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  • 1.
    Figueiredo, B.
    et al.
    Portuguese Lab Civil Engn LNEC, Lisbon, Portugal..
    Cornet, F. H.
    CNRS, IPG S, Strasbourg, France..
    Lamas, L.
    Portuguese Lab Civil Engn LNEC, Lisbon, Portugal..
    Muralha, J.
    Portuguese Lab Civil Engn LNEC, Lisbon, Portugal..
    Determination of the stress field in a mountainous granite rock mass2014In: International Journal of Rock Mechanics And Mining Sciences, ISSN 1365-1609, E-ISSN 1873-4545, Vol. 72, p. 37-48Article in journal (Refereed)
    Abstract [en]

    The design of an underground hydroelectric power scheme in northern Portugal has required the characterisation of the local stress field. Nineteen hydraulic tests have been concluded in two, 500 m deep, vertical boreholes. In addition twelve overcoring tests together with twelve fiat jack tests have been performed from an existing adit located some 1.7 km away horn the location of the hydraulic tests. Results have been integrated into a stress model that takes into account both topography and tectonics effects. Most of the data are consistent with a linearly elastic, gravity loaded model, provided a very soft geomaterial is considered. This implies that the stress field in this granite rock mass is controlled by gravity alone and shear stress relaxation along faults and fractures but is unaffected by present-day tectonics. (C) 2014 Elsevier Ltd. All rights reserved,

  • 2.
    Figueiredo, Bruno
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Tsang, Chin-Fu
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. Univ Calif Berkeley, Lawrence Berkeley Natl Lab, Berkeley, CA 94720 USA..
    Rutqvist, Jonny
    Univ Calif Berkeley, Lawrence Berkeley Natl Lab, Berkeley, CA 94720 USA..
    Niemi, Auli
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    A study of changes in deep fractured rock permeability due to coupled hydro-mechanical effects2015In: International Journal of Rock Mechanics And Mining Sciences, ISSN 1365-1609, E-ISSN 1873-4545, Vol. 79, p. 70-85Article in journal (Refereed)
    Abstract [en]

    This paper presents a numerical study of the hydro-mechanical behaviour of a fractured rock domain at 1000 m depth below the land surface as a function of different levels of fluid pore pressure. A 2D fractured rock domain is adopted based on data obtained from outcrop mapping, displaying multiple fracture sets, fracture intersections, dead-end and curved fractures. A continuum based numerical model is used to evaluate the effects of compressive boundary stresses, cracking by tension failure in the intact rock and fractures and shear displacement along fractures on its equivalent permeability. Two in situ stress boundary conditions are considered: an isotropic case SR1 with the two horizontal boundary compressive stresses having the same magnitude, and an anisotropic case SR2 with the ratio between these compressive stress components set to be 2. In the SR2 case, changes in the local stress and stress ratio distributions due to different fluid pore pressure levels are anisotropic and more significant than in the SR1 case, because of tension failures in the intact rock forming bridges between fractures. These failure regions opened new flow connections between fractures and thereby caused important anisotropic changes in the flow paths, and significant decrease in local gradients of fluid pore pressure. The equivalent permeability increases sharply when the fluid pore pressure is approximately 90% of the magnitude of the minimum stress at the boundaries of the fractured rock domain. Results show that the equivalent permeability of the fractured rock domain is most sensitive to the fractures normal stiffness, the permeability of the tension failure regions and the power-law exponent for permeability change.

