Fossil Creek Ped Tunnel

Fort Collins, Colorado

The City of Fort Collins initiated a project to increase foot, bicycle, and equestrian traffic by expanding the Fossil Creek Trail system through the construction of a pedestrian tunnel underneath the BNSF railroad. The Fossil Creek Pedestrian Tunnel is a 14- foot diameter tunnel that spans 70 linear feet underneath the BNSF railway.

A unique portal and earth retention system was considered to decrease the total tunnel length from approximately 120 ft to 70 ft.  Arch walls were constructed within the BNSF “B-zone” and make use of horizontal soil arching, acting similarly to an inverted tunnel lining.  Arch walls transfer lateral earth and surcharge pressures through compressive thrust, horizontally, to 48-inch diameter, reinforced drilled shafts constructed at the corner of each wall.  The unique wall geometry and decreased underpass length provided a significant cost savings to the City of Fort Collins.  Arch walls were designed to limit deep and large foundation elements and lateral support extending into the railroad embankment. Arch walls were constructed top-down in four lifts and include two reinforcing mats consisting of #8 vertical rebar and #5 horizontal rebar spaced at 12 and 6 inches, respectively.

The 70-foot long tunnel was constructed by jacking a 14-foot diameter, 1¼-inch thick steel casing pipe.  A shield was fabricated specifically for the project including the ability to utilize up to 5 sand shelves and breasting doors depending on encountered conditions.  Embankment mining was completed with mini-excavator, situated in the tail can behind the shield.  Segments 8-feet in length were utilized for transport and crane picks, aligned end to end with a circumferential band during construction, and welded together once excavation was completed.

Lithos staff served as the design engineer for the arch walls and tunnel and observed construction full time. The project was sucessfully completed on time and on budget.