BTS | LA METRO
part of the route passes through siltstone, which would be difficult to process with a slurry machine. The panel also said that if a one-pass liner system was used it should not preclude adding a second liner in future (if there was leakage of gas or water), for it to be acceptable. Additional investigation included reducing boring
Top and right and far right: Complete Red Line stations and artwork.
spacing to less than 100m, using continuous core borings, and the panel also recommended doing a large exploratory shaft to better understand the unique ground conditions (tar sands, gas, fossils). The 23m-deep box shaft (built with soldier piles and shotcrete) was located at Fairfax station, in Section 1, and considered very successful in removing the mystique as during the proposal stage bidders could visit. Palaeontology had to be considered in the contract
Above and below: Convex joint seismic design for Gold Line.
documents. The GBR has a ‘paleo layer’ within which excavation could only proceed in thin (6-inch (150mm)) lifts to avoid disturbing fossils. While that would slow things down, it was included in bids as it was an environmental mitigation. A palaeontologist was on site during investigations. The final geological profile for the Purple Line Extension included: old and young alluvium, liquefiable San Pedro Formation, and some siltstone on the east side. In the Wilshire/Fairfax area, the profile shows the top of the asphalt-impregnated soil. The TBMs were similar to those on the Gold Line. There
were two new TBMs for each section and the double gaskets were used, with cross gaskets, for lining. On Section 1 of the extension there was a need for soil
conditioning of the tar-infused soil. Additional tests were performed, including in Italy. The TBMs broke through into and were pulled through
stations. One change from the earlier Purple Line work was that columns were removed from the stations, resulting in a slightly narrower station with an arched
roof. The single-pass linings were the same size as on the Gold Line. Although heavy rail requires about a foot less diameter than light rail, the final size came out about the same as the tunnel was sized so that a remedial secondary liner could be added at a future date. For the Purple Line Extension, the cross gasket is on
the circumferential joint. Based on these experiences the cross gasket is now codified in LA Metro criteria to be on the longitudinal joint. Tunnelling went very well. Average settlement was less than allowable, including in sensitive areas such as Beverly Hills. Bentonite shield-gap grouting was judged to have contributed to reduced settlement, although not all contractors chose to use shield grouting. Tunnelling in gassy conditions also went well. There
were, however, a few exceedances of OSHA allowable gas limits in one reach of tunnel, requiring brief evacuation until Cal/OSHA gave the all-clear to return. In a portion of Reach 3 of the Section 1 tunnel, soil
vapour extraction was used to reduce hydrogen sulphide and methane concentrations. This had been planned for many years as a potential mitigation measure. The concept is that air is injected through some wells and extracted through others, neutralising hydrogen sulphide gas in the ground. The tunnelling works were then able to pass through the area without issues. Air in local building basements was monitored and any gas oxidised. Amanda handed back to Matthew, who continued the briefing to its conclusion.
PURPLE LINE: CONSTRUCTION SUCCESSES AND DIFFICULT INTERFACES Station construction took the same basic approach used for 35 years, although with a few points of note. During Covid shutdowns, complete street closures were possible in the Golden Triangle in Beverly Hills, which
Right and far right: Double gasket system. 16 | June 2026
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