Éditeur(s) : HAL CCSD
Résumé : The present PhD thesis is devoted to the numerical simulation and analysis of shear on cohesive granular materials. In the framework of the Contact Dynamics method, we developed a model of cohesive bonding, which mimics cohesion by cementation. This model involves three contact strength parameters (a tensile strength, a coefficient of sliding friction, and a coefficient of rolling friction) as well as a local criterion, which controls the creation and the breaking of cohesive bonds. All contacts are cohesive at the beginning of the numerical simulations, and contact rupture leads to an irreversible loss of tensile strength (debonding). We varied systematically the three contact strength parameters and studied their influence on the shear strength, the dilatancy, and the microstructure of the granular media. Our results show that internal friction and macroscopic tensile strength have different origins at the contact scale. We also find that the microstructure of the granular media, when sheared at the steady state (also known as the `critical state"), is strongly affected by the local coefficients of sliding and rolling friction. In particular, the mechanical role of the weak network changes with these two contact parameters, which results from the formation of a new structure of force transmission. On the other hand, we also applied the same numerical simulation method to the simulation of landslides. We developed a methodology, which takes into account the initial morphology of the hillslope, the strength of geomaterials, and a triggering factor, integrating in the same model the initiation and propagation phases of the landslide. To illustrate this methodology, we applied it to the simulation of a hypothetical example of avalanche triggered by the progressive rise of a water table.
Ce travail de thèse porte sur la modélisation numérique et l'analyse du cisaillement dans les milieux granulaires cohésifs. Dans le cadre de la méthode de la Dynamique des Contacts, nous avons développé un modèle de cohésion qui cherche à imiter la cohésion par cimentation. Ce modèle fait intervenir trois paramètres de résistance au niveau des contacts (une résistance à la traction, un coefficient de frottement de glissement, et un coefficient de frottement de roulement) ainsi qu'un critère de création et de rupture des liens cohésifs. Les contacts sont cohésifs au début des simulations et leur rupture entraîne la perte irréversible de la résistance à la traction. Nous avons fait varier les trois paramètres microscopiques et nous avons étudié leur influence dans la résistance au cisaillement, la dilatance, et la microstructure du milieu. Nos résultats montrent que le frottement interne et l'adhésion macroscopiques ont des origines microscopiques différentes. Nous observons aussi que la microstructure du milieu cisaillé à l'état stationnaire (aussi connu comme ``l'état critique") est fortement affectée par l'angle de frottement et la résistance au roulement au niveau des contacts. En particulier, le rôle mécanique du réseau faible change avec ces deux paramètres microscopiques, ce qui résulte de la formation d'une nouvelle structure de transmission de forces. D'autre part, nous avons appliqué cette méthode de simulation numérique à l'étude d'un processus naturel : les glissements de terrain. Nous avons développé une méthodologie qui tient compte de la morphologie initiale du système, de la résistance des géomatériaux, et d'un mécanisme de déclanchement, ce qui permet d'intégrer dans le même modèle les phases d'initiation et de propagation du glissement. Pour illustrer cette méthodologie, nous l'avons appliquée à la modélisation d'un exemple hypothétique d'avalanche déclenchée par la montée progressive du niveau d'une nappe d'eau.
https://tel.archives-ouvertes.fr/tel-00311303

tel-00311303
https://tel.archives-ouvertes.fr/tel-00311303
https://tel.archives-ouvertes.fr/tel-00311303/document
https://tel.archives-ouvertes.fr/tel-00311303/file/these_estrada.pdf

