May 15, 2015 06:08

Geosynthetics

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It’s hard to imagine preventing and controlling soil erosion today without the use of geosynthetics. These amazing, versatile and long-lasting materials have become vital to performing this task. They reinforce slopes and strengthen retaining walls, line retention ponds and channels, protect vegetation while it grows, keep silt from leaving construction sites, filter stormwater and much, much more.

have only been around for the last 60 years or so. Before that, all we had were the raw materials that Mother Nature provided. We manipulated those as best we could into the BMPs (best management practices) we needed. Not that there’s anything wrong with natural materials—straw, coir and wood shavings are still used today, especially when we need to have erosion-control devices that biodegrade.

“Geosynthetics have only been on the market since the mid-1960s, when Mirafi came out,” said Ron Whiteman, technical sales representative for Denver, Coloradobased Bowman Construction Supply, Inc.

Mirafi (a mashup of the words ‘miracle’ and ‘fibers’) is generally recognized as the first commercial geosynthetic. A product of TenCate Geosynthetics Americas, headquartered in Pendergrass, Georgia, it was actually invented by a company in Charlotte, North Carolina, that TenCate later acquired.

Geosynthetics have five major functions: soil reinforcement, sedimentation control, erosion control, filtration and drainage. The most familiar types of geosynthetics are geotextiles, also called ‘geofabrics.’ The world of geosynthetics also includes cellular confinement products, blocks that can be used to build walls.

There are two kinds of geotextiles: woven and nonwoven. The most common type of geotextile is nonwoven, needle-punched geofabric. It looks like felt and is black in color. Needles that look like fishhooks punch about 2,500 tiny holes per square feet in the fabric.

“What those holes do is allow filtration,” said Whiteman. “Water can pass through the nonwoven needle-punched fabric. If it’s used under a road surface, for instance, you won’t get ponding.”

Another major type of nonwoven geotextile is known as ‘spun bond.’ It’s called that because of the manufacturing process that produces it.

The fibers are spun, and then directly dispersed into a web by deflectors or air streams. Spunbond fabric is a bit cheaper to produce than the needle-punched kind.

Spun bond and needle-punched fabrics are the nonwovens typically used for separation and filtration. Spun bond is used mainly as a weed barrier. You don’t use needlepunched fabric that way, as weeds can poke up through the holes in it.

Woven geotextiles are primarily used for stabilization and confinement, and are generally sturdier. “The nonwovens hardly have any strength at all,” said Whiteman. “That’s why the woven geotextiles are out there; they’re very, very strong.”

Geosynthetics allow erosion-control contractors to solve difficult, multifaceted problems in creative ways, as the following projects will demonstrate.

Airport wall reconstruction

At Rocky Mountain Metropolitan Airport in Broomfield, Colorado, a mechanically-stabilized earthen (MSE) retaining wall, built just a few years earlier, was already failing. At the same time as this wall was starting to crumble, some new Federal Aviation Administration guidelines came into effect, necessitating that the safety area at the end of one of the runways needed to be enlarged. The runway is right next to that wall.

“In order to enlarge that safety area, the big MSE wall was going to have to come all the way down, and be completely rebuilt,” said Cal Kelley, an estimator and project manager at Kelley Trucking, Inc., a general earthmoving contractor in Golden, Colorado.

A further complication: a state highway runs by the airport right at that spot. This restricted the amount of land available to expand the runway.

“One corner of the embankment was confined by the roadway, so it was rather steep; essentially, we had a very large 1:1 slope,” explained Kelley. Correction—a large 1:1 slope that had to turn a sharp corner.

“The topography of the land, combined with the state highway being right there, dictated what we could do. This corner was kind of hemmed in, so to speak. Had there been more room, we would have been able to build a natural, gentle, 3:1 or 4:1 slope, something along those lines.”

