Chapter 3. Envelope materials

Transcription

1 Matrials for subsurfac land drainag systms 21 Chaptr 3 Envlop matrials Porous matrial placd around a subsurfac drain, to protct th drain from sdimntation and improv its hydraulic prformanc, should b rfrrd to as a drain nvlop. It is worthwhil to distinguish btwn th dfinition and function of an nvlop and that of a filtr. During th arly dvlopmnt of dsign critria for drain nvlops, xisting filtr critria wr oftn usd as a basis for rsarch. Hnc, th word filtr is oftn mistaknly usd in rfrnc to drain nvlops. A filtr is by dfinition a porous substanc through which a gas or liquid is passd to sparat out mattr in suspnsion (Mrriam-Wbstr, 1993). Filtration also is dfind as th rstraining of soil or othr particls subjctd to hydraulic forcs whil allowing th passag of fluids (ISO 10318, 1990). Hnc, a filtr, usd as a drain nvlop, would vntually bcom cloggd bcaus particulat mattr would b dpositd on or in it, rducing its prmability. Envlops hav th task to improv th prmability around th pip, and act as prmabl constraints to impd ntry of damaging quantitis of soil particls and soil aggrgats into drainpips. Yt th majority of small particls of soil matrial and organic mattr, suspndd in watr moving toward a drain, will actually pass through a proprly slctd and installd drain nvlop without causing clogging. Th rlativly coars nvlop matrial placd around th drain should stabiliz th soil mchanically and hydraulically, but should not act as a filtr. In addition to th functions dscribd abov, drain nvlops can improv th bdding conditions. This bdding function is primarily associatd with gravl nvlops in unstabl soils. Gravl provids a mchanical improvmnt in th drain-nvlop-soil systm, srving as bdding and sid support for larg diamtr plastic pips (Framji t al., 1987). Envlop matrials usd to protct subsurfac drains hav includd almost all prmabl porous matrials that ar conomically availabl in larg quantitis. Basd on th composition of th substancs usd, thy can b dividd into thr gnral catgoris: minral, organic, and synthtic nvlops. MATERIALS Granular minral nvlops Minral nvlops mainly consist of coars sand, fin gravl and crushd ston, which ar placd undr and around th drainpip during installation. If wll dsignd and installd, minral granular nvlops ar quit rliabl bcaus thy ar voluminous and can stor comparativly larg quantitis of soil matrial without noticabl malfunctioning. As such, thy hav providd satisfactory long-trm srvic undr most circumstancs. Traditionally, pit run naturally gradd coars sand or fin gravl containing a minimum of fins is th most common and widly usd

2 22 Envlop matrials drain nvlop matrial. Such matrial can b as prmannt as th soil itslf. Proprly dsignd gradd gravl nvlops fulfil all th mchanical and hydraulic functions of a drain nvlop and ar th idal nvlop from a physical standpoint. Gradd gravl should b a homognous, wll-gradd mixtur of clan sand and gravl fr from silt, clay, and organic mattr, which could advrsly affct its prmability. Th us of limston particls must b avoidd, bcaus a high prcntag of lim in gravl nvlops is a sourc of incrustation. In addition, th gradation of a gravl nvlop should b mad in accordanc to prscribd paramtrs (Sction Spcifications for gravl nvlops). Th us of gravl as drain nvlop has bcom a bit controvrsial. On of th conclusions of a symposium hld in Wagningn, Th Nthrlands in 1986 was th following: Gravl rmains for th tim bing th most rliabl filtr matrial. In viw of th cost of gravl th dvlopmnt of dsign critria for synthtic matrials mrits th highst priority (Vos, 1987). Howvr, at a confrnc, hld in Lahor, Pakistan in 1990 which was dvotd spcifically to th dsign and application of nvlops, it was concludd that nginrs who wr not familiar with synthtic nvlops, wr rluctant to rcommnd thir us (Vlotman, 1990). Considring th currnt tndncy, it may b assumd that synthtic nvlops will gradually rplac th application of gravl as nvlop matrial in futur drainag projcts. Organic nvlops Organic matrials, many of which ar by-products of agricultural production, hav succssfully bn applid as drain nvlops. Thy ar voluminous, so thy can b usd in cass whr both particl rtntion and hydraulic function ar important. Organic matrials may b applid dirctly on th drainpip in th trnch as loos blinding matrial, or may b prwrappd around th drainpip as Prwrappd Loos Matrials (PLMs). An intrmdiat typ of application has bn in strip-form, applid on top of th drainpip. This typ of application is now obsolt. Organic nvlop matrials includ chaff, cral straw, flax straw, ric straw, cdar laf, bamboo, corncobs, wood chips, rds, hathr bushs, choppd flax, flax stms, grass sod, pat littr and coconut fibr (Juusla, 1958; Framji t al., 1987). In northwstrn Europ (Blgium, Grmany, and Th Nthrlands), th most common organic nvlops wr mad from pat littr, flax straw and coconut fibrs. Th us of fibrous pat littr as a covr layr of drain tils has bn common practic for dcads until th nd of th 1950s. It was found that th hydraulic conductivity of th pat littr would oftn dcras drastically du to swlling of th nvlop undr prmanntly wt conditions du to.g. subirrigation (Rozndaal and Scholtn, 1980). During th subsqunt priod, flax straw has bn usd. It was applid originally as a covr strip and latr as prwrappd nvlop. Th coarsnss of th flax nvlop did howvr not always guarant th particl rtntion function. On a much smallr scal, othr organic nvlops hav bn applid. Ths matrials wr not always availabl in th rquird quantitis and thir handling was oftn laborious. Th us of straw was not succssful bcaus it usually dcomposd into a low-prmability layr around th pip. At th nd of th 1960s, coconut fibr (Figur 16) was introducd (Jarman and Jayasundra, 1975). Bing rlativly chap, it soon dominatd th markt bcaus high quality pat littr bcam scarc and xpnsiv (Mijr, 1973) and bcaus th flax industry dclind. Morovr, th finr coconut fibr was considrd a mor appropriat nvlop matrial than th coarsr-

3 Matrials for subsurfac land drainag systms 23 structurd flax straw. Vry soon it was discovrd that coconut fibrs wr oftn subjct to microbiological dcay (Mijr and Knops, 1977; Anthuniss, 1979, 1980, 1981). Th nvlops wr usually fully dcomposd aftr two to fiv yars, particularly if th ph of th soil xcdd th valu 6. Mor than a dcad latr, many farmrs complaind about minral clogging of thir drains. A rsarch projct was st up to invstigat th problm of minral clogging. Mor than 1000 xcavations wr mad and thy confirmd that th minral clogging problms, although FIGURE 16 Coconut fibr PLM nvlop partly du to th larg ffctiv por siz of th coconut fibr nvlop, mainly rsultd from th dcomposition of th organic substancs (Blom, 1987). In th mid-1980s, various attmpts wr mad to rtard or stop th dcomposition of organic nvlop matrials. In Grmany and in Franc a so-calld Supr-Cocos nvlop was introducd. Its fibrs wr imprgnatd with coppr sulphat (CuSO 4 ), to kill th bactria that caus th dcomposition (Anthuniss, 1983, 1984). In addition, som nvlops containd tiny coppr wirs. Supr-Cocos nvlops had limitd succss bcaus dcomposition was postpond for a fw yars only. In addition, nvironmntal lgislation mad installation of Supr-Cocos illgal in most countris, bcaus th chmical agnt lachd out rapidly. Coconut fibr nvlops ar still bing applid in northwst Europ du to thir comparativly low pric, but thir us is dclining in favour of synthtic matrials. Organic nvlops hav nvr bn popular in countris locatd in arid climats bcaus th comparativly high soil tmpratur activats microbiological activity and consquntly acclrats thir dcay. In th irrigatd lands of th arid tropics, organic nvlop matrials usually fail (Van dr Moln and Van Somrn, 1987). Th succssful application of organic nvlops in th Scandinavian countris, whr mainly fibrous pat and wood chips wr usd, was du to th rducd microbiological activity at lowr soil tmpraturs. Th srvic lif and suitability of organic matrials as nvlops for subsurfac drains cannot b prdictd with crtainty. Evntually, th majority of organic nvlops will dcompos, without any srious impact on th structural stability of th surrounding soil. Hnc, ths matrials should b applid only in soils that bcom mchanically stabl within a fw yars aftr installation of th drainag systm (Van Zijts, 1992). In addition, organic nvlops may affct chmical ractions in th abutting soil. This procss may rsult in biochmical clogging of th drain. If iron ochr clogging of drains is likly, rluctanc with th application of organic nvlops is justifid. Evn organic mattr that is accidntally mixd with trnch backfill matrial may svrly nhanc th risk of ochr clogging of th drain (Chaptr 5). Th rapid dcay of coconut fibr nvlops has stimulatd th sarch for affordabl, synthtic altrnativs. Th fact that synthtic nvlops can b mor asily manufacturd according to spcific dsign critria than organic ons has playd a significant rol in this dvlopmnt.

4 24 Envlop matrials Synthtic nvlops Prwrappd loos matrials A synthtic PLM is a prmabl structur consisting of loos, randomly orintd yarns, fibrs, filamnts, grains, granuls or bads, surrounding a corrugatd drainpip, and rtaind in plac by appropriat ntting and/or twins. Synthtic PLM nvlops ar usually wrappd around th corrugatd plastic drainpips by spcializd companis and occasionally in pip manufacturing plants. Th finishd product must b sufficintly strong to rsist handling and installation without damag. Synthtic PLMs includ various polymric matrials. Fibrs may b mad of polyamid (PA), polystr (PETP 1 ), polythyln (PE), and polypropyln (PP). Loos polystyrn (PS) bads can b wrappd around drains as PLMs in prforatd foil or in string ntting ( gogrids or gonts ). Th bads ar subjct to comprssion from soil loads that may rduc nvlop prmability (Willardson t al., 1980). In various Europan countris whr th drain dpth rangs from 0.9 to 1.2 m, th ffct of th soil load is howvr rlativly small. PLM nvlops mad from PP (wast) fibrs ar incrasingly usd in northwst Europ and in arid aras whr thy rplac xpnsiv gravl. Information on som nvlop matrials, which ar shown in Figurs 17-20, is givn blow. Figurs concrning th markt shars of various nvlop matrials ( turnovr ) ar givn for Th Nthrlands, in 1997, for illustrativ purpos only. Th data ar basd upon th installd lngths of wrappd drainpips. FIGURE 17 PLM nvlop mad from polypropyln wast fibrs (PP-300) PLM nvlops mad from polypropyln wast fibrs (PP- 300) (Figur 17) ar installd almost xclusivly in Blgium for privat drainag projcts (turnovr: 6 prcnt). FIGURE 18 PP-450 nvlop PP-450 nvlop (Figur 18) is a PLM nvlop, manufacturd from bulk continuous filamnts. Ths filamnts ar wast whn producing wovn PP fibr carpts. In Th Nthrlands, it is by far th most popular nvlop matrial (turnovr: 65 prcnt). 1 PETP is an acronym for polythyln trphtalat.

5 Matrials for subsurfac land drainag systms 25 PP-700 nvlop is a PLM matrial, mad from nw PP fibrs (Figur 19). Wrapping of pips with this nvlop is comparativly laborious, hnc th high pric (turnovr: 4 prcnt). It is mainly usd for largr pip diamtrs (xcding 160 mm). Du to th dclining availability of PP wast fibrs at comptitiv prics, wast PA fibrs ar usd occasionally. Contrary to PP fibrs, PA fibrs absorb watr as a rsult of which th coils may substantially incras in wight. In addition, it is mor difficult to procss PA fibrs to homognous prwrappd nvlops bcaus of problms with static lctricity. FIGURE 19 PP-700 nvlop FIGURE 20 PS-1000 nvlop PS-1000 is a PLM nvlop matrial that is manufacturd from comprssibl PS bads in ntting (Figur 20) and almost xclusivly installd in agricultural aras whr flowr bulbs ar grown (turnovr: 7 prcnt). In ths aras, th groundwatr contains a rlativly high amount of suspndd particls, and PS-1000 has provn a vry rliabl nvlop. In this application, th highr pric of PS is a good invstmnt; no farmr can afford to hav drainag systms fail. Synthtic matrials dtriorat whn xposd to solar (UV) radiation. Exprimnts with PLM nvlops, mad of PP fibrs in a tmprat climat hav indicatd that dtrioration can b hazardous within thr yars (Dirickx, 1998b). Th spd of th dtrioration will b doubl in smi-arid and arid rgions whr th avrag annual radiation is twic that in tmprat rgions. Howvr, onc installd, synthtic PLM nvlops, manufacturd from suitabl raw matrial (.g. rcycld PP fibrs) ar not subjct to dcomposition. Ths matrials ar thrfor rliabl and affordabl substituts for convntional gravl and organic nvlops. Prwrapping with loos matrials is limitd to diamtrs of 200 mm or smallr. Onc prwrappd around drains, PLM nvlops hav functional proprtis that ar similar to thos of gotxtils.

6 26 Envlop matrials Gotxtil nvlops According to pren (1998), a gotxtil is dfind as a planar, prmabl, polymric (synthtic or natural) txtil matrial, which may b wovn, non-wovn or knittd, usd in contact with soils and/or othr matrials in civil nginring for gotchnical applications. This dfinition includs application in agricultur sinc civil nginring incorporats drainag nginring in many countris. Wovn gotxtils ar manufacturd by intrlacing, usually at right angls, two or mor sts of yarns, fibrs, filamnts, taps, or othr lmnts. Non-wovn gotxtils ar shts, wbs, or batts, consisting of dirctionally or randomly orintd fibrs, filamnts, or othr lmnts. Ths lmnts ar bondd by mchanical, thrmal and/or chmical mans. Knittd gotxtils ar manufacturd by intrlooping on or mor yarns, fibrs, filamnts, or othr lmnts. Th fibrs, usd for production of gotxtils ar mad from th sam raw matrials as thos usd for PLMs, namly: polyamid (PA), polystr (PETP), polythyln (PE), and polypropyln (PP). Th fibrs of gotxtils may b monofilamnts, multifilamnts or taps; th lattr ithr flat, fibrillatd or twistd. Th combination of raw matrials, fibr configuration and waving, bonding or knitting tchniqus rsults in many typs of gotxtils which diffr widly in apparanc, physical, mchanical and hydraulic proprtis. In principl, gotxtils may b usd as nvlop matrial for drainpips bcaus thy possss two important proprtis that ar rquird for a drain nvlop, namly watr prmability and soil particl rtntion. Morovr, thy facilitat th watr accptanc of drainpips, and thy convy watr in thir plan, alongsid th pip wall. Wovn gotxtils, howvr, ar sldom usd for th manufacturing of drain nvlops. Th only justification for this fact must b thir comparativly high pric, bcaus thir spcifications ar indd favourabl. FIGURE 21 Typar nvlop In som Europan countris whr organic and synthtic PLMs ar usd, thr is prsistnt rluctanc to us gotxtils as drain nvlop bcaus it is argud that thir fin txtur may nhanc minral and ochr clogging. Yt in countris with a gotxtil industry lik Franc, Canada and th Unitd Stats, gotxtil nvlops ar applid succssfully at a larg scal. Laboratory xprimnts, fild trials and practical xprincs do not giv clar vidnc of th clogging risk of proprly slctd and proprly installd fin txturd gotxtils. Thr ar, howvr, circumstancs whr fin txturd gotxtils should prfrably not b usd (s Chaptr 5). An xampl of a gotxtil nvlop is Typar which is th brand nam of a non-wovn fabric, mad of continuous filamnts of 100 prcnt polypropyln without any xtranous bindrs (Figur 21). 2 pren is a draft Europan standard (EN) that is not yt finalizd.

