Challenges in Slope Stabilization
Battling heavy rains and hard-to-reach sites.
When it comes to slope stabilization projects, access is usually one of the most challenging aspects of the job.
So says John Strauss and many others who do slope stabilization on a day-to-day basis. Strauss is a project manager for Moore & Taber, a geotechnical construction contractor for geotechnical engineering firms based in Anaheim, CA.
Because of weather events in the past several years in California, slope stabilization projects have been more numerous—and more difficult. Rains from 2004 and 2005 were nearly triple the normal amount, Strauss notes, and the result was a number of slope failures occurring in areas inaccessible to equipment, such as the back of a residence or an ascending slope from a house. Thus, equipment had to be carried in to the site.
“Slopes typically had a failure either because of an area drain where the yard was a saturating spot, or you had sheet flow water coming over the top,” says Strauss. “Another issue that was prevalent is rodents. Gopher holes in those areas getting saturated with three times as much rain as normal weakens the soil, making it heavier, and the slope washes out.”
 |
Photo: Landscape Development |
One of the ways in which Moore & Taber addresses access issues is by using automatic conveyers—also called miniveyors—which are self-contained aluminum conveyers that come in 12-inch and 15-inch widths and are light and portable.
“They work in unison and can be hooked up four at a time and set up at different elevations,” explains Strauss. “We can do 90-degree turns between two conveyors. They are very versatile and effective.”
Strauss says most slope repair work requires the workers to get a keyway or bench started at the bottom. “If there is no access to get equipment in there and you have to remove that dirt, you have to stockpile it off of the slope before you can get started,” he says. “We use these miniveyors to transport the dirt to stockpile it onsite at another position on the slope. Then we’ll cut out the next bench and build a keyway that we just dug out and flip flop that.”
Angles can prove to be quite a challenge when constructing a slope stabilization project, Strauss points out. His company may be able to get some equipment such as Bobcats or small tractors up 1.5:1 slopes.
Strauss says there have been times when, because of traffic issues, his company has turned down work. “There were a couple of slopes we looked at that were on fairly busy four-lane roads, and we didn’t do them,” he says. “You couldn’t get any equipment in there, so you had to mobilize near the site, and the situation becomes frustrating to people who have to travel those roads.”
Rain during projects can present another monkey wrench. “You have to be quick and have your plastic ready to cover things up,” he says. “Most of our rain is fairly predictable, so when a storm is going to come, we’ll cover the slope up and try to maintain the drainage so it doesn’t saturate the area we’ve got cut out.”
Other challenges can come through slope angles. “We can get some equipment up the slopes that are maybe 1.5:1,” Strauss says. “But if it starts getting much steeper than that, you can’t climb them. We have to transport material with the conveyors, so we have to zigzag them up the hill or transport the material at a certain angle. If it’s too steep, the material would fall right off and won’t stay up, so angle to the slope is definitely the way to go.”
Strauss states that in California, a technical design is needed for slopes steeper than 2:1. “A lot of the older slopes from tracts built in the 1960s and 1970s are anywhere from 2:1 or 1:5:1, and therefore they are now out of code and need to have some new technical evaluation for repair,” he points out.
 |
Photo: Salix Applied Earthcare |
Because the heavy rains from 2004 and 2005 caused many slope problems, most of Moore & Taber’s slope stabilization efforts have been long-term repair projects. Strauss says repairs are done through three methods:
- Using pipe and board. Galvanized pipes measuring 2 to 2.5 inches in diameter are driven 7 to 10 feet into the ground and 2-foot by 12-foot pressure-treated pieces of wood are attached to form a barrier, which is buried in the ground to lend lateral stability.
- Using soil cement. The process involves benching out into the slope and re-compacting the soil with a 7% to 10% mixture of cement: wetting it, mixing it, and packing it to make a hard permanent surface.
- Excavating the soil with the benches and laying down geosynthetic material. The fabric is placed in every 2-foot lift, the soil is recompacted, and then benching out and cutting continues up the slope.
Choices are usually determined by the engineer, Strauss says. “Some of the engineers are more comfortable with one method and tend to recommend that,” Strauss says. “Some tend to stick to soil cement, others with pipe and board, but depending on the site and soil conditions, we may use a combination.”
