Rockfall Protection: Challenges in Design and Installation

“When working with rockfall protection, you don’t do the same thing twice,” says Tim Pfeiffer, senior geotechnical engineer for Foundation Engineering in Portland, OR. And because rockfalls can be triggered by a variety of manmade and natural causes, chances are that no one solution will apply to an entire job.

In addition to careful slope preparation (removing loose rocks and vegetation), successful rockfall protection involves a range of techniques used singly or in combination. For example, draping slopes with wire mesh (galvanized or PVC coated to blend with the terrain), reinforcing the mesh with aircraft cable or cable netting, installing rock bolts, revegetating slopes, and constructing catch basins, dirt berms, retaining walls, and catch fences. The rule of thumb holds that rockfall protection systems that dissipate rather than resist energy are the most cost-effective, efficient, and low maintenance.

Designs for rockfall protection systems must consider rock and soil types, the angle of the slope and conditions on top, and the toe of the affected area. Installation problems, which can be complicated by existing vegetation, access, aesthetics, and environmental issues or regulations, typically fall to the constructor responsible for installing rockfall-protection measures.

Techniques and Materials

The hexagonal wire mesh that is fundamental to slope protection, 0.118-in. galvanized wire or 0.146 in. when PVC coated, with openings measuring 3.25 x 4.5 in., is different from standard gabion wire in a number of aspects. Rockfall mesh is double twisted (one twist more than its chainlink cousin), which prevents stretching and, when properly installed, allows rock under the mesh to move farther down a slope. The double twist also ensures that a cut or a splice won’t cause an entire section of mesh to unravel and compromise the integrity of the remaining installation. Rockfall mesh is manufactured in rolls that are 3-15 ft. wide and up to 300 ft. long, whereas gabions are compartmentalized and manufactured as finished products according to customer specifications. Once draped on the slope, the mesh rolls are woven together with the same gauge wire and held in place by anchors – sometimes a cross-slope cable system, sometimes with tensioned steel bolts installed at the top and down the slope if the mesh is designed to adhere to the contour of the slope.

For anchoring mesh in fissured hard rock that is not subject to deterioration, Maccaferri Gabions Inc. in Williamsport, MD, recommends simple and/or double expansion bolts as anchors, 0.4-0.8 in. in diameter and at 3.3- to 6.6-ft. centers. For compact rock subject to deterioration, the recommendation is a series of 0.5- to 0.9-in.-diameter hooked steel reinforcing bars installed with mortar or resin. In compact soils, 3.3- to 4.9-ft. steel reinforcing bars, 65-100 ft. in diameter, can be driven into the ground and fitted with a hook to which the mesh is attached using a steel rope. In situations where the mesh is applied close to the slope with the goal of preventing rock fragments from falling, anchors the same size as the steel bars typically used for rock bolts, 1-1.25 in. in diameter, are installed every 162-323 ft.2 of covered surface.

Rockfall mesh can be also reinforced by a network of crisscrossed galvanized steel aircraft cables or overlaid with cable netting 0.24 or 0.31 in. in diameter, with openings of 0.5-1 in., the most common being 12 x 12 in. Installation of mesh or netting might require cranes or helicopters, especially on steep slopes or where access is limited. Rockfall mesh and cable netting can be anchored at the bottom or, when the plan is to leave room for debris to flow beneath the mesh, left open so the debris can be caught in a catch fence or a retaining wall at the toe of the slope.

Howard Ingram, president of Hi-Tech Rockfall in Forest Grove, OR, describes the advantage of mesh and cable netting installed together from an installer’s perspective. “Rockfall netting is basically 11-gauge hexagonal mesh wire, whereas cable netting is woven out of either ¼-or 5/16-inch cable,” he explains. “The hexagonal mesh is good for rocks, say, 18 inches to 2 feet in diameter. Once you go beyond that, the mesh is pretty much overmatched. A good slide can rip and tear the mesh, whereas the cable will retain up to 6-foot blocks. We did a project at Salt River Canyon, Arizona, in 1998 in which we hung cable-net drapery and wire mesh on a 0.75:1 slope. A boulder the size of a Chevy Blazer came loose and fell underneath the cable netting for quite a way until the netting retained and stopped it. State crews drilled holes in the rock through the cable net and the wire mesh, loaded up the rock with dynamite, blew it up into small pieces, and let it fall out of the bottom. Everything held.”

