I just got one cave-in protected on my support, and it lost durability. Went down to about 42% durability. I repaired it with a single nugget and it went up to... 46? or 48?
That means I'd have to put in more than I spent to build the support in the first place, if it went down even lower than 40%.
This is with a wooden support, for the record. Haven't tried a stone support yet.
solved as of this thread.