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Skanska Traylor Shea Joint Venture- Continues to Progress on Westside Subway Phase 1 Purple D Line Extension Project Scheduled for Completion in 2025 Despite Unique Tunneling Challenges
Skanska Traylor Shea Joint Venture
By Brian Hoover, Senior Editor / Photos
The Los Angeles subway system, known as Metro Rail, began in the 1990s with the Blue Line. It addressed the city’s traffic issues and marked the start of modern rail transit. The Red Line (1993), Green Line (1995), Gold Line, and Purple Line were later additions, connecting various neighborhoods. The ongoing Purple Line D Extension Project aims to expand the system further westward, reaching Beverly Hills and Westwood. The Purple D Line is a key part of Los Angeles’ subway system, connecting downtown Union Station to Wilshire/Western in Koreatown. Operated by Metro, the D Line helps reduce traffic and is part of the city’s eco-friendly transportation efforts. The City of Los Angeles has been planning the D Line route since the 1960s and conducted soil borings as early as the 1980s. Plans were stalled in 1985 after a methane seep caused a fiery explosion at a department store near the La Brea Tar Pits. The explosion brought about a moratorium on tunneling until operations resumed in 1993.
The Purple Line D Extension Transit Project began in 2015 and is being constructed in three sections (phases). Section 1 encompasses the Wilshire/La Brea Station, Wilshire/Fairfax Station, and the Wilshire/La Cienega Station, which is slated to open in 2025. Section 2 includes the Wilshire/Rodeo Station, the Century City/Constellation Station, and is scheduled to open in 2025. Lastly, Section 3, which is on track to open in 2027, includes Westwood/UCLA Station and the Westwood/VA Hospital Station.
Metro’s Westside Subway Extension, Section 1 is being constructed by Skanska Traylor Shea Joint Venture and is a 3.9-mile heavy rail subway in the Wilshire Corridor of Los Angeles. Since its start, they have worked on three underground stations, approximately 3.4 miles of twinbore tunnels, a Tunnel Boring Machine (TBM) Extraction Shaft, and complex systems and track work.
There were many expected and even unforeseen challenges during the construction of Westside Subway Extension, Phase 1, as the Skanska Traylor Shea Joint Venture successfully bored two twin tunnels, 22 feet in diameter, each approximately 4 miles long and 80-110 feet below ground surface. Much of the challenging tunneling conditions were created by the line’s proximity to the historic La Brea Tar Pits. Since the initial plans to build the line in 1960, the tunneling in this area has been deemed unsafe and by some, seemingly impossible. The joint venture team made it possible by mobilizing and utilizing two Herrenknecht multi-mode twin earth balance pressure tunnel boring machines (TBMs) affectionately referred to as Elsie and Soyeon. Elsie was responsible for boring the eastbound tunnel, beginning in October 2018. Soyeon also began operating in 2018, working simultaneously with Elsie for maximum production. These 1,000-ton, 400-foot-long, 22-foot-diameter TBMs have excavated miles of soil some 80+ feet below the Los Angeles surface. The Westside Subway Extension Phase 1 project is now approximately 80 percent finished with 100 percent of the tunneling safely completed.
Skanska Traylor Shea Joint Venture’s Purple D Line work has been managed by several joint venture team members, including Geoffrey Bender, Senior Project Manager at Skanska and Construction Manager of the Joint Venture. “The tunneling occurred in conditions that were classified as ‘gassy’ or even ‘extra hazardous,’ as our crews dealt with asphalt-impacted soils known as ‘tar sands,” says Bender. “Our team was prepared to encounter underground gases which included an extensive monitoring program and ventilation systems to ensure the safety of the public and all employees.” According to Bender, one such instance occured during Reach 2 tunnel mining. “Our joint venture encountered incidents of gas intrusion into the TBMs that resulted in precautionary shutdown of the boring machines. Whenever gas was discovered at a very low limit, immediate evacuation of tunnels and adjacent stations was required. Additionally, notification to and approval from Cal-OSHA to re-enter the underground work areas was necessary. The entire process could take anywhere from a few minutes to a few hours,” continues Bender.
“Added precautions included that cell phones were not permitted without special $2,000 explosion-proof safety cases. While the work and schedule were both impacted, the section of tunnel mining was completed without any safety incidents due to elevated gas levels. In addition to dangerous gases, the area also contained seismic faults and several geological variations from dense sands to silty sands, interbedded with layers of silts and clays.”
