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ALTO TUNNEL STUDY

NTPP Project #1001

Last updated: October 10, 2017

The Alto Tunnel Study looked at possible cost options for the tunnel's future:

  • Reconstruct and open as a multi-use pathway
  • Pursue further closure of the tunnel by filling it in to improve future stability
  • Leave the tunnel as is (no cost)

The study collected information requested by policy makers and the community to help make an informed decision about whether to reconstruct the tunnel for potential future bicycle and pedestrian use or if there is a need to fill and permanently close it at some point. The updated 2017 cost estimates are for Segment 8 of the 2010 Mill Valley-Corte Madera Corridor Study only.

Cost estimates for the other Segments of the Alto Tunnel route were not updated as part of this effort because no new information or evaluations of those segments were conducted as part of this scope of effort.  Costs to develop pathway approaches and other modifications can be reviewed in the 2010 Mill Valley-Corte Madera Corridor Study. At this time, no decision has been made as to the tunnel's future. 

NEW!! Draft Alto Tunnel Investigation and Cost Estimate Update - 2017

Released on September 12, 2017, review the draft report on the 2017 investigation of the interior reaches of the Alto Tunnel and the revised cost estimates for options for reconstruction as a multi-use pathway or permanent closure. The updated cost estimates that were developed as part of the Alto Tunnel Investigation cover portal-to-portal; no pathway approaches or other improvements outside the tunnel are included in the 2017 cost estimate update.  This 2017 estimate update includes only the Alto Tunnel (Segment 8) of the full Alto Tunnel corridor route further discussed in the 2010 Mill Valley-Corte Madera Corridor Study on Page 22-55. Costs to develop pathway approaches are discussed and cost estimated in the 2010 Mill Valley-Corte Madera Corridor Study.

Released 9/12/17: Draft Alto Tunnel Investigation and Cost Estimate Update - 2017 (7MB pdf. 9.14.17 corrected total estimate amount on p. 21 to be consistent w detailed project cost on p. 20)

News Release. September 12, 2017

Interpreted Tunnel Conditions for Cost Estimating Process:

(Click image below for full size pdf download)

Alto Tunnel Conditions

Presentation on September 27, 2017 6:30-8:30p at the Corte Madera Community Center

County staff and the consultant team presented to over 170 people attending what we found inside the Alto Tunnel and the updated cost estimates for both a reconstructed Alto Tunnel with a multi-use pathway and a fill-and-closure option. Photos and scans of the interior of the tunnel were shown and staff and engineers responded to 70+ questions and comments from community members about the investigation and cost estimate updates.  View the presentation and the video from the meeting. A summary of the questions and responses from community members will be posted soon.

The updated 2017 cost estimates are for Segment 8 of the 2010 Mill Valley-Corte Madera Corridor Study only. Cost estimates for the other Segments of the Alto Tunnel route were not updated as part of this effort because no new information or evaluations of those segments were conducted as part of this scope of effort.  You can review the costs to develop pathway approaches and other improvements in the 2010 Mill Valley-Corte Madera Corridor Study.

Purpose of the September 27, 2017 meeting:

  • Learn about how the investigation was conducted
  • Learn about the updated cost estimates for optional tunnel reconstruction and closure
  • Provide focused comments, input and questions on the updated cost estimates

Community members had a total of 4 weeks of review (September 12-October 11, 2017), which included 2.5 weeks to review the draft study before the public meeting on September 27th, and 1.5 additional weeks after the public meeting until the public comment period closed.

Next Steps

  • Comments and input from meeting and in writing will be considered for revisions to the draft study document. Final document to be released in late October/early November 2017.
  • Neighbors, community leaders, elected officials continue community conversations about the possible future of the Alto Tunnel
  • Use the collective information to help guide and inform decisions, if any, about the future of the Alto Tunnel 

NOTE: This meeting is NOT about whether or not to move forward with construction of a pathway or to fill the remaining tunnel voids.  It is an informational meeting about the existing tunnel condition, and cost estimate updates for pathway or closure options for informational purposes only. No decisions have been made, nor will be made at this meeting. This is an informational meeting to present information for future consideration.

   Underhill Borehole Site             Chapman Borehole Site Underhill Site Drilling Location  Chapman Site Drilling Location

Though SMART is not planned to be extended through the Alto Tunnel, CalPark tunnel, a shared multi-use pathway (current) and commuter train (future) tunnel, is a recent and similar example here in Marin. Curious how the CalPark Tunnel between San Rafael and Larkspur was constructed and funded? Read more on the CalPark Fact Sheet.

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.

On September 27, 2017, an informational meeting was held at the Corte Madera Community Center to present the findings of the Alto Tunnel Investigative Study and updated Cost Estimates for the reconstruction of the Alto Tunnel as a multi-use pathway or to fill and close. Over 170 members of the community attended. View the presentation , watch the video of the meeting, and read the summary of questions and responses from the meeting (coming soon).  Community members had a total of 4 weeks of review (September 12-October 11, 2017), which included 2.5 weeks to review the draft study before the public meeting on September 27th, and 1.5 additional weeks after the public meeting until the public comment period closed.

Purpose   

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 began Monday, January 30, 2017 and was completed in April 2017. The team worked at the Underhill site from 7a to 5p for approximately four weeks (with several storm delays) and the Chapman site for approximately 3-4 weeks. All work was conducted Monday through Friday; no weekends.  Additional outreach and courtesy notices were provided through a variety of information venues at least 2 days before work began. 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 presented findings and updated cost estimates to the public on September 27, 2017. 

Traffic

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

Road closures 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 was in coordination with the Town and emergency response officials.  Notification to residents and the community was given in advance to alert travelers of the change and to communicate alternative routes to Chapman Dr. residences, which was from the north during this period. See Map below; click for enlargement.

Chapman Dr Access Map during Geotechnical Investigation

Upcoming Efforts

The Alto Tunnel Study is wrapping up with the completion of the the geotechnical and structural analysis.  Comments on the draft Alto Tunnel Investigation and Cost Estimate Updates will be accepted until October 11, 2017.  The revised document will be finalized and posted in late October/November 2017. An email will be sent to the interested parties list when it is available.

Should any further County-sponsored meetings or actions on the Alto Tunnel be taken, notification will be provided to the community. 

Background

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 clando@marincounty.org or (415) 473-5078.

 

 
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