The engineers involved in the project considered a number of solutions. No one wanted the wall to fail again. Ultimately, they decided that the best solution was to rebuild the wall using a cellular confinement product (known as ‘geocell’). Geoweb, a polypropylene geocell system from Appleton, Wisconsin-based Presto Geosystems, Inc., was chosen.

“I’m sure they could have done another MSE or shotcrete wall,” said Kelley, “if they could have bought more land. But, with all the engineering and budgeting considerations, this was probably the optimal solution to the problem.”

A 1:1 slope is essentially a 45- degree angle. Kelley explained that topsoil is not going to hold very well at such a steep angle, and if you get any moisture at all, it’ll slide right off, and you won’t get any vegetative growth on that slope. The intention behind using the geocells was that they would hang onto the topsoil and the seed until the root bed could take hold and grow, providing an even better anchor for the wall.

The first step to the building process was to place a geotextile liner at the base. A layer of geocell was placed on top of that, along with select structural backfill (embankment dirt that had been mined onsite) and produced aggregate.

Thereafter, layers of geocell and structural backfill were piled every 18 inches throughout the entire height of the wall, which was laid back 90 feet.

“It was particularly tricky building this wall, because of that corner,” said Kelley. “You could only have X amount of pieces of geocell overlapping before you had to put three to six inches of backfill material in between the next overlapping piece of geocell.”

On the front face of the slope, four-inch geocells were hung and tied down with tendons. The tendons were anchored up at the top of the slope, and strung all the way down. The geocells hung off of those tendons, and were reinforced into the slope with small pieces of rebar. The rebar also linked the cells together.

The geocells were infilled with screened topsoil, amended with fertilizer and seeded. “Over that we placed another touch of topsoil, then mulch over the top of that,” said Kelley.

The wall will eventually be entirely covered in vegetation, which was one of the reasons for choosing geocell to build it. “We’ll be going into our first full growing season this spring, so we’re hopeful that it all takes root. It looks promising at this point.”

The project took several months to complete. “We had to embank 900,000 cubic yards of material,” recalled Kelley. “It was a pretty neat project that had to be done very meticulously, and thus was very time- and labor-intensive.”

When finished, in June of last year, the wall spanned 700 feet in length, 791 feet in width, and reached a height of 70 feet. Monitoring devices implanted in the wall have not detected any movement so far.

Golf course flood channel

Geosynthetics are the hero of another tale, finally solving a problem that had persisted for years at the Desert Rose Golf Course in Las Vegas, Nevada.

There were long-standing problems with this public golf course, built in 1964. The problems stemmed from the fact that this site has a unique feature.

Two major washes, the Las Vegas Wash and the Flamingo Wash, run right through it. These washes channel the monsoonal desert rains. “Two-thirds of the Valley’s watershed goes through the course from these two washes,” said Joe Damiani, principal civil engineer for the Clark County Regional Flood Control District. If you play the second hole, you’ll be right at the spot where both washes converge.

“Over the years, parts of the course had been washed out in different areas. We had tried some different types of treatments, such as riprap and cement, but it still kept getting eroded out.”

A concrete low-flow channel had been built to help re-direct the water from the two washes away from the course. “However, the channel wasn’t built to handle the capacity of flow that was actually needed,” Damiani said. “So we were always fighting an uphill battle.”

The greens weren’t the only things getting washed out. “The reason they did this project in the first place is because of the capacity of that channel,” said Nick Largent, a project engineer at Las Vegas Paving Corporation, the principal contractor. “During major rain events, it flooded the golf course and dumped a lot of debris onto the fairways and the sand traps. That was a pain to clean up, but the big rain events also flooded homes. That was the biggest concern.”

The District was already considering flood-control improvements, and had set aside $35 million for the purpose. Then, in 2012, a flood described as “deadly” devastated 74 homes near the course. After that, the District boosted the budget for the project to $50 million.

It was time to fix the problem, once and for all. “So we said, ‘Fine; let’s go ahead and design it properly, widen the channel to give it the full capacity it needs, and leave the grass in wherever we can,” said Damiani.

