Cenozoic evolution of Neotethys and implications for the causes of plate motions

Africa-North America-Eurasia plate circuit rotations, combined with Red Sea rotations and new estimates of crustal shortening in Iran define the Cenozoic history of the Neotethyan ocean between Arabia and Eurasia. The new constraints indicate that Arabia-Eurasia convergence has been fairly constant at 2 to 3 cm/yr since 56 Ma with slowing of Africa-Eurasia motion to <1 cm/yr near 25 Ma, coeval with the opening of the Red Sea. Ocean closure occurred no later than 10 Ma, and could have occurred prior to this time only if a large amount of continental lithosphere was subducted, suggesting that slowing of Africa significantly predated the Arabia-Eurasia collision. These kinematics imply that Africa's disconnection with the negative buoyancy of the downgoing slab of lithosphere beneath southern Eurasia slowed its motion. The slow, steady rate of northward subduction since 56 Ma contrasts with strongly variable rates of magma production in the Urumieh-Dokhtar arc, implying magma production rate in continental arcs is not linked to subduction rate

Surveillance et instrumentation des monuments historiques

Les sites et monuments historiques, comme tous les ouvrages naturels et humains, évoluent avec le temps du fait de l'action des éléments naturels ou de site. Cela amène donc la collectivité à se poser la question des moyens à mettre en oeuvre pour assurer la pérennité des ouvrages qu'elle souhaite conserver dans son patrimoine et la sécurité des personnes susceptibles de les visiter. En effet le risque principal auquel il faut faire face est celui de la " ruine " du site ou monument. Cette ruine résulte d'une instabilité qui peut brusquement mettre en péril l'édifice et ses visiteurs. La surveillance est le meilleur moyen d'obtenir à la fois un diagnostic juste et précis et de préconiser un traitement adapté au monument et à sa valeur historique. L'auscultation doit toujours être une opération limitée dans le temps et bien ciblée suite à l'analyse du phénomène étudié. En outre la surveillance doit employer des techniques simples et de grande sensibilité

An extension of analytical methods for building damage evaluation in subsidence regions to anisotropic beams

Ore and mineral extraction by underground mining often causes ground subsidence phenomena, and may induce severe damage to buildings. Analytical methods based on the Timoshenko beam theory is widely used to assess building damage in subsidence regions. These methods are used to develop abacus that allow the damage assessment in relation to the ground curvature and the horizontal ground strain transmitted to the building. These abacuses are actually developed for building with equivalent length and height and they suppose that buildings can be modelled by a beam with isotropic properties while many authors suggest that anisotropic properties should be more representative. This paper gives an extension of analytical methods to transversely anisotropic beams. Results are first validated with finite elements methods models. Then 72 abacuses are developed for a large set of geometries and mechanical properties

Modelling uncertainties in pillar stability analysis

International audienceMany countries are now facing problems related to their past mining activities. One of the greatest problems they have to deal with concerns the potential surface instability. In areas where a bord-and-pillar extraction method was used, deterministic methodologies are generally used to assess the risk of surface collapses. However those methodologies suffer from not being able to take into account all the uncertainties existing in any risk analysis. Through the practical example of the assessment of a single pillar stability in a very simple mining layout, this paper introduces a logical framework that can be used to incorporate the different kinds of uncertainties related to data, models as well as to specific expert's choices in the risk analysis process. Practical recommendations and efficient tools are also provided to help engineers and experts in their daily work.De nombreux pays se trouvent aujourd'hui confrontés à des problèmes liés à leur histoire minière passée, et notamment à celui de l'instabilité des terrains de surface. Des méthodes déterministes sont généralement utilisées dans les zones anciennement exploitées par la méthode des chambres et piliers, pour évaluer le risque d'effondrement de surface. Cependant, ces méthodes ne permettent pas de prendre en compte les différentes incertitudes qui existent dans toute analyse de risque. Cette étude, à travers l'exemple du calcul de stabilité d'un pilier de mine extrait d'un environnement très simple, présente une démarche logique pouvant être utilisée pour incorporer les incertitudes, liées aux données aussi bien qu'aux modèles ou aux choix spécifiques des experts, dans les processus d'analyse de risque. Des recommandations pratiques et des outils efficaces pouvant permettre d'aider les ingénieurs et les experts dans leur travail de tous les jours sont également présentés