7 Matrials for subsurfac land drainag systms 27 Wrapping of drains with gotxtils can b don for any diamtr. Gotxtil strips can b tid around th corrugatd drain, or pulld ovr it aftr th dgs hav bn swn togthr. Gotxtils that ar xposd to solar natural wathring ar also vulnrabl to dgradation. Rankilor (1992) rcommnds that xposur of gotxtils to natural wathring may not last longr than two months in tmprat rgions and only on wk in arid and smi-arid rgions. Gotxtils, manufacturd from organic raw matrial such as jut will dcay in a similar fashion as organic PLMs do, whil synthtic gotxtils, lik synthtic PLMs, do not. SPECIFICATIONS FOR DRAIN ENVELOPES In 1922, Trzaghi dvlopd filtr critria to control spag undr a dam. Ths critria hav sinc bn tstd for applicability for nvlops around subsurfac drains. Trzaghi rcommndd that th filtr matrial b many tims mor prvious than th soil bas matrial but that it not b so coars that th bas matrial would mov into th filtr. Trzaghi s dvlopmnt has srvd as a basis for much work don sinc that tim on gravl nvlop dsign. For drain nvlops, his dsign critria hav bn tstd and modifid, but his original concpts hav bn gnrally accptd. Van Somrn (FAO, 1972) rportd on th rsarch into and th guidlins for slction and application of drainag matrials (pips and nvlops) in various countris. In Blgium and Th Nthrlands, fforts wr mad to dvlop spcial dsign critria for prwrappd loos matrials (PLMs). Convntional dsign critria wr largly dtrmind by analogu modls in laboratoris, supportd by thortical considrations, and vrifid by fild trials. Monitoring th flow of watr and soil particls nar prwrappd drainpips in th fild was not an asy task without disturbing th systm. In addition, th data, mrging from fild xprimntation ar invitably blurrd bcaus it is sit spcific. Rsults achivd at som placs ar not ncssarily rplicabl at othr locations. Knops t al. (1979) publishd th first st of comprhnsiv guidlins for th slction of th thn usd prwrappd nvlops for us in Dutch soils. Subsquntly, a sris of rsarch projcts and concurrnt practical valuations, carrid out by various companis and institutions, hav producd dsign and application critria for drain nvlops mad of PLMs in Th Nthrlands (Huinink, 1992; Stuyt, 1992a; Van Zijts, 1992). Many fild survys hav bn mad into th possibl factors that affct pip sdimntation. Drain nvlops should mt spcifications but visual valuation of matrials is also important. Evn if th bst matrials hav bn usd and all spcifications ar mt, a drainag systm will not oprat proprly if nvlops xhibit som shortcomings du to carlss wrapping, handling or installation. Spcifications for gravl nvlops Spcifications for gravl nvlops ar discussd xtnsivly in numrous publications. This sction contains all th major issus. Sound dsign critria for traditional granular nvlops (gravl and coars sand) ar availabl and hav bn applid succssfully in practic (Trzaghi and Pck, 1961; Vlotman t al., in prss; Stuyt and Willardson, 1999). Th US Army Corps of Enginrs and th US Burau of Rclamation hav mad xtnsiv studis of gravl nvlops. Th rsult is a st of spcifications for gradd gravl nvlops, which hav bn succssfully usd by th Soil Consrvation Srvic (SCS, 1973), th US Burau of Rclamation (USBR, 1993) as wll as outsid th Unitd Stats.

8 28 Envlop matrials Th gradation curv of a proposd gravl nvlop should b matchd to th soil to b draind, as wll as to th pip prforations (Willardson, 1979). In addition, gravl should b intrnally stabl to avoid intrnal nvlop rosion. Th gnral procdur for dsigning a gravl nvlop for a givn soil is as follows: 1. mak a mchanical particl siz analysis of both th soil and th proposd gravl nvlop; 2. compar th two particl siz distribution curvs; and 3. dcid, by som dsign critrion, whthr th proposd gravl nvlop matrial is suitabl. Th involvd dsign critria consist of ruls that prscrib how to driv th particl siz distribution, rquird for a suitabl gravl nvlop, from particl siz distribution data of th soil, in ordr to guarant satisfactory srvic of th nvlop. Trzaghi s critria Th first critria, proposd by Trzaghi (US Army Corps of Enginrs, 1941) for what h trmd a filtr, ar: Th particl diamtr of th 15 prcnt siz of th filtr matrial (D 15 ) 3 should b at last four tims as larg as th diamtr of th 15 prcnt siz of th soil matrial (d 15 ): D 15 4 d 15 This rquirmnt would mak th filtr matrial roughly mor than tn tims as prmabl as th soil. Th 15 prcnt siz of th filtr matrial (D 15 ) should not b mor than four tims as larg as th 85 prcnt siz of th soil matrial (d 85 ): D 15 4 d 85 This rquirmnt would prvnt th fin soil particls from washing through th filtr matrial. Brtram (1940), Karpoff (1955), and Juusla (1958) suggstd similar or modifid filtr dsign critria for us with subsurfac drains. Critria of th US Soil Consrvation Srvic Th SCS (1971) has combind th rsults of th rsarch on gravl nvlops into a spcification for valuating pit run and artificially gradd granular matrials for us as drain nvlop matrials. Ths spcifications ar suprsdd by mor rcntly publishd spcifications (SCS, 1988), which distinguishd btwn filtr and nvlop. Th rcommndation for naturally gradd matrials or a mixtur of mdium and coars sand with fin and mdium gravl for us as nvlop is: D mm. D µm. D 5 75 µm. Additional critria ar suggstd to prvnt xcssiv finnss of an nvlop matrial, dsignd to b usd for finr txturd soils (SCS, 1988): 3 Th particl diamtr D x of th x prcnt siz by wight of th filtr matrial is dfind as th diamtr siv whr x prcnt passs. This also holds for th soil paramtr d x.

9 Matrials for subsurfac land drainag systms 29 D 15 < 7 d 85 but D mm. D 15 > 4 d 15. Critria of th US Burau of Rclamation For rigid, unprforatd pips, th US Burau of Rclamation trats th joint opning, th lngth of th pip sction, and th hydraulic conductivity of th nvlop matrial as a unifid systm. Thir Drainag Manual (USBR, 1978, 1993) contains graphs which considr all ths factors. Tabl 1, takn from this manual, givs rcommndd nvlop gradations for soils with diffrnt 60 prcnt passing sizs. TABLE 1 Gradation rlationships btwn soil and diamtrs of gradd granular nvlop matrial (aftr USBR, 1978, 1993) Soil, 60% passing (diamtr of particls, mm) For som fin-txturd and salty problm soils in Pakistan, th USBR critria producd gravl nvlops that wr obviously too coars, allowing xcssiv amounts of fin soil matrials to ntr th drains (Vlotman t al., 1990). Othr critria Sinc th dsign of gravl packs for wlls is similar to th dsign of nvlops for subsurfac drains, th critria dvlopd by Krus (1962) for gravl packs may also b usd for gravl nvlops. Ths critria ar basd on th ratio of th 50 prcnt siz of th pack (nvlop) matrial to th 50 prcnt siz of th aquifr (soil) and on th uniformity of th txtural composition (s Chaptr 6, Sction Physical proprtis of th soil) of both th aquifr and th gravl. Krus (1962) obsrvd that sand movmnt was rducd by dcrasing th uniformity of th gravl (i.. incrasing its uniformity cofficint) at all gravl-aquifr ratios and thrfor distinguishd btwn uniform soil and gravl pack up to a uniformity cofficint of 1.78 and non-uniform soil and gravl pack for largr valus. Th proposd maximum prmissibl gravl/aquifr particl siz ratios for th various combinations of txtural composition of both th aquifr and th gravl pack, to prvnt xcssiv movmnt of aquifr matrial, ar givn in Tabl 2. Bsids th 50 prcnt ratio of filtr to aquifr matrial, Pillsbury (1967) also usd th standard dviation rsulting from th diffrnc btwn th 95 prcnt and 50 prcnt sizs of th grading curv of th gravl nvlop dividd by 1.645, as a Gradation limitations for nvlop (diamtr of particls, mm) Lowr limits, prcntag passing Uppr limits, prcntag passing TABLE 2 Largst prmissibl gravl/aquifr siz ratios (aftr Krus, 1962) Txtural composition of aquifr Uniform (unstabl) Uniform (unstabl) Non-uniform (stabl) Non-uniform (stabl) Txtural composition of gravl pack Uniform (unstabl) Non-uniform (stabl) Uniform (unstabl) Non-uniform (stabl) Gravl/aquifr particl siz ratio (D50/d50)

10 30 Envlop matrials critrion for its ffctivnss. Pillsbury (1967) prsntd a graph of th 50 prcnt siz ratio nvlop-aquifr vs. this standard dviation which was dividd in two zons. Envlops that fall blow th limit lin wr judgd unsatisfactory. Basd on obsrvations of som drain nvlops that had faild in th Imprial Vally of California, Pillsbury rcommndd an nvlop-aquifr ratio of lss than 24. H concludd that concrt sand, satisfying th appropriat Amrican Socity for Tsting and Matrials (ASTM) standard with a 50 prcnt siz lss than 1 mm and a standard dviation gratr than 1.0 would b a satisfactory nvlop matrial undr most conditions. Shrard t al. (1984a, b) dvlopd filtr critria for protction of hydraulic structurs. Whil not intndd for application in subsurfac drainag, th principls may qually wll b applid for th dsign of gravl nvlops. Th authors stablishd that if a filtr did not fail with th initial flow of watr, it was probably prmanntly saf. Wll-gradd matrials wr mor succssful than uniform matrials. Shrard t al. (1984b) rportd on tsts with fin txturd soils and concludd th following with rspct to filtr and bas soil sizs: Sandy silts and clays (d 85 of mm) D 15 /d 85 5 is saf. Fin-graind clays (d 85 of mm) D 15 < 0.5 mm is saf. Fin-graind silts of low cohsion (d 85 of mm) D 15 < 0.3 mm is saf. Excptionally fin soils (d 85 < 0.02 mm) D 15 < 0.2 mm or smallr is saf. Sands and gravly sands containing fin sand fractions and having a D 15 of 0.5 mm or lss would b a suitabl filtr for vn th finst clays. For clays with som sand contnt (d 85 > 1.0 mm), a filtr with a D 15 = 0.5 mm would satisfy th D 15 /d 85 5 critrion. For finr clays, th D 15 / d 85 5 is not satisfid, but th finr soils tnd to b structurally stabl and ar not likly to fail. Finally, Shrard t al. (1984b) found that wll-gradd gravly sand was an xcllnt filtr for vry uniform silt or fin uniform sand, and that it was not ncssary that th grading curv of th nvlop b roughly th sam shap as th grading curv of th soil. Gravl nvlops that hav a D 15 of 0.3 mm and a D 15 /d 85 5 with lss than 5 prcnt of th matrial finr than mm will b satisfactory as nvlop matrials for most problm soils. Dilman and Trafford (FAO, 1976) rviwd critria for slction of gravl nvlop matrials and includd som commnts rgarding nvlop slction for problmatic soils. Dirickx (1992b) prsntd a summary of gravl nvlop critria from th Unitd Stats and th Unitd Kingdom. This summary clarly indicats that th critria from various sourcs do not match, vn if on taks into account th diffrnc btwn filtr (mchanical) function and nvlop (hydraulic) function. This fact has promptd nw rsarch projcts that hav yildd nw findings, i.. improvmnts of xisting critria, which may b usd to improv th dsign gravl nvlops (Vlotman t al., 1997). Anothr finding of intrst was that roundd and angular particls gav quivalnt rsults (Vlotman t al., 1992b). Spcifications for prwrappd nvlops Prwrappd nvlops may b organic PLM, synthtic PLM and gotxtil. Thir physical proprtis such as thicknss and mass pr unit of surfac ara ar important to chck th uniformity of th nvlops, and thir conformity with th rquird dsign standards. Charactristic opning siz, hydraulic conductivity and watr rpllnc dtrmin th hydraulic

11 Matrials for subsurfac land drainag systms 31 proprtis of prwrappd nvlops. Whn using loos granular matrials, particl siz distribution paramtrs may b usd as wll. Dpnding on what kind of drain pips is usd and how nvlop matrials ar wrappd around drainpips, som mchanical proprtis of nvlops such as comprssibility, abrasion damag, tnsil strngth and static punctur rsistanc may b part of th spcifications. In Th Nthrlands, rcommndations for th dsign and application of PLMs hav bn dvlopd on th basis of concurrnt rsarch projcts, thortical studis, mathmatical modlling, mpirical studis in xprimntal filds, analogu modlling in laboratoris and practical xprinc covring a 30-yar priod ( ) (Stuyt, 1992a). Thicknss Th thicknss of prwrappd nvlops srvs as a rfrnc for uniformity and conformity. In addition, nvlop thicknss is found a factor of importanc in thortical analyss as it influncs th soil rtntion capacity, th ntranc rsistanc of drainpips and th xit gradint at th soilnvlop intrfac. Th main task of an nvlop is soil particl rtntion. In this rspct, dsign critria for nvlop thicknss ar irrlvant. Thickr nvlops, howvr, may hav highr porositis, which xplain thir popularity whn chmical clogging is anticipatd. Thrfor, in th nvlop slction procdur, nvlop thicknss is an important paramtr, and oftn significant in trms of safty. Th thicknss of an nvlop should b a rlvant spcification if rduction of ntranc rsistanc is nvisagd or if rduction of ntranc rsistanc is th only objctiv to us an nvlop (s Chaptr 4, Sction Entranc and approach flow rsistanc). Although a thin nvlop may substantially rduc th ntranc rsistanc, th optimal rduction is obtaind at a thicknss of 5 mm, providd that th hydraulic conductivity of th gotxtil is not th limiting factor, which will gnrally not b th cas (Niuwnhuis and Wssling, 1979; Dirickx, 1980). A furthr incras of thicknss has no markd influnc on th ntranc rsistanc, although th ffctiv radius continus to incras sinc a comparativly prmabl nvlop rplacs soil matrial that is usually lss prmabl. Whn nvlops ar usd to rduc th xit gradint (s Chaptr 4, Sction Th xit gradint), th thicknss of th nvlop is also a rlvant dsign paramtr. Th dsign procdur for nvlop thicknss, as proposd by Vlotman t al. (in prss) shows that vn thin gotxtils ( 1 mm) may considrably rduc th xit gradint at th soil-nvlop intrfac. Th largr th diamtr of a drain, howvr, th smallr hydraulic gradints nar th drain will b. Hnc, thick or voluminous nvlops (i.. thicknss > 5 mm) ar gnrally considrd to b safr than thin ons, particularly if th drains ar occasionally usd for controlld drainag or subirrigation (subsurfac infiltration). For PLM, th spcification of a minimum thicknss was introducd to guarant a complt covr with a mor or lss homognous nvlop. According to th provisional EN-standard (CEN/TC155/WG18, 1994), th following minimum thicknsss ar rquird: Synthtic, fibrous PLMs: 3 mm (.g. PP fibrs). Synthtic, granular PLMs: 8 mm (.g. polystyrn bads). Organic, fibrous PLMs: 4 mm (.g. coconut fibrs). Organic, granular PLMs: 8 mm (.g. wood chips, sawdust).

12 32 Envlop matrials Th provisional EN-standard furthr spcifis that th man avrag thicknss of ach tst pic should not dviat by mor than 25 prcnt from that dclard by th manufacturr. Gotxtils ar availabl from vry thin, sht-lik fabrics to rathr thick, mat-lik matrials. Mass pr unit ara Th mass pr unit ara is not a slction critrion and thrfor not spcifid. Mass dtrmination can b carrid out as a control masur for uniformity and conformity. According to th provisional EN-standard, th mass also may not dviat by mor than 25 prcnt of th mass spcifid by th manufacturr in ordr to safguard a homognous product. Charactristic opning siz and rtntion critrion Th charactristic opning siz, drivd from th por siz distribution or poromtric curv of th nvlop, is th most important slction critrion bcaus it dtrmins th ffctivnss of th nvlop to rtain th surrounding soil matrial. Th rtntion of soil particls is normally not a problm sinc vry fin fabrics ar availabl. Laboratory rsarch as wll as practical xprinc, howvr, hav rvald that fin nvlops ar vulnrabl to minral blocking and clogging. Blocking of an nvlop is a dcras of th numbr of activ opnings in an nvlop that occurs whn it is brought in contact with a soil. Clogging, on th othr hand, is a dcras with tim of th numbr of activ opnings in an nvlop du to gradual accumulation of particls insid and on its surfac,.g. by particls suspndd in turbid watr. Thrfor, spcifications for nvlops should covr both soil rtntion critria and critria to prvnt clogging and blocking of th nvlop. Intnsiv rsarch has rsultd in critria for soil particl rtntion and in rcommndations with rspct to th problms of blocking and clogging. Th capability of an nvlop to rtain th soil matrial is xprssd as a ratio of som charactristic por siz of an nvlop to som charactristic particl siz of th soil in contact with this nvlop. In many countris, th O 90 is usd as th charactristic por siz for organic and synthtic PLMs and gotxtils alik, with a grat dal of succss. Th O 90 of a drain nvlop is th por siz for which 90 prcnt of th nvlop pors ar smallr. Th O 90 valu is usually obtaind by dry siving of wll-known sand fractions, whrby th nvlop itslf is installd as a siv and th rtaind amount of ach fraction is rcordd. Wt and hydrodynamic siving, also applid for this purpos, us gradd soil and mostly rsult in smallr O 90 valus than thos obtaind with dry siving. In 1994, a working group of scintists and nginrs in Europ dvlopd a nw classification systm for PLMs. Thy introducd thr classs of nvlops, dpnding on th ffctiv opning siz of th nvlop pors, O 90, as follows: PLM-XF xtra fin 100 µm O µm. PLM-F fin 300 µm O µm. PLM-S standard 600 µm O µm. In th provisional EN-standard (CEN/TC155/WG18, 1994) only two classs, namly PLM- F and PLM-S hav bn accptd. In Th Nthrlands, practical guidlins for nvlop application considr thr standard O 90 valus, namly 450, 700 and 1000 µm, 450 µm bing by far th most widly applid, and

