Slope Stabilization
Harnessing the best of nature and technology.
It might be difficult to improve upon the works of nature, but today’s slope stabilization experts are proving they can do exactly that as a blend of human ingenuity and bold techniques are producing results that are as aesthetically extraordinary as they are effective.
Some techniques incorporate the best features of nature, including a heavy reliance on vegetation to camouflage and complement hard structures. Yet, in Weehawken, NJ, it is the hard structure itself that has won praise for preserving a natural landmark loved by the community.
King’s Bluff, a 190-foot rock slope, required stabilization as part of a project extending the Hudson-Bergen Light Rail line into Weehawken. The work posed a number of unusual challenges, including a compressed time schedule and additional safety concerns created by the need for crews to work below the slope even as stabilization work was under way. However, the most daunting goal may have been the need to satisfy the concerns of Mayor Richard Turner, members of the Palisades Preservation Committee, and a wary public—all of whom were determined that the bluff’s aesthetic features would not be affected by the stabilization work.
“People have been looking at that slope for hundreds of years, and they want future generations to have the same view. But rock does weather and change, so you have to work with that. You have to come up with solutions if you want to preserve a slope, and there’s not a textbook with those solutions,” says Daniel Journeaux, president of Janod Inc., headquartered in Swanton, VT. “The original designs called for pattern bolting and wire-rope nets to stabilize and contain loose material, but it didn’t take historic and aesthetic concerns into consideration.”
He continues, “This was a very high-profile project from start to finish. We had reporters, the mayor, and the preservation committee watching us. People with binoculars were watching from the other side of the street as we worked. People were filming us. The people from OSHA were there. Even people in Manhattan were watching the project. There were just a lot of people interested in what we were doing throughout the entire process.”
One of a Kind
Journeaux has worked on hundreds of stabilization projects for highways, railways, mines, and quarries, but he considers the King’s Bluff project to be in a class of its own.
“It is art on a grand scale,” says Journeaux, whose focus has been to develop and improve techniques and equipment used in rock stabilization projects with an emphasis on cost efficiency.
Janod was involved in a slope stabilization project involving a pedestrian walkway and elevator near King’s Bluff when city officials sought the advice of the firm and its strategic partner, Golder Associates. The mayor had halted work at King’s Bluff because a particularly beloved portion of the slope, known as the Gorilla’s Head, was scheduled for removal before rock anchors and wire net were installed. “That was simply unacceptable to the mayor and his constituents,” Journeaux explains.
Janod representatives persuaded the city that they had a better approach, and the city responded with a design-build contract. The arrangement allowed the design to proceed even as work was performed.
“Time was of the essence,” Journeaux explains. “They were already delayed, so there was a cost factor. With the design-build contract we were able to get the ball rolling. When engineering geologists from Golder scaled the bluff, we were able to analyze the rock falls, and that helped us with the design. They worked closely with rock remediation technicians from Janod and Vertec [Contractors Inc., also of Swanton, VT] to determine the specific quantity of materials needed to get the job done.”
Unstable rock formations were stabilized with a layer of steel fiber–reinforced shotcrete applied through the dry-mix process. Following the installation of strategically located rock bolts to shore up those marginally stable rocks, additional shotcrete was applied by a nozzle man working from a rope.
The artistic touch was applied by a team that sculpted shotcrete to cover any signs of reinforcing materials and then colored it to match surrounding rock. That sculpting work was performed by Boulderscape, a professional rockscape company based in Capistrano Beach, CA.
Boulderscape specializes in replicating detailed rock formations for theme parks, golf courses, zoos, and public-sector retaining wall projects. The company’s sculptors have worked around the world. They typically work from an architectural firm’s conceptual designs, and their expertise frequently eliminates the need for rock castings.
“We had people from the New Jersey Transit Authority come to the site to evaluate a 16- by 49-foot test panel that we’d completed, and they couldn’t tell where the test panel began or ended,” Journeaux says with pride. “We said, ‘Perfect!’”
Journeaux acknowledges the treatment was more expensive than the traditional wire-rope nets, but not considerably more. And, he adds, the public was willing to pay a small premium to prevent the use of the invasive and unsightly alternative.
The King’s Bluff project underscores that the slope stabilization industry is still in its infancy in North America, according to Journeaux, and its maturation is proving that bigger isn’t always better.
