Unused sub-ducts will be sealed. I think the used ones are, I think, probably dealed as well. In any event, they will "poke out" of the actual ducting and they will be much less prone to inress. In comparison, ducting (especially very old stuff) is prone to brekages and distrurbances from tree roots, traffic vibration etc. That's especially true of very old ducts, which will not be constructed using modern plastic ducting.