As Bertha winds her way under Seattle and chews through an estimated 35 feet of soil a day, her cutterhead will exert 88,000,000 pounds of pressure on the forward wall of soil, grinding down the strata and depositing it on a conveyor belt directly behind it. Soil conditioners are added to the debris, rendering it an easier-to-manage goop as it is pushed back up to the surface, 80 feet above. To prevent cave-ins, Bertha automatically erects pre-fab concrete panels and seals them with grout to create the tunnel walls using a pair of erector arms.


Also behind the cutterhead is the trailing gear, a series of three cars containing the control room as well as the crew's workshop, power cells for the hydraulic wall-erecting arm with the backfill grout system, and electrical transformers, respectively. Since most of the boring process is automated, the operator and a pair of engineers, oversee the cutting process while the other 22 crew members survey the route ahead, maintain the cutterhead teeth, and operate the non-automated tunnel-wall erector.

The project is on track for its 2015 completion date. When it's done, the people of Seattle will not only have a new underground freeway capable of withstanding a 9.0 earthquake but also a massive new public space along the city's waterfront where the old SR-99 used to stand. It's a win-win. [Car and Driver - Washington State DOT -Images:WS DoT/ Flikr]