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Purple Line Tunneling Complete After 5 Years

Mario Tama via Getty Images

The Los Angeles County Metropolitan Transit Authority announced on April 2 that tunneling for the $2.4 billion D Line Subway Extension Project, also known as the Purple Line, has been successfully completed. Moving forward, Metro will collaborate with two joint ventures — one consisting of Skanska-Traylor-Shea, and the other composed of Tutor Perini and O&G Industries — to finalize construction on seven new underground stations. The initial phase of completed work is expected to be operational by 2025.

Metro called the work a remarkable feat of modern engineering. It highlighted the challenging geological conditions of the Los Angeles region and the nation that the builders had to navigate beneath, referring to the areas as dense and geologically formidable.

Contractors utilized cutting-edge tunnel boring machine (TBM) technology to excavate at a rate of approximately 40-60 feet per day. The TBMs, measuring 400 feet in length and 21 feet in diameter, employed closed-face, pressurized technology to minimize ground settlement during excavation. Additionally, the tunnel was lined with precast concrete segments bolted together to create sturdy rings, ensuring water- and gas-tightness and mitigating associated risks.

The challenging terrain posed further complexities for the contractors. They had to contend with abandoned oil wells, gassy ground, and the La Brea Tar Pits, exposing them to tar sands. To address these challenges, teams utilized horizontal directional drilling to identify and remove potential obstacles before they could damage the TBM equipment. This method was also used to navigate and avoid unmapped, abandoned oil wells beneath Beverly Hills High School.

“The D Line Subway Extension is one of the most complex engineering feats that Metro has undertaken,” said Lindsey Horvath, Chair of the LA County Board of Supervisors and Metro Board Member, in the release.

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