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Abstract For it¢s creep behavior，undrained shear strength of saturated clay is related to load rate. Strain-rate parameter (increase of undrained shear strength per log strain rate)is used to describe the change in undrained shear strength with strain rate by some researchers. Laboratory test data on a wide variety of clays show the value of this parameter is between 8% and 20%，most about 12%，and stress history，consolidation state (isotropic consolidation or anisotropic consolidation)，test type(compression test or extension test) have a little effect on this parameter. An anisotropic elastic viscoplastic model used to study the parameter is introduced. The theoretical equation of is deduced and shows it is only dependent on the ratio of secondary compression index to compression index，independent of the consolidation state and test type，which is shown by test results. Actually，this equation can be deduced from any model based on the delayed compression framework . For most clay，value of ratio of secondary compression index to compression index is about 0.03–0.05，and the value of calculated by the equation is 7.2%–12.2%. High value such as 20% shown by some tests for sensitive clay is related to the high value of ratio of secondary compression index to compression index caused by soil structure collapse. The validity of the equation is testified by some laboratory tests.
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Received: 25 October 2004
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