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
We present a 2-D Contact Dynamics discrete element model for simulating initiation and motion of rock avalanches, integrating hillslope geometry, Mohr-Coulomb rock behavior, pore pressure before avalanche triggering, and avalanche trigger. Avalanche motion is modeled as a dense granular flow of dry frictional and cohesive particles. On the basis of granular physics and shear experiments, we review some of the theories for the unexpectedly long runout of rock avalanches. Different causes are evoked, according to the strength (strong or weak) of the slip surface relative to the bulk. "Mechanical fluidization'' and "acoustic fluidization'' theories state that agitation of rock particles reduces frictional strength, increasing runout. Conversely, granular mechanics suggests that, as "shear-strain'' rate increases, granular material becomes more agitated, more dissipative, and more resistant. Another theory states that dynamic fragmentation of clasts creates an isotropic pressure that drives longer runout. In contrast, granular mechanics suggests that fragmentation may induce fluidization and strengthening of the granular material, while particle size reduction (among others) induces weakening of the granular flow and enhances long runout. Runout is also enhanced for column-like rock masses collapsing from steep hillslopes. Long runout may also be linked to thermal weakening mechanisms at the slip surface (e.g., thermal pressurization, and shear melting), which may lower drastically the shear strength. The model is illustrated with a hypothetical example of a rain-triggered avalanche, mobilizing shallowly dipping layers. Several phases are identified, including slope failure, avalanche triggering resulting from slip weakening, and avalanche motion in which rocks are folded and sheared.
ISSN: 0148-0227

hal-00420883
https://hal.archives-ouvertes.fr/hal-00420883
DOI : 10.1029/2008JF001072

Éditeur(s) : HAL CCSD Elsevier
Résumé : This paper presents a numerical investigation on the mechanical behavior of cemented granular materials by means of discrete element simulations. In particular, two conceptual elements of this behavior were explored: the yield surfaces and the plastic potentials, as defined in the framework of elastoplastic constitutive models. For this purpose, we implemented a contact model that mimics the mechanical behavior of cemented bonds, we constructed a set of polydisperse samples with different solid fractions, and we tested these samples by means of a biaxial device with four rigid walls. The yield surfaces and plastic potentials were thus obtained and analyzed as functions of the solid fraction of the material, drawing interesting conclusions, confirming previous experimental findings, and, consequently, evidencing the great potential of discrete element simulations in exploring the behavior of real cemented geomaterials.
ISSN: 0266-352X

hal-00853748
https://hal.archives-ouvertes.fr/hal-00853748
DOI : 10.1016/j.compgeo.2012.11.001

Éditeur(s) : HAL CCSD Bureau de recherches géologiques et minières
Résumé : Du relief au bassin
ISSN: 0292-8477

hal-00795450
https://hal.archives-ouvertes.fr/hal-00795450

Éditeur(s) : HAL CCSD
Résumé : International audience
In this paper, we compare the effect of rolling resistance at the contacts in granular systems composed of disks with the effect of angularity in granular systems composed of regular polygonal particles. For this purpose, we use contact dynamics simulations. By means of a simple shear numerical device, we investigate the mechanical behavior of these materials in the steady state in terms of shear strength, solid fraction, force and fabric anisotropies, and probability distribution of contact forces. We find that, based on the energy dissipation associated with relative rotation between two particles in contact, the effect of rolling resistance can explicitly be identified with that of the number of sides in a regular polygonal particle. This finding supports the use of rolling resistance as a shape parameter accounting for particle angularity and shows unambiguously that one of the main influencing factors behind the mechanical behavior of granular systems composed of noncircular particles is the partial hindrance of rotations as a result of angular particle shape.
AIP Conference Proceedings
Sydney, Australia

hal-00842799
https://hal.archives-ouvertes.fr/hal-00842799
https://hal.archives-ouvertes.fr/hal-00842799/document
https://hal.archives-ouvertes.fr/hal-00842799/file/Comparison_effects_rolling_Estrada_al.pdf

Éditeur(s) : HAL CCSD
Résumé : National audience
Colloque annuel du GdR MeGe
La Rochelle, France

hal-00844261
https://hal.archives-ouvertes.fr/hal-00844261

Éditeur(s) : HAL CCSD Geological Society
Résumé : International audience
Deformation mechanisms, long-term kinematics and evolution of fold and thrust belts subjected to erosion are studied through 2D analogue experiments involving large convergence. First-order parameters tested include (1) décollements and/or plastic layers interbedded at different locations within analogue materials and (2) synconvergence surface erosion. Weak layers, depending on their location in the model, favour deformation partitioning characterized by the simultaneous development of underplating domains in the inner part of the wedge (basal accretion) and frontal accretion where the wedge grows forward. Interaction between tectonics and surface processes influences this behaviour. Development of antiformal thrust stacks controlled by underplating shows small- and large-scale cyclicity. Thin plastic layers induce folding processes, which are studied at wedge scale. Recumbent and overturned folds, with large inverted limbs, develop in a shear-induced asymmetric deformation regime via progressive unrolling of synclinal hinges. Surface erosion and underplating at depth induce further rotation (passive tilting) and horizontalization of fold limbs. Model results give insights to discuss the mechanisms responsible for the large-scale structures (i.e. antiformal nappe stacks, klippen and kilometre-scale recumbent fold-nappes) encountered in several mountain belts such as the Montagne Noire (French Massif Central), the Galicia Variscan belt (Spain) and the northern Apennines (Italy).
ISSN: 0016-7649

