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Reduction of Landslide Shear Resistance by Gravel Fragmentation: Insights from DEM Modelling
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A novel hierarchical multiscale model has been applied to simulate the thick-walled hollow cylinder tests in dry sand and to investigate the corresponding shear failures. The combined FEM/DEM model employs the finite element method (FEM) as a vehicle to advance the solution for a macroscopic nonlinear boundary value problem incrementally. It is meanwhile free of conventional macroscopic phenomenological constitutive law which is replaced by discrete element simulations conducted in representative volume elements (RVEs) associated with the Gauss quadrature points of the FEM mesh. Our numerical simulations indicate that this multiscale approach is capable of replicating the evolution of cavity pressure during cavity expansion, before and after the onset of strain localisation, in qualitative agreement with laboratory tests. In particular, the curvilinear shear bands observed from experiments have been reproduced numerically. The information provided by both the mesoscale DEM and the macroscale FEM reveals a close linkage between significant particle rotations taking place inside the dilative shear bands and the highly anisotropic microstructural attributes of the associated RVEs.
This paper presents a composite particle model with sphere clumps for modelling non-spherical particles in discrete element method (DEM) simulations. The formulation of the sphere clumps uses random selections of particle radii and positions, thus it can theoretically simulate all possible non-spherical particles encountered in engineering projects. In this study, the generation of sphere clumps in space was discussed in detail, and different particle non-sphericities were compared. This numerical model has been employed to study the shear behaviour of granular assemblies under undrained triaxial compression conditions. It is evident that the use of composite particle model in the DEM can largely increase the shear strengths of granular assemblies. For granular samples consisting of various types of sphere clumps, different mechanical responses have been observed. In particular, the sphere clumps with high non-sphericity can lead to very high peak/residual shear strength, and high material internal friction angle. The use of sphere clumps mixture can modulate the shear behaviour, with its mechanical properties being close to those of real quartz sand grains.
The discrete element method has been employed to simulate vertical one-dimensional compression of an idealised soil. Direct measurement of the full stress tensor was possible and the results show that K 0 (the ratio of horizontal to vertical effective stresses) increases with void ratio, which is consistent with previous experimental studies. The anisotropic fabric induced during compression was quantified by considering the orientations and magnitudes of the normal contact forces. For the denser samples there was a definite bias towards more vertically oriented contacts, resulting in lower stresses being transmitted in the horizontal direction for a given vertical stress. In contrast, the contacts were oriented more isotropically in the looser samples, allowing more similar stresses to be transmitted in the horizontal and vertical directions.
This paper investigates the generation of hydrodynamic water waves due to rockslides plunging into a water reservoir. Quasi-3D DEM analyses in plane strain by a coupled DEM-CFD code are adopted to simulate the rockslide from its onset to the impact with the still water and the subsequent generation of the wave. The employed numerical tools and upscaling of hydraulic properties allow predicting a physical response in broad agreement with the observations notwithstanding the assumptions and characteristics of the adopted methods. The results obtained by the DEM-CFD coupled approach are compared to those published in the literature and those presented by Crosta et al. (Landslide spreading, impulse waves and modelling of the Vajont rockslide. Rock mechanics, 2014) in a companion paper obtained through an ALE-FEM method. Analyses performed along two cross sections are representative of the limit conditions of the eastern and western slope sectors. The max rockslide average velocity and the water wave velocity reach ca. 22 and 20 m/s, respectively. The maximum computed run up amounts to ca. 120 and 170 m for the eastern and western lobe cross sections, respectively. These values are reasonably similar to those recorded during the event (i.e. ca. 130 and 190 m, respectively). Therefore, the overall study lays out a possible DEM-CFD framework for the modelling of the generation of the hydrodynamic wave due to the impact of a rapid moving rockslide or rock-debris avalanche.
This paper presents a numerical investigation of the behaviour of dry granular flows generated by the collapse of prismatic columns via 3D Discrete Element Method (DEM) simulations in plane strain conditions. Firstly, by means of dimensional analysis, the governing parameters of the problem are identified, and variables are clustered into dimensionless independent and dependent groups.
Secondly, the results of the DEM simulations are illustrated. Different regimes of granular motion were observed depending on the initial column aspect ratio. The profiles observed at different times for columns of various aspect ratios show to be in good agreement with available experimental results.
Thirdly, a detailed analysis of the way energy is dissipated by the granular flows was performed. It emerges that most of the energy of the columns is dissipated by inter-particle friction, with frictional dissipation increasing with the column aspect ratio. Also, the translational and rotational components of the kinetic energy of the flows, associated to particle rotational and translational motions respectively, were monitored during the run-out process. It is found that the rotational component is negligible in comparison with the translational one; hence in order to calculate the destructive power of a granular flow slide, only the translational contribution of the kinetic energy is relevant.
Finally, a methodology is presented to calculate the flux of kinetic energy over time carried by the granular flow through any vertical section of interest. This can be related to the energy released by landslide induced granular flows impacting against engineering structures under the simplifying assumption of neglecting all structure-flow interactions. This represents the first step towards achieving a computational tool quantitatively predicting the destructive power of a given flow at any location of interest along its path. This could be useful for the design of engineering works for natural hazard mitigation. To this end, also the distribution of the linear momentum of the flow over depth was calculated. It emerges that the distribution is initially bilinear, due to the presence of an uppermost layer of particles in an agitated loose state, but after some time becomes linear.
