Holding Strong at Every Level

April 9, 2014

In rural Beaufort County, NC, a new bridge was being built over the Norfolk Southern Railway to replace an aging structure.

“The project consisted of raising the elevation and widening the bridge,” explains Staci Smith, regional engineer for ACF Environmental. “This led to the need to steepen the adjacent slopes to minimize the impact of the right of way.”

The steepened slope was reinforced with geogrid, and erosion control blankets were applied to the face of the slope. It quickly became apparent, however, that the blankets weren’t sufficient to handle water coming down the steep embankment, which was developing rills and gullies.

According to Doug Evans of Landsaver Environmental, based in Richmond, VA, the North Carolina Department of Transportation initially considered replacing the erosion control blankets with regular slope batting. “It’s like permanent turf reinforcement mats,” he says. “But it was the need to hold the topsoil onto the slope that led to the decision to use Presto Geosystem’s Geoweb system for slopes. Once you get into slopes that are that steep, it’s hard to keep the topsoil on.

“The Geoweb is a cellular confinement system,” he explains. “Because the slope was of such an erodible soil, we anchored the Geoweb with earth anchors at the top of the slope, then dropped the Geoweb down with tendons, down the face of the slope all the way down to the bottom.

“Then we infilled the Geoweb with a compost, mulch, and seed mixture to help ensure vegetation. Once you have vegetation growing on the face of the slope, it’s going to hold the soil better.”

He adds, “When the contractor reached out to us for a solution, I contacted Staci, and she helped put together this design using different types of geogrid and Geoweb on the face of the slope.”

The slope reached a height of approximately 25 feet, and more than 60,000 square feet of Geoweb material was used on the project, which was completed late in 2013.

One of the challenges was the limited space at the top of the slope, due to the widened bridge road.

“It was difficult, because at the top is an overpass going above train tracks,” says Evans. “So at the top of this 1:1.5 slope, there is a guard rail. Then there is a two-lane road going across the overpass. There is barely any area between the guardrail and the slopes. Normally, we would dig an anchor trench and bury the Geoweb 2 feet into the ground using an anchor trench and a deadman pipe. But here, we had to use earth anchors with cables.”

Horseshoe Cofferdam in Wisconsin
In late 2013, Lloyd Pedersen of Minnesota Ltd. was tasked with the job of checking the integrity of a pipeline, dating back to the 1960s, in Whitewater, WI. Part of the pipeline lies about 3 feet below the lakebed of Meade Lake, which is often dry. However, after a fairly wet summer, it held water about 24 to 36 inches deep.

“It’s a small lake; we call it a duck pond,” says Pedersen. Duck pond or not, he needed access to the pipeline, and this meant clearing out the water in the area of the integrity dig.

“You have to do something with all that water,” he explains. “You have to have some type of cofferdam, whether it’s sheet piling or an aqua barrier. We chose to go with the aqua barrier, from Dam-It Dams.

“We reached out to Jack Nichols of Dam-It Dams because we had seen his dams before. He was on another project, and he came up and gave us a little guidance about what they are made out of and we decided to go with him. At one point he showed up with a little mini-dam rolled up with the geotextile fabric and bags on the inside, just to show everybody how it worked, and that was impressive.

“We ordered 2,600 feet of dam to make a horseshoe-shaped dam on this project. It’s a bladder that is covered with a geotextile fiber. Once the dam was laid out, we pumped out the water, from the inside to the outside of the dam, and then went ahead and did our work on the pipeline.”

Pedersen notes that the dams come in different sizes. When fully inflated with water, they reach 4 feet, 8 feet, or 10 feet. This project required the 8-foot size.

Even though the water was at most 3 feet deep, Pedersen explains that a taller cofferdam has its advantages. “What happens is that the higher the dam is, the bigger footprint it has, and the lake bottom conditions will also play a part. This was kind of a mucky bottom, so a dam will settle in a bit. That’s why we went with a taller dam.”

