my friend was bushing rabbits with his 8 yr old russell last weekend and it dropped in a big place. never been dug and not been one for going to ground.
two others dropped in for a short while, but my mates russell has not been seen since. the lads have run 2 separate dogs with collars on through, dropped holed and pushed rods through tubes but no avail, they spent 3 whole days there.
for that reason, in my part of the world a terrier bushing is just not worth the risk, IMO