Importance of Free Ends in Triaxial Testing

Abstract
One of the main criticisms of the triaxial test is the nonuniformity of stress and deformation at all but small strains. The main cause of this nonuniformity is friction at the end platens, which causes both the barrelling effect and the concentration of dilation in local zones which in turn results in the premature development of a failure surface. These disadvantages are largely overcome by the use of lubricated end platens which, by removing the dead zones, allow the use of shorter test samples. Short samples with lubricated end platens have proved an advantage in triaxial research on particulate matter for a number of years. Use of this technique has the consistency of the test results. Preliminary tests on clays indicate that this is a promising method of reducing the testing time necessary for accurate pore pressure measurement. An additional benefit from their use is the reduction of piston friction. The use of electrical pressure transducers for measuring pore pressures is also described; this system is particularly suitable if automatic recording is required.

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