hal-00937644
https://hal.archives-ouvertes.fr/hal-00937644
DOI : 10.1144/jgs2013-012

Éditeur(s) : HAL CCSD American Physical Society
Résumé : International audience
This is the first of two papers investigating the mechanical response of cemented granular materials by means of contact dynamics simulations. In this paper, a two-dimensional polydisperse sample with high-void ratio is constructed and then sheared in a simple shear numerical device at different confinement levels. We study the macroscopic response of the material in terms of mean and deviatoric stresses and strains. We show that the introduction of a local force scale, i.e., the tensile strength of the cemented bonds, causes the material to behave in a rigid-plastic fashion, so that a yield surface can be easily determined. This yield surface has a concave-down shape in the mean: deviatoric stress plane and it approaches a straight line, i.e., a Coulomb strength envelope, in the limit of a very dense granular material. Beyond yielding, the cemented structure gradually degrades until the material eventually behaves as a cohesionless granular material. Strain localization is also investigated, showing that the strains concentrate in a shear band whose thickness increases with the confining stress. The void ratio inside the shear band at the steady state is shown to be a material property that depends only on contact parameters.
ISSN: 1539-3755

hal-00523453
https://hal.archives-ouvertes.fr/hal-00523453
DOI : 10.1103/PhysRevE.82.011303

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Orogenic wedges locally present chaotic tectonostratigraphic units that contain exotic blocks of various size, origin, age and lithology, embedded in a sedimentary matrix. The occurrence of ophiolitic blocks, sometimes huge, in such “mélanges” raises questions on (i) the mechanisms responsible for the incorporation of oceanic basement rocks into an accretionary wedge and (ii) the mechanisms allowing exhumation and redeposition of these exotic elements in “mélanges” during wedge growth.To address these questions, we present the results of a series of analogue experiments performed to characterize the processes and parameters responsible for accretion, exhumation and tectonosedimentary reworking of oceanic basement lithospheric fragments in an accretionary wedge.The experimental setup is designed to simulate the interaction between tectonics, erosion and sedimentation. Different configurations are applied to study the impact of various parameters, such as irregular oceanic floor due to structural inheritance, or the presence of layers with contrasted rheology that can affect deformation partitioning in the wedge (frontal accretion vs basal accretion) influencing its growth. Image correlation technique allows extracting instantaneous velocity field, and tracking of passive particles. By retrieving the particle paths determined from models, the pressure-temperature path of mélange units or elementary blocks can be discussed. The experimental results are then compared with observations from ophiolite-bearing mélanges in Taiwan (Lichi and Kenting mélanges) and Raman spectroscopy of carbonaceous material (RSCM) Thermometry data on rocks from the northern Apennines (Casanova mélange). A geological scenario is proposed following basic observations. The tectonic evolution of the retroside of doubly vergent accretionary wedges is mainly controlled by backthrusting and backfolding. The retro wedge is characterized by steep slopes that are prone to gravitational instabilities. It triggers submarine landslides inducing huge mass transfers. This erosion combined with backthrusting could favour exhumation of the ophiolitic fragments formerly accreted at the base of the wedge along the rough seafloor-sediments interface. Such an exhumed material can be reworked and deposited as debris- flows in proximal basins located at the foot of the retrowedge slope forming a tectono-sedimentary mélange. These syntectonic basins are continuously deformed and involved in prograding backthrusting-induced deformation.
ISSN: 0264-3707

hal-01395127
https://hal.archives-ouvertes.fr/hal-01395127
DOI : 10.1016/j.jog.2016.05.008