Slope Preparation Challenges

Like many rockfall contractors, Ingram is drawn to jobs that present challenges and offer opportunities to innovate. Installation of wire-mesh netting on a 409,000-ft.2, near-vertical slope above Highway 5 in the state of Washington required the removal of a stand of old-growth trees, each 50-60 ft. tall, and an assortment of other vegetation 20-30 ft. tall, without interrupting traffic on the interstate. The immediate challenge was to assemble a temporary barrier to catch the felled trees and the rock and debris that scaled off the slope prior to installing the wire mesh. “In all, we took out some 200-300 trees,” Ingram recalls. “We laid back the top of slope about 60-70 feet to about 0.5:1, leaving the bottom 150 feet to fall almost vertically.” He then used a 50-ft.-wide x 150-ft.-long section of cable-net drapery from Brugg Cable Products Inc. in Vancouver, WA, to catch debris. The drapery was held in place with a 156-ft.-tall crane. To protect motorists, the state department of transportation had originally designed a 150-ft. rockfall catch fence at the bottom of the slope to shield traffic while Ingram’s crews worked. Ingram thought he could do better. “The fence had foundations in the shoulder of the road. The thought was that we would put in new foundations and move the fence as we went along. Instead, we designed a new sliding barrier fence that we were able to slide along on a backhoe as we moved.”

Ingram’s crews tied back the trees, cut them into 15- to 20-ft. pieces, chopped the pieces into smaller sections, and kicked the sections down the slope. The underlying rock varied from solid rock to loose boulders up to 4-5 ft. in diameter. The loose rock was scaled off and some existing 50-year-old mesh was removed. The entire job took 90 days, 20 of it to remove the trees. Ingram says it would have taken longer if he hadn’t used his sliding catch fence, which he estimates allowed his crews to cut slope preparation time in half.

The steepness of the 200-ft.-high site, the fact that crews were working above live traffic, and the amount of the rockfall netting to be applied inspired Ingram to make another innovation. His crews assembled 15-ft.-wide rolls of galvanized rockfall netting (from Terra Aqua Gabions in Reno, NV) into 45-ft.-wide segments in the company yard and hauled them on-site on a specially constructed trailer. This eliminated crews’ having to splice the segments together in midair and actually reduced the number of splices by 75%. The mesh was anchored at the top of the slope with a variety of anchors, depending on soil conditions, including two different sizes of self-drilling E bows, 1.10 and 1.26 in., some with specially designed soil plates attached. “Where we weren’t in solid rock from the top down,” Ingram explains, “we dug down about 1.5 feet and installed a half-inch, 12- by 24-inch-wide steel plate on our anchors to provide a larger bearing surface in the soil and keep the anchors from moving.”

Another of Ingram’s projects involved the installation of Maccaferri light tan PVC-coated mesh together with galvanized Brugg cable net on a 320-ft.-high, 1:1 slope on California Highway 140, west of Yosemite National Park. This time the problem was access. “There was no access from the top of the slope,” he recalls, “and because high-tension wires ran through the middle of the area, we couldn’t use any equipment – a crane or helicopter – to hang the netting. So we winched up the material. We used a series of cables and pulleys and a tractor to slide the material right up the slope. For anchors we used 1-inch steel rods, 7 feet long, which is a little longer than what’s normal. Normally, especially with cable nets, all you need is just a pullout strength, which is 6 tons. Cable net with mesh behind it weighs 1 pound per square inch – about 300 pounds. In this case, the anchors went into solid rock, and with the long length it wasn’t hard to achieve a 6-ton pullout strength.”

Trouble on the Banks of the Rio Grande

For Edward Rector, geotechnical design engineer for the New Mexico Department of Transportation in Santa Fe, NM, aesthetics were a controlling factor in a multifaceted project on a section of state highway that runs next to the Rio Grande between Taos and Santa Fe. “This is a recreational area,” he states. “There is a lot of rafting that goes on in the river adjacent to the highway. The owners of the rafting companies objected to the placement of the catch fences we planned at the bottom of the cliffs to protect the highway from falling rocks. They wanted their clients to see an unspoiled wilderness. But the highway, which is right next to the river, is heavily used by commuters who’ve been exposed to a history of accidents because of falling rocks. We felt we needed to do something.”

