May 13, 2013 04:01

Defusing Slope Failure

Did you ever think that there would be feng shui solutions for building on slopes? Well, there are, and it’s a good thing, because nowadays more and more housing developments, office buildings, shopping malls and roadways are putting down roots in uneven soil.

Feng shui is defined as “a complex body of knowledge that reveals how to balance the energies of any given space to assure health and good fortune for people inhabiting it.” More specifically, feng means wind and shui means water. Feng shui is based on the understanding of nature, particularly on the idea that the land is alive and filled with energy.

The ancient Chinese believed that a specific land’s energy could either make or break the kingdom, so to speak. And when building on slopes, that is certainly true.

The rush to develop on steep natural or man-made slopes comes with serious considerations. Slope instability threatens structures at both the bottom and the top of a slope. Slope failures are responsible for more than $1 billion of damage to public and private property every year.

A slope failure typically occurs when a slope is overly steep, where fill material is not compacted, or where cuts in natural soils encounter groundwater or zones of weak material.

If you’re going to take on projects that require stabilizing slopes, you’ll need to make sure that you’ve got an experienced crew. Across the country, numerous construction companies will simply carve off a portion of a slope in order to create a flat foundation. Surprisingly, some companies, due to cost or time, don’t stabilize or support what is left of the slope. Sooner or later, the slope is bound to fail, sliding down on the very project that was just built.

And sometimes the job just isn’t done right the first time. Just because a slope has been stabilized doesn’t guarantee that the area won’t slide again. For example, if you began with a natural slope, then stabilized it improperly, the slope has been compromised and is less stable than it was originally.

Once a failure has occurred, the most appropriate stabilization measure will depend on site-specific conditions, such as the size of the slide, the soil type, alignment constraints and the cause of the failure.

When you think about it, slopes can be ticking time bombs. And man, for the most part, is the one who lights the fuse. Roger Moore, vice president at Ameritech Slope Constructors, Inc. in Asheville, North Carolina, dealt with such a bomb on a project for a homeowners’ association that sat above Lake Chatuge in Hiawassee, Georgia. “This is a nice area with a road that goes up a ridge and offers nice views of the lake,” details Moore.

The slope that sat above the road had previously failed. A grading contractor had graded it and cleared the debris, but it was sliding again. Ameritech found that when the roadway had originally been constructed, the construction team had run into rock. They had chosen not to remove as much rock as was necessary (possibly due to budget constraints) in order to build the roadway properly.

“They basically pushed some fill dirt out and around the rocks and made a curve, then built the road out to that edge. This is the section that needed to be stabilized,” Moore added.

The 70-degree angled slope needed to be addressed quickly. “It was going to slide down onto the busy roadway of the complex, possibly killing someone,” said Moore. How did Ameritech defuse the slope?

“The first thing we do when dealing with any type of slope failure is find out the geotechnical information: what is the slope like; is there a certain depth where we will encounter rock? What’s the nature and angle of the slope? Are we trying to stabilize something that is already moving or are we preventing something from happening?” Ameritech Slope Constructors chose to do a soil nail application combined with a Tecco Mesh application to repair the slope.

Tecco Mesh is a flexible, hightensile steel wire that adapts easily to slope surfaces and prevents slides. It is part of a slope stabilization system designed by Greobrugg, a geohazard solutions company based in Switzerland, which offers mesh, rock or soil nails, spike plates and connectors. The mesh can be stretched over sharp boulders and rocks without breaking.

The diamond-shaped three-dimensional mesh is so fine that it prevents earth masses and rock pieces from breaking out. The mesh also allows water to flow freely through it, so the slope retention system doesn’t contribute to runoff problems. It comes in rolls that measure 30m x 3.5m.

Ideal for fast installation over large areas, it is typically anchored only at the top of a slope, which allows the mesh material to drape freely down. The weight and friction of the material provides stability. The Tecco Stabilization System puts pressure on the slope to tighten it and hold its materials in place. Basically, it puts the slope into equilibrium.

When stabilizing a slope, two types of methods can be used to stop extensive slope damage: hardtype approaches such as retaining walls, riprap, dewatering techniques and anchor piles, or softtype approaches such as mats, sheeting, erosion control blankets and turf reinforcement mats (TRMs).

Many slope stabilization situations will dictate that a combination of both soft and hard approaches be used. Combining these different approaches is also important, because containing a running slope takes time. Most slopes are not stabilized in a day, and continue to slide during the project.

