The Metro Red Line was a new line (part of Segment 3) connecting downtown Los Angeles to North Hollywood. The project entailed construction of the lines section in soft ground, mixed face, and rock, and included boring and finishing twin tunnel line structures; a 376-foot-long, 70-foot-wide, 80-foot-deep excavation at the Universal City crossover; access shafts; twin running tunnels with a nominal finished diameter of 17-feet-10-inches, driven approximately 12,300 feet from north to south by separate tunnel boring machines (TBMs).
The team faced multiple challenges while boring this tunnel, including squeezing ground, which trapped the tunneling machine. The team excavated around the machine to free it, and then re-excavated and re-supported the tunnel before continuing. The squeezing ground condition occurred for several hundred feet. Additionally, the project site was located in densely populated downtown Los Angeles, with the tunnel passing under U.S. 101 and a portion located across the street from Universal Studios, requiring complex traffic management and environmental oversight to minimize impacts to the area.
The twin starter tunnels, approximately 21 feet in diameter, horseshoe-shaped and 300 feet long each, were excavated through mixed face geology. The soft ground portion of these tunnels was chemically grouted prior to the tunnels being mined. The tunnels were mined using multiple drifts, utilizing steel ribs, fiber-reinforced shotcrete, and cement grouted dowels spilling for support. The tunnels were successfully mined with the final concrete lining.
The twin special seismic sections, approximately 28 feet in diameter, in the shape of a modified horseshoe, and approximately 300 feet long each, were excavated through a fault zone of extremely fractured rock and clay gouge. Mining was accomplished utilizing multiple drifts, utilizing lattice girders, fiber-reinforced shotcrete, cement grouted dowels, and cement grouted spilling for initial support. The tunnels were successfully mined with the final concrete lining.