Slope Stability

Cliff failure. Simulated failure sequence of a 20 m high homogenous cliff with an undercut depth of 9 m.

Conventionally, the stability of rock slopes is assessed using limit equilibrium methods. Often these methods are limited to the study of planar or wedge instabilities and other complex failure kinematics (e.g., toppling) are generally not addressed. It follows that the failure mechanisms are often overlooked in the assessment of the hazard of instability and, subsequently, the risk to down slope structures. Furthermore, quantitative prediction of rock fall and run-out trajectories is of extreme importance in hazard assessment of natural and engineered slopes and in the design of protective measures. Although a more accurate representation of the slope behaviour can obtained by continuum-based numerical models, these models cannot fully describe the observed complex failure mechanisms characterized by a combination of intact material breakage, sliding along discontinuities, and rigid body instability.

Geomechanica’s Irazu simulation software can reproduce multiple modes of instability as well as the subsequent fragmentation, runout, and trajectories of failed material without assuming the mode or volume of failure a priori. Since Irazu can account for internal stress distribution, interaction between discrete blocks, and intact material fragmentation, it has the potential to largely overcome the drawbacks associated with limit equilibrium analyses and simulate the entire process’ dynamics. An Irazu model is able to capture several phenomena related to rock slope stability, including:

  • Planar sliding and toppling failure
  • Progressive failure (crack nucleation, growth and coalescence)
  • Rock falls
  • Dynamic fragmentation

Further Reading:

  • Lisjak A and Grasselli G (2011). "Rock slope stability under dynamic loading using a combined finite-discrete element approach". In Proceedings of the Pan-Am CGS Geotechnical Conference. Toronto. 2-6 October, 2011.
  • Lisjak A and Grasselli G (2010). "Rock impact modelling using FEM/DEM". In Proceedings of the 5th International Conference on Discrete Element Methods. London. 25-26 August, 2010.
  • Lisjak A, Spadari M, Giacomini A and Grasselli G (2010). "Numerical modelling of rock falls using a combined finite discrete-element approach". In Proceedings of the Rock Slope Stability Symposium. Paris. 24-25 November, 2010.
Simulation highlights
  • No need for assuming the failure mode a priori
  • Deformable and fracturable rock
  • Rock fall trajectories with dissipative impact model
  • Rock joint shear strength degradation
  • Static and dynamic loading
  • Discrete Fracture Network (DFN) capability with frictional or cohesive discontinuities

Failure of a 190 m high open pit slope with a pre-existing discrete fracture network (DFN). DFN properties were dip: -45 ± -4.5º, spacing: 4 ± 0.4 m, length: 8 ± 0.8 m, and bridge: 8 ± 0.8 m.