Taming the Slopes
A team of 20 Caltrans staff headed into Angeles National Forest for a series
By Kelly Markham
In many ways, San Gabriel Canyon Road north of Azusa is spectacular. It twists and turns through the chaparral and pines of Angeles National Forest (about 20 miles northeast of Los Angeles), offering up breathtaking vistas and the occasional coyote. The road, also known as State Highway 39, features jagged slopes bent at treacherous angles with boulders that appear poised to 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 safety concerns. For this reason, a six-mile section of the highway has been closed to the public since 1978 due to massive rockslides and mudslides following severe fires and storms. Caltrans District 7 (covering Los Angeles and Ventura counties) wanted to reopen the road in the future, but issues such as the falling-rock menace needed to be addressed first.
That’s precisely the challenge a multidisciplinary Caltrans team undertook in October 2009. The group, laden with gear from lasers to rappelling ropes, bivouacked on the closed portion of Highway 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.”
The rocks that make up the San Gabriel Mountains are millions of years old. The team’s testing technique 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 rockfall specialist Senior Engineering Geologist John Duffy. “Using (those) criteria, we selected four slopes with the highest potential for rockfall, and that’s where we did the testing.”
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 camera crew prepared to shoot by putting four cameras in sync. 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 crew determined the duration of the fall, point of impact, and length of the roll. 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.
The data collected along the highway 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.
In addition, the team discovered that rocks perched above Highway 39 were weaker than anticipated, and had 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 helped to narrow the types of mitigation that would be appropriate.
All of this is tremendously useful information. But the value of the Highway 39 rockfall testing goes beyond the data collected. The expedition also served as a videogrammetry test mission of sorts and 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.”