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NTPP Project #1001

Last updated: August 17, 2017

This Alto Tunnel Study is an effort to collect information on the feasibility of reconstructing the Alto Tunnel for potential future bicycle and pedestrian use.  As with the MV-CM Bike/Ped Study, 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 decisionmakers to help them make an informed decision about whether to initiate this project or not.

Save the Date for the Next Public Meeting: September 27, 2017.

Quick Reference Field Work Summary January 30-March 31, 2017

The field work portion of the Study has been completed.  Please join us later this summer to hear the findings of the investigation and review updated cost estimates.

Geotechnical & Structural Review and Cost Estimate Update Phase

This phase of the Study seeks further technical details about the previously non-inspected tunnel reaches 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. 

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

On January 18, 2017, an informational meeting was held at the Corte Madera Community Center to communicate details and answer questions.  View the presentation and field work schedule. Read the summary of the questions and responses from the meeting.

A biological site assessment report  (and appendices) was prepared on January 31, 2017 to evaluate for potential impacts to the environment and the study was determined to be CEQA categorically exempt for collection of information.

Following completion of the investigation and updated cost estimates, another meeting will be held during the Summer 2017 to present the findings.  


The purpose of this study is to observe the structural condition of the previously non-inspected reaches of the Alto Tunnel, as further discussed in Vol. II of the 2001 Alto Tunnel Scoping Study.  This information is necessary to better identify the scope of improvements that would be required to reconstruct the Alto Tunnel as considered in the 2010 Mill Valley/Corte Madera Corridor Study and to update previous cost estimates.  Today’s 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 for review and comment in summer 2017.

Alto Tunnel Study Field Investigation Site Area Map

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 
Alto Geotech Field Work Site Map

The investigative study

This effort is an investigative study, not a construction project.  We will observe the condition of the tunnel using laser mapping technology, thereby avoiding the higher cost and impacts associated with opening a portal and physically entering the tunnel or large access holes needed for remote cameras. (Click graphic to view larger .pdf) Alto Tunnel Borehole Cross Section

The study consists of drilling five small holes (each 5.5 inches in diameter) from two locations above the Alto Tunnel, then lowering a laser scanning probe into the holes, which will create images of the tunnel interior. DrillTech will drill the holes; McMillen Jacobs Associates will perform the scan.  The bore holes will be cased (lined) with steel as they are drilled and then sealed with a permanent cap to ensure safety and security.

Field Work Schedule

Field work is scheduled to begin Monday, January 30, 2017 and be completed by late March 2017. The team anticipates being at the Underhill site from 7a to 5p for approximately two weeks and the Chapman site for approximately 3-4 weeks. All work will be conducted Monday through Friday; no weekends.  More details on the field work dates will be posted to this webpage as they are finalized.  Additional outreach and courtesy notices will be provided through a variety of information venues at least 2 days before work begins. Weekly Progress Reports during field work: Report 1, Report 2, Report 3, Report 4, Report 5, Report 6, Report 7

January 30 through late March, 2017 (schedule updated 3/10/17) - Geotechnical & Structural Investigation phase field work:

1/30: locate/confirm utilities; nesting bird survey

1/31-2/28: Underhill Rd. (MV) Mobilization/drilling/demobilization

3/1-3/28: Chapman Dr. (CM)  Mobilization/drilling/demobilization - Expect the top of Chapman Dr. at Corte Madera Ave. to be closed 24/7 during drilling phase; all Chapman Dr. residences will be accessible from the north during this period.

3/31: Drilling field work completed

4/18-19: Scanning field work

The team will present findings and updated cost estimates to the public in mid 2017. 


To ensure drilling accuracy from one day to the next, it is necessary to keep drilling equipment properly aligned. Traffic at the Underhill Road site will not be impacted because all work will be done off of the roadway.  Due to the angle of the drilling boom and the narrow width of Chapman Dr., closure to vehicular traffic is necessary.

Expect a road closure at the top of Chapman Dr. at Corte Madera Ave. during the drilling phase (approximately March 1 through March 27, 2017, subject to change) with a detour/alternative route to lower Chapman Dr.  This temporary vehicular closure will be in coordination with the Town and emergency response officials.  Notification to residents and the community will be given in advance to alert travelers of the change and to communicate alternative routes to Chapman Dr. residences, which will be from the north during this period. See Map below; click for enlargement.

Chapman Dr Access Map during Geotechnical Investigation

Upcoming Efforts

This phase of the Alto Tunnel Study - the geotechnical and structural analysis - will occur over the next several months. A study report will be prepared in mid 2017 discussing the findings of the overall study.

Up-to-date information about the investigation will be posted here. 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.  Please contact Victoria with any questions before, during or after the investigation. 