As for revegetation, Strauss notes that many engineers are specifying low-water-requirement plants that root like ivy, dig into a hill, and cover it well with the ability to grow throughout the seasons. Some engineers opt to use soil cement when reconstructing a hill, creating a hard surface, he adds. “You’re not going to plant anything, but you can put that fill in some planter boxes or some other means so you can put potting soil and plants in afterwards,” Strauss says.
His company usually installs jute matting on the soil surface and covers it with plants, wood bark, or wood chips to create an attractive presentation, Strauss says.
He says the average duration of soil stabilization projects is two to four weeks—some can last a few months—and costs can range from $15,000 to more than $100,000, with most averaging $25,000 to $40,000.
“I applaud our crews for their skill, because there definitely is an art to how they come out and are finished to look nice,” Strauss says. “They look pretty bad when you start, and it gives homeowners peace of mind that their home is in good shape afterward. The slopes are solid and visually appealing as well.”
Moving Equipment to the Site
Access also was an issue in a slope stabilization project undertaken by Eric Woodhouse’s company, which was involved in remediating slopes following the 2005 weather events in southern California.
 |
Photo: Salix Applied Earthcare |
“We had slope failure everywhere,” notes Woodhouse, the Los Angeles division president of Earth Services for Landscape Development in Valencia, CA. He adds that some 40 inches of rain saturated an area that normally receives an average of 16 inches.
In one case, an 18-inch natural gas pipeline was sheared in January 2005 when 150,000 cubic yards of material moved after a hillside through which the 30-mile pipeline had become saturated with water. Eroding material sheared off the line, starting a fire that burned 200 acres near Piru, 20 miles west of Santa Clarita, CA.
The rains also helped put out the fire, and the pipeline was repaired. After the structural integrity of the area was restored, Landscape Development came in to do its work.
Woodhouse says the area in which his company was working is rural and mountainous, with the only access being on dirt roads. The only option for moving the hydromulching machinery was to hook it up to a Caterpillar D6 with chains on the back and lower the truck down to another level, Woodhouse says.
All of the company’s hydroseeding machines come on a rolloff skid so employees can set them down on the ground for loading. In this instance, workers set a unit down on the ground, tying it to a bulldozer and taking it down about a half-mile grade of 2:1 slope.
Then the 6x6 all-wheel drive 2,000-gallon water truck was lowered down and water was pumped down to the machine before it was pulled back up again, Woodhouse says.
“That was the major challenge on that project,” says Woodhouse. “It was one of the biggest and more technically difficult revegetation and repair remediation projects we did.”
 |
Photo: Landscape Development |
Once the site had been stabilized after the slope failure, Landscape Development had started by executing diversionary and surficial stabilization by using blankets, wattles, straw bales, and berming; constructing deeper swales on steep hillside areas; and hydroseeding the entire area with a native mix. Landscape Development used blankets as the most economical approach, based on the client’s budget, Woodhouse notes.
Knowing that flows would be more concentrated in the steeper areas and that some areas would require lining, the company terraced the slope between 10- and 20-foot elevation levels, Woodhouse says. A small spillway was created in each terrace, and the spillways were lined from terrace to terrace with coir blankets to prevent scour from runoff.
“There was a huge flat area up at the top of the hilly area where the pipe sheared off, so the water was flowing to two areas: down a steep section and then down another area that was like a 2:1 slope, and another area that was close to 1:1 and 1.5:1 in gradient,” Woodhouse says.
 |
Photo: Landscape Development |
Revegetation was one of the key elements in stabilizing the entire site, he says. “Our recommendations had been to hydroseed and blanket, but their budget constraints said no, so we had to engineer a seed mix that would give us a quick cover crop, along with chaparral and other natives.” That seed mix included buckwheat, native grasses, and some brome grasses for the initial cover crop. Sage and poppies were also included.
In choosing the seed mix, Woodhouse says, the company considered the “chaparral, brushy, woody type of cover on the hillsides. The concern was that those were going to take two to five years to establish and grow to become an effective erosion consideration. So we needed a cover crop, and that’s where we brought in the other grasses, some broadleaf, some lupine, and other things native to that area.”