  • 3.
    Fälth, Billy
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Hökmark, Harald
    Lund, Björn
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Simulation of co-seismic secondary fracture displacements for different earthquake rupture scenarios at the proposed nuclear waste repository site in Forsmark2016In: International Journal of Rock Mechanics And Mining Sciences, ISSN 1365-1609, E-ISSN 1873-4545, Vol. 84, p. 142-158Article in journal (Refereed)
  • 4.
    Latham, John-Paul
    et al.
    Department of Earth Science and Engineering, Imperial College, London, UK.
    Xiang, Jiansheng
    Belayneh, Mandefro
    Nick, Hamidreza M
    Tsang, Chin-Fu
    Department of Earth Science and Engineering, Imperial College, London, UK.
    Blunt, Martin J
    Modelling stress-dependent permeability in fractured rock including effects of propagating and bending fractures2013In: International Journal of Rock Mechanics And Mining Sciences, ISSN 1365-1609, E-ISSN 1873-4545, Vol. 57, p. 100-112Article in journal (Refereed)
    Abstract [en]

    The influence of in-situ stresses on flow processes in fractured rock is investigated using a novelmodelling approach. The combined finite-discrete element method (FEMDEM) is used to model thedeformation of a fractured rock mass. The fracture wall displacements and aperture changes aremodelled in response to uniaxial and biaxial stress states. The resultant changes in flow properties ofthe rock mass are investigated using the Complex Systems Modelling Platform (CSMPþþ). CSMPþþisused to model single-phase flow through fractures with variable aperture and a permeable rock matrix.The study is based on a geological outcrop mapping of a low density fracture pattern that includes therealism of intersections, bends and segmented features. By applying far-field (boundary) stresses to asquare region, geologically important phenomena are modelled including fracture-dependent stressheterogeneity, the re-activation of pre-existing fractures (i.e. opening, closing and shearing), thepropagation of new fractures and the development of fault zones. Flow anisotropy is investigated undervarious applied stresses and matrix permeabilities. In-situ stress conditions that encourage a closing offractures together with a more pervasive matrix-dominated flow are identified. These are comparedwith conditions supporting more localised flow where fractures are prone to dilatational shearing andcan be more easily exploited by fluids. The natural fracture geometries modelled in this work are notperfectly straight, promoting fracture segments that dilate as they shear. We have demonstrated theintroduction of several realistic processes that have an influence on natural systems: fractures canpropagate with wing cracks; there is the potential for new fractures to connect with existing fractures,thus increasing the connectivity and flow; blocks can rotate when bounded by fractures, bent fractureslead to locally different aperture development; highly heterogeneous stress distributions emergenaturally. Results presented in this work provide a mechanically rigorous demonstration that a changein the stress state can cause reactivation of pre-existing fractures and channelling of flow in criticallystressed fractures.

  • 5. Lei, Qinghua
    et al.
    Latham, John-Paul
    Xiang, Jiansheng
    Tsang, Chin-Fu
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Lang, Philipp
    Guo, Liwei
    Effects of geomechanical changes on the validity of a discrete fracture network representation of a realistic two-dimensional fractured rock2014In: International Journal of Rock Mechanics And Mining Sciences, ISSN 1365-1609, E-ISSN 1873-4545, Vol. 70, p. 507-523Article in journal (Refereed)
    Abstract [en]

    This paper aims to examine the validity of the discrete fracture network (DFN) method in representing a realistic two-dimensional fractured rock in terms of their geomechanical response to in situ stresses and hydraulic behaviour in a steady state fluid held. First, a real fracture network is extracted from the geological map of an actual rock outcrop, which is termed the analogue fracture network (AFN). Multiple DFN realisations are created using the statistics oldie analogue pattern. A conductivity parameter that was found to have a linear relationship with the conductivity of 2D fracture networks is included to further enhance network similarity. A series of numerical experiments are designed with far-field stresses applied at a range of angles to the rock domains and their geomechanical response is modelled using the combined finite discrete element method (FEMDEM). A geomechanical comparison between the AFN and as DFN equivalents is made based on phenomena such as heterogeneity of fracture-dependent stress contours, sliding between pre-existing fracture walls, coalescence of propagating fractures and variability of aperture distribution. Furthermore, an indirect hydro-mechanical (HM) coupling is applied and the hydraulic behaviour of the porous rock models is investigated using the hybrid finite element-finite volume method (FEFVM). A further comparison is conducted focusing on the hydraulic behaviour of the AFN and DFNs under the effects of geomechanical changes. The results show that although DFNs may represent an AFN quite well for fixed mechanical conditions, such a representation may not be dependable if mechanical changes occur. 

