By: Zachary Winson, Contributing Author
"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.
