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.




14 replies

  1. If the TBMs are lowered into the ground at Wilshire/La Brea, which direction will they go first? Since that’s the middle of Phase 1.

  2. Hi Anna,

    So Metro sells the TBM after the tunneling? Where do the proceeds go? If the TBM for Wilshire is supposed to be able to handle diverse conditions, is Metro going to hold onto that one for future projects given the material flexibility? Finally, if there is a market for used TBM, is that a market Metro looked at and then decided to procure a new one? If the sale is for pennies on the dollar like this article suggests (http://news.nationalpost.com/toronto/why-metrolinx-plans-to-leave-two-tunnel-boring-machines-in-the-ground-after-eglinton-lrt-line-is-finished) it might make sense to start a new tunneling project right after the Purple Line is done for significant cost savings. Thanks!

    • From the Purple Line Extension team:

      Metro’s contractor chose what they thought was the best machine for the job at hand. Herrenknecht Tunnelling Systems produces a technologically advanced and robust machine and their clients have easy access to spare parts and a large body of technological knowledge. Their machines are used throughout the world.

      Steve Hymon
      Editor, The Source

  3. What happens to the machine after it completes the job? can it be reused on other projects?

    • Hi Daniel,

      TBMs are built specifically to tunnel through the ground conditions of the project area, so often cannot be reused in another area. For example, one machine may be built to tunnel through tarry soils and can’t tunnel through harder, rocky conditions. Normally TBMs are sold after the project is complete so parts may be reused, and parts that cannot be used are recycled.

      Anna Chen
      Writer, The Source

      • Interesting. I’d assume the soil around La Brea is tarry, but then again the workers found various (possibly rocky) fossils while excavating sometime last year . Are the two machines built for different segments of the extension, or just to build the parallel tunnels at once?

        Also, I just have to say that this boring news is fascinating. 🙂

        • Hi Pat,

          I’d have to get more details from the construction manager, but as I understand it the area around La Brea has diverse conditions (sandy, tarry, etc), which the new TBMs will be built to handle! They are also meant to tunnel in parallel.

          Thank you,

          Anna Chen
          Writer, The Source

  4. TBM’s Average average 60 to 80 feet of progress every 24 Need to correct your info as TBM for Crenshaw Line is averaging over 60ft every 24 hours

    • From the Purple Line project team:

      The number is conservative because we have not begun tunneling yet. Our TBMs will also face differing geologies, including the tar sands, which Regional Connector and Crenshaw/LAX did not encounter. So comparing the amount of work done each day has a built in error factor. The 30 feet per 24 hours also includes downtime, maintenance and the time needed to pull the TBM through each station. In summary, 30 feet per day seems realistic to our tunneling team and includes a variety of factors.

      Steve Hymon
      Editor, The Source