To begin the massive boring operations, the joint venture crews first had to excavate the entire station area where the machines would make their initial launch. “Our launching pit, also the La Brea Station excavation site, measured 1,000 feet in length, 65 to 70 feet in width, and 80 feet in depth. Because this area was designated as ‘gassy,’ by Cal-OSHA, we again took measures to avoid any sort of combustion while performing the excavation,” says Bender. “We utilized conventional excavators like the Cat 395F and 335F on the surface to excavate the launching pit to a reachable depth. When it came to the machines moving material below surface equipment reach, the decision was made to use older heavy equipment such as Cat 953B and 973B track loaders. These units were retrofitted by the joint venture to remove almost all of the electronic components and replace them with mechanical ones.”
Bender says that modifications were also made to lower the overall operating temperatures of the equipment fleet and the older equipment allowed the joint venture to use less original electrical components that required modification. “Retrofitted CAT 325C excavators and Gradall excavators would feed the track loaders for station excavations to an awaiting Liebherr crane used to hoist spoils to the surface for off hauling, continues Bender. “Additionally, we used retrofitted compact equipment, like mini excavators and skid steer loaders, to excavate a cross passage between the two bored tunnels every 750 feet.”
The Skanska Traylor Shea Joint Venture utilized top-down excavation and bottom-up concrete construction using soldier piles and lagging, tiebacks, internal bracing, utility support, and HDPE membranes. The tunnel itself utilized a precast segmental lining which served as the initial ground support and final lining. Bender informally refers to the soil material extracted from the boring operations as ‘tunnel muck,’ and explains how this material is brought up from the TBMs to the surface to be disposed of. “Our crews injected a conditioning agent into the earth which changes the material into more of a slurry state or muck type consistency. We made use of conveyor systems, up to two miles long, to bring the tunnel muck up to street level. These conveyors dumped onto a vertical conveyor and then to another overland conveyor topside that discharged the spoils into a muck bin before being disposed of,” continues Bender. “At the height of construction, our joint venture crews numbered more than 350 self-performing employees with an additional 120 to 170 subcontractors also working onsite. During the tunneling operations, our joint venture TBM crews would travel two miles underground to the east before disassembling the boring machines to return to La Brea Station to resume tunneling the remaining two miles to the west.”
According to Bender, there were several other challenges, including stormwater runoff. “The three large, deep station excavations became retention basins during storm events due to stormwater sheets flow, and the porous temporary decking system that was Wilshire Boulevard. Our subcontractor, STS, pumped this runoff to the surface where it was treated and tested before being moved into the City storm drainage system. During the excavation of cross-passages, several locations experienced the intrusion of groundwater that exceeded design expectations,” says Bender. “Additional ground treatment, pumping, and excavation support were implemented on a location-bylocation basis to allow work to progress.”
Bender says that the Fairfax Station, across the street from the La Brea Tar Pits, also included a ‘Paleo Zone,’ which presented paleontological concerns for potential fossil discoveries during station excavation. “Because of the potential of archaeological finds, excavation for this station took place 6 inches at a time while in the 40-ft deep Paleo Zone. We found several artifacts through the project stations excavations, including a juvenile mammoth skull unearthed at the La Brea Station,” says Bender. “These discoveries required proper handling and transport for any and all encountered fossils within the excavation areas.”
When Bender was asked about any other interesting or unusual challenges that the joint venture crews encountered during the boring process, he immediately recalled an anomaly found on Wilshire Boulevard and San Vincente. “Early on and prior to mining a reach from one station to another, we bored a conduit down at tunnel level. Once the conduit was in the ground, we pulled a magnetometer through the conduit to look for any ferrous materials within the reach,” says Bender. “We did locate an old bridge foundation at San Vincente and constructed a grout curtain on each side of where the tunnel would be located to prevent any gas from penetrating the area. Next, we brought the boring machine up and had joint venture team members under pressure, like a scuba diver, placed in front of the machine to excavate by hand until they chipped into a piece of steel. The steel in this area was then removed so that the boring machine could continue and avoid any costly damage.”
Once the Skanska Traylor Shea Joint Venture $1.6 billion Westside Subway Phase 1 project is complete, Phases 2 ($1.37 billion) and 3 ($1.4 billion) will continue with Tutor Perini, and all phases will be open to the public in 2027. For more information on Phase 1 of the Purple Line D Extension Westside Subway Phase 1 Project, please visit https:// partners.skanska.com/usa/ clients/lametro/wse/outreach/ sitepages/home.aspx. Cc
