San Rafael, CA – On January 30, the Marin County Department of Public Works (DPW) will begin an investigative study of the 132-year-old Alto Tunnel, an old railroad passage from Mill Valley to Corte Madera that has been sealed for more than three decades.
The north entrance to the long-closed Alto Tunnel in Corte Madera.This follows a 2015 study that assessed the tunnel’s right-of-way. The tunnel would be an alternative and more direct route between central Marin to southern Marin than what is currently available for non-motorized traffic. A public meeting to answer questions about the upcoming field work will be held at 6:30 p.m. January 18 at the Corte Madera Community Center, 498 Tamalpais Drive.
The $558,620 study will assess the structural condition of the 2,200-foot tunnel, which was open from 1884 to 1971 and closed because of a series of internal collapses and subsequent engineered stabilizations. The investigative project would be the first look inside the tunnel in more than 35 years.
The results of the study are required to identify the scope of improvements necessary to reopen the tunnel. The process will help update the construction cost estimate of $40 million to $60 million, giving a more current number to take into account.
Without the Alto Tunnel, bicyclists and pedestrians traveling between Corte Madera and Mill Valley have the option of the Camino Alto/Corte Madera Avenue roadway or the Highway 101 “Horse Hill” bike path. The steepness of those routes, coupled with the presence of motorized vehicles, creates a challenge for many people. For that population, the option of having a relatively flat, non-motorized route between the two communities is safer and more attractive.
“Being able to actually see inside the tunnel and assess the geotechnical issues is an important step,” said Bob Goralka, DPW Principal Civil Engineer. “Having a tangible understanding of what’s going on inside will help guide future decisions.”
Work from two sites above the tunnel will allow engineers to study the interior sections of the tunnel, one site at 35 Underhill Road and the other at the intersection of Chapman Drive and Camino Alto. The project will take about three weeks at the Underhill site and five weeks at the Chapman site. Funding comes from the Non-Motorized Transportation Pilot Program, a federally-funded initiative enacted in 2005 for projects involving infrastructure for bicycling and walking.
The field work will require the drilling of five holes, each four inches in diameter, at the two sites and laser scanning probes lowered into the tunnel. Using advanced mapping technology will avoid the higher cost and impact of cutting a portal into the tunnel and physically entering it. The laser scanning course of action is the least impactful to the surrounding community and only minor traffic disturbances are expected.
Up-to-date information about the investigation will be posted on the study’s website.