What happens if I use borate treated lumber, not rated for ground contact, as ground contact? Okay, okay, I know the answer is a resounding DON'T DO IT.
The problem is that where I live (Maui) the only lumber available locally is borate treated, above-ground use only. Ground-contact rated lumber is available by special order only, which would be fine if I were building a huge project but is cost prohibitive for a couple of 4x12's and 4x4's. This is essentially a small backyard project, think of something like a mini-retaining wall, and some of the wood will be below ground. What if I were to coat the wood in something like an asphalt roofing cement? I know it won't survive long enough for my great-grandchildren to enjoy it, but realistically, how long is something like this going to last? I know there's a bajillion variables, so I'm really just looking for information so I can decide if it's worth it.
Well to me it sounds like the little project you are working on is not for living space at all and is just some kind of retaining wall.
I think as long as you are not retaining much, say 2' of dirt or something like that then if it were my project I would probably go for it knowing that is will last maybe 4-8 years.
Don't ask how I came up with those numbers, I am just pulling them from you-know-where.