“We’ve been used to needing bigger budgets to literally move mountains. Now we don’t have to do work to that degree, and many times, we don’t need to use that bigger budget,” he explains. “We can stabilize most slopes. There is a solution. Working on ropes to stabilize slopes is not only safer, but you’re right there in the middle of things. Your engineers are not at the bottom looking up. They are on top, looking down, and so they see the nooks and crannies. They gain a much better understanding of the slope. It gives you a different perspective.”
Understanding the importance of perspective is an essential skill for artists and sculptors, Journeaux says, and that was especially true for the Boulderscape team.
“Artists do look at the world differently than engineers do. We had the designers out there onsite, and a lot of people were doing a lot of thinking about how to make the finished project look as natural as the original,” he recalls.
“All Natural” Carries the Day
Across the country in Sacramento, CA, Salix Applied Earthcare’s bioengineering approach to slope stabilization projects emphasizes natural components as well as a natural look.
 |
PHOTO: SALIX APPLIED EARTHCARE |
The firm’s personnel have designed projects that have won the 2005 IECA Excellence in Technology Award, the National Park Foundation Environmental Conservation Award in 1999, and IECA’s Technological Advancement Award in 1994.
Salix Project Manager Kaila Dettman agrees with Journeaux that a fair measure of art must go hand-in-hand with the ever-advancing science of slope stabilization. A variety of factors determine how project managers strike an effective balance.
Client desires, applicable government regulations, the physical features of a site, and budgetary constraints all contribute to the decision-making process. For example, a hard structure, such as a retaining wall, might exceed a project’s budget and tip the scales in favor of a bioengineering solution. Yet, just as often, the aesthetic, environmental, and practical benefits of using vegetation also are deciding factors.
The Salix team has developed a preference for willows in slope stabilization projects based on company President John McCullah’s favorable experiences with the easily recognized tree. “It’s a fast-growing tree and easy to install with live cuttings during construction,” Dettman adds.
On one such project, a slope stabilization effort near Redding, CA, Dettman notes that naturally occurring vegetation was implemented to repair an unstable, erosion-prone site. The work took place along Highway 299, a main artery between the northern California coast and northern Sacramento Valley, and was the subject of a paper presented at EC05, IECA’s annual conference, in Dallas, TX.
“Some of the most challenging soils to stabilize on cut and fill slopes are those derived from decomposed granite. Fertility deficiency, vulnerability to dry raveling, and lack of vegetation once disturbed, calls for special measures to successfully establish vegetation and minimize erosion,” the Salix team and co-authors state in the paper. “Various methods exist for stabilizing slopes, many of which require expensive ‘hard’ structures. There is a need to develop cost effective and environmentally-sensitive techniques for decomposed granite slopes that provide stabilization and suitable soil conditions for long-term vegetation establishment.”
Noting that some slopes were healed naturally by the introduction of specific plants, Dettman explains, “We said, ‘If Mother Nature did that over time, we could speed that up with the right choices of plants.’ And, from a geotechnical standpoint, there are things vegetation can do well that a wall can’t.”
Working in conjunction with the California Department of Transportation District 2 and the University of California–Davis, Salix conducted a roadside field study on a slope with an angle of 1V:1.5H to “investigate methods for improving the stability and soil characteristics of granite cut and fill slopes.”
“Several different measures were installed at the test site including a soil-filled gabion wall at the toe, soil wraps (vegetated mechanically stabilized earth), geotextile flaps, and assorted bioengineering techniques such as live pole drains and brushlayering,” the paper states. “To test the impacts of compost admixtures on rooting depth, infiltration rates, and vegetation establishment, 16 test plots measuring 2 m (6.6 ft) wide and 4 m (13.1 ft) high, were installed and filled with different compost and soil mixtures. The admixtures were composed of 0%, 2%, 4%, and 8% compost by weight with four randomly assigned replications. All plots were covered with native straw. Rainfall simulators were used to apply rainfall at a rate of 60 mm (2.36 in)/hr for a 10 minute, 50 year return frequency event. To measure infiltration rates, an infiltrometer was placed in each test plot.”
