Los Angeles Regional Connector Will Open Up Options for Commuters - Civil + Structural Engineer magazine


By Richard Reitz

Commuting through Los Angeles is notoriously frustrating, but it is on the verge of getting a little easier with one of the city’s most ambitious transportation projects entering the home stretch.

For the past 12 years, Los Angeles County Metropolitan Transportation Authority (Metro) has been planning and building its Regional Connector, a 1.9-mile twin-tunnel underground rail system that will link three light rail transit lines and the heavy rail subway.

Targeted for completion in 2022, having stayed on schedule throughout the COVID-19 pandemic, this new Metro Rail extension will offer a transportation option that for many commuters will provide a viable alternative to congested roadways. It will also provide significant environmental benefits and economic development throughout Los Angeles County.

View on Hope Street. The glass building is the high-speed elevator. It shows a pedestrian bridge connecting the station to The Broad museum. This station is about 100 ft deep from this location. Photo: LA METRO

For many commuters, it means a one-seat ride for travel that was previously not possible. Present-day Metro Gold Line passengers, for example, will be able to travel directly from Azusa to Long Beach, or from East Los Angeles to Santa Monica, without transferring lines.

This station will be integrated with a new high-rise building by LA Times. The ticketing area shown will be inside the new building. Photo: LA METRO

“By reducing the need for transfer trains when traveling through and to downtown Los Angeles, the new connector is expected to save commuters up to 20 minutes of travel time on every trip,” said Bill Hansmire, WSP USA underground design manager. “This connection is going to open the door for many people to consider public transportation where this option was previously not practical or possible.”

WSP, in a joint venture with AECOM as the Connector Partnership, is providing transportation planning, rail design, and rail planning for the Regional Connector. Since joining the project to provide conceptual engineering and the draft environmental impact statement (DEIS) in 2008, the firm’s responsibilities have included tunnel and geotechnical engineering, tunnel ventilation, rail systems, travel forecasting, operational analysis, and mechanical, plumbing and electrical engineering.

WSP is working on behalf of Metro with Regional Connector Constructors, the joint venture of Skanska and Traylor Brothers, Inc. that is contracted by Metro to design and build the project. Mott MacDonald is serving as the final designer for the project.

It is estimated that the $1.81 billion Regional Connector will serve 88,000 riders every day, including 17,000 new riders. The project improves access to both local and regional destinations by providing continuous thru service between these lines and providing connectors to other rail lines via the 7th St/Metro Center Station, allowing passengers to bypass Union Station and transfer to the A (Blue), E (Expo), B (Red) and D (Purple) Lines.

‘Angeli’

The success of the Regional Connector project centered around the construction of two parallel 1.1-mile-long tunnels using a state-of-the-art tunnel boring machine (TBM) nicknamed “Angeli” and manufactured in Germany by Herrenknecht AG.

Angeli weighed about 1,000 tons, was 400 feet long and nearly 22 feet in diameter. It advanced about 50 to 70 feet per day on average and mined 10,900 feet between the two tunnels, excavating over four million cubic feet, or 240,000 tons, of earth.

Digging for the first half of the 1.1-mile tunnel concluded in July 2017, when Angeli broke through to a reception pit at 4th and South Flower streets in the city’s downtown. Angeli was then transported to the original starting point at 1st Street and Alameda in Little Tokyo to continue digging the parallel tunnel, which was completed in January 2018.

Angeli overcame several challenges, including a culturally sensitive building complex that was protected by performing permeation and compensation grouting; abandoned soldier piles and tieback anchors were encountered within the public right-of-way; the existing LA Metro heavy rail Re Line (B/D) was underpassed with a clearance distance as low as one quarter of the tunnel diameter (nominal 20 feet) without interrupting the B/D Line operations; sensitive stakeholders were coordinated for the ground borne noise and vibration; and Angeli precisely advanced with minimal clearances to existing bridge foundations and sanitary sewer manhole structure without disturbing.     

After both tunnels were fully excavated, “Angeli” was dismantled and removed from the site. Since that time WSP’s services have supported the completion of tunnel work and transitioned to focus on rail transit systems. WSP staff has been involved in the process leading to rail activation and will provide technical assistance after the start of operations.

Three cross-passages in the bored tunnels were excavated and had the final concrete lining placed along with comprehensive water/gas proofing process using hydrocarbon resistant membranes and chemical grout. Construction of tunnel walkways has also been completed.

Work has also progressed on the construction of three new underground stations, an underground crossover, cut-and-cover concrete box tunnels and U-structures:

  • Little Tokyo/Arts District Station – 1st Street/Central Avenue
  • Historic Broadway Station – 2nd Street/Broadway
  • Grand Av Arts/Bunker Hill Station – 2nd Place/Hope Street
  • Underground crossover cavern at Historic Broadway Station
  • Flower Street box tunnel connecting to the A Line
  • Alameda and 1st Street box and U-structure lags connecting to the L (Gold) Line

“Most of the stations, crossover cavern, and cut-and-cover structures have been fully excavated and concreting is complete or well advanced,” said Jason Choi, WSP project manager. “The Alameda lag is currently being excavated.”

