12 - geosynthetic

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GeosyntheticsGeosynthetics is the term used to describe a range of generally polymeric productsused to solve civil engineering problems. The term is generally regarded toencompass eight main product categories: geotextiles, geogrids, geonets,

geomembranes, geosynthetic clay liners, geofoam, geocells (cellular confinement)and geocomposites. The polymeric nature of the products makes them suitable foruse in the ground where high levels of durability are required. Properly formulated,however, they can also be used in exposed applications. Geosynthetics are availablein a wide range of forms and materials, each to suit a slightly different end use.These products have a wide range of applications and are currently used in manycivil, geotechnical, transportation, geoenvironmental, hydraulic, and privatedevelopment applications including roads, airfields, railroads, embankments,retaining structures, reservoirs, canals, dams, erosion control, sediment control,landfill liners, landfill covers, mining, aquaculture and agriculture.

GeotextilesGeotextiles form one of the two largest groups of geosynthetic materials. They areindeed textiles in the traditional sense, but consist of synthetic fibers (all are polymer-based) rather than natural ones such as cotton, wool, jute or silk. Thus,biodegradation and subsequent short lifetime is not a problem. These syntheticfibers are made into flexible, porous fabrics by standard weaving machinery or theyare matted together in a random nonwoven manner. Some are also knitted. Themajor point is that geotextiles are porous to liquid flow across their manufactured

plane and also within their thickness, but to widely varying degree. There are at least100 specific application areas for geotextiles that have been developed; however,
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the fabric always performs at least one of four discrete functions: separation,reinforcement, filtration and/or drainage.

GeogridsGeogrids represent a rapidly growing segment within geosynthetics. Rather than

being a woven, nonwoven or knitted textile fabric, geogrids are polymers formed intoa very open, gridlike configuration, i.e., they have large apertures between individualribs in the machine and cross machine directions. Geogrids are (a) either stretchedin one or two directions for improved physical properties, (b) made on weaving orknitting machinery by standard textile manufacturing methods, or (c) by bonding rodsor straps together. There are many specific application areas; however, they functionalmost exclusively as reinforcement materials.Geonet is a synthetic drainage material manufactured from the highest quality highdensity polyethylene (HDPE) resin to transmit fluids and gases uniformly under manyfield conditions. It is a proven solution for many applications, including animal wastecontainment ponds.Geomembranes are the most widely used products for solid waste containment(such as landfill liners), mining, and water containment applications.

Geosynthetic Clay LinersGeosynthetic clay liners, or GCLs, are an interesting combination of polymericmaterials and natural soils. They are rolls of factory fabricated thin layers ofbentonite clay sandwiched between two geotextiles or bonded to a geomembrane.Structural integrity of the subsequent composite is obtained by needle-punching,stitching or physical bonding. GCLs are used as a composite component beneath ageomembrane or by themselves in geoenvironmental and containment applications

as well as in transportation, geotechnical, hydraulic, and many private developmentapplications.

GeofoamGeofoam is a product created by a polymeric expansion process resulting in foamconsisting of many closed, but gas-filled, cells. The skeletal nature of the cell walls isthe unexpanded polymeric material. The resulting product is generally in the form oflarge, but extremely light, blocks which are stacked side-by-side providing lightweightfill in numerous applications. The primary function is dictated by the application;however separation is always a consideration and geofoam is included in thiscategory rather than creating a separate one for each specific material.

GeocellsGeocells (also known as Cellular Confinement Systems) are three-dimensionalhoneycombed cellular structures that form a confinement system when infilled withcompacted soil. Extruded from polymeric materials into strips welded togetherultrasonically in series, the strips are expanded to form the stiff (and typicallytextured and perforated) walls of a flexible 3D cellular mattress or slab. Infilled withsoil, a new composite entity is created from the cell-soil interactions. The cellularconfinement reduces the lateral movement of soil particles, thereby maintainingcompaction and forms a stiffened mattress or slab that distributes loads over a wider

area. The result is high bearing capacity even from inferior fill material, reduction ofstructural layer thickness and longer-term durability. Traditionally used in slope
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protection and earth retention applications, geocells made from advanced polymersare being increasingly adopted for long-term road and rail load support.

Drainage / Infiltration CellsRelated to Geocells, being also three-dimensional cellular structures, these are not

infilled with compacted soil. They provide an extremely strong, in comparison to theirweight, void volume supporting structure. This allows a more effective managementof water and also gases, replacing traditional implementations such as gravel, bluemetal or stone aggregate. These systems are rigid, to support extreme weight,modular and scalable. Normally made of plastic injection molded material, they areused in conjunction with geotextiles, that serve as the necessary separation andfiltration media. Implementations and applications can range from roof gardens,hydro static pressure relief for retaining walls, increased catchment area andefficiency for stormwater harvesting, among others.

GeocompositesA geocomposite consists of a combination of geotextiles, geogrids, geonets and/orgeomembranes in a factory fabricated unit. Also, any one of these four materials canbe combined with another synthetic material (e.g., deformed plastic sheets or steelcables) or even with soil. As examples, a geonet with geotextiles on both surfacesand a GCL consisting of a geotextile/bentonite/geotextile sandwich are bothgeocomposites. This specific category brings out the best creative efforts of theengineer and manufacturer. The application areas are numerous and constantlygrowing. The major functions encompass the entire range of functions listed forgeosynthetics discussed previously: separation, reinforcement, filtration, drainage,and containment.