13 Matrials for subsurfac land drainag systms 33 srvicing a grat varity of soils. Ths figurs wr accptd aftr Stuyt (1992a), using fild data, confirmd vidnc of th soundnss of th O 90 paramtr. In Blgium, th O 90 of a PLM nvlop should rang btwn 600 and 1000 µm for official drainag works. A frquntly usd rtntion critrion, also calld filtr critrion or bridging factor of an nvlop, is th ratio O 90 /d 90. In this ratio, d 90 is th particl diamtr of th soil in contact with th nvlop whr 90 prcnt of th particls, by wight, is smallr. Numrous othr rtntion critria hav bn proposd in th scintific litratur, which hav bn publishd in comprhnsiv tabls, by.g. Dirickx (1993) and Vlotman t al. (in prss). For th dsign nginr, howvr, th numbr of critria is confusing, th mor so bcaus many critria ar contradictory. This fact is slf-xplanatory, bcaus th critria wr dvlopd undr widly diffrnt boundary conditions, using many diffrnt tchniqus, quipmnt and so forth. Laboratory xprimnts hav unambiguously indicatd that th liklihood of soil particl rtntion is gratr whn a fabric is thickr. Hnc, th charactristic por siz of an nvlop may b largr for thickr nvlops, for qual rtntion. Indd, rtntion critria ar linkd to nvlop thicknss. From laboratory studis with analogu soil modls, Dirickx (1987), and Dirickx and Van dr Sluys (1990) drivd th following simpl rtntion critria for subsurfac drainag applications: O 90 /d 90 5 for thick nvlops 5 mm (PLMs). O 90 /d for thin nvlops 1 mm (gotxtils). For nvlops with a thicknss ranging btwn 1 and 5 mm, th O 90 /d 90 ratio may b intrpolatd stp-wis (Dirickx, 1992a) or linarly (Vlotman t al., in prss). Th stp-wis approach givs on valu of O 90 /d 90 for a rang of thicknsss and is somwhat mor practical than a linar approach which yilds a spcific valu of O 90 /d 90 for ach thicknss. Rtntion critria for thicknsss of PLMs and gotxtils btwn 1 and 5 mm, according to th stp-wis approach ar: O 90 /d 90 3 for thicknsss btwn 1 and 3 mm. O 90 /d 90 4 for thicknsss btwn 3 and 5 mm. Taking into account th rtntion critrion of a thin nvlop, most problms in subsurfac drainag will b prvntd by nvlops for which O µm. Fild obsrvations of Stuyt (1992a,b) confirmd, in a larg xtnt, th laboratory findings. Stuyt invstigatd th rlation btwn th O 90 siz of nvlop matrials and th thicknss of th sdimnt layr insid th pips using a miniatur vido camra fiv yars aftr thir installation. In total, 9634 m of drains wr invstigatd (184 latrals). Th pips had outr diamtrs of 60 and 65 mm. In Th Nthrlands, sdimnt layrs xcding 15 mm ar gnrally not tolratd. Th d 90 siz of th soils was approximatly 150 µm in most cass. Th corrlation btwn th thicknss of th sdimnt layr insid th pips and th O 90 siz of nvlop was significant (Tabl 3). Rgardlss of th O 90 siz, voluminous nvlops rtaind mor soil than thin nvlops. Envlops with largr O 90 valus, i.. having largr opnings, had poorr soil rtntion proprtis. Th raw matrial from which th nvlops wr manufacturd was not significant. Stuyt (1992a) also found that th abov-proposd O 90 /d 90 ratios wr valid for th invstigatd problm soils. Most of th applid nvlops in th xprimntal filds had rathr high O 90 /d 90 ratios (4 to 5).

14 34 Envlop matrials TABLE 3 Fittd valus for pip sdimntation dpth (mm) from a rgrssion modl, dpnding on ffctiv opning siz of th nvlop pors, O 90, and nvlop catgory (thin or voluminous) for obsrvations mad at thr xprimntal filds in Th Nthrlands (aftr Stuyt, 1992a) Exprimntal fild O 90 Uithuizrmdn Valtrmond Willmstad (µm) Thin Voluminous Thin Voluminous Thin Voluminous Exprimnts with turbid watr or watr chargd with soil suspnsions indicat that gotxtils ar vulnrabl to clogging whn O 90 /d 90 1 (Dirickx, 1990; Faur, 1991). Hnc, th ratio O 90 /d 90 = 1 is th lowr limit for soil particl rtntion, rgardlss of nvlop thicknss. Th phnomna of blocking and clogging of an nvlop ar howvr not so vidnt, nithr in laboratory xprimnts with soils, nor in fild xprimnts. Thrfor, th lowr limit O 90 /d 90 1 should b considrd a rcommndation rathr than a rigid dsign critrion. In th invstigation mad by Stuyt (1992a), nvlops with O 90 /d 90 nar 1 had such low sdimntation dpths that th nvlops appard to act as filtrs. Hnc, for thin gotxtils, th O 90 /d 90 ratio should prfrably b nar th uppr limit. On th othr hand, th uppr limit, st to 5 for voluminous nvlops (Dirickx, 1987) appars saf for voluminous PLMs sinc a maximum sdimntation dpth of 15 mm is tolratd in 60 and 65 mm outr diamtr pips (Tabl 3). In soils with som cohsion and, hnc, som structural stability, voluminous nvlops with O 90 /d 90 ratios as high as 7 hav bn applid succssfully. In Th Nthrlands and in Blgium, th succssfully applid rtntion critrion O 90 /d 90 for nvlops was thrfor adoptd as th major dsign paramtr. Rcommndations for nvlop applications ar also basd on som additional considrations (Huinink, 1992; Van Zijts, 1992) but th O 90 /d 90 critrion is th most important on. In summary, th following rtntion critria for both gotxtils and PLMs can b accptd: 1 O 90 /d for nvlop thicknss 1 mm. 1 O 90 /d for nvlop thicknss btwn 1 and 3 mm. 1 O 90 /d for nvlop thicknss btwn 3 and 5 mm. 1 O 90 /d for nvlop thicknss 5 mm. O µm. In ordr to minimiz th risk of minral clogging it is rcommndd that O 90 /d 90 1; furthrmor, nvlops that hav O 90 /d 90 ratios nar th uppr limit of th proposd rang of valus ar gnrally prfrrd. Locally mad fabrics such as carpt backing, which satisfis or may satisfy th abov rquirmnts aftr som modifications, ar qually suitabl as importd gotxtils. Thy may thrfor b trustd as nvlop matrials. Hydraulic conductivity Th hydraulic conductivity of nvlops should b gratr than that of th soil in ordr to rduc th ntranc rsistanc of drainpips, so that no hydraulic prssur will dvlop outsid

15 Matrials for subsurfac land drainag systms 35 th nvlop. From rsarch work of Niuwnhuis and Wssling (1979) and Dirickx (1980) it may b concludd that a substantial rduction in ntranc rsistanc is obtaind whn K / K s 10, whr K is th hydraulic conductivity of th nvlop and K s that of th soil (s Chaptr 4, Sction Drain with nvlop). Th hydraulic conductivity, prpndicularly to or in th plan of nvlop, can hardly b a problm bcaus nvlops ar much mor prmabl than th adjacnt soil that thy hav to rtain. Evn undr load, th hydraulic conductivity of comprssibl nvlops will mt th conductivity rquirmnts. If, howvr, nvlops ar brought in contact with soil, soil particls may fill pors and partly block thir opnings as a rsult of which th hydraulic conductivity at th soil-nvlop intrfac will dcras. In addition, nvlops may clog as a rsult of particl dposits and/or chmical prcipitats, and bcom lss prmabl with tim. Evaluation of blocking and clogging of nvlops is vry difficult. If th lowr limit of th rtntion critria is takn into account, it may nvrthlss b assumd that a favourabl hydraulic conductivity ratio is guarantd. Watr rpllnc PLMs do not xhibit wtting problms, yt gotxtils may do and watr rpllnc may b a problm. Watr rpllnc mans that a minimum watr had is rquird on top of th gotxtil, bfor watr starts to flow through it (Lnnoz-Gratin, 1992). Onc th watr has ntrd th pip through th nvlop, th rpllnc problm is solvd and will gnrally not rturn. Wttability rsistanc also dcrass whn th gotxtil is brought into contact with a moist soil. Rsarch work carrid out by Dirickx (1996a) showd that th wtting problm is mainly an initial problm of dry gotxtils. Th initially rquird had for th majority of th tstd gotxtils is smallr than 2 mm. For othrs, it rangs from 5 to 30 mm; on gotxtil rquird an initial had of 64 mm. Although initial watr rpllnc of nvlops dos not sm to b widsprad, gotxtils that xhibit this phnomnon should not b usd as drain nvlop to avoid th risk of soil structur dtrioration nar th nvlop du to th initial stagnation of watr. In accordanc with th standard on th dtrmination of rsistanc to watr pntration of txtil fabrics ISO 811 (1981), a tsting procdur has bn adoptd in th countris of th Europan Union, to xamin gotxtils on watr rpllnc in a qualitativ mannr (pren 13562, 1999). Mchanical proprtis Mchanical proprtis of nvlops ar mostly of scondary importanc. Gotxtils usd as drain nvlop do not prsnt spcific problms sinc thy ar dsignd for, and ar normally usd in mor challnging circumstancs. Morovr, problms that dvlop occasionally bcaus of handling (.g. taring) can b rpaird bfor installation. Th comprssibility of comprssibl nvlops has a major ffct on th charactristic opning siz and th hydraulic conductivity. Th opning siz normally dcrass in comprssd stat so that a safty factor is built in automatically. Th hydraulic conductivity dcrass also, yt th highly prmabl natur of th nvlop nsurs that th hydraulic conductivity ratio is mt in comprssd stat. Morovr, th comprssibility of coarsr nvlops, composd of coarsr fibrs, is small. Easily comprssibl thick nvlops, mad of fin fibrs should not b usd as drain nvlop.

16 36 Envlop matrials Abrasion is th waring of a part of th nvlop by rubbing against anothr matrial, ithr during transportation or installation of wrappd drainpips. Opn spots du to abrasion or whatvr othr caus, noticd bfor installation, should b rpaird in th fild, if thy ar not out of proportion. Abrasion during installation is lss likly to occur bcaus of th short tim that th wrappd pip is routd through th machin. Gotxtils ar wrappd around drainpips ithr manually or mchanically; thrfor, a crtain tnsil strngth is rquird. Dirickx (1994) proposd a tnsil strngth of 6 kn/m, dtrmind according to th wid-width tnsil tst (EN ISO 10319, 1996). Gotxtils must bridg th corrugations of larg drainpips and may not sag btwn th corrugations undr th soil load. Hnc, longation should b limitd, but this rquirmnt is only maningful if th gotxtil is tightly wrappd. Sinc this has nvr bn a practical problm, longation rquirmnts hav nvr bn put forward. Rsistanc to static punctur also is only applicabl for drains with larg corrugations whr a tightly wrappd gotxtil bridgs th corrugations. Th gotxtil should withstand th soil load btwn th corrugations, and puncturing by stons and hard soil clods. Ths phnomna ar simulatd by a static punctur tst. Through this tst, th forc rquird to push a flat plungr through a gotxtil can b dtrmind. Sinc such a problm has nvr occurrd in subsurfac drainag so far, no rquirmnts xist. AVAILABILITY AND COST Cost and availability of drainag matrials ar strongly intrrlatd. Costs vary continuously sinc ths ar dpndnt on various, partly unprdictabl factors lik currncy xchang rats and th cost of manual labour. For rfrnc, various indications of th cost of drainag matrials ar givn in this Chaptr. Th cost of gravl nvlops is not spcifid hr bcaus th local availability of suitabl granular matrial is rapidly dclining. In addition, th cost of installation is strongly dpndnt on local circumstancs. In th Intgratd Soil and Watr Improvmnt Projct (ISAWIP) in Egypt, local gravl nvlops wr four tims as xpnsiv as importd Canadian synthtic fabric nvlops (Mtzgr t al., 1992). In th Fourth Drainag Projct of th Intrnational Watrlogging and Salinity Rsarch Institut (IWASRI) of Pakistan, th cost of synthtic nvlops was found to b 40 prcnt lowr than that of gravl nvlops. Installation of synthtic nvlops was asir and fastr, too (IWASRI, 1997). Thus, vn if th pric of gravl is comptitiv, it gos hand in hand with high costs of ful and manual labour. It is thrfor irrlvant to considr th pric of th raw matrial only. Vlotman t al. (in prss) quot costs of gravl nvlops (matrial and transport) in various projcts in Pakistan. For all projcts, th costs of matrial and shipping of synthtic matrials was blow th cost of gravl. Unfortunatly, th high cost of gravl installation compard to that of installing prwrappd pips is not includd in this analysis. Th cost/bnfit ratio is crtainly in favour of PLM nvlops and gotxtils. PLM nvlops, manufacturd from PP fibrs and coconut fibrs dominat th markt in northwstrn Europ. PLM nvlops, manufacturd from pat fibrs ar now usd only occasionally. An indication of th cost of drainag matrials, i.. pips and PLM nvlops, in Th Nthrlands is givn in Tabl 4. Absolut prics ar not givn. Instad, th rlativ cost of pip and nvlop matrial is spcifid for various pip diamtrs and nvlop matrials. Th figurs

17 Matrials for subsurfac land drainag systms 37 ar basd upon corrugatd PVC pip, and ar quotd for contractors with high rats of turnovr. Th pric of installation of on mtr of wrappd drainpip mor or lss quals that of on mtr of unwrappd 60 mm pip. From Tabl 4, it can b sn that th pric of vn th chapst PLM nvlop compriss a substantial part of th pric of a pr-wrappd pip. This is particularly tru for smallr diamtr pips. In 1998, thr was a slight upward tndncy of th pric of polypropyln wast fibrs in Th Nthrlands. Ths fibrs ar no longr availabl in such hug quantitis as thy usd to b in th past. Dutch pip wrapping companis ar xprimnting with othr synthtic wast matrials in an ffort to b abl to markt comptitiv nvlops in th yars to com. TABLE 4 Th rlativ cost of PLM nvlops, xprssd as a prcntag of th cost of th nvlop plus a corrugatd PVC pip togthr as a prwrappd product, in Th Nthrlands in Th cost of installation is not includd. Th O 90 siz is spcifid within brackts Pip diamtr (mm) Coil lngth (m) Typar Coconut fibrs Polypropyln wast fibrs Rlativ cost of various nvlop matrials Polypropylnstr Poly- Coconut fibrs wast knittd fibrs sock Polystyrn bads in ntting Polypropyln fibrs Polypropyln fibrs (havy) (270) (1000) (300) (450) (400) (700) (1000) (700) (700) * * Th xtrnal diamtr of th wrappd 60-mm pip is 100 mm, i.. th thicknss of th nvlop is 20 mm. Th slction of an nvlop matrial is dtrmind by various factors. Th pric is obviously important. Th as of handling of th matrial is also a factor of considration. Coconut fibr nvlops will rlas substantial amounts of dust particls during handling and installation, particularly in dry wathr; PP fibr wrapping dos not. Prvious favourabl xprincs of farmrs ar important: thy tnd to ask for a similar nvlop whn ordring again. REVIEW OF LOCAL EXPERIENCE ON DRAINAGE MATERIALS Adquat charactrization of soil proprtis, fild conditions (.g. groundwatr tabl dpth) and physical proprtis of nvlop matrials is ssntial. In this contxt, th trm problm soils is rathr vagu and calls for furthr dfinition. This also holds for nvlop matrials: a gnric dscription lik PP nvlop is maninglss sinc it may covr th whol rang from thin gotxtils to voluminous PLMs. In an nvlop slction procss, a systmatic comparison with xprinc gaind lswhr is gnrally vry usful. Synthtic nvlops, ithr PLMs or gotxtils, hav provn to b rliabl and ar succssfully applid in Europ, th Unitd Stats, and Canada for th last 20 yars. Ths matrials hav also bn usd satisfactorily in larg-scal fild xprimnts in Egypt and Pakistan. In th lattr country, thy hav also bn usd as nvlop for intrcptor drains. This provs th transfrability of synthtic matrials from on rgion to anothr. In Framji t al. (1987), th us of nvlop matrials is summarizd for a grat numbr of countris. Ths data ar includd in th Tabl 5, which is supplmntd with additional

