Geosynthetics are man-made synthetic materials, where at least one of the components is made from a synthetic or natural polymer in the form of a sheet, a strip or a three-dimensional structure (woven, non-woven or woven). These products are used in geotechnical, environmental and civil engineering applications.
Geosynthetic products
Geosynthetics comprise a range of planar and three-dimensional structures. There are eight main product categories – geotextiles, geogrids, geonets, geomembranes, geosynthetic clay liners, geofoam, geocells and geocomposites.
Geotextiles: These are textiles comprising synthetic fibres rather than natural ones (cotton, wool, etc.) which make them less susceptible to biodegradation. These synthetic fibres are made into flexible, porous fabrics by standard weaving machinery, or matted together in a random non-woven manner. Geotextiles are porous to liquid flow across their manufactured plane and also within their thickness, but the extent of porosity varies widely. The classification is based on the method used to place the threads or yarns in the woven and non-woven fabric.
Woven geotextiles are cloth-like fabrics of controlled permeability formed by uniform and regular interweaving of threads or yarn in two directions. These products have applications in soil separation, reinforcement, load distribution, filtration and drainage. On the other hand, non-woven geotextiles are formed by random placement of threads on a mat and bonded by heat bonding, resin bonding or needle punching.
These are used in soil separation, stabilisation, load distribution and drainage. Geotextiles are the most commonly used geosynthetic products for agriculture, construction and transportation projects. These are relatively cost-effective products compared to other geosynthetics.
Geogrids: These are polymers formed into a very open, grid-like configuration, and they have large apertures between individual ribs in transverse and longitudinal directions. Geogrids are primarily used for soil reinforcement of unstable soil and waste masses. These products provide support for the construction of access roads, highways, dikes and other structures that previously required use of expensive piling methods on weak subgrades.
Geogrids are either stretched in one, two or three directions for improved physical properties; or made by weaving or knitting machinery by standard textile manufacturing methods; or bonding rods or straps together ultrasonically or through lasers.
Geonets: These are stiff polymer net-like sheets with in-plane openings used primarily as a drainage material within landfills, or in soil and rock masses. These are formed by a continuous extrusion of parallel sets of polymeric ribs at acute angles to one another. The two most common types of geonets are biplanar and triplanar. Alternatively, many very different types of drainage cores are available.
Geonets are typically used for road construction, restoration and repair to improve road surface quality, and decrease washboard formation and cracking. The major applications of geonets are in construction of roads, reinforcement of existing road and asphalt concrete pavements, prevention of cracking of roads, construction of high load platforms, patchwork, etc.
Geomembranes: These are relatively thin, impervious sheets of polymeric material used primarily for linings and covers of liquid- or solid-storage facilities. While geotextiles, geogrids and geocells are typically porous to allow water to filter through, geomembranes are used to control fluid movement. Thus the primary function is always containment as a liquid or vapour barrier or both. These products have low permeability and are used for scientifically closing waste containment areas and leachate pits. Geomembranes are more expensive in unit terms compared to other geosynthetic products.
Geosynthetic clay liners: These are rolls of factory-fabricated thin layers of bentonite clay positioned between two geotextiles or bonded to a geomembrane. They are used as a barrier to prevent liquid or waste contamination. They are also used as a composite component beneath a geomembrane or by themselves in geo-environmental and containment applications as well as in transportation, geotechnical, hydraulic, and many private development applications. The structural integrity of clay liners is maintained by needle punching, stitching or physical bonding.
Geofoam: This is expanded polystyrene or extruded polystyrene manufactured into large lightweight blocks, consisting of many closed, but gas-filled, cells. The primary function of geofoam is to provide a lightweight void fill below a highway, bridge approach, embank-ment or parking lot. Geofoam is also used in much broader applications, including lightweight fill, green roof fill, compressible inclusions, thermal insulation, etc.
Geocells: These are three-dimensional honeycombed cellular structures that form a confinement system when infilled with compacted soil. Typical geocells are made with ultrasonically welded high density polyethylene strips or novel polymeric alloy, and expanded on site to form a honeycomb-like structure, which may be filled with sand, soil, rock, gravel or concrete.
They are widely used in construction for erosion control, soil stabilisation on flat ground and steep slopes, channel protection, and structural reinforcement for load support and earth retention.
Geocomposites: Geocomposites consist of a combination of any of three geosynthetic materials – geotextiles, geogrids and geomembranes (for instance, deformed plastic sheets, steel cables, or steel anchors). They are primarily used for separation, reinforcement, filtration, drainage and liquid barriers. The application areas are numerous and constantly growing.
Advantages of geosynthetics
Application of geosynthetics offers many benefits including cheaper product costs, quick and effective protection against erosion problems, easy design methodologies, faster construction, consistency over a wide range of soils, increased safety factor, etc.
Further, geosynthetics also offer significant economic and environmental benefits. The economic benefits of geosynthetics include long-lasting, environmentally safe solutions for geotechnical engineering projects; project cost savings of around 30 per cent; minimal need for regular repair and thus direct reduction of maintenance costs; prevention of accidents; increased efficiency of structures; minimisation of pollution; and efficient use of natural resources. The cost of geosynthetic products is usually in the range of 3 to 5 per cent of the total project cost.
The environmental benefits of geosynthetics include long-term durability of civil engineering structures to prevent potential disasters (floods, droughts, earthquakes, global warming); long-lasting solutions at minimum cost (envisaged functional lifetime of over 100 years); and disposal without danger of contamination due to inert nature. Geosynthetics also assist the environment by acting as a containing barrier for toxic materials, and provide sealing and capping of pollution due to chemicals released after road and rail accidents into groundwater areas.
Conclusion
Given the multiple benefits of using geosynthetics in construction coupled with increased infrastructure spending, geosynthetics have become well-established construction materials for geotechnical, civil engineering and environmental applications in most parts of India.
As most construction projects are awarded on the basis of competitive bidding, the use of various geosynthetic products instead of more expensive materials (concrete, steel, etc.) should be emphasised. Going forward, growth in demand for geosynthetics will be driven by increased spending on infrastructure development activities, change in regulations pertaining to environmental protection, and increasing land scarcity.