Specification Part 3 - Road & Bridge Works - Urban Administration ...

701 GEOSYNTHETICS IN ROAD AND BRIDGE WORKS
701.1 Scope
This specification covers the various applications of Geosynthetic materials in road and bridge works
including supplying and laying as per special provisions.
“Geosynthetic is a general classification for all synthetic materials used in geotechnical engineering
application. It includes geotextiles, geogrids, Geonets, geomembranes and geocomposites etc.
i) Geotextiles: They are indeed textiles in traditional sense, but consists of synthetic fibers.
These fibers are generally non biogradable. The synthetic fibers are made into a flexible,
porous fabric by standard weaving machinery or are matted together in a random, or
nonwoven, manner. Some are also knit. Woven geotextiles are produced by weaving or
interlacing two or more sets of fibers usually at right angles. Non-woven geotextiles are
produced by mechanical bonding or needle punching of randomly oriented fibers. In
some cases, non-woven fibers may be bonded thermally or chemically. Geotextiles
generally available have thickness varying from 0.25 mm to 7.5 mm and have a mass per
unit area 150 to 2000 g/m 2 referred as gsm. The major point is that they are porous to
water flow across their manufactured plane or also within their plane, but to a widely
varying degree. They are used in various geotechnical engineering applications for
Geotextiles that have been developed; however, the fabric always perform at least five
discrete functions i.e., separation, reinforcement, filtration, drainage and moisture barrier
(when impregnated).
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Geogrids: These are primarily made of synthetic materials. Rather than being a woven,
nonwoven, or knit textile; geogrids are formed into a very open, grid like configuration,
i.e., they have large apertures. The geogrids are relatively high strength, high modulus,
low-creep- sensitive polymers with apertures varying from 10 to 100 mm in size. The
openings/holes in geogrids are either elongated ellipse, near squares with rounded
corners, squares or rectangles. Geogrids may be used for the purpose of separation but
invariable they are being used in some form of reinforcement.
Geonets: Geonets constitute another specialized segment of geosynthetics. They are
usually formed by a continuous of polymeric ribs at acute angle to one another. When the
ribs are opened, relatively large apertures are formed in a netlike configuration. Geonets
are like geogrids in configuration but they are used for the purpose of drainage. It is to be
noted that geonets are always used with a geotextile, geomembrane or other material on
their upper or lower surfaces. Clearly geonets are used for their drainage capability while
geogrids are used for reinforcement.
Geocomposite: A geocomposite consists of a combination of geotextile and geogrid; or
geogrid and geomembrane; or geotextile, geogrid and geomembrane or any of these
three materials with another material in laminated or composite form. Prefabricated
Vertical Drains or Band Drains, which are widely used for accelerating the consolidation
of soft soils also come under the category of geocomposites.
v) Geomembranes : Geomembranes are also synthetic materials but they are impervious
and made of thin sheets of rubber or plastic materials used primarily for lining and cover
of liquid-or solid-storage facilities.
701.2 Material Testing and Acceptance
Unless otherwise stated, the Geosynthetic materials shall conform to the requirements as under:
702.2.1 Geotextile: Geotextile shall be made of polyethylene or polypropylene or polyester or
similar fibers, either woven or nonwoven or knitted in variety, through machine made process of heat
bonding or needle punching or weaving techniques. These fabrics are required to pass water through
but retain the soil particles, which require specific cross-plane permeability or permittivity and apparent
opening size or equivalent opening size or 0 95 . The above two requirements along with the requirement
of strength and durability denote general characteristics of geotextiles to be used.