Exploring the micro-mechanics of triaxial instability in granular materials

The instability of granular materials due to water infiltration under fully drained conditions has been previously considered in experimental studies. While laboratory experiments can provide macro-scale insight into drained instability, the micro-mechanics under such conditions are yet to be explored. This study has employed the discrete-element method (DEM) to simulate constant shear drained (CSD) tests for an ideal soil. CSD tests were initiated from a range of packing densities and stress conditions. The DEM simulations were able to qualitatively replicate laboratory CSD tests. The choice of the loading control parameter was seen to play a central role in the macro-scale second-order work to identify an effective failure. All samples considered attained an onset of instability that coincided with fluctuations in the second-order work from a particle scale. The time of occurrence of the onset of instability was seen to depend on initial packing density and stress state. A change in the evolution of macro- and micro-mechanical quantities, showing either a sharp increase or decrease, was observed once the CSD conditions had been reached. Finally, conventional drained then constant volume (CDCV) tests were carried out where the appearance of instabilities and the evolution of macro and micro quantities were found to be different from those observed in CSD tests. The results presented in this study indicate that the constant shear drained loading conditions can result in more unfavourable situations than for the undrained loading condition. © 2016, ICE Publishing. All rights reserved.