3D LARGE OPEN PIT (LOP) MODELLING
There is a trend for open pits to be mined to greater depths than ever before. The greater depths are leading to higher underlying strains in the slope and failures arising from rock mass instability have become more prominent. These failures include instabilities resulting from the combined effects of induced slope damage and structure, as well as structurally controlled failures that pure kinematic analysis does not predict.
Successful slope modelling requires simulation of the complete stress path for every slope as well as an appropriate representation of the geometry of the problem. Naturally, the governing physics of the slope response must also be captured. There is also a requirement that the simulated displacements be correct; if a non-linear model of a slope is unable to replicate displacements, it will not have captured the extent and magnitude of yield in the slope, and vice-versa.
Figure 1: Example of a detailed 3D LOP-model
Requirements for successful 3D large open pit slope analysis
These fundamental requirements suggest:
- a 3D representation of the mine geometry
- incorporation of all geological structures and domains on scales relevant to the problem
- the correct consideration of initial stress field conditions
- the use of material models and higher order elements able to simulate the stress-strain behavior of rock from the intact to significantly yielded states.
The emergence of large 3D FE modelling tools with higher order elements and the inclusion of faults and geological contacts with an improved capacity to simulate dislocation and block movements allows much better simulation of many of these mechanisms.
To properly replicate slope behavior, the modes of material yield and slope failure must be able to be simulated. Figure 1 shows some conceptual slope failure modes and the yield mechanisms that contribute to them.
Figure 1: Failure modes to be simulated in proposed conceptual model (Haile 2006)
In Figure 2, the plastic strain distribution on the surface and along a vertical cutting plane of a large open pit model is shown. The FE results reproduce the phenomena listed in Figure 1.
Figure 2: Example of a LOP FE model with ~ 2 million DOF : Plastic strain distribution on the surface and along a vertical cutting plane
Example pictures of LOP wall failure and stability analysis
The pictures below illustrate the evaluation of the stability of a particular region of open pit walls using (in)stability indicators based on plastic strain, damage, displacements and averaged rock mass acceleration with a 3D non-linear, strain-softening, dilational finite element model.
Figure 3: Log. plastic strain distribution for an open pit during mining operations.
Figure 4: Displacement magnitude for an open pit during mining operations.