Minnesota Limited’s safety director, Jacob Heyne, adds, “Stability is what I think they were going for. It’s also a matter of attention to safety. You learn the hard way. You don’t want a dam breaking and putting the workers at risk. You want to be as safe as possible, and reach up for that next rung on the ladder of safety. The bigger the better, in this case.”

One of the advantages of the Dam-It Dams, according to Pedersen, is the way individual dams are combined to form a complete unit. “There is a collar that is used to pair them up next to each other. Other aqua dam companies will have individual dams, but then you have to overlap them, and you lose a lot of your linear footage. But with the Dam-It Dams, there is a collar that attaches them together, and it is then one continuous dam.”

Pedersen also appreciates the ease with which the dams are filled with water. “The installation went pretty smoothly. The nice thing is that you can fill them from one end. With some other dams, you fill from portholes on the top. This went together pretty well. The dams contain a textile fabric, with almost like a garbage-bag material on the inside, that gets filled from the fill tubes.”

They were in place for well over a month.

“When you work in the upper Midwest late in the year, you have the chance of freezing temperatures. To get them rolled up when we were finished, we had to put some heat on them to get the ice out and roll them back up. But I’ve seen them used in the dead of winter in Michigan, and they’ve worked pretty well. It’s really maintenance-free. You get them in place and go to work.”

Greening Manhattan
New York City isn’t known for a great deal of green space, but Dr. Clayton Rugh, who heads up Xero Flor America, is trying to change that.

When the Jacob Javits Convention Center owners considered installing a massive green roof, Xero Flor was invited to be part of the discussion, in part because of its success in outfitting the Ford Motor Co. Dearborn Assembly Plant with a green roof a decade ago.

“It’s our flagship project,” says Rugh of the Ford plant. “At the time it was installed, it was the largest green roof in the world. It’s about 10.4 acres of vegetation. It showed that we’re experienced and technically proficient at assisting with or completing a project of that size.” The Ford green roof still remains the largest in North America.

“The Javits design team actually flew to Detroit and walked around on the Ford roof, to see it for themselves. They could see firsthand that we successfully delivered the materials for the Ford project, and that the ownership at Ford was very pleased.

“At Javits, they didn’t want products that were brittle; they didn’t want products that were prone to degrade rapidly or would tear and come apart during installation. The strong, durable geosynthetic foundation to our system was a very valuable and attractive quality for the decision makers.”

Xero Flor America is the sole licensee for the United States for Xero Flor green roof products by the globally operating company XeroFlor Green Solutions of Germany.

“The Bonar product used on this project is a three-dimensional geosynthetic product,” he notes. “It’s a polypropylene and nylon assembly with a nonwoven thin fleece layer with extruded coils, like a Brillo pad, all along one side of this thin fleece fabric. These are synthetic polymers that are very durable, with a very high tensile integrity.

“We use this basic product as the drain layer, laying at the root surface, to get all the excess water to drain. Then there is a thicker fleece, like felt, about a quarter-inch thick. On this particular project, we also then have 1 inch of synthetic soil, a combination of pumice rock, a lightweight, durable aggregate with compost blended in, with organic material to provide soil-like function.

“Our vegetated mat is the very same geosynthetic that we use for the drain mat, only it’s reinforced with a stitched-on fleece layer. We call the product an XF301 mat. It is of very high durability, with built-in water retention. It’s a strong, robust product. It’s cultivated in a field very similar to sod, just like turf for your yard. We grow in it a group of plants that are a very hardy, flowering plant called sedum. Those plants are especially drought tolerant. They never need mowing or replanting.

“They’re quite pretty, actually related to cactus,” he says. “They have great water storage capabilities. We grow a wide variety of about 12 different species. There have fleshy leaves, and a variety of shapes and colors. The flowers themselves also have a variety of colors and shapes.

