An investigation of diffuse failure modes in undrained triaxial tests on loose sand

In this paper we present an experimental study performed as an attempt to exhibit non-localized, diffuse failure deformation modes in sand specimens submitted to test conditions leading to unstable failure. In particular, we are interested in diffuse modes which may appear in common triaxial compression tests on undrained loose sand specimens. A special series of triaxial compression tests are reported. These tests resemble the common triaxial compression tests but are modified in such a way that after the peak in the stress-strain curve, test control is changed from strain-control to stress-control. More precisely, a special kind of load-control is used, based on an original device which allows to recover displacement-control after a dynamic step in the post-peak regime. It is shown that the control used allows for the observation of the failure modes occurring in the unstable branch of the test.