Is P-Wave Velocity an Indicator of Saturation in Sand with Viscous Pore Fluid?

It is commonly assumed that within inundated sand the Skempton B value and P-wave velocity decrease with decrease in saturation. In centrifuge tests a common saturation procedure is to inundate the specimen with carbon dioxide while under a vacuum and then slowly introduce the viscous pore fluid. The B value and related saturation is difficult to measure in centrifuge models and P-wave velocity—saturation correlations have been used for this purpose. A laboratory emulation of centrifuge saturation procedures was made using a triaxial cell with top and bottom bender elements and a viscous methyl cellulose–water pore fluid. Contrary to expectations, the laboratory tests showed high P-wave velocities indicative of full saturation when B values were low. Numerical modeling of the laboratory tests indicated that if air bubbles within the pore fluid are numerous and closely spaced then there is a good correlation between saturation, B value, and P-wave velocity. However if the air bubbles are larger and only present in some of the pores then the P-wave velocity is not a good indicator of B value and average saturation. The laboratory tests also showed that placing the specimen under backpressure for several days increased saturation and related B values. It is suggested that this common laboratory procedure should be considered for saturating centrifuge test specimens.