Also during the project, Landscape Development introduced elements back into the soil in addition to the fertilizers, such as mycorrhizae and a vitamin B accelerator. The company also put sulfur back into the soil. “Given the conditions, it would have been too hard to introduce anything else to condition the soil,” Woodhouse says.
The slope stabilization was completed in three weeks of work over a two-month period during phases of the overall project, at a cost of $135,000.
Woodhouse drove past the area in the spring following the job and discovered the mix was “actually very pretty because we had different wildflowers and poppies and different colors: reds, yellows, oranges, and pinks. We had a full cover of grasses, so it turned out really well.”
Wire Mesh for a Green Wall
Steep slopes also presented challenges in a project executed by Toby Hutchins’ company, Carolina Mulch Plus in Asheville, NC. In the summer of 2006, Hutchins’ company was working on a two-month slope stabilization project that resulted from degraded rock in a housing development near the Blowing Rock area that is adjacent to about 50 miles of roadway.
The steepness of the terrain presented the biggest challenge. Hutchins estimates the slope to be 0.25:1—nearly a vertical wall. Lifts and baskets were used to transport workers up the slopes to work, he notes.
“We are revegetating a rock slope and trying to hold that rock in place,” he describes. “We’re working on a quarter-to-one slope, and the rock won’t stay on that slope, so we’re putting a Maccaferri wire mesh on the slope, drilling and putting anchors into the rock.”
 |
Photo: Salix Applied Earthcare |
Six inches out from the slope, the company installed another layer of wire mesh with Colbond Enkamat fastened to the back. Between the rock face and last layer of wire, the company filled in with compost and seed, creating a green wall.
Hutchins says with the combination of the Enkamat and wire mesh, “I don’t see where we’re going to have any erosion at all. The Colbond material has done an excellent job of holding the compost in place.”
In the revegetation efforts on the project, Carolina Mulch Plus used a combination of creeping red fescue and native seeds that produce low-growing, woody-type plants. “Once this grows back in, it should have very little maintenance to do to it at all,” Hutchins says. “This is more of an aesthetic situation where they had to go in and drill and bolt this rock in order to keep it from sliding off into the road. Through that process, they weren’t happy with the way the wire mesh looked against the rock.
“The other alternative was to go in and shotcrete it with a cement-type product they’d blow on the rock face,” he adds. “The developer is really conscientious about having things look natural, and that’s one of the reasons we decided to go with this process.”
Staying Ahead of the Rains
John Gentillon, CEO of San Diego Erosion Control and Storm Water Compliance Specialists in San Marcos, CA, says one of the most challenging slope stabilization jobs his company performed occurred several years ago on a residential development. The development was situated atop a knoll with two blue-line streams on either side; the drainage off the slopes went into the streams.
Gentillon’s company did an application on the major slopes, bed slopes with terraces that were about six or seven tiers in slope length.
“These were very large slopes and I knew it was going to be tough for us to do maintenance on them,” he says. “If we did have any large rain interval storms, there would probably be some issues polluting the creek.”
His company put in brow ditches or bed streams to force the water between the slopes to take it to where it was designed by the civil engineering, Gentillon says. The bare slopes then had to be protected, as the area was entering a rainy season.
Gentillon recommended to the developer that the prudent route would be to use an application of EarthGuard Stabilized Fiber Matrix based on 3,000 pounds of mulch per acre and 10 gallons of the EarthGuard product. His company then ran a double-sided Greenfix straw blanket over the top and another blanket to manage higher flows concentrated in the slopes. Gentillon estimates he used 1.5 million square feet of product for the project.
The project was completed in time for the beginning of the rainy season. An interval storm brought 5 inches of rain in 24 hours at the site.
“My idea worked,” Gentillon says. “None of the bench drains had any sediment material deposited in them, so my idea of spraying them with the EarthGuard product with an SFM-type blanket, then doing the blanket over the top for double-duty, saved the developer from having any issues with polluting the blue-line stream.”
