How deep can that go? Can you fit yourself and shovel in the
ring to go deeper? Is there any danger of collapse?
I went with an inner diameter of 1m, this is fine for a small shovel, a pump, a small jackhammer.
Concrete rings are used all the time in road construction work. I don't see a risk of collapse with those when stacking carefully. Even if one ring were to get a crack, where should it go? I felt safe in this regard