Lisjak A.,University of Toronto |
Grasselli G.,University of Toronto |
Vietor T.,National Cooperative for the Disposal of Radioactive Waste
International Journal of Rock Mechanics and Mining Sciences | Year: 2014
The stability of circular excavations in clay shales is a key issue in the drilling and tunnelling industries as well as in the field of deep geological waste storage. A large body of experimental evidence indicates that the damaged zone around these cavities is influenced by strong mechanical anisotropy induced by the layered material structure. The vast majority of numerical models adopted to date to analyse the stability of openings in layered rocks have been based on continuum mechanics principles using classic shear failure theory for elasto-plastic materials. However, a number of experimental observations demonstrate that clay shales may fail in a brittle manner under low-confinement conditions such as those characterizing the near-field of the excavation. Therefore, an alternative numerical approach based on non-linear fracture mechanics principles and the discrete element method is adopted to gain new insight into the failure process of this class of geomaterials. In order to account for the influence of clay shale microstructure on its mechanical behaviour a newly developed approach to capture the anisotropy of strength is proposed. With this numerical approach, the cohesive strength parameters of the fracture model are assumed to be a function of the relative orientation between the element bonds and the layering orientation. The effectiveness of the numerical technique is quantitatively demonstrated by simulating standard rock mechanics tests on an indurated claystone, namely Opalinus Clay. Emergent strength and deformation properties, together with the simulated fracture mechanisms, are shown to be in good agreement with experimental observations. The modelling technique is then applied to the simulation of the Excavation Damaged Zone (EDZ) around a circular tunnel in horizontally bedded Opalinus Clay. The simulated fracturing process is mainly discussed in the context of the damage mechanisms observed at the Mont Terri URL. Furthermore, the influence of in situ stress on resulting EDZ geometry is analysed together with possible implications for ground support and tunnel constructability. Modelling results highlight the importance of shear strength mobilization along bedding planes in controlling the EDZ formation process. In particular, slippage of bedding planes is shown to cause rock mass deconfinement which in turn promotes brittle failure processes in the form of spalling. The numerical technique is currently limited to two-dimensional analyses without any thermo-hydro-mechanical coupling. © 2013 Elsevier Ltd.
Vogt T.,Eawag - Swiss Federal Institute of Aquatic Science and Technology |
Vogt T.,National Cooperative for the Disposal of Radioactive Waste |
Schirmer M.,Eawag - Swiss Federal Institute of Aquatic Science and Technology |
Cirpka O.A.,University of Tubingen
Hydrology and Earth System Sciences | Year: 2012
River-water infiltration is of high relevance for hyporheic and riparian groundwater ecology as well as for drinking water supply by river-bank filtration. Heat has become a popular natural tracer to estimate exchange rates between rivers and groundwater. However, quantifying flow patterns and velocities is impeded by spatial and temporal variations of exchange fluxes, insufficient sensors spacing during field investigations, or simplifying assumptions for analysis or modeling such as uniform flow. The objective of this study is to investigate lateral shallow groundwater flow upon river-water infiltration at the shoreline of the riverbed and in the adjacent riparian zone of the River Thur in northeast Switzerland. Here we have applied distributed temperature sensing (DTS) along optical fibers wrapped around tubes to measure high-resolution vertical temperature profiles of the unsaturated zone and shallow riparian groundwater. Diurnal temperature oscillations were tracked in the subsurface and analyzed by means of dynamic harmonic regression to extract amplitudes and phase angles. Subsequent calculations of amplitude attenuation and time shift relative to the river signal show in detail vertical and temporal variations of heat transport in shallow riparian groundwater. In addition, we apply a numerical two-dimensional heat transport model for the unsaturated zone and shallow groundwater to obtain a better understanding of the observed heat transport processes in shallow riparian groundwater and to estimate the groundwater flow velocity. Our results show that the observed riparian groundwater temperature distribution cannot be described by uniform flow, but rather by horizontal groundwater flow velocities varying over depth. In addition, heat transfer of diurnal temperature oscillations from the losing river through shallow groundwater is influenced by thermal exchange with the unsaturated zone. Neglecting the influence of the unsaturated zone would cause biased interpretation and underestimation of groundwater flow velocities. The combination of high resolution field data and modeling shows the complex hydraulic and thermal processes occurring in shallow riparian groundwater close to losing river sections as well as potential errors sources for interpreting diurnal temperature oscillations in such environments. © 2012 Author(s).
