The SR 99 tunneling machine was built specifically for the ground conditions beneath Seattle. The machine’s cutterhead will chip away the ground as it rotates and carry excavated soil back through the machine using a spiral screw conveyor. Curved concrete panels are installed behind the machine’s front end to form rings that serve as the machine’s exterior walls. Ring by ring, the machine pushes forward while the tunnel takes shape in its wake. The machine will dig an average of 35 feet per day. A conveyor belt, that will eventually reach 9,000 feet in length, will move excavated soil from the front of the machine out of the tunnel to barges waiting at nearby Terminal 46.
The tunneling machine uses a laser as a reference as it moves forward through the earth. Projected from a fixed point behind the machine, the laser is received by a guidance system at the front of the machine that is precisely calibrated to the tunnel’s predetermined path. The guidance system is referenced by the machine’s operator to ensure the machine remains on course. The operator steers the machine by making slight adjustments with each push forward. To learn more about how the machine operates watch the tunneling machine video.