Los Angeles Regional Connector

Although the Regional Connector is just 3.1km long, it is transformative for all LA Metro users. It has joined the city’s existing L (Gold), A (Blue) and E (Expo) lines, now just called the A and E lines. This simplifies journeys from one side of the city to the other by providing a one-seat ride between Azusa and Long Beach, and between East LA and Santa Monica.

For many riders, it reduces the number of transfers from several to just one, saving up to 20 minutes on journey times. LA Metro estimates that the line will handle 60,000 or more trips each weekday and attract 16,000 new riders.
Stephanie Wiggins, CEO of LA Metro, says: “People are going to love the newfound connectivity available through the Regional Connector. We have built a beautiful, useful, state-of-the-art transit project that everyone in LA County will be able to use to get to the beach, rediscover downtown, or go museum hopping with their friends.”

The Regional Connector enables seamless through-journeys between the former Gold, Blue and Expo lines, now called the A and E lines. It features two entirely new underground stations at Historic Broadway and Grand Av Arts/Bunker Hill. The original at-grade Little Tokyo/Arts District station has been replaced with an underground station, freeing up street space.

Mott MacDonald was involved from 2014 as detail designer for Regional Connector Constructors (RCC), a joint venture between Skanska USA and Traylor Brothers. Our scope of work included:

• 3.1km of twin-bored, segmentally lined tunnels, each 5.7m (18'10") internal diameter
• Nearly 600m of cut-and-cover tunnel in LA’s financial district
• Three underground stations constructed using cut-and-cover
• A sequentially excavated crossover cavern
• Three mined cross-passages
• Ventilation, power and communication systems
• Civil, roadway and utility work
• Direct fixation track, resilient tie track, two double crossovers and floating slab track in areas sensitive to noise and vibration

Better connections

Beginning in the 1980s, four light rail transit (LRT) lines were built to serve the sprawling metropolis of Los Angeles, each one ending at the edge of the city’s downtown region. Plans were discussed for a rail link that would provide easy transfers between the city’s north-west and east-south LRT systems as early as 1984. However, lack of funding meant the proposal was put on ice.

The idea was revived in 2004 and feasibility assessments were undertaken, analysing route alternatives and ways to minimise costs, followed by an environmental study. In October 2010 the project was approved.

The Regional Connector links LA Metro’s entire 80-station system to southern California’s regional passenger rail system, Metrolink, which services 55 stations. Trains on both lines run every 10 minutes during weekday peak hours, every 12 minutes for most of the rest of the day and during weekend daytime hours, and every 20 minutes in the early mornings and later evenings.

By offering an alternative to congested roadways, the Regional Connector encourages a modal shift from car to rail transport. This brings potential environmental benefits, while improved access to transport and shorter journey times open new opportunities for people throughout Los Angeles County.

Underground challenges

Threading new underground infrastructure through congested urban environments is complex at the best of times. But downtown LA presented more challenges for this short metro link than most.

Space both above and below ground was limited. The tunnel boring machine (TBM) was able to pass beneath buried water, wastewater, electrical, telephone, fibre optic and natural gas utilities, but the cut-and-cover stations required utilities to be diverted where possible.

While excavation work was carried out below, utilities were suspended from the temporary roadway installed above

Where diversion was impractical, a means of leaving them in situ had to be found. The solution was to suspend the utilities from a temporary roadway that was installed to keep traffic flowing while excavation and construction were carried out below.

Cut-and-cover stations were built within walls constructed of bored concrete soldier piles. Capping beams tying the soldier piles together longitudinally provided support for steel girders bridging the station across its width. Decking allowed road traffic to run above while the utilities were slung below.

Industry recognition

The Regional Connector received Engineering News-Record’s (ENR’s) Best of the Best Projects award for the transit category in 2023.

The project also won ENR’s 2022 Excellence in Safety award, in recognition of its exemplary 7.7M hours worked without any lost time incidents.

It has won other awards too, from Tunnel Business Magazine, the Underground Construction Association, the Society for Mining, Metallurgy & Exploration and the International Tunnelling Association.

Connecting the stations

A single earth pressure TBM, named ‘Angeli’ by local schoolchildren, bored the twin tunnels, which were then lined with precast concrete segments. Angeli was launched from Little Tokyo, not far from the LA River, in early 2017 and reached the end of the drive in the financial district in June 2017. On completion of the first bore, Angeli was returned to Little Tokyo for the second drive.

The first part of the drive from Little Tokyo was in alluvial deposits of sand and cobbles. As it travelled towards the centre of downtown LA, Angeli moved into the Fernando formation, consisting predominantly of shale and siltstone. The loose ground at the start and end of each section of the tunnel drive required permeation and consolidation grouting to reduce the risk of ground loss and settlement. Cement grout injected into the alluvial material turned it into weak concrete. Ground treatment was also carried out in areas of low ground cover and near sensitive buildings.

