Pushback Generator: Key Functions

To access this screen:

Display the Optimization task bar and select Optimization >> Scheduler >> Maroma >> Settings >> Periods.

The pushback generator will:

Preserve distances between pushback rims on each bench as required by user defined mining width.
Ensure that all pushbacks include only contiguous sets of model blocks.
Obtain pushbacks of regular shapes avoiding narrow strips and recesses, sharp corners etc.
Obtain pit bottoms of size implied by mining width.
Avoid small local depressions and pikes.
Control pushback sizes whenever it is possible without violating mining width requirements.

To create a pushback sequence:

The pushback sequence is created in the following steps:

The final pit obtained from the Ultimate Pit OES is formatted to obtain pit bottoms of size implied by mining width and to remove small local depressions and pikes. This step is skipped if the first option in the Mining Width dialog is not selected.
Pushbacks are generated one by one starting with pushback 1. For each pushback, several “Pushback Candidates” satisfying conditions (1) - (5) are created and the one closest to the required size is selected. This process ends when there is not enough material left in the final pit to make a pushback of required size.
The material left at the completion of step 2 may form several disjoint small pits. In this final step, these small pits are joined to earlier pushbacks as far as possible.

To create a pushback "candidate":

A Pushback Candidate (a pushback that will be considered as part of the solution) is created in the following steps:

Using the Ultimate Pit OES as the guide, an area of contiguous blocks where the pushback would ideally be located to maximize NPV is identified.
This area is formatted bench by bench from top down to regularize the pit shape, see goal (2) above. The technique to achieve this bears some similarity to the techniques used in image processing.
The pit bottom bench of a required size is established.
The pit is formatted from the bottom bench up expanding the pit in some areas and shrinking it in others in the pursuit of goal (1), that is to satisfy the mining width requirements.
The last step is to process the pit from top down to ensure that it be contiguous and regularized at each bench.

Mining Width Control

“Mining Width” is the key parameter affecting pushback creation at all steps listed in the previous section. Relatively large values of this parameter may make it impossible to get pushbacks of desired size when strictly following the Ultimate Pit OES. The work around in such cases is to use pushback adjustment tool.
There are three options for pushback generation:
- Run the new generator with final pit formatting as the initial step.
- Run the new generator preserving the final pit defined in Ultimate Pit settings.
- Run the old generator.
The parameter defined as the percentage of the desired (user defined) size of the last contiguous pushback specifying the maximum size of a remnant pits that can be added to preceding pushbacks.

Note: Large additions to pushbacks can change their shapem and small remnants consisting of few blocks will be added automatically whatever the value of this parameter.
If Update Ultimate OES is checked, the Pushback Generator calls the Pit Optimization routine to produce OES starting at the last completed pushback as the initial topography and ending at the final pit. The rational to do so is this.
To maximize NPV of Pushback OES, we need to follow Ultimate Pit OES as closely as possible. For example, ideally for the first pushback we would like to take first 1000 blocks from the Ultimate Pit OES. However, once the pushback is formatted it may consist of only 800 blocks from among the first 1000 from the Ultimate Pit OES and 200 from the ones with sequence numbers larger than 1000. These 200 blocks are referred to as “out of sequence”. The remaining Ultimate Pit OES with blocks in the first pushback deleted is not any longer optimal for the remaining pit, so updating it can theoretically improve NPV. In this example “Percentage out of sequence blocks in the pushback” parameter set to 20 or less would trigger recalculation of the Ultimate Pit OES.
Minimum remnant parameter defines surface area size; if an area between pushback and final pit rims is smaller than this parameter, it must be added to the pushback.

Obstacle Removal

Suppose that pit optimization is conducted in the presence of obstacles removable at given costs and one or more of these obstacles is removed, according to the Ultimate Pit OES towards the end of the mine life.

If the Pushback OES, due to formatting, encroaches on these obstacles early on, NPV can be seriously compromised. To avoid this, we need to define obstacle boundaries as “Pushback Exclusion Zones” and submit them using the Pushback Settings Size Control screen.

Create Pushback Obstacle Adjustments can be used in such cases. The command is active only when a pit limits boundary file is defined in Pit Optimization Settings (Options tab) and the boundary file contains cost obstacles.