Andy, you bring out an important point about the dangers of tunneling work. Chappell's book (and Scott Nelson's relatively recent book) describe just how horrible and dangerous these jobs were. It's hard to imagine why anybody would do this work voluntarily. Nelson points out that convict lease laborers were used in the building of at least two tunnels. I imagine that many of the "free" laborers who did this type of work were actually victims of peonage who "owed their souls to the company store."
Jim Hauser