18 Envlop matrials 38 TABLE 5 Drainag matrials usd in a numbr of countris Matrial Pips Minral nvlops Organic nvlops Synthtic nvlops C l a y C o n c r t P l a s t i c S a n d a n d g r a v l S l a g G l a s s f i b r c l o t h C h a f f S t i c k s C o c o n u t f i b r s S a w d u s t S t r a w R i c s t r a w C d a r l a f B a m b o o P a l m f i b r s P a t f i b r s K n i t t d s o c k G o t x t i l s S y n t h t i c w a s t f i b r s Australia Blgium Canada China Colombia Costa Rica Cuba Czch Rp. Dnmark Egypt Ethiopia Grmany Franc Hungary India Iran Iraq Isral Irland Japan Jordan Nthrlands Pakistan Pru Poland Portugal Romania South Africa Spain Thailand Turky Uganda USA Zambia Zimbabw

19 Matrials for subsurfac land drainag systms 39 information from othr sourcs, includd that providd by th participants of th Intrnational Cours on Land Drainag (Wagningn, ). Som local xprincs that ar considrd to b informativ ar brifly discussd blow. Arid and smi-arid zons In th Mlka Sadi Pilot drainag schm in Ethiopia, trials wr conductd for valuating drainag nvlops. Thr diffrnt nvlops wr tstd in a pilot schm, comprising locally availabl rd ash, gravl and a factory mad fabric filtr. Th cost of gravl was six tims that of fabric filtr. Th prformanc of both gravl and rd ash wr suprior to that of th fabric filtr (Woudnh, 1987). In Egypt, voluminous nvlop matrials that ar producd locally, namly PP and PA wast fibrs (O 90 of 330 and 400 µm, rspctivly) prformd satisfactorily (Dirickx, 1992a). Occasionally, howvr, th wrapping of drainpips provs to b poor. Th yarn of prwrappd pips was slack and th nvlop matrial did not fully covr th pip. Aftr shipping and handling in th fild, bar spots mrgd at many placs. In addition, taping of th nvlop at ithr nd of coils was somtims inadquat as a rsult of which th nvlop was loos (DRI, 1997). In th north-wstrn irrigation districts of Mxico, locally producd corrugatd PE pips ar usd, with a diamtr of 100 mm for latrals and 150 mm for collctors. Thy must comply with ASTM standards (Chaptr 9). Collctor pips ar approximatly twic as xpnsiv as latrals. Polystr sock is usd as drain nvlop, th cost of which is 30 prcnt of th pric of th wrappd pip. An ncouraging rsult of rcnt nvlop tsting projcts in Pakistan is that synthtic matrials, producd in Pakistan, prformd wll in th laboratory and hav shown thir potntial for fild application. It is not unlikly that IWASRI will vntually rcommnd th Pakistan Watr and Powr Dvlopmnt Authority (WAPDA) to rplac gravl nvlops with locally manufacturd synthtic matrials. Locally manufacturd matrials wr found to outprform finr local and importd matrials, and hnc ar subjctd to additional fild trials. In th Mardan Scarp salinity control and rclamation projct in Pakistan, Dirickx t al. (1995) rcommnd nvlops with an O 90 ranging from 200 to 400 µm. In Pru, gravl and coars sand ar availabl vrywhr at vry rasonabl cost, and hav bn succssfully installd by hand and trnching machins. Th us of clay and concrt tils has not bn vry succssful. Many soils ar vry unstabl, and accurat installation of drains was complicatd. Installation by hand was quit slow, and th width of xcavation at th soil surfac was 6 to 15 tims that of th trnch box of a trnching machin. Concrt pips wr xpnsiv, bcaus thy had to b mad from sulphat rsistant cmnt. Most Pruvian soils that ar suitabl for agricultur hav a vry high contnt of calcium sulphat. Furthrmor, th rat of production of concrt pips was quit low. Btwn 1983 and 1985, 400 km of 65 mm and 100 mm corrugatd pip was installd. Ths pips wr manufacturd in Pru with an xtrudr, importd from Europ (D la Torr, 1987). Humid Tropics In Costa Rica, corrugatd pips wr importd from th Unitd Stats to drain fruit plantations, mainly bananas, notably in mdium to coars sands. In finr soils with low structural stability, th pips wr mostly prwrappd with gotxtils,.g. spun bondd polyamid (Murillo, 1987).

20 40 Envlop matrials In India, drainag matrials ar producd locally. Agricultural drainag systms ar solly installd on an xprimntal basis. In havy clay soils, drains ar installd without nvlop matrial, and th systms prform satisfactorily. Locally mad gotxtils ar usd with succss; problms ar rarly ncountrd (Oostrbaan, 1998). In th mid-1980s, th functioning of subsurfac drainag systms was invstigatd in pilot aras, using clay tils, installd in manually xcavatd trnchs (Singh, 1987). In 1998, th majority of th drainag systms is still bing installd by manual labour. Tmprat zons In Blgium, th us of clay tils was discontinud in 1975 whn thir application was suprsdd by corrugatd PVC pips. Sinc a potntial risk of minral clogging xists in narly all soils, nvlops ar usd vrywhr. Envlop matrials hav volvd from flax straw and coconut fibrs to loos synthtic fibrs. Currntly, loos synthtic PP fibr wrapping is almost xclusivly usd, but coconut fibr wrapping is still availabl. In th Scandinavian countris, sawdust from conifr trs is vry oftn usd as an nvlop matrial for agricultural subsurfac drainag systms. In unstabl soils in Dnmark th pip drain is protctd against minral clogging by a synthtic sht bnath th pip, and gravl or sawdust asid and on top of th pip. In Norway, 50 prcnt of th sawdust has usually dcayd aftr 20 yars. Still, som drains hav a srvic lif of ovr 30 yars, which will b du to th low tmpraturs in Scandinavia. Th sawdust is applid in a 50 to 70 mm thick layr (Mortnsn, 1987). Approximatly 60 prcnt of th installd drainpips in th thn Wst-Grmany wr prwrappd (Egglsmann, 1982). Organic nvlops lik pat, ry straw and coconut fibr wrappings hav bn xtnsivly usd. Evn nvlops mad from tannin-containing wood chips to prvnt or rduc ochr formation hav bn dvlopd (Egglsmann, 1978). Various kinds of synthtic fibr and granul wrappings hav bn applid, yt gotxtil and loos PP fibr wrappings ar th most widly usd matrials. Only 5 prcnt of th drainpips installd in Franc nd an nvlop matrial. Envlops hav volvd simultanously with drainpips and drainag mchanization. Originally, coconut fibr wrappings hav bn widly usd. Th risk of microbiological dcay of th coconut fibr wrapping has promptd th introduction of loos synthtic fibr wrappings and, at a latr stag, gotxtils. Currntly, gotxtils ar usd almost xclusivly (Lnnoz-Gratin, 1987). In Th Nthrlands, th rcommndations for th slction of PLMs ar as follows (Huinink, 1992; Van Zijts, 1992): Envlops containing pat fibrs and PP-450 should not b usd in cas of possibl iron ochr hazard and/or if th drains ar also usd for subsurfac irrigation purposs during th summr sason. Matur or ripnd clay soils with a clay contnt gratr than 25 prcnt do not rquir nvlops. For most othr soils, such as immatur clay soils with a clay contnt gratr than 25 prcnt, (loamy) sand, (sandy) loam, silt loam and pat soils, any nvlop may b slctd following th rcommndations, spcifid in Tabl 6. Excptions ar mad for clay soils with a clay contnt blow 25 prcnt, silts and vry fin sands which should b draind with PP-450 or, in cas of iron ochr, with PP-700 only.

21 Matrials for subsurfac land drainag systms 41 TABLE 6 Applicability of th most popular prwrappd drain nvlops in Th Nthrlands (adaptd from Huinink, 1992) Envlop matrial Soil typ 1 Soils with clay contnt > 25% down to drain dpth Soils with clay contnt <25%, loams and vry fin-txturd soils, structurally unstabl sands (mdian particl diamtr < 120 µm) Loamy sands and olic dposits Sandy soils (mdian particl diamtr > 120 µm) Paty soils and pats with clayy topsoils Soil profil maturd to drain dpth? Ys No Ys No voluminous nvlops (i.. thicknss 1mm) Cocos (O 90 = 700 or 1000 µm) Non 2 Ys Ys Ys Ys Pat/cocos mix, pat fibrs Non 2 Ys 3 Ys 3 Ys 3 Ys 3 Polypropyln fibrs 450 µm Non 2 Ys 3 Ys 3 Ys 3 Ys 3 Ys 3 Ys 3 Polypropyln fibrs 700 µm Non 2 Ys Ys Ys Ys Polystyrn bads Non 2 Ys Ys Ys Ys thin nvlops (i.. thicknss < 1mm) Glass fibr sht, Crx, Typar, Non 2 Ys 3,5 Ys 3,5 knittd sock nvlop 1 In layrd soil profils, nvlop slction should b basd on th layr with th lowst clay contnt. 2 No nvlop rquird; soil is structurally stabl and th risk of minral clogging of th drainpip is small. 3 Do not install this nvlop matrial if thr is a risk of iron ochr clogging, or if th drains ar usd for controlld drainag or for subirrigation purposs. 4 Us this nvlop matrial only if thr is a srious thrat of iron ochr clogging th drains. 5 Do not us a thin nvlop if th soil profil to drain dpth contains paty layrs. In Th Nthrlands, thin nvlop matrials ar usd with grat caution only, and only in highly unstabl vry fin sandy soils (mdian soil particl diamtr < 120 µm). For a varity of rasons, this catgory of nvlops has nvr bcom vry popular. Th pric of thin nvlops is not comptitiv, and most farmrs simply prfr nvlops to hav a visibl and substantial thicknss bcaus thy ar convincd that such nvlops provid bttr srvic than thin ons. Rliabl data, rtrivd from pilot ara rsarch projcts that convincingly prov that this traditional viwpoint is not always justifid, hav not had an apprciabl ffct. Tradition is indd a strong factor whn it coms to slcting drainag matrials, particularly nvlops. In th Marismas ara, locatd in th Guadalquivir stuary in southrn Spain, clay pips ar mainly usd although corrugatd plastic pips ar installd as wll. Th clay pips hav an insid diamtr of 80 mm, yt a squar outsid circumfrnc with a small longitudinal hol in ach cornr, which is introducd to assur thorough hating of th clay during th manufacturing procss. Th corrugatd PVC drains hav a diamtr of 50 mm. Th cost diffrnc btwn clay and PVC drains is small, and farmrs, thrfor, prfr th largr diamtr clay pips (Martínz Bltrán, 1987). Drains ar installd during th dry sason whn th groundwatr tabl is blow drain lvl. Drains do not rquir nvlops bcaus th Marismas soils ar vry stabl du to thir clay contnt gratr than 65 prcnt. Minral clogging of drainpips has nvr bn obsrvd xcpt for drains whos outlts into opn collctors wr submrgd during priods of havy rainfall. In silty loams and loamy clay soils of th Ebro basin in north-astrn Spain, corrugatd PVC drains with coconut fibr wrapping hav bn installd in th svntis. Thr is no information on th prformanc of ths drainag matrials. Corrugatd PVC drains and synthtic fibr wrapping hav bn usd in th sandy soils of th Ebro dlta as wll.

22 42 Envlop matrials

23 Matrials for subsurfac land drainag systms 43 Chaptr 4 Watr flow into and insid th drain FLOW TOWARDS THE DRAIN According to Ernst (1954), th flow towards a subsurfac drain can b dscribd by a vrtical flow (from th groundwatr lvl downward to drain lvl), a horizontal flow towards th vicinity of th drain, a radial flow to th drain and an ntry into it. Each of ths flows is subjct to a corrsponding rsistanc (Figur 22a). For stady-stat flow, th total rsistanc can thus b roughly classifid into vrtical, horizontal, radial, and ntranc rsistancs. Ths rsistancs can b masurd by stratgically locatd pizomtrs (Figur 22b). Pizomtrs consist of unprforatd narrow pips with a short filtr at th bottom nd in which th watr lvl rprsnts th hydraulic had in th soil nar th filtr nd. Diffrncs in hads ar a masur of th rsistancs mntiond. Th total loss of had, h t, is th sum of all diffrncs indicatd in Figur 22b: Th vrtical had loss, h v, is th diffrnc in watr lvl btwn pizomtrs 1 and 2, locatd midway btwn two drains, with filtrs at rspctivly groundwatr lvl and drain dpth. Th horizontal had loss, h h, du to (mainly) horizontal flow towards th drain, is th diffrnc in watr lvl btwn pizomtrs 2 and 3, with filtrs at drain lvl rspctivly midway btwn two drains and in th vicinity of th drain. FIGURE 22 Flow rsistancs towards a drain flowing at full capacity (a) and thir corrsponding had losss (b)

24 44 Watr flow into and insid th drain Th radial had loss, h r, is th diffrnc in watr lvl btwn pizomtrs 3 and 4, with filtrs at drain lvl rspctivly som distanc away from th drain and at th drain. Th ntranc had loss, h, is th diffrnc in watr lvl btwn pizomtr 4 and an opn standpip in th drain. Th rlationship btwn had loss and corrsponding rsistanc is givn by: h * = q L W * (1) whr h = diffrnc in had (m); L = drain spacing (m); q = spcific discharg (m/d); W = rsistanc (d/m); and = subscript v (vrtical), h (horizontal), r (radial), (ntry) or t (total). * Thus th total had loss is: h t = h v + h h + h r + h (2) Somtims th rsistancs W ar rplacd by th dimnsionlss quantitis α which ar indpndnt of th hydraulic conductivity of th soil: whr K = hydraulic conductivity (m/d); and α = gomtrical factor (dimnsionlss). Hnc, th total had is givn by: α * = K * W * or W * = α * / K * (3) h t = q L (W v + W h + W r + W ) = q L (α v / K v + α h / K h + α r /K r + α / K ) (4) This and othr drainag thoris ar usd for calculating drain spacings. Thy ar basd on a st of assumptions concrning th drain and th physical proprtis of th soils involvd. Although ths assumptions ar approximat, th outcom is usually sufficint for practical applications. On of ths assumption is that of an idal drain, without ntranc rsistanc, whrby th drain is considrd as an quipotntial. Gnrally, it is assumd that th drain surround (nvlop matrial and loosnd soil in th trnch) has such a high hydraulic conductivity compard to th undisturbd soil, that th ntranc rsistanc may b nglctd. Practical xprinc has shown that this cannot always b takn for grantd. Thr is still nd for a qury, both thortically and mpirically, in which cass substantial ntranc rsistancs may b ncountrd. Ponding and xcss soil watr during havy rains, in spit of th prsnc of a drainag systm, may also rsult from a low prmability layr nar th soil surfac that causs a suspndd or prchd watr tabl. Anothr caus may b compaction du to havy machinry, to slaking during havy rains and, on sports filds, to playing actions. This low prmability layr simply prvnts th watr from raching th groundwatr tabl, but has nothing to do with th subsurfac drainag systm itslf. Procdurs and programs for th dsign of subsurfac drainag systms ar in prparation by FAO. Thrfor, this analysis will b limitd to th influnc of th ntranc rsistanc and pip flow on drain prformanc.

25 Matrials for subsurfac land drainag systms 45 ENTRANCE AND APPROACH FLOW RESISTANCE Watr ntrs a ral drain through a finit numbr of prforations, which rprsnt at most only 1 to 2 prcnt of th total wall ara. Although a ral drain dos not altr th gnral radial flow pattrn, th stramlins convrg to th inlt prforations in th immdiat vicinity of th drain. This causs an ntranc rsistanc, W, lading to a had loss on ntry, h. As compard to flow to an imaginary, idal drain, th convrgnc of stramlins to th inlt prforations of a ral drain invoks an additional flow rsistanc and had loss. Th additional flow rsistanc is calld ntranc rsistanc and th corrsponding had loss is th ntranc had loss. According to Eq. (1) and taking into account Eq. (3) th rlationship btwn ntranc had loss and ntranc rsistanc is givn by: h qlw ql = = α (5) K Th ntranc rsistanc of a ral drain can b calculatd thortically for som simpl prforation shaps and pattrns, or can b obtaind if th flow pattrn towards both th idal and ral drain can b accuratly modlld (Sction Entranc rsistanc of drainpips). In most cass, th ntranc rsistanc is obtaind mpirically from th ntranc had loss. Thortically, th ntranc had loss can b obtaind dirctly from pizomtr radings outsid and insid th drain (Figur 22b). Practically, howvr, pizomtr 4 will b placd at som short distanc away from th drain to avoid th disturbanc of th soil causd by installing th drain (Figur 23) and thrfor, th masurd had loss involvs not only th ntranc had loss, but also part of th radial rsistanc. FIGURE 23 Approach flow and total had loss to valuat drainag prformanc in xprimntal filds

26 46 Watr flow into and insid th drain Entranc rsistanc, rsistanc of th disturbd soil and th radial rsistanc ar thortical concpts, which cannot b physically sparatd, nor sparatly masurd in th fild. Th masurd had loss is th lump sum of all th had losss which may b thortically considrd in th approach flow rgion. Cavlaars (1967) introducd th concpt of approach flow rsistanc (W ap ) and approach flow had loss (h ap ) for th flow in th approach rgion (Figur 23). Similar to Eq. (5), th rlationship btwn both quantitis for approach flow can b writtn as: h qlw ql ap = ap = α ap (6) Kap Th masurd had, h ap, rsults from ntranc rsistanc, rsistanc of th disturbd soil surrounding th drain, and th radial rsistanc in th undisturbd soil as shown in Figur 24 for a drain installd in a trnch. This also holds for trnchlss drainpip installation, but th disturbd zon will not b so clarly boundd compard to that cratd by a trnchr. FIGURE 24 Drain with or without nvlop, disturbd trnch backfill and undisturbd soil constitut th approach flow rgion Th had loss dtrmind in xprimntal filds is th approach flow had loss, though it is usually calld ntranc had loss, and is usd to calculat th ntranc rsistanc,.g. by Dilman and Trafford (FAO, 1976).

