Sediment shear waves: a comparison of in situ and laboratory measurements

Abstract

Surficial sediment shear wave velocity measurements were made at eight sites (water depths 7-22 m)

in the vicinity of La Spezia, Italy. Values of shear wave velocity measured in situ ranged from 16 m/s

in flocculent clays to 90 m/s in hard-packed sands. Values of laboratory measured shear wave

velocity were 6-22 m/s lower from the same sites. Low variability of measured shear wave velocities

allowed laboratory measurements to be corrected to in situ conditions using the following empirical

relationship: Vs, (in situ) = 10.46+1.17 Vs, (lab) The most likely causes for the lower laboratory shear

wave velocities were sediment disturbance during collection, transportation, storage and

measurement both by mechanical manipulations and by changes in confining pressure. Sediment

porosity, void ratio or wet density can be used to predict in situ shear wave velocity. Further studies

are required to refine and extend these empirical relationships, and to accurately define the high

gradients in shear wave velocity predicted for the upper few meters or sediment.