Impact des terrils houillers sur la qualité des eaux souterraines (bassin minier Nord-Pas-de-Calais, France) : approche géochimique et isotopique

L'exploitation du charbon dans le Nord-Pas-de-Calais (France) a engendré le dépôt d'importantes quantités de résidus miniers sous forme de terrils, essentiellement constitués de schistes houillers. Le lessivage de ces stériles par les eaux météoriques et l'oxydation des sulfures de fer contenus sont susceptibles de contribuer à l'enrichissement en sulfates et métaux associés de l'aquifère de la craie, principale ressource en eau de la région. Des analyses chimiques et isotopiques (S & C) ont été effectuées sur des prélèvements d'eau en amont et en aval hydraulique des sites d'étude ainsi que sur la fraction minérale des terrils. Afin de déterminer les modalités d'infiltration des eaux météoriques au sein des terrils, des prospections radio-magnétotelluriques, couplées à des mesures de perméabilité et de granularité ont été effectuées. Les analyses ont permis de mettre en évidence que le lessivage des terrils entraîne un flux d'ions sulfate et de carbone vers l'aquifère de la craie. L'approche géophysique a permis de mettre en évidence un phénomène d'infiltration des eaux météoriques. Elle a montré en outre l'existence de barrières de perméabilité en profondeur. Il apparaît donc que les eaux météoriques, lorsqu'elles s'infiltrent dans le terril, ne peuvent pénétrer à plus de quelques mètres de profondeur. L'existence de zones imperméables, en limitant l'infiltration des eaux en profondeur, limite également la quantité de sulfure potentiellement oxydable et donc la quantité de sulfates entraînée vers l'aquifère de la craie.In the Nord-Pas-de-Calais region (France), coal mining activity has induced a build-up of many mine tips. The tip materials are dominantly composed of siltstones, locally rich in iron sulfide. Weathering of pyrite might be expected to release sulfate ions and associated metals within the run-off waters down to the underlying aquifer, which is composed of a thick Cretaceous chalk formation. The objectives of this study were twofold: (1) to determine the possible role of the mine tips in the sulfate mineralization of the chalk aquifer and; (2) to assess the amount of waste material that can be leached and may supply sulfate ions to the water table.Two sites were selected for this study. Site 1 rests directly on the Senonian-Turonian chalk, whereas site 2 lies on sandy-clayey Cenozoic formations overlying the chalk formations (Figs. 1-2). Water samples were collected within the chalk aquifer (Fig. 3), which represents a free water table except for where the almost impermeable Cenozoic formations confine this table (site 2). Rock samples were collected at the surface and at a depth of <12 m at both sites. Various analyses were performed on these samples including mineralogical analyses carried out on both the bulk fraction and the clayey fraction, as well as elementary analyses of total carbon, total sulfur and CaCO3 contents. Elemental analyses were carried out by Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES: major and minor elements) or Inductively Coupled Plasma-Mass Spectrometry (ICP-MS: trace elements). Chemical and isotopic (C, S) analyses were performed on water sampled from upstream and downstream of the mine tips. In situ measurements were also carried out during sampling. Finally, to assess the degree of rainwater seeping through the coal mine tips, two radio-magnetotelluric surveys were carried out in September and November, 1999 following rainy periods. Permeability measurements and grain-size analyses of subsurface samples were also performed at site 2.The carbon and sulfur contents showed superficial leaching on the mine tips (Fig. 4). The use of sulfur isotopes as tracers of the sulfate origin allowed identification of two sources for the two sites: a "mine tip" source with a slightly negative d34 S (-2.8‰ to -3.9‰), which corresponds to the oxidation of sulfides contained by the Carboniferous shales, and another source (d34 S=-20‰) corresponding to the gypsum of the Cenozoic formations, which was only present at site 2 (Figs. 5 & 6).This study outlined different behavior for the tips of the two sites. At site 1, where there is a free water-table zone, the mine tip leachates carry sulfate ions directly to the water table, whereas in the case of a confined aquifer zone such as the one present at site 2, a proportion of the sulfate was reduced once exported to the water table (the redox potential showed negative values; Table 1). This suggestion of bacterially-mediated reduction is supported by the d34 S of the sulfate content in the water table. The bacterial activity was fueled by the organic carbon release that accompanies the sulfur leaching on the mine tips. This carbon contribution was confirmed by the 14C activity that characterized the chalk aquifer waters at the upstream region of the mine tip and noticeably decreased downstream. The decrease is a result of the supply of "dead carbon" from the mine tips (Fig. 7).The oxidation of pyrite also results in H+ production. However, the pH decrease observed downstream from the sites was very slight. Waters derived from leaching of the mine tip seeped through the buffered environment of the chalk aquifer. The distribution of metal content showed no surface to depth gradient for samples taken from both sites. The only evidence of "neutral acid mining drainage" (NAMD) was the sulfate amounts exported, and the increase in Mg, Ca, HCO3- and Sr contents observed downstream from the sites (Table 2a-b).The decrease in the apparent resistivity of radio-magnetotelluric profiles demonstrated that rain waters could deeply penetrate in some parts of the tip at site 2 (Fig. 9). Considering the constant nature of the waste material (grain size and porosity), a decrease in resistivity accompanied by an increase in conductivity between the two surveys indicated water seepage. Permeability measurements showed the occurrence of deep permeability barriers (Fig. 10), limiting not only rainwater seepage, but also the amount of mobilizable sulfide and consequently the amount of sulfates exported to the chalk aquifer. Grain size is not the only reason for the permeable or impermeable nature of waste material - the grain ordering and the compaction of levels at depth also have a role

