Please correct me if I'm wrong but John Henry wan't a spike driver, he was a steel (or drill) driver. The long steel star dril was held horizontally by another man, called the shaker, who held the drill point against the rock face and gave it a little twist with each blow so the drill would't bind in the hole.
John Henry said to his shaker,
He said shaker you'd better pray,
Cause if I miss the drill with this nine pound hammer,
Tomorrow'll be your buryin' day, Lord, Lord,
Tomorrow'll be your buryin' day.
The shaker, using both hands, held the drill over one shoulder so that if the driver (John Henry) missed the drill, the hammer would likely strike the back of the shaker's head. When enough holes had been drilled, they were packed with explosive charges and the rock was blasted away. When the rubble was cleared, the drill team would begin drilling new holes deeper into the developing tunnel.
If John Henry's drill is still there stuck in the rock, it would have to be hanging horizontally out of a hole at the end of an unfinished tunnel about fifteen feet deep.