Development of building vulnerability functions in subsidence regions from empirical methods

The extraction of ores and minerals by underground mining often causes ground subsidence phenomena. In urban regions, these phenomena may induce small to severe damage to buildings. To evaluate this damage, several empirical and analytical methods have been developed in different countries. However, these methods are difficult to use and compare due to differences in the number of criteria used (from 1 to 12). Furthermore, the results provided by damage evaluation may be significantly different from one method to another. The present paper develops vulnerability functions based on a concept that has been applied in other areas, such as earthquake engineering, and that appears to be a more efficient way to assess building vulnerability in undermined cities. A methodology is described for calculating vulnerability functions in subsidence zones using empirical methods. The first part of the paper focuses on existing empirical methods for damage evaluation, and selected necessary improvements or modifications are justified. The second part focuses on the development of a building typology in subsidence zones and its application in the Lorraine region, where many villages are subject to subsidence problems due to iron-ore mining. The third section describes and discusses the adopted methodology for determining vulnerability and fragility functions or curves. Finally, vulnerability functions are tested and validated with a set of three subsidences that occurred in Lorraine between 1996 and 1999

Development of building vulnerability functions in subsidence regions from analytical methods

Ore and mineral extraction by underground mining often causes ground subsidence phenomena, and may induce severe damage to buildings. This paper develops vulnerability and fragility functions to assess building damage in the context of mining subsidence hazards, comparable to functions used for other hazards. These functions are based on existing analytical methods for damage assessment. They take into account both the uncertainty of the geometric and mechanical parameters of the building and the soil–structure interaction phenomena that may have a critical influence on the building loading. The present paper discusses the methodology used to determine these functions, and the analytical method for damage evaluation is described. The second part is a detailed application of the methodology for a masonry building with or without reinforcement, for which both vulnerability and fragility functions are calculated. Finally, vulnerability functions are tested and validated with a set of three subsidences that occurred in Lorraine (France) between 1996 and 1999

Computer Simulation and Risk Analysis around Abandoned Underground Excavations Application to a French Case

International audienceThe Analysis of risks induced by Underground opening or unstable eliffs is one of the major and important tasks in earth science engineering. This analysis should take into account the effect of randomness, it should account also for the probability distributions of different parameters, and above all, the experience of the geotechnical engmeer

Comparison of building damage assessment methods for risk analysis in mining subsidence regions

The occurrence of subsidence phenomena in urban regions may induce small to severe damage to buildings. Many methods are provided in the literature to assess buildings damage. Most of these methods are empirical and use the horizontal ground strain as a subsidence intensity in the vicinity of a building. Application and comparison of these methods with a case study is the main objective of this paper. This comparison requires some harmonization of the existing methods and the development of a software, which combines the subsidence hazard prediction, the damage evaluation methods and a database of buildings with structural parameters as well as the geographical coordinates of the buildings An additional results is the development of a method for the prediction of the horizontal ground strain in the vicinity of each building. Results are given as a map of damaged buildings for the case study and the different existing methods with some statistical calculations such as the mean and the standard deviation of damage in the city. Comparison of these results allows identification of the “safer” method that give the higher mean of damage. The comparison of the calculated results and observed damage in Lorrain region show that, the Dzegeniuk et al. methods is more realistic in comparison of the other empirical methods