27 Matrials for subsurfac land drainag systms 47 Th ntranc rsistanc as dfind by Dilman and Trafford (FAO, 1976) is in fact an approach flow rsistanc and diffrs fundamntally from th thortical concpt of ntranc rsistanc. It can also b usful to xprss th approach flow had loss as a prcntag of th total had loss. To dtrmin th total had loss, ithr a pizomtr (pizomtr 1) as in Figur 22b or a wll tub as in Figur 23 can b installd midway btwn drains. Unlik th pizomtr, which is prforatd at th bottom ovr a limitd lngth only, th wll tub is prforatd ovr almost its ntir lngth. Th flow pattrn nar th drain is vry complx du to th disturbd soil whr physical charactristics ar htrognous and chang with tim and ar thrfor difficult to prdict. Th approach flow had loss, h ap, is affctd by th physical proprtis of this disturbd soil which surrounds th drain (K ap ), th drain spacing and th drainag matrials usd. A good nvlop matrial, howvr, can rduc α ap to such low valus that th drain will act as almost an idal drain. Th sam holds if th soil around th drain is highly prmabl, say K ap = 10 m/d. This is mostly th cas in backfilld trnchs in clayy soils or aftr trnchlss drainag in wll-structurd clays and clay-loams. Thus, ntranc rsistanc is sldom a problm in ths soils, vn in th absnc of a drain nvlop. Th rason for this bhaviour is that watr in th immdiat vicinity of th drainpip oftn follows prfrntial pathways. It will b routd through ithr th trnch backfill, if prsnt, or through cracks and fissurs, cratd by a trnchlss drainag machin. Th occurrnc of prfrntial flow is dtrmind by th conductivity ratio of th disturbd and th undisturbd soil. Th disturbd soil may hav a prmanntly highr hydraulic conductivity. Yt aftr sttling, som disturbd soils may bcom lss prmabl than th undisturbd soil. Soil disturbd in dry conditions will in most cass favourably affct drainag prformanc, rgardlss of whthr th soil is homognous or htrognous, and whthr th watr follows prfrntial flow paths or not. Any ffctiv subsurfac drainag systm rquirs good physical soil conditions in th immdiat vicinity of th drain. Only thn will drainag matrials, which ar by thmslvs appropriat, do a good job. In this contxt, good physical soil conditions is synonymous with a physically stabl and hydraulically prmabl soil. Such a soil, which consists of stabl soil aggrgats is oftn rfrrd to as a wll-structurd soil. Th installation of subsurfac drains causs major changs in th physical proprtis of soil matrial abutting th drain. Ths proprtis ar difficult to quantify, mainly bcaus thy cannot b accuratly obsrvd. Still, th physical proprtis of th soil ar crucial for th futur succss or th failur of th drainag systm. Aftr installation, a balanc has to b r-stablishd, as th soil will sttl around th drain in som way or anothr. Th major forc that govrns this procss is th drag forc of th flowing groundwatr that is discharging into th drain. Th forcs btwn soil particls and aggrgats that rsist this drag forc ar also important. Furthrmor, th rtntiv proprty of th pip or th drain nvlop plays an important rol. Dpnding on th way th drains wr installd (trnchr or trnchlss), th structur of th soil around th drain will b damagd, that is, waknd. Consquntly, th natural ability of th soil to rsist th dtrimntal forcs of th groundwatr will b undrmind. An additional complicating factor is th fact that th flux dnsity of th groundwatr is th highst whr th structural stability of th soil is oftn wakst, namly nar th drain, whr th flow convrgs.

28 (9) 48 Watr flow into and insid th drain Th soil may b locally compactd, spcially whn drains ar laid undr wt conditions. If drains ar installd with a trnchlss machin, which mploys a vrtical plough, th dtrimntal ffct on th structur of th soil dpnds on th dpth of installation and th soil watr contnt at th tim of installation. Up to a crtain dpth, th plough is abl to lift th soil, crating fissurs, and macropors. Yt, blow th so-calld critical dpth th ovrburdn of th soil prvnts it from bing liftd. Instad, th soil is pushd asid, compactd and smard and natural fissurs and macropors ar locally dstroyd (Van Zijts and Naarding, 1990). WATER FLOW INTO THE DRAINPIPE Th xit gradint Darcy s law dscribs th flow of watr through porous mdia undr laminar flow conditions and xprsss th proportionality btwn th discharg ovr a cross-sction and th hydraulic had loss, or btwn th discharg and th hydraulic gradint: whr dh Q = KA = KAi dl Q = discharg (m 3 /d); A = ara of cross-sction (m 2 ); K = hydraulic conductivity (m/d); dh = hydraulic had loss (m); dl = distanc ovr which dh is masurd (m); and i = hydraulic gradint or had loss pr unit of distanc (= dh/dl). (7) Th xit gradint i x is th hydraulic gradint at which watr lavs on mdium and ntrs anothr. Th flow mdia at th intrfac may b soil-watr, soil-air, soil-nvlop, nvlop-watr, or nvlop-air. Whn th watr ntrs th drain, th mdium it lavs can b th soil or th nvlop matrial. Th mdium it ntrs may b watr or air. If th stramlins ar paralll (Figur 25), th hydraulic gradint i is givn by: h i = = l In this cas, for a givn Q, th hydraulic gradint i is th sam anywhr in th flow rgion sinc A and K ar constants. Thus, th xit gradint i x or th gradint whr th watr lavs th soil is qual to th hydraulic gradint throughout th systm, which is a constant. Howvr, in cas of radial flow (Figur 26), th cross sctional ara pr unit drain lngth at a distanc r from th drain cntr is 2πr and th stramlins convrg. Th discharg pr unit drain lngth is givn by: and th hydraulic gradint by: Q AK ql = 2π rk dh dr (8) (9) dh ql i = = dr 2π rk (10)

29 Matrials for subsurfac land drainag systms 49 FIGURE 25 Horizontal flow whr q is th spcific discharg (for stady stat flow qual to rainfall or irrigation xcss in m/d), L th drain spacing (m), and ql th discharg pr unit drain lngth (m 2 /d). In this cas, th hydraulic gradint i is no longr a constant for a givn discharg pr unit drain lngth, but incrass with dcrasing r and vic vrsa. FIGURE 26 Radial flow Considring radial flow towards an idal drain, i.. a compltly prmabl drain, th xit gradint i x whr th watr lavs th soil and ntrs th drain will b gratr than anywhr ls in th flow systm. It is invrsly proportional to th drain radius (Figur 27). For non-idal drainpips, th flow lins furthr convrg toward th prforations in th drain wall, so that th xit gradint at th drain prforations will b vn gratr. Howvr, an idal drain with a smallr diamtr r o can rplac a prforatd ral drain in drain spacing calculations (Sction Plain drain). In thory, th xit gradint at th boundary of such a hypothtical (and smallr) idal drain quals th xit gradint at th prforations of a ral drain. Th concpt of radial flow is basd upon simplifying assumptions concrning th ral situation. Usually, howvr, th flow pattrn nar a drain is not fully radial; it may indd b vry diffrnt,.g. irrgular, dpnding on th hydraulic proprtis of th soil nar th drain. Hnc, th quipotntials in th groundwatr ar not ncssarily concntric, rlativ to th drain cntr. Instad, thy ar mor likly to b ccntric and vn irrgular. This fact oftn complicats th assssmnt of th actual xit gradint in ral situations. Th critical hydraulic gradint Flow of watr at a high xit gradint is rapid and powrful. It may xrt nough drag forc to ovrcom th rsistanc of th soil against shar. In this cas, movmnt of soil particls will

30 50 Watr flow into and insid th drain FIGURE 27 Exit gradint i x, xprssd as th ratio i x K/qL for radial flow as a function of th drain radius, r o i xk/ql (m -1 ) drain radius r o (m) start and local rosion will occur around th drain. Th hydraulic gradint at which ths phnomna occur, is calld critical hydraulic gradint. Th sharing rsistanc of a soil, which opposs th movmnt of soil particls or soil rosion, is givn by Coulomb s quation: τ f = c o + σ tanφ (11) whr τ f = sharing rsistanc pr unit ara (Pa); c o = cohsion (Pa); σ = ffctiv strss of th soil particls or intrgranular strss (Pa); and φ = angl of intrnal friction or sharing rsistanc. Cohsiv soils (lik clays) possss firm bonds btwn soil particls and ar mostly composd of soil aggrgats. Cohsionlss soils (lik sands) lack bonds btwn individual particls (c o = 0) and consist of individual soil particls, hnc: τ f = σ tanφ (12) Soil load and watr prssur dtrmin intrgranular strsss σ. Gratr soil loads and smallr watr prssur incras th ffctiv strss and rduc th risk of rosion. Howvr, stabl bridgs may occur in sands. Thy form archs, that span about 5-8 grain diamtrs (Pschl, 1969). Sand, thrfor, dos not normally ntr opnings lss than 5-8 grain diamtrs (xcpt for a fw grains that scap whil th archs ar bing stablishd). Watr flowing through a porous mdium xrts a prssur on th soil particls in th dirction of movmnt. This prssur is calld flow prssur. If th flow prssur acts in th dirction of gravity (downward flow) th ffctiv strss of th soil particls is incrasd and th risk of rosion is lssnd. If howvr th flow prssur acts against gravity (upward flow) th

31 Matrials for subsurfac land drainag systms 51 intrgranular strss may dcras substantially or vn b canclld, rsulting in a highly unstabl situation which is known as quick sand. Exampls of such flows ar mud volcanos bing formd in placs of strong upward watr movmnt. Flow prssur prpndicular to gravity causs a latral movmnt of soil particls whn th shar rsistanc is ovrcom. Th hydraulic gradint at which th structural strngth of th soil bcoms ngligibl is calld th critical gradint, i c. Th critical gradint dpnds on th ffctiv strss and on th cohsion of th soil. For cohsionlss soils without soil load, th critical hydraulic gradint quals approximatly unity. This situation occurs in cas of upward flow of groundwatr. For cohsiv soils, th cohsiv forc has to b considrd as wll. For ths soils, th critical hydraulic gradint will b gratr than that of cohsionlss soils. It is rlatd to th strngth of th cohsiv bonds btwn soil particls and/or aggrgats. If th flow prssur xcds th sharing rsistanc of th soil, rosion will occur bcaus th soil loss its structural strngth. Sinc th flow prssur is proportional to th acting hydraulic gradint, rosion will start as soon as th xit hydraulic gradint i x rachs th critical hydraulic gradint i c of th soil (Trzaghi and Pck, 1965). Intrnal rosion in which soil particls mov in th soil itslf is not considrd. It usually occurs in alkali soils, spcially whn th soil racts on phnolphthalin (ph abov 8.5). In such soils, intrnal rosion may occur if fin soil particls can dtach thmslvs from th sklton formd by th coarsr fractions. With th watr flow, thy mov through cracks and othr macropors in th soil. This may caus a turbid drain outflow, rsulting in a milky apparanc of such watrs and intrnal clogging of th soil sklton. Hydraulic failur gradint Th critical hydraulic gradint will b gratr in cas of ovrburdn load and with incrasing soil cohsion. In accordanc to ths assumptions, Samani and Willardson (1981) hav proposd th concpt of th hydraulic failur gradint, i f, which is th hydraulic gradint at which a confind or supportd soil cannot rsist th drag forc of th flowing watr. Th soil loss its structural stability and starts moving into, and possibly through nvlops. Thn th drainag systm is vry likly to fail bcaus this procss may substantially rduc th hydraulic conductivity of th nvlop. Samani and Willardson (1981) found that th hydraulic failur gradint dpnds on th plasticity indx of a soil (Chaptr 6). Th associatd rlationship was howvr not transfrabl btwn soils originating from humid and arid rgions. Yt, if th hydraulic conductivity is incorporatd in th i f -concpt a good corrlation was found btwn th hydraulic failur gradint and th combination of plasticity indx and hydraulic conductivity of th soil. This corrlation was valid both for humid and arid rgions. Vlotman t al. (in prss) usd th data of Samani and Willardson (1981) to driv an mpirical rlationship, which is only slightly diffrnt from th original on: i f = ( K ln I p ) whr i f = hydraulic failur gradint; K = hydraulic conductivity of th soil (m/d); and I p = plasticity indx of th soil. (13)

32 52 Watr flow into and insid th drain Th plasticity indx is a masur for th plasticity of a soil. It is dfind as th diffrnc in watr contnt, as a prcntag of th mass of ovn-drid soil, of a soil at its liquid limit and at its plastic limit (ICID, 1996): I P ( WLL WPL ) WDS whr W LL = mass of soil sampl at liquid limit (g); W PL = mass of soil sampl at plastic limit (g); and = mass of ovn-drid soil sampl (g). W DS = 100 (14) Eq. (13) considrs howvr only soil proprtis. Ovrburdn ffcts and nvlop typs ar not considrd, othrwis i f cannot b constant for a givn soil condition. Thrfor th i f -concpt is, in ssnc, th sam as th critical hydraulic gradint. Th i f -concpt can b usful as a dcision tool for th application of a voluminous nvlop to incras th radius r and so to rduc th xit gradint i x nar th drain to a valu infrior to th i f -valu of th soil. Still, th i f -concpt has nvr found widsprad application. Th xprinc obtaind so far with th i f as a tool for drain nvlop dsign is thrfor vry limitd. ENTRANCE RESISTANCE OF DRAINPIPES In th sction Entranc and approach flow rsistanc, it was stablishd that th had loss which is obsrvd nar a fild drain is associatd with th approach flow rsistanc, which is th lump sum of th ntranc rsistanc and th flow rsistanc in th adjacnt soil. Hnc, th ffct of (wrappd) subsurfac drains on drainag prformanc cannot b dtrmind as such. It is, howvr, important that th hydraulic proprtis of drainpips and nvlops on drainag prformanc can b assssd as wll. Ths proprtis ar thrfor discussd in this sction. Plain drain Th flow towards a drain can b stablishd if this flow can b modlld analytically. This can b don for radial flow. Th had loss, associatd with radial flow to an idal, full flowing drain in a homognous and isotropic soil (Figur 28a) with hydraulic conductivity K, rads: in which: h r ql ql r = qlw r = α r = ln (15) K 2πK r α = r 1 ln 2π r r o o (16) whr r = th radius of a circular quipotntial (m); and r o = th radius of th idal drain (m). Th radius r should b chosn such that th quipotntial has indd a circular shap, and th flow towards th drain is radial. That is, th ffct of th pip prforations on th chosn quipotntial must b insignificant. Th approach flow had loss, associatd with radial flow to a ral drain (Figur 28b) is givn by Eq. (6) which can also b writtn as:

33 Matrials for subsurfac land drainag systms 53 FIGURE 28 Radial flow to (a) an idal and (b) a ral drain h ap = ql ql + W ) = ( α + α ) K ( Wr r Sinc radial flow to an idal drain is dscribd by Eq. (16) th ntranc rsistanc rsults from: α = α α (18) ap In this cas, th ntranc rsistanc of a ral drain is th diffrnc btwn th approach flow rsistanc to a ral drain and th radial flow rsistanc to an idal drain. Th ntranc rsistanc α is fully associatd with th drainpip and thrfor is a constant dpndnt on th prforation shap and pattrn of th drainpip if th radial flow occurs ovr th whol drain circumfrnc. If radial flow occurs ovr only a sction of th drain circumfrnc (Figur 29), th flow rsistanc dpnds on th sctor ara whr th radial flow to th drainpip rally occurs (Boumans, 1963). Th actual ntranc rsistanc, α *, is invrsly proportional to th flow sctor: 2π α * = α (19) β whr β = angl of th sctor whr radial flow occurs (radians, 0-2π). Th transitional boundary of th soil with th pip prforations also affcts th ntranc rsistanc sinc th ntranc rsistanc is invokd by th convrgnc of stramlins to ths prforations. Th ntranc rsistanc incrass du to any typ of clogging, and dcrass bcaus of th washing out of soil particls. Th boundary btwn soil and pip prforations may hav manifold gomtrical configurations. Th following boundaris may xist (Figur 30): th prforations ar filld with soil; th soil forms a plan boundary with th prforations (plan boundary conditions); r (17)

