I'm building an attached deck and have already buried precast piers in the ground. Unfortunately, I had to dig so deep that the tops of the piers are below finished ground level. I'd rather not bury the 6x6 posts. The tops of the piers are 8"x8". Should I use sonotubes and pour concrete on top of the piers (assuming I can jury rig round sonotubes on top of a square pier top) to bring the piers above ground level? How about putting a post sleeve around the post to keep it away from the soil? I know metal and today's PT lumber aren't friendly with each other, so could I use plastic sleeves?
Pressure treated lumber is rated for ground contact. A common way of creating footers that I have seen is to pour concrete down in the hole, but below grade. Then just put the post on top of that and fill with dirt. I personally wouldn't do that, but I have heard of it being done. I would probably pour concrete in a tub on top of the footers and bring them up out of the ground 6-8 inches. The gravel isn't a bad idea.