Utah Opens State’s First Wildlife Bridge Overpass

Usually, bridges are built for cars, trains, or non-motorized transportation. But a new overpass designed by WSP USA was built specifically to link animal habitats separated by an interstate highway.

Located at the top of Parley's Canyon near the Salt Lake and Summit county line, the bridge is the first one built for this purpose in Utah … though if it proves successful in its intent, it may not be the last.

“It is the first overpass in Utah designed to accommodate wildlife and not humans,” said Joshua Sletten, WSP project manager and senior supervising engineer. “It is longer than most wildlife crossings and is intended for large animals like moose, elk and deer.”

The $5 million bridge is about 320 feet long, 50 feet wide, and spans three westbound lanes, four eastbound lanes, and the median along Interstate 80 (I-80). The overpass provides safe passage for animals entering and leaving the Snyderville Basin and the Wasatch Back. It is a corridor that has experienced hundreds of animal-vehicle collisions over the past 10 years.

“This bridge helps eliminate vehicle/wildlife conflicts through a corridor that had become known for collisions with animals,” Sletten said. “Today, wildlife is better protected, drivers are safer and congestion due to these collisions has been greatly reduced.”

WSP served as the prime designer, providing structural engineering as well as support for civil and geotechnical engineering, and landscape architecture. The firm worked on behalf of the Utah Department of Transportation (UDOT), the Utah Division of Wildlife Resources (DWR), Salt Lake City, Salt Lake County and Summit County.

Preliminary design began in January 2017, with construction starting in early 2018. The final landscaping boulder was placed on the bridge during a Dec. 13 ceremony, which marked its official opening as a wildlife crossing. That ceremony was also the last time people were permitted to walk across the overpass.

Three designs were considered for the bridge before UDOT chose an open, two-span plate girder option with a delta pier and spill-through abutments.

“This design was chosen based on a good balance of cost, form and function,” Sletten said. “Due to the terrain and width of I-80, it was a very challenging corridor for a wildlife bridge. Prior to the start of the design process, the corridor had been studied for nearly 10 years to identify how a crossing could be provided.”

Although the purpose is unusual, Sletten said its design and construction adhered to many of the techniques used for modern overpass structures.

Still, designers did factor in some variables, owing certain conditions that would be special for a bridge where the top surface does not tie into a road and it is entirely landscaped for wildlife.

“It did require innovative approaches to design for loads that are considerably different than what AASHTO [American Association of State Highway and Transportation Officials] standards typically require,” Sletten said. “This is a bridge that will not have snow removed, so it is possible that there could be times when several feet of snow may accumulate on the deck. The drainage system had to be designed to handle the melting of large amounts of snow in the winter to keep runoff from leaking onto traffic below.”

WSP worked closely with UDOT and DWR to understand length, width and surface parameters that would be attractive for wildlife.

Three-and-a-half miles of fencing along both sides of the highway on either side of the bridge will funnel animals towards the new overpass. Still, it will likely take another two to three years before wildlife develop an instinct to use it as a safe crossing. As more animals are trained to use the bridge, its impact will be significant for the well-being of wildlife populations and motorists alike.

Although a first for Utah, construction of wildlife-specific bridges are becoming prevalent in the western U.S. and Canada. Studying those existing bridges provided the Utah bridge designers with an understanding of one of the big challenges they faced—integration of soil and a drainage system into the bridge surface.

“Many freeway overpasses use large culverts with earth over the top,” Sletten said. “This site required an open look and flexibility for future widening of I-80. An open bridge provides that natural look and fits well within the environment.”

The designers examined the proper soil depth and type of soil needed to sustain the vegetation and maximize structural efficiency and economics. Prestressed concrete deck panels used in conjunction with a proven drainage system can properly drain the bridge and ensure minimal maintenance.

It was also important to make sure that the surface of the overpass blended naturally with its surroundings to entice animals to follow the crossing. After studying possible vegetation layouts, they chose an alternate approach.

“We ultimately decided the bridge would not have vegetation, as the topsoil required to sustain vegetation added unnecessary weight and could clog drainage systems,” Sletten said. “The clean, angular rock we specified for the surface is common to the mountainous region near the bridge.”

With its unusual purpose and design, the wildlife bridge has attracted a lot of favorable regional media attention and praise from wildlife advocacy organizations since its December opening.

The WSP design team was committed to designing and constructing a bridge that would be celebrated for its effectiveness in linking wildlife habitats while achieving beauty through simplicity.

“This project may help set the standard for other projects around the country, further helping to eliminate the significant problem of animal collisions on highways,” Sletten said. “I am proud to have helped steer and deliver this project to a successful completion for the community.”

Check out a video posted by UDOT about the wildlife bridge opening.

[To subscribe to Insights, contact the editorial staff at [email protected].]