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Purple Line Extension Tunnel Boring Machines and Submarines

Tunneling and deep-sea diving may seem like two different worlds, but they are shockingly similar. Like submariners in the ocean, a team of tunnellers on the Purple Line Extension Project will have to work under the water table. (Yes, there is water under your feet!)

The TBMs utilize high strength steel cutters and special steel alloy tools to cut the earth as it generates 100 tons of dirt, every 5 feet of advance. As the TBMs tunnel through the earth, the cutting tools wear away and will need to be replaced.  In some instances, this tool maintenance on the cutterhead can be done under regular atmospheric conditions.

Sometimes, this is not possible, and to change the tools under pressure, special hyperbaric chambers allow the workers to “dive” in air, similar to a diving bell, to make the repairs. In this case, the tunnelers are like deep sea divers, working under several atmospheres of pressure.

Above, the hyperbaric chambers, are what allows workers to safely travel between different pressure conditions in the excavation chamber and non-pressurized portions of the tunnel boring machine to perform routine inspections and repairs while the TBMs work underground.

Throughout the summer, crews will be busy delivering and assembling the TBMs, which are anticipated begin in tunneling early Fall 2018.

The TBMs will dig side by side starting at Wilshire/La Brea in the Miracle Mile section of Los Angeles and continue towards Wilshire/Western in Koreatown. Then they will be partially disassembled for transporting back to Wilshire/La Brea and then reassembled for the last leg of their journey through Wilshire/Fairfax and finishing just west of the Wilshire/La Cienega Station in Beverly Hills.

The Purple Line Extension will extend the subway from its current terminus at Wilshire/Western for nine miles to Westwood. The project is being constructed in three sections. The first, which is under construction, is 3.9 miles long with new stations at Wilshire/La Brea, Wilshire/Fairfax and Wilshire/La Cienega. The first section is projected to open in late 2023. The second section will extend the line to downtown Beverly Hills and Century City and the third section to Westwood.