Tunneling machines for Purple Line Extension under construction in Germany

The Purple Line Extension is expecting twin tunnel boring machines, but they will not arrive in Los Angeles until this fall. The TBMs will be lowered into the ground in 2018 at the Wilshire/La Brea Station and will be used to dig the tunnels for the first section of the project that is now under construction and will extend the subway for 3.9 miles from Wilshire/Western to Wilshire/La Cienega.

The TBMs are being assembled in Germany by the same manufacturer that built the machine being used to mine the tunnels for the Regional Connector Project (video here of that TBM in action). The machines are known currently by their numbers — S986 and S987 — although a contest to give them more personal names will begin later this year.

By any name, the machines are incredible pieces of machinery. The cutter heads at the front of the TBMs—the spinning disks that cut away the soil before them—have a circumference of 68.6 feet, allowing each machine to bore a tunnel 21 feet 10 inches wide. (The finished tunnel will be 18 feet 10 inches in diameter). Each one will weigh approximately 1,050 tons.

The machines do more than bore a tunnel under Wilshire Boulevard. They will also construct each tunnel as they move forward. Hydraulic systems in the TBMs push pre-cast tunnel segments into place, in essence building the shell of the tunnel while the cutter head continues boring. The TBMs also gather the soil from the cutter head and use a giant screw to carry the soil to the rear of the TBM, where it is carried away and brought above ground. With all these functions to perform, no wonder the TBMs equal the length of 10 school buses.

The TBMs work very quickly — if your definition of “quick” is the speed of a snail. At their fastest, the twin machines can move as much as four inches per minute. Overall the project expects to average 30 feet of progress every 24 hours or 150 feet per week. Once they begin tunneling, the TBMs will require approximately 22 months to complete their work. They will work five days a week, 24 hours a day.

At this time, workers continue to excavate the Wilshire/La Brea Station, install temporary street decking at the Wilshire/Fairfax Station and insert piles (the steel beams that will support the framework) for the Wilshire/La Cienega Station. 

The Purple Line Extension’s home page on metro.net is here. You can also follow the project on Twitter and Facebook. Here’s an album of recent construction pics on our Flickr page. The Purple Line Extension’s first and second sections are funded by the Measure R sales tax approved by L.A. County voters in 2008 and federal grants and loans.




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