LA METRO | BTS


code requires buildings in the methane zone to have additional geotechnical investigation, barrier systems for basements, permanent gas monitoring systems and subterranean ventilation.


RED LINE Red Line construction began in the 1980s. It was mostly cut-and-cover stations, using braced excavations, soldier piles and lagging, and dewatering depending on the groundwater conditions, Amanda said. Tunnel construction used open shields, except through


the Santa Monica Mountains, where hard-rock TBMs were used - with names ‘Thelma’ and ‘Louise’, giving a sense of when the works were done. Initial ground support was rock bolts and sometimes steel ribs. The final lining was cast-in-place (CIP) concrete.


CHALLENGES FROM GEOLOGY 1. Gases The design was required to mitigate methane and a two- pass system was used, for tunnels and stations: initial unbolted/expanded precast concrete segmental lining; an HDPE liner as a hydrocarbon-resistant membrane, confirmed by testing; and, the CIP lining. Tunnel construction in gassy ground follows the


California Tunnel Safety Orders, requiring methane concentration to be diluted to within safe limits, continuous monitoring during excavation, and equipment approved by the Mine Safety and Health Administration (MSHA). Equipment must be certified as intrinsically safe, non-sparking. No smoking or cameras are allowed in the tunnels, and no cell phones unless they have special covers. The main ventilation systems must exhaust gas and be made of rigid metal. Most of the time, a probe hole is to be maintain ahead of the face, unless a variance is obtained from Cal/OSHA. Similar mitigation approaches are used during the


operational life of the tunnels - HDPE barriers (in stations with welds tested extensively, using spark or vacuum- box testing), and continuous monitoring for gases and two-level alarms, with fan ventilation is activated at specified levels, and at higher levels the tunnels would be evacuated. Normal ventilation from the train piston- effect generally keeps any small amounts of gas diluted to acceptable levels. A few incidents were reported during Red Line construction, although generally-speaking things went well. In one case, tar leaked through the initial precast segments in a section of west of downtown.


2. Earthquakes A two-level seismic design approach was developed: 1) operating design earthquake (150-yr event), where damage to a structure is minor, enabling operations to resume after a brief inspection; and, 2) maximum design earthquake (2500-yr event), where the structure must maintain life safety, though may require repairs. There are seismographs in many operating stations,


and the state has an early-warning system. Trains are stopped upon detection of seismic activity (ground shaking or acceleration).


Left: Open shields used in early metro construction in LA.


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3. Soft ground Soft ground included alluvial soils above and below ground water, and also weathered and unweathered siltstones. The soft ground also posed challenges for open-face shield tunnelling. While most of the time, things were fine, there was some surface settlement due to ground loss. Another notable experience was a wood lagging fire early on in construction of the Red Line tunnel, under the 101 Freeway. Fortunately, permeation grouting had been specified under the freeway, so it did not collapse. Wood lagging is no longer allowed in the tunnels and the event resulted in the use of precast segments for initial excavation support. Public perception around construction issues in LA,


including safety of tunnelling, LA Metro created a Tunnel Advisory Panel (TAP), including Harvey Parker, Dan Eisenstein and Geoffrey Martin. They studied tunnels worldwide and considered the LA experience, concluding it was not too different at that time. They did, however, recommend future use of pressure-face TBMs. The Red Line was completed around 2000, but by then,


and despite the TAP’s findings, confidence waned due to perceived cost overruns and voters did not want to spend any more sales tax on subways, and so subway construction was stopped in 1997. The Eastside Heavy Rail Tunnel (HRT) project was in final design stage but was shelved, later re-emerging as a light rail line to East LA. The panel’s recommendations on TBMs and the success of the Eastside project were important in getting tunnelling moving again on the Westside.


GOLD LINE The Gold Line Eastside project was a light rail


Above: Generalised stratigraphy in the LA area.


Below: A few of the historic tunnels of Los Angeles.


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