Taming the Slopes: How an Intrepid Caltrans Expedition Took On the Rockfall Peril
A team of 20 Caltrans staff headed into Angeles National Forest for a series of groundbreaking rockfall tests. Turns out, the rocks had a little surprise for them.
In many ways, San Gabriel Canyon Road (SR-39) north of Azusa is spectacular. It twists and turns through the chaparral and pines of Angeles National Forest offering up breathtaking vistas and the occasional coyote. The road also features jagged slopes bent at treacherous angles with boulders that appear poised for plummet. Sometimes, geology and gravity conspire to send those boulders plunging, leaving a trail of airborne dirt and stone in their wake and landing with a dusty crash on the pavement below.
Needless to say, large rocks hurtling onto the highway present some safety concerns. For this reason, a six-mile section of SR-39 has been closed to the public since 1978 due to massive rockslides and mudslides following severe fires and storms. Caltrans would like to reopen the road in the future, but the falling-rock menace (and some other issues) must be addressed first.
That’s precisely the challenge a multidisciplinary Caltrans team took on in October. The group, laden with everything from lasers to rappelling ropes, bivouacked on the closed portion of SR-39 during an intense and unprecedented four-day rockfall testing expedition. About 20 people participated, including geotechnical staff, climbers, surveyors, a maintenance crew, and the District 7 video team.
Rockfall testing is exactly what it sounds like – testing how rocks fall. The idea is that if we know how rocks fall, if we understand rock behavior, we can develop mitigation strategies to protect motorists. To do that, we have to analyze the slope, the properties of rocks, the height of the rocks’ bounce, roll velocities and other complex variables that are likely to confuse anyone who doesn’t have a solid grasp of advanced physics. But the value is perfectly clear: safer roads.
“Caltrans needs to understand what the rockfall risk is so we know how to manage it,” explained Engineering Geologist Michael Salisbury, the project leader. “A vendor might try to sell us expensive high-energy barriers, but that doesn’t mean we need them. Rockfall testing helps us analyze the real risk.”
Videogrammetry – the New Tool on the Rockfall Scene
The rocks that make up the San Gabriel Mountains are millions of years old. The technique the team used to test them is a more recent development – one so new that Caltrans has never used it before. It’s called videogrammetry, and it determines the three dimensional coordinates of an object – in this case, a falling rock – by using two or more video images taken from different angles. The advantage of using videogrammetry over traditional techniques is that it provides more data and better accuracy.
But this simple explanation of videogrammetry doesn’t begin to capture the practical challenges involved. Rolling rocks down a mountainside isn’t as easy as it sounds. It requires piles of equipment, plenty of patience, and at least a couple of people willing to ascend jagged cliffs and pry boulders loose.
The first step in any rockfall test is selecting the slopes to be tested. “The Federal Highway Administration has a rockfall hazard rating system,” said Senior Engineering Geologist John Duffy, a rockfall specialist. “Using that criteria, we selected four slopes with the highest potential for rockfall, and that’s where we did the testing.”
Upon arriving at the first slope, climbers scaled the mountainside to set calibration points by painting them directly on the slope. They also selected and labeled the rocks to be rolled, choosing a variety of sizes and shapes. The video crew synched up four cameras and prepared to record. The survey team created a scan of the slope using a laser radar system to assist with image calibration. And then it was rock-rolling time.
Fifteen to 18 rocks per slope – weighing 300 to 3,000 pounds – were measured, dislodged and rolled while being filmed. The duration of the fall, point of impact, and length of the roll were determined. The rocks were weighed, the ground crew cleaned up the post-test mess, and then the entire operation moved on to the next slope to repeat the process.
Low-Velocity Rocks and Other Results
The data collected on SR-39 is still being analyzed, but the expedition itself revealed some unanticipated – and edifying – rock behavior.
“We rolled some pretty heavy rocks, and we just didn’t get the velocities we expected,” said Salisbury. “The terrain of the slope was more important than the size of the rocks. The big rocks often landed where the little ones did.”
This is good news. Not only does it mean a lower rockfall risk, it also means lower mitigation costs. Rather than purchasing expensive barriers designed for big, fast rocks, more affordable rockfall nets made of steel cable should do the trick.
Additionally, the team discovered that rocks perched above SR-39 are weaker than anticipated, and have a tendency to fracture. During some tests, the falling rock exploded into smaller rocks before coming to rest on the ground in a rain of stones. This observation is important because it helps further narrow down the types of mitigation that would be appropriate.
All of this is tremendously useful information. But the value of the SR-39 rockfall testing goes beyond the data collected. The expedition also served as a videogrammetry test mission of sorts that answered the question, “Can we make this work?” It’s a credit to the creativity and commitment of the testing team that the unequivocal answer to that question is yes.
“This testing helped us develop an effective new field technique. Eventually, we should be able to do the same thing with a smaller team throughout the state,” said Duffy. “There are 10,000 miles of roadway in California with a potential for rockfall. So there’s a real cause-effect relationship here. This kind of testing helps save lives.”