Jan. 17, 2011 05:02

Designer Fabrics That Fit the Earth

WHEN WE WERE KIDS, MOST OF US loved to ride the roller coaster. It was exciting—the wind in your hair, the dipping and soaring. Even though we’ve been on them many times, and knew what to expect, each time that car climbed to the high spot, hit the top and gathered speed on the way down, it gave us a thrill. And when it took those curves, it was a kick. Today we’re a little older and hopefully a little wiser, and the roller coaster is relegated to memory. Now, we’re in the business world, and the kicks we get are from completing challenging projects and knowing we did them well. If you’re in the soil erosion control business and are involved in keeping roadways from slipping, or in other allied businesses, put on your safety belts, make sure the bar is down and you’re secured in your seat because we’re going to take you on a roller coaster ride. This one is a little different than the one you went on when you were a kid, but just as exciting. So hang on to your seats as we climb the peak and make those sharp curves through the world of geosynthetics.

Geosynthetics . . . what are they?

Geosynthetics is a generic term that is used to describe a variety of permeable textiles manufactured for use with soil, rock, earth or other geotechnical-related material. They can be used as an integral part of an engineering project, structure or building system. Most geosynthetics are made from synthetic polymers such as polypropylene, polyester, polyethylene, polyolefin, polystyrene and the polys could go on and on. These polymers have high fiber strength and are very resistant to biological and chemical disintegration.

When you are on a job, underground problems may not be realized until you start digging. These unknowns add unexpected costs and possible budget overruns to a project.

Silt or sediment that erodes and migrates from your jobsite can clog storm drains, pollute nearby waterways and ultimately bring on Phase II penalties that not only add days, or weeks to your building schedule but could also shut your project down.

You may find that you have to excavate and then replace or compact a soft soil. Discovering these problems at this late date just ends up taking profits out of your bottom line. Soil failures top the contractors’ list of expensive callbacks and claims from property owners.

Beginning in the 1960s and with more experimentation through the ’70s, geosynthetics firmly established themselves as a practical and cost-effective material for subsurface construction and building. Since the 1980s, the industry has seen a steady growth in the use of these materials. Yet many in the industry believe that geotextiles, along with other related geosynthetic materials, have hardly established their position as a sub-surface construction material.

There are several types of geotextiles or geosynthetics. Five of the most common primary functions of geosynthetics are:

1 Filtration

2 Separation

3 Reinforcement

4 Drainage

5 Erosion control

Filtration: A function most widely performed by geotextiles. For example, silt washing out of fine soils can quickly clog foundation drains or even storm drains. Synthetic filter fabrics block the silt while allowing the water to pass through, protecting permanent drains and keeping jobsite runoff from polluting roadside curbs, storm drains and nearby waterways. Separation: One of the most common uses of geosynthetics is to provide a separation between two layers that have different soil properties. When you mix mud and gravel, you don’t get more gravel, you get more mud. Fabrics placed between a gravel roadbed and soft underlying soil will keep traffic from driving the gravel down into the mud, or forcing mud up into the gravel.

Reinforcement: No soil has any tensile strength. Synthetic fabrics can introduce a tensile component into the equation. The concept is similar to that of reinforcing concrete with steel. Since concrete is weak in tension, reinforcing steel is used to strengthen it. Geosynthetic materials function in a similar manner by providing tensile strength that helps hold the soil in place. The reinforcement provided by geotextiles or geogrids allows embankments and roads to be built over very weak soils, and allows for steeper embankments to be constructed.

Drainage: Although filtering applications are commonly referred to as drainage applications, they are different. All soils perform best in a well-drained condition. Placed appropriately, a thin synthetic sheet weighing just ounces a yard can replace tons of granular fill as a drainage medium. Composite products, such as filter fabrics that are wrapped or laminated over a draining center, make a fast draining assembly that is inexpensive, effective and quick to install.

