Increased mobility of bidisperse granular avalanches

The unexpected behaviour of long-runout landslides has been a controversial subject of discussion in the geophysics community. In order to provide new insight into this phenomenon, we investigate the apparent reduction of friction resulting from the presence of a second species of smaller particles in the bulk of the granular material that forms the avalanche. Results obtained by means of a two-dimensional soft particle discrete element numerical simulation are presented. The numerical experiments consider an avalanche of two-size particles, originally placed over an inclined plane. The friction coefficient for the particle-particle and wall-particle contacts is held fixed. The granular mass is allowed to evolve with time, until it comes back to rest on a horizontal plane. The position of the centre of mass is located, such that the runout length L(cm)/H(cm) could be measured, with L(cm) and H(cm) being the horizontal distance travelled and the height lost by the avalanche centre of mass, respectively. Many simulations were performed keeping the area of the avalanche constant, varying only the area fraction of small particles. The results show that the runout length increases with the area fraction of small particles, reaching a maximum for a given area fraction of small particles. A detailed analysis of the particle distribution in the granular mass indicates that the apparent friction coefficient is affected by the formation of a layer of small particles at the base of the avalanche. This layer is identified as the source of 'lubrication'. Furthermore, since there is a dependence of the runout on the fall height and the volume in real avalanches, some simulations with different areas and different fall heights were performed. The results show a tendency of the runout to increase with area, and to decrease with the initial fall height, which is in agreement with what is observed for geological events.