The City of Fort Collins, Colorado recently completed the construction of a 70-feet hiking/biking tunnel, underneath Burlington Northern Santa Fe (BNSF) Railway tracks, which will ultimately connect more than 30 miles of trails.
Colorado State is very well known for its outdoor activities, including hiking and biking. In order to keep track with popular demand, the City of Fort Collins is now building its own system of hiking and biking trails. The idea of extending the Fossil Creek Trail beneath the BNSF railway tracks with the construction of a tunnel, emerged after the aerial crossing solution was judged as impractical and the open-cut undercrossing was ruled out, as BNSF would not allow for traffic interruption.
Initial design concepts included a longer tunnel solution, with cast-in-place concrete portals, an approach that would require fill to be imported. An alternate plan to use secant pile walls closer to the railroad was also aborted, since horizontal shoring was prohibited based on American Railway Engineering and Maintenance-of-Way Association (AREMA) standards.
Finally, the design team resulted in a brand new concept – an arched wall constructed of sheet piles for initial support, followed by the construction of steel-reinforced shotcrete walls. This concept allowed a more constructible design, while reducing the tunnel alignment to 70-feet. "The arched walls transfer lateral earth and surcharge pressures horizontally through compressive thrust, to 48-inch diameter, reinforced drilled shafts constructed at the corner of each wall", says Robin Dornfest of Lithos Engineering, the designer company. The walls were designed to limit deep, large foundation elements and lateral support extending into the railroad embankment. The wall geometry and decreased underpass length reduced significantly the total project cost.
For the tunneling, a custom-made 14-feet diameter shield with breasting plates as well as a mini-excavator near the face were used by BT Construction, the project's contractor. During the process, a survey crew was employed to monitor settlement along the railway tracks. Contact grouting was performed from inside the tunnel to help minimize ground movement. Minor settlements did occur, but railroad traffic continued without disturbance and tracks were remediated right after the tunnel's construction.
The project began in summer 2016 and was successfully completed on time and on budget in April 2017.