Spatiotemporal slope stability analytics for failure estimation (SSSAFE): linking radar data to the fundamental dynamics of granular failure

Impending catastrophic failure of granular earth slopes manifests distinct kinematic patterns in space and time. While risk assessments of slope failure hazards have routinely relied on the monitoring of ground motion, such precursory failure patterns remain poorly understood. A key challenge is the multiplicity of spatiotemporal scales and dynamical regimes. In particular, there exist a precursory failure regime where two mesoscale mechanisms coevolve, namely, the preferred transmission paths for force and damage. Despite extensive studies, a formulation which can address their coevolution not just in laboratory tests but also in large, uncontrolled field environments has proved elusive. Here we address this problem by developing a slope stability analytics framework which uses network flow theory and mesoscience to model this coevolution and predict emergent kinematic clusters solely from surface ground motion data. We test this framework on four data sets: one at the laboratory scale using individual grain displacement data; three at the field scale using line-of-sight displacement of a slope surface, from ground-based radar in two mines and from space-borne radar for the 2017 Xinmo landslide. The dynamics of the kinematic clusters deliver an early prediction of the geometry, location and time of failure

Numerical investigation of force transmission in granular media using discrete element method

In this paper, a numerical Discrete Element Method (DEM) model was calibrated to investigate the transmission of force in granular media. To this aim, DEM simulation was performed for reproducing the behavior of a given granular material under uniform compression. The DEM model was validated by comparing the obtained shear stress/normal stress ratio with results published in the available literature. The network of contact forces was then computed, showing the arrangement of the material microstructure under applied loading. The number and distribution of the contacts force were also examined statistically, showing that the macroscopic behavior of the granular medium highly depended on the force chain network. The DEM model could be useful in exploring the mechanical response of granular materials under different loadings and boundary conditions

Multiscale Characterisation of Diffuse Granular Failure

International audienceWe study the evolution of structure inside a deforming, cohesionless granular material undergoing failure in the absence of strain localisation – so-called diffuse failure. The spatio-temporal evolution of the basic building blocks for self-organisation (i.e. force chains and minimal contact cycles) reveals direct insights into the structural origins of failure. Irrespective of failure mode, self-organisation is governed by the cooperative behaviour of truss-like 3-cycles providing lateral support to column-like force chains. The 3-cycles, which are initially in scarce supply, form a minority subset of the minimal contact cycle bases. At large length-scales (i.e. sample size), these structures are randomly dispersed, and remain as such while their population progressively falls as loading proceeds. Bereft of redundant constraints from the 3-cycles, the force chains are initially just above the isostatic state, a condition that progressively worsens as the sample dilates. This diminishing capacity for redistribution of forces without incurring physical rearrangements of member particles renders the force chains highly prone to buckling. A multiscale analysis of the spatial patterns of force chain buckling reveals no clustering or localisation with respect to the macroscopic scale. Temporal patterns of birth-and-death of 3-cycles and 3-force chains provide unambiguous evidence that significant structural reorganisations among these building blocks drive rheological behaviour at all stages of the loading history. The near-total collapse of all structural building blocks and the spatially random distribution of force chain buckling and 3-cycles hint at a possible signature of diffuse failure

Soft grain compression: beyond the jamming point

We present the experimental studies of highly strained soft bidisperse granular systems made of hyperelastic and plastic particles. We explore the behavior of granular matter deep in the jammed state from local field measurement from the grain scale to the global scale. By mean of digital image correlation and accurate image recording we measure for each compression step the evolution of the particle geometries and their right Cauchy-Green strain tensor fields. We analyze the evolution of the usual macroscopic observables (stress, packing fraction, coordination, fraction of non-rattlers, \textit{etc}.) along the compression process through the jamming point and far beyond. We also analyze the evolution of the local strain statistics and evidence a crossover in the material behavior deep in the jammed state. We show that this crossover depends on the particle material. We argue that the strain field is a reliable observable to describe the evolution of a granular system through the jamming transition and deep in the dense packing state whatever is the material behavior.Comment: 10 figure

