A simple discrete approach to explore the response of swelling and softening particles

The response of granular materials is significantly affected by the stiffness of the particles that constitute them. This stiffness can evolve and the particles can exhibit strong inter-granular strain. While generally overlooked in literature, these features emerge in numerous materials that can soften and swell. This is the case for clays, root systems, and polymer-based particles. This study investigates the macroscopic effects of concurrent particle swelling and softening, based on previous experimental results on hygroscopic granular assemblies. The material studied therein shows a dilation–compaction response in oedometric conditions, despite the constant load, driven by the evolving properties of the particles. To model this complex response, an intentionally simple discrete element approach is proposed, with variables depending on one main parameter. A sensitivity analysis – built on the proposed material model and implemented within the discrete element method approach – successfully reproduces the experimental observations and notably the dilation–compaction response. An analytical one-dimensional model is then developed to further explore the role of variables in such assemblies. The study highlights that a simple model can capture the experimentally observed meso-scale mechanisms, underlining how complex responses can emerge from elementary inter-particle interactions.