Undrained instability detection under general stress conditions

This work proposes a criterion for the detection of undrained instability under multiaxial stress conditions at elemental level. To do so, it develops the application of the proposed criterion to the implicit implementation of a multiaxial elastoplastic constitutive model, which considers the effect of inherent anisotropy for the simulation of monotonic loading. The general criterion proposed is validated through the simulation of undrained torsional shear tests in Toyoura sand. The application of the criterion shows precision at onset of both liquefaction and phase transformation mechanisms, in general stress conditions through the variation of the intermediate principal stress ratio b. By considering different ranges of the initial void ratio and confining pressure, the representation of the collapse surface in three-dimensional stress space and in critical state space is shown and the phase transformation surface is presented in the critical state space. The interaction of the collapse and phase transformation surfaces offers the possibility of better understanding the influence of the initial void ratio and the confining pressure in the mechanical behavior of geomaterials in undrained conditions and the occurrence of the related instability processes.