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Fact Sheet #4 - August 2010 (Tunneling)


Metro is evaluating the environmental impacts and mobility benefits of five alternatives for the Westside Subway Extension in the Draft EIS/EIR.  The five alternatives were identified through the Alternatives Analysis (AA) Study conducted in 2007-08.  They have been further developed through extensive analysis and public input during the Draft EIS/EIR that has been underway since early 2009.

Two of the alternatives, an extension of the Metro Purple Line to either Westwood/UCLA or Westwood/VA, are projected to have sufficient funding for construction and operations from anticipated local and federal funding sources and are included in the adopted Long Range Transportation Plan for Los Angeles County.  In the Fall of 2010, the Metro Board of Directors will be asked to select one of the alternatives as the Locally Preferred Alternative (LPA) to proceed into the Final EIS/EIR and Preliminary Engineering phase. We will also seek federal matching funds for construction, as well as input from the prior (AA) study conducted in 2007-08. The five alternatives are the following:

Alternatives Within Measure R/LRTP Funding

Alternatives Beyond Measure R/LRTP Funding

Tunneling in earthquake country

Many underground facilities – subway tunnels, sewers, storm drains and buildings – have been built in Los Angeles and throughout California near active fault lines. California has some of the strictest building standards when it comes to designing infrastructure to withstand earthquakes. One of the initial steps in planning a transportation project like a subway is to identify where faults are located and understand their characteristics. The goal in planning the subway is to avoid faults if possible. If that is not possible, then every effort is made to minimize exposure to the fault(s), often by crossing a fault in a perpendicular orientation. Running parallel increases the risk as more of the project is exposed to the fault. This can increase project costs as special engineering and construction techniques utilized in a fault zone must be employed along a greater portion of the project. Various special engineering techniques have been employed in fault zones to reduce the risk of damage, limit any damage that may occur, and allow for a swift return to regular operations should a seismic event take place. This can include constructing larger diameter tunnels or the use of enhanced tunnel linings and other measures to accommodate ground movement in fault zones.

Subways throughout the world have excellent records of withstanding major earthquakes over the last 25 years. The Metro Red Line tunnels cross the Hollywood fault north of the Highland Station. They have performed well during earthquakes with no damage or service interruptions, including after the Northridge earthquake in 1994. The Santa Monica fault runs through the Westside Subway Extension subway area. We are particularly focused on the fault in the area around Century City where its location may influence decisions regarding station and tunnel alignments. More testing and analysis will be conducted during the Final EIS/EIR and engineering phase to determine the best way to design and build the subway in this area.

Tunneling near oil fields

Greater Los Angeles is an oil producing area and there is significant local experience building here. During the draft environmental analysis, known oil fields and documented active or abandoned oil wells have been identified from oil well maps. This initial analysis indicated that the oil fields are much deeper than the potential subway tunnels. Shafts for active and abandoned oil wells are also located in the vicinity of project alignments along with other utilities such as sewer, water, gas and electric lines. These will be further mapped during Preliminary Engineering and relocated, if necessary, during the construction phase of the project. If any unmapped or unknown wells are encountered during construction, there are established procedures, regulated by government agencies, for dealing with them safely. Metro recently tunneled successfully through the former Boyle Heights oil field while constructing the Metro Gold Line Eastside Extension.

New tunneling technologies

Metro has enjoyed great success with the twin-bore tunnels used on the existing Metro system, particularly the recent tunnels on the Metro Gold Line Eastside Extension. These tunnels used the newest form of tunnel boring machine technology known as “earth pressure balance.” This technology maintains the pressure in the earth surrounding the tunnel. As a result, the tunnels for the Metro Gold Line Eastside Extension were constructed with no measurable surface subsidence and no substantiated property damage claims. They were also constructed on-time and on-budget.

Cross-section of typical twin-bore subway tunnels:

Metro is evaluating the best techniques for constructing the subway as well as other projects that may require tunneling. This includes looking at improvements to the twin-bore technology used on the Metro Gold Line as well as the potential use of a large single-bore tunnel. Previous tunnels built by Metro have used twin tunnels that are approximately 20 feet in diameter. Each tunnel accommodates a single train track with two side by side tunnels required to accommodate both directions of travel. Other projects in other countries have sometimes used a single, larger diameter tunnel that can accommodate two or more tracks in a single tunnel. The choice of a tunnel size for the project will be made following additional design and operations studies.

How to Stay Involved and Give Input

Metro invites you to stay involved throughout the study. You can find information as the work progresses, leave comments, and let us know how to contact you so we can keep you informed of upcoming meetings and other milestones by going to Contact Us . We also invite you to join our Facebook group, “Metro Westside Subway Extension.”