Theoretically, the Petersburg mine seems a sound tactic. As was mentioned earlier, this approach was tried at Vicksburg. I found a brief article that indicated that it was done at least twice at Vicksburg, with the second mine containing 1,800 pounds of black powder. Perhaps that is why at Petersburg they used 8,000 pounds of black powder.
There was a unit that trained for the attack but they were pulled off and the lead was given to an untrained unit. Even if this had not happened human nature may have derailed the attack. People are easily distracted by large explosions and big holes and this might not have been something that the attacking unit could prepare for. From my reading about the Crater I got the impression that a lot of the negative attitude towards the project was because it required constructing a shaft unlike what was typically encountered in coal mining. In order for the whole thing to work they needed to supply fresh air to the shaft as it was being dug and after it was finished. This problem was overcome and in the process advance mining technology for the future. So, even if it didn’t succeed as a military operation during the War, it did spin off technology that benefited future generations. Of course, I know nothing about mining history so I don’t know if what I read is accurate or if mines already operated with air pumps to allow longer and deeper shafts.
Just a small aside, apparently when the Union was trying to tunnel under Confederate lines at Vicksburg, the Confederate were trying to counter-mine to find the shaft. Instead of using soldiers to do this, they used slaves. There was a group of 8 slaves digging a countermine when the second Union mine was exploded. Seven of the 8 slaves were killed instantly, but the eighth on was flung 150 feet through the air and into the Union lines. He was able to pick himself up and dust himself off. The soldiers that he landed among installed him in a tent and then charged other soldiers a small fee to look at him; like a sideshow exhibit. The article is at url:
https://emergingcivilwar.com/2019/02/06/blowed-to-freedom-abraham-and-the-vicksburg-mine/