, however, it would appear difficult to correlate shear wave velocity to fines content since the mechanism of fines loss also affects the structure of the soil.

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