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Widely used in all major infrastructure projects in India, geogrids is a standardised technology that enjoys a market size of over Rs 5,000-6,000 million per year, avers Som S Sarkar.Geogrids are typically made from integrally jointed mono or bi-directionally orientated or stretched mesh made from polyethylene, polypropylene, polyester or similar polymer, with high secant modulus, in square, rectangular, hexagonal or oval mesh form. These have high junction, creep resistance and dimensional stability.Products that look like a grid and claim to have high tensile strength do not necessarily have the essential characteristics of a geogrid. To be called a geogrid, the right combination of grid geometry, dimensional stability and reinforcing strength resisting installation damage is required. This includes properties like junction strength aperture size, percentage of open area, rib thickness, long-term design load, tensile modulus and flexural rigidity.Significance of designA grid‘s design determines its interlocking capacity with the surrounding soil or granular particles, which in soil reinforcement dictates the amount of embedment length required to resist pullout from the soil. On the other hand, in base reinforcement, this interlock capacity governs the grid’s ability to confine granular particles against lateral movement, thereby increasing stiffness and bearing capacity within the granular base layer.Required dimensionsAs proposed by American Society for Testing and Materials (ASTM), the grid apertures must be large enough to allow the particles to penetrate through their opening. Also, they must be uniformly sized and distributed across the grid surface to assure continuity of the interlock throughout the grid’s plane. The most popular dimensions of rectangular apertures range between 25 mm and 75 mm for granular road base materials, while grids in walls typically contain rectangular or elliptical apertures with the longest dimension ranging from 3 to 12 inches in the direction of reinforcement. In both cases, however, the percentage of open area of the grids is over 30-50 per cent to assure adequate distribution of apertures across the grid.Rib and junction thickness in a grid provides adequate mass to the particles. This rib thickness should typically range between 2 mm and 7 mm as a very thin rib may not develop a strong, positive interlock. Additionally, dimensional stability of the grid geometry, strength of its ribsor stands, and strength through the junctions between longitudinal and transverse ribs are also critical parameters of successful performance.The significance of long-term design in a geogrid varies according to its application. For soil reinforcement in slope or wall, it is the most critical parameter and determines the strength to be maintained under loading conditions without creeping or aging. This reinforcement strength is mobilised through penetration of the particles through the ribs or strands, for which the grid develops horizontal soil bearings or passive resistance within the soil. The tensile load in the rib is passed through the grid junction to the adjacent inducing tensile reinforcement into the surrounding soil. To determine the long-term design strength of the grid along with its creeping characteristics, a specimen clamping rig inducing ‘through-the-junctionâ€� load transfer must be incorporated.Pullout resistancePullout resistance in geogrids inclu­des the embedment length required to prevent the failure of the enclosure. High flexural rigidity or stiffness is also an essential performance characteristic as it prevents folding or wrinkling, which often results in excess slack and deformation during placement and compaction of the first and most critical lift of fill over a weak sub grade.Engineered core of molecular orientation in manufactureThe specifications required to formulate a geogrid vary according to the specific performance requirements of each project. But the format and criteria for its applications should be consistent.Interlocking may be achieved by extrusion of HDPE/PP involving fusion of joints. Strength enhancement and creep sustenance is achieved through orientation of these planner products in one or both directions. For these purposes, it must be recognised that woven or laced products do not strictly conform to the above definition as they are unable to provide the effect of interlocking owing to poor junction strength. Such unjointed fabric forms should be treated as geotextiles, whose failure mode is by frictional failure, unlike interlocking or tearing failure in a true geogrid with an integral joint.Junction and modulus strengthA true geogrid is dimensionally stable with members and strong junctions with an integral junction. The manufacturing process has to assure the integrity of the junction of the grid structure and geometry to allow substantial junction strength, needed during placement and compac­tion of soil or granular fill over the grid.Additionally, HDPE/PP geogrids are pre-stressed to provide high tensile modulus or high reinforcing strength at very low elongation. Many net and fabric products exhibit a considerable stretch before breaking. These may have high ultimate strength but are not suitable for soil or base reinforcement as they stretch too much. A grid must be ‘strain compatibleâ€� with the medium it reinforces. It must generate reinforcing strength at low strain such as 4 to 5 per cent, before the soil or base layer undergoes significant or damaging strain that could lead to failure.Multiple usesEvery flyover/VUP/ROB and other embankment structures in India use geogrids today. Reinforced soil retaining walls have completely replaced age old masonry and RCC retaining walls. Earlier, retaining walls used to be mainly masonry or RCC retaining walls, but nowadays these are made with concrete precast panel or green facia having geogrid reinforcement inside. This helps in reducing approximately 35-40 per cent cost of the structure upto 6 m and even more beyond this height. These structures are also design friendly and usable in high-seismic zones.In sea reclamation projects, erosion on the sea front can be protected through a combination of different grades of geosynthetic materials. Here mainly geogrids, gabions and geotextiles are used in combination. For lining and separation applications geomembranes made of various materials like PVC, HDPE, LDPE, PP, etc, are popularly used. Similarly for waste management projects also geomembranes are used to separate the leechet at a collection points so it is not allowed to seep into the ground water.Limitation structures such as noise barriers, bastions and concertina can be made using a combination of geosynthetic materials. They can also be used to make blast walls, noise deflection barriers, bunkers, and so on.Geo-membranes, geotextiles, geocomposites and geogrids are already being used extensively in India and are laid out in the specifications of all major infrastructure projects. Indeed, this standardised technology enjoys a market size of over Rs 1,000 crore per year growing at a steady rate of 15-20 per cent annually.Characteristics needed to formulate a geogridChemical propertiesThe grid must resist the concentration of naturally occurring acids, alkalis and naturally pre-sent salts/minerals and aqueous media for planned lifetime use. It must resist diluted liquids and gasses it may come in contact with during the operation of the project.Chemical propertiesTo sustain its physical properties and ensure a long working life, the grid must be resistant to biological attacks. An essential element must be biological resistance to soil contaminants and soil microorganisms. As ground reinforcement, the grid’s mechanical properties should resist attacks from rodents, roots and insects.Characteristic propertiesProven engineered capability over sustained design life; quality assurance tested behaviour; ruggedness under site condition; durability; ease of handling and processing; capacity to form a soil grid composite.If you have an innovative technology to share, write in at [email protected]