QUEST on KQED Public Media.
The project consists of twin 30-foot wide tunnels. Upon completion of the project, Devil’s Slide will be the longest road tunnel in California at 4,342 feet northbound and 4,585 feet southbound. At the northern end, a 1,000-foot bridge will connect the tunnel to the valley at Shamrock Ranch.
Construction crews have experienced several setbacks. Crews excavated 10 types of rock that required different lining designs; water poured through the rock when crews were working, so they later had to “drain the mountain.” Creating enough ventilation was also a concern.
Over the last three years the crews excavated the rock, removing it to a nearby site. Then they installed initial support elements tailored to the type of rock in different regions of the tunnel. The crews then installed several layers of lining.
Initially, Caltrans had proposed building a bypass through the region in the late 1950s. But public opposition forced the bypass closed. In the 1970s, a Sierra Club lawsuit brought any such plans to a halt as environmental and local groups battled with Caltrans over the bypass through the 1970s and 80s.
In 1995, a large landslide closed the road for 158 days and cost almost $3 million to repair. Later that year the Federal Highway Administration ordered Caltrans to re-evaluate building a tunnel.
San Mateo County voters approved Measure T by 76 percent the next year, changing the county’s stated preference from the Martini Creek bypass to the construction of a tunnel. Caltrans broke initial ground on May 6, 2005.
After the tunnel is constructed, the old highway is set to be converted into a trail for hikers and bicyclists.
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Caltrans punched through the northern end of the tunnel on October 1st, 2010.