Geotextiles and Geomembranes

Geotextiles are permeable fabrics which, when used in association with soil, have the ability to separate, filter, reinforce, protect or drain. As the use of geotextile fabrics has expanded there has been the introduction of geotextile composites and the development of products such as geogrids and meshes. Overall these materials are referred to as geotextiles and related products. All have a wide range of applications and are currently used to advantage in many civil engineering applications including roads, airfields, railroads, embankments, retaining structures, reservoirs, canals, dams, bank protection and coastal engineering.

Geomembranes are impermeable membranes used widely as cut-offs and liners. Until recent years, geomembranes were used mostly as canal and pond liners; however, one of the largest current applications is to the containment of hazardous or municipal wastes and their leachates. In many of these applications geomembranes are employed with geotextile or mesh underliners which reinforce or protect the more flexible geomembrane whilst also acting as an escape route for gases and leachates generated in certain wastes.
The range of products and their applications has expanded rapidly over the last decade with geotextiles and geomembranes being specified world wide. This rapid growth is paralleled by a virtual explosion of technology. Current reference books and even manufacturers' sponsored publications tend to date very quickly and the need for a vehicle to bring together and discuss the growing body of technology now available has become evident.

Geotextiles and Geomembranes fills this need and provides a forum for the dissemination of information amongst research workers, designers, users and manufacturers. By providing a growing fund of information the journal increases general awareness, prompts further research and assists in the establishment of international codes and regulations.

Functions of Geotextiles

There are at least 80 specific applications area for geotextiles that have been developed.However, the fabric always performs at least one of five discrete functions:

Separation

Geotextiles function to prevent mutual mixing between 2 layers of soil having different particle sizes or different properties.

Drainage

The function of drainage is to gather water, which is not required functionally by the structure, such as rainwater or surplus water in the soil, and discharge it

Filtration

Filtration involves the establishment of a stable interface between the drain and the surrounding soil. In all soils water flow will induce the movement of fine particles. Initially a portion of this fraction will be halted at the filter interface; some will be halted within the filter itself while the rest will pass into the drain. The geotextile provides an ideal interface for the creation of a reverse filter in the soil adjacent to the geotextile. The complex needle-punched structure of the geotextile provides for the retention of fine particles without reducing the permeability requirement of the drain.

Reinforcement

Due to their high soil fabric friction coefficient and high tensile strength, heavy grades of geotextiles are used to reinforce earth structures allowing the use of local fill material.

Protection

Erosion of earth embankments by wave action, currents and repeated drawdown is a constant problem requiring the use of non-erodable protection in the form of rock beaching or mattress structures. Beneath these is placed a layer of geotextile to prevent leaching of fine material. The geotextile is easily placed, even under water.

Properties of Geotextiles

The properties of polymer material are affected by its average molecular weight (MW ) and its statistical distribution. Increasing the average MW results in increasing:

• Tensile strength

• Elongation

• Impact strength

• Stress crack resistance

• Heat resistance

Narrowing the molecular weight distribution results in:

• Increased impact strength

• Decreased stress crack resistance

• Decreased processability

• Increasing crystallinity results in:

• Increasing stiffness or hardness

• Increasing heat resistance

• Increasing tensile strength

• Increasing modulus

• Increasing chemical resistance

• Decreasing diffusive permeability

• Decreasing elongation or strain at failure

• Decreasing flexibility

• Decreasing impact strength

• Decreasing stress crack resistance

Rawmaterial for Geotextiles

The four main polymer families most widely used as the raw material for geotextiles are:

• Polyester

• Polyamide

• Polypropylene

• Polyethylene

The oldest of these is polyethylene, which was discovered in 1931 in the research laboratories of the ICI. Another group of polymers with a long production history is the polyamide family, the first of which was discovered in 1935.The next oldest of the four main polymer families relevant to geotextile manufacture is polyester which was first announced in 1941.The most recent polymer family relevant to geotextiles to be developed was polypropylene, which was discovered in 1954.

Types Of Geotextiles

In General, the Vast Majority of Geotextiles are made from polypropylene or polyester formed into fabrics as follows:

• Woven monofilament

• Woven multifilament

• Woven slit-film monofilament

• Woven slit-film multifilament

• Nonwoven continuous filament heat bonded

• Nonwoven continuous filament needle-punched

• Nonwoven staple needle-punched

• Nonwoven resin bonded

• Other woven and nonwoven combinations

• Knitted

Woven geo-textiles Concrete bases used for coastal works, water ways, embankments and in forming geocell for road.

Nonwoven geo-textiles
They are used for Filtration, drainage, reinforcement between soil stone or aggregate and in roads, railways works, erosion prevention and separation. As filter fabric for dams, under drainage system liners for pile foundation, coated pvc and bitumen s to skin traction

Knitted Geo-textiles

They are used in Knitted bags for protection of dam's riverbank etc. Warp knitted fabric of Kevlar yarns used in automobile and marine application. It is also used with foundations, soil, rock, earth or any other related material as an integral part of human man made project, structure or system.

History of Geotextiles

According to the historical record, it is believed that the first applications of geotextiles were woven industrial fabrics used in 1950's. One of the earliest documented cases was a waterfront structure built in Florida in 1958. Then, the first nonwoven geotextile was developed in 1968 by the Rhone Poulence company in France. It was a comparatively thick needle-punched polyester, which was used in dam construction in France during 1970.

In fact, the geotextile is one of the members of the geosynthetic family. Those members include the following items

• Geogrids

• Geonets

• Geotextiles

• Geomembranes

• Geosynthetic Clay Liners

• Geopipe

• Geocomposites

Overview of Geotextiles

Geotextiles have been used for thousands of years. Geotextiles were used in roadway construction in the days of the Pharaohs to stabilise roadways and their edges. These early geotextiles were made of natural fibres, fabrics or vegetation mixed with soil to improve road quality, particularly when roads were made on unstable soil. Only recently have geotextiles been used and evaluated for modern road construction.

A geotextile is defined as any permeable textile material that is used with foundation, soil, rock, earth, etc to increase stability and decrease wind and water erosion. A geotextile may be made of synthetic or natural fibers. In contrast, a geomembrane is a continuous membrane-type liner or barrier Geomembranes must have sufficiently low permeability to control migration of fluid in a constructed project, structure or system. A geotextile is designed to be permeable to allow the flow of fluids through it or in it, and a geomembrane is designed to restrict the fluid flow.

Geotextiles can be woven, knitted or non-woven. Different fabric composition and construction are suitable for different applications. The non-woven geotextile is an arrangement of fibres either oriented or randomly patterned in a sheet, resembling felt. These geotextiles provide planar water flow in addition to stabilization of soil. Typical applications include access roads, aggregate drains, asphalt pavement overlays, and erosion control.