California Department of Transportation: DevilÔÇÖs Slide Tunnels Project

Everything old is new again┬áKeith Regan learns from two senior bridge engineers how the California Department of Transportation is eager to work with contractors to bring new and little-used techniques into the mix to help address environmental and other constraints.  The stretch of State Route 1 that runs between the Northern California communities of Pacifica and Montara about 20 miles south of San Francisco is one of the most breathtaking parts of the famous Pacific Coast Highway, which has been memorialized in song, photographs and films. What makes the roadway beautiful, however, also makes it dangerous, and can at times leave it unusable for long periods of time. In the Pacifica area, Route 1 crosses through the DevilÔÇÖs Slide area, where the sheer steepness of the slope and the local geology leads to seasonal landslides into the sea below. Landslides are common during the annual rainy seasons, often forcing travelers to seek alternate routes. One of the more damaging of such slides occurred in 1995 and shut the roadway down for 158 days, causing damages that cost $3 million to repair. The only existing bypass route, meanwhile, was well inland and added as much as 40 miles of travel distance for a rapidly growing number of area residents driving to the San Francisco metropolitan area. That lengthy 1995 closure helped clear the way for the California Department of Transportation (Caltrans) to secure federal funding for an emergency solution. The answer involves a bypass of the steepest stretch of the highwayÔÇöwhich will be turned over to local communities and maintained as a hiking trail and bikewayÔÇöthat requires both tunneling and bridge work in highly sensitive environmental areas. When completed in 2011, the DevilÔÇÖs Slide Tunnels Project will involve two 4,200-foot-long tunnels beneath San Pedro Mountain, with the tunnels connected back to the highway by a pair of 1,000-foot-long bridges. The bridges, which are substantially complete, span a coastal wetland so environmentally sensitive that regulators prohibited even foot traffic in the area during construction, according to Kevin Harper, Caltrans project engineer and lead designer of the bridges. ÔÇ£The contractors could not erect temporary towers and could not have even foot traffic, let alone equipment, in the valley.ÔÇØ The area is also home to the endangered red-legged frog, Harper adds. Local constituents also wanted a bridge that was more aesthetically pleasing than the standard highway span. After all, the area is known for its graceful and scenic bridges, from the Bixby Creek Arch Bridge to the south to the Golden Gate Bridge a short drive to the north. In addition, the coastal location and saltwater-infused air would mean higher long-term maintenance costs if the bridge were to be made of steel. The solution that addressed all those factors was a cast-in-place concrete balanced cantilever segmental box girder bridge, a technique that Caltrans used as far back as 1974 but that had since not been used until recent times. The agency thought that the competitive bids for construction of the bridges would come from national firms because of the unique challenges the project posed, but instead it found that several local contractorsÔÇöincluding Disney Construction of San Mateo, which won the bidÔÇöwere willing to try their hands at the technique. ÔÇ£They saw it as something that Caltrans will be doing more of and were eager to develop the expertise,ÔÇØ says Harper. The contractor and its segmental construction engineers, Nutt, Redfield & Valentine of Sacramento, worked with Caltrans to modify the original design somewhat to allow for falseworks to be utilized on the approach spans that were not over the most sensitive areas of the valley. Meanwhile, the construction had to address an array of construction challenges, including a sweeping horizontal curve of some 200 feet that the bridge has to make in order to connect the tunnel entrances to the highway. ÔÇ£There were horizontal curves, vertical curves and a super-elevation,ÔÇØ says Moe Amini, Caltrans oversight engineer for the consultant-designed tunnels. In the end, the contractor brought the central span together, working from either side of the valley, within three-quarters of an inch of the targeted alignment. ÔÇ£It was really an impressive feat given all the different challenges,ÔÇØ Amini adds. Now that itÔÇÖs been revived by Caltrans, the segmental approach is quickly gaining favor again. It is also being implemented on the South Fork Eel River Segmental Bridge Projects about 200 miles further northÔÇöa project also known locally as Confusion HillÔÇöwhere the main span reaches more than 260 feet above the river, a height too great for even the most accomplished contractors to build falsework. Some Bay Area bridge replacement work has also utilized the balanced cantilever segmental approach. Precast segments were used on the new east spans of the San Francisco-Oakland Bay Bridge and cast-in-place segments at the Benicia-Martinez Bridge.Meanwhile, at DevilÔÇÖs Slide the focus has turned to the tunneling part of the work, which is also being done through a novel approach. Because of a number of factors, including a wide variation in the density of the rock being tunneled through, the department chose to use the New Austrian Tunneling Method (NATM), which is also known as the Sequential Excavation Method (SEM). Because Caltrans rarely oversees tunneling projectsÔÇöDevilÔÇÖs Slide is the first in 40 yearsÔÇöthe agency also created a Technical Advisory Panel to help guide design and construction. Excavation is now well under way, with some 2,000 feet of the northbound tunnel and 1,600 feet of the southbound tunnel having been dug as of late May 2009. ÔÇ£When we looked at the options and the cost considerations, this was the approach that made the most sense, and we felt that we could develop or find the expertise to help us guide the project,ÔÇØ says Amini. ÔÇ£We look to match the project constraints with the bridge structure type that best fits the site,ÔÇØ says Harper. ÔÇ£Lately weÔÇÖve been able to bring some techniques that havenÔÇÖt been used around here for a while back into use.ÔÇØ ÔÇô Editorial research by Vincent Kielty