34 54 Watr flow into and insid th drain FIGURE 29 Influnc of a partly radial flow pattrn on th ntranc rsistanc of ral drains β = 2 π β = π π < β < 2 π p = 1 p = 2 2 > p > 1 * α = α * α = 2 α * 2 α >α > α th soil nar th prforations is washd out and forms an archd boundary (archd boundary conditions); and th soil nar th prforations is washd out and forms an irrgular boundary. In th fild, th archd boundary is th most likly configuration (Pschl, 1969). According to Stuyt (1992a) this boundary may hav a mor complx thr-dimnsional configuration. Th opnings shown in Figur 30 may rprsnt ithr: gaps btwn til drains; circular prforations in plastic pips; and rctangular slots in plastic pips. FIGURE 30 Possibl boundaris btwn soil and drain opnings (aftr Cavlaars, 1970) Th shap of th outr pip wall (smooth or corrugatd) affcts th ntranc rsistanc, spcially if th prforations li in th vally of th corrugations which is normally th cas. Th gratst ffct stms from whthr th corrugations ar filld with soil or not. If th corrugations ar filld with soil, th gomtry of th boundary of th soil with th prforations is quit rlvant. For corrugations without soil th boundary with th corrugations will b dcisiv for th ntranc rsistanc. Th shap of th corrugations ( wav or block ) xrts only a minor influnc. For som pattrns and shaps of prforations in smooth outr pip walls, th ntranc rsistanc can b modlld analytically for plain and archd boundary conditions. Dirickx (1980) mad an xtnsiv rviw of th analytical solutions and chckd thir corrctnss with an lctrolytic modl. Th simplst but still sufficintly accurat solutions ar summarizd in

35 Matrials for subsurfac land drainag systms 55 Dirickx (1999). In many cass howvr, and for corrugatd drains, th ntranc rsistanc follows from modl rsarch. Accurat rsults can b obtaind with an lctrolytic modl sinc both boundary conditions and hydraulic conductivity ar known vry accuratly. This is not th cas whn sand modls ar usd, bcaus th configurations ar lss wll dfind. Analytical solutions and modl rsarch hav rvald that for circumfrntial opnings btwn clay and concrt tils, th ntranc rsistanc is largly rlatd to th gap spacing and th outr drain diamtr and only slightly to th gap width. Thus, incrasing gap width btwn tils is an inffctiv way to rduc th ntranc rsistanc whil th risk of soil invasion is nhancd. Th provision of sgmntd pips with hols also rducs th ntranc rsistanc. Such pips ar usd xclusivly in th Unitd Stats. Bcaus th gap spacing of til drains cannot b rducd, th only way to dcras thir ntranc rsistanc is th us of a largr diamtr til. Th most ffctiv way to dcras th ntranc rsistanc of drainpips with circular prforations is to incras th numbr and diamtr of th prforations. Although drains with continuous longitudinal slits do not xist, thir proprtis can b simulatd in mathmatical modls. As such, invstigation of thir proprtis is usful: incrasing th numbr of slits is mor ffctiv than incrasing th slit width and th drain diamtr. Hnc, incrasing th numbr of slit rows is th most ffctiv way to rduc th ntranc rsistanc of drains with discontinuous longitudinal slits. Th ntranc rsistanc of drains with discontinuous circumfrntial slits can b rducd by dcrasing th spacing btwn th rows of slits and by incrasing th drain diamtr. Th slit width is lss important. According to Childs and Youngs (1958), a ral drain can b rplacd by an idal drain with a smallr radius, th so-calld quivalnt or ffctiv radius, r f. Substitution of α r from Eq. (16) into Eq. (18) yilds: α 1 r ap = ln α 2π r + Similarly to Eq. (16), th radial flow rsistanc for flow to th idal substitut, which rsults in th sam flow rsistanc, is givn by: o f (20) 1 r α ap = ln (21) 2π r from which it follows that: r f = r 2πα ap 2πα = ro (22) As th ffctiv radius dpnds on th ntranc rsistanc, th ffctiv radius can b usd as an altrnativ to th ntranc rsistanc: th smallr th ntranc rsistanc, th largr th ffctiv radius. Valus of ntranc rsistancs associatd with various drainpips ar givn in Tabl 7. Th valus of Dirickx (1993) rsult from lctrolytic modl rsarch with th assumption that th corrugations of flxibl pips ar filld with soil, and that th soil forms a plan boundary with th prforations. Smdma and Rycroft (1983) do not quot any rfrnc yt th valus thy prsnt ar most likly stablishd from sand tank modls. Th tabl also contains th ratio r f /r o (= -2πα ) to show th ffct of ntranc rsistanc on th ffctiv radius of a drain.

36 56 Watr flow into and insid th drain TABLE 7 Entranc rsistancs and r f / r o -ratios of plain drainpips Typ of drainpip Clay and concrt Smooth plastic Corrugatd plastic Dirickx (1993) Smdma & Rycroft (1983) α rf / ro α rf / ro (dimnsionlss) (dimnsionlss) (dimnsionlss) (dimnsionlss) Although diffrnt ntranc rsistanc valus ar found in th litratur, sgmntd pips with gaps usually hav a gratr ntranc rsistanc than smooth plastic pips with mor uniformly distributd prforations. In turn, smooth plastic pips hav gratr ntranc rsistancs than corrugatd plastic pips with mor prforations and a gratr prforation ara. Drain with nvlop Sinc th ntranc rsistanc of drainpips can b of th sam ordr as th total flow rsistanc in th soil (Widmosr, 1968), any chang of prmability in th immdiat vicinity of th drain will markdly influnc drainag prformanc. Drain nvlops normally hav a gratr hydraulic conductivity than th surrounding soil. Hnc, thy contribut to th dcras of th ntranc rsistanc of drainpips. If an nvlop with thicknss d and a hydraulic conductivity K > K surrounds an idal drain (Figur 31), th total radial flow rsistanc is givn by: FIGURE 31 Thortical flow towards an idal drain surroundd by an nvlop 1 r 1 r = ln + ln α r 2π r 2πκ r (23) o whr r κ = radius of th soil-nvlop intrfac (m); and = K /K, is th rlativ hydraulic conductivity or th hydraulic conductivity ratio of th nvlop and th surrounding soil. Dfining th ntranc rsistanc prsnts no particular difficulty for drains without nvlops (Sction Plain drain). Howvr, th ntranc rsistanc of a drain with nvlop is affctd by both th hydraulic conductivity of th nvlop rlativ to that of th surrounding soil, as wll as by th nvlop thicknss. Whn an nvlop is usd, svral dfinitions of th ntranc rsistanc can b givn. Altrnativ 1 If th ntranc rsistanc is rlatd to th drainpip itslf, an nvlop dos not caus any chang in th ntranc rsistanc. Only th total flow rsistanc is changd. As long as th

37 Matrials for subsurfac land drainag systms 57 thicknss and th hydraulic conductivity of an nvlop allows for radial flow in th surrounding soil, th ntranc rsistanc α of a drainpip itslf is givn by: α α' = κ whil th radial flow rsistancs in th nvlop and in th soil form th othr componnts of th approach flow rsistanc, hnc: and, if th ffctiv radius, r f, is considrd: (24) α 1 r 1 r ' ap = ln + ln α 2π r 2πκ r + (25) f o 1 r α ap = ln (26) 2π r Hnc th ffctiv radius bcoms: r f r = r 1/ κ o (1/ κ ) 1 ' 2πα (27) Altrnativ 2 Th ntranc rsistanc may altrnativly b xprssd as th rsistanc of both drain and its surrounding nvlop. This is qual to combining th last two trms in Eq. (25) into: Th approach flow rsistanc now rads: 1 r ' α, = α + ln (28) 2πκ ro 1 r α ap = ln + α (29), 2π r For an idal drain whr α = 0, th ntranc rsistanc givn by Eq. (28) yilds th nvlop rsistanc to radial flow. Th ffctiv radius can b calculatd by combining Eqs. (26) and (29): -2πα, rf = r (30) Altrnativ 3 Widmosr (1968) dfind th ntranc rsistanc, α (,)W, as th diffrnc in flow rsistanc btwn a drain with an nvlop and an idal drain of th sam diamtr, r o. Thus: α (, ) W 1 r 1 r 1 r = ln + ln + α ' ln 2π r 2πκ r (31) o 2π ro

38 58 Watr flow into and insid th drain which, aftr som simplifications, finally rsults in: α ( W, ) ' 1 1 r r = α+ ln ln 2π κ r r o o (32) whil th approach flow rsistanc is givn by: 1 r α ap = α (, ) W + ln (33) 2π r Combination of Eqs (26) and (33) yilds th ffctiv radius: r f = r o -2 0 π α (, )W (34) Though Widmosr (1968) might hav givn th right dfinition on th ntranc rsistanc of a drain with nvlop, from th abov analysis it is obvious that th ffctiv radius of a givn drain with a wll-spcifid nvlop is indpndnt of whatvr dfinition is usd for th ntranc rsistanc. Corrugatd plastic drain pips with a prforation in ach corrugation and wrappd with a thin nvlop sht which spans th corrugations and kp thm fr from soil maks th drain surfac much mor prmabl and rducs th ntranc rsistanc considrably (Willardson and Walkr, 1979; Salm and Willardson, 1992). A substantial rduction of th ntranc rsistanc is obtaind if an nvlop is installd which has a hydraulic conductivity at last 10 tims highr than that of th surrounding soil. Th thicknss of th nvlop should, prfrably, b at last 5 mm (Niuwnhuis and Wssling, 1979; Dirickx, 1980). Mor favourabl spcifications do not significantly dcras th ntranc rsistanc any furthr. Still, gratr nvlop thicknss nhancs th ffctiv radius, bcaus th soil around th drain is rplacd by a comparativly mor prmabl nvlop. Th ffctiv radius of a wrappd drain incrass, if th hydraulic conductivity and/or th thicknss of th nvlop ar mad largr. Th us of a sufficintly prmabl nvlop (κ 10) which is adquatly thick (d 5 mm) around a plain drain rducs th ntranc rsistanc drastically. If κ 10 and d 5 mm, drains wrappd with nvlops which hav th sam xtrnal radius, r, hav almost th sam ffctiv radius, r f, rgardlss th pip radius, r o, and th nvlop thicknss, d (Figur 32). Thus, it may b mor cost fficint to slct th minimum drain diamtr rquird to satisfy th discharg capacity, and to wrap with a rlativly thick nvlop, than slcting a gratr diamtr pip, wrappd with a rlativly thin nvlop. This is bcaus largr diamtr pips ar much mor xpnsiv than a largr amount of nvlop matrial, rquird to arriv at th sam xtrnal diamtr r. Drain with a lss prmabl surround It is gnrally accptd that drainag works must b carrid out undr circumstancs that do not challng th structural stability of a soil. Th moistur contnt of th soil is a critical factor bcaus drainag works carrid out with trnchrs in wt conditions may rsult in dtrioration of th structur of th xcavatd soil. As a rsult, th drain surround bcoms lss prmabl than that of th surrounding natural soil. Trnchlss and mol drainag tchniqus can locally

39 Matrials for subsurfac land drainag systms 59 compact th soil around th drain or mol channl, inducing a lss prmabl zon around it. Invasion of soil particls into th nvlop and/or chmical dposits can rsult in a partial blocking of th pors and a dcrasd hydraulic conductivity of th xtrnal nvlop surfac. FIGURE 32 Effctiv radii, r f, for drains of diffrnt pip radii, r o, and providd with four continuous longitudinal slits as a function of th nvlop thicknss d for κ = Exprimntal rsarch shows that, if an nvlop has a 70 substantial thicknss,.g. >5 mm, 50 and if its hydraulic conductivity is 60 r (mm) lss than 10 prcnt of that of th surrounding soil, th ntranc rsistanc may b vry larg, and consquntly th ffctiv radius of th drain rducs to xtrmly 10 small valus. This is mainly du 0 to impdd flow in th lss prmabl layr surrounding th d (mm) drain. If th drain is wrappd with an nvlop, smaring and compaction of th surrounding soil influncs th ntranc rsistanc lss than nvlop clogging, yt th ffctiv radius may b rducd to intolrabl valus. r f (mm) r o (mm) A lss prmabl layr surrounding ithr a plain drain or a drain with a mor prmabl nvlop has an advrs influnc on th prformanc of drain matrials and must thrfor b avoidd at all tims. Mutual diffrncs btwn th ntranc rsistancs of various typs of drainpips may b important if ths drains ar installd without nvlop. Th hydraulic charactristics of th abutting mdia (ithr th soil or th nvlop and th soil) ar, howvr, much mor rlvant than th spcifications of ths pips. DISCHARGE CAPACITY OF DRAINPIPES Th discharg capacity of drainpips is an important componnt of any dsign procdur for land drainag systms, and is dscribd in all major drainag txtbooks. Th availabl information rangs from xhaustiv (Cavlaars t al., 1994) to straightforward tratmnt, which is limitd to th fundamntals only and som usful xampls (Smdma and Rycroft, 1983). In this guid, only th most rlvant matrial is discussd, following Dirickx (1993). Radrs who want to b informd furthr on th subjct ar rfrrd to th abov publications. Additional information on dsign procdurs (i.. formula) in various countris is givn in Framji t al. (1987). Pip diamtr nomographs, which ar quit usful for a quick scan analysis of th rquird pip diamtr(s), ar givn in Smdma and Rycroft (1983). A computr program for calculating th diamtr of drainpips is in prparation by FAO.

40 60 Watr flow into and insid th drain It is oftn financially attractiv to incras th pip diamtr of collctor drains and vn of latral drains in th flow dirction. In doing so, th diamtr is adjustd for th discharg, which incrass in th dirction of th outlt. This issu is discussd in dpth by Cavlaars (1979), and illustratd in a simpl cas by Smdma and Rycroft (1983). Th forthcoming FAO-publication on drainag dsign also includs th dsign of such composit drains. Th hydraulic dsign of drainpips is basd on formula that rlat th discharg of watr to th pip diamtr, th hydraulic roughnss of th pip wall and th hydraulic gradint. Diffrnt formula ar usd for smooth and corrugatd pips. Clay, concrt and smooth plastic pips ar considrd hydraulically smooth pips. Thir discharg capacitis can b calculatd from th Darcy-Wisbach quation. Th discharg capacity of corrugatd pips can b calculatd from th Chézy-Manning quation. For latrals, a minimum pip diamtr is advisabl to compnsat for lss accurat grad and alignmnt, and vntually for som sttlmnt that may occur, thus assuring th discharg capacity of th drainag systm. In Europan countris, a minimum diamtr of 50 or 60 mm is accptd; lswhr th minimum diamtr is 80 mm and in th Unitd Stats th smallst diamtr is 100 mm. For collctor drains th lngth covrd by a givn pip diamtr for a spcifid hydraulic gradint is calculatd. In th Chézy-Manning quation, th hydraulic roughnss (or friction rsistanc ) of th pip wall is xprssd as Manning s cofficint, n, or its rciprocal paramtr, k M. For drainpips with diamtrs ranging from 50 to 200 mm and small corrugations, th roughnss cofficint n = s m -1/3 (or th rciprocal valu k M = 70 m 1/3 s -1 ). From th rsults of Irwin (1982, 1984), Boumans (1988) stablishd that th k M -valu of largr diamtr pips with larg corrugations can b xprssd as: k M = 18.7d 0.21 S (35) in which d (m) and S (m) ar th intrnal pip diamtr and th pitch lngth, rspctivly. For most pips with larg corrugations, a roughnss cofficint n = 0.02 s m -1/3 (or k M = 50 m 1/3 s -1 ) can b accptd. Th typ of pip and th hydraulic gradint dtrmin th discharg capacity of drainpips. Th calculation of th discharg capacity of drainpips may b basd upon two principls (Wssling and Homma, 1967; Wssling, 1987): th transport principl with uniform flow, whrby a drainpip is assumd to transport a fixd discharg along its lngth, whil th pip itslf is flowing full; and th drainag principl with non-uniform flow, whrby a constant inflow of groundwatr into th drain along its lngth rsults in a discharg which incrass along th lngth of th pip.

