De La Vaissiere R.,Andra Inc |
Noiret A.,Andra Inc |
Cote P.,Institute Francais des science et Technologies des Transports |
Helmlinger B.,Solexperts France |
And 5 more authors.
Geological Society Special Publication | Year: 2014
The investigation of the induced fractures network around seals in drifts or shafts, and in particular its evolution, is a key issue for the performance assessment of an underground waste repository. Within this framework, a specific experiment was designed and implemented in the Meuse/Haute-Marne Underground Research Laboratory (URL). This experiment, called CDZ (Compression of the Damaged Zone), is dedicated to studying the effect of mechanical compression within the induced fractures zone of the Callovo-Oxfordian claystone (COx). An unequalled level of knowledge in the 3D structure of the fractures network has been attained. A multidisciplinary approach was applied to observe not only the initial state of the induced fracture zone but also its evolution during a loading cycle. The investigations show that the fracture network which composed the Excavation Damaged Zone (EDZ) was initially interconnected and open for flow and then partially closed progressively following the increasing mechanical stress applied on the drift wall. Moreover, the evolution of the EDZ after unloading indicates a self-sealing process. © The Geological Society of London 2014.
Vinsot A.,Andra Inc |
Leveau F.,Andra Inc |
Bouchet A.,Center Regional dInnovation du Biopole |
Geological Society Special Publication | Year: 2014
Deep argillaceous rocks are reducing environments. When exposed to air, reduced minerals of these rocks react with oxygen, modifying the surrounding chemical conditions. Thus, oxidation is an issue in studies about the confining properties of such rocks in the framework of geological disposal projects for radioactive waste. Previous studies in several underground research laboratories (URLs) in argillaceous rocks have shown that oxidation reactions mainly occurred in the excavation-induced fracture network surrounding the drifts. In the Callovian- Oxfordian argillaceous rock, at 2490 m in drifts from the Meuse/Haute-Marne URL, oxidized features were systematically looked for in 115 borehole cores. The concerned drifts were of various ages, from a few days to 6.5 years. After 5 months, oxidized features were encountered in numerous excavation-induced extensional fractures. In excavation-induced shear fractures, oxidized features were observed in a few borehole cores after 2 years, and they became frequent after 6 years. In all cases, the oxidized features observed were found on the fracture walls or were connected to them, and were less than 1.8 m from the drift walls. These observations about the oxidation front and its evolution over time provide insights regarding the properties of excavationinduced fractures with respect to oxygen transfer. © The Geological Society of London 2014.
Servida D.,University of Milan |
Moroni M.,University of Milan |
Ravagnani D.,GeoTer |
Rodeghiero F.,University of Milan Bicocca |
And 3 more authors.
Italian Journal of Geosciences | Year: 2010
In this work a report is given of the geological, petrographic and ore mineralogical features of several sulphide-bearing quartz breccia (SQB) bodies outcropping in a structurally complex sector of the Orobic Alps (Southern Alpine domain) and close to the uranium-rich mining area of Novazza. Although little studied, these breccia bodies were previously attributed to different genetic processes and/or geological domains (e.g., Crystalline Basement, Basal Conglomerate formation). The features of the SQB bodies suggest a hydrothermal origin likely related to the large-scale hydrothermal process causing the formation of the nearby uranium ore deposit at Novazza.
Dunand F.,GEOTER |
Gueguen P.,Joseph Fourier University
Natural Hazards and Earth System Science | Year: 2012
France has a moderate level of seismic activity, characterized by diffuse seismicity, sometimes experiencing earthquakes of a magnitude of more than 5 in the most active zones. In this seismicity context, Grenoble is a city of major economic and social importance. However, earthquakes being rare, public authorities and the decision makers are only vaguely committed to reducing seismic risk: return periods are long and local policy makers do not have much information available. Over the past 25 yr, a large number of studies have been conducted to improve our knowledge of seismic hazard in this region. One of the decision-making concerns of Grenoble's public authorities, as managers of a large number of public buildings, is to know not only the seismic-prone regions, the variability of seismic hazard due to site effects and the city's overall vulnerability, but also the level of seismic risk and exposure for the entire city, also compared to other natural or/and domestic hazards. Our seismic risk analysis uses a probabilistic approach for regional and local hazards and the vulnerability assessment of buildings. Its applicability to Grenoble offers the advantage of being based on knowledge acquired by previous projects conducted over the years. This paper aims to compare the level of seismic risk with that of other risks and to introduce the notion of risk acceptability in order to offer guidance in the management of seismic risk. This notion of acceptability, which is now part of seismic risk consideration for existing buildings in Switzerland, is relevant in moderately seismic-prone countries like France. © 2012 Author(s) CC Attribution 3.0 License.
Fullgraf T.,Bureau de Recherches Géologiques et Minières |
Ndiaye P.M.,University Of Cheikh Antar Diop Ucad |
Blein O.,Bureau de Recherches Géologiques et Minières |
Buscail F.,Geoter |
And 4 more authors.
Journal of African Earth Sciences | Year: 2013
Submarine basalt and trachyte of the Nandoumba group occur in eastern Senegal within the Bassarides branch of the Mauritanides orogen. The unit forms part of the parautochthonous domain which is stacked between underlying Neoproterozoic to Paleozoic foreland and overlying Variscan nappes. The crystallisation age of the volcanic to subvolcanic rocks has been determined by U-Pb single zircon SHRIMP method at 428 ± 5.2. Ma whereas zircon xenocryst ages vary from 500 to 2200. Ma. The shape of the xenocryst grains document proximal Neo- and Paleoproterozoic and distal Mesoproterozoic provenance areas for assimilated sediments. This is compatible with the Paleoproterozoic Birimian basement and Neoproterozoic cover rocks nearby whereas an origin from the Amazonian craton could be assumed for distal Mesoproterozoic zircons.Geochemical and Sm-Nd isotope whole rock analysis show that basalts of the Nandoumba group are similar to modern transitional to alkaline volcanic lavas in intraplate settings. Those basalts have a deep mantle source with a great contribution of a recycled mantle component such as EM1 and/or EM2. The basalts resemble in their composition those from the Meguma terrane of Nova Scotia which are of similar age suggesting a common source and therefore connection of Meguma with Gondwana during this period. Review of circum-Atlantic Silurian magmatism indicates ongoing fragmentation of NW-Gondwana that started in Cambro/Ordovician times. © 2012 Elsevier Ltd.