Éditeur(s) : HAL CCSD
Résumé : Time-series bathymetric data acquired between 1991 and 2011 have been used to evaluate the recent morphological evolution of the Nice upper continental slope (SE France, Ligurian Sea). Small-scale landslides lead to a retrogressive evolution of the continental shelf/upper slope transition but their frequency, size and impact are not well known. Mapping was undertaken to identify the morphology of landslide scarps and the location of the shelfbreak. Map comparisons were performed using ArcGIS “raster calculator”. Sediment remobilization on the upper slope (up to depths of 200 m) is fast and significant; landslide scars with volumes greater than 25,000 m3 can appear with a frequency less than 8 years. Shelfbreak migration toward the coastline can reach rates of 60 m over 7–8 years where the continental shelf is over 200 m wide. Furthermore, this quantitative analysis highlights alternations between periods of strongly erosive events and sedimentation periods. On the upper slope, eroded volumes can be multiplied by 10 during periods of enhanced landslide patterns (1999–2006). Such cycle-like landslide activity raises the issue of the triggering processes. On the Nice continental slope thick poorly consolidated beds rapidly deposited on a steep slope, earthquakes and rainfall leading to fresh water circulation below the shelf were identified as potential triggers. Our 4D bathymetric study suggests that over the last 20 years the greatest impacting factor may be freshwater outflows.
Engineering Geology for Society and Territory - Volume 4

hal-01218436
https://hal.archives-ouvertes.fr/hal-01218436
DOI : 10.1007/978-3-319-08660-6_42

Éditeur(s) : HAL CCSD American Physical Society
Résumé : International audience
This is the second of two papers investigating the mechanical response of cemented granular materials by means of contact dynamics simulations. In this paper, a two-dimensional polydisperse sample with high void ratio is sheared in a load-controlled simple shear numerical device until the stress state of the sample reaches the yield stress. We first study the stress transmission properties of the granular material in terms of the fabric of different subsets of contacts characterized by the magnitude of their normal forces. This analysis highlights the existence of a peculiar force carrying structure in the cemented material, which is reminiscent of the bimodal stress transmission reported for cohesionless granular media. Then, the evolution of contact forces and torques is investigated trying to identify the micromechanical conditions that trigger macroscopic yielding. It is shown that global failure can be associated to the apparition of a group of particles whose contacts fulfill at least one of the local rupture conditions. In particular, these particles form a large region that percolates through the sample at the moment of failure, evidencing the relationship between macroscopic yielding and the emergence of large-scale correlations in the system.
ISSN: 1539-3755

hal-00523451
https://hal.archives-ouvertes.fr/hal-00523451
DOI : 10.1103/PhysRevE.82.011304

Éditeur(s) : HAL CCSD American Physical Society
Résumé : International audience
We investigate a class of granular materials characterized by the possibility of interlocking between the particles. The interlocking is modeled by its effect through rolling resistance depending on relative rotation and normal force at the contact points and involving a single parameter analogous to the sliding friction coefficient. The model, which is introduced in the framework of the contact dynamics, method, is applied to simulate the simple shear of a large granular sample. We present a detailed analysis regarding the influence of rolling and sliding friction parameters on the macroscopic response in terms of shear strength, fabric properties, and force transmission. Interestingly, two distinct regimes can be distinguished in which the steady-state shear strength is controlled by either rolling resistance or sliding friction. The relative contributions of rolling and sliding contacts to the shear strength are consistent with the same two regimes. Interlocking strongly affects the force network by enhancing the arching effect and thus increasing the relative importance of weak contact forces and torques, which is reflected in a decreasing power-law probability distribution of the contact forces and torques below the mean. Due to the combined effect of friction and interlocking, the force-carrying backbone takes an increasingly columnar aspect involving a low fraction of particles. Our data suggest that the nature of the weak contact network is strongly affected by the formation of these columns of particles which do not need to be propped laterally. In particular, in the limit of high rolling resistance and sliding friction, the role of the weak network of contacts is no longer to prop the force chains, but, like the strong contact network, to actively sustain the deviatoric load imposed on the system.
ISSN: 1539-3755

hal-00412020
https://hal.archives-ouvertes.fr/hal-00412020
DOI : 10.1103/PhysRevE.78.021301

