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62 DeSlgning with Geotextlles Chap. 2 <br />TABLE 2.4 RELATIONSHIPS USED TO OBTAIN FABRIC OPENING SIZE <br />TO PREDICT EXCESSIVE LOSS OF FINES DURING FILTRATION' <br />Rclmi~c dtnsitc I < CU < j CU > j <br />Louse (U, < iOSr) O„ < ICL'lld,„1 O,r < (9dw)/CU <br />Intcrmtdiate O.t < 1.5(CU)(J,t O„t <(II Sdw)/CC <br />(509r < D, > ROGI <br />Uenx IU, > 80,r1 0„ < -'1CU)(d„ 1 O,r G (ISrlp)/CU <br />suoree: Alcor Giruud 1= I~ <br />'dta, Soil pan¢It svc corrtspnndine to 507 fintc CU. <uc(fintm of unifurmil} <br />(= A,,,/J,,,1'. A,.,. soil pamck ei1.t cortcopondinc m IO:r Ilntt. d,,,. sml panidc >ive <br />cnmspondme l0 60 c Inver. O„. apparcm nptnux ;ot of _cult.a ilc (if dales art <br />n01 given be Iht manuian urcr. thu r~lut a appruummel ~~ the AOS oie,t slue in <br />millimucnl <br />lion can be directly anncered by taking a soil sample and the candidate geotutilz(s) and <br />testing them in the labora[ar}~ in (I) gradiem ratio Iests ~ 16J to sec that the GR < 3.0, or <br />(2) long-term Oow tests (hJ to sec that the tcmtinal slope of the flow rate versus time <br />curve is essentially zero (recall Stnions 3.1.>.~ and ?.1.5.4). <br />Another approach to a suitable answer to the cloeging question posed is simply to <br />avoid situations that have been known to lead to severe cloggnlg problems. From esperi- <br />cnce it has been shoteq that three conditions are ncassan~ to have a high likzlihootl of <br />complete clogging: <br />1. Cohcsionless sand; and silts <br />?. Gap-graded panicle size dutribuuons <br />3. High h}'draulic ~:radlznts <br />I( these conditions arc present. one should avoid the use of :cotcxtilcs altogether. that <br />is, use a soil filter (although the chances are that it too will clog), or open up the geo- <br />texule to the point where some soil loss will occur. This of coupe can be done only i( <br />the upstream condltions will perm[[ such soil loss Rzcununcnded values for such situa- <br />tions are: <br />. 1Vovcn filbrics: POA ? G9a <br />- Nanvoven fabrics: porosity ? -l09c under the actual stress condltions under <br />which the geotexnle is serving <br />~_ ~ This discussion of soil-to-fabric contpatibilit}' assumes the establishment of a set <br />of mechanisms that arc In equilibrium with the flow regime being imposed on the sys- <br />-- tem. Yumcrous aucmpts at insight imo these phenomena have been attempted, most of <br />which suggest a number of possibilities (?8~: <br />n. ;,. <br />~'`~' Upstream soil filter <br />- Blocking <br />-' Arching <br />Panial clogging <br />