Several techniques for creating boundary-loading conditions for discrete element models of uniaxial and biaxial, tensile or compressive laboratory tests are presented. Planar walls are typically used to represent the loading platens and servo-controlled walls to apply a confining stress. While servo-controlled walls may be suitable for modeling confining stress on hard rock, the use of a string of bonded elements better represents confining pressure boundary conditions when modeling softer materials subject to large strains.
The discrete element method (DEM) is used increasingly to simulate and analyze problems involving geologic materials. This paper focuses on how to implement features found in twodimensional versions of DEM software to model biaxial tests on a variety of materials. The intension of this paper is not to provide fundamental insight into the behaviour of materials under different boundary conditions. Rather, the purpose is to show how features available in discrete element software can be used to best simulate appropriate boundary conditions when trying to simulate some commonly used tests for measuring material properties. Models of 2D biaxial or uniaxial tests are commonly used for comparison and calibration purposes with standard triaxial and uniaxial test data from the laboratory (Potyondy et al. 1996, Camborde et al. 2000, Boutt & McPherson 2002, Fakhimi et al. 2002, Nardin & Schrefler 2004, Holt et al. 2005). The discrete element input parameters are often iteratively selected for a simple biaxial model and the model results are compared to laboratory test data to determine appropriate combinations of DEM parameters that capture the salient macroscopic behaviour of the material in question. Therefore, the creation of an appropriate DEM model, including the sizes and relative positions of the elements and the model boundary conditions, is an important step needed before calibration of the DEM parameters can begin.