Kirkkonummi, Finland
Kirkkonummi, Finland

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Rinne M.,Fracom Ltd. | Shen B.,CSIRO | Backers T.,Geomecon GmbH
Journal of Rock Mechanics and Geotechnical Engineering | Year: 2013

The Äspö Pillar Stability Experiment (APSE) was conducted to study the rock mass response in a heated rock pillar between two large boreholes. This paper summarizes the back calculations of the APSE using a two-dimensional (2D) fracture propagation code FRACOD. To be able to model all the loading phases of the APSE, including the thermal loading, the code was improved in several ways. A sequential excavation function was developed to model promptly the stepwise changing loading geometry. Prior to the modelling, short-term compressive strength test models were set up aiming to reproduce the stress-strain behaviour observed for the Äspö diorite in laboratory. These models simulate both the axial and lateral strains of radial-controlled laboratory tests. The volumetric strain was calculated from the simulations and compared with the laboratory results. The pillar models include vertical and horizontal 2D models from where the stress in the pillar wall was investigated. The vertical model assesses the stability of the experimental rock volume and suggests the resultant stress below the tunnel floor in the pillar area. The horizontal model considers cross-sections of the pillar between the two large boreholes. The horizontal model is used to simulate the evolution of the stress in the rock mass during the excavation of the boreholes and during and the heating phase to give an estimation of the spalling strength. The modelling results suggest that the excavation-induced stresses will cause slight fracturing in the pillar walls, if the strength of the APSE pillar is set to about 123. MPa. Fracture propagation driven by thermal loading leads to minor spalling. The thermal evolution, elastic behaviour and brittle failure observed in the experiment are well reflected by the models. © 2013.

Shen B.,CSIRO | Stephansson O.,German Research Center for Geosciences | Rinne M.,FRACOM Ltd. | Amemiya K.,Hazama | And 3 more authors.
International Journal of Geomechanics | Year: 2011

The characterization of the excavation-damaged zone (EDZ) around an underground excavation is a major research topic for deep geological disposal of medium- to high-level radioactive waste. Rock fracturing because of excavation and thermal loading and its resultant rock mass permeability change in the EDZ are important for construction projects and long-term safety. A new function to predict rock mass permeability change in fractured rocks has been developed and added to the existing fracture mechanics code FRACOD. The new functions in FRACOD have been applied to predict the extent of EDZ and permeability change in the vicinity of the tunnel sealing experiment (TSX) tunnel of the Underground Research Laboratory (Canada), the zone of excavation disturbance experiments (ZEDEX) tunnel of the Äspö Hard Rock Laboratory (Sweden), and the deposition tunnels in crystalline and sedimentary rocks (Japan). The predicted EDZ and its permeability are consistent with the measurement data of the TSX tunnel and ZEDEX tunnel. The results from both tests indicate that FRACOD with the new function is capable of realistically predicting the EDZ and permeability change. © 2011 American Society of Civil Engineers.

Shen B.,CSIRO | Ko T.Y.,SK e and C | Lee S.C.,SK e and C | Kim J.Y.,SK e and C | And 8 more authors.
Computer Methods for Geomechanics: Frontiers and New Applications | Year: 2011

This paper describes a recent study on simulations of coupled Fracture (F) - Thermal (T) - Hydraulic (H) processes of rocks, with focus on the development a fracture mechanics code that predicts fracture initiation and propagation under thermal and hydraulic loadings. The new development is based on a numerical code FRACOD which is capable of simulating both mode I (tensile) and mode II (shear) fracture propagations that are common in rock masses. In this study, the thermalmechanical coupling in FRACOD was developed using an indirect method based on fi ctitious heat sources and a time-marching scheme. The hydro-mechanical coupling in FRACOD was focused on fl uid fl ow in explicit rock fractures using the cubic law. An explicit iteration method was used to simulate the fl uid fl ow process in fractures and its interaction with the mechanical deformation. Two verifi cation and applications cases have been included in the paper that demonstrate the effectiveness of the coupled functions.

Shen B.,CSIRO | Kim H.-M.,Korea Institute of Geoscience and Mineral Resources | Park E.-S.,Korea Institute of Geoscience and Mineral Resources | Kim T.-K.,SK Engineering and Construction SKEC | And 4 more authors.
Rock Mechanics and Rock Engineering | Year: 2013

