Image-Aided Element Shape Generation Method in Discrete-Element Modeling for Railroad Ballast

The discrete-element method (DEM) provides more realistic results compared with those of continuum analysis for unbound construction materials such as railroad ballast. This paper introduces a digital-image aided particle-shape generation method for DEM to consider the shape effects of aggregate particles on assembly behavior. The BLOKS3D DEM program is used in this research with the capability of representing angular aggregates as discrete elements with user-defined particle morphological properties from imaging-based shape indices. A three-dimensional (3D) image analysis approach is available to construct discrete elements with shapes close to the morphological properties of actual aggregate shapes. A large-sized shear box is then used in direct-shear tests to validate this image-aided particle-shape generation DEM. The purpose of validation is to match the laboratory test results with the DEM simulation results by using one single set of model parameters. To that end, the sensitivity of DEM model parameters are investigated by conducting DEM shear-box simulations using different combinations of model parameters realistically chosen on the basis of previous research studies. The validation process is finally accomplished by statistically demonstrating that the DEM shear-box simulation results using one set of parameters can predict the laboratory shear-box test results reasonably well under various normal stress levels.