Rector and his crews combined a mix of the standard and offbeat. In one section they dug a catch ditch, lined the ditch with used tires, and combined it with a geotextile-covered dirt berm. The idea was inspired by a temporary berm that state crews built to work on another section of roadway. “We originally thought about cable netting,” says Rector, “but there was no way to mesh 800 feet of vertical slope. This was a natural drainage, and the idea of the ditch was to combine a rockfall ditch with a berm to catch the rock.” On the slope side of the 12-ft. berm, Rector’s crews dug a 10-ft. ditch. The tires they used to line the ditch provided a flexible face to absorb energy and helped hold in the dirt. On the highway side, the berm was faced with a segmented concrete block wall. The ditch reduced the height of the enclosure motorists saw as they drove by, from 22 ft. to approximately 12 ft. On other segments of the road where there was a history of rockfalls, crews scaled loose rock, hung wire mesh, or constructed either Brugg rockfall catch fences faced with chainlink or additional rockfall berms covered with geotextile fabric.

“The section where we installed the Brugg catch fence was also 800 feet high,” notes Rector. “The top of the mesa is a basalt flow; parts of it were breaking off and rolling 800 feet onto the highway. There was very little we could do on the upslope portion because we don’t have the right of way and there’s no way you can mesh and keep boulders in place in those areas. We were limited to doing something in our right of way and decided the best thing would be something to catch the boulders as they rolled down.

“We sized the Brugg cable-net fences for the anticipated energy of the falling rocks and placed the fences on the slope where we believed we would catch the most rocks. In some places the fence is about 13 feet high and in other areas it’s about 10 feet. We placed I-beams in the ground at certain intervals – in most cases 20 feet apart. Because the terrain rolled, we had to custom-make the fence sections for each segment. In the first project, we painted the posts and put them fairly close to the bottom of the slope. On the second project, based on community input, we let the posts rust, which looks better, and we moved the fence a little farther up the slope to hide it behind vegetation. We also lowered the total height of the fence to about 10 feet, which looks better but doesn’t affect its function. We also put black PVC-coated chainlink, not wire mesh, behind the Brugg fence. On the first project we used galvanized chainlink.”

To complete the job, in areas where there were cut slopes and erosion was causing boulders to come loose, crews draped wire mesh. The first project was hung with Terra Aqua galvanized wire, the second with black PVC-coated mesh.

Asked about the cost-effectiveness of the project, Rector replies that he’s documented that the cable fences have caught boulders that might otherwise have ended up on the highway. And public acceptance? “It’s hard to know how the people traveling the highway feel because they’re not the ones who come to public hearings, but I believe they feel more secure.”

Down Mexico Way

The flexibility and strength of the wire-net rockfall fences Rector describes allow large amounts of kinetic energy to be dissipated rather than resisted and permit retention of large static loads. Often the most cost-effective in the short term, wire-net catch fences are also inexpensive to maintain. To design the appropriate type of fence, however, the site must be properly analyzed and rockfall conditions evaluated, including translational and rotational kinetic energy and anticipated jumping height of rocks. Wire-net catch fences have been engineered to withstand impact kinetic energies ranging from 30 ft.-tons to 369 ft.-tons. The primary goal is that the actual design load chosen for a site should require little or no repair of the system after repeated impacts, other than cleaning out rocks and debris that the fence collects.

A cable-net rockfall retaining system of this type was installed in a newly opened limestone quarry that supplied a cement factory near Orizaba, a mountainous section of Vera Cruz, Mexico. Rockfalls in the quarry caused residents, whose homes were located approximately 2,625 ft. from the mining operations, to be concerned. At the same time, a 5.18-ton boulder rolled down a mountain at an existing quarry, scraping a bus and causing more public worry. The first phase of the rockfall fence system designed by Brugg engineers in Santa Fe, NM, called for two permanent and one moveable fence (to be repositioned as the mining progressed) to catch boulders. Brugg used the Colorado Department of Highway’s Colorado Rockfall Simulation Program to provide statistical analysis of probable rockfall behavior at the quarry site. The maximum weight of rocks was calculated at 5.18 tons, the slope angle 48-60º, average jumping height of the rocks 3-4 ft., and the average velocity in relation to the maximum height and acceleration due to gravity at 32.85 ft./sec. Installation challenges included heavy jungle vegetation (large tropical trees, including banana and coffee plants and scrub oak), the inaccessibility of the site, and the high cost of transporting materials. Scrap steel railroad rails available at the cement plant were used as support columns for the fence and were finished as posts by plant workers. The final design was a 3,660-ft.-long, 10-ft.-high fence that could absorb 129 ft.-tons of energy.

