Granular materials are composed of solid, discrete particles and exhibit mechanical behavior that differs from those of fluids and solids. The rheology of granular flows is principal to a suite of natural hazards. Laboratory experiments and numerical models have adequately reproduced several features observed in terrestrial gravity driven geophysical flows; however, quantitative comparison to field observations exposes a failure to explain the high mobility and duration of many of these flows. The ability of a granular material to resist deformation is a function of the force chain network inherent to the material. This investigation addresses the evolutionary character of force chains in unconfined, two-dimensional, gravity driven granular flows. Our particular emphasis concerns the effects of stress localization on the substrate by dynamic force chain evolution and the implications for bed erosion in dense granular flows. Experimental systems employing photoelastic techniques provide an avenue for quantitative force analysis via image processing and provide dataset that can be used validate discrete element modeling approaches. We show that force chains cause extreme bed force localization throughout dynamic granular systems in spatial and temporal space; and that these localized forces can propagate extensively into the substrate, even ahead of the flow front.

Granular materials are composed of solid, discrete particles and exhibit mechanical properties that range from fluid to solid behavior. Some of the complexity exhibited by granular systems arises due to the long-range order that develops due to particle-particle contact. Inter-particle forces in granular materials often form a distributive network of filamentary force-accommodating chains (i.e. force chains), such that a fraction of the total number of particles accommodates the majority of the forces in the system. The force chain network inherent to a system composed of granular materials controls the macroscopic behavior of the granular material. Force transmission by these filamentary chains is focused (or localized) to the grain scale at boundaries such as the granular flow substrate. Recent laboratory experiments have shown that force chains transmit extreme localized forces to the substrates of free surface granular flows. In this work we combine analog and numeric experimental approaches to investigate the forces at the bed of a simplified granular flow. A photoelastic experimental approach is used to resolve discrete forces in the granular flows. We also conduct discrete element method (DEM) simulations, using input parameters derived from measureable physical material properties of experimental and natural materials, which successfully reproduce the analog experimental results. This work suggests that force chain activity may play an unexpected and important role in the bed physics of dense granular flows through substrate modification by erosion and entrainment, and that DEM numerical methods effectively treat force chain processes in simulated granular flows.

京都大学 / 新制・課程博士 / 博士(理学) / 甲第24167号 / 理博第4858号 / 新制||理||1695(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 早川 尚男, 教授 佐々 真一, 教授 山本 潤 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

Les risques naturels tels que les laves torrentielles constituent des menaces réelles pourles zones urbanisées de montagne. Les bâtiments et infrastructures peuvent être exposésà de grandes forces d’impact en cas d’évènement extrême. La réduction de cette menace,par des ouvrages de protection, impose de quantifier l’impact de ces écoulements sur lesstructures, qu’elles soient flexibles ou rigides.Tout d’abord, un écoulement granulaire sec, composé de particules non-sphériquesglissant sur un plan incliné, est modélisé en utilisant une loi de contact visco-élastiqueavec critère de rupture de Mohr-Coulomb. Des données expérimentales de la littératureont été utilisé pour calibrer et valider le modèle. À cette fin, la forme de la particule,l’épaisseur de l’écoulement et la forme finale du dépôt sur le mur sont considerés. Lavalidation est basée sur l’impact sur un mur rigide divisé en six segments. La principalecontribution de la force totale normale appliquée sur le mur est due à la composantedynamiques. La distribution hétérogène de la force normale sur chaque partie du murest due au développement des chaînes de force différent pour chaque arrangement desparticules.Ensuite, un filet est modélisé en utilisant des éléments cylindriques. L’impact sur lefilet est modélisé en utilisant le même modèle d’écoulement que précédemment. Le rôledes dissipateurs d’énergie apparaît essentiel pour réduire la force d’impact sur le filet etlimiter la force appliquée sur les points d’ancrage latéraux.Pour la première fois, des simulations montrent que pour un même écoulementgranulaire la force d’impact est plus élevée pour un obstacle rigide, avec une différencede 50% par rapport à un obstacle flexible. les simulations permettent de définir quelquesrecommandation pour le dimensionnement des filets. Il est constaté que l’utilisationviiid’un maillage de filet plus petit que D90 de l’écoulement est acceptable en termes decapacité à retenir les matériaux en écoulement. En plus, si le câble en bas du filet n’estpas fixé, le filet pourrait perdre totalement sa capacité de retenue. / Natural hazards such as debris flows are real threat to the urbanization of mountainousareas. Local communities and infrastructures can be exposed to large impact forces inextreme debris events. Mitigation of such threats requires, along other measures, theestimation of the impact of such flows on protection structures (rigid walls and flexiblebarriers). In this thesis, Discrete Element Method (DEM) is used to model the granularflow, the rigid walls and flexible barriers.First, a dry granular flow made of non-spherical particles flowing in inclined plane ismodeled using a visco-elastic contact law with Mohr-Coulomb failure criterion. Experimentaldata from the literature is used to calibrate and validate the model. The modelis calibrated based on the shape of the particle, the flow thickness and the final shapeof the deposit on the wall. Validation procedure is based on the impact on a rigid walldivided into six segments. The main contribution of total normal force applied on thewall is found to be due to the dynamic component. On the micro-scale, development offorce chains is believed to cause heterogeneous distribution of normal force on each partof the wall, for multiple same-test conditions.Next, a flexible barrier is modeled using cylindrical elements. The impact on thebarrier is modeled using the same flow model used for wall-impact problem. The use ofenergy dissipators is found to be essential for minimizing the impact force on the barrier,and thus controlling the force applied on the lateral anchors.By comparing a rigid wall and a flexible barrier for the same flow, we found thatthe rigid wall is exposed to higher impact force, due its high global stiffness comparedwith the flexible barrier. Next, different simulations are carried out to recommend designguidelines for the flexible barrier. It is found that using a mesh size as large as D90 of theviflow is acceptable in terms of mass retaining capacity. In addition, not fixing the bottomcable of flexible barriers might lead to the total loss of its retaining capacity in extremeevents.