Twin tunneling machines expected in LA by fall for Purple Line Extension

Written by Maggie Lancaster, assistant editor
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LACMTA

Twin tunnel boring machines (TBMs) are being constructed in Germany and are expected to arrive this fall to help construct The Los Angeles Country Transportation Authority (LACMTA) Purple Line Extension project.

The machines 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 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.

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 will 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.

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.

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