Éditeur(s) : HAL CCSD
Résumé : International audience
Powders and Grains 2009
Golden, United States

hal-00760571
https://hal.archives-ouvertes.fr/hal-00760571

Éditeur(s) : HAL CCSD American Physical Society
Résumé : International audience
Using contact dynamics simulations, we compare the effect of rolling resistance at the contacts in granular systems composed of disks with the effect of angularity in granular systems composed of regular polygonal particles. In simple shear conditions, we consider four aspects of the mechanical behavior of these systems in the steady state: shear strength, solid fraction, force and fabric anisotropies, and probability distribution of contact forces. Our main finding is that, based on the energy dissipation associated with relative rotation between two particles in contact, the effect of rolling resistance can explicitly be identified with that of the number of sides in a regular polygonal particle. This finding supports the use of rolling resistance as a shape parameter accounting for particle angularity and shows unambiguously that one of the main influencing factors behind the mechanical behavior of granular systems composed of noncircular particles is the partial hindrance of rotations as a result of angular particle shape.
ISSN: 1539-3755

hal-00594891
https://hal.archives-ouvertes.fr/hal-00594891
https://hal.archives-ouvertes.fr/hal-00594891/document
https://hal.archives-ouvertes.fr/hal-00594891/file/Estrada2010ReSubmitV2.pdf
ARXIV : 1105.4418

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The analysis of three cataclastic band sets from Provence (France) reveals that the band density, their conjugate angles, their ratio of shear displacement to compaction, and the amount of cataclasis within the bands differ and can be expressed as functions of tectonic setting and petrophysical properties. We identify (1) a dense and closely spaced network of shear-enhanced (reverse) compaction bands; (2) a regularly spaced less dense network of reverse compactional shear bands; and (3) a localized network of normal shear bands. The field data show that strain localization is favored in an extensional regime and is characterized by shear bands with a large shear to compaction ratio and a small conjugate band angle. In contrast, distributed strain is favored in a contractional regime and is characterized by compactional bands with a low ratio of shear to compaction and a large conjugate band angle. To explain the mechanical origin of this strain localization, we quantify the yield strength and the stress evolution in extensional and contractional regimes in a frictional porous granular material. We propose a model of strain localization in porous sands as a function of tectonic stresses, burial depth, material properties, strain hardening and fluid pressure. Our model suggests that stress reduction, inherent to extensional regime, favors strain localization as shear bands, whereas stress increase during contraction favors development of compactional bands.
ISSN: 0191-8141

hal-00856921
https://hal.archives-ouvertes.fr/hal-00856921
DOI : 10.1016/j.jsg.2012.11.011

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
We present Contact Dynamics discrete element simulations of the earthquake-triggered Jiufengershan avalanche, which mobilized a 60 m thick, 1.5 km long sedimentary layer, dipping similar to 22 degrees SE toward a valley. The dynamic behavior of the avalanche is simulated under different assumptions about rock behavior, water table height, and boundary shear strength. Additionally, seismic shaking is introduced using strong motion records from nearby stations. We assume that seismic shaking generates shearing and frictional heating along the surface of rupture, which, in turn, may induce dynamic weakening and avalanche triggering; a simple "slip-weakening'' criterion was adopted to simulate shear strength drop along the rupture surface. We investigate the mechanical processes occurring during triggering and propagation of an avalanche mobilizing shallowly dipping layers. Incipient deformation forms a pop-up structure at the toe of the dip slope. As the avalanche propagates, the pop-up deforms into an overturned fold, which overrides the surface of separation along a decollement. Simultaneously, uphill layers slide at high velocity (125 km/h) and are folded and disrupted as they reach the toe of the dip slope. The avalanche foot forms a wedge that is pushed forward as deformed rocks accrete at its rear. We simulated five cross sections across the Jiufengershan avalanche, which differ in the geometry of the surface of separation. Topographic and simulated surface profiles are similar. The friction coefficient at the surface of separation determined from back analysis is abnormally low (mu(SS) = 0.2), possibly due to lubrication by liquefied soils. The granular deposits of simulated earthquake- and rain-triggered avalanches are similar.
ISSN: 0148-0227

hal-00420882
https://hal.archives-ouvertes.fr/hal-00420882
DOI : 10.1029/2008JF001075