Improving the design of rockfall protection fences using a reliability-based approach

National audienceThis study proposes a reliability-based approach for the design of rockfall protection fences. From rockfall simulations using the code Rockyfor3D is deduced the probabilistic modelling of the impact angle and impact velocity of rocks. A discrete element model represents a low energy fence (software Yade-dem). The probability for the failure of the fence is evaluated, on a real study case, using a collocation method, based on a limited number of impact simulations

The effect of felled tree stems as bio-engineering type rockfall protection

International audienceIn mountainous regions forested slopes play an important protective role against rockfall. Up to now, most of the researches on rockfall protection forest have been and are focused on the dissipative effect of standing and living trees. There are hardly any studies on the protective capacity of tree stumps and lying stems against snow avalanches and rockfalls. Although, these techniques are more and more used throughout the Alps. In Austria, the felling technique Alpi has been developed, which allows a specialised lumberjack to create small rockfall barriers using one or two tree stems anchored on high tree stumps. Lying tree stems can be then used to increase efficiently the roughness of the soil and so to limit or avoid triggering and propagation falling rocks. But, due to the wood decay, the efficiency of such bio-engineering type protective works is decreasing with time. One of the questions that the forest and natural hazard managers have to answer is: what is the lifetime of such protective structures? In order to answer to this question we have developed a specific research on this thematic. The main objectives of this research program are to quantify the efficacy of these bio-engineering type rockfall fences depending on their characteristics (stump and stems density, position on the slope, tree species, etc.), and to evaluate their resistance over time. To achieve these objectives we have developed two types of experiments. The first one, performed during the summer 2009 on our experimental site test of Vaujany (France), are full scale rockfall experiments on four felled trees, which have been anchored on their stumps and are lying in an oblique direction to the slope. In total, fifty rocks (from 522 at 2242 kg) have been released one by one. For each rock, the trajectory has been filmed with high speed digital cameras. The second type of experiment is the uprooting of tree stumps (spruce, fir and beech) of different ages and diameter. To uproot the stumps we are using the same technique as for winching tests. For each experiment, stumps samples are taken in order to determine the wood decay ratio. Finally, a statistical analysis will be made in order to study the correlation between the force required to uproot a stump and its wood decay ratio. The first uprooting test campaign has been held during the summer 2009. First results of both experiment types will be presented. This study is part of the European Interreg IV Alpine Space project "MANFRED"

Caractérisation numérique d'une loi stochastique d'impact d'un bloc rocheux sur un éboulis

Les logiciels de trajectographie constituent un outil indispensable pour la détermination des zones soumises à un risque de chute de bloc en zones montagneuses. Une amélioration fondamentale à apporter à ces codes de calcul est la caractérisation fine du rebond du bloc sur le sol. Dans cette optique, une modélisation numérique de l'interaction entre le bloc et le sol par la Méthode des Elements Discrets est développée de façon à étudier l'impact d'un bloc rocheux sur un éboulis. Le traitement statistique des résultats issus des simulations numériques permet de définir une loi d'impact stochastique valable quels que soient le point d'impact et les conditions cinématiques initiales du bloc

Variational approach for nonsmooth elasto-plastic dynamics with contact and impacts

The objective of this article is the modelling and the numerical simulation of the response of elastoplastic structures to impacts. To this end, a numerical method is proposed that takes into account one-sided contact (Signorini condition) and impact phenomena together with plasticity in a monolithic solver, while accounting for the non-smooth character of the dynamics. The formulation of the plasticity and the contact laws are based on inclusions into normal cone of convex sets, or equivalently, variational inequalities following the pioneering work of Jean Jacques Moreau (1974) and Halphen and Son Nguyen (1975), who introduced the assumptions of normal dissipation and of generalised standard materials (GSM) in the framework of associated plasticity with strain hardening. The proposed time-stepping method is an extension of the Jean and J. J. Moreau (1987) scheme for nonsmooth dynamics. The discrete energy balance shows that spurious numerical damping can be removed and the scheme is practically unconditionally stable. Furthermore, the finite-dimensional variational inequality at each time-step is well-posed, can be solved by optimisation methods for convex quadratic programs, providing an interesting alternative to the return mapping algorithm. The paper is completed by numerical illustrative examples of impacts on metallic structures made of beams

Rockfall trajectory reconstruction: a flexible method utilizing video footage and high-resolution terrain models

