Designing a Safe Wildlife Crossing in Utah

Located at Parley’s Canyon near the Salt Lake and Summit county line, the bridge will be used as a wildlife crossing, serving as a link to habitats for animals entering and leaving the Snyderville Basin and the Wasatch Back. It would be the first wildlife bridge of its kind in Utah.

“Our team is committed to designing and constructing a bridge that is celebrated for both its effectiveness in linking wildlife habitats and for achieving beauty through simplicity,” said Joshua Sletten, WSP project manager and senior supervising engineer. “We want to provide a bridge that is visually stunning and unlike any recent project in the state.”

Currently, WSP is working as prime designer with its client, the Utah Department of Transportation (UDOT). There are three possible designs being considered for the bridge, which were delivered to the client for review in May. The bridge design options include an asymmetric two-span bridge, an open-spandrel steel arch bridge, or an anchored end span bridge.

The $5 million bridge will provide safe passage for elk, mule deer, mountain lions, moose and other wildlife, and will also help motorists traveling along the highway avoid accidents caused by wildlife crossing the highway.

“This corridor has had hundreds of animal-vehicle collisions over the past 10 years,” Sletten said. “Due to the terrain and width of I-80, the corridor is very challenging to provide a wildlife bridge. The corridor has been studied for nearly 10 years to identify how a crossing could be provided.”

Preliminary design work began in January, and is expected to be completed later this year. Construction will begin in the spring of 2018, targeting completion of the bridge in the fall of 2018.

The proposed bridge will be about 365 feet long, 45 feet wide, and span three westbound lanes, four eastbound lanes, and the median along I-80. It will be unlike most bridges typically constructed over roadways.

“The design is entirely different,” Sletten said. “The bridge will not experience traffic loads and the impact of millions of cars traveling over it.”

One noticeable difference is that the wildlife bridge will not be maintained in the winter. “That means over 10 feet of snow may accumulate on the deck,” Sletten said. “Melting of that snow in the winter will require special maintenance details to keep any runoff or water from leaking onto traffic below.”

Since it will be used for wildlife rather than typical highway traffic, the designers are exploring material options that will be more cost-effective and evaluate the benefit of higher-strength steel, higher-strength concrete, geofoam fill and possibly a steel deck to reduce overall structure weight.

Consideration is also being given to the types of vegetation that could be incorporated into the bridge.

“A properly designed vegetation layout will encourage wildlife to use the crossing by effectively reducing the amount of noise and light disturbance from I-80,” Sletten said. “Landscape architects will provide vegetation that will survive in this environment, harmonize with the local landscape, and require minimal maintenance.”

Wildlife fencing eight- to 10-feet in height will be installed along the corridor to prevent wildlife from entering the highway and guide the animals to the overpass. “Over a short period of time, the fencing will serve to ‘train’ the animals to use the bridge,” Sletten said.

Wildlife bridges are becoming more prevalent, especially in the western states and Canada. The bridges now in use are providing the designers with an understanding of one of the big challenges they face – the integration of soil and a drainage system into the bridge surface.

“Many freeway overpasses are using large culverts with earth over the top,” Sletten said. “This site requires an open look and the flexibility for future widening on I-80; therefore, an open bridge type that provides a natural look and fits well with the environment is used here. It is quite unique. I am not aware of another bridge exactly like this one.”

The designers are examining the proper soil depth and type of soil needed to sustain the vegetation and maximize structural efficiency and economics. Post-tensioned concrete deck panels used in conjunction with a proven drainage system may properly drain the bridge and ensure minimal maintenance.

The wildlife bridge plan has received widespread support from transportation professionals, animal and environmental groups, and the public.

Sletten said it has been rewarding to have the opportunity to work on a landmark bridge that will have a positive impact on the environment and animal habitat.

“We recognize the significance of the proposed bridge location and enthusiastically embrace the challenge to provide a bridge that is useful in purpose and in harmony with the environment, achieves a level of excellence in people’s minds, and adds lasting value to the community,” he said.