Experimental investigation of stick-slip behavior in granular materials

The mechanical behavior of granular materials is highly dependent on the arrangement of particles, particle groups, and associated pore spaces. Changes in the internal structure due to large deformation may cause changes in the mechanical behavior. The changes include: particle sliding, rolling, and interaction; shear band formation; and fabric anisotropy. During those changes, stick-slip behavior may take place between the granular particles. The objective of the thesis is to study the factors that influence the stick-slip behavior of granular materials. The influence of particle size, uniformity, confining pressure, density, and strain rate are investigated in this thesis. A series of axisymmetric triaxial tests were performed on glass beads to study the shear strength of granular materials. Sizes that were used are labeled as Very Small (d = 0.15 – 0.25 mm), Small (d = 0.75 – 1.00 mm), Medium (d = 1.55 – 1.85 mm), Large (d = 3.30 – 3.60 mm), and Well-graded (d = 0.09 – 1.55 mm). The confining pressures were 25, 100, 250, and 400 kPa. The load oscillations that appeared in the stress-strain results were analyzed to find the causes of the stick-slip behavior. To study the internal structure of the particles, two axisymmetric triaxial tests were performed on the glass beads under low confining pressure (25 kPa). The specimens were composed of Very Small, Medium, and Well-graded. The specimens were scanned before and after compression using a X-ray computed tomography system. In general, a slight post peak principle stress softening was observed as well as a continuous volume increase (dilation) even at relatively high strains. This appears to be caused by the uniform shape of the spherical particles. The load oscillations that appeared in the very small, small, and well-graded beads are due to the stick-slip phenomenon. From the computed tomography analysis, the specimens showed a bulging deformation mode. This is because the particles roll each other continuously during compression; they do not interlock. In the medium beads after compression, columns of beads were found in the specimen to support the theory of the stick-slip behavior

A nonlinear Lagrangian particle model for grains assemblies including grain relative rotations

International audienceWe formulate a discrete Lagrangian model for a set of interacting grains, which is purely elastic. The considered degrees of freedom for each grain include placement of barycenter and rotation. Further, we limit the study to the case of planar systems. A representative grain radius is introduced to express the deformation energy to be associated to relative displacements and rotations of interacting grains. We distinguish inter‐grains elongation/compression energy from inter‐grains shear and rotations energies, and we consider an exact finite kinematics in which grain rotations are independent of grain displacements. The equilibrium configurations of the grain assembly are calculated by minimization of deformation energy for selected imposed displacements and rotations at the boundaries. Behaviours of grain assemblies arranged in regular patterns, without and with defects, and similar mechanical properties are simulated. The values of shear, rotation, and compression elastic moduli are varied to investigate the shapes and thicknesses of the layers where deformation energy, relative displacement, and rotations are concentrated. It is found that these concentration bands are close to the boundaries and in correspondence of grain voids. The obtained results question the possibility of introducing a first gradient continuum models for granular media and justify the development of both numerical and theoretical methods for including frictional, plasticity, and damage phenomena in the proposed model

ENERGY DISSIPATION IN A SAND DAMPER UNDER CYCLIC LOADING

Various seismic and wind engineering designs and retrofit strategies have been in development to meet structures\u27 proper and safe operation during earthquake and wind excitation. One such method is the addition of fluid and particle dampers, such as sand dampers, in an eort to reduce excessive and dangerous displacements of structures. The present study implements the discrete element method (DEM) to assess the performance of a pressurized sand damper (PSD) and characterize the dissipated energy under cyclic loading. The idea of a PSD is to exploit the increase in shearing resistance of sand under external pressure and the associated ability to dissipate energy through interparticle contact sliding. The dissipated energy in the pressurized sand during cyclic motion results in a reduction of excessive displacement. The advantage of using the DEM is that applying a simple linear contact model for the entire contacts assembly and also utilizing the advantage of irregular-shaped particles to mimic the behavior of actual sand grains. The series of DEM simulations reported herein examine the effects of multiple factors on the magnitude of dissipated energy. These factors include stroke amplitude, grain size distribution, the magnitude of pressure imposed on the sand, and different configurations of the PSD. The results reveal that the main energy dissipation mechanism is generated through interparticle frictional sliding in the sand. Additionally, the magnitude of cumulative dissipated energy increases with the pressure level applied to the sand damper, as well as with the stroke amplitude of the loading. Moreover, operating the piston with multiple spheres leads to a significant increase in the magnitude of dissipated energy. However, the soil exhibits similar behavior to the case of one sphere where a strain hardening behavior was noticed. A noticeable increase in the piston capacity was observed when the sphere size was increased by 10%, and the rest of the response patterns remained unchanged. According to the results, by increasing the sphere friction, the piston capacity remains almost the same. It is also worth mentioning that when a wider range of particle sizes was employed, the capacity of the maximum force considerably increased. A significant increase in the piston capacity was clearly noticed when a boxed-shaped piston configuration was utilized at the origin of the pressurized sand damper instead of a single sphere. The results of the conducted simulations were quantitatively compared with experimental data obtained from physical modeling of a similar pressurized sand damper which revealed a fairly good agreement. This confirms the ability of the proposed framework to satisfactorily analyze complex geotechnical problems involving soil interaction and large deformations. The proposed sand damper model is shown to be a promising device that mitigates vibrations in structural systems subject to seismic and wind loading