July 15, 2014 11:34

Exploring the World of Geosynthetics

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Geosynthetics is really an umbrella term for the vast array of materials that make up the world of engineered products. They are used for controlling erosion, covering soil, or for separating layers of soil. These products can also be used as soil stabilizer, even as drainage aids. In addition, they are used to control weeds . . . and the list goes on. The truth is, the world of geosynthetics is expansive and complex.

 

To an outside observer, geosynthetics may seem like just a fancy word for those big mats that landscape and construction crews are always laying down.

They are mats, and they cover up dirt. Does it really make any difference if they’re made out of straw or plastic or paper? Yes, it does—different materials have different application in helping control erosion.

Some of the erosion control materials we discuss in this article consist of natural fibers: straw, wood or coir (coconut husk). These are not synthetic materials; however, the products are used in the same way geosynthetics are, so they should be mentioned.

The principle of soil stabilization has been around since Roman times. Mats of woven natural fibers were used on and around roads and on steep slopes to stabilize dirt.

The incorporation of a structured material into, or placed on top of the soil, better holds it and keeps it in place. This basic idea of soil stabilization is the basis of the geofabrics used today.

However, not all geofabrics are alike, and they cannot all do the same jobs. Whereas the mats of old began and ended with stabilizing soil and keeping it from eroding, the wide variety of products available today can do so much more.

They come in forms ranging from thin straw blankets, to thick polypropylene mats, to honeycombed plastic cells filled with soil.

The largest and most common group of geosynthetics for controlling erosion comes in the form of mats that can be laid out on the ground. These blankets can be rolled up and rolled out like, well, blankets. They are referred to as rolled erosion control products or RECPs.

RECPs can be broken down into the categories of temporary Erosion Control Blankets (ECBs) and permanent Turf Reinforcement Mats (TRMs). The primary difference between the two is the length of time for which they can be used, but there are many secondary differences in material makeup and appropriate uses.

ECBs are not technically geosynthetics. Unlike their more permanent counterparts, ECBs are made out of natural fibers, such as straw and coir and are, therefore, biodegradable.

If you have an area that is vulnerable to erosion, but plant material can grow on it, you’re going to need ECBs to cover the area until the plants take over. The length of time it takes an ECB to degrade is often its selling point.

You can allow desired plants to grow by cutting out X-shaped slits in the blankets and positioning plants to poke through the Xs. Since the blankets are water-permeable, the plants are able to get the sunlight and water they need to thrive, while undesired weeds get stuck under the blanket and will be unable to grow.

“Straw blankets will disintegrate much faster than coir. Coir can last 30 to 36 months,” said Disala Sondapperuma of Haymark Inc., in Spring, Texas. “Straw may have some applications, but when you need it to last longer, coir is a better choice.”

Blankets are typically classified as ultra short-term (three months), short-term (12 months), extendedterm (24 months), or long-term (36 months).

Nearly all other geofabrics are designed to be permanent and are made out of high-tech plastics, generally polypropylene. It’s this family of materials—the actual synthetics of geosynthetics—that make up the bulk of the industry, especially in large-scale industrial capacities. It’s varied enough that it is split into the subcategories of geotextiles, geomembranes, geogrids, geonets, geocells and geocomposites.

The type of geosynthetic most closely related to ECBs is, essentially, their more permanent version. They are designed for applications where erosion control is expected to be an ongoing issue that will always require man-made countermeasures. TRMs are nonbiodegradable, and commonly have a lifespan of 15 years or longer. Though they are almost all made out of polypropylene, their structures, and therefore uses, can vary.

“TRMs are the most common of the 100 percent geosynthetic erosion-control materials,” said Joel Sprague, P.E., senior engineer at Austin, Texas-based TRI/Environmental, an independent third party testing facility. “They’re designed to provide both short-term erosion protection against stormwater flows and, when combined with vegetation, long-term protection and extremely high resistance in very high flows.”

Sprague’s job is doing simulations of actual erosion control situations at their outdoor testing lab.

