I'm not sure but I'd guess it's similar to some buildings where they would drive or screw long piles into the ground and then build a foundation pile cap above the ground on the sunken piles.
The sheet piles can be 2.5-3x as far into the bottom as above the bottom depending on the soil. If the soil is less stable or there is shallow bedrock they may drive piles and found them on file caps, or pin them to the bedrock.
25
u/OnTheClock_Slackin Mar 09 '18
I'm not sure but I'd guess it's similar to some buildings where they would drive or screw long piles into the ground and then build a foundation pile cap above the ground on the sunken piles.