perform one or more of these roles, however; they are most frequently used as 

 filters which permit the passage of water through the fabric but not soil or 

 sand particles. Geotextiles used as materials separators prevent the mixing 

 of materials that should remain apart such as poor subgrade soil and good 

 subgrade gravel. Geotextiles have also been successfully used as reinforcing 

 in the paving of roads and to restrain lateral movements of embankments built 

 on soft soils. Koerner and Welsh (1980) give design guidance for many uses. 



The use of geotextiles has expanded rapidly in the past 20 years and 

 many different kinds are available today. However, there are constraints 

 that must be removed before geotextiles achieve unqualified acceptance. 

 One of these is lack of standardization. Many fabrics are made by suppliers 

 in different ways, out of different materials, and for different uses. In 

 choosing a fabric for a project, it may be necessary to consider tensile, 

 elongation, and puncture properties, plus factors such as fabric elasticity, 

 porosity, permeability, and resistance to abrasion, chemicals, light, 

 weather, and temperature as well as resistance to biological attack. 



Because geotextiles have many different uses in coastal structures, 

 drainage ditches, riverbank protection, and subgrade construction, no one 

 fabric is right for all applications. It remains to be determined just 

 what properties are important for each end use and what range of values for 

 each property is sufficient. However, based on the successful use of many 

 geotextile filters over the past 20 years, the promise of longevity is 

 exceedingly favorable. A prospective geotextile user should obtain advice 

 and information from engineers experienced in their use as well as from 

 more than one supplier. 



The term geotextile filter as used in this report refers to a permeable 

 fabric constructed of synthetic fibers designed to prevent piping (prevent 

 soil from passing through it) and remain permeable to water without signifi- 

 cant head loss or without permitting the development of excessive hydrostatic 

 pressure. 



b. Design Properties . A geotextile filter must be sufficiently 

 permeable to relieve the hydrostatic pressure differential between its 

 sides by allowing the passage of ground waterflow without detrimental head 

 loss, and it must prevent the passage, or piping, of adjacent granular or 

 fine soil. A geotextile is used to replace all or part of a conventional 

 filter system consisting of one or more layers of granular material. 

 Figure 82 illustrates a geotextile replacing a layer of gravel beneath a 

 revetment, showing how the filter is designed to prevent protected soil 

 from being washed through the overlying armor. It also demonstrates how a 

 geotextile can be incorporated into a toe protection apron. To be effective, 

 the geotextile must be designed to suit the grain size, ground water, and 

 wave conditions of each specific site as well as the type of structure in 

 which it is to be included. 



In order to function satisfactorily, the geotextile filter must have 

 the physical durability and filtering integrity to perform consistently 

 throughout the design life of the structure. Durability depends on the 

 chemical composition of the fibers, construction of the fabric, and physical 

 properties of the fabric in its completed (finished) form.- To ensure 

 durability, specifications for fabric should describe the basic chemical 



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