A bridge on the rocks

Nov. 18, 2002

"If that bridge went out, we'd be in a world of hurt," reported Eugene Calvert, director of transportation for Mendocino County. "It provides an absolutely critical transportation link between Mendocino County and Northern California. Without it, emergency services, interstate commerce and the traveling public would be severely limited. And a detour around the bridge would mean, at best, traveling over 30 miles of gravel road."

"If that bridge went out, we'd be in a world of hurt," reported Eugene Calvert, director of transportation for Mendocino County. "It provides an absolutely critical transportation link between Mendocino County and Northern California. Without it, emergency services, interstate commerce and the traveling public would be severely limited. And a detour around the bridge would mean, at best, traveling over 30 miles of gravel road."

Bridging a canyon is one thing. Bridging a steep, V-shaped chasm that is completely enclosed by forest--on the only north-south highway (U.S. 101) for more than 80 miles around--is quite another. And if that weren't enough of a challenge, that gorge sits in earthquake country, so any bridging solution must address serious seismic issues as well. Taking advantage of its "On Call" Phase II --Seismic Retrofit Bridge Engineering contract, the California Department of Transportation (Caltrans) called on DMJM+Harris to examine the bridge over Rock Creek in Mendocino County and devise a seismic retrofit solution. After looking at it, however, DMJM+Harris implemented a completely different approach.

"Beyond the seismic issues--and they were considerable--there were some very serious problems that no retrofit would fix," explained Neil Harris, project manager for DMJM+Harris. "The bridge deck was in pretty poor condition. Even if we repaired it, it still would not support current standard live loads. And because the bridge was a two-girder system, the existing deck could not be replaced while maintaining traffic flow, which was extremely important to Caltrans, the county and the state. Given the fact that there wasn't another road for almost 100 miles, this also was not a site where you could set up a practical detour. But the challenges didn't end there.

"The bedding rock under the bridge's tower footings was fractured. It could easily split if there were an earthquake. If that happened, no conceivable seismic retrofit measures to the bridge would prevent at least partial failure. And it was impossible to ascertain the precise fatigue cycles over the past 50 years for the steel girders on the bridge. So the overall life cycle of the existing structure was suspect. It became very clear to us that this wasn't a matter of retrofit, but replacement."

Calling in the replacement

One doesn't just decide to replace a bridge without hard facts. So the project team developed an approved seismic retrofit strategy that would allow the structure to withstand the maximum credible earthquake (MCE). Demonstrating to Caltrans the best possible solution given the constraints, the project team also pointed out that no retrofit solution would address all of the issues: bridge fatigue, live-load deficiencies, maintaining traffic flow during retrofit and the fact that the cost of the retrofit was estimated to be 60% of the cost of replacement. With those facts in hand, Caltrans elected to replace the bridge.

Several replacement bridge types were considered, from steel girder to precast/prestressed girder to cast-in-place concrete, but the steep canyon walls and fractured bedrock at the site made their conventional foundations problematic. The project team chose a slantleg foundation structure to take advantage of the high foundation-bearing capacities of the site. The slantleg scheme also provided a balanced superstructure span arrangement, along with an aesthetically pleasing and appropriate foundation.

Due to the site's remote location and the curvature of the alignment, steel and precast girder solutions were ruled out immediately. And because of the absolute need to sustain complete traffic operations during construction, it was imperative that staged or phased construction be used for the superstructure and substructure. The team chose two-column bents with a concrete box girder superstructure for the bridge. Since structure depth was not an issue and the span lengths were relatively short, a reinforced concrete box girder superstructure was chosen.

This one's mined

With the appropriate configuration chosen, it was time to focus on its implementation. The steep side slopes of the canyon, along with the near-surface rock, eliminated most foundation options. Unconventional foundation techniques were required to cope with the difficult site conditions. Pier shafts--shafts tunneled deep into the canyon walls at a perpendicular angle-were selected for two reasons. Not only do they provide an elegant method to support the bridge structure, they also serve to stabilize the existing slopes because of the force they exert on the canyon walls. As is often the case, though, solving one problem gave rise to a new one. Unfortunately, the steepness of the slopes and the distances beneath the bridge approaches prevented the use of large equipment to excavate the shafts.

"There just wasn't enough room to bring in heavy equipment," said Harris. "So, we took a page from engineering history and mined the shafts by hand. While that's certainly not a new construction technique--and hand mining is not exactly used every day in today's highly computerized and mechanized construction world--it proved to be the most efficient and effective way to solve the problem. But it's been quite a while since hand mining was used on a project like this."

But the team pushed the edge of the envelope in yet another way. The uniqueness of this project facilitated an unusual technical partnership. The Caltrans foundation engineering team has some of the best rock engineering credentials and experience in the state. With everyone focused on the ultimate goal of project delivery, DMJM+Harris teamed with the Caltrans foundation engineering team on the bridge's design. For this design challenge--which required close collaboration and thorough communication--Caltrans partnered with DMJM+Harris to develop the highest-caliber technical solution possible.

Connecting points

As with everything else on this proj-ect, the seismic aspect of the structure also was quite unusual. Although Caltrans was implementing new seismic design rules throughout the state (based on controlling displacement levels of structures, rather than forces), many of these requirements were not applicable for a structure of this type; this design just didn't fit into any typical category. So the project team proposed unique requirements to be used specifically for the Rock Creek Bridge. With those specifications accepted, the bridge superstructure is now designed to remain elastic (i.e., earthquake resistant) well beyond the anticipated design displacements.

The project team's approach, which successfully overcame technical challenges, also met more ordinary goals. While the intricacies and beauty of the engineering solution may fascinate members of the architecture/engineering industry, what mattered most to the client was the ability to deliver a safe, sturdy, long-lasting, beautiful bridge--without closing down an essential highway.

"Replacing the bridge rather than retrofitting was strategically important as it ensures longevity of access to the road via the bridge, said Calvert.  "But while that covers the practical aspect of the project, it doesn't address the intangible--yet equally important-aspects of the structure. The other benefit of this bridge is that it's a beautiful bridge. It's architecturally pleasing. Clearly, they did a great job when they designed it. Given the unspoiled surroundings of the bridge, that's pretty important to us."

By all accounts, the team that delivered Rock Creek met those expectations in full. The bridge was designed in approximately six months, on time and on budget. And whether it's the necessity to maintain traffic flow, address the issue of unstable rock formations, create an aesthetically appropriate design or overcome the unusual camber issues inherent in slantleg design, the Rock Creek Bridge was definitely a challenge. But in addition to all of the difficulties described, this project also faced environmental constraints.

Since the project site is close to the Standish Hickey State Recreation Area, there was great concern for the water quality of Rock Creek during construction. Significant limitations were placed on when specific construction activities could take place. In turn, this had a major impact on structural design. Because the structure was to remain in its Stage I design for about a year, Caltrans also required that the partially completed structure meet the same seismic design criteria as the completed structure. To the project team, each such constraint was another piece of the puzzle they had to put together; every environmental and seismic issue was carefully and fully addressed.

Bridging a canyon is never easy. But bridging Rock Creek in Mendocino County was an unprecedented challenge. This achievements has not gone unnoticed. In recognition of the technical achievements of this project, Caltrans and DMJM+Harris received the Consulting Engineers and Land Surveyors of California Honor Award for 2001.  They proved that the shortest distance between these two points was a lifeline.

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