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A theoretical study of pore-pressure distributions in drained, constant strainrate triaxial tests, an outgrowth and extension of earlier work by Gibson, Bishop, and Henkel, has been done to determine the experimental conditions under which pore-pressure gradients, and hence, effective stress differences throughout deforming triaxial test samples, are small.
It is concluded from the analysis of drained triaxial constant strain-rate tests that it is impossible to obtain a fully drained condition. In fact, in this test the long-term spatial pore-pressure distribution becomes parabolic. Unless strain rates are sufficiently low or permeabilities are sufficiently large to minimize the effect of this parabolic pore-pressure distribution, effective stresses in the deforming sample cannot approach uniformity and any property determinations based on the assumption of uniform effective stresses will be in error. A similar conclusion was reached by Gibson. It is also concluded that volumetric strains will be largest near the undrained end of a sample drained at one end, or largest in the middle of a sample drained at both ends. This strain distribution is a consequence of the parabolic pore-pressure distribution and suggests an explanation (apart from frictional end restraints) for any post-yield barreling observed in a drained triaxial test.
pore pressure, consolidation, triaxial, constant strain-rate tests, drained tests
Geologist, United States Department of the Interior, Geological SurveyDenver Federal Center, Denver, CO