The northbound Devil’s Slide Tunnel is 4,312 feet in length; the southbound tunnel is 4,223 feet long. The tunnels are equipped with 32 jet fans (eight pairs per tunnel), 1,197 high-pressure sodium lamps, 42 safety niches, and 10 emergency cross over passageways.
Photo: Michal Cialowicz/

The Devil’s Slide Tunnel construction story is one of danger, activism, and extreme engineering. The tale is about the Pacific Coast Highway, one of the world’s stunning scenic highways, being improved by the all-American political process of exhausting alternatives until a course of action is chosen. Behold, a most beautiful tunnel: a subterranean structure that preserves a coastline that’s a national treasure while emboldening economies and saving lives. It’s also the first tunnel built in California in nearly half a century.

Devil’s Slide Tunnel, San Mateo County, Calif.

Kiewit Corporation

Project focus
New, twin, 4,300-foot-long tunnels through variable geology bypass a winding, unstable section of California’s Pacific Coast Highway.

There’s a Devil’s Slide in Montana, one in Utah, and another in Hawaii. They’re severely geologically unstable mountainsides prone to deadly landslides. California’s Devil’s Slide loomed over a busy 1.2-mile section of the famous Pacific Coast Highway, also known as California’s State Route 1. The winding road has been seducing thrill seekers and tourists for 76 years. So notorious were the road’s hairpin turns that in the 1960s Hollywood producers staged a famous car crash scene over its cliffs – art imitating life.

Work on the Devil’s Slide section of California’s Route 1 began in 1935 when the old Pedro Mountain Road, which wound through coastal ranch lands, was improved. Three years after opening the Devil’s Slide section, Route 1 washed out. Rockslides and landslides from the onset made for an endless cycle of road repairs. Route 1 at Devil’s Slide was closed on eight occasions during its lifetime.

In 1995, the worst damage was suffered when a severe weather event destroyed a 160-foot section of Route 1. The failure created gashes several feet wide along the roadway and a severe 5-foot drop in elevation between one section and another. The highway was closed for 158 days, the longest closure on this deadly section of Route 1.

Within a month of the January storm a tunnel was pondered, proposed, then eventually fought for. Locals to the south of the slide, marooned by the closures, saw a tunnel as the best way to preserve the beauty of their coast while ensuring connectivity to San Francisco less than 10 miles up the coast.

As California’s highway department spent $3 million bolting the road into the cliff, locals were forced to take the long way to San Francisco. Instead of a 7-mile trip, commuters, tourists, and delivery services were rerouted over 45 miles of highways and toll bridges. Drives that took 10 minutes before the washout turned into hour-and-a-half journeys. Despite the multi-million-dollar fix, boulders, mud, sand, and gravel continued to slide into the roadway, creating life and death hazards for motorists and bicyclists.

Caltrans may be the most dynamic road building agency in the world. But, when it came to building this tunnel, it had to go back to tunneling school. The new Devil’s Slide Tunnel is the first highway tunnel built in California since the third bore of the Caldecott Tunnel system was excavated in 1964.

Twin segmental concrete bridges, built as part of the $439 million project, span Shamrock Valley at the northern portals of the Devil’s Slide Tunnels. Each bridge stands on two bridge piers and is approximately 1,000 feet long.
Photo: Michal Cialowicz/

The people’s tunnel
"Today we mark the start of a new chapter for both Caltrans and the local communities," declared Malcolm Dougherty, Caltrans director, on March 25, 2013, the opening day of the Devil’s Slide Tunnel. "No longer will local residents and businesses have to worry about severe winter storms closing the road." He also could declare with locals that Caltrans had gone from foe to friend.

A carnival-like feel permeated the tunnel on its opening day. Inside, citizens celebrated the birth of their handsome twin bores while walking about just hours before the old Route 1 section of Devil’s Slide was closed and the rerouted traffic was due to begin using the new superstructure. Old and young appeared on foot, bicycles, and driving classic cars.

National Public Radio’s local affiliate, KQED, reported from the tunnel’s grand opening, "Environmentalists who had agitated for the tunnels instead of a highway bypass are known as ‘tunnelistas.’" Among them was Ann Forrister, who was there under an alias: "My name is Captain Tunnel, and I am wearing red tights, black shorts, a red cape, tunnel vision glasses, a red hat. I’m celebrating the tunnel." Ann smiled when she explained how schemes to promote a tunnel were concocted at Tunnelista strategy meetings over copious bottles of California wine.

The alternative to a tunnel was a supposedly less expensive but more destructive freeway bypass. The bypass would’ve had the feel of a classic Bay Area superhighway rolling through the pristine coastline. So unacceptable was the "improvement" amongst locals that the bypass plan galvanized the opposition beyond the Tunnelistas.