41 Matrials for subsurfac land drainag systms 61 Application of both principls and pip charactristics yilds th following st of quations: Transport principl Drainag principl Clay, concrt and smooth plastic pips Q = 50 d s (36) Q = 89 d s (37) Corrugatd pips with small corrugations (usually pips ranging from 50 to 200 mm in diamtr) Q = 22 d s 0.5 (38) Q = 38 d s 0.5 (39) Corrugatd pips with larg corrugations (usually pips with a diamtr byond 200 mm) Q = 15 d s 0.5 (40) Q = 27 d s 0.5 (41) with Q = discharg (m 3 s -1 ); d = intrnal diamtr (m); and s = hydraulic gradint (dimnsionlss). All quations ar drivd for clan pips. Comparison of ths quations rvals that th assumption of th transport principl for th dtrmination of th diamtr of drainpips implis that a safty factor is automatically incorporatd in th dsign. Th quations basd upon th drainag principl yild largr discharg capacitis, and, as such, largr surfacs that can b draind with a givn pip diamtr. Adoption of som safty factor is indd rquird to incorporat th risk of possibl minral and/of chmical clogging of th pip in its hydraulic dsign. Usually, pips ar ovr dsignd to allow for subsqunt partial minral or chmical clogging, and for misalignmnt during installation. Whn applying th drainag principl, a safty factor must b imposd bcaus this principl is basd on a mor ralistic physical concpt, which lads to a mor conomical yt risky dsign. For practical application, th discharg capacitis as calculatd with th formula basd upon th drainag principl ar commonly rducd to 60 prcnt of th calculatd valus to includ a safty factor for possibl minral and/of chmical clogging of th pip (Cavlaars, 1974). This mans that, in th nd, both principls rsult in approximatly th sam discharg capacity (Dirickx, 1993). For collctor pips, th thortical capacity is usually only rducd to 75 prcnt. Hnc, an xtra safty of 15 prcnt is built in whn using th formula basd upon th transport principl. Additional rduction factors up to 50 prcnt may still b advisabl to compnsat for pip clogging, misalignmnt and an rronous assssmnt of th pip roughnss cofficint (El Atfy t al., 1990). Th rduction factor may b consrvativ (25 prcnt) if corrugatd plastics pip is installd in stabl soil, yt must b comparativly larg (50 prcnt) for til drains laid in unstabl soil.

42 62 Watr flow into and insid th drain Too small a drain or a drain partially filld with sdimnt causs a rducd transport capacity. Th pip sction will thn b too small for discharging th groundwatr proprly, and th watr in th drain will b flowing undr prssur. Watr may b standing abov th drain and th groundwatr tabl midway btwn drains will b too high. Too small a diamtr or a rduction in transport capacity can b obsrvd by a pizomtr to masur th watr had in th drain, and obsrvation wlls for th hight of th watr tabl transvrsal to and nar th drain.

43 Matrials for subsurfac land drainag systms 63 Chaptr 5 Th problm of clogging of pips and nvlops MINERAL CLOGGING Procsss in soils around drains A major problm that is oftn ncountrd on subsurfac drains is minral clogging of pips and nvlops. This physical procss occurs as th rsult of suddn drastic changs in soilwatr conditions nar th pips causd by thir installation. Immdiatly aftr installation, a nw quilibrium bgins to b stablishd at th vulnrabl ara nar th intrfac btwn th backfilld soil and th surfac of th drainpip or th surfac of th nvlop. Th ara is vulnrabl bcaus th physical strngth and th structural stability of th soil has bn waknd by th installation procss. Morovr, groundwatr starts flowing towards th drain, whrby th hydraulic gradints and th flux dnsitis, bing high in this ara, induc substantial drag forcs on th soil particls. Soil movmnt at th intrfac btwn soil and nvlop (or pip wall) causd by flowing watr is oftn rfrrd to as intrnal soil rosion. Zims (1969) mad an xtnsiv study of this phnomnon. H indicatd that soil particl movmnt at th intrfac btwn two mdia may b, in fact, causd by thr diffrnt physical phnomna, namly th washing out of fin soil particls (crating a natural filtr ), contact rosion and soil collaps. Th physical procss lading to th dvlopmnt of a natural filtr in a soil has bn discussd by various authors (Stuyt, 1982, 1992a; Cavlaars t al., 1994). Anothr phnomnon, which advrsly affcts watr ntry into drains, is th dvlopmnt of a so-calld filtr cak. Th phnomna just mntiond may b charactrizd, in brif, as follows: Natural filtr. If only fin soil particls ar washd out, a coarsr soil sklton is lft bhind that bridgs ovr th opnings in th drain or in th nvlop. Th formation of a natural filtr, for instanc in soil backfilld on top of a granular nvlop, is illustratd in Figur 33. Th drag forc of th watr that flows toward th drain causs small soil particls to mov into and through th nvlop whil thos of largr sizs ar rtaind (Tim 1). Aftr som tim, a highly prmabl natural filtr will dvlop in th soil adjacnt to th nvlop (Tim 2), with an nhancd hydraulic conductivity. If coarsr particls ar washd out also, th formation of a natural filtr in th soil may b suprsdd by xcssiv soil particl movmnt, which will locally undrmin th physical strngth of th soil sklton. This procss, in turn, promots contact rosion. Contact rosion mans that particls of narly all sizs ar washd out locally, rsulting in modification of th sklton which transfrs th ffctiv strsss within th soil. Th rsult of contact rosion is shown in Figur 34. Hr, th drag forc of th watr that flows toward th drain causs soil particls of all sizs to mov into and through th nvlop (Tim 1). Aftr som tim, macropors will dvlop at th intrfac btwn th nvlop and th soil (Tim 2).

44 64 Th problm of clogging of pips and nvlops Filtr cak. A filtr cak is a dns layr of soil particls which dvlops if suspndd, fin soil particls accumulat at or nar th intrfac btwn th soil and th nvlop. Th gratr part of this ara is oftn locatd in th soil rathr than in th nvlop (Stuyt, 1992a). Figur 35 shows th dvlopmnt of a filtr cak in th cours of which fin soil particls mov toward but do not ntr th nvlop (Tim 1). Many particls accumulat in th soil nar th intrfac btwn th soil and th nvlop (Tim 2). This condition occurs whn th nvlop opnings ar too small and act as a filtr for th small soil particls moving with th watr. Th hydraulic conductivity of filtr caks is oftn considrably smallr than that of th original soil, bcaus fin soil particls clog th soil pors at th soil-nvlop intrfac. Soil collaps. Whn th drag forc of th watr surpasss th cohsiv forcs and intrgranular strsss of a soil, th soil collapss and may consolidat. Soil collaps is illustratd in Figur 36. It shows that, aftr installation of th drain, th cohsion of th soil prvnts soil matrial from moving toward and into th nvlop (Tim 1). At a latr stag, soil aggrgats ar dislocatd and soil particls mov through th nvlop towards th drain (Tim 2). Som scondary bridging may occur at th soil-nvlop intrfac that stops furthr soil movmnt into th nvlop. FIGURE 33 Natural filtr (aftr Stuyt, 1982) FIGURE 34 Contact rosion (aftr Stuyt, 1982) Soil collaps implis local soil structural failur, disprsion of soil aggrgats and movmnt of soil particls of all sizs at th intrfac btwn th soil and th nvlop. Soil collaps is most likly to occur in havy, cohsiv soils at high

45 Matrials for subsurfac land drainag systms 65 hydraulic gradints. Th drag forc of th watr and th soil load, inducd at drain dpth, may vn caus th saturatd soil matrial nar th drain to b prssd through th nvlop and into th pip prforations, as a muddy substanc (Van dr Louw, 1986; Stuyt, 1992a). FIGURE 35 Filtr cak Until rcntly, contact rosion was considrd harmful to th succssful functioning of subsurfac drains (Stuyt, 1982). Latr obsrvations howvr indicatd that a low rat of contact rosion is favourabl in that it promots th formation of a macropor ntwork around th drain. This ntwork plays an important rol in th convyanc of watr into th drain. Stuyt (1992a) mad a srious attmpt to gain insight into th physical procsss, associatd with minral clogging. A CT scannr was usd to obtain thr-dimnsional (3-D) digital imags of soil cors, containing 300 mm long sctions of wrappd drainpips with th surrounding soils. Aftr a srvic lif of fiv yars, 45 drain sctions wr rtrivd from thr xprimntal filds, locatd in aras in Th Nthrlands whr th soils at drain dpth consist of vry fin sands: indd problm soils with low structural stability. Each CT-squnc is a 3-D, gomtrically prcis mapping of th intrior dnsity variations insid drain nvlops and th surrounding soils. In th 3-D imags, two major typs of soil pors could b distinguishd, namly txtural pors insid soil aggrgats and macropors (voids, cracks) which sparat ths aggrgats. In 40 prcnt of all cass, th avrag macroporosity in th trnch was lowr than that in th subsoil. Two typs of soil structural faturs wr found in th subsoil: horizontal layring and vrtically orintd macropors (Figur 37). FIGURE 36 Soil collaps (aftr Stuyt, 1982)

46 66 Th problm of clogging of pips and nvlops FIGURE 37 Exampl of a layrd subsoil (lft) and of a subsoil with vrtically orintd macropors, that had dvlopd at formr root channls (right) (aftr Stuyt, 1992b) FIGURE 38 Imag aras displaying drain nvlops and activ macropors (aftr Stuyt, 1992b) In Figur 37, only th rlativly prmabl aras in th soil around th drain ar dpictd. Thr is no rlation btwn soil prmability and th intnsity of th gry shading. Th lattr is inducd by imag procssing tchniqus in ordr to facilitat visual intrprtation of th highly complx imag. Parts of th Plxiglas rims of both th sampl containr and th sampl holdr of th CT scannr wr cut away by imag procssing tchniqus. Not all th prmabl aras dpictd in Figur 37 ar physically connctd to th drain and, as such, convying watr into it. Using a 3-D imag analysis tchniqu, th aras that ar connctd to th drain - th so-calld activ macropors - could b dtctd. In Figur 38, ths activ macropors ar displayd. Th dpictd sampls in Figur 38 ar th sam as th ons displayd in Figur 37. It can b clarly obsrvd that only a minority of all th dtctd prmabl

47 Matrials for subsurfac land drainag systms 67 aras is activly convying watr into th drain. Ths activ macropors ar partly dvlopd through contact rosion procsss that must hav takn plac during soil sttlmnt aftr installation. FIGURE 39 Illustration of th htrognity of minral clogging pattrns as dtctd insid voluminous nvlops (aftr Stuyt, 1992b) Subtl banding is vidnt undrnath th drain, indicating comparativly prmabl soil layrs, and th drain trnch contains som gomtrically complx macropors (Figur 38 lft). Watr accss to th drain on th right procds through a sris of paralll, vrtically orintd macropors. Th htrognity of minral clogging of voluminous nvlops, as dtctd on fild sampls is illustratd in Figur 39 in th form of transformd CT-imags that dpict th nvlops as flat surfacs. Aras that ar not sriously cloggd ar gry. Cloggd nvlop aras ar not dpictd and appar whit. Contrary to thortical assumptions, th ffct of an nvlop on th watr flow pattrn towards a drain is oftn limitd, as is its ffct on th radial and th ntranc rsistanc. Study of all watr flow pattrns into th drains rvald that thr is no vidnc that nvlop spcifications hav a significant ffct on th gomtry of such pattrns. Variation of th flow rsistanc nar a subsurfac drain is thrfor likly to b largly associatd with structural faturs of th soil, i.. its macroporosity and th gomtric arrangmnt of th macropor ntwork nar th drain. Th so-calld ffctiv opning siz, O 90, appard to b th only crucial dsign paramtr for an nvlop. Unlik any othr nvlop spcification, th O 90 valu had a significant ffct on th rat of minral clogging of drainpips (Stuyt, 1992a). Envlops largly act as soil rtainrs or prmabl constraints that physically support th soil nar th drains. Givn th importanc of th physical proprtis of soils in rlation to th procss of minral clogging, good installation practic will favourably affct th srvic lif of wrappd drains. On th othr hand, wll-dsignd nvlops cannot cancl th unfavourabl physical proprtis of th surrounding soils, nor can thy compnsat for poor installation practic. Installation undr gnral wtnss must thrfor b avoidd as much as possibl. Pip clogging Sdimntation in drainpips dos not dpnd only on th intrinsic charactristics of th soil. Othr factors such as th conditions and th quality of installation and inadquat maintnanc of th drains,.g. high prssur jtting, can caus sdimntation in drains. Minral dposits in drains ar du to soil grains passing th nvlop (if any) and th opnings in th pips. Fin particls (< 20 µm) ar usually carrid in suspnsion, causing a turbid outflow. Sand rmains in plac and - if abundant - will caus pip clogging. In flat country, with drain gradints around 0.2 prcnt (0.2 m pr 100 m) vn vry fin sand (mdian particl siz 50

48 68 Th problm of clogging of pips and nvlops µm) will stay nar th ntry point in th pip. Slf-claning of th pip may b xpctd only at much stpr gradints. CHEMICAL AND BIOCHEMICAL CLOGGING In subsurfac drains, thr ar four known typs of dposits that ar associatd with bactrial activity. Ths ar ochr dposits, mangans dposits, sulphur slim and iron sulphid. Glatinizd, voluminous oxidizd iron dposits, namd ochr, ar th most srious and widsprad. Othr known dposits ar lim and gypsum, which mostly occur in subsurfac drains of irrigatd aras as a rsult of th chmical composition of th soil and th quality of th irrigation watr. Iron ochr Th glatinous slims, associatd with ochr dposits ar usually yllow, rd, or tan in colour. Ochr is filamntous (from bactrial filamnts), hydratd (mor than 90 prcnt watr), and its dry mattr has a high iron contnt (2-65 prcnt dry wight). Thy usually contain an organic matrix (2-50 prcnt dry wight) (Ford, 1979, 1982a). Thr ar two main catgoris of ochr problms: 1. Ochr as a tmporary problm, calld autochthon (of local origin). Tmporary ochr as a clogging factor may disappar ovr a priod of thr to fiv yars. It usually occurs rapidly and can b oftn dtctd at drain outlts soon aftr drain installation. If th drains can b maintaind in working ordr, th concntration of F 2+ raching thm will gradually dcras. 2. Ochr as a prmannt problm, calld allochthon (of forign origin). Prmannt ochr is th most hazardous condition bcaus it continus to b a clogging agnt for th srvic lif of th drainag systm, rgardlss of tratmnt. Prmannt ochr occurs in soils that contain xtnsiv quantitis of rsidual iron and natural nrgy. Th solubl rducd iron originats from surrounding aras, hnc th nam, and is transportd by spag into th draind ara. Thr ar ochr locations whr th solubl iron originats 4 to 6 km from a drainag sit. Thus, it is important to considr topographical trrain faturs whn stimating th potntial for prmannt ochr formation. In gnral, sits considrd to hav prmannt ochr potntial should not b til-draind without modifications in dsign and provisions for continuous maintnanc. Ochr can b found in th soil abutting th drain nvlop, th nvlop itslf, th pip prforations and within th drain pip. Most clogging in corrugatd pips can b tracd to saling of th prforations and accumulations within th vallys of th corrugations. Within th pips, th havist accumulation of ochr appars to b in th lowr third of th drain lngth, although th lowr third is usually not th rgion of maximum ochr formation. Ochr can usually b dtctd at drain outlts or in manhols as a voluminous and glatinous mass. Howvr, it may b prsnt in th drains, whil not visibl at th outlt. Ochr formation Th dvlopmnt of ochr rquirs rducd or frrous iron (F 2+ ) flowing into drains as raw matrial. Th minimum concntration of frrous F 2+, ncssary for growth of th iron bactrium Lptothrix, is 0.12 mg/l (0.12 ppm) (Ford, 1980).