Vulnerability assessment for mining subsidence hazard

International audienceAfter the last mining subsidence events, which occurred in the iron-ore field in Lorraine (France) in 1996, 1997 and 1999, and because of the thousand hectares of undermined areas, the assessment of vulnerability of buildings and territories became necessary for risk management. The ten last years highlight evolutions of the vulnerability concept and its assessment method between the first risk management decisions and current risk assessment. These evolutions reveal a lot about the complexity of the vulnerability concept and about difficulties to develop simple and relevant methods for its assessment. The objective of this paper is to present this evolution and to suggest different improvements compared to other concepts and methods developed in other countries (USA, Poland...) and for other hazards (flood, industrial...). These improvements take into account more various kinds of elements in the vulnerability meaning (buildings, people, roads, public facilities and public functions) as well as they deal with method of assessment in relation to the subsidence intensity and vulnerable elements.Depuis les affaissements miniers survenus en Lorraine en 1996, 1997 et 1999, et en raison des milliers d'hectars sous-minés, l'évaluation de la vulnérabilité des habitations et des territoires est devenu une nécessité pour permettre une meilleure prévention des risques. Les dix années écoulées depuis les premières décisions prises en réaction au risque d'affaissement minier montrent une évolution importante des méthodes employées. Ces évolutions sont révélatrices de la complexité du concept de vulnérabilité et de la difficulté à trouver des méthodes pertinentes et opérationnelles pour l'évaluer. Cet article a pour objectif de présenter cette évolution et de donner des perspectives d'amélioration au vu des méthodes employées dans d'autres pays (Etats-unis, Pologne) et vis-à-vis d'autres aléas (sismique, inondation, accidents industriels). Ces perspectives portent sur les différents enjeux qui contribuent à la vulnérabilité (personnes, bâtiments, infrastructures, équipements publics...) et sur les méthodes de caractérisation de la vulnérabilité en fonction des enjeux et de l'intensité de l'affaissement

Auscultation et surveillance des pertubations hydromécaniques d'ouvrages souterrains par la mesure et l'analyse de la propagation d'ondes dans les roches

International audienceThe excavation of a drift creates a disturbed zone, which is called EDZ (Excavation Disturbed or Damaged Zone). The study of the mechanical characteristics of this zone is essential to estimate the stability of such a drift at short, mean or long term. The propagation of ultrasonic waves is used as a mean of analysis in rock mechanics since the 1960s. Initially, it was used to determine the dynamic elastic properties. Then, it was gradually adapted to the study of the other properties of the material such as cracking, porosity, saturation, etc. With the computer tools continuously more powerful, a more elaborated treatment of the signals became possible, which allows characterization of the "EDZ". The Tests carried out in laboratory, allowed us to correlate P-wave and mechanical parameters of the studied media in elastic and plastic phases. Besides, the mechanical modeling of the EDZ around a drift allows understanding the mechanical phenomena on the scale of the work. Finally, the results of an in-site experiment allowed us to locate the EDZ around an underground mine pillar. It contributed to have a better understanding of the mechanical models available and to estimate the pillar stability.Lorsqu'une galerie est excavée, quelle que soit la méthode d'excavation, la roche encaissante subit des perturbations dans une zone dite EDZ (Excavation Disturbed or Damaged Zone). Ces perturbations peuvent se traduire par différents effets allant d'une faible variation de la pression interstitielle à un endommagement important, voire la rupture de l'ouvrage et peuvent mettre en danger la stabilité d'une partie ou de l'ensemble de l'ouvrage souterrain concerné. L'étude des caractéristiques mécaniques de cette zone " EDZ " est essentielle pour estimer la stabilité de l'ouvrage à court, moyen ou long terme. La propagation des ondes ultrasoniques est utilisée comme moyen d'analyse en mécanique des roches depuis les années 1960. Initialement, elle a été mise au point pour déterminer les modules élastiques dynamiques des roches. Ensuite, elle a progressivement été adaptée à l'étude d'autres propriétés du matériau telles que : fissuration, porosité, saturation, etc. Avec les outils informatiques sans cesse plus puissants, un traitement des signaux plus élaboré est devenu possible, ce qui permet d'envisager la caractérisation de l' " EDZ " par cette méthode. Des essais en laboratoire à la fois acoustiques et mécaniques, nous ont permis de relier qualitativement les paramètres de propagation des ondes et les caractéristiques mécaniques du milieu étudié pendant les phases élastique et plastique. Par ailleurs, la modélisation mécanique de l'EDZ autour d'une galerie permet de comprendre les phénomènes mis en jeux à l'échelle de l'ouvrage. Enfin, les résultats d'une expérimentation in situ ont permis de caractériser la distribution de la zone endommagée autour d'un pilier de mine, ce qui a contribué à la vérification des modèles mécaniques existants et à l'estimation de la stabilité de l'ouvrage