“We chose this method because it was a budgetary issue for the homeowners’ association. They had a tight budget and it was less expensive than building a retaining wall,” said Moore. “We first drilled the nails in—keep in mind, you have to design what kind of spacing you need for the nails.”

“For this job, we placed the nails into the slope at varying distances, from about 10 to 20 feet. By drilling into the more stable ground and rock, we created a hold and anchor action. We grouted around the nail, creating a kind of locking mechanism between the soil and the nail. Then we installed the Tecco Mesh over the top.” Each nail has a plate that goes over the top of the nail. The plate is what locks on to the mesh.

Once the mesh is tightened down against the plate, the whole system acts like a girdle. “It now will hold everything, meaning all the soil and rock that is trying to fail. Finally, we hydroseeded so you wouldn’t see the mesh; the grass grows right over it,” said Moore.

Hydroseeding is a technique that has been used for the last fifty years. Spraying on mixtures of water, seed, fertilizer and mulch has proven to be an effective way to quickly establish vegetation.

Moore added, “We like the Tecco Mesh because it is a very green product and you can actually color the mesh with a powder coating to match your environment. Let’s say you’re using it on a rocky slope and you don’t want it to show up or be real shiny, you can put a powder coating on it to match the rock, or any environment.”

It’s also considered a long-lasting method because the nails are epoxy-coated, which protects them from corrosion. And because of its galvanized coating, the mesh has about a 100- to 125-year life span.

The bomb at this site was defused, and the roadway is now safe.

But in North Carolina, many slopes are not safe; most of them are coming to the end of their lifecycle. Due to normal weathering and erosion, numerous natural and fill slopes are decomposing and sliding. “There’s a lot of slope repair and slope stabilization going on here, because we’ve bought about all the time we can with these slopes,” said Doug Blalock, corporate secretary and project manager of Charles Blalock and Sons in Raleigh, North Carolina.

Blalock and Sons had to deal with a bomb in Jackson County, North Carolina, that had already exploded. “It was a fill slope, 20 feet high and 150 to 200 feet long, located on a state highway that literally had blown out,” said Blalock.

What made matters worse is that they couldn’t close the roadway below, so they had limited access and very little room to work in.

“One thing you have to consider anytime you have a slide is that you still have a very volatile area. Just because the slope has moved doesn’t mean it has completed the process. When the slope and the general area around the slope is still in crisis, you don’t want to go in and excavate a lot of the instability out of it. What you do want to do is scotch it up and make a wall,” said Blalock.

The company fixed the slope by drilling holes anywhere from 24 inches to 48 inches wide into the slope’s existing bedrock. They inserted I-beam steel pilings into the holes, and sealed them with concrete so they didn’t move. They connected the pilings by inserting timber blocks. (Timber blocks are, instead of two-by-fours, more of a two-by-eight or two-by-ten piece of wood. The blocks can be anywhere from eight to 16 feet long.)

“We just slide those timber lines in-between the pilings, letting the pilings be the guide or clamp. If you can imagine putting your fingers straight down on the table and you now have to do something to bridge the gap between each finger, that’s how we use the timber blocks,” said Blalock. The combination of the concrete-sealed pilings and the timber blocks becomes the new strength and wall of the slope.

“And then over the face of all of that, we pour concrete. This creates a face that preserves the wood so that it doesn’t rot. The concrete also gives the slope a little more strength and seals it. But really, your strength is in the piling and the wood,” explains Blalock.

As you can see, there is no one-size-fits-all kind of solution when it comes to slope stabilization. As each project may be different, so are the solutions. For example, some slopes were not stabilized properly the first time around; some failures were caused by the soil retaining too much water. Still another problem can be with the soil itself.

Any strategy in stabilizing a slope should be tailored to each particular slope. Slopes differ when it comes to geology, soil composition, vegetation and geographic factors. Location should also be taken into consideration. Soil varies in different parts of a state or area.

Andrew Pinkley, a senior staff engineer for Buchart, Horn Inc. in Memphis, Tennessee, deals with soil issues. The soil composition in his state is a wind-blown loam that was deposited there in the ice age. “It’s very stable as long as it’s in place and nothing disturbs it. But once you start dumping water on it, or start excavating it and it gets loose, we have a lot of problems.”

Pinkley said, “When it comes to slope stabilization, we have several different methods. Mats are probably the most commonly used way to stabilize a slope.”

