The painstaking work performed on the Devil’s Slide Bridges in Pacifica, Calif., involved building two parallel bridges over an environmentally sensitive area where temporary towers, falsework, equipment and even foot traffic was prohibited.

The project involved construction of large-span, cast-in-place concrete bridges connecting a highway across the valley into 4,000-ft-long tunnels that pass through the mountain over a coastal wetland, which is a habitat for the endangered red-legged frog.

Devil’s Slide is a big landslide that goes thousands of feet above the roadway. Previously, there were no bridges at the location, and the road ran along the coast.

“What we’re doing is bypassing the road being on the cliffs of the ocean,” Kevin Harper, Caltrans senior bridge engineer, told Roads & Bridges. “This will make for a more reliable roadway. Landslides have been occurring for years, and the road had long closures. Lots of commuters get landlocked, which adds hours to their commute.”

There was an approximately 445-ft main span over the area that could not be touched during construction. The horizontal curve has a pretty tight radius, coming across and then rolling out into a tangent coming into the tunnels, and a cast-in-place segmental was basically the only option that could handle the constraints.

“The main challenge was building over that large span without putting any equipment or personnel down in the valley below,” Harper said. “Plus, it was on a horizontal curve, and the aesthetics created a structure where every surface is covered. Aesthetics and environmental constraints were very challenging.”

The curve required the cantilevers to be built with three-dimensional cambers to assure that the segmental cantilever tips would meet over the valley.

An aesthetics committee was formed, comprising local politicians and activists in the community.

“They wanted something beyond ‘normal,’” Harper said. “They didn’t want a typical Caltrans concrete bridge. They wanted something beyond that. We could not do arch bridges because of the horizontal curvature, but the bridges sort of try to mimic an arch.”

The aesthetics of every aspect of the bridges are unique and complicated the construction. Near the piers, the variable-depth superstructure vertically bifurcates into two distinct elements with an upper-deck-level box girder and a lower box girder strut element that gives the bridges a graceful shallow arch look.

The bridges will be open to traffic in 2011.