Exploring the micro-mechanics of triaxial instability in granular materials

Abstract

14The 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. 17This 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 second-21 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 macro-and 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. 30Keywords: Discrete-element modelling; fabric/structure of soils; particle-scale behaviour 31 32

DEM Study on the Instability Behaviour of Granular Materials

“…Similarly, at a high magnitude of deviatoric stress (𝑞 = 180 kPa), 𝑞 decreases instantly at the onset of CSD loading in the high void ratio (𝑒 0 = 0.585) specimen indicating that the sample cannot withstand large magnitude constant 𝑞 load. Similar behaviour of deviatoric stress for different void ratio samples during CSD loading was reported by Perez et al (2016) and Lashkari et al (2019).…”

“…The mechanical anisotropy which is dependent on the force vector can be distinguished into, normal force anisotropy 𝑎 𝑛 (due to normal contact forces) and tangential force anisotropy 𝑎 𝑡 (due to tangential contact forces). Perez et al (2016) reported that the 𝑎 𝑛 , 𝑎 𝑐 anisotropic coefficients are more influenced by stress ratio under CSD loading. Hence these anisotropic coefficients have been emphasized in this study.…”

“…The Discrete Element Method (DEM) developed by Cundall and Strack (1979) is an alternative to the continuum mechanics approach for modelling and analysing the soil mechanical behaviour at a particle scale level. Even though many experimental CSD analyses have been conducted on the instability of granular materials, only, limited studies were performed to simulate the CSD test using micromechanical based approaches such as DEM (Ning et al 2013;Perez et al 2016;Lashkari et al 2019). Perez et al (2016) and Lashkari et al (2019) have adopted a stress controlled loading in their analyses and identified the instability behaviour based on Hill's particle scale criterion and strain rate increment.…”

“…Even though many experimental CSD analyses have been conducted on the instability of granular materials, only, limited studies were performed to simulate the CSD test using micromechanical based approaches such as DEM (Ning et al 2013;Perez et al 2016;Lashkari et al 2019). Perez et al (2016) and Lashkari et al (2019) have adopted a stress controlled loading in their analyses and identified the instability behaviour based on Hill's particle scale criterion and strain rate increment. Furthermore, they simulated the evolution of micromechanical quantities (coordination number, anisotropic coefficients) and highlighted the behavioural change of micro parameters post the instability point.…”

The effect of consolidation on undrained behaviour of granular materials: experiment and DEM simulation

“…where _ e is shear strain rate (1/s), d is the particle diameter (m), p 0 is the mean stress (kPa), and r g is the particle density (kg/m 3 ). The particle density (r g ) was 9•766 × 10 8 kg/m 3 , which is approximately 3•69 × 10 5 times larger than 2650 kg/m 3 for real geomaterial as reported by Perez et al (2016). To optimise the simulation time, the particle density was scaled up in the literature, and notably, Thornton (2000) used a scale-up factor of 10 12 .…”