This paper describes a boundary element code development on coupled thermal-mechanical processes of rock fracture propagation. The code development was based on the fracture mechanics code FRACOD that has previously been developed by Shen and Stephansson (Int J Eng Fracture Mech 47:177-189, 1993) and FRACOM (A fracture propagation code - FRACOD, User's manual. FRACOM Ltd. 2002) and simulates complex fracture propagation in rocks governed by both tensile and shear mechanisms. For the coupled thermal-fracturing analysis, an indirect boundary element method, namely the fictitious heat source method, was implemented in FRACOD to simulate the temperature change and thermal stresses in rocks. This indirect method is particularly suitable for the thermal-fracturing coupling in FRACOD where the displacement discontinuity method is used for mechanical simulation. The coupled code was also extended to simulate multiple region problems in which rock mass, concrete linings and insulation layers with different thermal and mechanical properties were present. Both verification and application cases were presented where a point heat source in a 2D infinite medium and a pilot LNG underground cavern were solved and studied using the coupled code. Good agreement was observed between the simulation results, analytical solutions and in situ measurements which validates an applicability of the developed coupled code. © 2012 Springer-Verlag.

Shen B.,CSIRO | Rinne M.,Fracom Ltd. | Kim T.K.,SK E and C | Lee J.M.,CSIRO | And 7 more authors.
72nd European Association of Geoscientists and Engineers Conference and Exhibition 2010: A New Spring for Geoscience. Incorporating SPE EUROPEC 2010 | Year: 2010

The paper describes a recent numerical code development and laboratory investigations on coupled thermal-mechanical processes of rock fracture propagation. A series of laboratory tests were conducted on rock strength and fracture toughness within a temperature range from -80 C to 400 C, and key temperature dependent parameters were obtained on granite specimens. The numerical development is based on a fracture mechanics code FRACOD that has previously been developed by some of the authors of this paper. The code simulates complex fracture propagation in rocks governed by both tensile and shear mechanisms. For the latest development an indirect boundary element method, namely the fictitious heat source method, is implemented in FRACOD to simulate the temperature change and thermal stresses in rocks. This method is particularly suitable for the thermal-mechanical coupling in FRACOD where the displacement discontinuity method is used for mechanical simulation. An example case is presented where a borehole drilled into the rock formation. Depending on the initial reservoir rock temperature, cooling fractures may or may not occur in the borehole wall from the same differential temperature between the borehole wall and the reservoir rock. © 2010, European Association of Geoscientists and Engineers.

Andersson J.C.,Vattenfall | Feng X.,Chinese Academy of Sciences | Pan P.,Chinese Academy of Sciences | Koyama T.,Kyoto University | And 11 more authors.
Rock Mechanics in Civil and Environmental Engineering - Proceedings of the European Rock Mechanics Symposium, EUROCK 2010 | Year: 2010

This paper presents results of the 1st stages ofTaskAof the Decovalex 2011 project, the numerical modeling of the Äspö Pillar Stability experiment performed by the Äspö Hard Rock Laboratory of the Swedish Nuclear Fuel and Waste Management Company (SKB). The objective is to perform back calculation of the Äspö pillar behavior using state of the art numerical modeling techniques for the material behavior. The work is divided into three stages and it is the first stage of thework that will be presented in this paper. Seven international teams from six different countries participated in the task and contributed to the results presented in this paper, concerning back calculation of uniaxial and triaxial compressive core testing and elastic back calculation of the stress path for excavation-induced stresses. The results are useful for understanding the occurrence of spalling in the upper part of the pillar during excavation and the stress path modeling gives the first approximation of the yielding strength of the pillar. The calculated results agree well with observations measured during experiment. © 2010 Taylor & Francis Group.

Siren T.,Posiva Oy | Kemppainen K.,Posiva Oy | Shen B.,CSIRO | Rinne M.,Fracom Ltd.
Harmonising Rock Engineering and the Environment - Proceedings of the 12th ISRM International Congress on Rock Mechanics | Year: 2012

Fracture propagation code (FRACOD) is a two-dimensional Displacement Discontinuity Method (DDM) computer code that was designed to simulate fracture initiation and propagation. The latest development introduced in the code allows the possibility to simulate anisotropy of rock medium using strength anisotropy instead of just explicit joints and bedding planes. Anisotropy related to new fracture initiation is described by direction dependent Mohr-Coulomb and direct tensile strength criteria. The fracture propagation function is converted to anisotropic by formulating F-criterion to be direction dependent.A case example is presented where the code is used in fracture mechanics prediction of Posiva's Olkiluoto Spalling Experiment (POSE). The POSE experiment will be described in more detail in an accompanying paper by Kemppainen et al. elsewhere in this proceeding. In the case example laboratory results of the anisotropic behaviour of the rock are used in the simulations. Special attention in modelling is paid to analysing the loading conditions under which spalling will occur. The results show that the fracture propagation is very sensitive to changes in the anisotropy direction, friction angle and cohesion. However, the fracture toughness is observed not to be a very sensitive parameter. Modelling results suggest minor spalling on the pillar surface while observations from the field shows slight fracture slipping of an existing fracture. © 2012 Taylor & Francis Group, London.

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