The area in which the fences were to be installed had to be cleared of vegetation by hand using machetes. Post holes were also dug by hand, with crews using gasoline-powered percussion tools when they ran into rock. Concrete for the steel column foundations used for the permanent fences was mixed by hand, with posts set approximately 4 ft. below the ground. Because of the terrain, sections of the nets had to be angled and were thus individually fabricated. Square metal tubing was used for foundation sleeves for the one movable fence. The project took five months to install using an average of 14 installers and an additional 12 people to transport materials to the site.

Tough Conditions in the US

Considered routine especially in Europe, rockfall protection has slowly gained momentum in the United States. Repeated rockfalls in a residential section of downtown Portland, OR, for example, convinced city crews that the area required some type of long-term stabilization. Tim Pfeiffer says his first look at the job was confined to a small 450-ft. section of highway, but it slowly became obvious that the entire slope, rising to 90 ft. and 120 ft. in some sections above the road, was unstable. This observation was substantiated by a series of rockfalls, some so serious they had closed the road.

“The problem began with the wet winter of 1997,” Pfeiffer remembers. “The moisture caused fairly significant damage, and the city undertook some scaling and cleanup, but when rocks continued to fall and threaten the road, they decided to engineer a repair. We did some research and found out the cut was between 75 and 100 years old and had most likely been coyote blasted – a very small hole was dug at the base of the hill, packed with explosives, and shot all at once. This left the slope in very poor condition. Combine this with 75 or 100 years of weathering and vegetation and what have rocks that were also in very poor condition. Some time before, regular chainlink fence had been laid over the face, but the rock either knocked the fencing down or tore it out, leaving rocks and debris spread across the road. The idea was not to stabilize the slope but to keep the rocks and soil off the highway.”

Based on reconnaissance of the site, Pfeiffer was faced with expanding the project on the fly. “First we decided to cover more area with mesh and net, which meant ordering more material. Because of the vegetation, you couldn’t always see the condition of the rock. This led to the decision to rip off what was growing so we could evaluate the condition of the slope.” The job of scaling and vegetation removal fell to crews from PKO Inc. of Roseburg, OR. They undertook the slow job of tearing out ivy, felling alders 12 in. in diameter, and in cases where remaining root wads hung to the slope, attaching cables and pulling out the roots along with the rocks to which they were attached.

A layer of mesh (Terra Aqua’s double-twisted, hexagonal wire covered with a black PVC coating to blend in with existing rock) was draped in 250-ft.-long rolls and then covered with 12- x 12-in. Brugg cable netting to catch the larger rocks. Top anchors were 6-ft.-long bars grouted into the rock. Neither the netting nor mesh was anchored at the bottom in order to allow debris to collect behind a catch fence installed at road level. “In the past,” says Pfeiffer, “we commonly used only the mesh on a slope like this. In this case, we were looking at a pretty high slope and a larger volume of debris. In my experience the netting has been good for rocks up to a yard or so. But the size and volume of loose rock we left on this slope was very high, and the slope was still in very bad shape, even after the scaling. This caused us to be concerned.”

The catch fence installed by PKO’s crews at the bottom of the slope was a 12-ft.-high, low-capacity Brugg cable net (load limit of 30 ft.-tons). The Brugg cable was combined with regular chainlink to keep smaller rocks and debris from littering the road; fence posts were standard design H-beam sections. “We use Brugg net fences most often where we have very large, wide-open slopes and you can’t really retain the rock itself, but you have to attempt to catch it before it hits the road or damages a structure,” explains Pfeiffer.

