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Sec. 2.2 Geoterrtlle Functions and Mechanisms 61 <br />where ~ = penniltivity <br />k, =cross-plant permeability coc(ticizm <br />t =thickness at a specified normal pressure <br />The testing Cor geoteztilc permittivity was covered in Section ?.1.4.4. <br />2.2.3.2 Soil Retention <br />As one allows for greater Ilosv of wa[zr through the gcutt.~ulc, the ~~oid spaces in <br />it must be made larger. There is, however. a limit-that brine when the upstream soil <br />particles star to pass through the fabric voidi alone with the flowing water. This leads <br />to an unacceptable situation called "foil piping," wherz the finer soil panicles arc carticd <br />through the hbric, leaving larger soil voids behind. The water vtlocu}• then increases, <br />accelerating the process, until the soil structure beeins to collapse. This collapse often <br />Izads to minute sinkhole-type patterns which grow larger with time. <br />This entire process is prevented by making the geotexulc voids small enough to <br />retain the soil on the upstream side of thz fabric. It is the tinzr soil fraction which must <br />be initially rc[ained and they are [he tareetzd soil sizz in the design process. Fonunatzly, <br />filtmuon concepts arc well establishzd in the design of sod filtzn and those same ideas <br />will be used to design an adtquatz (abric filter. <br />Thzre art a number of approaches to accomplish soil rztentwn, all of which use <br />the soil panicle size characteristics (usually the finer fraction) and compare it to the <br />AOS (or FOS) of the fabric. The simplest of these methods examines the pcrczntagc of <br />soil brine retamcd passing the \b '_00 iie~c (= 0.074 mml. Acatrdine to Task Force <br />2J, the lollowing is recommchdcd )'_~): <br />1. Soil <_ 509c passing the No. 200 sieve <br />AOS of the fabric ? Na. 30 sieve (OS9 mm) <br />2. Soil > >09c passing the i~o. 200 sieve <br />AOS of the (abric ? No. 50 sieve 10.297 mm) <br />Slichtlc morn restrictive is the recommendation of Carroll for the AOS sieve size in <br />mm. which is the following ~26~: <br />where d.. is the panicle size ai «'hich S~S"r of the sample is finer. Finally, the most con- <br />servati~e method is after Giroud ~27~. who presents a table for recommended Ovs values <br />(i.c., the opening size in millimeters corresponding to the AOS value) in terms of rcla- <br />uve density (DR), coefficient of uniformin (CU). and aezraec panicle size (d,u) (see <br />Table 2.4).,Thcsc three approaches ,hould he used on dtc basis of the criticality of the <br />suu;uion bung considered since each is somewhat more restricuvc than the other. <br />2.2.3.3 Long-Term Compatibilih~ <br />Perhaps the most asked quzsuon in the use of gcoic~ulcs m hydraulic relatzd <br />structures is: "Will it clog?" Obviomly. some soil particles will embed themselves <br />within the structure of the (abric, but ihz question really asks it the (abric will com- <br />plerel}' clog such that the flow of water through it will be completely shw a((. The ques- <br />