There has been some awesome stuff posted on the forums about back-filling post holes with a combination of gravel and sand. This allows the posts to not have soil contact, and allows water to drain away, prolonging the life span of the posts.
I am building a 36 foot x 20 foot structure with a gabled roof with 6 posts. I want to tightly back fill the post holes with gravel and sand, and also have some gravel and sand beneath the post so it is not sitting on soil.
My question is, will the gravel/sand backfill have enough strength to resist lateral pressure that wind exerts on a building?
I have this topic discussed but I haven't been able to tell if it is specifically for fence posts, or if it could also work for larger buildings.
I figure if the back-fill loosens up over time, that I could get a wedge shaped rock and hammer it into the back fill, tightening it back up.
I asked this same question of a professional pole building contractor here in wet Oregon. He said for extra support he likes to use a cement collar (a ring of cement around the top section of the pole). This ring would be tapered several inches above ground for water run off (if necessary) and to keep soil from the wood where it meets the ground. In Oregon water gets into the wood, but is able to escape as the cement collar is only 10" deep. Then as you say, rock is used beneath and around for good drainage. The cement collar is a bit more ridged than the rock so it helps strengthen the large poles used in horse barns, etc. I did some searching and found other types of pole collars, as well as using wood or metal beams to cross brace poles. If you are interested in more details of how the bracing is done just let me know and I will go into more detail. Apparently this bracing was how it was done in the old days.
Around here in MD the collar idea is used for fence posts and adds a lot of stability, even when the ground is saturated. Many of the wood pole building installers now coat their poles with latex where they go in the ground. Not very green I suppose, and I wonder how moisture that gets in the wood, would ever get out.
I would be very concerned about putting wood in the ground, even with sand and rocks around. Hopefully you have a soil type with lots of loam and very little clay so it drains quickly. Here, it is all clay and the sand around the pole would simply create a pond of water that would never dry out. A few years ago I planted 300 tulip bulbs in the fall, and every one rotted by springtime. I think I would opt for raised earth, stone filled ditch, plinth, or some other method than putting wood in the ground. Also put the building up higher than surrounding soil (grading) for good drainage, and large roof overhangs and gutters/downspouts to get the water away from the building site. There is another good thread on here about raised earth foundations.
Where do you live? Climate? I only have mobile access and the mobile site doesn't display any into about the poster.
I look forward to hearing from others that know more than I.
Location: SW Ohio, 6b, heavy clay prone to hardpan
posted 4 years ago
Brett makes a very good point and one I was going to mention as well. Heavy, poorly drained soils (like the clay I have here in SW Ohio) will create a pool of water around the post that saturates the sand/stone fill, and accelerates rot. Locust can last several years but most other wood stays water saturated and softens quickly. Water pools in post holes so well here, that if you sink a green post it will often sprout. We actually has nine, fifteen foot tall green posts (we put up a temporary tarp shelter for a large gathering) put out some twigs and leaves, a few weeks after we placed them. They were cut green and placed in a three foot deep post hole backfilled with crushed gravel with fines. These were placed on high ground that drained down into a creek about thirty yards away. Even with the slope and nearby drainage, the post sprouted like they were prized cuttings placed in a vase.
From what I can see on soil maps, you have a similar soil in southern Illinois.
If that is true and you are also dealing with heavy clay, I wouldn't consider placing wood in the ground unless you are planning a major water management project around the building. TBH, I still wouldn't do it. For anything permanent, here, we put stone/concrete in the ground and the wood on top of that.
Below is a very easy to understand explanation of using plinth stones by JC White Cloud whom is very smart and experienced about these things. In the link I mentioned above in my previous comment, he mentions large stones (maybe like a plinth?) under structural members, and smaller stones to fill out the remainder of the foundation to hold your wall bottom plate.