Instability in Granular Materials: Experimental Evidence of Diffuse Mode of Failure for Loose Sands

The influence of three loading paths on the collapse of loose sand is analyzed with a particular attention paid to the onset of collapse and the mode of failure exhibited. Experimental results on conventional undrained triaxial compression tests, constant shear drained tests, as well as quasi-constant shear undrained path are presented, compared, and analyzed. It is now recognized that some collapses can occur before the Mohr-Coulomb plastic limit criterion is reached, and our recent results obtained with the new arrangement built up highlight that these collapses occur under a diffuse mode of failure. An extensive experimental series of tests shows that the first negative value of the second-order work computed using experimental data corresponds to the loss of controllability. Moreover, it is shown that the stress ratios at collapse and the corresponding mobilized angles of friction are very close for all types of tests. For similar void ratios, the onset of collapse is thus largely independent of the loading path under drained and undrained conditions but depends on a stress state to bring the material inside the unstable domain and also on the current direction of the stress increment. Indeed, it appears that the orientations of the stress increments at collapse for all tests are the same, what explains, according to the second-order work criterion, that collapse occurs at the same stress ratio. A potentially unstable domain, depending on the stress increment direction, can thus be defined.