So you ended up with the pole in the ground about 33", are you happy with that depth if it is only to be used as one pole for a hammock? The other end going to a tree.
Actually, I know already where gas comes in. Power I get from lines above ground. And I sure as heck know where the sewer line is because not 2 months in this house I had to replace it.
I don't have a solid engineering answer. Making it up as I go. But this I do know...where the pole meets the ground is a fulcrum. If I tie the hammock at 6' above the ground, and the pole extends 3' into the ground, there is a 2:1 translation of force at 6' above to the bottom, all other things being equal, which they aren't. My estimate of that force, for my weight, above ground is about 175 lbs. So I'm looking at 350 lbs below ground. Concrete would not "give" in the hole the way pea gravel would, because it is a solid mass, and a better choice I'm guessing when you have lateral forces on the pole.
If it were just me and I didn't have an engineer who knew what he was talking about I could quiz, I'd sink a 4' hole, drop a 10' pole in there,
fill the hole with concrete, and set the pole at a distance where attaching at 5' above ground is enough.
But with concrete maybe 33" is enough. If after doing this you thought the pole leaned too much under weight it would straight forward to put a diagonal brace from pole to ground, against the shear force. If the brace was firmly planted on a good rock or dug-in concrete block that would surely provide all the backup you needed. Or so my gut tells me.
I agree, sometimes those projects go better is they are multi-use.
------- AKA "4D's" ----------------
"Oh yeah, to keep it on topic. I sewed on my hammock today"
"A night above ground makes the next days ride even more enjoyable"
"As for his secret to staying married: "My wife tells me that if I ever decide to leave, she is coming with me." -- Jon BonJovi
Happy Wife = Happy Life" -- author unknown
In my girlfriend's daughter's treehouse version, I used carriage bolts 8" and 10" and suspended the supports below with steel cables.
The cables form a loop where they protrude through the bottom of the treehouse support. These are for attaching hammocks among other things. You can see one attachment point in the upper left of the first photo.
The second image shows some of the cable suspension arrangement. I just finished this a few weeks ago.
Designer of the Semi-Gathered Bridge hammock
This would avoid other braces that would be in the way.