Clearing the western road to Yosemite — finally

On the steep wall along Ferguson Ridge, an American flag waves above imposing rows of metal “drapery” that stretch hundreds of feet downward.

The cable netting is a safety feature covering the face of a massive, 9-year-old rock slide, but it’s also a gawker’s delight. It stands out amid rugged oak trees and ancient rock in stunning Merced River Canyon.

For the next few years, the sights will only get more interesting along the well-traveled western route into Yosemite National Park.

The time has finally arrived to deal with the 110,000 tons of rock and debris that buried a small part of Highway 140 in 2006. The state this year already has moved a lot of debris in this $133 million project to open up this bottleneck by 2019.

In the next few years, crews will build a reinforced concrete rock shed over nearly 800 feet of the roadway. Stout protection? The rock anchors can each withstand 600,000 pounds.

And there will be 300 rock anchors in this structure.

“It will take rock slide stacked up 35 feet deep,” says resident Caltrans engineer Corey Casey. “It is amazing.”

The Highway 140 rock shed would only be the second of its kind in California. The first was opened last year along Highway 1 near Big Sur, and it is less than half the length of the structure planned in the Merced River Canyon.

“Rock sheds are used in Canada and Europe,” says Grace Magsayo, Caltrans project manager. “They are specifically designed to handle a lot of force.”

Authorities worked years on this project before deciding on this option. Along the way, they installed temporary bridges across the Merced River to funnel traffic away from the rock slide, studied the area’s environment and monitored the movement of the debris.

Officials held meetings with nearby communities of Mariposa and El Portal, gaining background on one of the key arteries into the world-class destination of Yosemite. Nearly 4 million people visit the park each year.

Slower road for 9 years

Over nine years, many local residents actually became accustomed to one-lane traffic, controlled by signals allowing vehicles to pass for a few minutes in one direction, then a few minutes the other way.

“Some people have said, ‘Why not just leave it the way it is?’” says Damian Riley, president and chief executive officer of the Mariposa County Chamber of Commerce and Visitor Center. “But a greater majority welcomes this project. People are excited.”

A 2007 Caltrans study showed area business was off by 50% and more in the nearby communities, many of which rely heavily on tourism.

The Yosemite concession operator estimated a $4 million loss in the first year. The number of tour buses passing through dropped from 250 to 34 in that year.

But things have picked up since then. Lodging, restaurants and other businesses are seeing more customers, says Riley. Though the south entrance on Highway 41 has a little higher volume of traffic, Highway 140 is not far behind.

National Park Service figures show Highway 41 had a peak of 72,000 vehicles in July last year. The west entrance from Highway 140 recorded 56,000 vehicles. Big Oak Flat to the northwest also gets a little more vehicle traffic than Highway 140. To the east, the high Sierra entrance at Tioga Pass, which closes each winter, has a lowest year-round volume.

“Generally, it’s an easier drive into Yosemite Valley from Highway 140 even with the traffic light at the rock slide,” Riley says. “Highway 140 is the all-year route. It’s lower in elevation and often does not have snow when other routes do.”

Science of slide

After the slide, authorities began monitoring the debris pile, using monitors similar to the ones placed in the volcanic crater at Mount St. Helens in Washington.

Movement and falling rock seem to have stopped in the last few years, says Magsayo of Caltrans. She says Caltrans studies suggest the area has stabilized and may not become active again for more than two centuries.

“The rock shed will definitely protect this area,” she says. “But the likelihood of another large slide here is very small.”

At the same time, the Merced River Canyon is steep and V-shaped. The canyon walls make rock slides inevitable, according to geology professor John Wakabayashi of Fresno State. Another rock slide could happen somewhere else in the canyon.

“When the walls are steep, things are going to fall,” Wakabayashi says. “That’s what you get with this kind of canyon.”

The Ferguson Ridge area between Mariposa and El Portal is particularly susceptible to rock slides because of its weak bedrock.

“Weaker bedrock is more prone to sliding,” Wakabayashi says.

Some of the rock that tumbles down is a relic from a time when dinosaurs roamed the planet. More than 150 million years ago, it was ocean sediment along the continent. Over time, geological processes turned the sediments into fine-grain rocks known as phyllite.

Anyone driving Highway 140 can see layers of these rocks — gray, green or even red and lustrous, similar to slate. They look like dark, chunky stacks of dominoes tilted at odd angles along the river and above the road.

Nature can jolt these rocks down the canyon walls with earthquakes, heavy rainfall seasons or freezing and thawing.

Not a tunnel

Caltrans had discussed other alternatives to fully reopen the road and simply leave rock slide in place, such as a bypass tunnel. But that idea was too invasive, leaders decided.

Building the rock shed and keeping the same road alignment would disturb the least amount of land, which helps protect such creatures as the threatened limestone salamander.

Crews have recently removed core samples from the remaining rock slide debris to assess how useful it might be as a building material, says Caltrans spokeswoman Angela DePrato, who was at the rock slide site last week.

“Whatever we can use here will save taxpayer money,” she says.

DePrato says the cable netting was placed with the help of helicopters and specially trained state climbers, who scaled the canyon wall to do their work.

The damaged portion of Highway 140 will have to be rebuilt, and the rock shed will be constructed over it. The construction phase of the project is expected to begin next year and conclude in three years.

In Mariposa, it’s all good news to Jake Wackerman, owner of a restaurant called 1850. His business relies mostly on locals who enjoy the gourmet hamburgers, seafood and fried chicken, as well as beer and wine.

But he’d love to add more tourist business, he says, mostly during the 120-day warm season.

“The road project is great,” he says. “It’s a marvel of engineering, and they’re moving fast. Around here, our numbers are up from last year. When that project is finished, I think it will only get better.”

This story was originally published August 1, 2015 at 9:00 AM with the headline "Clearing the western road to Yosemite — finally."