Inside the Devil's Slide tunnel project

March 5, 2008 7:22:37 PM PST
We're getting our first look inside the Devil's Slide tunnel project on Highway 1 along the San Mateo County coastline. Tunneling began in November and won't be done for another three years, which can't come fast enough for commuters.

There are two side-by-side tunnels. The northbound tunnel has been excavated to about 181 meters and the southbound tunnel to about 90 meters. Work goes on inside the tunnels 24 hours a day on a state of the art project that requires more than a thousand pages of plans.

Caltrans engineer, Skip Sowko, is the project manager. He says tunneling requires constant adaptation to deal with the different layers of rock along the way.

"You deploy what you need at that specific point to take care of the encountered rock. You're not like just designing in the beginning and sticking with it. You're adapting as you go," says Sowko.

The softer rock is excavated with a carbide-tipped piece of equipment called a road-header.

"It's got a big gnarly ball on the end and it basically grinds away at the rock," says Sowko.

The harder stuff is blasted out, then engineers have to decide what kind of supports to use to hold up the tunnel walls as work continues forward. There are many different choices from bolts to girders.

"So every one meter, 1.2 meters, they're making that decision, 'what do we do?' and they got to make that within a few minutes. So they make that decision and they move forward," says Sowko.

After excavation and support, a lightweight concrete is sprayed on. That's true of the tunnel face as well, in between excavations. Sokow says the tunnels seep five gallons of water per minute, but that's normal. Some of the water inside is brought in to cool the carbide tips. Giant 300-horsepower fans ventilate the tunnel air and laser surveying equipment makes digital contour and surface maps every step of the way.

Commuters and area business owners are keeping a close eye on what they can see from the outside.

"It's a dream come true because I used to have to commute around it when Devil's Slide was closed in the past and it was torture," says Judy Macias of Half Moon Bay.

Eventually, the tunnels will connect with the new bridge on the south side of the hill, and you should be able to drive the new roads in early 2011.

You can see a compilation of raw video inside the Devil's Slide tunnel project in the video player above.