Back analysis on the pillars behavior during the flooding of the Lorrain iron ore basin (France)

Abstract : The several subsidence events that have occurred during second half of the 1990s in the French Lorraine iron-ore basin have triggered a strong impact on local people. Considering the various instabilities that developed during or shortly after the flooding of large sectors of the underground mine workings, many buildings were indeed severely damaged, generating the expropriation of several hundreds of families in various villages. To face this crisis situation, a risk analysis and mitigation method was elaborated in order to identify the mining sectors presenting a risk of failure at term with possible harmful effects on surface. For each of those sectors, some risk mitigation processes were implemented7 (evacuation, treatment, monitoring) in order to ensure public safety and to assist the decision makers in their objective of land-use planning. The method was applied to the whole iron-ore basin, including the northern sector, which was not yet flooded at that time. Various sectors potentially sensitive to future pillar failure and subsidence events were thus identified within the northern part of the ore basin before its submergence which began in December 2005 to be completed in March 2008.Taking into account the many events having affected the southern part of the basin, during or just after the flooding of the mine workings, one could have awaited that several instabilities would have developed in the northern part during the flooding process. However, except few isolated cases (cities of Angevillers and Rochonvillers) that have generated very limited disorders, no major ground movement was detected during the flooding process as well as during the 3 following years. Based on data analysis and experience feedback, the proposed paper intends to better understand the difference of behaviour between the northern and southern parts, in terms of sensitivity to flooding and of subsidence development. The analysis will be focused on the predisposition of the pillars to failure, manly by adapting to the context of the Lorraine iron ore basin the 'Salamon formula', very largely used in the Anglo-Saxon mining world. After a short summary of the historical, geological and mining contexts of the Lorraine iron ore basin, the databases built for the study (gathering a sufficient number of samples 'broken pillars' and 'stable pillars') will be described. The principal results identifying predisposition of the pillars to failure, according to the various mining will be then discussed. As perspective, some considerations will be carried out about the possibility to integrate the 'time effect' in the analysis. This may give some crucial information about the chronology of failure (which sector seems predisposed to fail before one another) but also about the velocity of the movement (sudden collapse or continuous subsidence).
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Christophe Didier, Nielen van Der Merwe, Frédéric Poulard. Back analysis on the pillars behavior during the flooding of the Lorrain iron ore basin (France). 1. Congrès international sur la gestion des rejets miniers et l'après-mine (GESRIM 2012), Apr 2012, Marrakech, Morocco. ⟨ineris-00973658⟩

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