• Builders plan to cover the crossing with “engineered†soil inoculated with local microbes early next year so more than 5,000 native shrubs and wildflowers can be planted.
• But the crossing won’t be connected to the mainland until Stage 2, when many tons of earth must be moved to create a safe elevated passage over one of the state’s busiest freeways.
It doesn’t take a hawk eye to recognize that the Wallis Annenberg Wildlife Crossing over the 101 Freeway in Agoura Hills is not your normal Caltrans project.
For one thing, there’s the color. Most Caltrans structures are the light gray of natural concrete. But to reduce reflectivity and help the crossing blend in more with the surrounding land, the new crossing’s 27 million pounds of concrete have been colored a shade dubbed “dusty mocha†by the project’s lead designer, Robert Rock, a landscape architect for Chicago-based Rock Design Associates.
The official name, however, is Federal Standard Color 595-33105, a hue “inspired,†Rock said, by the soil around the crossing that will eventually provide wildlife safe passage between the Santa Monica Mountains to the south and the Simi Hills and Santa Susana Mountains to the north when the project is completed.
For the record:
9:23 a.m. Oct. 29, 2024An earlier version of this story incorrectly reported that Robert Rock works for Living Habitats. He now has his own firm, Rock Design Associates. It also incorrectly reported that Katherine Pakradouni was the founder of the project’s nursery. She was the former manager.
When the project began in 2022, the end date was projected to be late 2025, but severe rain and heat the past two years caused delays that have pushed completion to sometime in 2026, said Rock. Weather extremes can wreak havoc on construction projects, especially when they involve so much concrete, he added.
Nonetheless, there’s been plenty of progress. The crossing now spans the 101, and up on the deck — 21 feet and 8 inches above the freeway — there’s a marked difference from the noise on the ground, where the traffic is so loud it’s hard to have a shouted conversation (unless all you’re saying is “What?!â€).
On Oct. 16, Caltrans senior bridge engineer Darwin Vargas led a group to the top of the structure, promising serenity once we stepped on the deck. He was right. At the top, the deck looks like a moonscape, and the roar of rushing cars and trucks was barely discernible, thanks to the thick, 12-foot-tall sound walls on either side of the structure, which is slightly wider than an American football field and about three-quarters its length.
On that day, workers were using power tools (and even those sounds were muffled) to smooth the sides and surface of the concrete deck, preparing it for the next steps — waterproofing the concrete and covering it with heavy rubber sheets about 2 inches thick. After that will come a “giant mattress of gravel†12 inches deep, Rock said, laced with perforated pipes to collect runoff and direct it into Liberty Canyon Creek.
It’s hard to tell as you’re driving underneath, but the crossing is slightly slanted, about two feet lower on the north end than the south to help drain off any moisture, because the last thing you want on a concrete structure is a lot of standing water.
This space will be off-limits to humans once the crossing is completed, with locked gates to keep people out and discreet fencing to guide wildlife away from nearby homes into the wild hills beyond. The crossing is designed to feel like an extension of the hills and valleys in the area to re-create the seamless corridor that existed before the noisy, busy freeway made passage so dangerous.
Once the gravel is in place, the builders will add about 6,000 cubic yards of “engineered soil†to the structure, Rock said. It’s a sandy loam mixed with lightweight volcanic aggregate to provide the volume and moisture needed to support plant life while reducing the weight on the structure and allowing for easy drainage.
That quantity of soil is enough to cover three-quarters of an American football field in about 2.5 feet of soil, he said, but on the crossing, the “ground†will be higher in some places than others, contoured to mimic the dips and dents one might find on nearby hills.
The engineered soil will also be inoculated with mycorrhizal fungi spores collected within five miles of the crossing, the same spores added to the soils growing the 5,000-plus hyper-local native shrubs and wildflowers that will be planted on the crossing. The plants are considered “hyper-local†because they are grown from seeds that were collected within a five-mile radius of the structure, Rock said.
“We’ve got to have the right microbes in order for these locally sourced seeds to flourish,†said Ted Hartsig, a soil scientist for the engineering company Olsson, one of many contractors working on the crossing.
If all this sounds a little fiddly, well, just understand that it’s tough to mimic Mother Nature. Success is in the details, and there are more than a 100 people led by Rock trying to ensure that every detail is done correctly.
After all the soil is in place, they plan to start planting early next spring, Rock said. The project has two nursery sites now: a seed bank near Paramount Ranch and the main nursery off Las Virgenes Road in Calabasas, where thousands of seeds have been planted in tiny flats and then repotted to grow large enough to be planted.
Those seeds were collected in 2022 by the nursery’s former manager, Katherine Pakradouni, who has since moved on to another position. The people who helped her set up the nursery, Jewlya Samaniego and Jose Campos, now run the site together and put in so many long hours that some people have questioned whether they live there. They don’t, Samaniego said laughing, but there are time it feels that way, especially during the heavy rains last winter and the scorching heat this summer.
Native plants don’t need much water once they’re established in the ground, but growing in little pots that dry out quickly, that’s another matter, Campos said. During the highest heat, they had to water the plants twice a day to keep them alive, and they finally created a shaded area to protect tender seedlings from the heat.
Most of the plants growing at the nursery will be planted on the structure next year: native shrubs such as white sage, California fuchsia, California buckwheat, sagebrush, purple sage, black sage, toyon and laurel sumac, along with a variety of native grasses.
The hope is that the plants will be robust enough — thanks to the beneficial microbes — to crowd out the black mustard and other invasive plants growing nearby, Rock said. Once established, the plants shouldn’t need much water, but the crossing will have an irrigation system installed that can be activated to wet down the crossing in the event of a wildfire.
Stage 2 involves creating a tunnel over Agoura Road to the south to create a gradual slope that will allow animals to wander off the crossing into the Santa Monica Mountains. To the north, a deep gash in the hill will also be filled with soil, to create a sloping entry from the Simi Hills onto the crossing.
Those areas will have similar plantings along with native oaks and other native trees, which can’t be planted on the crossing because their roots run too deep, Rock said.
The nursery is growing those plants from seed too. The plants have been moved to the growing area called the “seed bank†near Paramount Ranch, he said. The goal is to collect seeds from them until they’re ready to be moved to their permanent homes on the shoulders that link the crossing to the hills, which likely won’t be until 2026.
Rock said they’re doing the planting in stages so the plants on the crossing have a year to get established before wildlife can reach them. Otherwise, the young plants could be nibbled to death by hungry travelers.
The design work for Stage 2 is nearly completed, Rock said, but it will likely take a good part of 2025 to get the tunnel over Agoura Road completed.
Then there will be lots of earth to move. Much will come from the north side of the crossing, where fill dirt was deposited from the 101 expansion in the 1950s and 1960s, said Rock.
The shoulders will have to be filled in slowly, a little at a time on each side, said Vargas. Too much soil on one side could destabilize the structure, causing it to topple, he said, “which would be really, really sad.â€
Once the fill soil is in place, the builders will add a final layer of top soil, a rich mix that was dredged from the bottom of Malibu Lake 70 years ago and has been resting near Paramount Ranch ever since, teeming with the beneficial microbes so important to this project’s success.
All that won’t happen for another year at least. On Oct. 16, the only animals on the structure were human, but high above, circling in the brilliant blue sky, a hawk was keeping tabs.
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