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.

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.

Granular flows are a complex process, involving a wide range of grain sizes, materials, varied viscous fluids, among others. For this reason, the simulation of granular flows requires a certain level of simplification, allowing the isolated study of its governing variables and extending the global observations to field events. Here, we present the planar setup as an alternative for studying simplified processes associated to granular flows. The planar setup consists of two windows separated by a thin gap and enclosing a granular assembly. We present two examples where the planar setup is adapted for the study of the competing action of segregation and disaggregation in a fractured grain under shear flow, and for the study of the stability scenarios of a flow impacting a permeable obstacle. The close visualization of the kinematics at the particle scale provides an ideal opportunity for describing the mechanisms behind the grain disaggregation or controlling the obstacle stability. Both examples highlight the advantages of the planar setup for the study of granular flows.

The dynamic properties of loose sands under low stresses are an unexplored topic in soil dynamics because these soil conditions are uncommon in most geotechnical structures on Earth. However, low densities and low-stress conditions prevail on other planets, like, for instance, the surface of Mars, for which particular attention is presently given through the InSight NASA mission. This work presents a new procedure for measuring the dynamic properties of loose sand under low stress by using the dynamical mechanical analysis (DMA) tester, a technique commonly used in asphalt engineering but not in geotechnical engineering. Compared to traditional geotechnical methods (resonant column and cyclic triaxial tests), DMA investigates a broader range of strains using a single apparatus. In this work, we assess the dynamical properties of loose fine sand Dr ∼ 0.2, considered a possible Mars regolith analog, by varying the input strain from γ = 10-6 to γ = 10-2 while applying confining pressures from ρ3 =3 kPa to ρ3 = 30 kPa. The results validate the proposed procedure, showing an increment of the shear modulus as the confining pressure increases. Furthermore, they highlight DMA's advantages for studying the dynamic properties of granular soils under low stress and strain.

We present a transdisciplinary perspective for multiple actors interested in disaster-risk-reduction strategies in inhabited volcanoes, based on the novel approach of complexity and collaborative research. Our reflection is based on an ongoing research process that puts into dialogue scientific knowledge and local knowledge from Andean campesino communities inhabiting the Doña Juana Volcano-Páramo in the Latin American tropics. These local communities have experienced historically complex social, economic, and environmental conflicts in their territories; currently, most of them live in the buffer zones of the National Natural Park Doña Juana-Cascabel, which is part of Colombia’s national system of natural protected areas. We address inhabited Andean volcanoes as complex systems, which combine multi-temporal and spatial processes that allow for the emergence of nonlinear and heterogeneous social and ecological interdependencies. By transgressing disciplinary boundaries, we see an opportunity for building horizontal dialogues with communities inhabiting active volcanoes, understanding resilience, and eventually developing situated and collaborative disaster-risk-reduction strategies. The resulting vision is proposed as a fundamental methodology in settings such as south-western Colombia, marked by convoluted social dwelling patterns, and unequal biosocial and socioeconomic histories. Detached from positivistic and victimising perspectives, we seek to build knowledge on the antecedents, inheritance, persistence, and preservation of systems, ultimately enabling response-abilities for decision-making. The motivations, knowledge, and self-organisation capacities of the volcano inhabitants bring about new possibilities of doing locally while thinking globally. The transdisciplinary research approach allows us to situate our exchanges with the local community, and envision future collaborative paths towards promoting community-oriented forms of appropriation and transformation of a volcanic lifeworld.

Compaction is probably the industrial process more intensively used in road construction. The current compaction practice is based on the useful methodology proposed by Proctor (1933). This methodology intends to reproduce field compaction in the laboratory by applying a controlled mechanical energy to the soil and relating the dry density with the water content. However, recent developments in soil compaction analysis in the laboratory and field seek a better understanding of the link between the mechanical stresses applied to the soil and the strains it undergoes. This requirement needs analyzing the stresses applied to the soil by the compaction machine, the distribution of stresses within the soil layer undergoing compaction, and the behavior of the partially saturated soil. This article explores the feasibility of using a series of simplified analytical equations to model compaction within a mechanical framework. The proposed model focuses on the compaction produced by a cylinder applying a static load. The methodology combines the Hertz contact stress theory with the Fröhlich stress distribution and the Barcelona Basic Model for partially saturated soils. The results of the proposed analytical model successfully agree with the experimental observations obtained using a geotechnical centrifuge model and confirming its potential use for compaction control and monitoring.

Debris flows involve the high-speed downslope motion of rocks, soil, and water. Their high flow velocity and high potential for impact loading make them one of the most hazardous types of gravitational mass flows. This study focused on the roles of particle size grading and degree of fluid saturation on impact behavior of fluidsaturated granular flows on a model rigid barrier in a small-scale flume. The use of a transparent debris-flow model and plane laser-induced fluorescence allowed the motion of particles and fluid within the medium to be examined and tracked using image processing. In this study, experiments were conducted on flows consisting of two uniform and one well-graded particle size gradings at three different fluid contents. The evolution of the velocity profiles, impact load, bed normal pressure, and fluid pore pressure for the different flows were measured and analyzed in order to gain a quantitative comparison of their behavior before, during, and after impact.

Abstract: Granular flows are typically studied in laboratory flumes based on common similarity scaling, which create stress fields that only roughly approximate field conditions. The geotechnical centrifuge produces stress conditions that are closer to those observed in the field, but steady conditions can be hardly achieved. Moreover, secondary effects induced by the apparent Coriolis acceleration, which can either dilate or compress the flow, often obscure scaling. This work aims at studying a set of numerical experiments where the effects of the Coriolis acceleration are measured and analyzed. Three flow states are observed: dense, dilute, and unstable. It is found that flows generated under the influence of dilative Coriolis accelerations are likely to become unstable. Nevertheless, a steady dense flow can still be obtained if a large centrifuge is used. A parametric group is proposed to predict the insurgence of instabilities; this parameter can guide experimental designs and could help to avoid damage to the experimental apparatus and model instrumentation.