Avalanches of a granular medium reinforced with flexible fibers

The addition of a small fraction of flexible fibers to a granular medium is known to enhance the bulk mechanical strength of the material. However, their effect on the free-surface flow of grains remains largely unexplored. To address this gap, we investigate the influence of flexible fibers on dry granular avalanches. Using a drum partially filled with a grain-fiber mixture and rotated at low speeds, we observed successive avalanches and analyzed their dynamics. Specifically, we examined the effects of fiber volume fraction and fiber length on both the statistical properties and the individual dynamics of avalanches. Our results show that the average starting and stopping angles increase with fiber volume fraction, demonstrating that the addition of fibers stabilizes the granular slope. Fiber length is found to have only a minor influence on slope stabilization, i.e., on the increase in avalanche angles induced by fibers. While the increase in avalanche angles aligns with the bulk rheology of grain-fiber mixtures, the role of fiber length appears different in the case of free-surface flows. Increasing the fiber volume fraction also alters the distribution of avalanche angles, leading to a broader distribution and a higher probability of small-amplitude avalanches. Analysis of individual avalanche dynamics further reveals that fibers affect avalanche duration, suggesting enhanced energy dissipation within the system. To capture these effects, we propose a theoretical model that links avalanche dynamics to the rheological properties of the material. This framework provides an estimate of how fibers influence the inertial term in the rheology of grain-fiber mixtures, thereby extending existing empirical formulations for their behavior.