The Ferguson Rock Shed as it would appear to eastbound motorists on State Route 140, as imagined in an artist's rendering. Artist's rendition courtesy of District 10
Note: Public Information Officer Warren Alford wrote this story for the most recent edition of the District 10 newsletter.
Corey Casey’s first thought as he looked up at the jumbled pile of rubble where the foundation for a rock shed was supposed to be was, “That’s the end of that project.” His second thought was, “OK, we need to call the team together (Paul Elliott, the senior resident engineer, and Grace Magsayo, the projet manager) and create a plan to get this back on track.”
The Ferguson Project, as it is known, is the largest public works project in tiny Mariposa County’s history, likely rivaled only by the building of State Route 140 itself in the early 1920s – 1922 being the date stamped on most of the historic bridges along the route. The history of the project includes a couple of massive slides at the famously unstable slope in the Wild and Scenic Merced River canyon.
The route serves the communities of Mariposa, Midpines and El Portal. It provides the lifeblood of tourism revenue for the rural county and is the best all-season route for the 4 million Yosemite National Park visitors annually.
During the darkness sometime in the early morning hours on a December morning in 2006, a small shift of the earth caused a massive rockfall burying State Route 140 in Mariposa County under 800,000 cubic yards of rock, enough to fill nearly 44,500 full-sized dump trucks.
Fast forward to that fateful Dec. 18 morning in 2015, when Casey gazed up at the jumbled pile of rubble burying the excavation site of the project underway to restore full-access to the route after many years of planning.
Minutes earlier, he had been driving to the remote location in the Merced River canyon near Yosemite- his mind racing to plan a climb up the slope to inspect damage to protective netting from a small slide that had been reported earlier.
Sensitive monitoring equipment installed to measure turbidity (basically stirred-up dirt) picked up a pulse of muddy water for a two-and-a-half-minute period at approximately 5 a.m. that pinpoints the exact time of the slide. Casey’s stomach was queasy with the thought that despite the safety protocols in place at the time, he would have been swept off the mountain along with the other workers had the slide occurred a couple of hours later – the blink of a gnat’s eye in geologic time.
The Ferguson site as it appeared in November 2015, after the second big slide.
Photo by District 10
The Ferguson site as it appeared in June 2015, before the second big slide.
Photo by District 10
The project team immediately mobilized. In a few short weeks, they secured a $1.5 million change order, secured a contract for a geotechnical determine if a project was still feasible, flew drill rigs onto the slope to install sensitive monitoring equipment, and began the process of planning designing a new project.
Casey thought he would never scale the slope again, but soon found himself back installing monitoring equipment in an ambitious effort to get the project moving forward. Unfortunately, geologic time being what it is, the study to determine slope stability needed time to generate enough data to be meaningful.
Once it was determined that a rock shed was viable for a project, it became clear immediately that the original plan for traditional “cast-in-place” construction was no longer viable. Caltrans designers from District 10 and Central Region (including Grant Schuster and Caroline Reyes), developed an innovative solution to construct a 750-foot, segmentally constructed launched rock shed structure on the same alignment that will be built outside of the rock fall zone and then launched into place.
The installed segments will provide shelter for workers so that the entire process takes place under the protective cover of the structure as each segment is moved into place.
“We’re basically building the same structure we had planned initially, but in a completely different way, said Casey. “This will be the first launched structure like this built along a curve.”
Watch how the rock shed would be constructed in what Caltrans engineering team agree is the safest, most efficient way.
Animation courtesy of District 10
Project alternatives considered along the way included exploring options to bore a tunnel through the mountain, build a bridge around it, or building a protective rock shed over the roadway. Building the rock shed was determined to the best solution as it maintained the existing road alignment, resulting in less land disturbance and helping protect such creatures as the threatened limestone salamander. The new design dramatically increases protection for workers and will help protect the project schedule and budget.
Grace Magsayo, the current deputy director of Planning and Project Management, was assigned the Ferguson Project as one of her first assignments upon transferring to District 10 from District 3 back in 2006. She said she assumes that the successful completion of the project will be a dramatic bookend to her career at District 10 when the project is completed.
“I joke that this project is between a rock and a river,” said Magsayo in reference to the sensitive species that occupy the upland area and the federally protected wild and scenic river down below. “Our team has been pushed to the limit of innovation and courageousness, somehow squeezing a project in between these two constricting factors.”
Magsayo credits the fact that team members from both District 10 and Central Region Design have made this project their cause, indicating that some have even postponed retirement to see the project through. Magsayo points to District Director Dan McElhinney’s experience with projects like the Devil’s Slide and Bay Bridge as key to the effort to keep the Ferguson rock shed moving forward despite the complexity of the project.
“This project is going to put Mariposa and El Portal on the map,” said Magsayo. “This will be the longest rock shed in California and only the second one that Caltrans has built.”