Bioengineering Techniques
John McCullah, a watershed geologist and a certified professional in erosion and sediment control (CPESC), owns Salix Applied Earthcare in Redding, CA. The company specializes in landslide repair and training.
Three years ago, his company trained local Native Americans to use modified brush layers and live pole drains to address a slope stabilization effort on US Forest Service land in Usk, WA, north of Spokane and near the Idaho border.
Restoration of LeClerc Creek—designated as a bull trout stream to protect the threatened species—had been under way, but one of the biggest threats to the creek had been an old logging road.
The Forest Service had built a new logging road with a new access up on a ridge, several miles long and near the creek. Plans were in place to remove the old logging road, but when workers got to one end of it, they found a large area that had been bare, with erosion and landsliding right into LeClerc Creek.
“The slide was dumping onto part of that old logging road, and they couldn’t remove the logging road until they fixed the landslide,” McCullah explains.
A local Native American tribe, the Kalispell, was under contract with the Forest Service to do some of the work, so McCullah was hired to come in and teach them some of the techniques he uses. While heavy equipment was available to help address the problem, bioengineering techniques were necessary, McCullah says. Those techniques included live pole drains, modified brush layers, and a vegetative rock toe.
The live pole drain is a biotechnical and reclamation technique akin to a subsurface drain that drains moisture away from an unstable site, McCullah says. “On these landslides, gullies form after a significant amount of runoff so water collects in the bottom of the gully,” McCullah says. “It runs down unimpeded and continues to gully.”
McCullah says live pole drains wouldn’t work in large gullies where there are highly concentrated flows. “These gullies are a foot deep and a foot or two wide. There are occasional ephemeral gullies. You put a bundle of willow in and put soil over them,” he explains. “The willow grows. There’s evidence of these things that are seven to nine years old where during a big storm you’ll see the willow bushes growing up in the middle of this gully.
“During the storm, you’ll go to the bottom of the gully and see water weeping out from the bottom of the bushes. After four years, you can’t even see the gully anymore because the willows have completely covered the gully. It’s a good way to treat landslide gullies, especially before you repair the rest of the landslide.”
McCullah compares modified brush layers to “a bunch of little check dams that look like little eyebrows that go across the slope and break up the slope length.
“They slow the velocity of the overland flow of water. You build a bunch of these little catches that catch soil and water and allow growth. I think of them like a Pachinko board, a Japanese pinball machine.”
At the site, McCullah and the workers were dealing with slopes steeper than 1:1. “There were 100-foot slopes above the road, and below the road slopes about 60 feet down to LeClerc Creek,” he says. “The road had actually been built by filling in. It was a large fill slope that was filled into the creek. When the road got removed, it was an opportunity to remove a lot of the road fill and restore the creek to its original width.”
The first challenge on the project was stabilizing the landslide, which was north-facing and consisted of decomposed granite soil, McCullah says.
He says another alternative would have been to build some type of retaining wall, but the slope above the road was so steep, “It was beyond its angle of repose. It was native soil but it was so steep and eroding. There was no chance for vegetation to get established.”
While a pine tree could be viewed “here and there,” the area was bare, with eroding soil and large gullies.
“Of interest is that the Forest Service did go in there about four years prior and built a bunch of traditional check dams in these gullies,” McCullah says. “They used pressure-treated logs and built a whole series of check dams that either collapsed or filled up and actually caused more gullying as water went around the check dams.”
In doing the work, McCullah and the tribal workers had to wait for the right season. “Bioengineering is best done as late in the fall as possible,” he says. “Another challenge was they were holding up some of the road removal until this landslide was stabilized.”
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Salix’s bioengineering techniques took relatively little time. McCullah conducted training over three days. The pole drains and modified brush layers took about three days to construct with a crew of four. A heavy-equipment excavator came in with rock and the crew built a 4-foot-high vegetative rock toe over two days. Over the course of another two days, crews planted willow pole plantings through the rock while it was being constructed. A year later, the road was removed.
A large rain event resulted in material coming down from a landslide and collecting behind the rock toe. The rock toe had stabilized the slope, willows grew, and there was no more transportation of material to the creek.
