Is P-Wave Velocity an Indicator of Saturation in Sand with Viscous Pore Fluid?
Publication: International Journal of Geomechanics
Volume 7, Issue 6
Abstract
It is commonly assumed that within inundated sand the Skempton 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 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 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, 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 value and average saturation. The laboratory tests also showed that placing the specimen under backpressure for several days increased saturation and related values. It is suggested that this common laboratory procedure should be considered for saturating centrifuge test specimens.
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Acknowledgments
The writers wish to acknowledge the support from the Ministry of Transportation/UBC Professional Partnership program, the Canadian Council of Professional Engineers, Trow Associates Inc., and the National Scientific and Engineering Research Council through Strategic Liquefaction Grant No. NRCNSERC 246394. They also wish to acknowledge the help from Ali Khalili and Bill Leung in setting up the laboratory tests, from Peter Cundall of Itasca for providing help in understanding the numerical model, and from Ryan Phillips of C-CORE for helpful discussions on centrifuge testing aspects. The triaxial testing system with bender elements was purchased using Canada Foundation for Innovation (CFI)—New Opportunities Infrastructure Funding, Project No. UNSPECIFIED6869.
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© 2007 ASCE.
History
Received: Dec 13, 2005
Accepted: Apr 6, 2007
Published online: Nov 1, 2007
Published in print: Nov 2007
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