A million tons of rock and dirt have fallen onto Highway 1
BIG SUR — Geologists and engineers crowded a conference room in San Luis Obispo this week to address the latest assault upon California’s most revered roadway.
Yet another stretch of Highway 1, that improbable serpentine hemming the continent’s western edge, had abruptly disappeared.
No one in the room was shocked or surprised. The scientists and builders knew what they were up against.
A week earlier, sensors in the mountains had picked up increased ground movement at a site 10 miles north of Ragged Point. On-site crews and equipment were evacuated, gates closed and locked.
On the morning of May 20, a hillside near a small ravine known as Mud Creek collapsed, sloughing an estimated 1.5 million tons of rock and mud over the highway and into the ocean.
The landslide was a third of a mile wide and 40 feet at its deepest. What once was a steep drop into the Pacific was now a broad, sloping bench extending almost 250 feet beyond the shoreline. By some estimates, the collapse had added 15 acres to the coast, a little more than 11 football fields including the end zones.
And the worst might not be over, said field inspectors who had just returned from the slide. Listen closely, and you’ll hear a sound of water running like rain through the rocks and dirt. The slide at Mud Creek is still moving.
“This is a big one,” said Rick Silva, an engineer with the California Department of Transportation, who had phoned in for the meeting. “It might be a once-in-a-career slide.”
But Silva and the gathered geologists and engineers were unfazed.
Their worst-case scenario had always been the 1983 slide that buried Highway 1 near Julia Pfeiffer Burns State Park for 14 months with 4 million cubic yards of rocks and dirt.
This one might pose even bigger problems, but now they’re more familiar with these notoriously temperamental mountains. And they’re confident they will once again succeed in carving a new route through the rubble, opening the coast for businesses, residents and the million annual tourists drawn to its hairpins and panoramas with a bucket-list fervor. It will take time though. The Santa Lucia Mountains, extending from Cambria to Carmel, are particularly vulnerable to landslides.
This coastal range reveals one of California’s defining tectonic features — the subduction of the Farallon Plate under the North American Plate — and the bedrock there, says Noah Finnegan, a geologist with the University of California, Santa Cruz, has “a tortured history.”
“They have been pervasively fractured and broken up,” he said. “They have a hard time holding a steep slope.”
In such an environment, water and gravity are great levelers. The steep and narrow canyons of the coast are frequently scoured by debris flows, torrents of water skimming off topsoil and rocks at disastrous velocities. The formidable hillsides are subjected to deeper subsurface forces that come into play as groundwater shifts the frictional properties of the soil.
It’s not uncommon, Finnegan said, that big slides occur long after the last rainfall: “Groundwater response is often delayed.”