Author's Bio: Carol Brzozowski is a journalist living in Coral Springs, FL.
January-February 2007
Challenges in Slope Stabilization
Battling heavy rains and hard-to-reach sites.
When it comes to slope stabilization projects, access is usually one of the most challenging aspects of the job.
So says John Strauss and many others who do slope stabilization on a day-to-day basis. Strauss is a project manager for Moore & Taber, a geotechnical construction contractor for geotechnical engineering firms based in Anaheim, CA.
Because of weather events in the past several years in California, slope stabilization projects have been more numerous—and more difficult. Rains from 2004 and 2005 were nearly triple the normal amount, Strauss notes, and the result was a number of slope failures occurring in areas inaccessible to equipment, such as the back of a residence or an ascending slope from a house. Thus, equipment had to be carried in to the site.
“Slopes typically had a failure either because of an area drain where the yard was a saturating spot, or you had sheet flow water coming over the top,” says Strauss. “Another issue that was prevalent is rodents. Gopher holes in those areas getting saturated with three times as much rain as normal weakens the soil, making it heavier, and the slope washes out.”
|
Photo: Landscape Development |
One of the ways in which Moore & Taber addresses access issues is by using automatic conveyers—also called miniveyors—which are self-contained aluminum conveyers that come in 12-inch and 15-inch widths and are light and portable.
“They work in unison and can be hooked up four at a time and set up at different elevations,” explains Strauss. “We can do 90-degree turns between two conveyors. They are very versatile and effective.”
Strauss says most slope repair work requires the workers to get a keyway or bench started at the bottom. “If there is no access to get equipment in there and you have to remove that dirt, you have to stockpile it off of the slope before you can get started,” he says. “We use these miniveyors to transport the dirt to stockpile it onsite at another position on the slope. Then we’ll cut out the next bench and build a keyway that we just dug out and flip flop that.”
Angles can prove to be quite a challenge when constructing a slope stabilization project, Strauss points out. His company may be able to get some equipment such as Bobcats or small tractors up 1.5:1 slopes.
Strauss says there have been times when, because of traffic issues, his company has turned down work. “There were a couple of slopes we looked at that were on fairly busy four-lane roads, and we didn’t do them,” he says. “You couldn’t get any equipment in there, so you had to mobilize near the site, and the situation becomes frustrating to people who have to travel those roads.”
Rain during projects can present another monkey wrench. “You have to be quick and have your plastic ready to cover things up,” he says. “Most of our rain is fairly predictable, so when a storm is going to come, we’ll cover the slope up and try to maintain the drainage so it doesn’t saturate the area we’ve got cut out.”
Other challenges can come through slope angles. “We can get some equipment up the slopes that are maybe 1.5:1,” Strauss says. “But if it starts getting much steeper than that, you can’t climb them. We have to transport material with the conveyors, so we have to zigzag them up the hill or transport the material at a certain angle. If it’s too steep, the material would fall right off and won’t stay up, so angle to the slope is definitely the way to go.”
Strauss states that in California, a technical design is needed for slopes steeper than 2:1. “A lot of the older slopes from tracts built in the 1960s and 1970s are anywhere from 2:1 or 1:5:1, and therefore they are now out of code and need to have some new technical evaluation for repair,” he points out.
|
Photo: Salix Applied Earthcare |
Because the heavy rains from 2004 and 2005 caused many slope problems, most of Moore & Taber’s slope stabilization efforts have been long-term repair projects. Strauss says repairs are done through three methods:
- Using pipe and board. Galvanized pipes measuring 2 to 2.5 inches in diameter are driven 7 to 10 feet into the ground and 2-foot by 12-foot pressure-treated pieces of wood are attached to form a barrier, which is buried in the ground to lend lateral stability.
- Using soil cement. The process involves benching out into the slope and re-compacting the soil with a 7% to 10% mixture of cement: wetting it, mixing it, and packing it to make a hard permanent surface.
- Excavating the soil with the benches and laying down geosynthetic material. The fabric is placed in every 2-foot lift, the soil is recompacted, and then benching out and cutting continues up the slope.
