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U.S. Department of Transportation U.S. Department of Transportation Icon United States Department of Transportation United States Department of Transportation

Public Roads - January/February 2013

January/February 2013
Issue No:
Vol. 76 No. 4
Publication Number:
Table of Contents

An Eight-Lane, Four-Bore Hole in One

by Doug Hecox

After 2 years of digging, Caltrans is teeing up to complete the newest portal of the Caldecott Tunnel, which promises congestion relief between Alameda and Contra Costa Counties.

The fourth bore of the Caldecott Tunnel between Oakland and Orinda, CA, is shown here under construction. When it opens in late 2013, the 4-year, $402 million project will help reduce congestion in the San Francisco metropolitan area.

For decades, highway construction has been typified by armies of orange-clad workers armed with shovels, jackhammers, hardhats, trucks, and other tools of the trade. But in the hills between Oakland and Orinda, CA, one of the Nation's most labor-intensive highway projects wields a much different kind of tool: a 130-ton (118-metric-ton) roadheader. This piece of tunneling equipment is one of the largest roadheaders ever used in the United States.

The roadheader is helping the California Department of Transpor-tation (Caltrans) expand a main thoroughfare that carries State Route (S.R.) 24 through the hills. When completed in late 2013, the new bore will hold two new lanes of traffic, expanding the existing Caldecott Tunnel from three bores to four and from six lanes to eight.

After nearly 16 months of slowly grinding through tons of rock and soil, the roadheader reached daylight on November 29, 2011. The fourth bore is the first new open passage since the Caldecott Tunnel's third bore opened in 1964, and its rapid, innovative construction methods represent a new milestone in tunneling technology in this country.

The Original Tunnel

Oakland, which is in Contra Costa County on the east side of San Francisco Bay, has long found its economy eclipsed by growth in San Francisco. The Gold Rush of 1849 enriched San Francisco, sending its population -- less than a thousand residents in 1848 -- soaring to more than 34,000 by 1852.

In the 1880s, Oakland mer-chants realized that the city was losing business and residents to San Francisco. To keep its economy afloat, Oakland sought to expand commerce east toward Orinda. However, traveling from Oakland over the Berkeley Hills to Orinda by horse-drawn wagon or stagecoach was difficult at best. A trip by wagon or on horseback from northern Alameda County to the east required travelers to skirt the hills by first heading north to Richmond or Martinez, or south to Hayward or Dublin. The trip was slow, dangerous, and costly.

Oakland officials wanted to build a tunnel through the hills to streamline commerce into and out of the area. In 1903, Alameda County excavated a tunnel through the hills just below the crest near the eastern end of Broadway, the area's primary thoroughfare. The tunnel -- known as both the Kennedy Tunnel and the Broadway Tunnel -- was 1,040 feet (317 meters) long and 17 feet (5.2 meters) in diameter, and timber beams supported the walls.

The original tunnel could barely accommodate two loaded wagons passing each other. Making travel even more difficult, the tunnel was accessible only by steep, narrow, and winding roads that were susceptible to washouts and landslides during inclement weather. Also, because of a miscalculation by its designers, diggers missed connecting the two ends of the tunnel, leaving it with a 4-foot (1-meter) elbow in the middle.

A retired Caltrans engineer described the tunnel as wet and dark. Until a lantern system was installed, travelers used to set wads of newspaper on fire to signal to drivers on the other side to wait until they had passed through.

Expanding Need, Expanding Tunnel

Despite its limitations, the original tunnel served area residents adequately for years. Then, along came the automobile. The onset of increasing automobile and truck traffic eventually rendered the existing geometry inadequate. By 1930, an estimated 30,000 vehicles per day were using the route and the aging tunnel. Plans were already underway for a larger tunnel system. In March 1928, officials in Alameda and Contra Costa Counties met to begin planning a trio of tunnels through the hills. The scarcity of funding during the Depression Era ultimately forced officials to scale back, resulting in construction of one new tunnel with a single bore and two travel lanes.

Beginning in 1934, construction began on a new, larger, more modern tunnel that would offer drivers two lanes in each direction. Despite two mid-construction cave-ins that delayed the tunnel's completion by several months, the Broadway Low Level Tunnel opened to traffic in 1937. It shaved 10 miles (16 kilometers) and an hour's travel time off the prevailing route, greatly cutting time and cost for travelers and area farmers who depended on the route for basic commerce.

