Tunneling through an urban environment like that of Los Angeles is complex enough. Add to that a medley of poor soils including tar that for millennia sucked in animals to their deaths, creating archaeological conditions; a variety of dangerous gases; and two seismic faults and it becomes a task that until not so long ago was impossible—and even forbidden after an infamous 1985 explosion at a department store.
The once impossible has become reality thanks to a pair of earth pressure balance (EPB) tunnel-boring machines (TBMs), a customized conveyor system, extensive monitoring and political will on the part of owner Los Angeles County Metropolitan Transportation Authority (Metro). A joint venture of Skanska USA Civil, Traylor Brothers Inc. and J.F. Shea Construction completed tunneling last year for its $1.64-billion design-build contract to build the first of three sections of the Los Angeles Metro Purple Line extension, which is on track for a 2025 completion.