Why Highway 1 near Big Sur is always collapsing into the ocean

A stretch of Highway 1 in Big Sur is closed indefinitely after the southbound lane eroded during last weekend's rainstorm, bringing uncertainty to the communities that rely on the road for transportation.

As Caltrans works to repair the road, which is closed from Limekiln State Park to Palo Colorado in Monterey County, the agency is coordinating convoys for residents and essential workers to enter and exit the area for food and supplies. The state is now working on a temporary fix that would allow residents and visitors to pass using the structurally stable northbound lane of traffic.

It's one of several closures along this stretch of Highway 1 in recent years, as slides during the rainy season wash rocks down.

That's because the area's mountainous terrain and steep cliffs — the very thing that makes the area a world-famous road trip destination — are what makes the road so prone to rapid erosion, or slip-outs.

How geology explains the slip-outs

It might not have crossed your mind when you're driving on that part of Highway 1, but the road was built on highly unstable ground, full of faults and fractures in the ground that leave the area prone to rockslides, especially after heavy rainfall.

In short, the Big Sur coastline is "a road builder's nightmare," said Dick Norris, marine geologist at University of California San Diego's Scripps Institution of Oceanography.

The Big Sur coast.

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According to Norris, much of Highway 1 was built downhill from the location of previous rockslides — and once a slide has happened, it's more likely to recur.

"I think that probably many of the current slides are old slides that are being reactivated," Norris said. "Then of course, the material that the roadbed is built on is oftentimes an old slide deposit."

As we've seen in the last several years, this unstable soil can lead to intensive, and often costly, repairs.

"The ideal thing I suppose would be to have the road far inland from where it is: instead of on the cliff, farther up, in towards the interior," Norris said. "As we know, the whole point about why Big Sur is so beautiful is that it's so bloody steep. And the steepness reflects both the active erosion that is going on there all the time, and then, of course, a lot of that erosion is happening through these big slides."

The composition of these rocks is similar to much of the California coast — though Norris, who's done field work in Big Sur, said the region is especially affected by slides since closing the often-unstable Highway 1 can strand residents.

This unstable terrain across the region leads to something like a long-term game of Whac-A-Mole — once one segment of Highway 1 is fixed, another section inevitably erodes before long.

A section of California's famed Highway 1 collapsed into the ocean following heavy rains over the weekend.

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What workers do after a collapse

The first step for engineers after any slip-out or slide occurs: assessing the damage.

Caltrans workers first went out to survey the collapsed highway on March 31, the day after the slip-out, and determined that the site was stable enough to run convoys along the stretch twice a day, according to Caltrans District 5 spokesperson Jim Shivers — though Caltrans is still exercising caution.

"While stable, it is expected to see some further recession as the slope comes to a state of equilibrium," Shivers wrote in an email to LAist.

Next, engineers will stabilize the edge of the road, drilling into parts of the rock that is known to be competent, i.e. stable enough to support pillars. Then, they will insert dowels into the rock and reinforce the area with shotcrete, or concrete that is projected pneumatically.

"Once this work is completed, a traffic signal can be put in place and utilized to allow unlimited access to the traveling public," Shivers wrote, noting that the edge stabilization work needs to take place before Caltrans' planned temporary solution.

This traffic signal would allow for more regular traffic to pass on the stable northbound lane without the assistance of workers. Currently, the only public access allowed on the eroded part of Highway 1 — the only way for Big Sur residents to leave for supplies or goods — takes place on twice-daily convoys.

Caltrans engineers are still determining how best to repair the stretch of road after the temporary traffic signal is installed. Possible solutions include building a bridge, wall or viaduct, according to the agency.

There is some precedent for this repair: When a similar failure took place just south of this location in 2011, engineers ended up fixing the stretch of road by constructing a viaduct.

A hole is visible where a section of southbound Highway 1 broke off and fell in the ocean at Rocky Creek Bridge on April 02, 2024 near Big Sur, California.

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When to expect Hwy 1's full reopening

Shivers wrote that Caltrans currently can't say when this stretch of Highway 1 will reopen with a temporary traffic signal — though they anticipate more information will soon become available.

"The goal is to have a tentative schedule within the week," Shivers wrote.

As for the permanent fix, Shivers wrote that it may still be months before Caltrans will know how long the repairs and construction will take, as its method and design still need to be finalized.

That's typical for projects like this one, which often use pillars, bridges or retaining walls to keep the road stable and keep landslides at bay, said Norris.

"I don't envy [Caltrans] at all, but they are obviously getting to be experts at it since they've been building repairs for decades," Norris said.

Caltrans is also working on repairing a separate stretch of Highway 1 at Paul's Slide that eroded in January 2023. That fix is projected to be finished by late this spring.