Kriging Surrogate Models for Predicting the Complex Eigenvalues of Mechanical Systems Subjected to Friction-Induced Vibration

This study focuses on the kriging based metamodeling for the prediction of parameter-dependent mode coupling instabilities. The high cost of the currently used parameter-dependent Complex Eigenvalue Analysis (CEA) has induced a growing need for alternative methods. Hence, this study investigates capabilities of kriging metamodels to be a suitable alternative. For this aim, kriging metamodels are proposed to predict the stability behavior of a four-degree-of-freedom mechanical system submitted to friction-induced vibrations. This system is considered under two configurations defining two stability behaviors with coalescence patterns of different complexities. Efficiency of kriging is then assessed on both configurations. In this framework, the proposed kriging surrogate approach includes a mode tracking method based on the Modal Assurance Criterion (MAC) in order to follow the physical modes of the mechanical system. Based on the numerical simulations, it is demonstrated by a comparison with the reference parameter-dependent CEA that the proposed kriging surrogate model can provide efficient and reliable predictions of mode coupling instabilities with different complex patterns

Statistical evaluation of cross-classifications derived from rearranged community data matrices

In order to enhance interpretation of two-way contingency tables (cross-classifications) derived from two hierarchical classifications, new indices are suggested to evaluate the relative contribution of nodes in either hierarchy to the nodes or to a partition of groups derived from the other hierarchy. Using these tools, cut-levels in both hierarchies can be found to define optimal partitions, and groups from both partitions can be associated in order to identify their mutual relationships. The method is illustrated with an actual example from vegetation ecology

On the influence of multiple contact conditions on brake squeal

This study focuses on squeal noise prediction for an automotive brake system. For this purpose, a stability study of a finite element model of the brake system is carried out. For the determination of the squeal propensity of a brake system via finite element models, the commonly used approach consists in considering only a friction coefficient at the pad-disc interface. However, numerous other contacts exist in a brake system. In the present study, the influence of several contacts between the caliper, the bracket, the pad and the piston is studied. It turns out that the consideration of these numerous contacts has a real impact on the stability results and can not therefore be neglected. Indeed, a high dispersion of results for the system’s eigenvalues indicating strong modifications of the stability behavior and thus of the squeal propensity is observed when different contact conditions are considered. This study insights the necessity to take into account of all contact conditions during the design process of brake systems

Hierarchical Factor Classification for Contingency Tables

Abstracts, Barcelona, Universidad Pompeu Fabr

Mise en place d’une technique numérique pour l’estimation de vibrations non-linéaires auto-entretenues avec frottement: Application à la prédiction du crissement de frein automobile

International audienceCe papier présente une approche non-linéaire permettant d'approximer le comportement vibratoire non-linéaire dans le cas où plusieurs instabilités sont impliquées dans la réponse du système. Cette méthode dénommée MASA (Modal Amplitude Stability Analysis) est complémentaire de l'ana-lyse aux valeurs propres complexes. Dans un premier temps, les aspects théoriques de la méthode sont présentés. Puis, le modèle éléments finis du frein servant d'illustration est présenté. Enfin, la méthode non-linéaire est appliquée sur le modèle afin de prédire la réponse dynamique de celui-ci

Generalized Modal Amplitude Stability Analysis for the prediction of the nonlinear dynamic response of mechanical systems subjected to friction-induced vibrations

International audienceThe numerical prediction of the dynamic behaviour of mechanical systems subjected to friction-induced vibrations is still a tedious problem. Different methodologies exist nowadays to study it.The first one is the complex eigenvalue analysis, which is widely used by the scientists and the industrials to predict the appearance of instabilities despite its disadvantages. Other methodologies, namely temporal integration and frequential approaches, have been developed to determine the transient and/or the steady-state response toassess the history of the dynamic response, and so to identify the unstable modes involved in the nonlinear dynamic response as well as the vibration levels. However, because of their complex implementation, their high numerical cost and sometimes the strong assumptions made on the form of the solutions, these methods are not widely and currently used in industry. To cope with the limitations of the CEA, namely the over- or under-predictability and the lack of information about modal participations in the nonlinear dynamic response, developing complementary tools is necessary. Thus, this paper is devoted to the extension and generalization of a nonlinear approach, called the modal amplitude stability analysis, to the multi-instability case. The method, called the Generalized Modal Amplitude Stability Analysis (GMASA), allows to identify the evolutions and contributions of unstable modes involved in the nonlinear self-sustaining vibration response and to estimate the limit cycles. The method is applied on a phenomenological system for which it is easy to provide an understanding of the unstable mode(s) contribution to the nonlinear dynamic response of the system and for which the calculations can be performed with reasonable computational times. Thus, the efficiency and validity of the GMASA approach are investigated by comparing the GMASA results with those of the reference results based on temporal approach