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Estimating Rock Strength of Moderately Fractured EDZ in Hard Rock Tunnels: Äspö Hard Rock Laboratory

  • Author / Creator
    Lu, Yun
  • Underground openings create various excavation-induced responses. The most noticeable response is the readily observed excavation damaged zone (EDZ) where the mechanical properties of the rock mass have been irreversibly altered, i.e., damaged. Within this damaged zone the stresses and displacements cannot be predicted using elastic theory. This damaged zone contains a region of reduced stresses and increased displacements and forms a rock mass ring that can contribute to the stability of the underground opening. This concept is well known and provided the stability of EDZ support ring is maintained, large underground excavations can be created at a great depth. However, predicting and quantifying the extent and properties of the rock mass mechanical damage that can occur in the EDZ has remained a challenge. The damage that occurs in the EDZ in typical strong rocks (ISRM Class R3 to R6) can be characterised as stress-induced and/or blast-induced fractures. These fractures can range from the millimetre to the metre scale. It is well known that the addition of cracks to a solid changes its mechanical properties and characteristics. Hence to quantify these changes both geometry and properties of the induced fractures must be known. Once these changes are quantified, the biggest challenge is translating these changes into rock mass mechanical properties. Recent developments with numerical methods have shown that discrete element modelling offers the most realistic methodology for estimating rock mass properties, provided the characteristics and geometry of the discrete fractures are known with confidence. The methodology, which has been termed as the Grain-based Model (GBM), can incorporate both intact blocks and discrete fractures. The GBM utilizes a voronoi tessellation scheme to simulate the microstructure of rocks by creating randomly composed blocks that are similar to the polygonal grains of intact rocks. The GBM can be calibrated to the conventional laboratory tests (UCS test, direct tension test and triaxial compression test) on intact rocks. It is shown that such calibration is not path dependent, i.e., calibrating to the direct tensile strength is able to predict the compressive strength. Once the GBM calibration with the intact properties was carried out, large-scale discrete fractures were added to the GBM to determine the effect of these fractures on the rock mass mechanical properties. To validate the GBM approach, several models were developed and the mechanical responses were compared to published results. The models increased in complexity from a single inclined fracture to two regular and uniformly spaced fracture sets. The responses from the GBM models were in good agreement with the results from the physical models. When the fractures are not uniformly spaced there is no simple means to establish the complex geometry of the fractures. At the Äspö Hard Rock Laboratory in Sweden, a unique experiment was carried out to establish the geometry of the excavation-induced and naturally occurring fractures found at the boundary of tunnel excavated by careful drill-and-blast technique. The fractures were measured at the mm scale. A methodology was developed to import the geometry of the fractures into the GBM. This model was then loaded to establish the mechanical properties. The results from the models were compared to the rock mass strength derived using the empirical Hoek-Brown failure criteria. The finding from the research demonstrates that the GBM and provides a reasonable approach to establish the rock mass strength at the tunnel scale. The methodology also provides an approach for establishing the effect of the blast-induced damage on the rock mass strength.

  • Subjects / Keywords
  • Graduation date
    2014-11
  • Type of Item
    Thesis
  • Degree
    Doctor of Philosophy
  • DOI
    https://doi.org/10.7939/R3KH0F70T
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.
  • Language
    English
  • Institution
    University of Alberta
  • Degree level
    Doctoral
  • Department
    • Department of Civil and Environmental Engineering
  • Specialization
    • Geotechnical Engineering
  • Supervisor / co-supervisor and their department(s)
    • Martin, Derek (Civil and Environmental Engineering)
  • Examining committee members and their departments
    • Sego, Dave (Civil and Environmental Engineering, University of Alberta)
    • Chalaturnyk, Rick (Civil and Environmental Engineering, University of Alberta)
    • Fairhurst Charles (Civil, Environmental, Geo - Engineering, University of Minnesota)
    • Joseph, Tim (Civil and Environmental Engineering, University of Alberta)
    • Jar, Ben (Mechanical Engineering, University of Alberta)