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Lisjak A.,Geomechanica Inc. | Tatone B.S.A.,Geomechanica Inc. | Mahabadi O.K.,Geomechanica Inc. | Grasselli G.,University of Toronto | And 6 more authors.
Rock Mechanics and Rock Engineering | Year: 2015

The analysis and prediction of the rock mass disturbance around underground excavations are critical components of the performance and safety assessment of deep geological repositories for nuclear waste. In the short term, an excavation damaged zone (EDZ) tends to develop due to the redistribution of stresses around the underground openings. The EDZ is associated with an increase in hydraulic conductivity of several orders of magnitude. In argillaceous rocks, sealing mechanisms ultimately lead to a partial reduction in the effective hydraulic conductivity of the EDZ with time. The goal of this study is to strengthen the understanding of the phenomena involved in the EDZ formation and sealing in Opalinus Clay, an indurated claystone currently being assessed as a host rock for a geological repository in Switzerland. To achieve this goal, hybrid finite-discrete element method (FDEM) simulations are performed. With its explicit consideration of fracturing processes, FDEM modeling is applied to the HG-A experiment, an in situ test carried out at the Mont Terri underground rock laboratory to investigate the hydro-mechanical response of a backfilled and sealed microtunnel. A quantitative simulation of the EDZ formation process around the microtunnel is first carried out, and the numerical results are compared with field observations. Then, the re-compression of the EDZ under the effect of a purely mechanical loading, capturing the increase of swelling pressure from the backfill onto the rock, is considered. The simulation results highlight distinctive rock failure kinematics due to the bedded structure of the rock mass. Also, fracture termination is simulated at the intersection with a pre-existing discontinuity, representing a fault plane oblique to the bedding orientation. Simulation of the EDZ re-compression indicates an overall reduction of the total fracture area as a function of the applied pressure, with locations of ineffective sealing associated with self-propping of fractures. These results are consistent with hydraulic testing data revealing a negative correlation between pressure values and an increase in the EDZ transmissivity. © 2015 Springer-Verlag Wien

Nussbaum C.,Federal Office of Topography Swisstopo | Bossart P.,Federal Office of Topography Swisstopo | Amann F.,ETH Zurich | Aubourg C.,University of Pau and Pays de lAdour
Swiss Journal of Geosciences | Year: 2011

Excavated in the Opalinus Clay formation, the Mont Terri underground rock laboratory in the Jura Mountains of NW Switzerland is an important international test site for researching argillaceous formations, particularly in the context of deep geological disposal of radioactive waste. The rock laboratory is intersected by naturally formed tectonic structures, as well as artificial fractures primarily formed as a consequence of tunnel excavation and the associated stress redistribution. The description and characterisation of tectonic and artificial structures is, in many cases, of key importance for interpreting the results of the various in situ experiments conducted in the rock laboratory. Systematic small-scale mapping of the tunnel walls and floor, and adjacent niches, provides basic information about the geometry and the kinematics of the geological fractures intersecting the underground laboratory. A compilation of all tectonic structures identified is presented in this paper. The underground laboratory is located in the backlimb of the Mont Terri anticline, a NNW-vergent imbricate fault-bend fold, which is characterised by a pronounced along-strike asymmetry resulting from variously oriented inherited faults. The total shortening accommodated by this structure was estimated by mass (area) balancing to be approximately 2. 1 km. The Mont Terri area is significantly affected by N- to NNE-striking normal faults of the Eo-Oligocene Rhine-Bresse transfer zone and by ENE-striking faults of Late Variscan age. Depending on their orientation with respect to the transport direction towards the NNW, these faults served as oblique and frontal ramps during the subsequent Jura thrusting in the Late Miocene. The various fault systems identified in the underground rock laboratory clearly correlate with the regional-scale structures. In addition to classical structural analysis, the anisotropy of magnetic susceptibility was measured to determine the magnetic fabric and strain imprint of the Opalinus Clay. Results indicate a well developed magnetic fabric with a magnetic foliation close to the bedding, and with two distinct magnetic lineations which are probably related to the Mont Terri anticline folding and layer-parallel shortening prior to the folding. Strain imprint is more pronounced in the overturned forelimb, which is consistent with the structural data. © 2011 Swiss Geological Society.

