SCHEDULE: BID LATE SUMMER 2002
An international invitation to tender for 1.7 mile (2.7km) of twin tube running tunnels on the new light rail transit (LRT) line to East LA in Los Angeles will be called in late summer 2002 by the City’s Metropolitan Transportation Authority (MTA). Along with two cut-and-cover stations, the underground section passes beneath the busy commercial and residential section of 1st Street and the rising topography of Boyle Heights. Otherwise the 6 mile (9.6km) long line is largely an at grade extension of the Pasadena LRT line from Union Station. The new line was approved by the MTA Board in February 2002 and the underground contract, including the two station boxes, will be the first to tender. Following a bid in late summer, the contract is scheduled to be awarded by early 2003 and work towards an opening date for the full line of mid-2008. The Pasadena LRT line, currently in construction, is due to open in 2003.
Complying with recommendations from its Tunnel Advisory Panel comprising Dan Eisenstein and Geoff Martin, MTA will award the tunnel contract as a conventional design-bid-build procurement in efforts to better control the risks associated with tunneling and let the above ground sections and the system-wide installations as design-build contracts. Design of the underground works is underway by an integrated team of MTA staff supplemented by the East Side LRT Partners JV led by Parsons Brinckerhoff Quade & Douglas.
The LRT project replaces the East Side heavy metro rail underground line extension of the Red Line through Union Station that was cancelled in 1998 amid intense public, political and internal debate. The tunnels for heavy and light rail systems are similar at about 25ft (6m) diameter, with heavy rail trains comprise six cars for a capacity of 160 passengers/car and travel at 70 miles/h, while LRT trains travel more slowly, negotiate tighter alignment curves, pass through at-grade intersections, and have three cars/train with a capacity of 76 passengers/car resulting in shorter, less expensive stations.
As with the original project, the tunnels of the new alignment of the LRT pass below the water table and through the alluvial sands, gravel, silts and clay deposits of the Los Angeles River basin. To avoid ground instability, collapse and surface settlement issues that plagued construction of the earlier Red Line Metro tunnels, EPB TBM excavation is specified for the East LA LRT tunnels with positive pressurised face support required at all times. To meet the contract schedule two EPBMs are required to progress the tunnels concurrently from the 1st/Boyle Ave station eastward to the Evergreen Cemetery portals. The criteria for achieving zero surface settlement damage along the urban alignment is set at 1% face loss in normal ground condition and 0.5% in over-consolidated materials.
Also for the first time on the LA transit systems, the East LA LRT tunnels are designed with a one-pass precast concrete bolted and gasketed segmental lining. Previously, the so called two-pass LA tunnelling method, comprising a primary lining followed by a secondary in-situ concrete lining, with a PVC membrane in between, was designed specifically to prevent water seepage and perhaps more importantly, the ingress of methane and hydrogen sulfide gases. To meet the same stringent requirements on the EPB LRT tunnels, a team at the University of Illinois under the directorship of Prof. Ed Cording, has designed a one-pass segmental lining that will allow the lining to ‘rock’ with heavy earthquake movement and provide an effective barrier to gases. The 10in (250mm) thick precast segments have convex-convex faces on the radial joints to allow yield to seismic movement and are fitted with a double gasket system to prevent water and gas infiltration (inset). Also included is a cast-in grout injection tube with a port on the inside face of the rings through which the joint between the two gaskets can be grouted should there be evidence of leaks or gas infiltration.
During an interview in LA, Dennis Mori, Executive Officer, Engineering & Construction for MTA explained that the cost of the full 6-mile, eight-station extension is estimated at some $826 million. “Of that MTA has applied for some 50% in federal funding with matching funds allocated from local revenues. As with design, it is intended to create and integrated team of MTA staff and engineers from private companies for construction management and site supervision.”
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