Mike, years ago they used to make a "below grade rated" treated wood
that was a genuine Wolmanized product. It was rated to be used in your
application, and for decking on boat docks, etc.
Haven't seen that stuff in years, and they probably don't even make it
anymore with today's EPA requirements. But if you could find that, it
should last a long time.
But today's treated lumber is rarely more than a low pressure dip of
green wood in a pool of mildly preservative liquid. It is better than
nothing, but that would be where I would leave it.
I have seen some pretty good preservation from some of the old
buildings I have worked on. In some areas where the wood structure was
going to flood, they put creosote treated wood (think telephone poles
and railroad ties) that had been sawn into rough boards into the wet
areas, but scabbed them onto regular wood. In other words, the wet
area was scabbed, not the whole piece.
Alternately, when I was a kid we used to put a 1/4" or so layer of
roofing tar on fence posts when we replacing posts. We figured out how
deep they fence post would go, then went about 6" higher with the tar.
That worked great. But it was a fence.
But the real solution is what Joe said. Regrade your terrain to get
the joists out of the dirt/water exposure. You need to get the
moisture from the dirt and water away from the wood. No wood will ever
last a long time sitting in dirt.
Thanks guys, really useful comments.
Looks like I need to get my spade out then...