Construction image of the cavern. Photo: LA METRO

The excavation of Little Tokyo/Arts District Station reached final grade before the TBM launching in January 2017 and the 1st Street cur-and-cover excavation was completed in parallel with bore tunnels in 2018. The Historic Broadway station reached its full depth of about 90 feet in June 2018, and the crossover cavern excavation, which would be the last technically difficult underground challenge, according to Hansmire, was completed in March 2019 nine months after the commencement from the final grade of the Historic Broadway Station.

In addition, the Grand Av Arts/Bunker Hill Station, with a maximum depth of 105 feet, completed its excavation in February 2017. The 1,350-foot-long cut-and-cover excavation along the Flower Street was completed in July 2019. Concreting the final structures within those excavations has been substantially advanced, and the backfill and surface restoration works are expected to begin later this year.

“Among those stations, the construction of Historic Broadway Station would be considered as the most challenging one,” Choi said. “The station excavation was hugging the basement of an adjacent historic building, which had a portion vacated into the excavation to be underpinned. The existing 10-foot-diameter storm drain running the entire length of the station had to be replaced and protected in place during construction. The entrance structure had to be designed and constructed to adapt future development of the property owner. And, the crossover cavern construction had to be commenced from the station excavation.”    

The Broad and the Disney Concert Hall are in the background. Photo: LA METRO

Firsts for LA Metro

The Regional Connector tunnel includes some “firsts” for LA Metro. Most notably, it was the LA Metro’s first cavern excavated in weak rock for a rail crossover using the sequential excavation method (SEM) of tunneling.

“The location has a very narrow public right-of-way with a historic adjacent structure,” Hansmire said. “Cavern construction avoided many costly constructability problems if done as cut-and-cover construction. The crossover was essential to efficient rail operations. To not construct the crossover would have resulted in unacceptably long single tracking times when one line is shut down for maintenance or other reasons.”

One challenge during design was prediction of ground movements and behavior to protect existing adjacent buildings and utilities in one of the world’s most active areas for seismic activity.

A cutting-edge construction schedule orchestrated three major activities at one place: twin bore tunneling by a single TBM, deep station excavation, and SEM excavation. During excavation, the project team – including SEM experts and experienced tunnel engineers from the contractor, the final designer, the construction manager, and LA Metro’s technical consultants – met daily to assure ground stability, ground support quality and performance, and adjacent building and utility responses.

Upon completion of contact grouting in July 2020, the cavern excavation and the final lining installation completed successfully without exceeding predicted movements and imposing structural impacts on the adjacent structures.    

Commuters will notice something unusual at the Grand Av Arts/Bunker Hill Station. For the first time, the LA Metro system is using high-speed elevators in a station instead of escalators. The new station, located about 100 feet below the surface, will feature six high-speed elevators with hoisting equipment located below the elevators—rather than above—to avoid the visual impact at the station entrance pavilion. At that depth, the elevators will transport passengers from the rail concourse level to the surface more quickly than escalators.

Another innovation involved the location of a fan plant at the junction of the two lines coming into Little Tokyo, which will provide ventilation for the underground transit lines. WSP’s design analysis confirmed that this plan would work at this location, which resulted in a significant cost savings for the client.

“When it was determined an additional tunnel ventilation fan plant was needed at that junction, it created an opportunity to use the TBM launch pit beneficially,” Hansmire said. “So rather than being backfilled, it became the perfect location for the fan plant, thus avoiding additional excavation.”

Utility Protection and Relocation

One major challenge during the construction of the tunnels was working around or relocating the existing underground water and utility infrastructure – some of which has been in place for more than a century, and none of which could afford to be disrupted. In addition, construction needed to minimize disruption of the downtown roads located directly above the worksites.

“It has been our biggest issue, requiring some very careful coordination as we balance the competing demands of utility work that requires closing lanes of traffic, while accommodating roadway traffic during those construction times,” Hansmire said.

The significant success of the tunneling work was recognized in July with a national honor from the Underground Construction Association, which named the Regional Connector its Project of the Year.

Environmental Justice

Meeting the commitments made for mitigation of environmental impacts, particularly in the Little Tokyo neighborhood, posed challenges that kept the designers on their toes. Noise and vibration impact of construction and rail operations required extensive efforts to properly establish design-build performance requirements, and to work with the stakeholders and the final designer to ensure mitigation is achieved.

“Connecting two existing transit lines that use systems equipment of different designs and ages demanded careful planning that required expertise and experience in rail transit systems,” Hansmire said. “While the path was complicated, what was set in motion years ago when we started planning the project came together as expected, and with the tunnels completed and the underground stations being concreted, that vision is fast becoming a reality.”

Stakeholders that will benefit from this extension include several major Los Angeles cultural institutions along the route, including the Walt Disney Concert Hall, the Colburn School and The Broad art museum.

“The all-encompassing multidiscipline scope of the Regional Connector project has been a professionally rewarding opportunity to use my career-long project management and technical experience of many projects,” Hansmire said. “The Regional Connector project encompasses all technical disciplines for underground rail transit … everything is included.”



Source link

Leave a Reply

Your email address will not be published. Required fields are marked *