A section of Angeli, the tunnel boring machine used for the twin-bored tunnels for the Regional Connector, which was launched at Little Tokyo (Credit: Metro)

A section of Angeli, the tunnel boring machine used for the twin-bored tunnels for the Regional Connector, which was launched at Little Tokyo (Credit: Metro)

A section of Angeli, the tunnel boring machine used for the twin-bored tunnels for the Regional Connector, which was launched at Little Tokyo (Credit: Metro)

Stations were excavated ahead of the TBM drive. ‘Soft eyes’ were constructed in the head walls formed by the soldier piles to allow Angeli to break through.

The tunnel alignment involved boring near to existing bridge caissons and a deep sewer, as well as closely below the existing LA Metro Red Line tunnels. Sensors were placed on the existing tunnels to check no settlement was occurring and to allow the subway to remain operational safely.

In autumn 2017, on the first tunnel drive between Spring Street and Main Street near the LA Police Department headquarters, Angeli hit three uncharted steel piles left over from a much earlier construction project. The TBM managed to bore through them but the teeth and wheels on the cutter head and the screw conveyor were damaged, necessitating $1M of repairs. Months of delay to the programme were initially anticipated, but Angeli was back in action only a month later and reprogramming of the tunnelling operation enabled the lost time to be recovered.

Seismic concerns

California is known for its seismic activity, with approximately 500 active faults. Like other West Coast US transit system owners, LA Metro requires a two-level analysis, which goes beyond California Building Code (CBC) requirements.

The Regional Connector includes LA Metro’s first crossover cavern – a wide-span section of tunnel where trains can cross between tracks. Because there was no precedent for an underground space of this size in the city, we studied the interaction between soil and structure under simulated earthquake motions, using two-dimensional, fully coupled dynamic time history analysis.

500 active seismic faults in California

This type of simulation is used to predict the behavior of a system under dynamic loading. The cavern was modelled using mathematical equations describing not only the physical behavior of the cavern itself but also of the ground around it. Results of the analysis informed both the temporary support and permanent designs for the cavern.

LA’s largest cavern

Measuring 17m wide by 10.4m high and 91m long, the crossover cavern was constructed using the sequential excavation method (SEM).

The name describes the process: excavation starts with small headings that are progressively enlarged and ultimately connected to achieve the final tunnel shape. On completion of every excavation stage, a lining of fast-drying fibre-reinforced sprayed concrete is applied to support the exposed ground.

The crossover cavern adjacent to Historic Broadway Station was constructed using the sequential excavation method. Settlement control was critical as several major buildings and utilities were in the potential zone of influence.

Our seismic analysis showed how thick the initial ‘flash lining’ needed to be. A thicker, slower-drying sprayed concrete primary layer was then applied on top of this. A further, secondary layer was applied once the final tunnel profile had been created.

During excavation, surface settlement was closely monitored, with a maximum of 15mm allowed. To minimise the risk of settlement, a pipe arch canopy was inserted above the crown and ahead of the tunnel face to support the ground.

Alleviating risk with BIM

Today, it is hard to imagine any major project being designed and built without the use of building information modelling (BIM). But it was groundbreaking when work on the Regional Connector started in 2012. BIM was used to design stations and cut-and-cover tunnels – the first 3D design delivery to be used for the LA Metro system.

BIM is commonly used for digitally creating, managing and sharing information on a construction project, but on the Regional Connector it proved particularly useful in overcoming challenges with existing utilities.

Overlaying the digital model with 2D or 3D utilities maps made it simple to locate and plan around underground obstructions. We included all existing and proposed utilities in the project corridor. These models were used in planning, design and construction sequencing.

BIM was used to map the interfaces between existing and new structures, and to coordinate structures and services throughout the project – Historic Broadway Station is shown here.

Only good vibrations

Where rail tracks run close to buildings, noise is a potential problem. Vibrations from train wheels rolling along the rails are transmitted into the ground – and potentially also into buildings, which can amplify the vibrations into audible noise. The Regional Connector improves access to performing arts institutions including the Walt Disney Concert Hall and Colburn School of Music – but it was imperative that performances there would not be spoiled by the noise of passing trains.

Our solution for damping vibration was floating track slab, whereby track is fixed to precast concrete modules that are fitted with rubber isolation pads. These hold the track assembly clear of the tunnel lining, minimising the transmission of vibration and thereby avoiding the risk of structural resonance and noise.

Prior to construction, a mock-up was built and tested to verify the accuracy of our predictive design modelling. After construction was completed, permanent vibration monitoring equipment was installed in the tunnels for continuous testing and verification.

“We’ve done a lot of testing up on the surface, both in and outside the Disney Concert Hall, and been able to prove that there’s no noise, vibration or impact on the stakeholders,” confirms Mat Antonelli, LA Metro’s construction director.

Putting art on the line

LA Metro commissioned eight artists to create artworks for the three Regional Connector stations, making them cultural destinations in their own right.

Migrations by artist Clarence Williams lines the tracks at Historic Broadway Station