May 16, 2016 04:32

Keep It Down with Coir, Jute Mats & Blankets

keep it down
When you need to keep a slope from blowing or washing away, you have a plethora of options. Erosion control is a wide field and the array of tools and methods at your disposal is vast. Between rip-rap, gabions, hydroseeding, articulated concrete blocks and geo-everything, it’s enough to make your head spin.

When it comes time to make the call, many times you’ll find that less is more. The soil doesn’t need a concrete vest, or a sea-foam green layer of seed; sometimes it just needs to be tucked in. For those sites that call for some sort of rolled erosion control products (RECPs), rolling out (and stapling down) the brown carpet can be enough to provide the lasting protection that site needs.

Of course, revegetation is never as simple as that, and estimating what’s required to regrow an area isn’t easy. Will that slope need one growing season or two, in order to meet the EPA’s guidelines? What if it doesn’t rain enough? What if it rains too much?

Each project will have its own slope and site conditions. Each will require a specific degree of protection, for a specific length of time.

Using a mat that isn’t strong enough, or won’t last long enough, can lead to project failures. These are financially costly and can be damaging to a company’s reputation.

The best way to avoid that headache is to know what your mats or blankets can and cannot do. To get a better handle on the different types of RECPs and their capabilities, I talked with the people who manufacture them.

Mark Myrowich, CEO of Erosion Control Blanket in Riverton, Manitoba, Canada, explained the design requirements for mats and blankets. “There are two purposes to blankets,” he said. “Firstly, they hold the seeds in place, which also means holding water and dirt in place. Secondly, they stop the raindrop impact from slamming into the dirt and breaking those dirt particles apart.”

This means that blankets have to be strong enough to shelter seeds from the vagaries of wind and rain for that slope and soil type. Ideally, they provide this support just long enough for the vegetation to mature to the point where it can live on its own.

That, in turn, means that RECPs have to hit a sweet spot in terms of durability and price point. Blankets that don’t provide enough support risk revegetation failure, and blankets that last longer than you need are unnecessarily expensive.

For this reason, mats and blankets are generally organized by their expected duration. They range from the ultra short-term (less than 90 days) to the permanent turf reinforcement mats, or TRMs, which are expected to last indefinitely.

At the short end of this list, you generally find stitch-bonded erosion control blankets (ECBs), composed of a fabric which is stitch-bonded to a netting. “The four fibers that you put into a stitch-bonded blanket would be straw, wood/excelsior, coconut and polypropylene,” said Myrowich. As for netting, “You can have plastic nets or biodegradable nets, and the biodegradable nets on stitched blankets are usually made of jute.”

Most short-term blankets are composed of straw, which has its pros and cons. “You use straw for its low cost, its large fibers and its ability to retain moisture in the ground,” said Myrowich. Straw also has a comparatively low tensile strength, making it suitable for milder slopes and gentler conditions. Furthermore, if a slope isn’t graded properly, pooling water can cause straw to rot, creating bare patches in the revegetation effort.

To understand how mats and blankets shake out when it comes time to apply them, I hunted down Adam Lyman, a contractor based in the town of Nampa in southern Idaho. Lyman is the manager of Syman LLC’s design department.

Due to the peculiarities of the geography he faces, Lyman is particularly suited to discussing the limits of RECPs. Southern Idaho is especially rough terrain for mat and blanket use. The area is rife with highly rocky areas, which disrupt the smooth slopes that RECPs generally require.

It also has very sandy soil, which makes staking down blankets a real problem. “There are some times when putting down six-inch staples is pointless,” said Lyman. “So we’ll use 18-inch wood stakes to actually have something that will anchor that blanket in.”

The anchoring problems were so prevalent that four years ago, the company switched to hydroseeding in areas where they used to lay blankets. So, between Idaho’s tough terrain, and the state’s direct regulatory supervision by the EPA, Lyman uses blankets when, and only when, they’re needed.

Even with those limitations, Syman LLC uses straw on slopes gentle enough to allow it. Lyman finds that while they are cost effective, they do add a little extra work at the end of a job. “It seems like with synthetic netting, the fabric will most often decompose before the plastic netting,” he said. “You’ll find a wad of netting sitting on top of the vegetation.”

Photodegradable nettings like these are made of plastic that has undergone treatment so that it will literally break down with sufficient exposure to sunlight. “There’s also oxodegradable plastic, which is treated with mineral salts that make it break down over time; you just need to have oxygen,” said Myrowich. “Then there are bioplastics, which are made out of plant-based materials, and can be composted.”

Most ECBs are available in a version that uses jute for one or both nets. “They’ll last about one or two years before they just degrade to nothing,” said Mike Jotzke, technical service manager for East Coast Erosion Control in Bernville, Pennsylvania. The plant fibers also add value as they decay. “Because they are natural materials, they do return a little nutrition to the soil,” he said.

Another plant-based netting option is coir, which is made from coconut husks. Coir has longer fibers than jute and tends to have a higher tensile strength. Both can be blended with straw to make the fabric of an ECB that little bit stronger.