Another factor about the soil in Tennessee is that in the last couple of years, the state has had a considerable amount of rainfall. Heavy rainfall can quickly affect slopes made up of this porous or silt-like soil. When the soil becomes saturated, water takes the place of air between the grains of soil. The soil becomes heavier, since water is heavier than air.

Water saturation greatly increases the probability that mass movement of a slope will occur. Due to the excessive amount of water Tennessee has received recently, the state is experiencing its fair share of mudslides.

Jason Robertson of Erosion Solutions in Athens, Tennessee, said, “Water is our main concern now when dealing with slope stabilization. We do a lot of slope stabilizations and slope repairs— that’s our business. Ninety-nine percent of the time when there’s a slope failure here, it’s because of water. And anytime you get water behind a slope, you’re asking for big trouble.”

Erosion Solutions looks at repairing a slope with a different focus.

The first thing they do is find out how saturated the failing slope is and where the water is coming from, i.e., is it water runoff or groundwater infiltration?

Once they establish the water source or sources, they must divert it away from the slope before they start any type of repair. “You can fix a sliding slope a number of ways, but if you don’t address the water issues, it will ultimately fail again,” said Robertson.

“To keep the water away, we use concrete, grout and riprap. We’ll take concrete and make a riprap ditch, grout it and make a channel flow to get the water off the slope. We also use slope drains and a lot of sediment tubes.”

Using sediment tubes on contour lines slows the water down, because it doesn’t have a chance to gather up steam and start cutting channels in the slope. They also use erosion control mats where feasible. Sometimes a bonded fiber matrix is used to stabilize the area and to get some vegetation growing, but their first order of business is to eliminate the water flow.

“What people don’t understand is that when you start a really big cut and get down into some rock, there’s always going to be some water next to the rock. The water oozes out of the cut slope and loosens all the dirt that surrounds the rock. Then it starts getting really, really heavy. And if the slope is steep, it will want to slide,” said Robertson.

Erosion Solutions recently dealt with a slope failure that was basically a slope made up of solid rock with a lot of over-burden dirt on top. With all the water saturation on the top of the dirt, it finally slid off.

“When you go in there to repair it, you’re going to run into a bunch of mud. You’ve got to get rid of that mud and get down to either sod, rock, or hard ground before you can start your stabilization process. This is the time you have to decide whether you’re going to use grass, soil nails, concrete, or even more rock,” said Robertson.

Typically, Erosion Solutions will hydroseed a slope and put erosion mats on top of that. After hydroseeding, they take sediment logs or straw wattles and again put them on the line contours of the new slope, to help slow or stop any water from getting in.

Another concern on a project is how well the Professional Engineer (PE) and the contractor work together. “On many jobs, you’ll have a lot of engineers, who are book-smart, working with a guy who’s been on a dozer all his life. The contractor understands how water flows; more importantly, he’s been doing it for a while,” said Robertson. “When you put the contractor together with the PE, you hope that both parties want to achieve the same results. If they respect each other’s expertise and welcome their input, they’ll come up with the right solution every time.”

Without the rain, a typical slope in Tennessee is just a normal soil slope. If it fails, Erosion Solutions will dig all the back soil out and put down Geotech Style. Geotech Style is a landscaping fabric also known as geotextile fabric. This is a permeable fabric used with soil for separation, filtration, reinforcement, protection or drainage control. It is typically made from polypropylene or polyester.

“You roll it out and staple it down with saw pins that are about six inches long. Once in place, the fabric protects the slope like armor,” said Robertson. It also allows planting on steep slopes to grab hold and grow, further securing the slope.

One of the products Robertson would like to try in slope stabilization is the Posi- Shell Cover System. Posi- Shell is a spray-applied mineral mortar coating, similar to stucco, used for intermediate cover and erosion prevention. “It’s like a thin layer of cement that you can vegetate—you can actually grow grass through it.”

Any new technology is worth keeping an eye out for, but more importantly, Robertson said, “I think the best future ‘product’ is good teamwork between all levels. People really need to come together and find common ground.”

Perhaps we should be more in tune with the Chinese art of feng shui when choosing to construct on that lovely slope, for when it slides, it will hit and take out everything in its path. And with a lot of construction on slopes these days, every one of them has the potential of being a ticking time bomb.

Also in Soil Erosion News


In many ways, we are fortunate that, in our chosen profession, we are able to help people when certain disasters occur: the tornadoes in Missouri, Oklahoma, Louisiana and Georgia, the flooding in Louisiana, the snows in the northeastern part of the country, the rain in California, and the snow in Colorado....

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