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The original item was published from 1/12/2017 11:50:26 AM to 1/21/2017 12:00:01 AM.

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Posted on: January 12, 2017

[ARCHIVED] Notice of Alto Tunnel Geotechnical /Structural Investigation Informational Meeting 1/18/17, 6:30pm

Alto Tunnel Study

Alto Tunnel Geotechnical and Structural Investigation

The Alto Tunnel Geotechnical and Structural Investigation will seek further technical details about the tunnel to inform a refined cost estimate for potential reconstruction of the tunnel if it were to be used as a bicycle and pedestrian pathway.  

On April 28, 2016, a Request for Proposals was released for professional services for review of the geotechnical & structural condition of the tunnel and to update cost estimates for reconstruction of the tunnel for consideration of potential future use as a bicycle and pedestrian facility. These Questions & Answers for Proposers were prepared prior to submittal of proposals.  Proposals were due on May 26, 2016.  The Town of Corte Madera and City of Mill Valley participated in the proposal review and interview process of the consultant team to assist the County of Marin in their selection process.

On October 25, 2016, the Marin County Board of Supervisors approved a contract with McMillen Jacobs Associates for geotechnical and structural investigative services and to update previous cost estimates.

An informational meeting will be held on January 18, 2017 at the Corte Madera Community Center at 6:30 pm to communicate details and answer questions.  Following completion of the investigation and updated cost estimates, another meeting will be held to present the findings.  A more detailed timeline of upcoming milestones will be provided soon.


The purpose of this study is to observe the structural condition of the Alto Tunnel.  This information is necessary to better identify the scope of improvements that would be required to reopen the Alto Tunnel as considered in the 2010 Mill Valley/Corte Madera Corridor Study and to update previous cost estimates.  This specific Alto Tunnel study is just the next step toward developing more reliable information regarding the overall improvements and costs that would be required to reopen the tunnel. The draft study’s results will be presented to the public for review and comment in summer 2017.

The Investigative Study

This effort is an investigative study, not a construction project.  The condition of the tunnel will be observed using remote cameras and laser mapping technology, thereby avoiding the higher cost and impacts associated with opening a portal and physically entering the tunnel.  The study consists of drilling five small holes (each four inches in diameter) from two locations above the Alto Tunnel, then lowering a laser scanning probe into the holes, which will transmit images of the tunnel. Work at the following locations were chosen to minimize the number of holes and to reduce the disruption to area residents and to traffic, particularly during work and school commute periods (see area map below):

•In the vacant County-owned lot south of 35 Underhill Road (just south of Upperhill Road)

•Adjacent to Chapman Drive at Camino Alto  

A final work plan detailing what is planned at each site and when will be presented at the January meetings.  Drill Tech will drill the holes; McMillen Jacobs Associates will perform the scan.  

Field Work Schedule

Work is expected to begin Monday, January 30, 2017 and be completed by late March 2017. The study team anticipates being at the Underhill site for approximately three weeks and the Chapman site for approximately five weeks. Drill work will occur only during daytime work hours.  More details on the field work dates will be posted to the County webpage as they are finalized and updated.  The team will then summarize their findings and present them to the public in summer 2017.

Traffic impacts

During the investigation, traffic impacts will be kept to a minimum. Traffic for all users at the Underhill Road site will not be impacted because all work will be done off of the roadway. More information about the need to control traffic at the Chapman site will be presented at the January 18th meeting.

Stay Informed

Up-to-date information about the investigation will be posted at  The County’s outreach consultant, Victoria Eisen of Eisen|Letunic, will be your point of contact during this effort.  She can be reached at or (510) 525-0220.  You may also contact the Ms. Carey Lando, Senior Project Planner, at or (415) 473-5078.

If you have additional questions or want to learn more about the upcoming geotechnical investigation work, please join us at the Community Meeting next week:

Wednesday, January 18, 2017

6:30-8 pm

Corte Madera Community Center

498 Tamalpais Drive, Corte Madera

The results of the geotechnical and structural investigation and updated cost estimates will be presented at public meeting later this year in mid-2017.

Additional Info about the Alto Tunnel Study
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