Jeff,
I'm not an engineer, just a home builder/remodeler, so I can't support this very well. Maybe others can help me out? Here is how I understand compaction.
Undisterbed soil is considerd 100% compact. It's also called virgin soil. There are exceptions to this, but overall, if it has never been dug in before, it's probably undesturbed and 100% compact.
When moving dirt and building up pads, the goal is to get that new soil as close as possible to 100% compaction. When I worked in California, the inspectore would send a signal into the ground, similar to how radar works. He's measure that signal to determine the amount of compaction in the soil. Our goal was something like 97% in order to pass the inspection.
Even with heavy equipmen, sheeps foot rollers ,building it up in lifts, and adding water, we didn't always pass the inspection. It's not easy to get soil compacted enough to pass a California inspection!!!
Using a Wacker brand, or jack-hammer styler compactor dos work, but it's slow going and takes allot of room. There are hydraulic tampers that pack the earth with a much smaller footprint, but again, it's even harder to get compaction with them. When I did pipe jobs, we ran the Wackers in the trench on top of a layer of sand. The sand was jetted for compaction, but teh dirt fill had to be hand packed. This was extremly dificult to do and time consuming. We literally spend days filling and compacting pipelines that only took one day to dig and install!!!!!
It would be extremly hard to get good compaction on a post that is dug with a backhoe and filled with dirt. It's not something that you will notice while building, and maybe if the weather never gets bad, you might never notice.
Here's what might happen. Rain water will penetrate dug earth much, much faster then virgin soil. It will literall fill up that hole just like a cup of water. The bigger the hole, the worse this is! Now the soil that might have 70-85% compaction is allot less. This soil will take a very long time to dry out. Now lets say there is a wind storm, thunder storm, or some sort of extreme weather with strong winds that come in. With these very common conditions, you can have total failure in your posts.
Locally, it's fairly common with carports. There was one that I know of that just came out of the ground last week. I don't know the exact details or soil conditions, but the wind pulled up the posts on two corners. Then the building twisted and snaped off the posts on the other side. The roof colapsed with those two poles half way out.
Eddie
