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Mahabadi O.K.,University of Toronto | Mahabadi O.K.,Geomechanica Inc. | Tatone B.S.A.,University of Toronto | Grasselli G.,University of Toronto
Journal of Geophysical Research: Solid Earth | Year: 2014

This study investigates the influence of microscale heterogeneity and microcracks on the failure behavior and mechanical response of a crystalline rock. The thin section analysis for obtaining the microcrack density is presented. Using micro X-ray computed tomography (μCT) scanning of failed laboratory specimens, the influence of heterogeneity and, in particular, biotite grains on the brittle fracture of the specimens is discussed and various failure patterns are characterized. Three groups of numerical simulations are presented, which demonstrate the role of microcracks and the influence of μCT-based and stochastically generated phase distributions. The mechanical response, stress distribution, and fracturing process obtained by the numerical simulations are also discussed. The simulation results illustrate that heterogeneity and microcracks should be considered to accurately predict the tensile strength and failure behavior of the sample. Key Points Influence of heterogeneity on fracture patterns Influence of rock microcracks on the strength of the material Accurate results using reliable inputs and rock heterogeneity and microstructure © 2014. American Geophysical Union. All Rights Reserved.

Ha J.,University of Toronto | Lisjak A.,Geomechanica Inc. | Grasselli G.,University of Toronto
49th US Rock Mechanics / Geomechanics Symposium 2015 | Year: 2015

This work demonstrates the potential of using the hybrid finite-discrete element method (FDEM) to model thermal and mechanical coupling mechanisms around a wellbore in shale formations. The simulated thermal and mechanical stresses were first validated in separate models by comparing them to closed-form solutions. Then, the stability of a borehole in Opalinus Clay (Mont Terri, Switzerland) was analyzed. The prominent mechanism affecting the wellbore stability was found to be the mud pressure applied to the excavation boundary. At lower in-situ stresses, the extent of fracturing due to tensile stresses from cooling the rock by -100 °C was greater than at higher stress regimes. This was due to the greater stress differential between the mud pressure and the stress field in the higher stress regime. The higher mud pressure counteracts the thermal stresses and thus less fractures develop. It was also observed that the choice of constitutive model (i.e., isotropic versus anisotropic) had little effect on the temperature change required for fracture initiation, but it did influence the fracture pattern due to preferential planes of weakness present in the bedded Opalinus Clay. Copyright 2015 ARMA, American Rock Mechanics Association.

Tatone B.S.A.,Geomechanica Inc. | Grasselli G.,University of Toronto
49th US Rock Mechanics / Geomechanics Symposium 2015 | Year: 2015

Discontinuities are prevalent in most rock masses and represent planes of preferential deformation and fluid flow. Shear displacements can significantly alter the hydro-mechanical properties of discontinuities and, thus, the hydro-mechanical properties of the entire rock mass. The importance of the network of discontinuities in controlling rock mass behavior has long been known and has led to an abundance of research on discontinuity shear strength and transmissivity. Although well-studied, key aspects of discontinuity behavior and characterization have received lesser focus, namely the evolution of asperity damage and discontinuity void space morphology as a result of shearing. In this paper, a methodology that combines the use of two recent technologies (micro-X-ray computed tomography and combined finite-discrete element modeling) is described to study these aspects of discontinuity behavior. Copyright 2015 ARMA, American Rock Mechanics Association.

Lisjak A.,University of Toronto | Lisjak A.,Geomechanica Inc. | Garitte B.,National Cooperative for the Disposal of Radioactive Waste | Grasselli G.,University of Toronto | And 2 more authors.
Tunnelling and Underground Space Technology | Year: 2015

The Opalinus Clay formation is currently being investigated as a potential host rock for the deep geological disposal of radioactive waste in Switzerland. Recently, a test tunnel was excavated at the Mont Terri underground rock laboratory (URL) as part of a long-term research project ("Full-scale Emplacement (FE) experiment") aimed at studying the thermo-hydro-mechanical (THM) effects induced by the presence of an underground repository. The objective of this paper is twofold. Firstly, the results of the rock mass monitoring programme carried out during the construction of the 3. m diameter, 50. m long FE tunnel are presented, with particular focus on the short-term deformation response. The deformation measurements, including geodetic monitoring of tunnel wall displacements, radial extensometers and longitudinal inclinometers, indicate a strong directionality in the excavation response. Secondly, the deformational behaviour observed in the field is analyzed using a hybrid finite-discrete element (FDEM) analysis to obtain further insights into the formation of the excavation damaged zone (EDZ). The FDEM simulation using the Y-Geo code is calibrated based on the average short-term response observed in the field. Deformation and strength anisotropy are captured using a transversely isotropic, linear elastic constitutive law and cohesive elements with orientation-dependent strength parameters. Overall, a good agreement is obtained between convergences measured in the field and numerical results. The simulated EDZ formation process highlights the importance of bedding planes in controlling the failure mechanisms around the underground opening. Specifically, failure initiates due to shearing of bedding planes critically oriented with respect to the compressive circumferential stress induced around the tunnel. Slippage-induced rock mass deconfinement then promotes extensional fracturing in the direction perpendicular to the bedding orientation. The simulated fracture pattern is consistent with previous experimental evidence from the Mont Terri URL. © 2014 Elsevier Ltd.

Lisjak A.,Geomechanica Inc. | Mahabadi O.K.,Geomechanica Inc. | Kaifosh P.,Geomechanica Inc. | Vietor T.,National Cooperative for the Disposal of Radioactive Waste | Grasselli G.,University of Toronto
Rock Engineering and Rock Mechanics: Structures in and on Rock Masses - Proceedings of EUROCK 2014, ISRM European Regional Symposium | Year: 2014

Experimental evidence based on microseismic data clearly show that fluid-pressure-driven fractures interact with preexisting discontinuities, thus highlighting the strong influence of rock mass structures on hydraulic fracture development. However, these mechanisms are not well accounted for by analytical models and conventional continuum-based numerical approaches. The purpose of this paper is to assess the use of an alternative hybrid finite-discrete element (FDEM) code, enhanced with hydraulic fracturing capabilities, to model pressure-driven fracturing in jointed rock masses. The proposed approach is first validated by comparing the emergent pressure response and fracture patterns simulated under homogeneous isotropic conditions with available analytical solutions. Then, the effect of rock mass discontinuities on the hydraulic fracturing process is investigated for several joint geometries. © 2014 Taylor & Francis Group, London.

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