BTS | LA METRO
Regional Connector LRT The Regional Connector is another light rail project, in downtown LA. It includes about 3km of tunnel and connects four light rail lines. It has three underground stations. The first part of the tunnel has about 1.5 diameters of overburden and is in soft ground below the groundwater table, so it was tricky tunnelling. There was also a culturally sensitive building above. Compensation- grouting was mandated. Another interesting feature involved the track crossover.
Top and right: Tunnel construction in gassy conditions.
Right: On the Red Line there was some tar leakage through initial support precast segments (not bolted).
The HDPE waterproofing system was improved. It
can be difficult to install well and this was improved by compartmentalization using water bars. Around each compartment is a re-groutable hose with grout ports. This is useful during construction, such as on Crenshaw/ LAX when used to create dry stations for operations, but also for any future midlife refresh. On the tunnelling side, the pressurised-face approach
Below: Map showing route of completed Red Line.
and pressurised grouting were used, and also pressurised bentonite in the shield gap around the tunnel. We again used double-gasketed segments, with an enhancement on Crenshaw/LAX: a cross gasket on the longitudinal joint, enabling the tunnel to be compartmentalised in 1.5m lengths and future grouting in the gap between segments.
Such layout is typically provided for with cut-and-cover structures, but nearby were the Police Department building, the Higgins Building and others with basements extending into the street. There was also a large-diameter storm drain. Fortunately, instead of alluvial material the geology comprises siltstone and claystone of the Fernando Formation. A mined crossover cavern was designed using sequential excavation. Its impressive dimensions (LxWxH) are: 290ft (88m) x 58ft (17.5m) x 36ft (11m). Some high-rise buildings from the 1980s had three or
four storeys of basement, constructed with soldier piles and tiebacks, into the street. From a new shaft (using liner plates and ribs) and then with sequential excavation found and cut the tiebacks. The void was infilled with artificial soil to later allow tunnelling through the area. On sustainability, we reused TBMs. The Gold Line TBMs
were reused, with some refurbishment, in Seattle and the Regional Connector. For discussion of the Purple Line Extension project, Matthew handed back to Amanda.
THE PURPLE LINE EXTENSION The project is 9 miles (14.5km)-long, entirely underground in twin bore tunnels with seven stations. It is being built in three sections (1-3) and the route goes out to the University of California, Los Angeles (UCLA), and the Veterans Administration Hospital. There are many venues for the Olympics in 2028, mainly at the end of the extension. The project goes through a number of jurisdictions, requiring coordination with third parties. Planning began in 2007, and Section 3 (length 2.5
miles (4km)), at the end of the line, is projected to finish construction in 2027. Funding restrictions were lifted, following the success
of the Gold Line works, as noted. Improvements in technology beyond pressure-face TBMs also helped, including methods to reduce gas in the ground during tunnelling, and better monitoring of the environment. LA Metro also convened another peer-review panel, in 2005, through the American Public Transportation Association (APTA) which can help supply peer experts for particular problems. The panel concluded, based on available data, that it would be safe to tunnel in the methane-zone area but recommended use of only slurry TBMs, because they could contain the gas and release it at the surface, and that the two-pass lining system should be maintained. On that basis, the US Congress reversed the legislation,
in 2006. Following further geotechnical exploration, LA Metro held a follow-up panel, in 2012. The panel concluded that EPB machines could be used, especially as
14 | June 2026
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