<p>The granular column collapse is a simplified system of the complex dynamics observed in gravity-driven natural mass-movements (i.e., landslides, debris flows, rock avalanches) and industrial applications (i.e., pharmaceutics, concrete, and food industry). In this system, a granular column is built with an initial height and initial width and then is allowed to collapse by self-weight onto a horizontal plane, while observing the variation in runout as a function of its initial geometry. Despite its wide use in the study of mass-movements mobility, either dry or with a liquid, little is known on the internal physics during collapse and its variation when immersed in an ambient fluid. This work presents a planar setup that allows the study of fully and partially immersed granular columns, with little disturbance at release [1]. The use of a planar configuration allows the monitoring of the moving mass and its deformation patterns, providing a unique insight into the particle-fluid interactions at release and during collapse that were not possible before. These observations are of great importance for the understanding of particle-fluid interactions at a mesoscale and can shed light into larger processes like a submarine and subaerial landslides. This work addresses these interactions by varying the geometry and measuring the mobility in dry and immersed conditions. The associated deformation patterns are observed both at the column-scale and at the particle-scale, 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 converges toward an equivalent portion of surface particles with little influence by the initial column geometry. These observations validate the planar setup for the study of granular columns, provides a novel interpretation in the momentum transfer in particle-fluid systems, and sets a validation case for future numerical simulations.</p><p>[1] Pinzon & Cabrera, Planar collapse of a submerged granular column. Physics of fluids, v31, 2019.</p>
Granular fronts are a common yet unexplained phenomenon emerging during the gravity driven free-surface flow of concentrated suspensions. They are 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 visco-plastic 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, like fresh concrete or debris flow. In these materials, granular and fluid phases have the natural tendency to segregate 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 material properties 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 non-dimensional number is defined, and is found to be effective in predicting the formation of a granular front.
In the SW flank of the Doña Juana Volcanic Complex, Colombia, the dynamic geomorphic system responds to the complex interaction between volcanic, climatic, and tectonic driving forces, where the recent landscape (last ~20 years) is being shaped as a function of denudational processes. Despite the rapid rates of landforms development, the geomorphology of this area is poorly documented. To overcome the lack of information we mapped the area using a GeoSAR DEM and an Unmanned Aerial Vehicle based DEM. This paper presents two maps, a 1:25,000 scale map and a 1:5,000 detailed map of landforms along the Humadal Creek. Detailed categorization of landforms (at 1:5,000) allowed us to identify geomorphic processes in the village of Las Mesas and rural areas that triggered hazards for the communities. The morphologic evolution interpretation of this volcanic tropical area serves as a tool for future geohazard assessment in inhabited areas with important information gaps.
<div>The granular column collapse experiment, which consists of the rapid removal of lateral support to a column of granular material, is an important benchmark case for the physical and numerical study of transitional mass flows. While other researchers have focussed on the link between the aspect ratio of the column to mobility of the flow, these experiments are also an important platform to evaluate frameworks for triggering of slope failures.</div><div>&#160;</div><div>Critical state soil mechanics centers around the theory that initially dense soils will dilate, and initially loose soils will contract upon shearing. If the soil is sheared at a rate which exceeds the rate which fluids can be expelled or drawn into the pore space between particles, the shearing is considered to be occuring at constant volume and termed &#8220;undrained&#8221;. This state is associated with a rise in pore fluid pressure and a reduction in intergranular normal effective stress. The authors have conducted experiments varying the time scales of the volume change and dissipation processes. In these experiments, a novel transparent soil mixture comprised of quartz and mineral oil was utilized to visualize the saturation regime of soils during the granular column collapse experiment. Particular attention was paid to triggering mechanisms and the transition between the metastable state and avalanche regimes. The transparent material allowed visual confirmation of the volume change during shearing and important insights were gained into the role of the unsaturated soil condition in temporary strength. These observations have implications beyond the column collapse experiment, including the initiation of debris flow experiments as well as analysis of triggering mechanisms of unstable slopes in the field.</div>
The granular column collapse experiment is an important benchmark case for the physical and numerical study of transitional mass flows. Unlike columns of dry granular materials, the presence of a relatively incompressible fluid, such as water, in the voids of saturated columns complicates the shear behavior of the column by becoming a function of the coupled shear and volumetric behavior of the grain–fluid system. Dilative or contractive behavior at the pore level will cause a decrease or increase, respectively, in the pore fluid pressure. These changes in effective stress, in turn, will define stability or instability and length of runout. Here we use the new opportunity provided by transparent soil to observe air entry within saturated columns to explore the hypothesis that the entry pressure provides the maximum contribution of capillary pressure at incipient failure, thereby providing a quantitative control on the stability of dilative granular columns. Furthermore, the mobility of densely packed saturated columns subject to collapse was significantly influenced by air entry. An analytical model, based on this assumption of limiting capillary pressure, is able to describe the stability of the experimental columns as well as the larger dataset from the literature, reframing the previous empirical stability threshold using limit equilibrium and soil material parameters. Our results demonstrate the importance of stress-dilatancy and air-entry phenomena on the rapid shear behavior of saturated granular materials.
