The trick with concreted fence posts is to bed the bottom end
of the post in several inches of gravel before pouring concrete.
Any moisture entering the post can leave out the bottom, and the
bottom isn't held in direct contact with moisture. PT posts
will last a very long time when done properly.

Depending of course on the water table. They'd be in the
water all winter where I am.

Bob



It rains all fall, winter, and spring, but the water table stays down 6-8
feet or so. I think I'd be safe with a 30 inch hole with 6 inches of gravel
at the bottom. Maybe less gravel.