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A coupled computational fluid dynamics and distinct element method (CFD-DEM) is proposed to simulate undrained triaxial tests on granular soils. In this method, the compressible volume-averaged Navier–Stokes equations of continuity and momentum with an additional state equation are used for modelling liquid flows. In addition, the volume-averaged Navier–Stokes equations are modified by including the velocity of moving grid in their convective terms to settle the problem of variable boundaries in the process of undrained triaxial compression. Then the capability of the developed method is mainly examined by comparisons with the constant volume method of simulating undrained triaxial tests, which shows that the results of the CFD-DEM method can compare favourably with those obtained from the constant volume method. Finally, the influences of strain rate, initial pore pressure and fluid compressibility on the undrained behavior of granular soils are carefully studied. [ABSTRACT FROM AUTHOR]

Abstract: This paper aims at investigating the influence of grading on the undrained behavior of granular materials. Series of undrained triaxial tests were carried out with two different materials, glass balls and Hostun sand. For each material, samples with different gradings and similar relative densities were prepared. Experimental results show that the undrained shear strength decreases when the coefficient of uniformity increases from 1.1 to 20. The conditions of instability for the selected granular materials were also analyzed, based on the sign of the second-order work during undrained triaxial loading. The results demonstrate a significant influence of the grading: increasing the coefficient of uniformity heightens the potential of static liquefaction and the materials become more unstable. Furthermore, numerical tests using the three-dimensional discrete element method (DEM) were conducted on assemblies of spheres. The DEM inter-particle parameters were derived from the experimental test results on glass balls. The DEM simulations showed similar behaviors compared to experimental results and confirmed the influence of the grain size distribution on the stress–strain relationship and instability phenomena. [Copyright &y& Elsevier]

A modeling method for the particle-based flexible membrane boundary for the drained/undrained triaxial test is proposed herein. First, the particle placement manner to construct a cylindrical membrane is described. Second, the vertical and radial stiffnesses/moduli of the entire membrane are derived and related to the interparticle contact stiffness, thereby enabling the calibration of the interparticle stiffness and correcting the confining pressure acting on the sample. Third, the application strategy of confining pressure is presented for the drained test, and a two-stage loading strategy is proposed to explore the excess pore pressure incrementally for the undrained tests. Using this membrane boundary, triaxial compression tests under the drained/undrained condition by discrete element method (DEM) were conducted on the samples with different initial void ratios and membrane stiffnesses. Simulations reproduced the behavioral transition by increasing the initial void ratio, the influence of membrane stiffness and the characteristics of shear banding, demonstrating the effectiveness of the proposed method. Differences from the referencing results with the rigid wall boundary were observed and attributed to the additional confining pressure from the membrane radial modulus, the persistence of the shear band allowed by the membrane flexibility, and the membrane penetration. [ABSTRACT FROM AUTHOR]