Excavation-induced damage development

Irazu simulation of the excavation of a full-scale emplacement test tunnel in a bedded clay shale formation (Opalinus Clay, Mont Terri underground rock laboratory, Switzerland).

The stability of underground openings is an issue of the utmost importance for a number of geoengineering applications, including the geological disposal of nuclear waste, borehole drilling, and the construction of tunnels at depth. The underground storage of solid radioactive waste is currently being assessed in several countries worldwide. However, one main concern is that the favourable long-term isolation properties of the host rock may be negatively affected by the rock mass damage associated with the excavations comprising the underground repository, namely the Excavation Damaged Zone (EDZ). In the oil and gas industry, the stability of boreholes is a long-standing problem, which results in substantial yearly expenditure. In the presence of instabilities, drilling and production are impacted by several additional costs in the form of time and equipment losses, washouts, problematic logging, and sidetracking. Similarly, during tunnelling operations, construction and ground support difficulties have been reported when excavating road or hydroelectric tunnels in highly stressed brittle rock formations.

The vast majority of numerical models adopted to analyze the stability of underground openings are based on continuum mechanics principles using classic shear failure theory for elasto-plastic materials. These methods, however, are limited in their ability to fully reproduce typical brittle failure mechanisms, including strain localization, fracturing and micro-seismicity, as well as large displacements and rotations in fractured rock masses.

Geomechanica’s Irazu simulation software offers unique advantages when the extended loss of continuity inside the rock, for example due to the progressive material breakdown, makes continuum models inappropriate. Furthermore, the adopted discrete element algorithms are perfectly suited to capture finite displacements and rotations of discrete bodies, including complete detachment, and to automatically recognize new contacts as the simulation progresses. Since the Irazu simulations explicitly reproduce the spontaneous nucleation and propagation of cracks within the rock mass, unique insight into the brittle fracturing process around underground excavation can be obtained. That is, the entire evolution of rock mass damage and associated failure and deformational mechanisms (e.g., shearing, spalling, dilation, bulking) can be simulated, thus leading to a much improved understanding of the problem.

Quantitative information, such as fracture length, orientation and aperture, stress redistribution, and displacements, can be readily obtained from the model. From a constructability viewpoint, the Irazu modelling results can be then used to determine the most appropriate excavation method and sequencing, to optimize the support design, and ultimately to control damage development. In the context of the geological storage of nuclear waste, the assessment of performance and long term safety of a repository system, including the simulation of radionuclide transport towards the biosphere, may benefit from the geometric information (e.g., fracture aperture and network interconnectivity) that can be extracted from Irazu simulations for a range of different geomechanical scenarios.

Further Reading:

  • Lisjak A, Grasselli G and Vietor T (2013). "Numerical investigation of failure mechanisms around unsupported circular excavations in clay shales". International Journal of Rock Mechanics and Mining Sciences.
  • Lisjak A (2013). "Investigating the influence of mechanical anisotropy on the fracturing behaviour of brittle clay shales with application to deep geological repositories". Ph.D. Thesis, University of Toronto. Toronto, Canada (URL )
Simulation highlights

  • Elastic models including isotropic and transversely isotropic
  • Isotropic and anisotropic strength models
  • Model heterogeneity with arbitrary number of material types
  • Discrete Fracture Network (DFN) capability with frictional or cohesive discontinuities
  • Arbitrary in situ stress conditions (isotropic, anisotropic, gravitational)
  • Staged excavation and liner installation
  • Micro-seismic monitoring function

Irazu simulation of the excavation of a full-scale emplacement test tunnel in a bedded clay shale formation (Opalinus Clay, Mont Terri underground rock laboratory, Switzerland).