I did some googling, and it seemed minimal. That said, we chose cedar for its rot resistance and lack of chemicals. I added the visqueen barrier to prevent contact between the wood and the soil, so the only possible leaching is going to be between the bottom of the studs and the existing clay back into the trimix. I think it'll be slim to none at the end of the day.
I did get 2 more yards, should have taken the extra big scoop dude offered, but I was scared of breaking my truck. 4000# is a lot for an old half ton frame.
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But it did it. It was a little squishy, but nothing resembling sketchy. I was surprised once again. To my knowledge, I never got into the bump stops either.
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Hindsight being what it is, I wish I'd taken the extra generous scoop.
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I'm just a couple inches shy of where I want to be, story of my life. Next is to get some sort of hail protection up.