Major progress on Devil's Slide tunnel

Even on the most idyllic sunny day on the San Mateo County coast, it's chilly, dark, dusty, muddy and noisy deep inside San Pedro Mountain, where construction crews are digging twin tunnels to carry traffic around Devil's Slide.

But despite the gloomy atmosphere, workers are making major progress on the $325 million tunnel project, which includes a pair of arched bridges and an operations center. The buildings and the bridges are finished, and the tunnel diggers are expected to bust through the north end of the mountain by this fall. A little more than a year later, the finished tunnel should open to traffic.

No one will be happier than residents of the coastal communities who depend on that scenic but unstable stretch of Highway 1.

The Devil's Slide tunnel will be California's first highway tunnel built since 1964, when the third bore of the East Bay's Caldecott Tunnel opened. Tunneling is dirty and dangerous work, and progress is slow. Workers wear not only the usual hardhats, reflective vests and other construction safety gear, they also carry flashlights or headlamps, respirators and emergency "self rescue kits" that provide oxygen in the event of an accident like a gas leak, collapse, fire or power failure.

Using heavy equipment that looks like something out of a science fiction film, crews working northward are slowly but steadily chipping away at the innards of the mountain, which is composed of a variety of soils from hard shale to loose dirt and rocks.

"Sometimes it goes slower, and sometimes it goes faster," said Ivan Ramirez, the Caltrans engineer overseeing the excavation work. "But a good average is about 10 feet per day."

The diggers

Each of the roughly 4,200-foot bores - 30 feet wide and 24 feet high - is about 90 percent of the way through the mountain. The contractor, Kiewit Pacific, is using the New Austrian Tunneling Method, which involves analyzing the kind of soil that's being excavated and using different types of machinery to get through it.

The main diggers are the road header, which features a long arm with a pair of 2 1/2-foot-wide spinning wheels - covered with carbide-tipped spikes - and an excavator workers refer to as the T. rex.

The road header is used on harder soils and rocks. It bites into the earth and gnaws it into smaller pieces. A series of rotating metal plates moves what's left onto a conveyor belt that lifts the muck into a truck that carries it to a disposal site on the side of the mountain, just outside the south portal. The smaller T. rex works similarly, but on softer soils.

Once the hole is extended, workers secure it by installing arched metal ribs and coat the tunnel walls with sprayed concrete, applied by a remote-controlled robot. The thickness varies from 4 to 14 inches, depending on the stability of the soil, Ramirez said. Next, a bright yellow sheet of plastic waterproofing material is installed to protect the tunnel.

"The No. 1 enemy of tunnels is water," Ramirez said, "so we try to keep it out."

Two layers of reinforcing steel bars come next, and then the whole thing is smothered in a thicker, smoother layer of concrete, using an overhead gantry that straddles the tunnels and slides on rails. Although it's known as the final coat, enamel panels and a brighter paint job will be applied before the tunnel is opened to traffic.

One bore each way

When the tunnels open, there will be a northbound bore and a southbound bore, each with one lane of traffic plus an 8-foot shoulder. It will be far less scenic than the current stretch of Highway 1, perched on a rocky promontory high above the Pacific Ocean, but much safer - and far more reliable.

In January 1995, Devil's Slide washed out after a series of storms. It took five months to repair the road, with the ensuing traffic problems cutting tourism to the San Mateo County coast by 50 percent. In 2006, heavy rains again caused the slide to slip and sent rocks the size of small cars tumbling onto the highway. The road was closed for four months while crews stitched it back together using large steel cables and rods in a $7 million emergency repair job.

Lots of delays

Construction of the bypass began in May 2004, ending 30 years of bickering and bureaucratic delays. Excavation of the tunnel started in fall of 2007, and crews have been working around the clock, seven days a week, to finish their holes through the mountain.

"It's going to be exciting to see the tunnel finally break through and tie in to the bridge," said Tom Grey, a senior engineer with Caltrans.

E-mail Michael Cabanatuan at mcabanatuan@sfchronicle.com.

This article appeared on page A - 1 of the San Francisco Chronicle