KQED reported, "Even though Caltrans claimed the new alignment would also be striped for two lanes, it would be paved to accommodate four lanes. The steepness of the road required as many as 11 cuts and fills up to 106 feet wide and 275 feet high. ‘I was appalled when I saw this,’ said [Bill] Bechtell, a retired geotechnical and civil engineer and member of the Sierra Club who opposed the freeway bypass. ‘I live in Montara and it would have destroyed our community, with the freeway coming through a state park and adjacent open space. You would have the noise from truck traffic and it was not something I wanted to see happen.’" (Read the KQED report at

Weeks after the ’95 failure, Doug Hamilton, a consulting geologist living in nearby Atherton, Calif., became the rare local who put scientific reasoning behind the tunnel concept. At first, Hamilton hadn’t an official connection to the project. However, months after the storm’s destruction he was retained to do a feasibility study on a tunnel with Woodward Clyde Consulting, a geotechnical firm since acquired by URS, and Parsons Brinckerhoff.

"Clear from the onset was that a tunnel was going to be less impactful on the landscape than the proposed bypass," recalled Hamilton, who later worked on a subcontracting team for HNTB, the tunnel’s designer of record. "And our report concluded that a tunnel wasn’t cost prohibitive as Caltrans was claiming."

Caltrans Director Malcolm Dougherty (center) walks with other officials through one of the Devil’s Slide Tunnels on its opening day, March 25, 2013.
Photo: Michal Cialowicz/

Caltrans sponsored the inland bypass with its sharp turns, massive cuts, and filled embankments. The massive road building agency had the political and financial power. But the people began an uprising. Early on, the Sierra Club was strongly opposed, so too was the Committee for Green Foot Hills representing the Bay Area and Palo Alto. Local residents in Half Moon Bay and Montara, the ones marooned for months when the road failed, were most vociferous. With the road still in shambles, locals added to the momentum created by Zoe Tucker, another local who organized serious political support by creating "Measure T," a political call to action demanding a tunnel be built. Passage of the measure – overwhelmingly – in 1996 cleared the way for additional federal funding.

Geology, geology, geology
"Devil’s Slide shows up on maps going back to the late 19th century," explained Hamilton, now in his 80s. His groomed beard, stiff posture, and love of Baroque music belie his wild Western ways. In 2001, while working on behalf of Caltrans gathering samples of rock near the tunnel’s southern portal, Hamilton parked his Chevy S-10 pick-up at the edge of a cliff, tied a climbing line to its rear bumper and, with his trusty rock hammer, rappelled over jagged rocks with “a not very adequate rope assembly.” Never mind that his permit required him to have three other climbers spotting him. Hamilton admits to ignoring the clearly posted no climbing signs. The San Mateo County Sheriff and numerous deputies awaiting his return to the top of the cliff were not amused.

Hamilton discovered that the Devil’s Slide’s formation consists of a granite rock similar to the granite found in the Sierra Nevadas and Yosemite except that the inland granite is much harder as a result of glacial activity. Hamilton explained, "Devil’s Slide granite looks like the other granites but it’s soft and breaks apart without much effort. It’s been altered by hydrothermal alteration which makes it weak as if it were made of clay."

Hamilton’s geological adventures over the Devil’s Slide cliffs also produced archeological discoveries. “At the very bottom of the cliffs there are remnants of the Ocean Shore Railroad which traversed the face of the landslide back in the early 1900s.” Along the way to the bottom Hamilton also passed the remains of fatal car wrecks. “There are Model-T vintage autos alongside some plastic remains of newer cars like Toyotas.”

Kiewit Corporation received Notice To Proceed in February 2007. Less than six years later the work was done. "We finished the heavy lifting in October of 2012," said Bob Stier, Kiewit’s chief for the underground. "Understandably, Caltrans wanted to check and recheck their vital communication lines running between the tunnel and an offsite Operations Maintenance Center that controls the tunnel’s jet fans, cameras, and fire protection systems."

Stier explained that the key to completing the tunnel project successfully at Devil’s Slide was understanding the tunnel’s surrounding geology. In fact, according to the tunneling chief, it’s the only way any tunnel project is going to succeed whether it is a deepwater, transit, or highway tunnel. In short, the rule of tunneling seems to be geology, geology, geology.

"Knowing the geological conditions as well as we did allowed us to sustain a steady rate of excavation. Superior knowledge of geological conditions is the only way to keep to schedule on a tunnel project," Stier said. "Geology dictates which gear is procured – where and when we use the heavy equipment underground." Stier’s been with Kiewit for six years and has been tunneling for more than a quarter of a century. Currently he’s directing the construction giant’s work in New York City including the 2nd Avenue subway tunnel project.

"We never sacrifice safety for speed," said the cautious Stier. "Confined areas, big gear, [and] many miners working in tight quarters demand caution while excavating."