Four randomly chosen plots were divided in half, with big squirreltail (Elymus multisetus) planted on the upper half and nothing planted on the lower half. A variety of techniques were applied to the slope to stabilize the roots, decrease the angle, shorten the length, control surface erosion, and relieve pore pressure, the paper notes. “Gabion baskets were lined with a coconut fiber geotextile and placed along the toe of the slope to form a wall. To facilitate vegetation establishment, the gabions were filled with native soil, hand compacted, and planted with willow poles.”
Meanwhile, soil wraps were installed around the test plots and the four gabions at the bottom of a spring that had created a gully on the slope face.
“The wraps were constructed by laying 900 g/m coir netting fabric 2.5 m (8.2 ft) beyond the outer slope edge, applying fill, compacting it to 0.5 m (1.6 ft) on top of the blanket, and stretching the remaining fabric over the compacted fill with an excavator. The fabric was keyed in at an inslope angle and staked in place along the outer edge,” the paper explains. “The technique was complex and required training and some trial-and-error to develop the most effective system for installation. Properly compacting the slope face and stretching the fabric was challenging and time-consuming.”
 |
PHOTO: JANOD INC. |
Brushlayering was installed between the soil wraps and at other points along the slope. The team constructed a scupper drain, with its outlet onto the soil-wrapped slope, to divert sediment-heavy runoff away from the test plots. Staked rice-straw wattles along the drain and willow stakes provided stability for the drain and dissipated flow energy. Bundles of willow cuttings tied with polypropylene rope and placed in a trench created live pole drains. Those drains, secured with live stakes and backfilled, were installed to draw excess moisture from unstable areas and direct to places able to dissipate flow energy.
Additional techniques were needed the following spring to further stabilize the slope. “It was determined that the site was typical of newly constructed slopes adjacent to native granite parent material that are susceptible to mass failure due to forces that cause inherent slope instability. As pore pressure built at the contact between the face of the parent material and the compacted fill, small pockets of less compacted soil served as a collection and conveyance feature for subsurface flow. Following complete saturation, the forces pushed out from within the compacted fill and the release of flow caused slumping. A new practice was introduced to address inherently unstable soil upslope from the test plots,” the paper notes.
That new practice was geotextile flaps, which involves “allowing the coir netting to lie as a flap over the face of the slope instead of wrapping the netting back over the soil, as performed with soil wraps.” The flaps were with wood lathes along their edges and long steel anchors driven into the slopes.
The technique “provided horizontal subsurface conveyance to release pressure within the slope, as well as surface erosion control via the overhanging flaps of material.
“The steel anchors provided an opposing force inward, meant to stabilize the soil by confinement and compensate for a lack of soil cohesion. Willow poles were laid on the top of each flap to form brushlayers in various locations before the soil was applied to enhance establishment,” the paper notes. “It was anticipated that the willow roots would anchor the soil in place, provide additional subsurface conveyance and foliage would supply surface protection.”
The study determined that the bioengineering techniques “throughout the site were successful based on high survival rates of willows after two winters.”
“The test plots demonstrated that increased compost concentration in the admixture was directly correlated to increased infiltration rate (R2 = 0.9824). Examination of individual grasses revealed that roots had grown at least 4 ft into the decomposed granite. The addition of compost proved to be successful for reducing overland flow and promoting vegetation establishment in adverse soil conditions.”
“The coir flaps were critical to the success of the project, providing structural support that may have only been possible with an expensive retaining wall. Effectiveness and constructability make coir flaps an important consideration for use in non-cohesive soils subject to failures that occur as a result of saturation and mass soil movement,” the paper concludes.
“The stable soil provided secure sites for germination and growth thus promoting vegetation establishment. It is important to note that conventional practices generally do not condone attempting to establish perceived ‘water-loving’ vegetation in areas with droughty and nutrient-poor conditions. This project demonstrated that bioengineering installations are successful for a variety of site conditions.”
Dettman elaborates, “It’s proof that willows can be used on a slope with good results and not just limited to streambanks. They put out enough roots to help stabilize the surface, and they also use a lot of water.”
 |
PHOTO: SALIX APPLIED EARTHCARE |
Landslide Lessons
Dettman indicates the deadly mudslides that befell California in early 2005 prompted numerous inquiries about slope stabilization. At least 10 people were killed in a mudslide that buried homes in La Conchita, northwest of Los Angeles, triggered by relentless rains.
