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Caltrans Embarks on $272M Devil’s Slide Project

Wed July 11, 2007 - West Edition
Jennifer Rupp

By Jennifer Rupp


Route 1 on California’s coastline runs between the towns Pacifica and Montara, crossing a steep, unstable geological formation known as the Devil’s Slide region. Rockslides and land slippage are responsible for frequent road closures in this area. The latest slide happened in April 2006. Highway 1 was closed for four months and it cost approximately $9 million to fix.

After years of evaluation and public input, Caltrans propagated a three-part plan to provide a safe, dependable highway between Pacifica and Montara.

Phase 1

The first contract is known as the South Rock Cut Project, which consisted of the re-alignment of Route 1 and a retaining wall.

The wall was constructed by spraying concrete onto steel reinforcing. Shotcrete, as it is called, is applied using a hose and nozzle device similar to a firefighter’s hose. The wet concrete was then sculptured by hand to give the surface a more natural texture. Finally, the concrete was stained to look more like natural rock than concrete.

The retaining wall was constructed after cutting part of the hillside away to create room to re-align Route 1 for entry/exit at the south portal.

Construction began in April 2005 and was complete in June 2006. The prime contractor was Condon Johnson and Associates Inc.

Its winning bid was $6.8 million.

Phase 2

The second phase is the Devil’s Slide Project Bridges, which connect the north portal of each tunnel to Highway 1. Both bridges pass over a valley at Shamrock Ranch that contains a pond and wetlands.

During construction, the wetland area will be fenced off to protect an environmentally sensitive area.

Each bridge has a total length of approximately 1,000 ft. (305 m). Two sets of twin piers on each side of the valley produce main spans approximately 445 ft. (135 m) long.

At the highest point the bridges are 125 ft. (38 m) above the valley floor and are curved to provide a smooth transition from each portal to the existing alignment of Highway 1.

Concrete box sections of the bridge will be built out from each of the four piers. This technique is known as the cast-in-place cantilever method. It avoids building temporary support towers in the environmentally sensitive area of the valley during construction.

During the public input phase of the project, bridge appearance was raised as an important issue. To provide a design that met with public approval and to complement the natural surroundings, particular care was taken over aesthetics. For example, tapered piers and curved struts give a graceful shallow arch look to each bridge. An open design tubular steel pedestrian/bicyclist guard rail will preserve spectacular views from both bridges.

John Cunliffe of the Caltrans Public Affairs office reported that four massive, 2,000 ton (1,800 t) concrete bridge footings were poured on Aug. 28. More than 120 truckloads of concrete filled the 48 ft. long by 40 ft. wide by 15-ft. high (15 by 12 by 4.6 m) forms.

“The whole operation went very smoothly and according to plan,” said Caltrans Construction Manager Ed Der. He commended the contractor, Disney Construction, for a job well done.

The cost of the bridges project is approximately $40 million dollars and it will take 24 months to construct. Work began in April 2006 and is slated to be complete by the middle of 2008. At this time, the bridge construction is running on schedule.

Phase 3

The largest phase of the reconstruction is the Devil’s Slide Tunnels Project, priced at $272 million with a 1,500-day duration. The project calls for construction of two tunnels beneath San Pedro Mountain, each 30-ft. (9 m) wide and 4,200-ft. (1,280 m) long.

Approximately one-quarter mi. south of the tunnel is the site of an Operations and Maintenance facility. An earthen embankment and vegetation-covered roof will help the facility blend with natural surroundings.

Plans, specifications, and estimate for the tunnel contract were completed November 2005. The contract was advertised for bid in June 2006 and bid opening took place in November of 2006.

The contract for the tunnels project was approved on Jan. 3, 2007, awarding the bid to Kiewit Pacific, a large construction company with operations throughout the United States and Canada.

Kiewit Pacific will use two Voest Alpine 120-ton (109 t) “roadheader” machines for part of the tunneling work at Devil’s Slide. This Austrian made monster is approximately 50 ft. (15.2 m) long and costs more than $3 million.

In late March 2007, the first phase of preparation work for the tunnels began at the South portal site with excavation to construct an underground holding tank. The tank will collect water from the seasonal waterfall next to the portal and surface rainwater in the area. A culvert under Highway 1 will channel water into the ocean.

The concrete holding tank has approximate dimensions of 190 ft. long by 16 ft. high by 16 ft. wide (58 by 4.9 by 4.9 m). The tank will hold approximately 260,000 gal. (984,000 L). Following completion of the ’built in place’ tank, a temporary detour is needed to make room to build a 340 ft. (104 m) long retaining wall on the west side of the highway.

Tunnel boring will progress from south to north using excavation techniques that rely on inherent rock strength for support — known as the New Austrian Tunneling Method. Both tunnels will be bored at the same time, one tunnel face approximately 60 yds. (55 m) ahead of the other. Staggering the bores reduces the likelihood of damage from blasting in alternate tunnels.

The work schedule is 24 hours a day, 7 days a week resulting in a 24-month timetable to break through to the north portal.

Throughout the public input and evaluation phases, lasting more than 30 years, concerns about effects on wetlands, wildlife, plants, noise, and visual impacts received highest priority. Among the wildlife, species of concern include the Peregrine falcon, the Mission blue butterfly, the California red-legged frog, and the San Francisco garter snake.

The bypassed section of Route 1, together with 70 acres of state right of way, will be available for public access and recreational use following the planned tunnel opening in late 2010. Caltrans will monitor and improve the state of wetlands, wildlife, and plants until independent resource and regulatory agencies mitigation work is complete and the wetlands are naturally self sustaining. CEG

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