Devil’s Slide Tunnel, Project Updates and Geotechnical Info

Work started from the south portals in September of 2007. The tunneling methods being used is the New Australian Tunneling Method (NATM), and excavating is being performed by an Alpine 1 Road header (pictured at right, Photo by Kim Komenich of the San Francisco Chronicle) for the softer material, and conventional drill and blast for solid rock. In early March of this year,  ABC7 News reported that the Northbound tunnel had been excavated to 181-m, and the Southbound tunnel to 90-m. The work continues around the clock.

At the North end of the project, there are two gracefully sweeping bridges over 1,000-ft long that connect the North tunnel portals with Highway 1 just South of Pacifica. In the last week or two, work on the bridges has reached about 95% completion. They constructed them from each side of the canyon, and met in the middle within 20mm. They are now performing the final deck pours and finishing work and expect to be done in July. One interesting aspect of the bridge is that the inside is hollow. They have included tubes for post-tensioning cables in the event that they find the bridge is sagging or something at a later date. Once the bridges are completed, the tunneling contractor will begin excavating from the North side as well to meet somewhere in the middle.

Miscellaneous Geotechnical Tidbits and Other Info

  • Alpine 1 Roadheader – Custom for the project. Used for softer material.
  • Using fiber-reinforced shotcrete for the tunnel lining
  • No GPS underground of course, laser total station to control accuracy
  • Continuous LIDAR scans and photos of the tunnel as excavation progresses
  • 3D Modeling from the LIDAR and imagery used to adjust tunnel design on the fly (first time this has been done)
  • Every 1.2 m, team must decide on what methods of support to use based on the type of material encountered
  • Tunnels seep 5 gal of water per minute
  • Scheduled to be completed in early 2011

Links and Video

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Download the KML File to review the project’s location in Google Earth.