“The homes were at the base of a very unstable slope, and there were roads cut in the middle of the slope. Roads are one of the primary reasons these slopes fail,” she says. “People are asking questions and looking for ways to stabilize slopes, and we’re definitely interested in trying to provide some answers.”
Radhey Sharma, Ph.D., an Iowa State University civil, construction, and environmental engineering professor, is an international expert on unsaturated soils. He notes dramatic California mudslides periodically grab national headlines but that a limited understanding of unsaturated soil behavior causes billions of dollars each year in the collapse of building foundations, highway embankments, and other land movements as well.
Sharma, who was recently awarded the Telford Medal, the highest honor in civil engineering, co-authored a new theoretical framework for understanding and modeling the hydraulic and mechanical behaviors of unsaturated soils.
 |
PHOTO: SALIX APPLIED EARTHCARE |
 |
PHOTO: SALIX APPLIED EARTHCARE |
Sharma indicates there is much to learn about the complex micromechanics of unsaturated soil and the precise point when the bond between particles disappears and sets off a cascading slope failure.
“If we can learn more at the micro level, the goal is to help us understand the effects at the macro level,” explains Sharma. “There are certain parameters that can be linked, but how those links function is where the work is yet to be done.”
While the dynamics might not be fully understood yet, Sharma suggests the California experience underscores the value of “common-sense approaches.”
“One answer is not living in those areas rather than being adventurous and saying, ‘We know everything and we can handle it,’” Sharma says. “We don’t know everything.”
Sharma sees a critical need to transfer the highly technical research of unsaturated soils into information that engineers, project designers, and government officials can apply to routine slope stabilization projects.
“In most universities, even at the graduate level, there is no course that talks about unsaturated soil mechanics, and one consequence is when people go out into the field they don’t understand its importance. I spoke at a conference about the topic and I could see some blank faces, even among the Ph.D.s,” Sharma says. “There is a lag between research and what is going on in the field, but that is always the case. It can take five to 10 years to close that gap, but the positive thing is that people are beginning to recognize the importance.”
Dettman draws parallels to her chosen field. “There are many tried-and-true bioengineering techniques out there that really work well, but people are experimenting with other ways to establish native vegetation. Compost is the hot topic right now. Instead of throwing seeds on a bare slope and expecting them to grow, which is asking the seeds to do a little much, compost can provide protection, nutrients, and moisture instead of applying chemical fertilizers. People are using more environmentally friendly materials in erosion control.
“We’re still learning. There are a lot of projects that have gone in during the past five years that we’ll be learning a lot from even five and 10 years from now. Erosion control is an ever-growing industry because of the [population growth and urban sprawl] and the need to restore areas with problems. This industry is still evolving; there’s a lot of work to do.”
Dettman encounters a growing acceptance of bioengineering solutions to erosion. In fact, more and more clients are requesting the approach that uses natural options to improve upon nature.
“There are still a lot of skeptical people, especially on stream projects. They think that if you use vegetation it will reduce flow and cause floods,” Dettman explains. “But some engineers have really come to love bioengineered approaches, especially if the vegetation is well maintained. That may require some extra effort, but the benefits vastly outweigh the costs.”
She adds, “People think vegetation and the results that come from it are unpredictable, but that isn’t true if you involve all the right disciplines at the start.”
She returns to the example of the willow. “Some people will say live willow cuttings just don’t work. They’re working off their experience, but it means those cuttings weren’t planted right. Installation is very important, of course, and that’s why you have to have a hydrologist, biologist, and botanist on hand. We want to get everyone onsite to put their thoughts, knowledge, and experience into the project. When you do that, it’s a much more successful project in the end and you don’t have to go back and fix things over and over.”
Journeaux says the heightened public attention only served to make the King’s Bluff project more memorable. “It was fun and we came out shining because we applied our techniques like we always do, whether it was in terms of safety and security or our standards for quality,” he adds.
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Journeaux, who has been asked to speak to various audiences about the King’s Bluff project, intends to apply the lessons of that work to other projects in the future.
“We applied proven techniques—rock bolts and shotcrete—with a technique used at places like Disneyland, zoos, and swimming pools. We went outside the box to preserve a rock formation the way people wanted it,” he says. “There will definitely be an opportunity to use that process again.”
