I don't know how far down they drive them, but they hold the whole dock up. I will get pics of the job and maybe we can find out how deep they go.
They are thick like telephone poles as you can probably see across the canal. The dock itself is bolted to the wall, and then they set in the big poles down into the bottom.
Okers.
When they replaced the bridge here, they drove steel I-beams down into the ground until they hit the greywhack substrate here.
(Greywhack, that neat silty rock that shatters into a billion sharp little pieces and can be crumbled with your bare hands. AFTER it's shredded your fingers of course.)
The stuff is only 20 foot down, but the creek follows the contours of a deep gouge in the underlying rock.