Erosion Control: Federal and state regulators are getting tough on silt-laden run-off from construction sites. To comply with the strict permit provisions, contractors need to use “Best Management Practices” (BMPs) that can stand up to regulatory scrutiny. Natural and synthetic fabrics are an important part of the strategy to keep soil from washing off slopes, to trap silt run-off, and to protect newly planted seed, and help in the revegetation of any exposed soil.

There is hardly a construction project that can’t benefit from the use of a geosynthetic material. These products can be divided into several different subcategories: Geomembranes: This is the largest category of geosynthetics.

These thin sheets of rubber or plastic are primarily used for linings and covers for solid waste facilities, as well as settling ponds and catch basins.

Geogrids: Unlike geotextiles, geogrids are manufactured in an open shape with grid-like patterns that are used in soil reinforcement.

Geo-cells: These are three dimensional, honeycombed cells filled with soils or gravels. The geo-cell is typically used for erosion control and soil stabilization. They provide compartmentalized cells to prevent the movement of soil and soil-like materials, thereby creating a very rigid layer of material. Geo-cells can also be filled with cement for use in heavy traffic areas.

Geonets: These are made of a geosynthetic material that consists of intersecting ribs that form a three-dimensional net-like product. They are used to improve drainage by creating a thin plane for water to move through.

Now, let’s look at the wide range of projects where geosynthetics shine as a construction material.

Subgrade Stabilization: Geosynthetics can improve the loadbearing capabilities and reduce road rutting when working over weak soils. Geosynthetics provide a separation barrier that will prevent dirt and fill from punching into the subgrade when exposed to construction traffic. The subgrade can then develop its full load-bearing capacity and the fill can better distribute the loads from traffic.

Asphalt Overlay: In roadway construction, the problem of moisture breaking through the surface from rain and melting ice and snow is the primary culprit in premature road failures. When a geotextile is installed between the old and new asphalt layers, it helps prevent moisture seepage by absorbing the asphalt coat sprayed on the old pavement, providing a permanent moisture barrier. Pavement life is extended, and maintenance costs reduced.

Sediment Control – Silt Fence: Geotextiles can control runoff from construction sites, and can be successfully used on newly-graded slopes and in areas where sedimentation control is a concern. They also have a place in controlling silt and suspended soils from construction near shorelines of lakes and reservoirs.

Steep Slope Reinforcement: With the use of geotextiles and geogrids, construction on slopes with far steeper angles is possible. The ability of the geotextile to reinforce and stabilize the soil allows for more efficient land use.

Internal Reinforcement– Retaining Walls: Used internally, geotextiles and geogrids in reinforced soil walls create a stable fill area behind the wall face. Geosynthetics allow for a significant reduction in concrete use, which decreases the cost of the wall’s construction and reduces the load-carrying requirements of the wall facing element.

Subsurface Drainage: Geotextile fabrics are used in subsurface drainage system applications as a permeable separator to keep soil out but permit water to pass freely. Uncontrolled groundwater can cause severe damage.

Soil Erosion Control / Turf Reinforcement: Erosion control mats are used to reinforce and stabilize the soil and turf in and along ditches, channels and slopes. The three-dimensional structure of the mat strengthens it and allows it to retain soil while promoting root and stem growth through the mesh.

Erosion Control Under Rip Rap: Geotextiles are used between the soil and the rip-rap to prevent the erosion of the soil base through the protective armoring layer. The geotextile is used instead of the old conventional graded aggregate filter. In this type of application, large stone, rip-rap or large concrete mattresses are used to protect the soil against erosion and constant wave attack. Geotextiles in this particular installation can be a more cost-effective means than that of the conventional aggregate filter system, and also allows for greater control during underwater applications.

There are two key factors that must be adhered to. First, the geotextile used must have sufficient permeability to allow for the passage of water to relieve the hydrostatic pressure that will build up behind the armor layer. Second, the geotextile must be capable of retaining the silt and soil particles under the armor as well.

With federal and state agencies pushing for better erosion control practices, the erosion control market has a product that, when used and installed properly, can meet the ever changing needs of the erosion control professional.

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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