The final phase of construction has started on the $92-million Wallis Annenberg Wildlife Crossing project in Agoura Hills, in southern California, 35 miles northwest of downtown Los Angeles.
The initial phase, completed earlier this summer, entailed building a massive bridge over 10 lanes of Highway 101 near Liberty Canyon Road to connect open space on both sides.
Construction crews placed more than 26 million pounds of concrete, installed 82 bridge girders, and constructed sound walls and rock features designed to be covered with vegetation.
Because the span had to support plant life, they first installed a waterproofing membrane atop the bridge deck, followed by perforated pipes and a filter fabric for drainage. Next, they laid lightweight aggregate, subsoil, boulder microhabitat and finally topsoil and native plants.
Crews spread 6,000 cubic yards of “living soil” across the one-acre bridge deck in preparation for fall planting. The goal was to create a native soil that will help local plant life flourish.
The effort required collaboration with the team’s soil scientists, biologists, engineers and mycologists who identified, harvested and cultivated native soil biology and beneficial fungi from the site.
The habitat on the bridge will be comprised of coastal sage scrub plant species native to the Santa Monica Mountains. More than 50,000 native plants, including native trees, shrubs and perennials, are being used to recreate the habitat and provide continuous coverage across the crossing.
Crews have now turned their attention to the final phase, which includes building a secondary structure over nearby Agoura Road, a two-lane thoroughfare. The work will involve significant earthmoving, restoration of natural hydrology, protection of heritage oak trees and installation of some utilities and relocation of others along the freeway corridor.
The project is expected to be completed by fall 2026. When finished, the spans will make up the world’s largest urban wildlife crossing.
Together, the structures will provide a corridor for local wildlife, such as mountain lions, deer, coyotes and other animals to safely pass between the Santa Monica Mountains and the Simi Hills. Without a safe and sustainable wildlife crossing, animals in the area are essentially trapped.
California Gov. Gavin Newsom said in a statement the project has been made possible by the “visionary work” of the state and federal and private partners.
“The Wallis Annenberg Wildlife Crossing will soon protect Los Angeles’ native wildlife and over 300,000 drivers daily, as well as provide a cutting-edge model for urban wildlife conservation,” he said.
Wildlife crossings are critical to the success of California’s ambitious goal of supporting transportation infrastructure while conserving 30 per cent of lands and coastal waters by 2030.
The project is a public-private partnership led by Caltrans, the National Wildlife Federation and other conservation groups. The venture has drawn international attention for its scale and ambition.
Other partners on the project include the National Park Service, the Santa Monica Mountains Conservancy/Mountains Recreation & Conservation Authority, Resource Conservation District of the Santa Monica Mountains. Living Habitats LLC collaborated with Caltrans on the design. C. A. Rasmussen, Inc., based in Valencia, Calif., built the crossing over Highway 101.
Although the highway crossing structure is larger than the one being built over Agoura Road, the smaller project is more complex. The Agoura Road structure spans a smaller local road but is as wide as the structure over the highway – and requires a more extensive foundation.
To connect the habitat areas north and south of the freeway, the project team notes a large amount of earthmoving work is necessary.
“The landscape connects to the freeway and must reconstruct critical hydrology and restore habitat while protecting the existing creek watersheds and several large, heritage oak trees,” the team states. “In addition, a series of important utility lines parallel to the freeway must be relocated – a monumental, multi-agency effort.”
Plans for the wildlife crossing came about as a result of mountain lions being killed when crossing the highway. Studies by the National Park Service found the road was not only proving deadly for animals trying to cross, but it had also created islands of habitat that are isolating local wildlife – from mountain lions to bobcats, birds and lizards – and could lead to their demise.
Hundreds of millions of dollars have been dedicated to acquiring and preserving thousands of acres of open space in the Santa Monica Mountains over almost half a century. Of all the area roads, multiple research and planning efforts identified Highway 101 as the most significant barrier to the ecological health of the region.
According to the project team, the work will preserve biodiversity across the region by connecting an integral wildlife corridor, and help save a threatened local population of mountain lions from extinction.