Choices are usually determined by the engineer, Strauss says. “Some of the engineers are more comfortable with one method and tend to recommend that,” Strauss says. “Some tend to stick to soil cement, others with pipe and board, but depending on the site and soil conditions, we may use a combination.”
As for revegetation, Strauss notes that many engineers are specifying low-water-requirement plants that root like ivy, dig into a hill, and cover it well with the ability to grow throughout the seasons. Some engineers opt to use soil cement when reconstructing a hill, creating a hard surface, he adds. “You’re not going to plant anything, but you can put that fill in some planter boxes or some other means so you can put potting soil and plants in afterwards,” Strauss says.
His company usually installs jute matting on the soil surface and covers it with plants, wood bark, or wood chips to create an attractive presentation, Strauss says.
He says the average duration of soil stabilization projects is two to four weeks—some can last a few months—and costs can range from $15,000 to more than $100,000, with most averaging $25,000 to $40,000.
“I applaud our crews for their skill, because there definitely is an art to how they come out and are finished to look nice,” Strauss says. “They look pretty bad when you start, and it gives homeowners peace of mind that their home is in good shape afterward. The slopes are solid and visually appealing as well.”
Moving Equipment to the Site
Access also was an issue in a slope stabilization project undertaken by Eric Woodhouse’s company, which was involved in remediating slopes following the 2005 weather events in southern California.
|
Photo: Salix Applied Earthcare |
“We had slope failure everywhere,” notes Woodhouse, the Los Angeles division president of Earth Services for Landscape Development in Valencia, CA. He adds that some 40 inches of rain saturated an area that normally receives an average of 16 inches.
In one case, an 18-inch natural gas pipeline was sheared in January 2005 when 150,000 cubic yards of material moved after a hillside through which the 30-mile pipeline had become saturated with water. Eroding material sheared off the line, starting a fire that burned 200 acres near Piru, 20 miles west of Santa Clarita, CA.
The rains also helped put out the fire, and the pipeline was repaired. After the structural integrity of the area was restored, Landscape Development came in to do its work.
Woodhouse says the area in which his company was working is rural and mountainous, with the only access being on dirt roads. The only option for moving the hydromulching machinery was to hook it up to a Caterpillar D6 with chains on the back and lower the truck down to another level, Woodhouse says.
All of the company’s hydroseeding machines come on a rolloff skid so employees can set them down on the ground for loading. In this instance, workers set a unit down on the ground, tying it to a bulldozer and taking it down about a half-mile grade of 2:1 slope.
Then the 6x6 all-wheel drive 2,000-gallon water truck was lowered down and water was pumped down to the machine before it was pulled back up again, Woodhouse says.
“That was the major challenge on that project,” says Woodhouse. “It was one of the biggest and more technically difficult revegetation and repair remediation projects we did.”
|
Photo: Landscape Development |
Once the site had been stabilized after the slope failure, Landscape Development had started by executing diversionary and surficial stabilization by using blankets, wattles, straw bales, and berming; constructing deeper swales on steep hillside areas; and hydroseeding the entire area with a native mix. Landscape Development used blankets as the most economical approach, based on the client’s budget, Woodhouse notes.
Knowing that flows would be more concentrated in the steeper areas and that some areas would require lining, the company terraced the slope between 10- and 20-foot elevation levels, Woodhouse says. A small spillway was created in each terrace, and the spillways were lined from terrace to terrace with coir blankets to prevent scour from runoff.
“There was a huge flat area up at the top of the hilly area where the pipe sheared off, so the water was flowing to two areas: down a steep section and then down another area that was like a 2:1 slope, and another area that was close to 1:1 and 1.5:1 in gradient,” Woodhouse says.
|
Photo: Landscape Development |
Revegetation was one of the key elements in stabilizing the entire site, he says. “Our recommendations had been to hydroseed and blanket, but their budget constraints said no, so we had to engineer a seed mix that would give us a quick cover crop, along with chaparral and other natives.” That seed mix included buckwheat, native grasses, and some brome grasses for the initial cover crop. Sage and poppies were also included.