FunctionsThe juxtaposition of the various types of geosynthetics just described with theprimary function that the material is called upon to serve allows for the creation of anorganizational matrix for geosynthetics; see Table 1. In essence, this matrix is thescorecard for understanding the entire geosynthetic field and its design relatedmethodology. In Table 1, the primary function that each geosynthetic can be calledupon to serve is seen. Note that these are primary functions and in many cases (ifnot most) cases there are secondary functions, and perhaps tertiary ones as well.For example, a geotextile placed on soft soil will usually be designed on the basis of

its reinforcement capability, but separation and filtration might certainly be secondaryand tertiary considerations. As another example, a geomembrane is obviously usedfor its containment capability, but separation will always be a secondary function.The greatest variability from a manufacturing and materials viewpoint is the categoryof geocomposites. The primary function will depend entirely upon what is actuallycreated, manufactured, and installed.
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Table 1 - Identification of the Usual Primary Function for Each Type ofGeosynthetic

Type ofGeosynthetic (GS)

Separation Reinforcement Filtration Drainage Containment

Geotextile (GT) X X X X

Geogrid (GG) X

Geonet (GN) X

Geomembrane (GM) X

Geosynthetic ClayLiner (GCL)

X

Geopipe (GP) X

Geofoam (GF) X

Geocells (GL) X X

Drainage cell (DC) X X X

Geocomposite (GC) X X X X X

Geosynthetics are generally designed for a particular application by considering theprimary function that can be provided. As seen in the accompanying table there arefive primary functions given, but some groups suggest even more.

Separation is the placement of a flexible geosynthetic material, like a porousgeotextile, between dissimilar materials so that the integrity and functioning of bothmaterials can remain intact or even be improved. Paved roads, unpaved roads, and

railroad bases are common applications. Also, the use of thick nonwoven geotextilesfor cushioning and protection of geomembranes is in this category. In addition, formost applications of geofoam, separation is the major function.

Reinforcement is the synergistic improvement of a total systems strength createdby the introduction of a geotextile, geogrid or geocell (all of which are good intension) into a soil (that is good in compression, but poor in tension) or otherdisjointed and separated material. Applications of this function are in mechanicallystabilized and retained earth walls and steep soil slopes; they can be combined withmasonry facings to create vertical retaining walls. Also involved is the application ofbasal reinforcement over soft soils and over deep foundations for embankments and

heavy surface loadings. Stiff polymer geogrids and geocells do not have to be held intension to provide soil reinforcement, unlike geotextiles. Stiff 2D geogrid and 3Dgeocells interlock with the aggregate particles and the reinforcement mechanism isone of confinement of the aggregate. The resulting mechanically stabilizedaggregate layer exhibits improved loadbearing performance. Stiff polymer geogrids,with rectangular or triangular apertures, in addition to three-dimensional geocellsmade from new polymeric alloys are also increasingly specified in unpaved andpaved roadways, load platforms and railway ballast, where the improved loadbearingcharacteristics significantly reduce the requirements for high quality, importedaggregate fills, thus reducing the carbon footprint of the construction.

Filtration is the equilibrium soil-to-geotextile interaction that allows for adequateliquid flow without soil loss, across the plane of the geotextile over a service lifetime


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compatible with the application under consideration. Filtration applications arehighway underdrain systems, retaining wall drainage, landfill leachate collectionsystems, as silt fences and curtains, and as flexible forms for bags, tubes andcontainers.

Drainage is the equilibrium soil-to-geosynthetic system that allows for adequateliquid flow without soil loss, within the plane of the geosynthetic over a servicelifetime compatible with the application under consideration. Geopipe highlights thisfunction, and also geonets, geocomposites and (to a lesser extent) geotextiles.Drainage applications for these different geosynthetics are retaining walls, sportfields, dams, canals, reservoirs, and capillary breaks. Also to be noted is that sheet,edge and wick drains are geocomposites used for various soil and rock drainagesituations.

Containment involves geomembranes, geosynthetic clay liners, or somegeocomposites which function as liquid or gas barriers. Landfill liners and covers

make critical use of these geosynthetics. All hydraulic applications (tunnels, dams,canals, reservoir liners, and floating covers) use these geosynthetics as well.

"Base for Plantation on dome" Use of geotextile was made in little unusedapplication in India Pavilion at Shanghai Expo 2010. Plantation was to made on avery large dome (more than 34 m diameter). Non woven geotextile was used in twolayers. one layer was provided as a base layer and over the same another layer wasprovided which was punctured at the places where plants were to be planted throughstappling process. The gap between two layers was used for irrigating the plants andsupplying nutrients(Dr. K M Soni).

Advantages The manufactured quality control of geosynthetics in a controlled factory

environment is a great advantage over outdoor soil and rock construction.Most factories are ISO 9000 certified and have their own in-house qualityprograms as well.

The thinness of geosynthetics versus their natural soil counterpart is anadvantage insofar as light weight on the subgrade, less airspace used, andavoidance of quarried sand, gravel, and clay soil materials.

The ease of geosynthetic installation is significant in comparison to thick soillayers (sands, gravels, or clays) requiring large earthmoving equipment.

Published standards (test methods, guides, and specifications) are welladvanced in standards-setting organizations like ISO, ASTM, and GSI.

Design methods are currently available in that many universities are teachingstand-alone courses in geosynthetics or have integrated geosynthetics intraditional geotechnical, geoenvironmental, and hydraulic engineeringcourses.
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12 - geosynthetic

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