“These are grown on geosynthetics in the field. When they’re mature, we roll them up, just like a piece of sod. We take them up to the roof and roll them out. It’s basically a living, green roof on one side of the mat. The day it’s rolled out, you have a functioning, living green roof.

“Underneath the vegetated mat, the next layer is 1 inch of designer soil, called XeroTerr. That’s laying on about a quarter-inch-thick synthetic fleece. It’s polyester and polypropylene recycled fibers. Those are needled into a relatively thick fleece fabric.

“Below that fabric is the other geotextile. In this case, the fabric side of the geotextile is facing upward, so that the coils are facing down. This will allow an open space of excess water to move from the roof to the drains. It also allows aeration through the system, and drainage for the system.”

As extra protection to prevent penetration of roots into the roof itself, a thin sheet of 20-mil low-density polyethylene was added as well.

From a stormwater standpoint, a green roof offers a significant benefit. “A 1-inch rain on 1 acre is 25,000 gallons of water,” Rugh notes. “So when you have an 8-acre building, with every one-inch rain, 200,000 gallons of water are running off a conventional roof.

“But a green roof stores a significant fraction of the incoming rainfall, and it also slows down water that does run off. Instead of having a big splash runoff, you have this slow percolation of stormwater runoff.

“Another benefit is that if you have to replace an 8-acre roof every 15 to 20 years, that’s an expensive undertaking. Having a green roof will make that roof last at least two times longer, if not three times longer.”

Rugh reports that very little maintenance is required. After a modest amount of watering after installation, a once-yearly application of fertilizer in the spring is all that is required.

The Javits vegetative roof installation began in October 2012 and is expected to be completed in July 2014. When done, it will total 292,000 square feet, nearly 8 acres, easily the largest green space in Manhattan outside of Central Park.

From Landfill to Mass Transit
Thurston County, WA, lies roughly 60 miles south of Seattle and has been growing steadily through the years. A large landfill had been closed for some 25 years and, with available space in the region becoming scarce, the county wanted to take advantage of the landfill area by building a parking lot for commuters.

“One thing they wanted to expand on is mass transportation,” says Mark Lally, Pacific Northwest manager for Tensar International. “They want to get people out of their cars. This particular area is a very environmentally aware area; it’s a very green space. One of the reasons this landfill closure and building this parking lot was so important was because it’s very near the state capitol, and there are a lot of people coming in and out of there. So they wanted to utilize this space that otherwise would just sit vacant. It is a prime location, so it made sense for the county to turn that old closed landfill into a parking lot.”

The site became the Hawks Prairie Park & Ride lot for bus riders.

“Although there was mass transit nearby, it simply wasn’t sized to meet ridership demand, so they needed to expand to a bigger, better mass transit hub,” says Lally.

“Landfills are typically difficult to build on, because you’re never quite sure what’s underneath. They are prone to what is known as differential settlement. In other words, some areas might settle faster than others. As the material begins to decompose under the soil, it begins to lose its bearings, and it begins to collapse in on itself. It just rots away.”

Because of this differential settlement, site stabilization and pavement reinforcement were critical elements of the project.

With portions of the landfill ranging in height from 25 to 40 feet, the site was preloaded with 148,000 tons of fill to accelerate consolidation of the subsoil and refuse in the landfill. Initially, half of this amount was applied, then allowed to settle for six months. The remaining half was then added, with another six months for settlement.

Once this level of compaction was complete, Tensar TriAx Geogrid was put in place to stabilize the subgrade. Lally explains that the geogrid used on this construction is not the same as that used for retaining walls.

“There is a difference between geogrids used in retaining walls and geogrids used for subgrade improvement or for pavement optimization. In walls, geogrids are called uniaxial grids, because all the stress is basically in one direction.

“Our geogrids, which are triaxial, with a hexagonal arrangement, take a load and disperse it 360 degrees. It’s kind of like snowshoeing on a mountain range. The snowshoe takes a load and disperses it over a wider area. That’s what triaxial grids do-they alleviate the pressure. They disperse it over a wider area like a snowshoe effect.

