Geotextiles dimensions

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. 

The fineness of the fibre is t5q)ically, depending on the product, about 6 dtex, 12 dtex or 18 dtex, which correspond to fibre diameters of approx. 30 pm, 40 pm and 50 pm. Likewise the thickness of the slit film monofilaments typically lies within the range of a few 10 pm, ihe width amounts to a few millimetres. These dimensions and the resulting low surface to volume ratio, may render geotextiles susceptible to the ageing processes. 

Fig. 8.8. Schematic drawing (not to scale) of the puncture test. The tapered tran-cated cone with its dimensions (left), the arrangement of the tmncated cones on the base plate of the pressure vessel (right) and the test equipment with inserted geomembrane and protective nonwoven geotextile as well as the sand bed are indicated. The test procedure is described in the text...

G. Heerten, Dimensioning the filtration properties of geotextiles considering long-term conditions, Proc. 2nd Int. Conf. on Geotextiles, Las Vegas (1982). 

The dimension of the fibre, yam or filament has a direct impact on the mechanical and hydraulic properties of the geotextile. Fibre dimension is expressed according to its diameter, described in deniers or dtex, depending on the measuring system used (American or International System). Thread dimension is generally defined by its yam density. In the case of nonwoven geotextiles made from short staple fibres, fibre dimension is completed by the length of the cut fibres. 

Needlepunch technique and use of mix of fibres of different dimensions makes it possible to control pore size openings with great precision. Openings can be very small or large, thus giving nonwoven geotextiles an advantage over other products. 

The dimensions of natural fibres represent an alternate important aspect. The geometrical dimensions of these fibres, especially the fibre length, depend mainly on the fibre s location within the plant. Fibres from fruits and seeds are a few centimetres long whereas fibres from stems and leaves are much longer (>1 m). Table 4.4 compares some related properties and characteristics of natural fibres for geotextile applications. 

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