Armand G.,Andra Inc |
Leveau F.,GEOTER |
Nussbaum C.,SWISSTOPO |
De La Vaissiere R.,Andra Inc |
And 4 more authors.
Rock Mechanics and Rock Engineering | Year: 2013
The performance and safety assessment and technology demonstration are the main objectives of research programs for feasibility studies for deep geological repository of radioactive waste. In this context, the French national radioactive waste management agency (ANDRA) started to develop the Meuse/Haute-Marne underground research laboratory (URL) at Bure, nearly 300 km East of Paris. The host formation consists of a Callovo-Oxfordian claystone found between 420 and 550 m below ground, overlain and underlain by poorly permeable carbonate formations. One of the major concerns related to performance assessment is the excavation-induced fractures which may provide groundwater preferential pathway for radionuclide migration. The extent of the fractures possibly acting significantly in the radionuclide migration is known as the excavation damaged zone (EDZ). A scientific study on the EDZ characterization is performed at the main level of the URL (-490 m). Observations such as structural analysis on core, overcored resin-filled samples, geological survey of the drift face and sidewalls, were made to better understand the fracture network characteristics, extent and its generation. Pulse and constant head test hydraulic conductivity measurements were performed with multi packer system to estimate the extension of the EDZ hydraulic conductivity. Fractures exhibited high transmissivity near the excavation walls, but farther from the exaction walls, shear fractures showed hydraulic conductivity values reflecting values of undisturbed or slightly disturbed rock mass condition. The major findings in terms of geometry and properties of excavation-induced fractures are discussed in detail in this paper. For example, it is observed that the shape of the fracture network depends on the orientation of the drift in relation to the orientation of the in situ stress field. © Springer-Verlag Wien 2013.
Bosch D.,Montpellier University |
Bruguier O.,Montpellier University |
Caby R.,Montpellier University |
Buscail F.,GEOTER |
Hammor D.,Annaba University
Journal of Geodynamics | Year: 2016
Laser-ablation U-Th-Pb analyses of zircon and allanite from magmatic and metamorphic rocks of the Adrar des Iforas in Northern Mali allow re-examining the relationships between the different crustal units constituting the western part of the Tuareg Shield, as well as the timing of magmatic and metamorphic events in the West Gondwana Orogen. Granulite-facies metamorphism in the Iforas Granulitic Unit (IGU) and at In Bezzeg occurred at 1986 ± 7 Ma and 1988 ± 5 Ma respectively. This age is slightly younger, but consistent with that of the HT granulite facies event characterizing the In Ouzzal granulitic unit (IOGU), thereby substantiating the view that these units once formed a single granulitic belt of c. 800 km long. High-grade metamorphic basement units of the Kidal terrane surrounding the IGU contain Paleoproterozoic magmatic rocks crystallized between 1982 ± 8 Ma and 1966 ± 9 Ma. Inherited components in these rocks (2.1 Ga and 2.3-2.5 Ga) have ages similar to that of detrital zircons at In Bezzeg and to that of basement rocks from the IGU. This is taken as evidence that the Kidal terrane and the IGU formed a single crustal block at least until 1.9 Ga. East of the Adrar fault, the Tin Essako orthogneiss is dated at 2020 ± 5 Ma, but escaped granulite facies metamorphism. During the Neoproterozoic, the Kidal terrane underwent a long-lived continental margin magmatism. To the west, this terrane is bounded by the Tilemsi intra-oceanic island arc, for which a gneissic sub-alkali granite was dated at 716 ± 6 Ma. A synkinematic diorite extends the magmatic activity of the arc down to 643 ± 4 Ma, and, along with literature data, indicates that the Tilemsi arc has a life span of about 90 Ma. Backward docking to the western margin of the Kidal terrane is documented by migmatites dated at 628 ± 6 Ma. Subduction related processes and the development of the Kidal active margin was responsible for the development of a back-arc basin in the Tafeliant area, with deposition of sediments intruded by basaltic and dacitic lavas, one of which was dated at 623 ± 6 Ma. East of the IGU, in the Tamaradant domain, metagabbros and meta-anorthosites emplaced within greywackes have geochemical characteristics typical of subduction-related environments (enrichment in LILE and HFSE depletion). A metadiorite intruding the Tamaradant sediments gave an age of 630 ± 6 Ma, which is tentatively taken as evidence for a Pan-African age for the subduction processes that took place east of the IGU. Collision of the Kidal terrane with the eastern margin of the West African Craton is best dated by a syncollisional tonalite, which provides an age of 604 ± 5 Ma. Late kinematic processes shortly followed at 599 ± 4 Ma as exemplified by the emplacement of a monzogranite belonging to the complex Central Batholith. The geochronological and geochemical evidences provided by this study allow proposing that the Kidal terrane, the IGU and Tamaradant domain of the Adrar des Iforas once belonged to a single terrane, which probably extended northward to include the IOGU, and which was later dissected by major lithospheric scale faults during the late Pan-African orogenic phases. © 2015 Elsevier Ltd.