Pfeiffer and Jim Pynch, owner of PKO, worked together on another project on State Highway 101 in Oregon where a section of highway is cut into a near-vertical 500-ft. rock face that drops directly to the Pacific Ocean. Draping the 190 ft. of highly visible cliff above the highway with mesh to protect the road from rockfalls was not an option. Instead, Pynch’s crews called in a 250-ton crane equipped with a drill basket and mounted with a track drill to install 30,000 ft. of rock bolts over a half mile of cliff. The concept of rock bolts, which requires that key blocks in a rock face be identified and then stabilized with tensioned steel rods, is that a properly anchored key block will hold surrounding rocks. Pynch’s crews scaled what loose rock they could from the cliff above the road, protecting workers with temporary mesh netting draped on the face, then drilled through strategic rocks to install the steel anchors and grout. “In cases like that, you’re designing as you go,” says Pfeiffer. “You pick a large slab of rock, what you hope is a key block, and go from there.” Once the scaling and the installation of the anchors were complete, Pynch’s crews removed the temporary netting and installed approximately 60,000 ft.2 of Terra Aqua’s galvanized wire mesh.

In Pynch’s long and specialized rockfall career, other challenges have included such logistically difficult jobs as installing rockfall protection for two Tacoma Public Utilities hydropower stations situated deep in La Grande and Cushman canyons and reachable only by aerial tram. “We did all the studies we could possibly do,” says Okezie Imo, senior specialist at the power company, “and came to realize that it wasn’t the ground that was moving; it was the rocks.”

To protect the power houses during construction, Pynch’s crews built a temporary 24-ft.-high, double-layered, 0.75-in. plywood wall at the bottom of the slope. The tram towers also had to be protected during scaling and debris removal, so Pynch constructed 30-ft.-high plywood barriers anchored with two-by-fours and timbers. All materials had to be brought in on the tram and debris hauled out again after scaling was completed. Pynch used concrete anchors for one of the tower barriers, hauling a trailer load of ready-mixed cement on the tram, then transferring the cement by foot across a trail that crews constructed across the slope. “We had to pipe in water,” says Pynch, “then build our forms. We poured about 50 yards of concrete that way.” To help protect against the weathering that had destabilized the slope, French drains were also dug across the slope face.

Finally Pynch installed Terra Aqua galvanized hexagonal mesh at the top of the 0.5:1 slope and drilled rock bolts at the bottom, where a 10-ft. anchored Brugg cable-wire catch fence was also installed. The fence was constructed of cable strung between steel posts, then covered with galvanized mesh, a combination that would protect the buildings from both boulders and debris. The galvanized mesh that Pynch installed on the slope was dropped from the top. The slope was such that the mesh layers overlayed themselves at the top of the slope where the area to be covered was only 75 ft. but splayed out to a width of 300 ft. at the bottom. “We rolled the wire off by hand at the top and then rappelled down the slope with ropes to straighten it out and put it in place,” describes Pynch. The mesh was held with both rock bolts and poured-in-place concrete anchors.

What’s Next?

Are there any changes in store for the rockfal-protection industry? Not much but continued innovation, believes Ingram. “Those of us who work in the industry will continue to experiment with applications of existing materials, and we’ll continue to see new technology, such as ring nets like the kind used during World War II to protect harbors against submarines and torpedoes. Manufacturers have redesigned and come up with machines that can manufacture the nets out of wire, which is stronger than the cable they’re typically made from.”

“I’ve seen changes from state to state in the US with installation,” says Pynch. “In Oregon, we’re told the same thing now as years ago when we started: The top of the wire is anchored tight to the ground using rock bolts. In Washington they try to hold the wire off the ground. They put in a rock bolt anchor then a concrete pad underneath the first down cable in order to hold the top of the wire out. In Tennessee, they put a double row of rock bolts at the top. The bottom bolt they hold a foot off the ground and attach the top cables to that row of bolts, which keeps the top of the wire off the ground. The idea is that the wire will catch any loose rocks that are above.”

Ingram thinks anchor systems are not likely to change except as engineers and contractors experiment with applications. “Anchors are a really sticky part of these rockfall systems. The engineers tell us we have to have a certain pullout strength, but they don’t tell us how to achieve it. It’s left to guys like me to figure out how to get the required strength in a particular soil or rock condition. Customizing…that’s what the business has been and will continue to be about.” 
About the Author

Penelope B. Grenoble

Penelope B. Grenoble writes on issues concerning waste operations, equipment, and technology.