Fuelled by technological innovations and the growing commitment of countries to reduce their carbon footprint, offshore wind has steadily been gaining ground on non-sustainable sources of energy. According to the International Energy Agency (IEA) [2], it is foreseen that wind will be the principal source of energy in Europe by 2027. However, in an effort to protect the marine environment from sound pollution [1], regulations applicable to offshore wind endeavors have become more stringent. To sustain the growth of the offshore wind sector, more durable installation methods are required. Prolongation of the impact duration has been identified as a suitable method to protect the marine environment from sound pollution [7]. However, the market introduction of this technology is hampered by a lack of understanding of its effects on soil-structure interaction. Therefore, concerns exist on (mono)pile drivability, as well as the performance of the foundation under axial and lateral loads. To address this issue, a series of centrifuge experiments is performed in the centrifuge facility of Delft University of Technology (DUT). The experiments are conducted at 50g acceleration and show the effect of blow prolongation on the drivability of miniature steel, tubular pile (outer diameter, D = 42 mm; wall thickness, t = 2 mm) in dry GEBA sand at 80% relative density. The blow-prolongation technology that is assessed is IQIP’s BLUE Piling (BP) Technology [4]. Details on the actuator used to simulate BP technology in the centrifuge are provided by the work of Quinten et al. (2022) [6]. Prior to dynamic installation, the pile was allowed to settle in into the sample under 1g conditions. Subsequently, an second self-weight penetration phase is initiated by increasing the centrifuge acceleration to 50g, while the ram acts as dead-weight on top of the pile. Following the self-weight penetration phase, the centrifuge is intermittently stopped and reinitiated to (re)set the actuator. The BP ram has a mass of 1.889 kg ram and stroke of 40 mm. The results of the BP experiment are compared against those from centrifuge experiments involving impact hammering (IH), the most widespread method of installation for monopiles in the offshore sector. A detailed description of the actuator, the miniature impact hammer of DUT, is provided by Quinten et al. (2022) [5]. The model pile was pre-embedded at a depth of 50 mm under 1g conditions prior to the dynamic installation phase. The hammer operates at a driving frequency of 10 Hz and is equipped with a ram of 0.140 kg, which is released from a height of 40 mm. Comparison of the results of both experiments, reveals striking differences in the cumulative settlement behavior of the pile as well as the pile stresses. The cumulative pile displacement charts, as shown in Figure 1, show significant differences between the two installation methods. For BP (Figure 1a), a series of 3 single blow experiments was conducted. For IH (Figure 1b), the experiment lasted for a total of 37 consecutive blows. The realized pile set during the experiment is comparable between the two tests. The average normalized displacement equates to 0.5D and 0.03D per blow for BP and IH respectively. The soil level inside the pile cavity was measured following the execution of both experiments. The associated measurements indicated that both piles were driven in a fully coring mode. When considering the prototype pile dimensions and soil conditions, this finding corresponds well with the work of Jardine et al. (2005) [3]. Further differences between IH and BP were observed in terms of (peak) pile stress. The driving forces are reduced from 25 kN for IH to 6 kN for BP, respectively. The driving factor behind this reduction is the decrease in interface stiffness between the ram and the anvil. The latter completely offsets the effects associated with the use of a significantly heavier ram mass in the case of BLUE Piling. When the driving forces are expressed as a percentage of the pile yield limit, aforementioned figures respectively equate to 42% and 10%. Extrapolated over the full installation sequence, the latter would contribute to a reduction in the fatigue accumulated during installation.
The results presented here form the first step towards understanding the effect of blow duration soil-structure interaction for blow prolongation technology. For the set of installation parameters and boundary conditions considered in this study, it is shown that the differences in pile installation behavior can be captured using centrifuge modeling. The prolongation of blow duration results in a significantly different overall installation behavior. When looking at the driving forces, the decrease of the interface stiffness between the ram and anvil produces the anticipated decrease in peak driving force. A sustained physical modeling effort is required to ultimately lay the basis for a predictive installation framework for blow-prolonging technology, which would arguably accelerate its adoption by the industry. The latter should help the reek the associated benefits, particularly in terms of fatigue reduction and sound remediation in the near future.
The collapse of a granular column is a benchmark case for the study of granular materials. In this work, the collapse dynamics of two-dimensional columns of elongated grains are numerically investigated in dry and immersed conditions. The focus is set on the orientation of the grains and its influence on the behavior of the column. An energy-based approach suggests the influence of grain shape on mobility is limited to the prefactor of a power-law.
We present the late Holocene eruption history of the poorly known Doña Juana volcanic complex, in SW Colombia, which last erupted in the twentieth century. This represents a case study for potentially active volcanism in the rural Northern Andes, where tropical climate conditions and a fragmented social memory blur the record of dormant volcanoes. We reconstructed the volcanic stratigraphy of the central-summit vent area by integrating new mapping at 1:5000 scale with radiocarbon ages, sedimentology analysis, and historical chronicles. Our results revealed cyclic transitions from lava-dome growth phases and collapse to explosive Vulcanian and possibly subplinian phases. Pyroclastic density currents were generated by dome collapse producing block-and-ash flows or by pyroclastic fountain/column collapse and were rapidly channelized into the deeply incised fluvial valleys around the volcano summit. The pyroclastic density currents were ∼4−10 × 106 m3 in volume and deposited under granular flow− or fluid escape−dominated depositional regimes at high clast concentrations. In places, more dilute upper portions reached a wider areal distribution that affected the inhabited areas on high depositional terraces. The coefficient of friction (ΔH/L) is higher for block-and-ash flows and dense lava−bearing fountain/low-column-collapse pyroclastic density currents compared to pumice-bearing, column-collapse pyroclastic density currents. Associated mass-wasting processes included syneruptive and intereruptive debris flows, with the last one documented in 1936 CE.