July-August 2005
Slope Stabilization
Harnessing the best of nature and technology.
I
t might be difficult to improve upon the works of nature, but today’s slope stabilization experts are proving they can do exactly that as a blend of human ingenuity and bold techniques are producing results that are as aesthetically extraordinary as they are effective.Some techniques incorporate the best features of nature, including a heavy reliance on vegetation to camouflage and complement hard structures. Yet, in Weehawken, NJ, it is the hard structure itself that has won praise for preserving a natural landmark loved by the community.
King’s Bluff, a 190-foot rock slope, required stabilization as part of a project extending the Hudson-Bergen Light Rail line into Weehawken. The work posed a number of unusual challenges, including a compressed time schedule and additional safety concerns created by the need for crews to work below the slope even as stabilization work was under way. However, the most daunting goal may have been the need to satisfy the concerns of Mayor Richard Turner, members of the Palisades Preservation Committee, and a wary public—all of whom were determined that the bluff’s aesthetic features would not be affected by the stabilization work.
“People have been looking at that slope for hundreds of years, and they want future generations to have the same view. But rock does weather and change, so you have to work with that. You have to come up with solutions if you want to preserve a slope, and there’s not a textbook with those solutions,” says Daniel Journeaux, president of Janod Inc., headquartered in Swanton, VT. “The original designs called for pattern bolting and wire-rope nets to stabilize and contain loose material, but it didn’t take historic and aesthetic concerns into consideration.”
He continues, “This was a very high-profile project from start to finish. We had reporters, the mayor, and the preservation committee watching us. People with binoculars were watching from the other side of the street as we worked. People were filming us. The people from OSHA were there. Even people in Manhattan were watching the project. There were just a lot of people interested in what we were doing throughout the entire process.”
One of a Kind
Journeaux has worked on hundreds of stabilization projects for highways, railways, mines, and quarries, but he considers the King’s Bluff project to be in a class of its own.
“It is art on a grand scale,” says Journeaux, whose focus has been to develop and improve techniques and equipment used in rock stabilization projects with an emphasis on cost efficiency.
Janod was involved in a slope stabilization project involving a pedestrian walkway and elevator near King’s Bluff when city officials sought the advice of the firm and its strategic partner, Golder Associates. The mayor had halted work at King’s Bluff because a particularly beloved portion of the slope, known as the Gorilla’s Head, was scheduled for removal before rock anchors and wire net were installed. “That was simply unacceptable to the mayor and his constituents,” Journeaux explains.
Janod representatives persuaded the city that they had a better approach, and the city responded with a design-build contract. The arrangement allowed the design to proceed even as work was performed.
“Time was of the essence,” Journeaux explains. “They were already delayed, so there was a cost factor. With the design-build contract we were able to get the ball rolling. When engineering geologists from Golder scaled the bluff, we were able to analyze the rock falls, and that helped us with the design. They worked closely with rock remediation technicians from Janod and Vertec [Contractors Inc., also of Swanton, VT] to determine the specific quantity of materials needed to get the job done.”
Unstable rock formations were stabilized with a layer of steel fiber–reinforced shotcrete applied through the dry-mix process. Following the installation of strategically located rock bolts to shore up those marginally stable rocks, additional shotcrete was applied by a nozzle man working from a rope.
The artistic touch was applied by a team that sculpted shotcrete to cover any signs of reinforcing materials and then colored it to match surrounding rock. That sculpting work was performed by Boulderscape, a professional rockscape company based in Capistrano Beach, CA.
Boulderscape specializes in replicating detailed rock formations for theme parks, golf courses, zoos, and public-sector retaining wall projects. The company’s sculptors have worked around the world. They typically work from an architectural firm’s conceptual designs, and their expertise frequently eliminates the need for rock castings.
“We had people from the New Jersey Transit Authority come to the site to evaluate a 16- by 49-foot test panel that we’d completed, and they couldn’t tell where the test panel began or ended,” Journeaux says with pride. “We said, ‘Perfect!’”
Journeaux acknowledges the treatment was more expensive than the traditional wire-rope nets, but not considerably more. And, he adds, the public was willing to pay a small premium to prevent the use of the invasive and unsightly alternative.
The King’s Bluff project underscores that the slope stabilization industry is still in its infancy in North America, according to Journeaux, and its maturation is proving that bigger isn’t always better.