March 2013: Release RFP for land surveying and mapping

June 4, 2013:  BOS approval of contract for field surveying & mapping

April-mid 2013-15: Property research, evaluation and documentation

Mid 2013-15:  Field Surveys of Alto Tunnel corridor

Late 2015: Public Workshop on property research and mapping findings

April 28, 2016: Release RFP for geotechnical investigation

May 26, 2016: Proposals due

October 25, 2016: BOS approval of contract for geotechnical investigation

January 18, 2017: Informational meeting on geotechnical phase process; 6:30p at the Corte Madera Community Center

January 30 through late March, 2017 (updated 2/22/17) - Geotechnical and Structural Investigation Study phase field work

Spring 2017: Analysis of scan images; update cost estimates

Mid 2017 (tent.): Informational meeting on results of study & updated cost estimates


Studying the feasibility of potentially opening the Alto Tunnel to provide a bicycle and pedestrian connection between Corte Madera and Mill Valley was identified as one of the top priority projects in the county, based on public input in the 2006 Marin County Bicycle Plan Update public workshops. Currently, bicyclists use either Camino Alto or the U.S. 101 “Horse Hill” bike path via local neighborhood streets. As discussed in the 2010 Mill Valley to Corte Madera Bicycle and Pedestrian Corridor Study, it is challenging for cyclists and pedestrians alike to travel between the two communities due to a number of factors including steep topography, lack of facilities, and speed of vehicles. This route is also part of the regional North/South bicycle route. 

Completed in 2010, the Mill Valley to Corte Madera Bicycle and Pedestrian Corridor Study evaluated improvements to three alternative routes between Mill Valley and Corte Madera, including (1) the Horse Hill/Alto Hill Pathway and connecting roadways, (2) reopening the Alto Tunnel for bike/ped use, and (3) the Camino Alto/Corte Madera Avenue on-street route.  Conceptual improvements were evaluated through an inclusive public process.  During this process, several additional issues were identified with the Alto Tunnel route that needed additional study.  These additional issues included the contingency levels (40%) on the project cost estimates, as well as property/right-of-way questions.

Learn more about the Alto Tunnel in the 2010 Mill Valley to Corte Madera Bicycle and Pedestrian Corridor Study project webpage.

The Alto Tunnel corridor is in the incorporated areas of Mill Valley and Corte Madera. The County of Marin is the lead agency on this multi-jurisdictional corridor study and works closely with staff from Mill Valley, Corte Madera, Transportation Authority of Marin, and County Parks & Open Space.

The Marin County Board of Supervisors authorized additional federal Non-Motorized Transportation Pilot Program (NTPP) grant funds to refine cost estimates and investigate issues identified in the MV-CM Bike/Ped Study.  This additional research will further evaluate property and right-of-way issues, as well as collect additional geotechnical information on the condition of the tunnel itself.  This information will help narrow the construction cost estimate contingency level to achieve a higher level of confidence in how much it would cost if the tunnel were reconstructed for bicycle and pedestrian use. 

A Request for Proposals for land surveying and mapping was released on March 4, 2013 to provide current and accurate mapping of the Alto Tunnel corridor between Redwood Ave./Tamalpais Dr. in Corte Madera and Vasco Court in Mill Valley.  Concurrent with the land surveying and mapping efforts, the Marin County Real Estate Division conducted title research, documentation of ownership interests, and potential subsurface rights of property owners above the tunnel structure. This phase is complete.

Summary of the Alto Tunnel Corridor Right of Way Research - 3.25.16

 The first phase of the Alto Tunnel Study includes detailed research and analysis of property records, field surveys and mapping of the Alto Tunnel Corridor between Redwood Ave./Tamalpais Dr. in Corte Madera and Vasco Court in Mill Valley.  The results of this phase of study include a narrative document summarizing the results of that property research, a Right of Way map showing the segments along the corridor that the County holds and those segments that others hold, a Boundary Map, and a Topographic Map of the Alto Tunnel corridor.  

The narrative summary and information posted to this webpage is for information purposes, as the County is only partway through the Study and it is not yet finalized.  A Revised draft summary of the right-of-way research conducted by the County of Marin incorporated minor clarifications in response to public feedback and input.  Some of the clarifications include specifically identifying the private properties above the tunnel, as requested by some parcel owners.  Other clarifications are stylistic in nature and do not change the position or findings based upon the research conducted and information obtained or provided since the initial release of the draft research summary.  The information collected to date, as well as public input and additional information received during the course of the entirety of the Alto Tunnel Study process will help inform the refinement of cost estimates in the next phase of the Study. Over 90 people attended an informational meeting on Tuesday November 17, 2015 at 5:30p, Mill Valley Community Center to hear about the initial right-of-way research along the Alto Tunnel corridor and to provide their feedback to County staff. View the presentation (pdf) from the November 17, 2015 informational meeting.  

Background and Technical Documents

How to Stay Informed

County staff will utilize the Interested Parties email lists for public notifications and information on the Alto Tunnel Study. If you are not on this list and would like to add your contact information for future correspondence; OR if you are on the list and do not want to receive Alto Tunnel Study information, please contact the County’s Project Manager, Ms. Carey Lando (see below).

A Public Workshop in Fall 2015 provided an opportunity to submit input and learn about the findings of the property research and land surveying.  As with the MV-CM Bike/Ped Study, County staff will maintain a fair and transparent process for providing information to all members of the public via website posting, notices and public meetings. An Informational Meeting on January 18, 2017 will be held to communicated details and answer questions about the upcoming geotechnical and structural investigation phase of the study.  Findings will be presented at a public meeting in mid 2017.

For questions about this study, please contact Ms. Carey Lando, Senior Project Planner, at or (415) 473-5078.