  • 6.
    Place, Joachim
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Ghafar, Ali Nejad
    KTH Royal Inst Technol, Dept Civil & Architectural Engn, S-10044 Stockholm, Sweden..
    Malehmir, Alireza
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Draganovic, Almir
    KTH Royal Inst Technol, Dept Civil & Architectural Engn, S-10044 Stockholm, Sweden..
    Larsson, Stefan
    KTH Royal Inst Technol, Dept Civil & Architectural Engn, S-10044 Stockholm, Sweden..
    On using the thin fluid-layer approach at ultrasonic frequencies for characterising grout propagation in an artificial fracture2016In: International Journal of Rock Mechanics And Mining Sciences, ISSN 1365-1609, E-ISSN 1873-4545, Vol. 89, p. 68-74Article in journal (Refereed)
    Abstract [en]

    Grouting the fractures encountered when constructing underground facilities is of primary importance for environmental, safety and economic reasons. The success of grouting operation, however, depends upon several parameters governing the grout propagation. Experimental benches replicating fractures have therefore been designed to study processes related to grout propagation. In this paper, we investigate the ultrasonic transport properties of such an idealized fracture whose 100 mu m aperture is about 0.02 the wavelength, and filled with various fluids flowing under external forcing. As the artificial fracture is made of two solid and parallel walls separated by a thin fluid layer, we use the thin fluid layer concept to study the compressional (P-) wavefield transmitted across and reflected off the fracture, with no mode-conversion considered. We demonstrate that air and various fluids (water, grouts of varied w/c - water to cement ratio) can be distinguished when injected into the fracture, both at atmospheric pressure or under over-pressure as done in real grouting cases in the field. Then, using an analytical solution, we verify our experimental data and predict the results that can be obtained with a different fracture aperture. Our results illustrate that replicating such ultrasonic measurements both in space and time would allow to monitor successfully the grout propagation within an artificial fracture. The detection of the filtration of the suspended cement particles of the grout, the formation and erosion of filter-cakes, are also in the scope of the method.

  • 7. Torabi, Anita
    et al.
    Zarifi, Zoya
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Energy release rate of propagating deformation bands and their hosted cracks2014In: International Journal of Rock Mechanics And Mining Sciences, ISSN 1365-1609, E-ISSN 1873-4545, Vol. 67, p. 184-190Article in journal (Refereed)
    Abstract [en]

    Cataclastic bands and their mechanical properties within two mineralogically different sandstones have been studied. In order to investigate the propagation energy of these bands, we have used dynamic elastic moduli calculated from laboratory measurements of ultrasonic velocities using a portable ultrasonic measuring system. As a product of cataclasis and compaction, elastic moduli (uniaxial compressive and shear moduli) of the bands are different from their corresponding host rocks leading to heterogeneity and anisotropy within sandstone reservoirs. Poisson's ratio decreases in the cataclastic bands of sandstone with high quartz content as a result of compaction, while it increases within cataclastic bands of sandstone with high clay and phyllosilicate content (more than 30%). Using the calculated propagation energy rates, we investigate which mode of deformation can be more dominant in the setting of our samples, propagation of deformation band as a result of strain hardening within the cataclastic band or crack propagation inside the deformation band due to strain softening prior to the propagation of band. We further estimate the maximum length of cracks inside a deformation band which can precede further growth of the deformation band. (C) 2013 Elsevier Ltd. All rights reserved

  • 8.
    Tsang, Chin-Fu
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Barnichon, J. D.
    Birkholzer, J.
    Li, X. L.
    Liu, H. H.
    Sillen, X.
    Coupled thermo-hydro-mechanical processes in the near field of a high-level radioactive waste repository in clay formations2012In: International Journal of Rock Mechanics And Mining Sciences, ISSN 1365-1609, E-ISSN 1873-4545, Vol. 49, p. 31-44Article in journal (Refereed)
    Abstract [en]