“We’ve been used to needing bigger budgets to literally move mountains. Now we don’t have to do work to that degree, and many times, we don’t need to use that bigger budget,” he explains. “We can stabilize most slopes. There is a solution. Working on ropes to stabilize slopes is not only safer, but you’re right there in the middle of things. Your engineers are not at the bottom looking up. They are on top, looking down, and so they see the nooks and crannies. They gain a much better understanding of the slope. It gives you a different perspective.”
Understanding the importance of perspective is an essential skill for artists and sculptors, Journeaux says, and that was especially true for the Boulderscape team.
“Artists do look at the world differently than engineers do. We had the designers out there onsite, and a lot of people were doing a lot of thinking about how to make the finished project look as natural as the original,” he recalls.
“All Natural” Carries the Day
Across the country in Sacramento, CA, Salix Applied Earthcare’s bioengineering approach to slope stabilization projects emphasizes natural components as well as a natural look.
|
PHOTO: SALIX APPLIED EARTHCARE |
The firm’s personnel have designed projects that have won the 2005 IECA Excellence in Technology Award, the National Park Foundation Environmental Conservation Award in 1999, and IECA’s Technological Advancement Award in 1994.
Salix Project Manager Kaila Dettman agrees with Journeaux that a fair measure of art must go hand-in-hand with the ever-advancing science of slope stabilization. A variety of factors determine how project managers strike an effective balance.
Client desires, applicable government regulations, the physical features of a site, and budgetary constraints all contribute to the decision-making process. For example, a hard structure, such as a retaining wall, might exceed a project’s budget and tip the scales in favor of a bioengineering solution. Yet, just as often, the aesthetic, environmental, and practical benefits of using vegetation also are deciding factors.
The Salix team has developed a preference for willows in slope stabilization projects based on company President John McCullah’s favorable experiences with the easily recognized tree. “It’s a fast-growing tree and easy to install with live cuttings during construction,” Dettman adds.
On one such project, a slope stabilization effort near Redding, CA, Dettman notes that naturally occurring vegetation was implemented to repair an unstable, erosion-prone site. The work took place along Highway 299, a main artery between the northern California coast and northern Sacramento Valley, and was the subject of a paper presented at EC05, IECA’s annual conference, in Dallas, TX.
“Some of the most challenging soils to stabilize on cut and fill slopes are those derived from decomposed granite. Fertility deficiency, vulnerability to dry raveling, and lack of vegetation once disturbed, calls for special measures to successfully establish vegetation and minimize erosion,” the Salix team and co-authors state in the paper. “Various methods exist for stabilizing slopes, many of which require expensive ‘hard’ structures. There is a need to develop cost effective and environmentally-sensitive techniques for decomposed granite slopes that provide stabilization and suitable soil conditions for long-term vegetation establishment.”
Noting that some slopes were healed naturally by the introduction of specific plants, Dettman explains, “We said, ‘If Mother Nature did that over time, we could speed that up with the right choices of plants.’ And, from a geotechnical standpoint, there are things vegetation can do well that a wall can’t.”
Working in conjunction with the California Department of Transportation District 2 and the University of California–Davis, Salix conducted a roadside field study on a slope with an angle of 1V:1.5H to “investigate methods for improving the stability and soil characteristics of granite cut and fill slopes.”
“Several different measures were installed at the test site including a soil-filled gabion wall at the toe, soil wraps (vegetated mechanically stabilized earth), geotextile flaps, and assorted bioengineering techniques such as live pole drains and brushlayering,” the paper states. “To test the impacts of compost admixtures on rooting depth, infiltration rates, and vegetation establishment, 16 test plots measuring 2 m (6.6 ft) wide and 4 m (13.1 ft) high, were installed and filled with different compost and soil mixtures. The admixtures were composed of 0%, 2%, 4%, and 8% compost by weight with four randomly assigned replications. All plots were covered with native straw. Rainfall simulators were used to apply rainfall at a rate of 60 mm (2.36 in)/hr for a 10 minute, 50 year return frequency event. To measure infiltration rates, an infiltrometer was placed in each test plot.”