The new tunnel served the needs of 1930s motorists seeking fast, convenient transportation on S.R. 24 between rural Contra Costa County and Oakland. With the completion of the San Francisco-Oakland Bay Bridge in 1936, many real estate developers boasted that the new tunnel would enable someone to live in Orinda, or the neighboring communities of Concord and Walnut Creek, and work in San Francisco and have less than an hour's commute.

Excavation of the fourth bore began on the eastern side (Orinda) in August 2010 using a traditional excavator and on the western side (Oakland) in March 2011 using an electricpowered roadheader (shown here).

The pace of the area's population growth quickened in the mid-1940s as soldiers returning from World War II purchased homes, started families, and opened businesses. Through the 1950s, Contra Costa County's population grew by 137 percent. In addition, the populations of Concord and Walnut Creek, once sleepy farming communities, exploded by more than 400 percent during the same period.

By some accounts, the Broadway Low Level Tunnel saved Oakland and preserved the economy of Contra Costa County. In 1960, the tunnel was renamed in honor of Thomas Caldecott, who had been Alameda County supervisor, president of Joint Highway District 13, and one of the tunnel's earliest proponents.

As the region's population continued to grow, the volume of traffic matched its pace, forcing State planners to begin a third bore. Groundbreaking for the third bore began in 1960, about 150 feet (46 meters) north of the second bore. Engineers designed the new bore slightly larger to comply with modern construction and safety standards, and included extra room to accommodate work on a fourth bore, should one be needed in the future. The third bore, which expanded the tunnel to six lanes, opened to traffic in 1964. This expansion made it possible to switch the direction of traffic in the middle bore so that four lanes were always available for the heavier traffic flow direction with two lanes serving the lighter flow.

The Fourth Bore

Continued regional growth prompted work on the Caldecott Fourth Bore Project in 2010. The project is a partnership among the Federal Highway Administration (FHWA), Caltrans, the Metropolitan Transpor-tation Commission, the Contra Costa Transportation Authority, and the Alameda County Transportation Commission. The $402 million project depends on $180 million from the American Recovery and Reinvestment Act of 2009, as well as funding from bay area bridge tolls and a local transportation sales tax passed by Contra Costa voters in 2004.

Given current traffic of more than 160,000 vehicles per day on S.R. 24, the project will relieve congestion and provide motorists with two dedicated tunnels for each direction of traffic. When it opens to traffic in late 2013, the fourth bore will eliminate the need to reverse the direction of traffic twice a day in the middle bore and will dedicate four lanes each to westbound and eastbound traffic. The bore will accommodate two 12-foot (3.7-meter)-wide traffic lanes, with a 10-foot (3-meter)-wide shoulder on the right side and a 2-foot (0.6 meter)-wide shoulder and a 3-foot (0.9-meter)-wide emergency walkway on the left.

Furthermore, the fourth bore will include numerous design improvements, such as cross-passages between the third and fourth bores to act as safety exits for drivers in the case of an emergency in one or the other bore. A waterproof lining will prevent ground water seepage and roadway drainage problems. The new tunnel will be nearly 50 feet (15 meters) wide and about 40 feet (12 meters) high, with a roadway clearance of 16 feet (5 meters) -- design improvements to accommodate today's larger vehicles and enhance airflow through the tunnel.

"This ambitious fourth tunnel allows the State to address longstanding traffic challenges," says Federal Highway Administrator Victor Mendez. "Residents and commuters will spend less time driving, and the project is creating hundreds of good-paying jobs for area workers."

On August 9, 2010, with more than a hundred spectators looking on, Administrator Mendez climbed into the driver's seat of the roadheader and began grinding into the hillside, beginning the tunnel's fourth bore.

The Tunneling Process

Boring a 3,248-foot (990-meter)-long hole through a mountain is no simple matter. In each of the previous tunnels, workers had to dig their way through the mountain using shovels, explosives, and an abundance of manpower, time, and patience. For the fourth bore, the project team called in a more powerful tool to perform the job: roadheaders. These machines each have a long, extendable arm and a barrel-shaped, high-velocity rotating cutting head with hardened steel teeth that grind into the rock.

On August 9, 2010, FHWA Administrator Victor Mendez manned the controls of a roadheader during the ceremonial groundbreaking of drilling for the fourth bore of the Caldecott Tunnel.

After being shipped from Europe, through the Panama Canal, to the Port of Oakland and assembled onsite, the roadheaders performed the jobs of hundreds of early tunnel workers faster and more safely. To do the work as expeditiously as possible, tunneling occurred simultaneously from the Oakland and Orinda sides.

