Modelling of Hard Rockmasses with Non-Persistent Joints to Assess the Stress Induced Damage of Deep Excavations

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Authors
Vazaios, Ioannis
Keyword
Deep Tunnelling , Hard Rockmasses , Brittle Failure , Excavation Damage Zone (EDZ) , Numerical Modelling , Finite-Discrete Element Method (FDEM) , Discontinuum Modelling , Strain Bursting
Abstract
Brittle fracturing within low confinement environments is the dominant failure mechanism of hard rockmasses under high magnitude in situ stresses around underground excavations. With an emphasis on the behaviour of slightly to moderately jointed rockmasses, this body of research investigates the influence of pre-existing joints on the fracturing mechanisms occurring and the excavation induced damage surrounding an underground opening. To achieve this goal, advanced numerical approaches involving the generation of discontinuity networks and discontinuum analysis methods were employed. Discontinuity geometrical data obtained from laser scans of an unsupported tunnel served as input parameters for the generation of discrete fracture networks (DFNs), and the material properties of the massive granite encountered at the Canadian Underground Research Laboratory (URL) were used to develop a grain-based numerical model using the distinct element method (DEM) enriched with DFN geometries. The created synthetic rockmass models (SRM) were used to determine the effect of pre-existing structure on the driving failure mechanisms. Field observations from the URL were also used to calibrate a tunnel scale model by applying the finite-discrete element method (FDEM) to simulate the tunnel response within a massive rockmass, and serve as the reference model for subsequent analyses by integrating DFNs. SRM results provided useful insights and an S-shaped strength envelope derived from bi-axial tests revealed the role of low confinement in brittle fracturing. From the same tests, evaluation of the rockmass modulus resulted in a semi-empirical equation for estimating it based on discontinuity parameters. Following the validation of the ability of the FDEM method to capture brittle processes observed at the URL, results from the jointed models demonstrated the influence of discontinuities on the material response during an excavation. The strong impact of the joint network geometry and field stresses on the excavation induced damage and overall stability of an opening was revealed. Regarding strain bursting phenomena, the ability of discontinuities to dissipate and/or enhance them was examined. The capability of this approach to capture the complex energy storage and energy release, and fracturing mechanisms was demonstrated along with its potential practical application in engineering design.
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