Four randomly chosen plots were divided in half, with big squirreltail (Elymus multisetus) planted on the upper half and nothing planted on the lower half. A variety of techniques were applied to the slope to stabilize the roots, decrease the angle, shorten the length, control surface erosion, and relieve pore pressure, the paper notes. “Gabion baskets were lined with a coconut fiber geotextile and placed along the toe of the slope to form a wall. To facilitate vegetation establishment, the gabions were filled with native soil, hand compacted, and planted with willow poles.”
Meanwhile, soil wraps were installed around the test plots and the four gabions at the bottom of a spring that had created a gully on the slope face.
“The wraps were constructed by laying 900 g/m coir netting fabric 2.5 m (8.2 ft) beyond the outer slope edge, applying fill, compacting it to 0.5 m (1.6 ft) on top of the blanket, and stretching the remaining fabric over the compacted fill with an excavator. The fabric was keyed in at an inslope angle and staked in place along the outer edge,” the paper explains. “The technique was complex and required training and some trial-and-error to develop the most effective system for installation. Properly compacting the slope face and stretching the fabric was challenging and time-consuming.”
|
PHOTO: JANOD INC. |
Brushlayering was installed between the soil wraps and at other points along the slope. The team constructed a scupper drain, with its outlet onto the soil-wrapped slope, to divert sediment-heavy runoff away from the test plots. Staked rice-straw wattles along the drain and willow stakes provided stability for the drain and dissipated flow energy. Bundles of willow cuttings tied with polypropylene rope and placed in a trench created live pole drains. Those drains, secured with live stakes and backfilled, were installed to draw excess moisture from unstable areas and direct to places able to dissipate flow energy.
Additional techniques were needed the following spring to further stabilize the slope. “It was determined that the site was typical of newly constructed slopes adjacent to native granite parent material that are susceptible to mass failure due to forces that cause inherent slope instability. As pore pressure built at the contact between the face of the parent material and the compacted fill, small pockets of less compacted soil served as a collection and conveyance feature for subsurface flow. Following complete saturation, the forces pushed out from within the compacted fill and the release of flow caused slumping. A new practice was introduced to address inherently unstable soil upslope from the test plots,” the paper notes.
That new practice was geotextile flaps, which involves “allowing the coir netting to lie as a flap over the face of the slope instead of wrapping the netting back over the soil, as performed with soil wraps.” The flaps were with wood lathes along their edges and long steel anchors driven into the slopes.
The technique “provided horizontal subsurface conveyance to release pressure within the slope, as well as surface erosion control via the overhanging flaps of material.
“The steel anchors provided an opposing force inward, meant to stabilize the soil by confinement and compensate for a lack of soil cohesion. Willow poles were laid on the top of each flap to form brushlayers in various locations before the soil was applied to enhance establishment,” the paper notes. “It was anticipated that the willow roots would anchor the soil in place, provide additional subsurface conveyance and foliage would supply surface protection.”
The study determined that the bioengineering techniques “throughout the site were successful based on high survival rates of willows after two winters.”
“The test plots demonstrated that increased compost concentration in the admixture was directly correlated to increased infiltration rate (R2 = 0.9824). Examination of individual grasses revealed that roots had grown at least 4 ft into the decomposed granite. The addition of compost proved to be successful for reducing overland flow and promoting vegetation establishment in adverse soil conditions.”
“The coir flaps were critical to the success of the project, providing structural support that may have only been possible with an expensive retaining wall. Effectiveness and constructability make coir flaps an important consideration for use in non-cohesive soils subject to failures that occur as a result of saturation and mass soil movement,” the paper concludes.
“The stable soil provided secure sites for germination and growth thus promoting vegetation establishment. It is important to note that conventional practices generally do not condone attempting to establish perceived ‘water-loving’ vegetation in areas with droughty and nutrient-poor conditions. This project demonstrated that bioengineering installations are successful for a variety of site conditions.”
Dettman elaborates, “It’s proof that willows can be used on a slope with good results and not just limited to streambanks. They put out enough roots to help stabilize the surface, and they also use a lot of water.”
|
PHOTO: SALIX APPLIED EARTHCARE |
Landslide Lessons
Dettman indicates the deadly mudslides that befell California in early 2005 prompted numerous inquiries about slope stabilization. At least 10 people were killed in a mudslide that buried homes in La Conchita, northwest of Los Angeles, triggered by relentless rains.
