The work of these sandhogs is the subject of the Sandhogs reality TV series from the History Channel. Summarizing the work, “Walking Boss” Morgan Curran (a very colorful character on the show) says this about how the project got started:
As he heads down the narrow mine shaft to work, “Walking Boss” Morgan Curran says crews had to excavate a cavernous staging area, then install an elevator, lay down railroad track, connect power cables and put in a pump to get out the muck as they blast and dig. “We came through the wall of the pit, and we blasted our way here until we got enough room to make way for these tunnels that you see behind me, and then brought a [rock] boring machine in and did one side and then the other side.” (Photo by VOA News)
According to History.com, the tunnel is 600-800-ft underground and 10 to 24-ft in diameter. 5,000 sandhogs have worked on the project with 24 losing their life due to construction accidents in the tunnel.
Also coming online with the new tunnel will be a massive 9-story underground water filtration plant underneath a golf course to connect up with the tunnel.
Story at VOA News by way of ASCE Smartbrief.