Geotechnical engineers, the people who have the most in-depth knowledge of the unstable nature of the Last Chance Grade slide on U.S. Highway 101, always seem to be the ones most concerned with the risk the region is facing.
During a Friday tour of Last Chance Grade that included Congressman Jared Huffman, June James, a Caltrans geotechnical engineer, described how even the recently completed $4.8 million retaining wall will prove futile against the forces of nature.
“These walls are sliding within the landslide, because the landslide is too large to stabilize,” James said, adding that the two main slides characterizing Last Chance span 2,200 feet of road.
Despite the fact that the recently completed wall has 80-foot vertical piles and 100-foot horizontal tiebacks, it still dropped eight inches vertically and slid 11 inches horizontally from July 2012 to September 2013, James said. The slide is just entirely too deep to tie the road to anything stable. Movement was detected from 160 to 125 feet deep and even faster movement at 40 feet below the road surface.
“The walls are not built to stabilize the landslide; they’re just to maintain the geometry of the road,” James said.
Looking for the big picture, Huffman asked James, “What is the endgame for the hillside?”
With a smile, James gestured toward the Pacific and responded: “You see the sea stacks in the ocean?”
Failing Del Norte since 1894
Talitha Hodgson, the Caltrans project manager for all things Last Chance, detailed the history of the notoriously slippery slope during the Friday tour:
A County Road was built on Last Chance slide in 1894.
In 1923, Last Chance became a part of State Route 1, the old Redwood Highway.
From 1933 to 1937, the road was realigned to the current Route 101, but even before that work was completed, the California Division of Highways documented that the roadway through Last Chance slide was “expensive to maintain because of the extremely unstable formation.”
Even back then, the state considered moving the route to the east side of the ridge, away from the hillside certainly slipping into the Pacific, but dropped the idea due to high cost and the damage to California State Parks’ resources (read: old-growth redwoods).
For as far back as records go, there have been major landslide repairs at Last Chance one to three times per decade.
In 1972, the road entirely failed, killing the parents of local Realtor Kurt Stremberg, who now sits on a citizens committee advocating for the rerouting of 101 around the Last Chance slide.
The death of Stremberg’s parents and decades of knowing that a long-term solution was needed culminated in several project study reports and geotechnical reports that sought an alternative.
The final determination came in the form of Caltrans’ 2003 Value Analysis Report. It’s finding?
“That VA decided the best alternative was to clean up the alignment,” Hodgson said, meaning straighten out the curves, lessen the grade, but ultimately continue to plan on repairing Last Chance Grade indefinitely.
Since 1980, $36 million has been spent on maintenance and construction projects at Last Chance with an additional $7 million spent on studies.
Another $8.5 million is already planned to be spent on a soldier pile wall and a soldier nail wall.
“These projects are just slowing things down,” Hodgson said.
Again looking at the big picture, or perhaps thinking about his relatively new role as steward of federal taxpayer dollars since almost all of these repairs are funded by federal emergency repair dollars, Huffman noted: “By spending all that money you’re guaranteeing that you’re going to be spending more money in the future.”
That’s why Caltrans is currently undergoing an Engineered Feasibility study to find an alternative, so “at least we’ll have a plan,” Hodgson said.
Caltrans has also recently started an Economic Impact study to determine how the longterm closure of Highway 101 at Last Chance would affect the economies of the North Coast — the type of study that proved crucial for the bypass of the Confusion Hill slide in northern Mendocino County in 2009.
“We have a plan in place, but we’re also not going to wait for this to fall into the ocean to do that,” said Roger Goddard, acting district manager for North Coast Redwoods District of California State Parks.
Route through redwoods
After the physical tour of Last Chance Grade and its recent improvements (completed without accidents, in rain and snow, and under budget, noted resident engineer Karen Sanders), Huffman and the rest of the group returned to the parks office. There, they watched a virtual tour of the landscape with a keen eye on the possible alternative routes through state parks and timber land owned by Green Diamond Resource Company.
The virtual tour guide, David Best, GIS coordinator for Redwood National Park, said that one of the park’s main concerns is protecting the Damnation Creek watershed, which is entirely composed of old growth without any evidence of logging.
Likely routes traverse through the Wilson Creek watershed, which is entirely owned by state parks and Green Diamond.
“You’ll notice the amount of old growth between Green Diamond land and the (existing) road in the park is not substantial,” Best said.
Neal Ewald, senior vice president of Green Diamond, who oversees all of the company’s operations in California, noted the company owns old-growth redwoods stands in the Wilson Creek watershed that could be used as mitigation for old-growth redwoods that might be cut on state parks land.
Huffman noted that all of the alternatives traverse through old-growth redwoods, although there seemed to be an option to veer far enough north before cutting over to the existing highway and avoid cutting old growth entirely.
Although possible, there were doubts in the room about laying down that much new road in the Wilson Creek watershed and the ability to have the road climb certain slopes.
Best said that landslides will still occur on the western edge of the Wilson Creek watershed, but there isn’t the same problem of coast erosion where the toe of the hillside is being eaten away by the ocean.
Why so slidey?
With sea level expected to rise, coastal erosion could cause slides at Last Chance to increase in frequency, James said.
Charles Fielder, director of Caltrans District 1, said that in some areas of California coastline, rocks have been installed to prevent erosion, but “the rock you would need to hold this back would just be unfathomable,” Fielder said.
One of the biggest drivers of landslides in the region is its location in California’s only rainforest. Oftentimes, the added water weight makes the Last Chance landscape — large blocks of sandstone riding on a thin layer of clay — too heavy not to slide. The rain also makes working conditions at Last Chance Grade difficult.
“When we talk about rain in Del Norte, we’re talking about 70 inches of rain per year; sometimes 2 inches a day,” said Josh Runnion, structures representative for Caltrans. “Your visibility and safety issues go through the roof.”
A previous version of this story said that Kurt Stremberg serves on a county committee advocating for the rerouting of Highway 101, but it is a citizens committee.