Labiouse V.,Ecole Polytechnique Federale de Lausanne |
Vietor T.,National Cooperative for the Disposal of Radioactive Waste
Rock Mechanics and Rock Engineering | Year: 2013
In the context of nuclear waste disposal in clay formations, laboratory and in situ simulation experiments were performed to study at reduced scale the excavation damaged zone (EDZ) around tunnels in the indurated Opalinus Clay at Mont Terri, Switzerland. In the laboratory, thick-walled hollow cylindrical specimens were subjected to a mechanical unloading mimicking a gallery excavation. In samples cored parallel to bedding, cracks sub-parallel to the bedding planes open and lead to a buckling failure in two regions that extend from the borehole in the direction normal to bedding. The behaviour is clearly anisotropic. On the other hand, in experiments performed on specimens cored perpendicular to bedding, there is no indication of failure around the hole and the response of the hollow cylinder sample is mainly isotropic. The in situ experiment at Mont Terri which consisted in the overcoring of a resin-injected borehole that follows the bedding strike of the Opalinus Clay showed a striking similarity between the induced damaged zone and the fracture pattern observed in the hollow cylinder tests on samples cored parallel to bedding and such a bedding controlled Excavation Damaged Zone is as well consistent with the distinct fracture patterns observed at Mont Terri depending on the orientation of holes/galleries with respect to the bedding planes. Interestingly, the damaged zone observed in the hollow cylinder tests on samples cored parallel to bedding and in situ around URL galleries is found to develop in reverse directions in Boom Clay (Mol) and in Opalinus Clay (Mont Terri). This most probably results from different failure mechanisms, i.e. shear failure along conjugated planes in the plastic Boom Clay, but bedding plane splitting and buckling in the indurated Opalinus Clay. © Springer-Verlag Wien 2013.
Sellin P.,Swedish Nuclear Fuel and Waste Management Company |
Leupin O.X.,National Cooperative for the Disposal of Radioactive Waste
Clays and Clay Minerals | Year: 2014
Geological disposal is the preferred option for the final storage of high-level nuclear waste and spent nuclear fuel in most countries. The selected host rock may be different in individual national programs for radioactive-waste management and the engineered barrier systems that protect and isolate the waste may also differ, but almost all programs are considering an engineered barrier. Clay is used as a buffer that surrounds and protects the individual waste packages and/or as tunnel seal that seals off the disposal galleries from the shafts leading to the surface. Bentonite and bentonite/sand mixtures are selected primarily because of their low hydraulic permeability in a saturated state. This ensures that diffusion will be the dominant transport mechanism in the barrier. Another key advantage is the swelling pressure, which ensures a self-sealing ability and closes gaps in the installed barrier and the excavation-damaged zone around the emplacement tunnels. Bentonite is a natural geological material that has been stable over timescales of millions of years and this is important as the barriers need to retain their properties for up to 106 y. In order to be able to license a final repository for high-level radioactive waste, a solid understanding of how the barriers evolve with time is needed. This understanding is based on scientific knowledge about the processes and boundary conditions acting on the barriers in the repository. These are often divided into thermal, hydraulic, mechanical, and (bio)chemical processes. Examples of areas that need to be evaluated are the evolution of temperature in the repository during the early stage due to the decay heat in the waste, re-saturation of the bentonite blocks installed, build-up of swelling pressure on the containers and the surrounding rock, and degradation of the montmorillonite component in the bentonite. Another important area of development is the engineering aspects: how can the barriers be manufactured, subjected to quality control, and installed? Geological disposal programs for radioactive waste have generated a large body of information on the safety-relevant properties of clays used as engineered barriers. The major relevant findings of the past 35 y are reviewed here.
Gaus I.,National Cooperative for the Disposal of Radioactive Waste
International Journal of Greenhouse Gas Control | Year: 2010
Before implementing CO2 storage on a large scale its viability regarding injectivity, containment and long-term safety for both humans and environment is crucial. Assessing CO2-rock interactions is an important part of that as these potentially affect physical properties through highly coupled processes. Increased understanding of the physical impact of injected CO2 during recent years including buoyancy driven two-phase flow and convective mixing elucidated potential CO2 pathways and indicated where and when CO2-rock interactions are potentially occurring. Several areas of interactions can be defined: (1) interactions during the injection phase and in the near well environment, (2) long-term reservoir and cap rock interactions, (3) CO2-rock interactions along leakage pathways (well, cap rock and fault), (4) CO2-rock interactions causing potable aquifer contamination as a consequence of leakage, (5) water-rock interactions caused by aquifer contamination through the CO2 induced displacement of brines and finally engineered CO2-rock interactions (6). The driving processes of CO2-rock interactions are discussed as well as their potential impact in terms of changing physical parameters. This includes dissolution of CO2 in brines, acid induced reactions, reactions due to brine concentration, clay desiccation, pure CO2-rock interactions and reactions induced by other gases than CO2. Based on each interaction environment the main aspects that are possibly affecting the safety and/or feasibility of the CO2 storage scheme are reviewed and identified. Then the methodologies for assessing CO2-rock interactions are discussed. High priority research topics include the impact of other gaseous compounds in the CO2 stream on rock and cement materials, the reactivity of dry CO2 in the absence of water, how CO2 induced precipitation reactions affect the pore space evolution and thus the physical properties and the need for the development of coupled flow, geochemical and geomechanical models. © 2009 Elsevier Ltd. All rights reserved.