Oak Street Outfall

Oak Street Outfall

Fort Collins, Colorado – Downtown Storm Sewer System

Two segments turned this routine pipe-laying job into a challenging project.

The first segment was a 96-inch diameter tunnel 160 feet long, which passed beneath three sets of railroad tracks and a busy road. Most of the tunnel was in claystone bedrock, but the start of the drive encountered weak, saturated clays and gravels. The tunnel was excavated with a pipe jacked digger shield.

The Fort Collins Downtown Storm Sewer System involved construction in difficult geologic conditions beneath existing railroad tracks, streets, and utilities. The picture shows
the digger shield entering the Riverside embankment.

The second challenging segment was a tunnel under College Avenue. Since the alignment passed underneath a busy and historic area, settlement, sinkholes, and traffic disruptions were unacceptable. While the alignment was adjusted to keep the crown mainly within stiff clay, the rest of the face consisted of sand, gravel, and cobbles with nested boulders. Construction during winter kept all but the tunnel invert above the water table. The College Avenue tunnel (650-feet) was driven by the same digger shield, but ground support was ribs and boards with a filter fabric backing. The carrier pipe is 78-inch Hobas pipe grouted into place.

Extensive collaboration was required between D&A staff and the contractor to choose a tunneling method that would allow tunneling through boulders while keeping control of the face. Further collaboration was required with the client to address permitting issues and the crossing of railroad tracks.

Client:  City of Fort Collins

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