Field Work to begin on Alto Tunnel Geotech Investigation Study

The Alto Tunnel Study is an effort to collect information on the feasibility of reconstructing the Alto Tunnel for potential future bicycle and pedestrian use. The County is the lead agency in partnership with and with the cooperation of the City of Mill Valley and the Town of Corte Madera.  At this time, no decision has been made to reconstruct the tunnel. The County is gathering technical information and details requested by decision makers to help them make an informed decision about whether to initiate this project or not.

The field work begins this week; please see planned schedule below, but understand that unexpected factors could delay or accelerate this timeline.  The Underhill site is located on a vacant parcel on Underhill Road, west of Upperhill Road and adjacent to #35 Underhill Road.  The Chapman site is at the Corte Madera Avenue/Chapman Drive intersection. 

Tentative Alto Tunnel Study Schedule

  • Sun 1/29: Conducted bird surveys at both sites
  • Mon 1/30: Located utilities at both sites
  • Tue 1/31-Wed 2/1: Prepare Underhill site for drilling
  • Thu 2/2-Mon 2/13: Drilling at Underhill site
  • Tue 2/14-Wed 2/15: Move out of Underhill site
  • Thu 2/16-Fri 2/17: Prepare Chapman site for drilling
  • Mon 2/20: Holiday, no work
  • Tue 2/21-Mon 3/13: Drilling at Chapman site
  • Tue 3/14-Wed 3/15: Move out of Chapman site
  • Thu 3/16-Fri 3/17: Perform tunnel scan at both sites


  • The incoming storm this week may affect work to set up equipment at the Underhill site.  Crews will be on site Wednesday to install Best Management Practices (BMPs) for work during the rainy season, but the start of drilling may be delayed. Stay tuned to the webpage for the latest information!
  • Thurs 2/16-Wed 3/15: Chapman Drive will be closed to all users at Corte Madera Avenue, including weekends.  All residences will be accessible from the north during this period.  Signs will be posted.
  • All work 7am-5pm (no power tools or heavy machinery from 7-8a at Underhill site)

Please see the project website and the Jan 18, 2017 community meeting presentation for more information about the study. Please contact Carey Lando, County of Marin Senior Project Planner at (415) 473 5078 or email or the County‚Äôs outreach consultant, Victoria Eisen of Eisen|Letunic, at or (510) 525-0220 with further questions.