California’s Pacific Coast Highway’s Devil’s Slide section opened in 1937. The winding stretch of dangerous road was closed on eight occasions, including in 1995 when a landslide wiped out the roadway for 158 days.
Photo: Michal Cialowicz/

The confines of a tunnel heightens the danger of the simplest action. That reality makes the decision to explore new tunneling techniques even more remarkable. "We built this tunnel using an assembly line philosophy. Various stages of excavation were performed simultaneously. Drill-and-shoot operations were conducted in a bore as multiple digger arms were used in another – first in the United States," explained an understandably proud Steir.

Kiewit’s challenge was the three faults they had to dig through to excavate the northbound and southbound highway tunnels, each approximately 4,300 feet long. Tunneling on both bores began at the southern end of the project and moved northward. The northbound highway tunnel remained a couple of months ahead of the southbound highway tunnel. Fault A consisted of weathered quartz diorite; fault B consisted mostly of sandstone conglomerate, the hardest of the stone to be encountered. Fault C was mostly interbedded siltstone, sandstone, and clay stone, the weakest rock in the tunnel.

The Devil’s Slide Tunnel was a showcase for the New Austrian Tunneling Method (NATM), also called Sequential Excavation Method (SEM). These two methods call for different types of excavation support structures inside the tunnel depending on the ground conditions crews are moving through. Weak stone conditions required elaborate support systems. Stronger rock required less intensive supports. NATM allows for as much natural support as possible, thus minimizing unnecessary use of forms, shields, and grout. In total, Kiewit employed five tunneling designs to accommodate varying types of rock.

The biggest challenge to the union laborers and operators working in the tunnel was the varying rock found in the face of the tunnel’s excavation. "My Crews were never able to get into a good rhythm," said a tough miner named Ron Myers, a special international representative for the Laborer International Union of North America (LIUNA). "One day we’re using a road-header and the next day we’re back to drilling and shooting. The only gear we used consistently were the muck trucks." At the peak of construction Kiewit employed 300 craft and staff in the tunnels – most were union operators or Myers’s miners.

“We’re miners,” said a blunt Myers. “A lot of workers show up at a mining job and say, ‘I’m a miner.’ They’re not a miner. Maybe they worked on an excavator pushing dirt, but that doesn’t make them a miner. Moving dirt is entirely different than digging deep into the earth.”

“Kiewit realizes one very important thing. If you’ve got experienced shifters and walkers, you can drive some tunnel. Kiewit takes stock in that,” said Myers, who is referring to his foreman when he talks of walkers and shifters. “Kiewit realized that the tunnel training school produces qualified miners. Deadbeats getting hurt on the job drives up your comp insurance. The laborers — the majority of those working underground — take a lot of pride in keeping the job safe.

“Kiewit donated equipment and sent a guy to check the entire electrical system underground. That saved us a lot of money,” said an appreciative Myers of his deep subterranean campus. “Since then we’ve broken in quite a few new hands. This training is vital. You have no second chances in a mine. It’s dangerous down there.”

Myers speaks from experience having worked in mining shafts deeper than the Devil’s Slide Tunnel is long. Dedicated to his craft, he asked for and got Kiewit’s help improving the Tunnel Training School, an abandoned copper mine in Arizona that’s now a proving ground for those aspiring to work underground for a living. “Contractors saw the benefit. They’d ask, ‘Did you go to the Tunnel Training School?’ If they answer yes, they’re told, ‘You’re hired.’”

Dawn Dobson, Kiewit’s Project Manager on the tunnel job said for all the hard rock her teams dug through her key takeaway was, "On any project success depends on honest communications. The team met daily to agree on the current construction approach that was faced during the upcoming 24-hours of operations. Communication at the field level has been the key ingredient to the success of this project. Integrated communication between Caltrans and the contractor, field personnel, and construction management were critical as operations required constant excavation changes as new ground conditions were encountered."

Perhaps the extraordinary community forged in digging this tunnel was what charged the tunnel’s opening day climate with good energy. Community, Caltrans, contractor, and labor came together to build a new national landmark.

View additional photos below. Visit Caltrans’ Devil’s Slide Tunnel Project website at

Dan McNichol, Ph.D., is author of "The Roads That Built America." He can be contacted at All photos (copyrighted) by Michal Cialowicz of


Twin segmental concrete bridges, built as part of the $439 million project, span Shamrock Valley at the northern portals of the Devil’s Slide Tunnels. Each bridge stands on two bridge piers and is approximately 1,000 feet long.
Photo: Michal Cialowicz/


Captain Tunnel (aka Ann Forrister) walks with author Dan McNichol throught the Devil’s Slide Tunnel on opening day.
Photo: Michal Cialowicz/


Local consulting geologist Doug Hamilton discusses tunnel construction with author Dan McNichol.
Photo: Michal Cialowicz/


Cross section through the Devil’s Slide Tunnel shows the complex geology. Click here for an enlarged image.
Photo: Michal Cialowicz/


Caltrans’ proposed route for the Devil’s Slide Bypass, as an alternative to the tunnel. Click here for enlarged image.
Photo: Michal Cialowicz/