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Single bore solution for LA freeway missing link 16 May 2017

Shani Wallis, TunnelTalk

A single-tube, double-deck, TBM-bored toll tunnel is recommended as the preferred alternative for realizing the missing, and long awaited, section of the SR 710 freeway in Los Angeles, California.

At a meeting today in Los Angeles, the recommendation will be presented to the Congestion, Highway and Roads Committee of the Los Angeles County Metropolitan Transportation Authority (Metro), in support of the performance measures and scoring used to compare various alternatives leading to the preferred alternative. In a document seen by TunnelTalk, dated 17 May 2017 and signed by Metro CEO Phillip A Washington, recommendation of a 4.2 mile (6.57km) single tube TBM drive of 58.5ft (17.8m) o.d. is presented as the principal element to complete the 6.3 mile of missing freeway infrastructure to connect the southern stub of State Route 710 in Alhambra, to the northern stub of Interstate Route 710 in Pasadena (Fig 1).

 Fig 1. Route of the missing I-710 freeway
Fig 1. Route of the missing I-710 freeway
Fig 2. Preferred single-tube proposal
Fig 2. Preferred single-tube proposal

The 53.5ft (16.3m) i.d. bored tunnel will run at 40ft to 250ft (13m-76m) below the surface and accommodate two lanes on each road deck. Cut-and-cover transitions either end and a further 1.4 miles of at-grade work completes the link. With ventilation structures at each end, there will be no intermediate ventilation structures or on-off ramp junctions along the underground route.

Fig 3. Double-deck cut-and-cover transitions
Fig 3. Double-deck cut-and-cover transitions

The Metro announcement suggests that the project will be built at a preliminary cost estimate of US$3.15 billion, in 2014 dollars, and take between four and five years to build. Applying a toll to the freeway plan provides a revenue source and the opportunity to adopt a public private partnership (PPP) model for project procurement.

The announcement comes at the end of decades of indecision and study after strong protest in the 1970s effectively killed off plans to build the freeway link on the surface. It was passage of the tax-increase fund-raising Measure R on the 2008 election ballot, that funding in the amount of $780 million was earmarked for the current environmental study of the SR 710 North project. In 2011, Metro, in partnership with Caltrans, the California State Department of Transportation, initiated the current environmental study to address severe congestion and adverse impacts on the communities created by the missing link. The study considered a range of five alternatives, including no-build options with improvements to traffic management controls to upgrade surface traffic conditions, options for bus and light rail rapid transit systems, as well as a twin-tube bored tunnel alternative to the single-tube option.

The completed studies recommend the single double-decked bored toll tunnel solution with a restrictions to exclude trucks of more than 33,000 tonne and ban vehicles carrying flammable or hazardous materials to optimize traffic operations through the tunnel.

Evaluation of the performance factors of the five alternatives, on a scale of 1 to 10 (Table 1), indicated that both tunnel alternatives – a single bore or twin-tube configuration:

  • Showed above average performance compared to the other alternatives that reported below average performances when transportation objectives (e.g. minimize travel time; improve connectivity and mobility; and reduce congestion on local street system) and the corresponding performance measures were evaluated.
  • Provided substantial benefits in terms of reducing congestion, particularly on local streets compared to the transit alternatives, which showed negligible operational improvements on the overall transportation network (freeway and local/arterial roadways).
  • Showed considerable increase in transit ridership. The transit alternatives provided a better performance related to the increased transit ridership objective when compared to the other alternatives, with minimal benefits reported for the overall transportation network.
  • Demonstrated positive effect on minimizing environmental (human and natural) impacts by avoidance, or practical and feasible mitigation measures to off-set impacts.
  • Showed considerable advantage over the other alternatives when evaluating the remaining non-transportation related objectives (e.g. minimizing right of way impacts and maximizing cost efficiency of public investments).

Table 1.  Performance evaluation of the alternatives
Table 1. Performance evaluation of the alternatives

The Metro recommendation states that, of the alternatives, the single bore freeway tunnel (SBFT) presents the highest net present value at about $1.5 billion. The SBFT option is expected to carry 90,000 vehicles/day, remove 42,000 vehicles/day from local streets within the study area and save motorists using the tunnel some 13 minutes in rush hour commutes. In addition, the SBFT is preferred as it:

  • Improves local and regional mobility
  • Improves air quality for affected cities within the study area
  • Reduces congestion and cut-through traffic with 42,000 fewer vehicles on local streets/day
  • Saves travel time and reduces delay, with up to 4,000 fewer vehicle hours traveled/day during peak period within the study area
  • Realizes fewer vehicle miles traveled on local streets, reducing arterial travel by 280,000 vehicle-miles travelled/day
  • Improves connectivity and mobility by increasing throughput by 66,000 vehicles/day on the freeway and increase person throughput by 49,000 daily
  • Creates the greatest number of jobs at approximately 42,000 and generates the highest projections of employment earnings at approximately $29 million (in 2010 dollar values) per year, surpassed only by the twin-tube tunnel alternative.

The Metro recommendation states that the “results of the technical studies completed over the past few years clearly capture the mobility benefits included in the single bore freeway tunnel (SBFT), with tolls and truck restrictions. This alternative reduces regional and local congestion associated with north-south travel demand within the study area, and delivers the best transportation performance and benefits with the least environmental impacts.”

Work will now continue to complete the final EIR/EIS by the first quarter of 2018, after which Caltrans will certify the project by filing the NOD (Notice of Determination) and ROD (Record of Decision). During the draft EIR/EIS public consultation period, between March and August 2015, more than 8,000 comments were received. Of these, 1,328 comments supported, and 237 comments opposed, the tunnel alternative. All comments received will be further addressed in the final EIR/EIS. Metro staff will report again to the Metro Board of Directors in November/December 2017. As a project status update, the report includes no financial review in the current recommendation statement.

Recommendation of the single-tube, large-diameter, double-deck TBM bored tunnel will have been influenced by other similar TBM projects around the world and in particular, by the SR-99 elevated viaduct replacement double-deck TBM bored tunnel in Seattle. Although the project suffered serious mechanical faults and delays, the repaired and upgraded TBM did succeed in completing its 9,270ft (2.85km) long x 17.48m o.d. drive beneath the heart of Seattle, breaking through last month on 4 April 2017.

Another recently completed single-tube, double-deck TBM bored highway tunnel is the undersea fixed link under the Bosphorus Strait in Istanbul Turkey, which completed its daring challenge on 22 August, 2015. Other TBM bored road tunnels of note include the Tuen Mun - Chek Lap Kok subsea highway link, which is being excavated using a TBM of 17.6m diameter, and the 16.2km section of the Tokyo Ring Road project in Japan where four TBMs of more than 16m diameter are working on the twin-tube subsurface highway.

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