Eurographics 2008 - Short Papers, 21-24. 2008 The interactive simulation of 3D terrains has been approached from several perspectives. Due to the complexity of the system involved, most of the models proposed focus on a visually realistic animation of the scene, rather than on a physically-based accurate simulation of a granular system. Those models lack generality when interacting with the environment; in most cases, no reaction forces are computed, considering only soil deformation. This limitation reduces their usability in applications such as driving simulators. We propose the use of a theoretical discrete model that considers normal forces for 3D real-time simulation of granular systems. We also extend this model to consider horizontal forces, allowing a wider range of interactions. Several numerical tests have been implemented and detailed results have been analyzed which show a good model performance.