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1. Comparing Realistic Particle Simulation Using Discrete Element Method and Physics Engine

   https://doi.org/10.1061/9780784482803.050  

   He, Hantao; Zheng, Junxing; Li, Zhaochao (February 2020, Geo-Congress 2020)

   Hambleton, J. P. (Ed.)

   Discrete element method (DEM) has been widely applied to simulate granular soil behavior. However, traditional DEM uses sphere clusters to approximate realistic particles, which is computationally demanding when simulating many particles. This study explores the use of physics engine, a platform developed for simulating physical processes in video games, to simulate realistic particles. This paper compares realistic particle simulation methodologies using physics engine and discrete element method, including contact models, parameter settings, computational speeds, and simulation results. The results show that the physics engine and DEM achieve similar simulation outputs, while the physics engine runs significantly faster than DEM, because PhysX uses both CPUs (central processing units) and GPUs (graphics processing units) of computers, triangular face tessellations to represent realistic particles, and a simplified contact model to accelerate simulations. This study provides geo-mechanicians and DEM modelers with one more option for them to consider when they simulate realistic particles. 

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2. Chrono::GPU: An Open-Source Simulation Package for Granular Dynamics Using the Discrete Element Method

   https://doi.org/10.3390/pr9101813  

   Fang, Luning; Zhang, Ruochun; Vanden Heuvel, Colin; Serban, Radu; Negrut, Dan (October 2021, Processes)

   We report on an open-source, publicly available C++ software module called Chrono::GPU, which uses the Discrete Element Method (DEM) to simulate large granular systems on Graphics Processing Unit (GPU) cards. The solver supports the integration of granular material with geometries defined by triangle meshes, as well as co-simulation with the multi-physics simulation engine Chrono. Chrono::GPU adopts a smooth contact formulation and implements various common contact force models, such as the Hertzian model for normal force and the Mindlin friction force model, which takes into account the history of tangential displacement, rolling frictional torques, and cohesion. We report on the code structure and highlight its use of mixed data types for reducing the memory footprint and increasing simulation speed. We discuss several validation tests (wave propagation, rotating drum, direct shear test, crater test) that compare the simulation results against experimental data or results reported in the literature. In another benchmark test, we demonstrate linear scaling with a problem size up to the GPU memory capacity; specifically, for systems with 130 million DEM elements. The simulation infrastructure is demonstrated in conjunction with simulations of the NASA Curiosity rover, which is currently active on Mars. 

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3. Physical and Mechanical Properties of Ottawa F65 Sand

   https://doi.org/10.1007/978-3-030-22818-7_3  

   El Ghoraiby, M.A. (November 2019, Proceedings of LEAP-UCD-2017 workshop)

   This paper presents the results of soil characterization and element tests of Ottawa F65 sand. The data presented is intended to be used as calibration material for the prediction exercise conducted as part of the Liquefaction Experiments and Analysis Project (LEAP 2017). The databank generated includes soil specific gravity tests, particle size analysis, hydraulic conductivity tests, maximum and minimum void ratio tests, and cyclic triaxial stress-controlled tests. An effort was made to ensure the consistency and repeatability of the test results by reducing the sources of variability in the sample preparations and increasing the number of tests. The uniformity of the soil was evaluated by conducting tests on samples from five different batches. The results showed that the sand is uniform among the five batches. Due to significant variability in previously reported maximum and minimum void ratio results, the effects of the test operator were studied by comparing test results obtained from three different operators. For the triaxial tests, a constant height dry pluviation method was used for sample preparation. To eliminate the effect of the human error in maintaining a constant drop height and to ensure consistency of the sand fabric between different samples, a device was developed to facilitate the sample preparation. The cyclic triaxial experiments were performed using three different soil densities, and a liquefaction strength curve was obtained for each density based on a 2.5% single amplitude axial strain criteria. The developed databank in this study was made publicly available for the community through DesignSafe. 

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4. Effect of Particle Size Distribution on Monotonic Shear Strength and Stress-Dilatancy of Coarse-Grained Soils

   Basson, M.S. (January 2023, Geo-Congress 2023)

   Natural soil deposits can consist of particles with a wide range of sizes. In current practice, the assessment of shear strength and stress-dilatancy behavior of coarse-grained soils is based on methods developed for poorly graded sands, without explicit consideration for differences in gradation. This paper investigates the influence of the range of particle sizes on the monotonic shear strength and the stress-dilatancy response of poorly- to well-graded soils. Using the 3D discrete element method (DEM), the applicability of commonly used sand-based stress-dilatancy frameworks is assessed for a range of gradations. This DEM investigation employs clumps of spheres to accurately simulate the particle shapes on specimens with coefficients of uniformity (CU) varying between 1.9 and 6.9. These specimens were subjected to isotropically consolidated drained triaxial compression at various relative densities and confining stresses with the objective of isolating the effects of particle size distribution from those of particle shape. The peak and critical state shear strengths and the dilatancy responses of the specimens with different gradations are evaluated. For the same state parameter, the results indicate an increase in the shear strength and rate of dilation as the range of particle sizes increases. However, the critical state line shifts downward, and its slope decreases as CU is increased. The DEM results are compared to Bolton’s stress-dilatancy relationship to highlight the inadequacies of using clean sand-based frameworks in capturing the behavior of well-graded soils. 

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5. Numerical Simulation of a Free Fall Penetrometer Deployment Using the Material Point Method

   https://doi.org/10.1016/j.sandf.2020.04.002  

   Zambrano-Cruzatty L, Yerro A. (January 2019, 2nd International Conference on the Material Point Method for Modelling Soil-Water-Structure Interaction)

   This paper proposes a numerical framework to model the deployment of a Free Fall Penetrometer (FFP) device in dry sands using the Material Point Method (MPM). Seabed characterization is required to assess a number of geotechnical problems in the nearshore and offshore areas, and FFP deployment is becoming a popular method to characterize shallow sediments. A moving mesh technique is used to ensure the accurate geometry of the FFP device throughout the calculation and the soil-FFP interaction is modelled with a frictional contact algorithm. The FFP device is simulated as a rigid body, which enhances the performance of the computation and reduces its computational cost. Numerical results are compared to experimental data, and are very promising. 

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