“Triaxial geogrid is typically used under soft subgrades. So, for example, if you wanted to build a road over an area that has very soft soils, you would use the triaxial geogrid for subgrade improvement. Or you could use triaxial geogrids for component reduction for pavements or for new building foundations.

“In subgrade stabilization or pavement optimization applications, geogrids can be used to minimize the thicknesses of aggregate or rock layers. A typical project would include a 30% to 60% reduction in aggregate.

“The other purpose of this geogrid is that it provides a bridging effect, so that in the event something is undermined, the geogrid will bridge it and support it.”

For the parking lot area of this project, the TriAx Geogrid was applied on top of a 6-inch layer of fill that acted as a cushion for the landfill liner. Another 12 inches of fill was placed onto the geogrid, followed by a geomembrane liner and an additional 30 inches of fill. This area was then paved, with 4 inches of crushed surfacing and 3 inches of asphalt.

Areas of the landfill for bus lanes required additional support. Here, 12 inches of fill was placed over the existing liner, followed by the geomembrane. Then two separate layers of TriAx Geogrid were placed, 16 inches apart, within 60 inches of fill. This area was paved with a 3-inch layer of asphalt, reinforced with Tensar’s GlasGrid System, which was then covered with another 3-inch layer of asphalt.

By the time the project was completed in late 2012, the site totaled 8.1 acres and contained 46,000 square yards of TriAx Geogrid.

“Before this technology,” Lally says, “it would have been difficult to build on top of a landfill. But now we can build over these areas, whereas in the past the risk would have been much higher.”

He adds that there was also quite a bit of erosion and sediment control needed on the job. “There was inlet protection during construction to prevent sediment contamination of the local water supply, silt fence, and straw-coconut turf reinforcement mats. After construction, on areas that weren’t covered by asphalt or road, they used turf reinforcement mats to encourage vegetative growth. This project was just chock full of fabrics, grids, erosion control, pavement reinforcements-it was the full gamut.”

In January 2013, the county officially opened the new Hawks Prairie Park & Ride lot to the community, increasing the region’s park-and-ride capacity by more than 75%.

The Strength of Geomembrane With Concrete Cover
“There have been a lot of different liner materials tried in irrigation canals across the West over the last 15 or 20 years,” says Joe Kaul, owner of Kaul Corporation in Golden, CO. “Only a few have been very successful. The original way to line these canals was to use earthen materials such as clay, which involves the lowest cost. But over time, with freeze-thaw cycles, it begins to crack, and then it starts to leak. It’s just not a very efficient way to convey water.

“Some water is piped, but quite often there is too much volume, and there are turnout issues. These irrigation canals have a lot of different water users, so open channels are most often used because of the volume they can handle, and because of the frequency of different turnouts for different users who take their water rights from the channel.”

Kaul points to a 10-year canal-lining demonstration study by the US Bureau of Reclamation, which he terms “the most extensive study of its kind that I am aware of.” The study examined 34 different canal-lining sections that were constructed across four states to test durability and effectiveness. Most of the test sections ranged between 15,000 and 30,000 square feet.

The project divided the 34 units into four generic categories:

  • Fluid-applied membrane
  • Concrete alone
  • Exposed geomembrane
  • Geomembrane with concrete cover

Examining factors such as construction cost, durability, maintenance cost, and effectiveness of seepage reduction, the study concluded that the geomembrane with concrete cover seems to offer the best long-term performance.

Initial construction costs tended to be a bit higher for the geomembrane with concrete cover, but durability, low maintenance costs, and effectiveness equaled or exceeded those of the other categories. Each of the other options seemed to have significant shortcomings in one or more of these areas.

As an example of the geomembrane with concrete cover, Kaul describes the Jackson Gulch series of irrigation canals utilizing the Geoweb cellular confinement system manufactured by Presto Geosystems.