In choosing the seed mix, Woodhouse says, the company considered the “chaparral, brushy, woody type of cover on the hillsides. The concern was that those were going to take two to five years to establish and grow to become an effective erosion consideration. So we needed a cover crop, and that’s where we brought in the other grasses, some broadleaf, some lupine, and other things native to that area.”
Also during the project, Landscape Development introduced elements back into the soil in addition to the fertilizers, such as mycorrhizae and a vitamin B accelerator. The company also put sulfur back into the soil. “Given the conditions, it would have been too hard to introduce anything else to condition the soil,” Woodhouse says.
The slope stabilization was completed in three weeks of work over a two-month period during phases of the overall project, at a cost of $135,000.
Woodhouse drove past the area in the spring following the job and discovered the mix was “actually very pretty because we had different wildflowers and poppies and different colors: reds, yellows, oranges, and pinks. We had a full cover of grasses, so it turned out really well.”
Wire Mesh for a Green Wall
Steep slopes also presented challenges in a project executed by Toby Hutchins’ company, Carolina Mulch Plus in Asheville, NC. In the summer of 2006, Hutchins’ company was working on a two-month slope stabilization project that resulted from degraded rock in a housing development near the Blowing Rock area that is adjacent to about 50 miles of roadway.
The steepness of the terrain presented the biggest challenge. Hutchins estimates the slope to be 0.25:1—nearly a vertical wall. Lifts and baskets were used to transport workers up the slopes to work, he notes.
“We are revegetating a rock slope and trying to hold that rock in place,” he describes. “We’re working on a quarter-to-one slope, and the rock won’t stay on that slope, so we’re putting a Maccaferri wire mesh on the slope, drilling and putting anchors into the rock.”
|
Photo: Salix Applied Earthcare |
Six inches out from the slope, the company installed another layer of wire mesh with Colbond Enkamat fastened to the back. Between the rock face and last layer of wire, the company filled in with compost and seed, creating a green wall.
Hutchins says with the combination of the Enkamat and wire mesh, “I don’t see where we’re going to have any erosion at all. The Colbond material has done an excellent job of holding the compost in place.”
In the revegetation efforts on the project, Carolina Mulch Plus used a combination of creeping red fescue and native seeds that produce low-growing, woody-type plants. “Once this grows back in, it should have very little maintenance to do to it at all,” Hutchins says. “This is more of an aesthetic situation where they had to go in and drill and bolt this rock in order to keep it from sliding off into the road. Through that process, they weren’t happy with the way the wire mesh looked against the rock.
“The other alternative was to go in and shotcrete it with a cement-type product they’d blow on the rock face,” he adds. “The developer is really conscientious about having things look natural, and that’s one of the reasons we decided to go with this process.”
Staying Ahead of the Rains
John Gentillon, CEO of San Diego Erosion Control and Storm Water Compliance Specialists in San Marcos, CA, says one of the most challenging slope stabilization jobs his company performed occurred several years ago on a residential development. The development was situated atop a knoll with two blue-line streams on either side; the drainage off the slopes went into the streams.
Gentillon’s company did an application on the major slopes, bed slopes with terraces that were about six or seven tiers in slope length.
“These were very large slopes and I knew it was going to be tough for us to do maintenance on them,” he says. “If we did have any large rain interval storms, there would probably be some issues polluting the creek.”
His company put in brow ditches or bed streams to force the water between the slopes to take it to where it was designed by the civil engineering, Gentillon says. The bare slopes then had to be protected, as the area was entering a rainy season.
Gentillon recommended to the developer that the prudent route would be to use an application of EarthGuard Stabilized Fiber Matrix based on 3,000 pounds of mulch per acre and 10 gallons of the EarthGuard product. His company then ran a double-sided Greenfix straw blanket over the top and another blanket to manage higher flows concentrated in the slopes. Gentillon estimates he used 1.5 million square feet of product for the project.
The project was completed in time for the beginning of the rainy season. An interval storm brought 5 inches of rain in 24 hours at the site.
“My idea worked,” Gentillon says. “None of the bench drains had any sediment material deposited in them, so my idea of spraying them with the EarthGuard product with an SFM-type blanket, then doing the blanket over the top for double-duty, saved the developer from having any issues with polluting the blue-line stream.”