Blended fabrics offer a middle ground between straw and coir in terms of tensile strength, longevity and price.

Also in this middle ground are blankets made of excelsior, or wood fiber. “Excelsior is the wood shavings from aspen trees,” said Jotzke, “six-inch strands that are cut with barbs on them so they stick together.” According to him, they last about two years, and are the next step above straw mats.

If you need a fabric any stronger than that, you’re probably looking at a full jute or coir fabric mat as the next step up. The two are sometimes referred to interchangeably, and the mix-up is understandable. Both have stronger woven varieties, both are resistant to saltwater, both are highstrength biodegradable fabrics and both are imported from Asia.

Jute comes primarily from Bangladesh, and the difference in weaves is the primary factor in strength and durability of jute blankets. “There’s basket weave, which is just a standard over/under type of pattern,” Jotzke said. “Or there’s the leno weave, which is more like a braided weave.” Leno is the stronger and more durable of the two.

Coir comes from India and Sri Lanka, and it starts as a bundle of unwoven, cured fibers. “Those are rolled into a bale, and we put that into a hopper machine, and distribute it into a blanket, like the straw,” said Jotzke. That generates the stitched type of blanket.

The cured fibers can be diverted from the hopper for a different result. “They will spin the fibers into a yarn, and that yarn is then woven into the mat,” he said. Woven mats are stronger and more durable than stitched.

“The coir mats normally come in 400 gram, 700 gram and 900 gram blankets. Those refer to the grams in each square meter of fabric, and more grams mean smaller holes and stronger fabric,” Jotzke said.

Jute and coir tend to get used where a mat needs to stand up to abuse. “We did a project a few years ago in a really windy area of Southern Idaho, where they had some really fine sand, and we hydromulched it,” said Lyman. “Unfortunately, some of the public was using the area for mountain biking, and so people rode over where we’d mulched and it cut our mulch up in the sand. Then the wind got under it and blew it all away in sheets.”

So the company went to the city and got the project specifications for the area changed. Then they rolled out a blanket that could hold up to what the bikers were dishing out.

“We went back and put four-foot wide rolls of jute matting on all of it,” Lyman said. “We used a lot of stakes, because it was such loose sand.” It worked well, and now they apply the same solution to areas like those which are easily eroded by foot traffic.

Having an ace in the hole like that can be helpful, as natural netting is an increasingly attractive green option. With environmental regulations ever tightening, projects specify natural net for its biodegradability, but that isn’t all.

“Natural fiber netting is flexible, so it contours to the soil a little bit better,” said Jotzke. “The other big selling point is that the joins are movable; they aren’t fixed.” That flexibility serves two purposes. Firstly, it alleviates the problem of animals trapping themselves in the netting, which can occur with plastic nets. Secondly, if a project calls for live staking, you can move the joins, rather than having to cut through them, preserving the structural integrity a bit better.

Sometimes, though, a project calls for revegetation in a spot where plants will never be able to hold out on their own. These situations call for a permanent structural aide to keep the plants alive and thriving in spite of the tough conditions.

Fortunately, TRMs are designed just for these purposes. Although they come in many varieties, TRMs are usually composed of polypropylene, and provide slope stabilization and erosion control on the steepest inclines and the highest flows. The range of TRMs is large enough for its own article, but it’s worth mentioning that their design calls for a netting tight enough that animals don’t usually get trapped in them.

Naturally, choosing which blanket best suits a site’s needs is only one part of the problem. They also need to be installed correctly. Some inexperienced contractors, or those unscrupulous ones, will underbid a project by underestimating the amount of labor required to pin in a blanket to manufacturer’s specifications. Then, when the chips are down, the costs go up or the blankets fail.

Good installation starts with trenching the mat in at the top of the slope or the head of the flow. Then it needs staples, in the pattern recommended by the manufacturer. “I’ve done some repair jobs where we find that there are almost no staples and I’m thinking, ‘Yeah, you probably got a really good price on installation,’” said Lyman. “If I weren’t paying the labor to put the staples in, I could offer a ridiculously low bid, too.”

Occasionally, site conditions call for even tougher measures. When there’s a problem site that has blown other blankets away, Lyman has an extra secure ‘super blanket’ technique that he uses. “We will trench it for wattles first, put the blanket on, and then go back and staple the blanket,” he said. “Then we’ll put wattles in our trenches on top of the blanket. I haven’t had a problem when I do that, even in really bad sand.”

Ultimately, blankets and mats are not one-size-fits-all solutions. On highly rocky terrain, you may find that hydroseeding works better. In a channel with a very high flow, you might find that hard armor is a necessity, and a TRM simply won’t do the job.

That said, ECBs and TRMs are excellent tools to have at your disposal, and they can be tailored to suit many projects. When a project plan says “put blankets in,” they’re calling on your expertise and awareness to decide what fits best. Doing a little extra research on your own, and thinking twice over the decision, may make a big difference for your company, your client, and the land you protect.

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