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Fossil Creek Drainage Improvements

The City of Fort Collins needed help increasing the conveyance capacity of Fossil Creek to remove 140 homes from the 100-year floodplain. Seems simple enough, until you consider the existing 50-foot high Union Pacific Railroad embankment.

We started with a comprehensive alternatives analysis and complex hydraulic modeling. This indicated the installation of two parallel 120-inch diameter tunnels under the embankment was the best solution. Research determined that prior to the construction of the embankment, a wood timber bridge occupied this area, and it was likely still in place. A railroad track map gave a location for the abutments of the bridge, and we positioned the tunnels to avoid the pilings.?

Another challenge was uncovered during the geotechnical investigation. Extremely low bearing capacity soil was found at the proposed invert elevations, offering little supporting strength. After additional hydraulic analyses were performed, the invert of the tunnels was raised, and we recommended the installation of two, 12-inch steel pipes through the embankment. The pipes serve as rails where the tunnel piping rests to span areas of soft soils.

Raising the invert of the tunnels now meant the tunnel installation machine, with maximum thrust of up to 1.2 million pounds, had little soil to push against. A monolithic thrust block composed of 113 cubic yards of reinforced concrete was constructed, and acted in unison with a series of four, two-foot diameter caissons that were drilled into bedrock. Through careful design and installation, this block did not have to be removed after the tunnels were complete, and is now part of the erosion control downstream.