Bioengineering Techniques
John McCullah, a watershed geologist and a certified professional in erosion and sediment control (CPESC), owns Salix Applied Earthcare in Redding, CA. The company specializes in landslide repair and training.
Three years ago, his company trained local Native Americans to use modified brush layers and live pole drains to address a slope stabilization effort on US Forest Service land in Usk, WA, north of Spokane and near the Idaho border.
Restoration of LeClerc Creek—designated as a bull trout stream to protect the threatened species—had been under way, but one of the biggest threats to the creek had been an old logging road.
The Forest Service had built a new logging road with a new access up on a ridge, several miles long and near the creek. Plans were in place to remove the old logging road, but when workers got to one end of it, they found a large area that had been bare, with erosion and landsliding right into LeClerc Creek.
“The slide was dumping onto part of that old logging road, and they couldn’t remove the logging road until they fixed the landslide,” McCullah explains.
A local Native American tribe, the Kalispell, was under contract with the Forest Service to do some of the work, so McCullah was hired to come in and teach them some of the techniques he uses. While heavy equipment was available to help address the problem, bioengineering techniques were necessary, McCullah says. Those techniques included live pole drains, modified brush layers, and a vegetative rock toe.
The live pole drain is a biotechnical and reclamation technique akin to a subsurface drain that drains moisture away from an unstable site, McCullah says. “On these landslides, gullies form after a significant amount of runoff so water collects in the bottom of the gully,” McCullah says. “It runs down unimpeded and continues to gully.”
McCullah says live pole drains wouldn’t work in large gullies where there are highly concentrated flows. “These gullies are a foot deep and a foot or two wide. There are occasional ephemeral gullies. You put a bundle of willow in and put soil over them,” he explains. “The willow grows. There’s evidence of these things that are seven to nine years old where during a big storm you’ll see the willow bushes growing up in the middle of this gully.
“During the storm, you’ll go to the bottom of the gully and see water weeping out from the bottom of the bushes. After four years, you can’t even see the gully anymore because the willows have completely covered the gully. It’s a good way to treat landslide gullies, especially before you repair the rest of the landslide.”
McCullah compares modified brush layers to “a bunch of little check dams that look like little eyebrows that go across the slope and break up the slope length.
“They slow the velocity of the overland flow of water. You build a bunch of these little catches that catch soil and water and allow growth. I think of them like a Pachinko board, a Japanese pinball machine.”
At the site, McCullah and the workers were dealing with slopes steeper than 1:1. “There were 100-foot slopes above the road, and below the road slopes about 60 feet down to LeClerc Creek,” he says. “The road had actually been built by filling in. It was a large fill slope that was filled into the creek. When the road got removed, it was an opportunity to remove a lot of the road fill and restore the creek to its original width.”
The first challenge on the project was stabilizing the landslide, which was north-facing and consisted of decomposed granite soil, McCullah says.
He says another alternative would have been to build some type of retaining wall, but the slope above the road was so steep, “It was beyond its angle of repose. It was native soil but it was so steep and eroding. There was no chance for vegetation to get established.”
While a pine tree could be viewed “here and there,” the area was bare, with eroding soil and large gullies.
“Of interest is that the Forest Service did go in there about four years prior and built a bunch of traditional check dams in these gullies,” McCullah says. “They used pressure-treated logs and built a whole series of check dams that either collapsed or filled up and actually caused more gullying as water went around the check dams.”
In doing the work, McCullah and the tribal workers had to wait for the right season. “Bioengineering is best done as late in the fall as possible,” he says. “Another challenge was they were holding up some of the road removal until this landslide was stabilized.”
Salix’s bioengineering techniques took relatively little time. McCullah conducted training over three days. The pole drains and modified brush layers took about three days to construct with a crew of four. A heavy-equipment excavator came in with rock and the crew built a 4-foot-high vegetative rock toe over two days. Over the course of another two days, crews planted willow pole plantings through the rock while it was being constructed. A year later, the road was removed.
A large rain event resulted in material coming down from a landslide and collecting behind the rock toe. The rock toe had stabilized the slope, willows grew, and there was no more transportation of material to the creek.