The area's sedimentary geology -- primarily sandstone, siltstone, conglomerate, chert, shale, and mudstone -- challenged progress, but with the assistance of the roadheaders, workers could dig forward using a sequential excavation process in which the length of each excavated section was determined based on the surrounding geology. Hard, unfractured rock, for example, would allow crews to tunnel for about 7 feet (2 meters) before stopping to install support measures. Broken shale, in contrast, would permit excavating only about 2 feet (0.6 meter), at which time the surrounding rock would become too unstable, requiring a pause to reinforce the surrounding walls.

Once a section had been excavated, work crews applied about 8-12 inches (20-30 centimeters) of shotcrete, which is a sprayed-on concrete reinforced with plastic fibers, onto the freshly excavated walls and installed rock dowels (steel rods) around the perimeter of the excavation to provide support. Then they placed an arch-shaped steel lattice to retain the tunnel's shape and, once again, applied a coating of shotcrete. Then they would continue boring the next section. This sequential excavation enabled workers to advance anywhere from 3 to 13 feet (0.9 to 4 meters) per day, depending on the hardness of the rock.

After each round of excavation, workers installed steel rock dowels and sprayed shotcrete (shown here), a sprayable form of concrete, onto the newly excavated section of tunnel to provide structural support.

"Comparing the actual construction rate of the fourth bore to that of prior tunnels, notably the third, can be misleading," says Sarah Skeen, a geotechnical engineer with FHWA's California Division and the project's Federal program oversight manager. "Each bore was different, and relied on different techniques such as excavation support systems. There was wide variation in safety requirements affecting tunnel work, such as gas classification and ventilation, and differing rules about worker safety, like crew-size limits and the number of working-hours per day."

Current safety requirements in effect for the fourth bore included a more extensive ventilation system during tunnel excavation. Other safety features included the practice of installing presupport bracing (the steel rods and lattice described above) along the length of the bore to ensure stability and sealing the rock with shotcrete before each round of excavation to protect workers from exposed rock. Further, dust-control measures required crews to allow time for dust particles in the air to settle before work resumed.

This tunneling method "uses the strength of the surrounding rock to stabilize the tunnel," says Bill Bornman, a construction manager at Caltrans. "By contrast, a hundred years ago, tunnel builders would use enormous timber beams to support the ground."

Over the course of 16 months, work continued 24 hours a day, 5 days a week. Trucks hauled an estimated 238,000 cubic yards (182,000 cubic meters) of excavated material to Treasure Island in San Francisco Bay, where Caltrans is stockpiling it for future use.

To avoid the "elbow" problem that occurred in the Broadway Low Level Tunnel, the fourth bore crews used a combination of new and traditional methods to ensure that the two ends of the tunnel would be precisely aligned when they met. "Lasers projected on the face of the tunnel outlined exactly where the roadheader should excavate," Bornman says. "After each round of excavation, high-tech scanning equipment provided a 3-D profile to indicate precisely how much ground had been excavated. Crews using standard survey equipment would crosscheck the alignment as excavation progressed."

When the two crews broke through in November 2011, the two sides of the tunnel were less than an inch (a couple centimeters) off the target, which is a testament to the precision of this modern tunneling technique.

The Next Chapter

When the fourth bore is completed in late 2013, it, like the Caldecott Tunnel's first two bores, will rank among the Nation's premier examples of highway engineering. After more than 75 years of active use, the tunnel's first two bores have not undergone any major redesign or modernization, and were declared eligible for the National Register of Historic Places in 1998.

"With significant Federal, State, regional, and local support, the Caldecott Fourth Bore Project serves as a prime example of how all levels of government have come together to improve California's infrastructure," says Caltrans Director Malcolm Dougherty.

Relying on 21st-century technology to solve 20th-century traffic problems first envisioned in the 19th century, the Caldecott Tunnel is one of the Nation's most storied engineering achievements. When its fourth bore opens to traffic, the next chapter in its history will begin.

In November 2011, crews operating the roadheader from the eastern side broke through the final chunk of rock to complete the excavation. Shown here is the rotary cutting head emerging from the other side.


Doug Hecox is a spokesperson with FHWA's Office of Public Affairs. He has a journalism degree from the University of Wyoming, teaches journalism and public relations writing at American University, and has authored two books.

For more information, visit or contact Doug Hecox at 202-366-0660 or