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Sec. 2.2 Geolextlle Functions and Mechanisms 63 <br />These are shown schematically in Figure 2.12. Obviously, a number of them are work- <br />ing together simultaneously, and just what mechanism dominates under what conditions <br />of soil type, fabric type, and flow regime is still in need of further research. <br />2.2.4 Drainage <br />Fabrics being placed in a situation whereby water is transmitted in the plane of their <br />structure provide a drainage function. Thus a definition of drainage is <br />Drainage: the equilibrium Cabric-to-soil system which allows (or (rte water flow (but not <br />soil loss) in the plant of the fabric over an indefini[ely long time period. <br />All fabrics can provide such a function but to widely varying de;rees (29(. For example. <br />a thin woven fabric, by virtue of the fibers crossing over and . •der one another, [rans- <br />mits water within the spaces created at these crossover poiu.; but to a very modes[ <br />degree. Conversely, the thick, needled nonwoven fabrics have considerable void space <br />in [heir structure available For water transmission. Furthermore (as a preview of the ma• <br />tcrials to be discussed in Chapter 5), geocomposites can transmit much more water than <br />can thick, bulky geotextiles. Obviously, proper design will dictate jus[ who[ material is <br />nccessan•. <br />• It will be noted that [here is considerable overlap with the preceding section on <br />filtration and except for the consideration of direction of flow, the soil retention and <br />long-term compatibility concepts remain the same. <br />2.2.4.1 Permeability <br />Rc(errtng now to in-plane permeability for the drainage function, we must con- <br />sider that the fabric's thickness will decrease with increasin_e nomtal stress on it. Far thts <br />reason we define a term callzd "transmissivity" as follows <br />B = kot (2.17) <br />where B =transmissivity <br />k„ = in-plane permeability coefficient <br />t =thickness at a specified notmat pressure <br />The testing method for geotextile transmissivity was covered in Sccuon 2.1.4.5. <br />2.2.4.2 Soil Retention <br />The criteria used to design the opening spaces o(a geote.xtile such that tt retains <br />the adjacent soil were covered in Section 2.2.3.2. The concepts and design guides are <br />precisely the same for the drainage function. <br />2.2.•3.3 Long•Term Compatibility <br />As with the filtration function, the compatibility of the soil with the geotextile <br />must be assured over the lifetime of the system being built. The criteria discussed in <br />Section 2.2.3.3 hold (or drainage situations exactly as they do for filtration situations. <br />• <br />