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At the heart of the city of San Francisco lies the Central Subway light-rail line. Extending the Muni Metro T Third Line improves public transportation to and from some of San Francisco’s busiest, most densely populated areas, offering travelers an alternative to heavy traffic congestion.


In addition to the technical difficulties with drilling two tunnels across a city like San Francisco, the close proximity of active subway routes – with only eight feet of separation in the soft ground sections of the alignment – added to the challenge.

Technical Highlights

  • Two parallel 1.6 mile, pre-cast concrete, segmentally lined, twin-bored subway tunnels.
  • Tunnel boring machine (TBM) launch shaft and extraction shaft
  • Consolidation and compensation grouting work to prevent building settlement
  • Five emergency cross passages
  • Perimetral slurry walls for the future stations
  • Completion works for the associated connecting structures (launch shaft for cutters)
  • A final service shaft approximately 39 feet in diameter


  • Award of Merit 2015 from Engineering-News Record (ENR)
  • The Outstanding Transportation Project 2015 award given by the local section of the American Society of Civil Engineers (ASCE)