One angle to look at the question regarding poles in the ground is to ask yourself this question: How many old houses do you know that have this construction. This will give you the answer how long the poles in the ground will last.
I would not recommend underground poles for anything that is supposed to last more than 10 years. Wooden fences are made this way and is the end of the pole is charred, the wood cut in the right time of the year there is a chance they last 10-20 years and are then replaced.
For houses, even if you are building on a solid rock you should use stones if you want the base of the house to last.
If the ground is something else than rock, it should be dug about one meter deep (this is an estimate and completely depends on the situation) and filled with sand and a large rock put on top. You only have to use rocks in the corners, or every six meters (this is how long logs you should use on top of the rocks).
Here is an image, taken under a house:
http://www.moreeni.net/talo/lattian_alta.jpg
You can see that the house is resting on rocks that are dug underground and logs on top of them.
On the bottom of this page is and image of a house that has regular stones and log on top. (The page is in finnish) If you click on the links to the left there are some other example images of different houses.
Here is another one. The stones are just laid on top of each other:
http://www.perinnemestari.fi/?id=65&id2=73
My take on this is that wood and earth should not be in contact unless you want to build something temporary.
Good luck with your project
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