In September 2013, torrential rains and flash floods hit the mountains of northern Colorado. Floodwaters ripped through County Road 47, tearing up large sections of the road and destroying the Little Thompson River Bridge which essential isolated the Big Elk Meadows community. The immediate challenge was to restore temporary access to the community. A permanent solution designed to withstand the ferocity of the river while safeguarding the areas aesthetic beauty remained the ultimate challenge. Achieving both a cost effective and sustainable option required out of the box thinking.
The Federal Highway Administration's Office of Federal Lands proved ready by uniquely combining the Every Day Counts initiative, Geosynthetic Reinforced Soil Integrated Bridge System (GRS-IBS), and Rockery Wall Systems. Individually both technologies have a long history in pre-modern construction. Engineering these solutions together solved a modern day dilemma.
The GRS-IBS abutment is constructed using alternating layers of compacted granular fill material and fabric sheets of geotextile reinforcement to provide support for the bridge. Due to the simplicity of the GRS-IBS construction, standard materials used, and the modular nature, on-site changes can be easily accommodated. While the IBS does not require a deep foundation, in the case of Big Elk Meadow there was a need to raise the bridge and ensure safety from flooding for half a century-even from more severe floods than those of 2013.
Due to the physical destruction caused by the flood, the engineers had access to an abundance of rocks and boulders-many rocks the size of Volkswagen Beetles. The cost-effective and sustainable solution was to use them to construct a 24-foot near-vertical rockery foundation to support the GRS-IBS.
By standard design, the rockery sloped inward one foot for every four-foot rise thus forming a near-vertical face. Traditionally, structures exposed to flowing water use a flatter slope of 1 foot back for every 1-1/2 foot in rise. Being a first-of-its-kind near-vertical installation in a stream, designers did not know the hydrodynamic effect of a 500-year flood. So, they modeled the foundation and the flood in a hydraulic laboratory to account for the hydraulic sheer, lift, and push to identify the best size of rock.
To ensure a good factor of safety, engineers used even larger rocks than the modeling required and grouted the stacked rockery to fill 25% of the voids, which would help keep the structure stable for decades to come.
The bridge's 24-foot support wall is notable not only for its cost-effective and sustainable construction, but also because it is incredibly tall and steep. In fact, this is the first bridge ever to employ foundations of what engineers call near-vertical rockeries exposed to stream flow. But for residents, the real relief lies in the fact that the completed roadbed now sits safely above the 500-year flood level
An additional 14-foot upper portion consisted of GRS abutments placed directly on the rockery foundation's backfill completed this innovative combination. Then, in keeping with the aesthetics of the rockery foundation, smaller rocks walled the GRS abutment face in lieu of using conventional concrete masonry units.
A set of six side-by-side prestressed precast box girders span the gap between the GRS's two abutments. This new 64-foot two-lane bridge will then be topped with an asphalt wearing surface.
In the manpower- and equipment-poor environment of the post-flood Colorado rebuild, such cost savings were not only important, but absolutely essential. Engineers realized significant cost savings by employing the sustainable practice of using on-site construction materials. The new bridge also provides a flatter approach from the road, improving safety at the nearby intersection of County Road 47 and US 36.
However, the unique look of the structure may bring more pride to engineers and residents than any other factor. The bridge has a lovely, natural aesthetic that appeals to locals and visitors alike. In fact, six similar bridges on County Road 43 have replicated its successful design
This was the first time that contractor American Civil Constructors (ACC) built a bridge of this type, and the first time ever that a near-vertical rockery was used to build a bridge embankment in a stream. Since no design methods for determining the effect of flooding on rockery stability existed, engineers began the repair on a learning curve.
In the end, the project proved that thoughtful engineers can deliver sound, balanced, sustainable solutions through unique engineering innovations, and that elegance and affordability can exist hand-in-hand. Today, the bridge is more than a showpiece; it provides a replicable model that others can use to guide smart, creative bridge designs within the unique context of any landscape-or the strict requirements of any budget.