A versatile cost-effective material

Geosynthetics and have proven to be among the most versatile and cost-effective ground modification materials.

The overall measurement of the world market for geotextile usage is estimated at around 700million m2 annually and their use is rapidly expanding in both public and private construction projects as innovative uses and new products continue to appear in the market. The reason why geosynthetics is gradually gaining popularity in construction is that the polymers have the capability of solving geotechnical problems to a greater degree than conventional techniques.

A look back into history reveals that in the early 1970’s the materials were introduced in the construction industry as civil engineering fabrics where they were only used in civil engineering applications, later they were referred to as geotechnical fabrics which were used in soil engineering. However, in 1982 the term ‘geotextile’ became sufficiently suitable with the textile materials finding use in the soil environment. These materials became known as ‘geomembranes’ used in geotechnical environments. To encompass the polymeric material a new term was introduced ‘geosynthetics’ (polymeric) materials that are now used in the construction industry.

As the geosynthetics industry continues to grow, new and innovative products are being brought to market. They come in various names depending on their nature, characteristics and material used. The range is wide; geotextiles, clay liners, geocells, geofoams, geomembranes and many others. The main difference between them is how they are made or applied in order to achieve unique characteristics. For instance geotexiles consist of synthetic fibers that are used for separation, reinforcement, filtration, drainage, and protection applications. On the other hand geomembranes which represent the largest group of geosynthetics are rather impermeable sheets of plastic that exist in two main groups of geomembrane; calendered and extruded. Calendered geomembranes are formed by working and flattening a molten viscous formulation between counterrotating rollers, which are used in unique applications that provides tensile strength and enhances tear and puncture resistance. Extruded geomembranes are simply manufactured by melting polymer resin, or chips, and forcing the molten polymer through a die using a screw extruder with a main usage of enhancing the interface friction between the geomembrane and adjacent soils or other geosynthetics. They are extensively used in containment as a liquid or vapor barrier or both, also in landfills, surface impoundments, canals, and other containment facilities.

The application areas of these materials are numerous and persistently growing. The major functions are separation, reinforcement, filtration, drainage or containment.

Main areas of use
In the area of geotextiles the most common are the woven and non woven fabrics which as the names imply are filaments that are either woven or randomly distributed in layers. Approximately 75% of geotextiles are nonwoven in the form of heat-bonded staple made of synthetics polymers weighing from 120gm to 250gm/m2. On the other hand the market range of the woven fabrics stands at about 15% made from inexpensive lightweight woven slit polypropylene film weighing up to 2000gm m2. The remaining 10% is accounted for geogrids, geonets and erosion control mats worldwide.

In Africa, geomembrane lining systems are the most common geosynthetic material used due to their suitability in drainage on roads and rail, leach pads, landfills and mine sites. As Eng. John Alexander of Geosynthetics Technology Ltd, explains, drainage geocomposites on the other hand are the most ignored, due to lack of knowledge of their existence by specifiers and cost savings.

The biggest benefit realized is through using geotextiles in construction over wet or clayey areas, especially in remote areas where transport and haulage is very expensive, they have both technical and financial benefits as a new design methodology.

In road construction geosynthetics are used to strengthen base sections, improve surfacing and enhance drainage. The materials reinforce, separate and stabilize subgrade and base materials providing the benefits of reducing aggregate requirements, simplifying construction, increasing roadway life and reducing future maintenance. Geosynthetics also prevent accidents as they are able to withstand large lateral deformations and vertical differential settlements. Road structures can at times fail prematurely due to lateral displacement and weakening of base course aggregate. Geogrids or geotextiles are used to enhance performance or reduce the thickness of a permanent road, even when constructed on a reasonably competent foundation. They provide tensile strength in the earth mass in locations where shear stress would be generated in road works. Moreover, they allow rapid dewatering of the roadbed, and preserve their permeability without losing their separating functions.

Geotextile or geogrid is an important component and excellent alternative solution for railroads battling poor subgrades and the problems they can pose. Geosynthetics are a standard feature in modern track construction for they make a more effective construction method. In railway track construction earthworks in particular, geosynthetics have become requisite. They are used for separation, filtration and reinforcement in, under and next to the track bed. They prevent materials from wandering off sideways due to shocks and vibrations from running trains.

Erosion and drainage

Geosynthetic components are an integral part of erosion and sediment materials ranging from temporary products such as hydraulic mulch geofibers, plastic erosion control meshes and nettings, erosion control blankets and silt fences to high performance turf reinforcement mats, geocellular confinement systems, erosion control geotextiles and fabric formed revetments. These products are designed to help solve erosion- and sediment-control problems and to provide long-term stabilization by establishing and maintaining vegetative cover. The fabric, which lies beneath various forms of riprap cover (i.e. rock, gabions and mattresses, concrete pattern-placed units, etc.), allows free drainage of water while holding back fines thereby preventing shoreline and other erosion. When it comes to drainage and filtration, layers are used in civil applications to remove moisture or fine particles from the system. Geosynthetics provide a widely convenient, effective and economic alternative to traditional aggregate and sand filtration layers. Their significance is to remove the water and other fluids while providing excellent soil retention, assuring long-term free flowing drainage without soil loss. The applications include retaining walls, sport fields, dams, canals, reservoirs, and capillary breaks. Also the sheet, edge and wick drains are geocomposites used for various soil and rock drainage situations.

Landfills

Geosynthetic reinforcement in landfill applications involve the conventional reinforced soil structures and veneer stabilization with re-inforcements placed along the landfill slope and anchored at the crest. Landfill covers and base liners are placed horizontally to the geosynthetic reinforcements, which are attached to the solid waste, and fiber reinforcement to augment the shear strength of the soil liner material and the interface shear strength between the soil liner and texture geomembranes.

Dr. Filshill Archie of InterGeo Services says that one of the largest markets for geosynthetics is the use of geotextiles for paved and unpaved roads, since they act as separators between subbase and aggregates in roadway construction. This can come in very handy in Africa where most roads are unpaved and often unpassable during the rainy season. Their function is to eliminate the aggregate base lost into the subgrade which can reduce the durability of the roadway and create potholes. They also prove great value when constructing roadways over soft soils due to their high strength and embankments that can reduce the quantity of removal of unsuitable soils, adding that, they increase the value of the life cycle analysis.

Amongst the benefits and reasons why civil engineering in Africa should divert and embrace the proven durable and long lasting methods in construction is because geosynthetics are stress free when it comes to installation. They also increase efficiency while minimizing pollution of natural resources by stabilizing areas that are frequently and intensively used by people, animals or vehicles through establishing vegetative cover. Geosynthetics are also known to be resistant to chemicals and biological organisms usually found in soils. They stabilize against degradation due to short-term exposure to ultraviolet radiation.

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