ISSN: 0149-6115
Published Online: 11 October 2007
Page Count: 12

Evaluation of the First Mode of Vibration and Base Fixidity in Resonant-Column Testing

Khan, Zahid H.
Research Assistant and Professor,University of Western Ontario, Civil Engineering Department,ON,

Cascante, Giovanni
Associate Professor,University of Waterloo, Civil Engineering Department,ON,

El-Naggar, M. Hesham
Research Assistant and Professor,University of Western Ontario, Civil Engineering Department,ON,

(Received 3 March 2006; accepted 20 June 2007)

Abstract

In resonant column testing (ASTM standard), the shear strain distribution along the height of a specimen is assumed linear and fixed at the base. To investigate these assumptions, specimens of dry sand, mine tailings, and cemented sand are tested at different confinement and shear strain levels. The measured mode shapes for dry sands and mine tailings are linear at low and high strain levels; however, for a stiff cemented-sand specimen the first mode shape presents slippage at the end platens. Resonant frequencies decrease up to 50 % while the damping ratios increase up to 200 % because of this slippage. The coupling between the specimen and end-platens is enhanced using three different agents: gypsum cement, portland cement, and epoxy resin. The epoxy resin produces the best coupling, whereas portland and gypsum cements are effective only at low confinements and strain levels. Even after eliminating the slippage at the end-platens, the shear wave velocity of aluminum and PVC probes decreases with the increase in specimen stiffness because of the lack of base fixidity. To correct this apparent reduction, a new model and calibration procedure based on a two-degree-of-freedom system are proposed.

Keywords:
dynamic properties, resonant column, mode shapes, two-degree-of-freedom system, base stiffness

Paper ID: GTJ100305
DOI: 10.1520/GTJ100305
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