So you defer to S. Ball and no longer speculate ?
He may have known the secret to keeping a keg of powder dry in a damp environment ,and a flash pan too. That would have been a sight to see attempted . And quite the keg I'd imagine.
Depending on stability of the ground 90 feet below ground level it might have remained intact. Dry? Not so much.
Rats at 90 feet? humans were weren't they? IF Ball rat proofed all works involved , perhaps no rats.
I'd need to read his confidence in ascending after such a detonation rigging and how nothing was loose or dislodged or potentially dislodged enough to do the rigging without risk of something falling and tripping a flashpan rigged to go off..(Well you know how it was supposed to go off ...).
Descending after time elapsed on soft walled works ? A good job for someone else. Even if the powder should have been damp.
Old accounts of hand dogging well diggers in clay hint that non supported clay walls are by no means eternal. And among the worse for instability and risk of sudden collapse/closing.
en.wikipedia.org