Airport/Port: Regional Connector Transit Project
Project of the Year
Submitted By: Skanska USA Civil
Owner: Los Angeles County Metropolitan Transportation Authority
Lead Design Firm: Mott MacDonald
Contractor/Engineer: Regional Connector Constructors Skanska Traylor JV
Prelim. Engineering and Owner’s Engineering Repr.: AECOM / WSP – The Connector Partnership JV
The 2023 Oscar awards featured a Best Actor and Best Supporting Actor who reinvented themselves and came back for a second act. The tunnel-boring machine Angeli is the LA performer who did much the same, but entirely underground.
The 22-ft-dia TBM in 2017 launched out of a 45-ft-deep pit in Little Tokyo, burrowing through an initial 400 ft of alluvium above the water table and only 5 ft of earth above it. Then it navigated a 600-ft curve downslope, tunneled through a local bedrock called the Fernando formation, passed beneath the Red Line subway and the historic Second Street Tunnel and close to batter piles for the Fourth Street Bridge.
After completing the tunnel between Little Tokyo and downtown in five months, Angeli was taken apart, brought back to the launch site and did it again for the second twin tunnel of the $1.7-billion, 1.9-mile Regional Connector project that will connect existing rail lines for seamless rides through Los Angeles County.
Angeli navigated a maze of underground utilities and unknown obstructions, including three steel piles and 47 steel tiebacks that damaged the screw conveyor; the piles’ origins are “still unknown to this day,” says Justin Waguespack, vice president with Skanska USA Civil, which with Traylor Brothers Inc. led Regional Connector Constructors. “It could’ve been anything, being in downtown LA.”
The project includes 1.9 miles of twin-bored tunnels, 22 ft in diameter.
Photo courtesy Skanska
Working collaboratively with LA Metro, the team expedited the replacement of the screw conveyor and added a shift to the mining operation to gain back two months on the schedule.
BIM-assisted design and Shape Accel Arrays—a system of wireless sensors—helped Angeli navigate through extremely tight tolerances and situations of very shallow cover as the team built bored, mined and cut-and-cover tunnels with three underground stations, including all the necessary systems and related infrastructure.
The three underground stations are approximately 400 ft in length and vary from 40 to 112 ft in depth. One of the stations also includes extensive underpinning and relocation of a large storm drain facility. Using a sequential excavation method (SEM), RCC excavated a 23-ft-high, 58-ft-wide and 287-ft-long crossover cavern—one of the largest ever excavated in Los Angeles.
A total of 3,700 ft of cut-and-cover tunnel was constructed in the congested financial district of Los Angeles.
Photos courtesy Skanska
Initially, RCC planned to excavate the cavern starting with the left bored tunnel and upper and lower right drifts, before backfilling part of the left tunnel to enable excavation of the upper and lower left drifts. However, several of the drifts needed to be completed before the TBM could begin to mine the right bored tunnel. Rather than risk delaying the schedule, RCC opted to bore three drifts with two centrally shared walls. As long as the minimum heading distance of 60 ft was maintained between drifts, all the drifts could be excavated simultaneously.
The project also includes over 3,700 ft of cut-and-cover tunnel construction in the congested financial district in Los Angeles. That section and the three stations were constructed during 55-hour weekend closures using temporary traffic decking. Crews used top-down excavation and bottom-up concrete construction using soldier piles and lagging, tiebacks, internal bracing, utility support and HDPE membrane. The track work includes the installation of direct fixation track as well as noise and vibration measures at sensitive locations.
Angeli averaged 60-70 linear ft per day of mined tunnel with a maximum achievement of 200 ft, says Waguespack. Crews worked two 10-hour shifts.
Richard McLane, Traylor Brothers chief mechanical engineer, notes tunneling advancements that have facilitated tunnel construction in LA County. A 1985 methane-fueled explosion at a dress store in 1985 halted tunnel construction in the Westside area for years afterward. But newer technologies facilitated the work downtown, including a Shape Accel Array (SAA). SAA proved instrumental in keeping Angeli on track as it passed under the shallow initial ground cover and through the dense web of underground infrastructure. A 400-ft-long array of accelerometers positioned just above the TBM measured movement and potential deformation or settlement, transmitting data to the team’s command center. “It sends automatic alerts if something is out of tolerance,” says McLane. Settlement generally must be no more than a half-inch.
The team deployed natural gas detectors to warn of naturally occurring methane gas. The job had a rating of “potentially gassy,” but there was only one minor incident, says Waguespack.
Photo courtesy Skanska
Clean and Connecting
While LA Metro grapples with concerns about the safety and cleanliness of the overall transit system due to drug use and homelessness, the Regional Connector project itself epitomized safety and community and climate awareness.
The stations are built to meet CalGreen, the state’s green building code, says Waguespack. “Lights turn off and on automatically; there are LED lights,” he says. The team has worked with union halls and local communities to get through 6.5 million work hours with zero lost time.
The safety team worked with upper management and the superintendents, developing a four-week look-ahead system to ensure careful planning and timely materials delivery. Each onsite employee underwent an extensive four-hour jobsite orientation that includes drug screening, site safety conditions, underground training and quality control.
The eight-story, 100-ft-deep 2nd & Hope Station boasts a 70-ft-tall underground atrium.
Photos courtesy Skanska
Each morning, the foremen conducted a daily hazard analysis with crews. They attended a weekly foreman’s safety meeting to discuss problems and achievements, reaffirm safety planning schedules and discuss methods of addressing concerns. Managers visited the site bimonthly to discuss safety plans with workers and invite them to share their thoughts.
At 2nd and Broadway, an existing 123-in. reinforced concrete horseshoe arch storm drain required replacement. RCC opted for a Hobas centrifugally cast, fiberglass reinforced, polymer mortar pipe rather than reinforced concrete. The Hobas pipe has a 126-in. internal diameter, the largest of its kind in the world, offering a stronger, more efficient alternative to the existing reinforced concrete pipe; the material is six times lighter than reinforced concrete, says Waguespack.
Crews installed 320 linear ft of Hobas pipe in 20-ft sections, with each new segment lowered onto temporary support structures until secured with lateral and longitudinal braces. It was then surrounded by 18 in. of 100-psi slurry, providing structure, support and flexibility to help the Hobas pipe remain durable and reliable for the foreseeable future.
”The old brick storm drain went right over Broadway station,” recalls Waguespack. “We had to come up with a way to both keep the drain active but at the same time construct the station. When we bid, it was unknown if we could support the brick drain. The safest and best way was to apply to get the Hobas pipe approved as a permanent material for LA County.”
At 2nd & Hope Station, design required providing access from the street-level triangular plaza and the Broad museum and Grand Avenue podium level to the eight-story, 100-ft-deep underground station that extends 400 ft from the Disney Concert Hall toward Flower Street. Crews built a pedestrian bridge with glass railing and tree planters, connecting the second-floor upper plaza level to the Broad museum as well as to the street plaza. The station is accessible via a pair of 45-ft-tall stainless steel and glass towers with a 94-ft by 37-ft-wide fritted glass shade canopy. Six elevators descend to a 70-ft-tall underground atrium. A concourse walkway leads to an open level elevated above a 35-ft-tall open platform area with arched ceiling and end wall porcelain art panels.
The project was bid with an 18% disadvantaged business participation goal at the original $987-million estimate. With scope changes increasing that number to about $1.3 billion, “we still hold and exceed that 18% goal,” says Waguespack.
Angeli now rests at a yard owned by the joint venture in Riverside. “She’s still there, and we hope to use her on a future project,” says Waguespack.