BTS | LA METRO
Right: Shielded hard rock TBMs were used under Santa Monica Mountains.
discharge. The project was successful with surface settlement effectively negligible, and some planned compensation grouting was found to not be needed. The success of the Gold Line project turned around
opportunities for metro tunnels in LA. Voters were again ready for tunnels, with sales tax approvals. The subway system also receives federal and other local funding. Amanda handed over to Matthew to discuss more
recent projects.
Right and below: Red Line designed to exclude gases.
RECENT PROJECTS AND INNOVATIONS More recent projects, over 2010s and 2020s, cover the Regional Connector, Crenshaw/LAX and the Purple Line Extension - the “jewel in the crown”, he said.
Crenshaw/LAX Crenshaw/LAX is a light rail project in Crenshaw district, not far from the airport and the sea. It has twin-bore tunnels linking three underground stations. For excavation support on LA Metro projects, the
extension that also passed through gassy ground at an old oil field. It had two underground stations. The TBM specifications allowed either slurry or earth pressure balance (EPB) shield tunnelling, provided they could operate in the geology and gassy-ground conditions. The design required tail-shield grouting and other measures, as discussed earlier, plus employment of compressed air to access the face. There would be single-pass lining (gasketed, and annulus to be fully grouted). Amanda noted that Matthew gave her a copy of the
draft BTS Tunnel Specifications at that time, which helped in document development. The precast lining was to the base design from the suspended Eastside HRT project, at 19ft (5.7m)-diameter, designed with the same concept of redundancy. The segments used a double gasket and a convex-to-convex radial joint, so as able move (‘rock’) during a seismic event while maintaining the gasket seal. Mike King did the final design for the segments. Before that, extensive testing of the joint system was performed at the University of Illinois. The TBMs selected by the contractor, Traylor Brothers,
had 200ft (61m)-long screw conveyors. The contractor had worked through the idea with shield manufacturer Herrenknecht, looking to handle gas in the muck by allowing their discharge well away from the heading. Additional ventilation could then be supplied at the screw
Top right:
Initial Ground support with Rock bolts, followed by CIP Concrete lining.
Bottom right:
Long geological section with tunnel alignment through mountains.
12 | June 2026
standard approach is bracing with steel pipe struts and soldier piles, top-down construction, concrete road decking, and shoring – following prescriptive design criteria with standard drawings that designers follow and constructors must implement. Two stations (Expo, MLK) were built with cutter soil
mix walls. These deep trenches mix soil under bentonite support. The soils stays and the bentonite is displaced by concrete, to leave a ‘soilcrete’ mix into which, before setting, steel soldier piles are pushed. The finished product is a wall with steel soldier piles and a soilcrete face, able to be trimmed by excavator bucket. When academic research, led by Dr. Youseff
Hashash, compared excavation support systems, use of conventional soldier piles and timber lagging saw wall deflection of about 25mm and surface settlement of about half that. With cutter soil mix walls, it was found that wall deflection was about 6mm-13mm, half that of the conventional lagging. Interestingly, though, surface heave was observed; with the wall being so stiff that on excavating a large hole the elastic relief in the adjacent ground dominates.
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