This paper presents the Discrete Element Method (DEM) simulations on the instability behaviour of granular materials during Constant Shear Drained condition (CSD). CSD condition was implemented by decreasing mean effective stress on an assembly of particles under strain controlled loading. In this study, the instability condition was predicted at the particle scale level using particle second order work increment (Nicot et al., 2012). The DEM contact parameters have been calibrated to capture the macroscopic responses and the instability behaviour consistently with the laboratory experimental observations. Using the same contact parameters the effect of different range of initial states at the beginning of CSD condition such as different initial mean effective stress (𝑝 0 ′ ), void ratio (𝑒 0 ) and deviatoric stress (𝑞) on the instability behaviour were analysed. In addition, the micromechanical parameters such as coordination number, anisotropic coefficients (geometric, mechanical) have been extracted to assist in characterising the instability behaviour during CSD condition. The initial stress state of the soil (i.e. at the onset of CSD) condition has shown a significant influence on the evolution of anisotropic coefficients, an evident behaviour change was noted once the CSD condition is imposed. A continuous increase of geometric anisotropy, and a gradual decrease of mechanical anisotropy was observed after the instability condition is reached.
In this paper, the Discrete Element Method (DEM) is coupled with the Lattice Boltzmann Method (LBM) to model the cone penetration test of saturated granular media. The coupled numerical model was calibrated using one-dimensional consolidation theory. The results obtained from the 1D consolidation test simulation showed good agreement with the analytical equation proposed by Terzaghi. A series of LBM-DEM simulations were then carried out to understand the effect of penetration rate on the behavior of saturated granular materials during the cone penetration test. The model has predicted a significant influence on the excess pore fluid pressure and an insignificant influence on the cone resistance responses and has qualitatively captured the effect of penetration rate consistent with the experimental data. The simulation results showed that excess pore fluid pressure increased with an increase in penetration rate. The particle displacement and fluid velocity contours have provided insights into the particle behavior and fluid pressure fluctuations during CPT. The increase in excess pore fluid pressure has been attributed to the fluid pressure gradients created by the cone in the fluid system based on the penetration rate. The pore pressure distribution plots have shown a maximum pore fluid pressure below the cone region and over the cone shoulder position. A consistent evolution pattern of fabric anisotropy has been observed throughout the depth in all the penetration rate conditions. The fabric components (∅ 22 ) and (∅ 11 ) have dominated around the cone area and at the boundary region, respectively. This indicates the preferential orientation of contacts in the vertical direction at the cone region and the horizontal direction at the boundary region. The simulation results have demonstrated that the LBM-DEM model can efficiently simulate the cone penetration test and associated pore fluid pressure, including the cone-particle-fluid interactions during CPT.
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