Bossart P.,Federal Office of Topography Swisstopo | Ling L.X.,European Underground Research Infrastructure for Disposal of nuclear waste in Clay Environment | Blechschmidt I.,National Cooperative for the Disposal of Radioactive Waste | Ohlsson M.,Swedish Nuclear Fuel and Waste Management Company | And 2 more authors.
Geological Society Special Publication | Year: 2014

This paper describes how four scientific and safety relevant issues have been addressed in special-purpose research laboratories focusing on the geological disposal of high level and longlived radioactive waste. These are: (a) the effects of heat on the engineered barriers and the geological environment; (b) the geochemical characterization of pore-water in argillaceous rocks; (c) the diffusion and retention of radionuclides; and (d) the full-size sealing of a waste emplacement. They are illustrated by experiments conducted in five underground research laboratories (URLs), three of which are in clay formations (Mol in Belgium, Centre de Meuse-Haute-Marne in France, and Mont Terri Rock Laboratory in Switzerland) and two in granite (Aspö Hard Rock Laboratory in Sweden and Grimsel Test Site in Switzerland). This paper highlights how the various types of experiments are related and how their results have been applied to foster progress. The most complex experiments have revealed artefacts and technical or methodological difficulties associated with interactions among multiple phenomena, the occurrence or intensity of which cannot be analysed by simple models. In turn, these difficulties have prompted experiments targeted at elementary phenomena, thereby encouraging the development of new investigation protocols and monitoring tools. More than 30 years of investigations in special-purpose URLs show the benefits of in-situ experimental programmes in the context of radioactive waste management. The laboratories have opened up avenues for research and advanced knowledge and technology. Thanks to a large component of international cooperation, they have made it possible to mobilize the financial and human resources required for this type of research. They have, above all, shared thoughts and promoted interdisciplinary studies around the same subject. They make common strategies possible at international level. © The Geological Society of London 2014.

Kupferschmied N.,ETH Zurich | Wild K.M.,ETH Zurich | Amann F.,ETH Zurich | Nussbaum C.,Federal Office of Topography Swisstopo | And 2 more authors.
International Journal of Rock Mechanics and Mining Sciences | Year: 2015

Opalinus Clay, a Mesozoic clay shale, has been chosen as host rock formation for the disposal of radioactive waste in Switzerland. For this study, borehole damage zones were utilized as a proxy for an excavation damage zone that forms around a circular, mechanically excavated tunnel in intact Opalinus Clay. Pilot boreholes were resin-impregnated and over-cored at different times after drilling, to provide insight into the time-dependent formation of fractures on both the micro- and the macro-scale. Observed fractures were characterized in terms of failure mode, their relation to the rock anisotropy, and the in-situ stress tensor. The analyses show that fractures that form in the short term initiate as shear fractures at the pilot-borehole wall and propagate parallel to bedding. Typically, a dominant shear fracture tangential to the pilot borehole wall was observed. Upon propagation of these shear fractures, wing cracks, horsetail splays and second-order shears form sub-parallel and sub-perpendicular to bedding planes, forming a complex fracture network, which extends a quarter pilot-borehole diameter into the rock mass. In the longer term, tangential shear fractures tend to propagate in a direction opposite to the initial propagation direction. In addition, new bedding-parallel fractures deeper in the rock develop, leading to the formation of thin slabs, buckling of the slabs when unsupported and eventually progression of the buckling zone deeper into the rock mass. Buckling is associated with the formation of extensional fractures normal to bedding in the center and lateral to the buckling zone. The zone of buckled rock slabs was found to have an extension of more than one borehole diameter at the time of preservation with resin. In the short term, the axis connecting the maximum failure depth on opposing sides of the borehole is parallel to the minimum stress direction in a plane normal to the borehole axis. In the long term, this axis rotates significantly towards the maximum stress direction, primarily as a consequence of tangential shear fracture propagation, slab formation and buckling. Dissipation of excess pore pressures may be the key process underpinning longer-term fracture propagation and formation. © 2015 Elsevier Ltd.

Lutz S.,Federal Office of Topography Swisstopo | Beutler G.,University of Bern | Schaer S.,Federal Office of Topography Swisstopo | Dach R.,University of Bern | Jaggi A.,University of Bern
GPS Solutions | Year: 2014

The International GNSS Service (IGS) issues four sets of so-called ultra-rapid products per day, which are based on the contributions of the IGS Analysis Centers. The traditional (“old”) ultra-rapid orbit and earth rotation parameters (ERP) solution of the Center for Orbit Determination in Europe (CODE) was based on the output of three consecutive 3-day long-arc rapid solutions. Information from the IERS Bulletin A was required to generate the predicted part of the old CODE ultra-rapid product. The current (“new”) product, activated in November 2013, is based on the output of exactly one multi-day solution. A priori information from the IERS Bulletin A is no longer required for generating and predicting the orbits and ERPs. This article discusses the transition from the old to the new CODE ultra-rapid orbit and ERP products and the associated improvement in reliability and performance. All solutions used in this article were generated with the development version of the Bernese GNSS Software. The package was slightly extended to meet the needs of the new CODE ultra-rapid generation. © 2014 Springer-Verlag Berlin Heidelberg

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