The granular column collapse is a simple model to study natural disasters such as landslides, rock avalanches, and debris flows because of its potential to provide solid links of physical and mechanical properties to these catastrophic flows. Such flows are commonly composed of different grain-size distributions, namely, polydispersity. Owing to the complexity of different particle-size phases, explanations of the collapse dynamics, run out distance, and size-segregation behavior of granular flows remain elusive. A binary-size mixture of granular materials is well-known as a simplified version of particle-size distribution. This paper explores the effects of the large-particle content on the collapse mobility, deposition morphology, and size segregation of binary-size mixtures composed in each column geometry. Although the kinetic energy and deposition morphology are nearly insensitive to the content of large particles for each column geometry, the large and small particle-size phases govern differently on total kinetic energy. Remarkably, the contribution of these two particle phases to the kinetic energy is similar when the large-particle content reaches around 10% for all column geometries. By quantifying the difference of the apparent friction coefficient of small and large particle phases, the size-segregation degree of binary-size mixtures is evaluated. The results noted that the segregation degree increases exponentially with increasing the large-particle content, but it is nearly independent of the column geometry. These findings complement insights into the flow properties of geological hazards, leading to offering valuable evidence for the management of natural disasters such as landslides and debris flows.

This study develops a dynamic model to better describe the frictional-dilatancy behavior of underwater granular motion. We employ the compressible Navier-Stokes equations as the continuum framework, and introduce mu(J) rheology in treating the constitutive law of the immersed granules. Within the compressible Navier-Stokes framework, the change in granular volume fraction that occurs when the granules undergo shear-induced volumetric dilation (contraction) is considered using the frictional-dilatancy law from soil mechanics. On introducing frictional dilatancy, the constant coefficient of friction at startup in mu(J) rheology, which governs the yielding limit of particles, is replaced by a particle-volume-fraction-dependent evolutionary variable. The proposed model enables an accurate description of the properties of quasi-static deforming granular mass. The validity of the model is verified by classical immersed granular collapse. A comparison with experimental and previous simulation results demonstrated that the introduction of frictional-dilatancy law delays the initiation of submarine granular flow.