Tunnel boring machines reach milestone at Purple Line Extension project

June 10, 2020

Both tunnel boring machines (TBM) working on the Purple Line Extension project in Los Angeles, CA broke through at the future Wilshire/Fairfax subway station near the La Brea Tar Pits in the Miracle Mile. The breakthroughs mark the completion of 5 km (3 miles) of tunneling on the 6.4 km (4 mile) project.

The 907 t (1,000-st), 122-m (400-ft) long TBM named “Soyeon” started at the Wilshire/La Brea station in October of 2019 and broke through the eastern side of the planned Wilshire/Fairfax station site on May 21. Metro’s first TBM “Elsie” broke through to Wilshire/Fairfax on April 4, the Metro reported.

Reaching this dual milestone is a significant win for Metro. The underground soil conditions in this area of Los Angeles represent some of the most challenging for the entire project. The agency’s modern, high-tech TBMs have mined through a unique combination of soils and geologic conditions, including tar sands and methane gas.

The breakthrough also marks another milepost in the decades-long effort to extend L.A.’s subway farther west underneath Wilshire Boulevard, one of the busiest and most congested urban thoroughfares in the United States.

“This has been an enormously complex operation that Metro and its contractor Skanska Traylor Shea have successfully conducted,” said city of Inglewood Mayor and Metro Board Chair James T. Butts. “When this project is ultimately completed, we will have fast, frequent and reliable subway service connecting downtown L.A. and West L.A. in just 25 minutes. That will be a game-changer for all of L.A. County.”

Public works projects like the Purple Line Extension Project are considered essential activities during the COVID-19 public health crisis. Metro and its contractors are committed to ensuring the safety of all project workers.

The TBMs were originally lowered into the ground at Metro’s Wilshire/La Brea station site in the Miracle Mile area of Wilshire in October 2018 and advanced about 18 m/d (60 ft/day). They worked five days per week, 20 hours a day.

Metro’s TBMs are pressurized, closed-face machines that minimize ground settlement during excavation. The tunnel is lined with precast concrete segments that are bolted together to form a ring. Segments are also gasketed to make the joints between segments water- and gas-tight.

When tunneling is finished for this entire project section, both of Metro’s TBMs will have mined nearly a half-million cubic yards of earth.

“We are proving once again that we can successfully mine through some of the most challenging conditions that any subway project in the world is likely to face,” said Metro CEO Phillip A. Washington. “We have a world-class project team that has extensive tunneling expertise. We are well on our way to delivering this vitally important transit project to taxpayers.”

Metro’s TBMs will next head farther west to the final Section 1 station at Wilshire/La Cienega. The TBMs are expected to reach the end of this first four-mile tunneling section in the city of Beverly Hills this fall.

The TBMs were manufactured in Germany by Herrenknecht AG. Metro has contracted with Skanska Traylor Shea (STS), a joint venture to design and build the first section of the project.

The $9.3-billion Metro Purple Line Extension is a 14.5-km (9-mile) underground subway project that will extend the Metro Purple Line from its terminus in Koreatown to the Westwood/VA Hospital in West Los Angeles. Section 1 is expected to be completed in 2023, Section 2 in 2025, and Section 3 in 2027.

 

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