I am trying to have my yard graded after having a new home built. The problem is that my garage is set low on the property in flood zone. A landscaper had recommended placing a 2 by 12 along side of garage to protect garage from new soil. He would then grade accordingly. The only pressure treated 2 by 12s I have found are for above ground use. How limited would there lifespan be? Would I be better off with two 4 by 6's that could be used below ground?