To optimise strategic decisions and maximise project value it is important to be able to model multiple scenarios. Modelling scenarios in significant detail can be time and cost prohibitive; modelling too coarsely can lead to the wrong decisions. It is important to find the right balance in modelling for efficiency and accuracy.
One compromise that usually needs to be made in strategic planning for open pit mines is to use pit shells for scheduling rather than pit designs; as pit designs are time consuming. To generate pit stages for scheduling, mine planners will often use “nested” pit shells (a series of pit shells generated at increasing commodity prices). The smaller shells tend to be higher margin and hence lead to higher project values when mined first.
There are times when these nested pit shells do not represent realistic pit stages. In the output below, the blocks in plan are coloured by pit shell with the hot colours representing initial shells (higher priority) and colder colours representing final shells (lower priority). Note that there are a number of “pods” with the same pit shell number. Many strategic scheduling packages model these pods as a single stage, when in reality they are mined separately. However, even if the software could separate these pods from each other, this deposit cannot be mined by nested pit shells. This project is space constrained and the pits need to be backfilled. Thus mining areas need to be completed quickly to provide waste storage capacity. Mining by nested shells does not complete areas; rather it mines some material then comes back later to complete.
In response to this Snowden developed an automation tool to generate more practical stages for strategic scheduling whilst still allowing for preferential mining orders. We call it Stage Automator. Stage Automator takes the inventory within a shell and an ordered string file from the mine planner to code stages into the block model, ensuring practical mining stages are created by adhering to:
- Minimum and maximum ore tonnages within each stage.
- Spatial contiguity of the blocks within the stage i.e. separate out blocks that do not touch each other into separate stages.
- Removing stages with fewer than a specified number of tonnages.
- Wall angle precedences associated with your ordering of stages.
Running this tool on the example rapidly produced the following rationalised pit staging. This enabled practical stages to be scheduled and provided confidence to the strategic analysis. The time saved was able to be devoted to running more scenarios (including the consideration of alternative staging plan) to find the best overall project strategy.