Although I am a Realtor, my husband is a builder/contractor with 25 years experience.
He suggests that you begin by testing the soil, making sure that is not wet or soft.
Your holes should be dug below the freeze line - generally 42 inches - 44 inches deep.
You'l need 12 foot - tubes/footings and he suggest that you add 4 rebar (sp?) in each sonitube...or another alternative is to use what is called a "big foot" tube.
You can give him a call if you need additional information at 203-515-0280. His name is Nicholas.
Hope this helps.
I would want someone who really knows to give the best answer here. The reasons is simple. You not only have to worry about frost heaving. You also need to worry about wind stresses and weight from the structure trying to move everything around. 8' of empty space with nothing touching the ground is a lot of stress on any post trying to put it all together.