Seismic Risk and Open Pit Slopes


The potential effects of earthquakes on the stability of open pit mines were addressed in this research work, also incorporated in Chapter 10 of the design guidelines book. The outcome of this study concluded that:

  • Topographic effects are important reasons why pit slopes are more stable than natural slopes.
  • Effect of seismic shaking on slope FOS best correlates with PGV.
  • Typically used seismic coefficients do not match well the results of dynamic elasto-plastic analyses but are conservative.
  • PGV has been shown to be a better indicator of seismic stability than PGA, which is a logical outcome given that velocity relates directly to stress. However, more work is required to use PGV to develop more representative (i.e., less conservative) seismic coefficients.  Additional work would involve 3D simulations, which Itasca showed would reduce the amplification factors with respect to the 2D simplifications.  Also, some more generic geologies into the simulations.  If successful, this work could have direct benefit to Sponsors in terms of more reliable and less conservative estimates of seismic stability.

Further, it was reported that:

  • although earthquakes have produced numerous large and small landslides in natural slopes, there are few, if any, recorded instances in which earthquakes have been shown to produce significant slope instabilities in hard rock open pit mines.
  • the process responsible for earthquake-induced landslides in natural rock slopes is generally considered to be topographic amplification, which is an increase in shaking associated with ridges and topographic changes. It is believed to be a function of slope geometry and seismic wavelength resulting from at least two interacting processes: focusing and de-focusing of seismic waves from the free surfaces of hills and canyons; and excitation of whole topographic edifices, which occurs when the wavelength of the incoming seismic wave is similar to the width of the topographic feature.
  • topographic amplification and consequent slope failure does not occur in large open pit slopes because the slopes are outside the range of geometries that would experience topographic amplification or, if amplification does occur, the slope geometries are such that the amplification is too weak to promote slope failure.