“A series of projects sprung up on the western slopes of Colorado that have all used the same cross-section, which is a cushioned geotextile, a 30-mil PVC liner, and another cushioned geotextile. There is one above and one below. The Presto Geoweb material with tendons is anchored over the top of that geotextile and then infilled with concrete. The tendons are tied to a pipe deadman anchor system and essentially suspend the Geoweb section over the liner system without having to anchor and puncture the geomembrane liner. Typical embankment slopes are 1.5:1 with narrow bed slope widths.

“The concrete offers the best long-term cover; it’s durable and prevents problems with sediment buildup. The sediment can come along with the water that is being conveyed, or it can slough off the hillsides adjacent to the canal, and it needs to be cleaned out. The concrete offers a cover for the geomembrane system underneath.”

Gary Kennedy, superintendent with the Mancos Water Conservancy District in Colorado, explains that the region has been rehabilitating its canal systems for a number of years.

“We looked at regular lining material with shotcrete over the top of it. We also looked at tiles-individual tiles that are put into place. At Jackson Gulch, we decided to go with the Geoweb system, filled with concrete, for several reasons.

“One was due to the expansion and contraction with the freeze-thaw cycle. It’s a high-elevation area. We’re at about 8,000 feet in elevation. We do get a lot of freeze-thaw on our project. Ice is one of our biggest problems. We went with the Geoweb system so we could have whole concrete cracks. We knew that it would give us a structure that, once it did crack, would give us a tile effect and give us some movement.”

As Kaul described it, there are two kinds of concrete-concrete that’s already cracked, and concrete that is going to crack.

“The important thing here,” he says, “is that it cracks in a controlled manner, along the walls of the Geoweb cells. The Geoweb system functions as a form for the concrete and isolates the concrete into smaller blocks, eliminating the need for steel reinforcement and expansion joints.  Since the concrete is isolated in smaller blocks, a degree of flexibility is introduced. The cell walls are perforated, and when the concrete is poured, it flows through the perforations, and this locks the concrete into the Geoweb cells.

“So when it cracks, it cracks along the line of the cells, allowing the system to flex and conform to minor subgrade movement. We know it’s going to crack. What you don’t want is for it to crack and not be reinforced down below. What can happen with typical reinforced concrete is that a whole section can crack, uplift, and become displaced. This doesn’t happen with the Geoweb system because of its flexibility.

“In addition, the Geoweb cells allow for a consistent concrete infill depth on a slope, which is not easily done otherwise. The 3D structure also allows for a higher slump concrete to be used, facilitating faster concrete placement. One other benefit is that concrete-filled Geoweb is more flexible than standard concrete and will last longer in freeze thaw situations.”

Kennedy explains that the initial section of Geoweb-lined canal was constructed in 2011, over a course of 1,600 linear feet. “We just put in our second section of lining of this type,” he says. “The first section we were very pleased with, so we went ahead and continued with that product. A lot of people in the area have looked at it and are very interested in trying to put in the same kind of lining system.”

Kevin Moran, a civil engineer with the Bureau of Reclamation, notes that about 60 miles away from the Jackson Gulch project, another lengthy irrigation canal is being constructed. Approximately 5,800 feet of the canal will use the Geoweb system, while another 6,000-foot stretch of the channel is being built using shotcrete.

“The Bureau of Reclamation has designed and funded projects using both construction methods,” he says. “We hope to see advantages to the geosynthetic cell system in that it helps control shrinkage cracks, allows flexing and slight movement, ensures the design thickness of concrete, and permits placement of the concrete via the chute of a transit mixer or pump truck, which is faster than through a nozzle and pump for shotcrete. We are also expecting that the maintenance will be minimal and less than the section of ditch lined with shotcrete.”
About the Author

Steve Goldberg

Steve Goldberg writes on issues related to erosion control and the environment.