07 June 2012 ~ 1 Comment

Preparing for the Central Subway Tunnel: Major Work Commencing Next Week

Pouring concrete - launch box guide walls - June 2012

At the site of the tunnel launch box, crews are constructing shallow, below-ground walls to prepare for the major upcoming excavation.

Next year, a pair of tunnel boring machines (TBMs) will begin excavating the Central Subway tunnel, building the backbone of Muni’s extension of the T Third Line through vibrant and bustling SoMa, downtown, Union Square and Chinatown. Next week, major work to prepare for tunneling is ramping up in SoMa, with additional tunnel-preparation work planned to begin this year at new sites along the project alignment.

This new stage of construction will be the first to build permanent elements of the Central Subway, kicking off the next phase of progress on this critical transit upgrade.

The work, to be carried out by tunneling contractor Barnard Impregilo Healy Joint Venture, involves excavating the site where tunneling will begin, stabilizing the ground at key points along the project alignment and relocating utilities at the tunnel’s end point.

This blog post provides an overview of the tunnel-related work slated to take place in 2012. Future posts will contain additional details about what to expect at specific construction sites, including work timelines, construction impacts and descriptions of construction techniques.

A variety of measures, including street sweeping, noise monitoring and traffic controls, will be employed to reduce construction impacts. Access to local businesses, residences and driveways will be maintained throughout construction. However, traffic, transit, parking and pedestrian walkways will be impacted, and travelers through these areas can expect an increase in noise, dust and truck traffic while work is in progress.


This map provides an overview of the construction to prepare for tunneling that will occur in 2012. A PDF of the map can be downloaded here.

Central Subway Tunnel: 2012 Construction Overview

The following work is planned to occur this year to prepare for tunneling in 2013:

  • Launch Box: Next week, the tunneling contractor will begin excavating and constructing a launch box at the site where tunneling will begin. The launch box will be built on 4th Street between Bryant and Harrison streets, beneath the I-80 overpass. At nearly 500 feet long, 50 feet wide and up to 40 feet deep, this major excavation will take up most of the block. Next year the TBMs will start digging the Central Subway tunnels from the launch box, traveling north under 4th Street and then Stockton Street.
  • Headwalls: The contractor will install below-ground walls, called headwalls, at subway station sites in SoMa and Union Square. Headwall installation will take place on 4th Street between Folsom and Howard streets and on Stockton Street between Market and Post streets.
  • Grouting: Ground stabilization work, known as jet grouting or compensation grouting, will occur along the project alignment in SoMa and Union Square. This work involves drilling shafts into the ground, monitoring ground conditions and, if necessary, injecting a mud-like substance that acts as a stabilizer.
  • Utility Relocation: The Central Subway tunnel will extend past the end of the line in Chinatown to North Beach, paving the way for a potential future extension of the line. Utility relocation in North Beach this year will prepare for excavation of the site where the TBMs will be removed from the ground. Ongoing communications and outreach are underway to determine a schedule for this work.

Find Out More

The Central Subway team will continue to work closely with communities, local businesses and organizations to inform them of construction impacts and to ensure appropriate mitigation measures are in place. As work commences at new locations along the alignment, additional information on local impacts will be made available here, on Facebook, on Twitter, on our project Google Map and in media alerts. To schedule a briefing on tunnel-related construction for your community group or organization, please contact Brajah Norris at 415-701-5263 or email our team at central.subway@sfmta.com.

More information about the Central Subway tunnel and the tunnel-related work slated for 2012 is available in the following documents:

We appreciate your continued patience while construction is in progress.

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