Welcome
You did mention that it's a deck being built. To me that means above ground timberwork however will need some posts in the ground.
I know there's regulation for deck construction, certain sized beams for certain spans, certain distance between floor board slats etc.
Found a link that explains the terminology.
Anatomy of a deck
Try to place the posts where there's no roots or lessor roots. Try to have a slight gap in the decking timber so water can go through it easier.
With your walkway also come above ground with a deck style walkway. A boardwalk. These are becoming very common.
boardwalk - Google Image Search