As Xander Bakker is suggesting, I would suppose there would be some preferences that may lead to the optimal shapes. The first thing that comes to mind is a pragmatic "what it the minimum sized strip of land that can be mined?". This may be governed by something as simple as the minimum-sized bulldozer/excavator that it is practical/economical to work with. This may come into play as the output of the split may include a long thin sliver.
Also, for economies of scale, would it be best if the polygon was all compact, or would it instead be OK to sprawl?
Or is there there a central processing location, then the whole site needs to be split into 5 with each "slice" touching the central area?
If you have not already, I'd come up with all the factors that need to be considered. This is important as once those are established, that may lead to totally different solution processes on the technical side to come up with the split. There are several folks on GeoNet who are really good at the many ways to "slice and dice" a polygon, but to get them involved the criteria would need to be known, as the solutions are varied and explaining them can get complicated.
Chris Donohue, GISP
