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Tech Tour: Annenberg Wallis Wildlife Crossing

ASCE LA YMF toured the Wallis Annenberg Wildlife Crossing in Agoura Hills on May 30, 2024. The crossing consists of a 2-span bridge over 10 lanes of US-101 traffic and will be the largest of its kind in the world once completed. The wildlife crossing system is valued at $92 million. The site sits at Liberty Canyon, where the only thing between the open spaces of the Santa Monica Mountains and the Simi Hills is the freeway. The lack of any connection for wildlife has genetically isolated populations, and the project will bridge the gap. We toured the site after all the girders had been placed and diaphragm/deck construction had begun.

In 2018, a collared mountain lion known as P-64 was found to be crossing US-101 using the nearby culvert for Liberty Canyon Creek, being one of the only ones to ever to do so. P-64 sadly passed away a few months later after burn injuries from the 2018 Woolsey Fire.

We had the C.A. Rasmussen (prime contractor) superintendent lead the tour, accompanied by the Caltrans resident engineer and two assistant resident engineers. From the onset, the team made it clear. This was not your typical bridge project.

The funding nature of the project created its own unique challenges. The primary funding source was the National Wildlife Federation, as well as the Wallis Annenberg Foundation. This in turn created unique constraints of donor-related conditions, such as that the freeway crossing be built as soon as possible. Given the three parts to the project, the freeway crossing, the Agoura Road frontage road crossing, and the the utility relocation via covered bridge (there are currently 66kV Southern California Edison transmission lines running overhead, the lower elevation Agoura Road and utility covered bridge would have made more sense to construct first from a constructability perspective. Because the freeway crossing had to come first, the southern slope had to be stabilized using a soldier pile wall to withstand the new loading condition from the southern Abutment 1. The soldier pile wall was a change from the originally designed mechanically stabilized earth slope.

Next, excavations up to 15 feet deep were required to install spread footing foundations, including for Bent 2 within the freeway median, requiring lane shifts and temporary railing. Abutment 3 to north has variable geology, which led to large amounts of seepage weeks after the last storm due to an especially wet winter. Without any deck level access, the contractor had to lift everything from freeway level to deck level, including girders and deck construction materials. With dead load controlling the bridge loading, hollowed pretensioned box girders placed side-by-side were precast to withhold the massive weight of more than 5 feet of soil. High strength 5000 psi grout was used to fill the gaps between each girder. Pumped concrete was planned for the deck pour. The lifting equipment has its own challenges. The large span and width of the bridge creates large moments on the cranes, which in turn requires either large weights or bases, which in turn requires traffic lane closures. Caltrans only allows these closures to be done at night when traffic is minimal, but the mobilization of the crane is often the critical path to these nighttime operations.

We also got to see some of the detailing in the falsework up close, including how it is designed for errant vehicle impact loading, and the innovative sand trap bearings to easily and safely allow for deconstruction after girder and deck construction.

The Caltrans resident engineer also talked about some of the project's special provisions, from the specific color of the concrete, to the bridge sound walls, to the engineered soil media that cover the crossing, to the massive stakeholder collaboration in developing such a unique facility. He also shared how biologists have found that birds have been some of the first to use the wildlife crossing even as it is under construction; the bridge serves as a shield from the busy freeway lights and noise below.

After a beautiful sunny day on the bridge, we enjoyed burgers at the nearby Tavern Tomoko. Thank you to all that came out, to C.A. Rasmussen for hosting, and to Caltrans RE team!


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