I am building a deck. How about setting 6x6 p.t. posts down on a concrete footing at 4 feet deep (the frost line here) and fill with gravel and soil, and then let the posts run high for the railings? This is instead of doing concrete piers and setting posts on them above ground. I may be doing posts like this near the house to avoid flashing needs there, so I could use the stiffness from the posts being done like a pole barn. Any issues with frost pulling on the sides of posts, or does it only push up at the bottom?
It works for pole barns, it will work for your deck. However, be careful of the quality and/or treatment of your PT lumber - I've seen pressure treated lumber rot off in the ground despite being pressure treated. I think the thicker sections (such as 6x6) are rather difficult to get throughly treated all the way through. I've seen some pole barns use 3 2x6 laminated (though that was ALSO a trick to be able to have PT in the ground and non-PT above the ground, by having a staggered splice in the layers after it was above ground.) Presumably there are more carefully done ones that are "post or piling" grade and less well done ones that are "landscape timber" but in many cases there's just one type in stock, take it or leave it - so you might want to look into it more and figure out when you should leave it and shop elsewhere, or special-order something different.
In my opinion, it is better in the long run to use concrete footings and concrete piers.
You can use pressure-treated but do not use typical pressure treated lumber, you should use something with a treatment rated for ground-contact/in-ground use. The advantage to burying your posts is little to no lateral bracing is needed and it is less work over all.
The advantage of concrete piers is longevity and easier maintenance. It's a lot easier to replace a post that is not buried.
Whatever you decide, you should check your local building codes to see if you will comply...
I've been a deck builder for 20 years,i always dig a hole below frostline install the 6x6 post and fill around it with concrete.....I have never had a deck settle or heave with this practice! Not only does it give you a very stable post/posts to build on but it also keeps the deck from swaying without braces. I've done over 4000 decks,never had a problem! You have to make sure you have ground contact treated posts,home depot/Menards treated isn't enough....make sure it's the real dark green treated not the pale green color...
I have seen decks built as you described, with the posts in concrete. Very recently I saw where concrete was poured in the bottom of the hole for a footer, and then the post was put on top of that with just dirt around it. This was to code and approved by an inspector. Post in the ground 20-30 years. I have built 3 decks recently and have put the posts on footers above the ground because I feel better about keeping the wood above the dirt.