The question: how could they increase the capacity of the channel while still keeping the area it runs through as a functioning, aesthetically pleasing golf course?

“The golf course architect and the city came to us looking for geosynthetic solutions that we’d used on previous, similar jobs,” said Tom Tueller, a sales engineer at West Chester, Ohiobased Contech Engineered Solutions, LLC. “We suggested a product that can handle fast water velocities as well as reinforce turf.”

The product was Pyramat, a high-performance turf reinforcement mat (HPTRM) made by Propex Global in Chattanooga, Tennessee. (Contech distributes and promotes the product.) It’s a three-dimensional, woven-polypropylene engineered geotextile designed for erosion-control applications on steep slopes and vegetated waterways.

The product consists of hexagonal fibers woven into a matrix. The fibers themselves are made of polypropylene monofilament yarns, woven into a uniform series of pyramid-shaped projections (hence the ‘pyra’ part of the name).

“The majority of the product was going to be put underneath the sod, to help get better root establishment,” said Tueller.

“Because the site had such a long history of erosion issues, they wanted to make sure that the vegetation rooted in really well. They also needed something with a very high tensile strength. This stuff is so strong, you could drive over it with a backhoe and it’ll stay in place.”

“There was another part, a wetland, that they weren’t going to put sod over,” said Tueller. “This area is the course’s ‘rough;’ they were going to plant a few trees and shrubs there. We put mat over the topsoil. I told them they could either leave the mat as it is, or put a little bit of topsoil over it. It would help hold the soil in place during a storm event, keep it from uprooting everything.”

One of the goals was to maximize the amount of grass available for play, so the concrete low-flow channel was also matted and sodded.

Besides preventing soil erosion, the mat also helped create a better, more playable golf course. Now, if a golfer hits a ball into the channel, instead of bouncing around on the concrete, he could still play it.

Two different types of Pyramat were used with two different UV ratings, one for the sodded areas, and another in the wetlands area.

Damiani, “and we were able to shore them up.”

“One thing we learned in this process is that the sod and the mat actually work together,” said Largent. “The system doesn’t really The product would be more complete itself until the sod is exposed in the latter area, so the mat with a higher UV rating was used there.

It usually takes about two weeks for sod to root in. But the rain had other ideas. It came down pretty hard several times during the rooted in. Once you get it all together as a package, it works really well to control erosion.”

And the results? “It’s been performing really well,” said Damiani.

“We’ve had some minor rain events that have gone through, but it’s held up nicely. We’re anticipating that this solution is going to last a long, long time.” The Desert Rose Golf Course is scheduled to reopen in September of this year.

Largent has had occasion to work with a number of geosynthetic materials over the years, and has a high opinion of them. “They’re pretty easy to work with. It used to be installation phase. This caused some problems.

“We’d roll out the sod, then get a rain event four or five days later, and have to roll it back up again,” said Largent. Several rolls of Pyramat had to be replaced, even though it hadn’t been damaged. “The product stayed in place just fine. It held the soil down underneath it really well. The replacement had more to do with the interaction between the sod and the mat, with the sod not being completely grown in yet.”

In the concrete channel, the mat had been bolted to the sides. Even so, the rain caused problems. A few times during the storm events, downstream water was able to get underneath and rip it right out.

The people working on this project were able to turn this to a positive. “The rain events showed us where the weaknesses were,” said that some of the rolled products had to be placed in a certain direction on a slope. They’re getting to the point now where there’s not a particular way you have to install them, so that process is getting a lot easier.”

He said that the manufacturers always have suggestions on how the materials should be installed. “It’s still kind of a specialized market, so the companies we’ve dealt with have been happy to send reps to meet with us. They’ll explain exactly how we need to do things, set things up on a project.”

The importance of geosynthetics in soil erosion work can’t be overstated. New and improved products are coming out all the time. The more you know about them, the more you appreciate the ways they can be employed to make your work not only easier, but more enduring.

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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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