“The homes were at the base of a very unstable slope, and there were roads cut in the middle of the slope. Roads are one of the primary reasons these slopes fail,” she says. “People are asking questions and looking for ways to stabilize slopes, and we’re definitely interested in trying to provide some answers.”
Radhey Sharma, Ph.D., an Iowa State University civil, construction, and environmental engineering professor, is an international expert on unsaturated soils. He notes dramatic California mudslides periodically grab national headlines but that a limited understanding of unsaturated soil behavior causes billions of dollars each year in the collapse of building foundations, highway embankments, and other land movements as well.
Sharma, who was recently awarded the Telford Medal, the highest honor in civil engineering, co-authored a new theoretical framework for understanding and modeling the hydraulic and mechanical behaviors of unsaturated soils.
|
PHOTO: SALIX APPLIED EARTHCARE |
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PHOTO: SALIX APPLIED EARTHCARE |
Sharma indicates there is much to learn about the complex micromechanics of unsaturated soil and the precise point when the bond between particles disappears and sets off a cascading slope failure.
“If we can learn more at the micro level, the goal is to help us understand the effects at the macro level,” explains Sharma. “There are certain parameters that can be linked, but how those links function is where the work is yet to be done.”
While the dynamics might not be fully understood yet, Sharma suggests the California experience underscores the value of “common-sense approaches.”
“One answer is not living in those areas rather than being adventurous and saying, ‘We know everything and we can handle it,’” Sharma says. “We don’t know everything.”
Sharma sees a critical need to transfer the highly technical research of unsaturated soils into information that engineers, project designers, and government officials can apply to routine slope stabilization projects.
“In most universities, even at the graduate level, there is no course that talks about unsaturated soil mechanics, and one consequence is when people go out into the field they don’t understand its importance. I spoke at a conference about the topic and I could see some blank faces, even among the Ph.D.s,” Sharma says. “There is a lag between research and what is going on in the field, but that is always the case. It can take five to 10 years to close that gap, but the positive thing is that people are beginning to recognize the importance.”
Dettman draws parallels to her chosen field. “There are many tried-and-true bioengineering techniques out there that really work well, but people are experimenting with other ways to establish native vegetation. Compost is the hot topic right now. Instead of throwing seeds on a bare slope and expecting them to grow, which is asking the seeds to do a little much, compost can provide protection, nutrients, and moisture instead of applying chemical fertilizers. People are using more environmentally friendly materials in erosion control.
“We’re still learning. There are a lot of projects that have gone in during the past five years that we’ll be learning a lot from even five and 10 years from now. Erosion control is an ever-growing industry because of the [population growth and urban sprawl] and the need to restore areas with problems. This industry is still evolving; there’s a lot of work to do.”
Dettman encounters a growing acceptance of bioengineering solutions to erosion. In fact, more and more clients are requesting the approach that uses natural options to improve upon nature.
“There are still a lot of skeptical people, especially on stream projects. They think that if you use vegetation it will reduce flow and cause floods,” Dettman explains. “But some engineers have really come to love bioengineered approaches, especially if the vegetation is well maintained. That may require some extra effort, but the benefits vastly outweigh the costs.”
She adds, “People think vegetation and the results that come from it are unpredictable, but that isn’t true if you involve all the right disciplines at the start.”
She returns to the example of the willow. “Some people will say live willow cuttings just don’t work. They’re working off their experience, but it means those cuttings weren’t planted right. Installation is very important, of course, and that’s why you have to have a hydrologist, biologist, and botanist on hand. We want to get everyone onsite to put their thoughts, knowledge, and experience into the project. When you do that, it’s a much more successful project in the end and you don’t have to go back and fix things over and over.”
Journeaux says the heightened public attention only served to make the King’s Bluff project more memorable. “It was fun and we came out shining because we applied our techniques like we always do, whether it was in terms of safety and security or our standards for quality,” he adds.
Journeaux, who has been asked to speak to various audiences about the King’s Bluff project, intends to apply the lessons of that work to other projects in the future.
“We applied proven techniques—rock bolts and shotcrete—with a technique used at places like Disneyland, zoos, and swimming pools. We went outside the box to preserve a rock formation the way people wanted it,” he says. “There will definitely be an opportunity to use that process again.”