This type of analysis showcases the potential of the Distinct Element Method to investigate the phenomenology of dry granular flows and to gather unique information currently unachievable by experimentation.
One of the ultimate goals in landslide hazard assessment is to predict maximum landslide extension and velocity. Despite much work, the physical processes governing energy dissipation during these natural granular flows remain uncertain. Field observations show that large landslides travel over unexpectedly long distances, suggesting low dissipation. Numerical simulations of landslides require a small friction coefficient to reproduce the extension of their deposits. Here, based on analytical and numerical solutions for granular flows constrained by remote-sensing observations, we develop a consistent method to estimate the effective friction coefficient of landslides. This method uses a constant basal friction coefficient that reproduces the first-order landslide properties. We show that friction decreases with increasing volume or, more fundamentally, with increasing sliding velocity. Inspired by frictional weakening mechanisms thought to operate during earthquakes, we propose an empirical velocity-weakening friction law under a unifying phenomenological framework applicable to small and large landslides observed on Earth and beyond.
Two puzzling traits of giant rock avalanches (sturzstroms) are the decrease of the effective friction coefficient as a function of the volume (volume effect) and the remarkable preservation of large geological structures during the flow, demonstrating that the upper cap of a sturzstrom travels coherently on top of a basal shear layer. Hence, frictional heat is rapidly produced along the shear layer, which could explain the formation of sheets of molten rock inside certain landslide deposits. It has been conjectured that a molten layer could potentially self-lubricate the base of the sturzstrom. To theoretically investigate this scenario, we consider the model of a rock slab sliding on an inclined surface. We present a set of coupled differential equations to calculate the frictional heat produced, the properties of the molten layer (thickness, temperature, and velocity distribution), and the motion of the slab. Our simulations illustrate the onset of self-lubrication and show the volume effect when the melt viscosity is low, corresponding to a simulated mafic composition of the rock. For a felsic composition (and to some extent also for intermediate melts) we find that the melting introduces more resistance at the beginning of the melting process, in close similarity with frictional melting in tectonic faults. However, in contrast to faults, the rock avalanche is capable of overcoming the initial resistance in at least two situations: if the rock is rigid and the landslide is sufficiently thick or else if the material of the landslide is disintegrated. The simulations also show that although self-lubrication is a viable possibility to explain the runout of sturzstroms, there are rather stringent conditions for the formation of a molten layer of good lubricating qualities. More generally, we suggest that the properties of the Coulomb frictional law at the interfaces may change radically during sliding and that the assumption of constant friction does not represent a good model in landslide calculations.
Peculiar properties of cemented geomaterials are endowed by inter-particle bonds, of which the contact behaviour has recently been found to be size dependent. This study uses the distinct-element method to investigate the size effect on the compressive resistance of an inter-particle bond, which is modelled as an assemblage of tiny bonded particles. The model parameters are carefully calibrated according to laboratory observation of a realistic reference material. The simulations demonstrate that the compressive resistance of the bond largely depends on bond slenderness and boundary curvature as well as the bond contact area. The combined effect of three size parameters can be captured in a single formula. Given the bond contact area in common, a thin bond between large particles tends to fail at higher compressive force than a thick bond between small particles because the coalescence of micro-cracks inside the bond is hindered more effectively in the thin case, resulting in ultimate failure due to compressive crushing rather than tensile cracking. A full consideration of the size effect on the bond contact strength may change the picture of progressive bond breakage in cemented geomaterials, which deserves further investigation once a size-dependent bond contact law is obtained.
Large rockslide-debris avalanches, resulting from flank collapses that shape volcanoes and mountains on Earth and other object of the solar system, are rapid and dangerous gravity-driven granular flows that travel abnormal distances. During the last 50 years, numerous physical models have been put forward to explain their extreme mobility. The principal models are based on fluidization, lubrication, or dynamic disintegration. However, these processes remain poorly constrained. To identify precisely the transport mechanisms during debris avalanches, we examined morphometric (fractal dimension and circularity), grain size, and exoscopic characteristics of the various types of particles (clasts and matrix) from volcanic debris avalanche deposits of La Réunion Island (Indian Ocean). From these data we demonstrate for the first time that syn-transport dynamic disintegration continuously operates with the increasing runout distance from the source down to a grinding limit of 500 µm. Below this limit, the particle size reduction exclusively results from their attrition by frictional interactions. Consequently, the exceptional mobility of debris avalanches may be explained by the combined effect of elastic energy release during the dynamic disintegration of the larger clasts and frictional reduction within the matrix due to interactions between the finer particles.
1 Introduction.- 2 Case Histories, Geomorphological Facts.- 3 Comments on Mechanisms of Release.- 4 Mechanisms of Disintegration.- 5 Mechanisms of Displacement.- 6 From Analysis to Prediction.- 7 Secondary Effects.- Review of Highlights.- References.