Many examples of rockfall simulation software provide great flexibility to the user at the expense of a hardly achievable parameter unification. With sensitive site-dependent parameters that are hardly generalizable from the literature and case studies, the user must properly calibrate simulations for the desired site by performing back-calculation analyses. Thus, rockfall trajectory reconstruction methods are needed. For that purpose, a computer-assisted videogrammetric 3D trajectory reconstruction method (CAVR) built on earlier approaches is proposed. Rockfall impacts are visually identified and timed from video footage and are manually transposed on detailed high-resolution 3D terrain models that act as the spatial reference. This shift in reference removes the dependency on steady and precisely positioned cameras, ensuring that the CAVR method can be used for reconstructing trajectories from witnessed previous records with nonoptimal video footage. For validation, the method is applied to reconstruct some trajectories from a rockfall experiment performed by the WSL Institute for Snow and Avalanche Research SLF. The results are compared to previous ones from the SLF and share many similarities. Indeed, the translational energies, bounce heights, rotational energies, and impact positions against a flexible barrier compare well with those from the SLF. The comparison shows that the presented cost-effective and flexible CAVR method can reproduce proper 3D rockfall trajectories from experiments or real rockfall events.</p

Analyse expérimentale et numérique de la dynamique d'arbres verts de petits diamètres soumis à l'impact d'un bloc rocheux

Afin d'évaluer le rôle de protection des forêts contre les chutes de blocs, un modèle éléments finis d'impact d'un bloc sur une tige de bois vert a été développé. Le modèle réalisé sous CAST3M intègre les non-linéarités géométriques et matérielles régissant l'impact. En particulier, un algorithme de gestion du contact tige/bloc issu des Méthodes aux Eléments Discrets a été développé. La calibration est faite à partir d'essais d'impact de type pendule de Mouton. Ce modèle met en évidence les échanges énergétiques et les modifications de cinématique du bloc lors de l'impact

Using the Discrete Element Method for rockfall protection forests efficiency assessment

L'évaluation de l'efficacité de la forêt pour la protection contre les chutes de blocs n'est possible que suite à l'étude détaillée de l'impact d'un bloc sur un arbre. Dans cette optique, un modèle basé sur la Méthode des Eléments Discrets a été développé et utilisé pour identifier les paramètres principaux contrôlant la cinématique du bloc après impact sur l'arbre. L'objectif final de ce travail est de construire un méta-modèle de l'interaction entre un bloc et un arbre utilisable en pratique dans les modèles de simulation de la propagation des blocs

Modélisation de l'impact d'un bloc rocheux sur un terrain naturel, application à la trajectographie des chutes de blocs

This thesis is dedicated to the study of falling rocks bouncing on natural soils in order to improve the models used for rockfall hazard assessment. The impact of a rock on a scree slope is modelled using the Discrete Element Method. The comparison between numerical results and half-scale experimental results of impacts on a coarse soil shows that the numerical model allows accurately predicting the bouncing of the rock for a reduced number of model parameters to be calibrated. The numerical results also emphasize that the interaction between the impacting particle and the granular soil is composed of three stages : the partial energy exchange from the impacting particle to the soil, the propagation of a shockwave from the impact point and the wave reflection on the substratum. The study of energy exchanges during these three stages allows defining the impacting particle bouncing occurrence diagram as well as three impact regimes. The statistical analysis of numerical results using Bayesian inference leads to the definition of a stochastic impact model. This model is relevant for rock velocity after impact depending on both rock velocity before impact and soil particles layout near the impact point. Finally, the stochastic impact model is integrated into trajectory analysis models which allows defining a global probabilistic approach of rockfall hazard assessment including protective structures design.Ce travail de thèse porte sur la caractérisation du rebond d'un bloc sur un terrain naturel dans la perspective d'améliorer les modèles de détermination de l'aléa de chute de blocs. L'impact d'un bloc rocheux sur un sol composé d'éboulis est modélisé par la Méthode des Elements Discrets. La comparaison entre les résultats de simulation et les résultats d'essais à échelle réduite d'impact sur un sol granulaire grossier met en évidence que le modèle numérique développé assure une prédiction pertinente du rebond pour un nombre réduit de paramètres de simulation à calibrer. L'analyse de l'impact à l'aide du modèle numérique montre que l'interaction entre l'impactant et le sol peut être décomposée en trois phases : le transfert énergétique initial du bloc vers le sol, la propagation d'une onde de compression du point d'impact vers l'intérieur du sol et la réflexion de l'onde de compression sur le substratum. L'étude des échanges énergétiques lors de ces trois phases conduit à la définition d'un diagramme d'existence du rebond délimitant les domaines d'arrêt et de rebond de l'impactant et à l'identification de trois régimes d'impact. Le traitement statistique des résultats de simulation par des méthodes statistiques basées sur l'inférence Bayésienne permet également de définir une loi d'impact stochastique. Cette loi est représentative de la variabilité des vitesses du bloc après impact en fonction des paramètres cinématiques incidents et de l'arrangement géométrique des particules du sol au voisinage du point d'impact. Enfin, suite à l'intégration de la loi stochastique d'impact dans le contexte de l'analyse trajectographique, une approche probabiliste globale permettant la caractérisation détaillée de l'aléa de chute de bloc ainsi que l'implantation et le dimensionnement d'ouvrages de protection est proposée