Geotextiles filtration function

Filtration function (a) Without geotextile and (b)With geotextile. 

Similar to the filtration function, the requirements for drainage are soil retention, adequate flow capacity, and long term soil-to-geotextile flow equilibrium, in respect of minimum clogging of the pore spaces. Apart from the flow capacity within the plane of the geosynthetic material, the other aspects have been dealt with above. We will therefore now consider the inplane flow characteristics. 

The soil retention function of the geotextile separator can be seen as similar to the geotextile filter, in that for a suitable separator the AOS of the geotextile must be related to the particle size distribution and, as stated under the filtration function, this aspect will be considered in more detail later when describing the relationship between structure, properties, and functionalities of geotextiles. 

It has to be stressed that geotextiles cannot always replace the granular filter completely (see Sec. 19.2.3). A granular layer can often be needed to reduce (damp) the hydraulic loadings (internal gradients) to an acceptable level at the soil interface. After that, a geotextile can be applied to fulfill the filtration function. 

According to the filtration function, a geotextile filter must be able to retain fine particles of base soil (retention criterion) and avoid the development of excessive pore water pressure on a soil geotextile interface (ie, fi ee flow of liquid through the geotextile plane) (permeabihty criterion). 

Finally, the geotextile must have the required filtration/hydraulic characteristics to provide the coincident functions of separation, filtration, and drainage. Properties required for separation and filtration functions are related to opening characteristics... 

Fabrics perform one or more function in each installation generally there is one primary function. The five basic primary functions have been identified as separation, stabili2ation, reinforcement, filtration, and drainage. When the geotextile is impregnated with an impermeable material such as an asphaltic... 

The above factors are also applicable when geotextiles are used for filtration. However, with filtration the intention is to form a natural filter cake at the fabric/soil interface that will retain the fines consequently the pore-size requirement is different than that for the separation function. For filtration, the AOS of the textile must be smaller than the larger sizes of soil particles, but of sufficient dimension for the small soil particles to pass through, otherwise the geotextile will become clogged or blind and its permeability adversely affected. The fabric must have a pore-size distribution with a large number of larger size pores, so that a relatively unhindered water flow is maintained where some of the smaller size pores become blocked. 

Separation filtration (a) and drainage functions (b) of geotextiles in road construction. 

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