were destroyed. The creek bed
dropped another 10 to 15 feet
in some areas. At the Academy
Boulevard bridge, storms in 2008
damaged a previously installed drop
structure, which is a man-made
structure that controls the energy and
velocity of water that passes through
it to a lower elevation.
To make matters worse, the
continuing degradation of the
channel downstream of the Academy
Boulevard bridge threatened the
bridge’s stability, Jacobsen said. The
bridge piers were built on footings
in a sandy bed, and a significant
flood could have collapsed the entire
bridge. Therefore, an additional
challenge was to find a way to
protect the bridge from scour, or
erosion at the bridge foundation,
during floods.
Walter Pennington, a water
resources engineer and manager
of Ayres Associates’ Denver-area
office, wrote a memo to Jacobsen
in February 2009 explaining the
situation. “Future local pier scour
will likely undermine (the Academy
Boulevard) bridge footings, resulting
in an unstable foundation and/or
bridge failure,” Pennington wrote.
“It is my professional opinion that
the channel stability and the bridge
conditions … be addressed
immediately.”
Ayres Associates had
previously worked with the City
to develop a master plan for Sand
Creek and identify critical locations
for improvements, which included
Academy Boulevard. So when
problems occurred at that location,
the City retained Ayres Associates
again to design a grade control
structure across Sand Creek
downstream of Academy Boulevard
and provide other engineering
services for the improvements.
Specific to protecting the bridge
piers, Ayres Associates designed
riprap, or rock protection, to
strengthen the bridge foundation
and provide 100-year flood
protection. This was where the
project departed from customary
design, said Paul Clopper, Ayres
Associates project engineer. With the
Academy Boulevard bridge, the
severity of the streambed
degradation and local pier scour
posed unique problems that
traditional engineering methods
couldn’t solve. The traditional repair
would involve constructing a thick
layer of riprap surrounding the
bridge piers. But to offer adequate
protection at this site, the riprap
would have been large, and the
thickness for 100-year protection
wasn’t practical or consistent with
the master plan, Clopper said.
Ayres Associates proposed
applying an innovative partially
grouted riprap technique that was
developed in Germany in the
1980s but never applied in the
United States. The process involves
partially filling voids between rock
with a Portland cement-based grout.
With this method, the armor layer
uses smaller riprap stones, which
are thinner, easier to install, and less
expensive. The partially grouted
riprap also can withstand hydraulic
forces associated with a 500-year
flood event at the site. Partial
grouting increased the riprap’s
stability while maintaining flexibility
and porosity.
The method, untested in the
United States, was initially greeted
with some skepticism, Pennington
said. “Anytime you do something
new, it’s hard not to go with the
traditional or proven methods. It
took some back and forth to get the
City to feel comfortable with the
new technique and understand the
associated benefits.”
The City did not have the
luxury of becoming intimately
TRENDS
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