The column collapse experiment is a simplified version of the complex granular flows observed in both natural and industrial contexts. Due to its simple setup and rich behavior, the column collapse has been studied experimentally and numerically by several authors. The purpose of the analyses presented in this paper is to verify whether some of the results presented in these publications are affected by grain-size effects. In order to do so, we simulate two-dimensional granular columns by means of a discrete-element method, i.e., contact dynamics. Specifically, we study the influence of the grain size, as compared to the system size, on the resultant deposit geometry and collapse duration. We show that (i) some of the previously published results may be affected by grain-size effects, (ii) in order to avoid these effects, the system-size to grain-size ratio must be larger than 75 for short columns and larger than 50 for tall columns, and (iii) the quantities that are the most affected by grain-size effects are the column mobility and the collapse duration. Our findings serve as a tool for comparing results obtained by different researchers and draw guidelines on the number of grains that must be used in order to avoid grain-size effects.

A granular front emerges whenever the free-surface flow of a concentrated suspension spontaneously alters its internal structure, exhibiting a higher concentration of particles close to its front. This is a common and yet unexplained phenomenon, which is usually believed to be the result of fluid convection in combination with particle size segregation. However, suspensions composed of uniformly sized particles also develop a granular front. Within a large rotating drum, a stationary recirculating avalanche is generated. The flowing material is a mixture of a viscoplastic fluid obtained from a kaolin-water dispersion with spherical ceramic particles denser than the fluid. The goal is to mimic the composition of many common granular-fluid materials, such as fresh concrete or debris flow. In these materials, granular and fluid phases have the natural tendency to separate due to particle settling. However, through the shearing caused by the rotation of the drum, a reorganization of the phases is induced, leading to the formation of a granular front. By tuning the particle concentration and the drum velocity, it is possible to control this phenomenon. The setting is reproduced in a numerical environment, where the fluid is solved by a lattice-Boltzmann method, and the particles are explicitly represented using the discrete element method. The simulations confirm the findings of the experiments, and provide insight into the internal mechanisms. Comparing the time scale of particle settling with the one of particle recirculation, a nondimensional number is defined, and is found to be effective in predicting the formation of a granular front.

Landslide generated tsunamis are a primary natural hazard to coastal communities and infrastructure, but the present state of knowledge is limited to the specific details of momentum transfer from the landslide to the water body for either subaerial or fully submerged conditions. Here we report a series of novel experiments and an analytical model that bridges this gap, by exploring the collapse of a granular column for a wide range of water depths. We show that the maximum seaward wave amplitude is governed by a single dimensionless ratio, the relative depth of submergence. Based on the experimental observations, we propose a continuous function that quantifies the maximum wave amplitude by considering the momentum flux from the initially vertical granular column to the initially still fluid. Predictions made using the momentum function are in good agreement with observations of the present study and with other experimental studies of granular column collapse at larger scales. The analytical model allows the prediction of maximum wave amplitude over the full range of submergence conditions from subaerial to partially submerged and fully submerged collapses, with potential applications for tsunami hazard assessment.

Granular materials are used in several fields and in a wide variety of processes. An important feature of these materials is the diversity of grain sizes, commonly referred to as polydispersity. When granular materials are sheared, they exhibit a predominant small elastic range. Then, the material yields, with or without a peak shear strength depending on the initial density. Finally, the material reaches a stationary state, in which it deforms at a constant shear stress, which can be linked to the residual friction angle ${\varphi }_{\mathrm{r}}$. However, the role of polydispersity on the shear strength of granular materials is still a matter of debate. In particular, a series of investigations have proved, using numerical simulations, that ${\varphi }_{\mathrm{r}}$ is independent of polydispersity. This counterintuitive observation remains elusive to experimentalists, and especially for some technical communities that use ${\varphi }_{\mathrm{r}}$ as a design parameter (e.g., the soil mechanics community). In this Letter, we studied experimentally the effects of polydispersity on ${\varphi }_{\mathrm{r}}$. In order to do so, we built samples of ceramic beads and then sheared these samples in a triaxial apparatus. We varied polydispersity, building monodisperse, bidisperse, and polydisperse granular samples; this allowed us to study the effects of grain size, size span, and grain size distribution on ${\varphi }_{\mathrm{r}}$. We find that ${\varphi }_{\mathrm{r}}$ is indeed independent of polydispersity, confirming the previous findings achieved through numerical simulations. Our work fairly closes the gap of knowledge between experiments and simulations.

By means of the Discrete Element Method, we investigate the relationship between the power‐law exponent of the grain size distribution (GSD) and the mobility of granular transitional flows. This is done with the aim of testing a recently posed hypothesis, suggesting that high power‐law exponents are related to the large mobility observed in natural mass flows (Lai et al., 2017, https://doi.org/10.1002/2017gl075689), as a consequence of a lubrication mechanism in which small grains act as rotational bearings reducing the shear strength of the flowing mass. We show that both the mobility of the flow and the shear strength of the material are independent of the GSD power‐law exponent, as long as the system‐to‐grain size ratio of the studied system is sufficiently large, as it is expected in natural granular flows. This confirms that the power‐law exponent, or the proportion of small and big grains, are not plausible origins of the large mobility of granular avalanches.

Submerged mass flows are commonly considered a natural geohazard that interacts with terrestrial and submarine infrastructure and communities and are linked to the generation of tsunamis. Previous studies have investigated submerged mass flows by means of the benchmark case of a granular column collapse. This study explores the mobility and collapse dynamics of submerged granular columns within a planar configuration as a function of the column aspect ratio a. The use of a planar configuration allows the monitoring of the moving mass and its deformation patterns and provides a novel insight into the particle-fluid interactions at release and during collapse not possible before. Special attention was devoted to the column saturation and release mechanism. The column mobility is observed to be directly controlled by a and is linked to a clear distinction between triangular and trapezoidal deposits. The column collapse is found to be described by two characteristic times, marking transition points through the collapse stages. The associated deformation patterns reflect both at a column scale and at localized particle groups during collapse, reflecting in the velocity scaling of a deformable and moving granular mass and the occasional ejection of particles at its surface. We observed that the area of the released portion decreases during collapse and converge toward an equivalent portion of surface particles with little influence by a. The experimental results provide a novel insight into the study of submerged mass flows and set a reference behavior for future numerical validations.