DT wrote:

I've done several trenchs using them. They will get stuck on any rock that is
wider than the trench. They also have trouble with large roots that cross the
path. For my last 120' trench I rented a mini excavator and it was much better,
but a lot slower. I wouldn't want to trench 800' with an excavator.

Just rent a bigger trencher... Vermeer has one that will cut through
granite.