When operating, the SR 99 tunneling machine's rotating cutterhead scraped away soil, carrying it back through the machine using a spiral screw conveyor. Curved concrete panels were installed behind the machine’s front end to form rings that serve as the machine’s exterior walls. Ring by ring, the machine pushed forward while the tunnel took shape in its wake. A conveyor belt that eventually reached 9,000 feet in length moved excavated soil from the front of the machine out of the tunnel to barges waiting at nearby Terminal 46.
The tunneling machine used a laser as a reference as it moved forward through the earth. Projected from a fixed point behind the machine, the laser was received by a guidance system at the front of the machine that was precisely calibrated to the tunnel’s predetermined path. The guidance system was referenced by the machine’s operator to ensure the machine remained on course. The operator steered the machine by making slight adjustments with each push forward. To learn more about how the machine operated watch our tunneling machine video (links to YouTube).