Many materials can be described as granular materials composed of soft or ultra-soft grains. Mostfood products, metal powders, microgels and many suspensions are soft-grain systems. Despite their different mechanisms of deformability, depending on their composition and structure, their common feature is that they can undergo large strains without rupture. As a result, these materials can reach high packing fractions beyond random close packing through both grain rearrangements and grain shape change. Until now, because of the lack of proper numerical and experimental tools, their compaction behavior under stress, volume change behaviour under shear and microstructure have mostly remained unexplored.In this work, we present a numerical technique to model soft granular materials in which the grains can undergo extensive shape change and large deformations. It combines an implicit formalism of the Material Point Method and the Contact Dynamics method [1-3]. In this framework, the large deformations of individual grains as well as their collective interactions are treated consistently. In order to reduce the computational cost, this method is parallelised using the Message Passing Interface (MPI) strategy. Using this approach, we investigate the uniaxial compaction of 2D packings composed of elastic grains. We consider compressibility rates ranging from fully compressible to incompressible grains. The packing deformation mechanism is a combination of both grain rearrangements and large deformations, and leads to high packing fractions beyond the jamming state. We show that the packing strength declines when the grain compressibility decreases, and the packing can deform considerably. We also investigate the evolution of the connectivity of the grains and grain deformation distributions in the packing.REFERENCES[1] S. Nezamabadi, F. Radjai, J. Averseng, J.-Y. Delenne, “Implicit frictional-contact model for soft particle systems”, Journal of the Mechanics and Physics of Solids, 83: 72-87, 2015.[2] S. Nezamabadi, T.-H. Nguyen, J.-Y. Delenne, F. Radjai, “Modeling soft granular materials”, Granular Matter, 19: 8, 2017.[3] S. Nezamabadi, X. Frank, J.-Y. Delenne, J. Averseng, F. Radjai, “Parallel implicit contact algorithm for soft particle systems”, Computer Physics Communications, In press.