“We’ve done more testing on vegetated materials with TRMs than anybody, and we’ve been able to show that they’re quite effective. The erosion resistance is equivalent to or better than rock. We’ve done testing after one year of growth and we can’t develop enough flow to cause failure.”

TRMs can also be considered to be a part of the largest subcategory of geosynthetics, which is geotextiles. Geotextiles, sometimes also called filter fabrics, are also the most relevant to most of those who use geosynthetics. They’re available in two main subtypes: woven and nonwoven.

But before getting into the differences between woven and nonwoven geotextiles, we should outline what they have in common.

Dave Snyder, geosynthetic marketing manager for Hanes Industries in Winston-Salem, North Carolina, says, “Geotextiles perform three primary functions—separation, filtration and reinforcement.” These three functions back each other up.

A geotextile will separate the native soil from an eroding force. This separation keeps the native soil together and more structurally sound than if it were allowed to mingle with the forces and materials on the other side.

In most landscaping applications, a mat usually separates the soil on the bottom from some type of mulch on top. In more industrial applications, the geotextile may be covered with crushed rocks, gravel or something similar.

It’s important to always keep synthetic geotextiles covered, or at least out of the sun, because their plastic makeup leaves them susceptible to damage from ultraviolet radiation.

All geotextiles are water-permeable, so the filtration ability is paramount. In applications where these fabrics are used around plants, it allows the water and nutrients to reach the soil and soak down to the roots.

While a geotextile must allow water to pass through freely, it must also keep the native soil from passing through it. If soil can work its way from one side to the other, the material would not be an effective barrier. The result would be the loosening of dirt that would undermine the reinforcement of the soil.

Geotextiles need to reinforce the area they are laid on. “Essentially, what a geotextile mat has to do is take a load that is placed on top of it, and spread that load over a larger surface area,” said Snyder. “For instance, when constructing a gravel road, a woven geotextile would be placed on top of the native soil, which is then covered in gravel. The combination of gravel and getotextile takes the concentrated force from car tires and spreads it out over the native soil. It’s similar to how a snowshoe does less damage to snow than regular boots.” This effect curbs erosion and damage to the underlying soil, and thereby keeps the road in good condition.

All geotextiles provide some combination of separation, filtration and reinforcement, but no product is a one-size-fits-all. What product works best depends on the specifics of the job it’s intended for. There are hundreds of geosynthetic products on the market today, and no one of them can really be said to be the best for every single application.

Woven geotextiles are made of synthetic fibers woven together, just like any cloth. This stable network of interlocking fibers provides strong reinforcement capabilities.

Water can still flow through the material, and can even be filtered in an appropriate direction. Compared to nonwoven types, they have a higher tensile strength. So if a job calls for something like road reinforcement, woven is probably a good way to go.

Nonwoven geotextiles are very similar to their woven siblings, but their different structure leads to a few differences. In production, nonwoven geotextiles are made by taking polypropylene fibers and needle-punching or heat-bonding them together into cohesive mats.

What’s important is to be knowledgeable about what’s needed on a particular project. The specifications for the project will determine what products are most appropriate.

When evaluating a project’s requirements and deciding which products to use, you’ll find that all products have tradeoffs. For example, a mat with the perfect filtration qualities may not have the tensile strength you require. But there is no need to find everything yourself. Here’s where your distributor comes in; they should be able to help you find the right product.

“This industry has evolved enough now that distributors who are truly in the business will have the right product to match your application,” Snyder said. “They’re going to focus on getting you exactly what you need for your application. They will have a cost-effective product for you that will last the length of time you need it to last. They will know the prevalent specifications. They’ll know what works best in a particular area and make sure to have it available.”

It’s a fortunate resource for those doing erosion control work. Even with all the various products outlined here, there’s still so much more to geosynthetics that we could devote another entire article to them.

There are geosynthetics with complex structures that go beyond the form of mats. Geocells form a honeycomb structure filled with soil that is excellent at soil reinforcement on roads. Geonets are tough netting that directs water runoff like nothing else does; geogrids are thin plastic structures that provide excellent reinforcement.

Goesynthetics have proven to be versatile and cost-effective ground modification materials. They have become essential elements as barriers in environmental applications.

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