    The present paper provides an overview of key coupled thermo-hydro-mechanical (THM) processes in clay formations that would result from the development of a high-level radioactive waste repository. Here, in this paper, clay formations include plastic clay such as the Boom Clay of Belgium, as well as more indurated clay such as the Callovo-Oxfordian and Upper Toarcian of France and Opalinus Clay of Switzerland. First, we briefly introduce and describe four major Underground Research Laboratories (URLs) that have been devoted to clay repository research over the last few decades. Much of the research results in this area have been gained through investigations in these URLs and their supporting laboratory and modeling research activities. Then, the basic elements in the development of a waste repository in clays are presented in terms of four distinct stages in repository development. For each of these four stages, key processes and outstanding issues are discussed. A summary of the important areas of research needs and some general remarks then conclude this paper.

  • 9.
    Tsang, Chin-Fu
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. Lawrence Berkeley Natl Lab, Berkeley, CA USA.
    Figueiredo, Bruno
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Niemi, Auli
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Importance of stress effects on inputs to fracture network models used for subsurface flow and transport studies2018In: International Journal of Rock Mechanics And Mining Sciences, ISSN 1365-1609, E-ISSN 1873-4545, Vol. 101, p. 13-17Article in journal (Refereed)
  • 10.
    Öhman, Johan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Air and Water Science.
    Niemi, Auli
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Air and Water Science.
    Tsang, Chin-Fu
    Probabilistic Estimation of Fracture Transmissivity from Wellbore Hydraulic Data Accounting for Depth-Dependent Anisotropic Rock Stress2005In: International Journal of Rock Mechanics And Mining Sciences, ISSN 1365-1609, E-ISSN 1873-4545, Vol. 42, no 5-6, p. 793-804Article in journal (Refereed)
    Abstract [en]

    A new method is introduced that incorporates the use of hydrological and rock mechanical data in assigning transmissivities for fracture-network models. The hydrological data comes from fixed-interval packer tests carried out in a borehole and the rock-mechanical data are the prevailing in situ depth-dependent stress-field and the stress-closure relationship of fractures.

    In the model, the fracture transmissivity distribution is considered to be constituted of two components, one deterministic stress-induced component and the other a stochastic component that describes the intrinsic variability of fractures in a network. The outcome is a tensorial description of fracture transmissivities in an anisotropic stress-regime, where the transmissivity for an arbitrarily oriented fracture in the network is determined by its orientation in relation to the ambient stress-field. These transmissivities are conditioned such that the overall results satisfy the hydraulic packer test data. The suggested procedure is applied to an example data set from a site at Sellafield, England.

    The results show that the probabilistic approach, relying on hydraulic data alone, may underestimate the true variability in fracture transmissivities, since the typically vertical boreholes entail a sampling bias towards horizontal fractures that are predominantly subject to vertical stress. The suggested method helps to account for the true underlying three-dimensional variability that is incompletely resolved by using the hydraulic borehole data alone. This method is likely to have the largest impact at low stress-levels, in strongly anisotropic stress-fields, for borehole directions parallel to one principal stress, and for fracture network geometries characterized by sets orthogonal to the three principal stresses.

  • 11.
    Öhman, Johan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Niemi, Auli
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Tsang, Chin-Fu
    Öhman, J., A. Niemi, and C.-F. Tsang (2005), Probabilistic Estimation of Fracture Transmissivity from Wellbore Hydraulic Data Accounting for Depth-Dependent Anisotropic Rock Stress. Int. Journal of Rock Mechanics and Min. Sciences. 42(5-6): 793-804.2005In: International Journal of Rock Mechanics And Mining Sciences, ISSN 1365-1609, E-ISSN 1873-4545, Vol. 42, no 5-6, p. 793-804Article in journal (Refereed)
    Abstract
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