The Fabric of Erosion Control
Geotextiles have been used for thousands of years. Geotextiles were used in roadway construction in the days of the Pharaohs to stabilise roadways and their edges. These early geotextiles were made of natural fibres, fabrics or vegetation mixed with soil to improve road quality, particularly when roads were made on unstable soil. Only recently have geotextiles been used and evaluated for modern road construction.
A geotextile is defined as any permeable textile material that is used with foundation, soil, rock, earth, etc to increase stability and decrease wind and water erosion. A geotextile may be made of synthetic or natural fibers. In contrast, a geomembrane is a continuous membrane-type liner or barrier Geomembranes must have sufficiently low permeability to control migration of fluid in a constructed project, structure or system. A geotextile is designed to be permeable to allow the flow of fluids through it or in it, and a geomembrane is designed to restrict the fluid flow.
Geotextile-related materials such as fabrics formed into mats, webs, nets, grids, or formed plastic sheets are not the same as geotextiles. Although geotextiles have historically been made of natural plant, modern geotextiles are usually made from a synthetic polymer (such as polypropylene, polyester, polyethylenes and polyamides) or a composite of natural and synthetic material. Plant fibre-based erosion control geotextiles are subject to decomposition and have a limited shelflife before their inherent durability suffers. On-site use of these blankets degraded in this way can produce an ineffectual installation. The synthetic polymers have the advantage of not decaying under biological and chemical processes, but being a petrochemical-based product they use non renewable resources in their construction, and cause environmental pollution in their manufacture and use, and have associated health risks.
Geotextiles can be woven, knitted or non-woven. Different fabric composition and construction are suitable for different applications. The non-woven geotextile is an arrangement of fibres either oriented or randomly patterned in a sheet, resembling felt. These geotextiles provide planar water flow in addition to stabilization of soil. Typical applications include access roads, aggregate drains, asphalt pavement overlays, and erosion control.
Woven geotextile looks like burlap. It is a fabric made of two sets of parallel strands systematically interlaced to form a thin, flat fabric The strands are of two kinds - slit film which are flat, or monofilaments which are round. The way these two sets of yarns are interlaced determines the weave pattern that in turn determines the best application for that woven fabric. Weave patterns come in a virtually unlimited variety that do affect some properties of the fabric. Woven geotextiles are generally preferred for applications where high strength properties are needed, but where filtration requirements are less critical and planar flow is not a consideration. These fabrics reduce localized shear failure in weak subsoil conditions, improving construction over soft subsoil and providing access to remote areas through separation.
Ideally, vegetation can form the best erosion control, but this is often difficult to establish. The use of hydroseeding or seed impregnated fabric can be an effective method to establish vegetation. Hydroseeding, sometimes referred to as hydromulching is a process of planting grass that is fast, efficient and economical. A mix is made of mulch, seed, fertilizer, and water. This mix is then pumped and sprayed onto the ground where the slurry with a consistency of papier-mache provides an ideal germination medium.
Geotextiles are only superior to hydroseeding: 1) when the growing season is short and plants cannot stabilize the slope quickly, 2) at high altitudes, or 3) where major storms are a frequent occurrence. Too often, synthetic geotextiles are uses in situations where hydroseeding would be a far more appropriate choice. This overuse of geotextiles is because of the ease of use and low maintenance required.
Geotextile use will sometimes mask slope failures until erosion is too far advanced to effectively and cheaply remediate the slope. When advanced erosion is detected it means costly restoration. In contrast when a hydroseeded area has crust failure, whether from weather, human or animal activity, the damage is visible early and can be cheaply repaired.
Erosion control covers a variety of conditions from high velocity stream flow to heavy wave action, to less severe conditions.; All conditions should be considered before selecting a method of control.
Natural fibre geotextiles degrade to form an organic mulch and help in quick establishment of vegetation. Different fibres will degrade at different rates eg coir geotextiles degrade in 2-3 years while jute degrades in 1-2 years. Coir is therefore useful in situations where vegetation will take longer to establish, and jute is useful in low rainfall areas because it absorbs more moisture.
In many arid and semi-arid areas the action of the wind causes considerable erosion. Geotextiles made from natural fibre such as coir, or jute can be used for wind erosion control, dust control, sand dune formation and stabilization. Jute is particularly useful for dust control because of the hairiness of the fibres.
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