Published: Jan 1981
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Considerable effort has been devoted to assessing differences in behavior between the conventional triaxial test (H = 2 to 2½ D) with end restraint and the “frictionless” cap and base test (H = D) with greased end membranes. While it is generally acknowledged that a minimal end membrane thickness is required for a frictionless test on sand to prevent effective bearing of particles through end membranes and into cap and base, it has not been recognized that beyond some optimal thickness a Poisson's effect dominates, causing an artificial reduction in sand strength. Therefore sand strength decreases gradually as end membrane thickness increases, first as end restraint is reduced and then as the Poisson's effect takes over. With the aid of stacks of multiple greased membranes, the optimum end membrane thickness (thickness where end restraint and Poisson's effect are near minimum values and counterbalance each other) can be established and the Poisson's effect avoided. Given that optimum end membrane thickness for the frictionless test on sand varies with sample height and confining pressure as well as particle size, and that the frictionless test requires correction or central measurement of sample deformation (overall deformation includes sand, membranes, extruding grease, etc.), the conventional test is preferable whenever feasible. Certainly the conventional test can be used when strength alone is required, since it yields a strength comparable to that of the frictionless test with optimum end membrane thickness on sand.
frictionless cap and base test, conventional triaxial test, end restraint, end membrane thickness, optimum thickness, Poisson's effect, sand
Assistant professor, Clarkson College of Technology, Potsdam, N.Y.