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Laboratory tests with a Hardin oscillator were used to study the time-dependent characteristics of the damping ratio of isotropically consolidated specimens of kaolinite and calcium bentonite. Damping was determined using a steady-state method and from the decay of free vibrations. After completion of primary consolidation, the dynamic response was studied as a function of time for both drained and undrained conditions.
The damping ratio decreased approximately 12 percent for kaolinite and 25 percent for bentonite per logarithmic cycle of a dimensionless time ratio during secondary compression. To evaluate the effects of time in clay soils, at least one test should be continued to five to ten times the time at the end of primary consolidation.
Errors due to diffusion of air into the specimen were eliminated by using a mercury jacket around the specimen. The apparatus damping constant was found to vary significantly from test to test because of minor variations in the apparatus setup, and hence the apparatus must be recalibrated for each test.
clays, damping ratio, resonant-column tests, time effects, soils
Research civil engineer, Soils and Pavements Laboratory, U. S. Army Engineer Waterways Experiment Station, Vicksburg, Miss.
Professor, North Carolina State University, Raleigh, N.C.