The column collapse experiment is a simplified version of natural and industrial granular flows. In this set-up, a column built with grains collapses and spreads over a horizontal plane. Granular flows are often studied with a monodisperse distribution; however, this is not the case in natural granular flows where a variety of grain sizes, known as polydispersity, is a common feature. In this work, we study the effect of polydispersity, and of the inherent changes that polydispersity causes in the initial packing fraction, in dry and immersed columns. We show that dry columns are not significantly affected by polydispersity, reaching similar distances at similar times. In contrast, immersed columns are strongly affected by the polydispersity and packing fraction, and the collapse sequence is linked to changes of the basal pore fluid pressure P. At the collapse initiation, negative changes of P beneath the column produce a temporary increase of the column strength. The negative change of P lasts longer in polydisperse columns than in monodisperse columns, delaying the collapse sequence. Conversely, during the column spreading, positive changes of P lead to a decrease of the shear strength. For polydisperse collapses, the excess of P lasts longer, allowing the material to reach farther distances, compared with the collapses of monodisperse materials. Finally, we show that a mobility model that scales the final runout with the collapse kinetic energy remains true for different polydispersity levels in a three-dimensional configuration, capturing the scaling between the micro to macro controlling features.@en

Geophysical mass flows involve particles of different sizes, a property termed polydispersity. The granular column collapse is a simplified experiment for studying transitional granular flows. Our research focuses on the role that polydispersity has on the velocity and runout distance of dry and immersed granular columns, undergoing a numerical and experimental study. Our results highlight that polydispersity does not have a strong effect on the collapse of dry columns. On the contrary, the collapse sequence of immerse granular columns strongly depend on the polydispersity level.

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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 φr. 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 φr is independent of polydispersity. This counterintuitive observation remains elusive to experimentalists, and especially for some technical communities that use φr as a design parameter (e.g., the soil mechanics community). In this Letter, we studied experimentally the effects of polydispersity on φr. 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 φr. We find that φr 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.

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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.

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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.@en

On the night of March 31, 2017, a series of mass movements took place in the Mocoa Basin and devastated the northwestern part of its urban area. The mass movements were triggered by a 4-day rainfall accumulation and high rainfall intensities on the night of the event. After the trigger, about 420 mass movements transported material as landslides, debris flows, and hyperconcentrated flows along the Taruquita and Taruca creeks and Sangoyaco River on the northern side, and as landslides and mudflows along the Mulato River on its southern side, resulting in major damages to both its population and infrastructure. Previous reports by Cheng et al. (2018) and García-Delgado et al. (2019) described the event from a hydrological and geological perspective, connecting them with the damage caused by the event, but leaving aside an analysis of the series of mass movements and its characteristics. This work presents an extended review of these mass movements and studies them via a statistical analysis, providing a detailed review of the event’s characteristics and their relationship with the resultant damage. These characteristics are compared with the existing hazard maps and they provide valuable information and awareness on hazard management for communities settled in the northern Andes.@en