Exploring the micro - mechanics of triaxial instability in granular materials

14 The instability of granular materials due to water infiltration under fully drained conditions has been 15 previously considered in experimental studies. While laboratory experiments can provide macro-scale 16 insight into drained instability, the micro-mechanics under such conditions are yet to be explored. 17 This study has employed the Discrete Element Method (DEM) to simulate constant shear drained 18 (CSD) tests for an ideal soil. CSD tests were initiated from a range of packing densities and stress 19 conditions. The DEM simulations were able to qualitatively replicate laboratory CSD tests. The 20 choice of the loading control parameter was seen to play a central role in the macro-scale second21 order work to identify an effective failure. All samples considered attained an onset of instability that 22 coincided with fluctuations in the second-order work from a particle scale. The time of occurrence of 23 the onset of instability was seen to depend on initial packing density and stress state. A change in the 24 evolution of macroand micro-mechanical quantities, showing either a sharp increase or decrease, 25 was observed once the CSD conditions had been reached. Finally, conventional drained then constant 26 volume (CDCV) tests were carried out where the appearance of instabilities and the evolution of 27 macro and micro quantities were found to be different from those observed in CSD tests. The results 28 presented in this study indicate that the constant shear drained loading conditions can result in more 29 unfavourable situations than for undrained loading condition. 30