I also
think it would kill the tree. If it didn't the tree roots would likely start coming up into the
berm.
That being said, 20+ years ago I let some workers talk me into filling around a 70' poplar to a depth of about 5' where I was developing a patch of
lawn for the kids to play in.
They stacked rock against it -they said for protection. Within a year or two 10' of bark split and made a huge vertical sore on it.
Ants have worked it. Not a pretty sight.
It has stunted the growth and the top looks thin. It has lost maybe a few more than average limbs for a tree it's age, but it's still alive.
I wouldn't do it again though.