Flagstaff Road Bridge – Boulder County, CO

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In the spring of 1995, record rainfall along the Colorado Front Range contributed to a significant landslide on a portion of the steep embankment of Flagstaff Mountain above Boulder. The landslide undermined a section of the winding and narrow roadway and threatened the homes below. LORIS brainstormed ideas with Boulder County personnel to address this engineering challenge. In addition to a standard convention of using tied-back retaining walls to correct the problem, LORIS also evaluated the option of constructing a bridge over this troublesome spot in the road. Boulder County selected the bridge concept because it was less expensive to build than the retaining-wall option and would also minimize the visual impact to the mountain.

LORIS’ solution consisted of a cast-in-place cantilevered bridge deck supported on caissons drilled into bedrock to cost-effectively cross this chronic landslide zone. The bridge stopped at the centerline of the roadway so traffic could remain open during construction, thus avoiding 40-minute detours. The finished bridge was well received by the community with minimal complaints during construction. Our creative design approach saved Boulder County more than $200,000 over conventional slope stabilization methods.

The Flagstaff Road bridge was awarded the ACEC/Colorado 1997 Engineering Excellence Award for its innovation in solving a difficult, potentially dangerous problem.