I don't know if it will help but, when a subway is dug, they recover the machines in the end
I know his is surface digging machines but they recovered them.
For "The Channel Tunnel" between France and England, they used two digging machine per tunnels.
Some of them were buried deeper and abandoned,some others were unbuild, one of them was recovered and is exposed on a roundabout.
Since they were made to measure for this tunnel, they were not made to be reused elsewhere.
I still do believe that a digging machine could not be reused (as it cost money to build they could want to reuse them).
Maybe it could get damaged (needs repairs), or lost if a critical malfunction append (simply considered as destroyed).
If destroyed, it could take some turns to extract the old one before using a new Miner robot on the mine.
for example
Number of mines | Complet rate | Failure rate |
1 mine | 100% | 33 % |
2 mine | 80% | 44 % |
3 mine | 60% | 55 % |
4 mine | 40% | 66 % |
5 mine | 20% | 77 % |
6 mine | 10% | 100 % |
after the first repair it could restart with a Higher failure rate
Number of mines | Complet rate | Failure rate |
1 mine | 90% | 34% |
2 mine | 65% | 56 % |
3 mine | 40% | 78 % |
4 mine | 15% | 100 % |
And so on
Number of mines | Complet rate | Failure rate |
1 mine | 85% | 40% |
2 mine | 55% | 70 % |
3 mine | 25% | 100 % |
And a critical failure rate that increase at each mine and never reset on repair or maybe a bonus of 20% for 1 turn only (it just got repaired)
Number of mines | Critical failure rate |
1 mine | 0% |
2 mine | 5% |
3 mine | 10% |
4 mine | 15% |
5 mine | 20% |
... | ... |
11 mine | 50% |
... | ... |
16 mine | 75% |
... | ... |
20 mine and more | 95% |
So the player could have some mines before a failure or a complete destruction of a miner.
Maybe the failure rate could grow much faster as it dig a deeper layer of the planet.
5% on surface layer, 6% on first underground layer, 7% next and so on.
It would make it harder to dig deeper.