Grain size distributions, from a single grain size to well-graded distributions, on dry and immersed flows

Landslides and debris flows are massive geophysical processes that could occur in subaerial or submerged conditions. They involve granular materials in a wide range of Grain Size Distributions (GSD). In such processes, granular materials are subjected to large deformations, reaching a state where strains accumulate at constant shear stress. This state is known in the geotechnical community as the residual or critical state. The influence of the GSD on the residual state has been a matter of discussion between conflicting experimental and numerical observations. In this work, we confirm, at a grain scale and in dry conditions, that the residual shear strength is independent of the GSD. Moreover, we experimentally validate this result on dry granular flows, comparing the influence from monodisperse materials (materials with one grain size) to well-graded materials (materials with multiple grain sizes) on the mobility of a granular column. In this configuration, a granular column is let to collapse by self-weight and spread horizontally. The column mobility can be interpreted as a macro representation of the material's effective shear strength. Furthermore, we explore the effect of the GSD in immersed columns, finding a strong dependence of the GSD on the flow dynamics arising from the evolution of basal pore pressure P. At the flow initiation, negative P changes beneath the column produce a temporary increase in the column strength. This positive change lasts longer and with a larger amplitude in granular flows with well graded materials than in monodisperse ones. Then, during the column horizontal spreading, positive changes of P provoke a decrease in shear strength. For column collapses of graded materials, the excess of P lasts longer, allowing the collapses to reach farther distances compared with collapses of monodisperse materials. Finally, considering the relevance of mobility in granular flows, we propose a mobility model that scales the final runout with the collapse kinetic energy. This model works for both dry and immersed flows with different GSDs and has been validated for results from different authors, methodologies, and grain characteristics. Our results offer a novel perspective on the influence of GSD on the complex relationships between solid and fluid phases in granular flows, highlighting features that can be extended to massive natural processes.