49 Matrials for subsurfac land drainag systms 69 It must b in solution in th groundwatr rathr than locatd on soil particls. It is prsnt mostly as iron hydroxid (F(OH) ) or as iron sulphid (FS ), and will prcipitat whn 2 2 oxidation taks plac aftr contact with air,.g. nar and insid subsurfac drains (Smdma and Rycroft, 1983). Many soils contain substantial quantitis of iron, yt th conditions, rquird to crat ochr problms in drains vary considrably. Bactria ar rquird to convrt th insolubl frric iron (F 3+ ), which is locatd on soil particls, to a solubl form (F 2+ ) which can b transportd to th drains by groundwatr advction. Frrous iron (F 2+ ) can only xist in groundwatr if th oxygn in th soil has bn dpltd,.g. aftr a soil is floodd for a considrabl tim, or whn micro-organisms hav usd all availabl oxygn. If this condition is mt, iron-rducing bactria rduc th insolubl frric iron (F 3+ ). This biological action of th bactria is nrgy intnsiv, and nrgy sourcs must thrfor b prsnt. Th major sourcs ar organic matrial lik rmnants of plants and plant roots, and crtain acids lik malic, citric, tannic and lactic acids. Hnc th highr th organic contnt in th soil, th fastr and mor widsprad th convrsion from F 3+ to F 2+ by bactria will b. Solubl F 2+ flowing in groundwatr ntrs a diffrnt nvironmnt as it approachs th drain and passs through th drain nvlop. If som oxygn is prsnt in this ara, crtain filamntous and rod-shapd bactria will prcipitat som of th F 2+ as insolubl F 3+ and incorporat it into ochr. Iron-prcipitating bactria must b prsnt for xtnsiv clogging to occur, vn whn othr conditions ar just right for chmical prcipitation of th iron. Iron alon dos not hav srious sticking proprtis. Th raction insid drains is a combination of bactrial prcipitation and th incorporation of chmically prcipitatd iron into th sticky slims of th bactrial masss involvd in th ochr matrix. Thr is a typ of ochr that forms only at low ph, in pyritic soils (acid sulphat soils). Ths soils ar found in many coastal aras as wll as in min dumps and in crtain shals. Pyrits ar formd from iron and hydrogn sulphid in floodd marin dposits. Whn such soils ar draind, th pyrits first oxidiz to F 2+ and sulphats. Ths sulphats chang to sulphuric acid, which lowrs th soil ph blow 3.5. Th rod-shapd bactrium Thiobacillus frrooxidans, which can function only in an acid nvironmnt, thn convrts th solubl iron into ochr. In Egypt, Iraq and Pakistan no srious ochr problms hav bn rportd. Th absnc of ochr thr is du to th gnrally alkalin soil nvironmnt. In alkalin soils, frrous iron (F 2+ ) cannot xist in solution in th groundwatr. In Isral, svr ochr problms hav bn ncountrd whn draining crtain swampy aras. Th drainag systms wr dsignd such that anarobic conditions wr maintaind by placing an lbow at th drain outlts to crat submrgnc. Ths systms hav opratd succssfully for svral yars (Hnkin, 1987). Th sam procdur was introducd in Th Nthrlands in th 1960s, yt with limitd succss (Huinink, 1991). Prdiction of ochr problms Th following on-sit obsrvations may giv clus to potntial ochr problms insid drains (Ford, 1979): 1. Soil typs that appar to show th highst potntial for ochr formation ar fin and silty sands, organic soils, soils with organic pans and minral soil profils with mixd organic mattr.

50 70 Th problm of clogging of pips and nvlops 2. Sits bing utilizd for sprinkling of swag fflunt usually furnish sufficint nrgy for rduction ractions. Such sprinkld soils ar potntially srious for ochr hazard if th profils ar subjctd to long trm flooding. 3. Som topographical faturs indicat possibl ochr problms. If thr is land of highr lvation clos to th proposd drainag sit, prmannt ochr potntial may b a problm du to prmannt spag. Vallys at th bas of scarpmnts ar typical for prmannt ochr. 4. Flood plains of rivrs and gullis ar suspct, particularly if th sit is a mixtur of sand and organic mattr. 5. Dprssions containing organic rsidus ar ochr pron sits. 6. Blu-colourd clays or bog-lik, dcomposabl organic mattr btwn 0.6 and 1.2 m blow th soil surfac suggst prmannt ochr sits. 7. Oil-lik films floating on surfac watr in canals may indicat ochr and may contain ochr forming bactrial filamnts. 8. Glatinous ochr that has prcipitatd on ditch banks and/or canal bottoms is an important indicator for potntial ochr problms. 9. Th amount of F 2+ in groundwatr is usually highr in soils with organic pans and a ph blow six. 10.Basd on practical xprinc, th last likly candidats for ochr problms ar silty clays, clay loams and clay soils. 11.In arid aras, ochr is sldom a problm. Ochr potntial ratings It is possibl to stimat th maximum potntial for ochr bfor installing drains, as wll as to stimat whthr spcific soil typs or profils can b considrd suscptibl (Ford, 1982b). Analysing th soils for total iron is of no valu bcaus th valus do not indicat solubl F 2+ or th complx intractions btwn th soil ph and th soil typ. Th F 2+ -contnt of th groundwatr flowing into a drain is a rliabl indicator of th potntial for ochr clogging. Th simplst way to dtrmin th frrous (F 2+ ) iron contnt of th groundwatr is using papr indicator strips, which ar immrsd in a groundwatr sampl. Th colour can b usd to assss th concntration of th frrous iron. Th concntrations ar colour-codd into th following classs: 2, 5, 10, 25, 50, and 100 mg F 2+ /l. Ford (1982a) dvlopd a rliabl yt laborat tsting procdur to assss th ochr clogging potntial of soil profils bfor installing drains. This procdur is indpndnt of ph and soil typ. Th mthod has bn dvlopd and tstd xtnsivly at numrous locations in th Unitd Stats (Ford, 1982a). Using this mthod, it is possibl to dtrmin whthr a soil layr may rlas much or littl frrous (F 2+ ) iron, onc watr saturatd, and whthr th frric iron (F 3+ ), which is adhrd to th soil particls, can b asily rducd to solubl F 2+. Scholtn and Vn (1987) hav compard th ochr potntial ratings, assssd with th Ford mthod, with th mthod using indicator strips. Thy found a strong corrlation of th dtctd frrous iron contnt, dtrmind with both mthods. Howvr, th contnt indicatd by th strips is consistntly highr than th contnt indicatd by th Ford mthod (ratio 3 to 4). Yt, for routin masurmnts, th simpl mthod with indicator strips will suffic. In spit of th insufficint numbr of radings in thir invstigations, Scholtn and Vn (1987) prsnt a tabl (Tabl 8) to assss th ochr potntial. Th figurs in this tabl ar in rasonabl agrmnt with th figurs, proposd by Ford (1982a).

51 Matrials for subsurfac land drainag systms 71 How to minimiz ochr clogging of drains Thr is no known conomical, longtrm mthod for ffctivly controlling ochr clogging in drains. Although options ar limitd, th mphasis must b on living with th problm. Th following rcommndations may b usful (Ford, 1982a, 1982b). 1. Prcipitating iron in th soil by promoting oxidation. Iron cannot b dissolvd in groundwatr until it is rducd. Hnc, all masurs that minimiz th dvlopmnt of anarobic conditions ar accptabl. Soil aration prvnts rduction. Closr spacing and shallowr dpth of drains may b bnficial for crtain sits. 2. Siz of th prforations in drainpips. Th largr th pip prforations, th longr th priod bfor drain discharg may b svrly rstrictd. Ochr adhrs to frayd plastic dgs of prforations. Clanly cut inlt prforations ar ssntial. Small prforations limit th ffctivnss of jt claning as a mthod for claning drains installd with synthtic nvlops. 3. Drain nvlops. A gradd gravl nvlop is bst. It may howvr still clog undr conditions of svr ochr potntial. Soil compatibl, coars structurd PLMs may also rduc th risk of clogging by ochr. Rlativly thin synthtic nvlops lik gotxtils prsnt th gratst risk. Survys of slctd drainag sits show that ochr clogging of drains, wrappd with synthtic matrials occurs first in th slots and vallys of pip corrugations, and can b prsnt in amounts sufficint to caus drain failur. Ths matrials clog rlativly asily by ochr dposits bcaus th iron prcipitating bactria asily grow across th voids in th fabrics. Of all thin synthtic nvlops, knittd polystr nvlops ar th last vulnrabl to ochr clogging. 4. Organic nvlop matrials. Envlops, manufacturd from pin, oak and cyprss sawdust dlayd ochr dvlopmnt at drain inlt opnings for xtndd priods in Florida (Unitd Stats). Sawdust crats an anarobic nvironmnt and appars to b toxic to ochr nhancing bactria. Sawdust may contain aromatic hydroxyl compounds that complxs iron. Th us of pat and othr organic nvlop matrials should b avoidd. Thy usually incras ochr problms and nhanc clogging. 5. Submrgd outlts. Submrgd drains in groundwatr with high ochr risk prvnt th solubl frrous iron (F 2+ ) to oxidiz to th insolubl clogging frric iron componnts (F 3+ ) (Rozndaal and Scholtn, 1980). This is an old rcommndation that has bn usd with som succss whn th ntir drain is prmanntly undr watr. Th drain lin must b compltly undr watr ovr its ntir lngth throughout th yar. This may rquir that th drains b installd on a flat grad or horizontal. Ochr rmoval from drains TABLE 8 Ochr potntial according to th Ford-mthod and th mthod of indicator strips Ochr potntial Frrous (F2+) in groundwatr (mg/l) Ford mthod Indicator strips Vry high >10 >25 High Modrat Littl Ngligibl <0.5 <1 Data on jtting of drains, wrappd with synthtic nvlops, ar scarc. In Th Nthrlands, mdium prssur jtting of ochr cloggd drains has gnrally not bn vry succssful. Th dwatring capacity of jttd drains was not significantly nhancd, or only for a vry short priod. Jtting watr must pass through th pip prforations and b dflctd by th nvlop in ordr to clan th vallys. In structurally unstabl soils, th prssur at th nozzl should not

52 72 Th problm of clogging of pips and nvlops xcd 20 bar, othrwis th soil nar th drains may dstabiliz and flow into th drain (s Chaptr 7, Sction Maintnanc of drainpips). Th largr th pip prforations, th bttr th potntial for claning th vallys and nvlop. Jt claning is unsatisfactory if dlayd until th ochr has agd and bcom crystallin and/or sticky. Dry rodding (with a scratchr at th nd without xtra watr) can also b applid succssfully, providd that: th opration is carrid out whn th ochr is still slimy, bfor it had th opportunity to hardn during a prolongd dry priod (summr); and rodding is don whil th drain is carrying watr (wt priod). Thus th (still slimy) ochr is asily loosnd and will b carrid away by th drain discharg (Cavlaars, prsonal communication). As ochr clogging is usually most svr shortly aftr installation, it is rcommndd to jt th drains during th first yar if ochr problms ar suspctd, rathr than wait until th drains ar cloggd. Drains should discharg into opn ditchs rathr than through closd collctor systms. Th accss of singl drains through opn outlts gratly facilitats jtting. Hrringbon or similar drain dsigns should hav ntry ports for jt flushing. Lim and gypsum dpositions Whras ochr is a prominnt problm in humid tmprat rgions, which has bn invstigatd xtnsivly on a larg scal for many dcads, th dposition of slightly solubl salts, such as calcium carbonat (CaCO 3 ) and calcium sulphat as gypsum (CaSO 4.2H 2 O), within drainpips and nvlops is a not systmatically invstigatd problm. Thr is ampl scop for systmatic invstigation on lim and gypsum dpositions with pip drains; this would includ an invntory of th xtnt of th problm and th conditions undr which it is likly to dvlop. Lim and gypsum dposition is most likly a chmical procss. Th hard and crystallin dposits ar likly to build up comparativly slowly so that advrs ffcts only appar aftr a long tim. Th problm may occur in gypsifrous soils and soils with a high contnt of calcium carbonat, which ar common in arid and smi-arid aras, or rsult from th salts applid with th irrigation watr. Dpnding on th dissolvd Ca 2+ -contnt of th groundwatr, it may howvr also occur in non-irrigatd aras lik Blgium whr CaCO 3 is rportd to hav cmntd th gravl around a drainpip of a road drainag systm to a compact, imprvious mass. Calcarous dposits in and around drains installd in soils that convy groundwatr rich in dissolvd Ca 2+ also ar rportd in Franc (CEMAGREF, 1983). In arid rgions, Cavlaars t al. (1994) found gypsum in xcavatd drains. No dposition of lim was howvr found in horizontal drainag systms, in spit of th lim dposition hazard - incrustation - of tub wlls. Prcipitation of lim and gypsum may tak plac if th concntration of calcium compounds (carbonats, bicarbonats or sulphats) xcds thir solubility. Many watrs, particularly in arid rgions, ar partly or narly saturatd with calcium bicarbonat, (Ca(HCO 3 ) 2 ), which, upon concntration, prcipitats in th soil as CaCO 3. Prcipitation of CaCO 3, and of CaSO 4 will occur if th soil solution is concntratd by watr rmoval during plant growth, and th solubility of th rlativly insolubl CaCO 3 and th mor solubl CaSO 4 is xcdd. This physical procss dos not xplain th prcipitation of CaCO 3 in th drain nvlop and at th prforations which may rsult from th convrsion of Ca(HCO 3 ) 2 through th loss of carbon dioxid, (CO 2 ). For tub wlls, th prcipitation hazard may b xplaind by th prssur dclin in th groundwatr at th ntranc of th nvlop or th tub opnings. Complt prvntion of th dposition of CaCO 3 and CaSO 4 in a horizontal drainag systm will not b possibl, yt som masurs can b takn to rduc th prcipitation hazard of ths

53 Matrials for subsurfac land drainag systms 73 calcium salts. Kping drainag systms undr watr will rduc th risk of mor concntratd soluts nar th drainag systm and th rlas of CO 2 from th groundwatr. Mangans dposits Mangans, if dissolvd in groundwatr undr suitabl rducing conditions, can form a bactrially nhancd, glatinous black clogging dposit. Sulphur prcipitat Sulphur slim is a yllow to whit stringy dposit formd by th oxidation of hydrogn sulphid that may b prsnt du to rduction of sulphats dissolvd in groundwatr. Sulphur bactria oxidiz th H 2 S to H 2 O and lmntal sulphur S. Globuls of lmntal sulphur and masss of whitish, sticky slim ar dpositd within th filamnts of ths bactria and forms a prcipitat of sulphur at th drain outlts (Martínz Bltrán, 1978; Ford, 1980). Sulphur slim has not bn a srious problm in most agricultural drains. It is found most oftn in muck soils. It may also b prsnt at sits dsignd for subirrigation through drainpips if th wll watr usd for irrigation contains hydrogn sulphid (H 2 S). Iron sulphid Iron sulphid (FS 2 ) may b found undr chmically rducd conditions,.g. whn drains ar burid in mixd soil profils, in gullis and rivr plains, or whn topsoil or organic dbris ar usd to covr th drains during installation. It is a glatinous black prcipitat formd by th raction btwn frrous iron (F 2+ ) and hydrogn sulphid (H 2 S). It will usually not stick to light sandy soil particls. It bcoms a clogging agnt if it is prsnt in amounts that can block soil pors. In gnral, iron sulphid should not b a srious problm for most installations that do not blind th drains with topsoil or dbris of organic mattr. PENETRATION OF ROOTS INTO DRAINPIPES Fild data concrning root pntration ar scarc. Pntration of roots of fild crops is rar in arabl lands. Such roots may tmporarily obstruct drain discharg and slightly nhanc pip siltation, but thy will di aftr harvsting. Roots ar mor challnging in drains installd undr prnnial plants lik trs and shrubs,.g. undr shltrblts, which bordr orchards. Thy may fill th ntir drain ovr a considrabl lngth, trapping suspndd matrials and sriously obstructing drain discharg. Installing unprforatd pip sctions at locations whr such roots occur may prvnt th problm (s Chaptr 2, Sction Rigid pips). In arid countris, drains ar installd at 1.5 to 2 m dpths and occasionally dpr, hnc, root growth into th drains is lss likly as compard with drains that ar installd at shallow dpths. Quantitativ information on root growth insid drains is scarc. In Blgium, during a dry spll, dp rooting cabbag causd problms in a shallow drainag systm that was usd to control a prchd watr tabl. In Egypt, th Eucalyptus tr is known to caus troubl (Cavlaars t al., 1994). In Isral, th roots of crtain typs of Tamarix trs tnd to clog drains. Th roots of Tamarix and of som othr typs of trs cannot b rmovd, spcially whn gravl nvlops hav bn usd (Hnkin, 1987).

54 74 Th problm of clogging of pips and nvlops In Pakistan, all trs locatd within a distanc of 35 m from th drains wr rmovd as a way of prcaution in th Mardan Scarp projct. In Spain, vry fin roots of salin shrubs (Suada fruticosa) which grow on th banks of collctor ditchs wr found to grow into latrals, causing srious clogging. This problm may b solvd by installing unprforatd pip sctions with a minimum lngth of 3 m at th downstram nd whr th latrals discharg in ths ditchs (Martínz Bltrán, 1987). In Surinam, an Asiatic vin calld kudzu causd substantial problms of root growth insid drains (Van dr Moln, 1972). In Pru, sugar can was rportd to grow into pips at a dpth of 1.5 m (Cavlaars, 1987). In Th Nthrlands, th occurrnc of roots in agricultural lands is linkd to th typ of crop, th typ of nvlop, and th sit that is draind. Roots pntratd asily into drains wrappd with organic nvlops (a mixtur of pat and coconut fibrs), glass fibr sht nvlops, knittd sock nvlops, and a PLM nvlop consisting of polystyrn granuls. Thin synthtic nvlops howvr providd good protction. Root pntration was gnrally lowr whn th nvlop thicknss was gratr (Stuyt, 1992a). Fruit trs (appls, pars) do not caus many problms, yt poplar (Populus canadnsis) is known to b harmful.

55 Matrials for subsurfac land drainag systms 75 Chaptr 6 Guidlins to prdict whthr an nvlop is rquird Du to th drag forc of watr flowing toward a drain, soil particls may b carrid into th drain from all sids. Drainpip siltation may b du to particl invasion of cohsionlss soil, to soil disprsion of cohsiv soil at drain lvl, or to downward transport of disprsd or suspndd matrial through soil pors, cracks and voids. This procss can nvr b prvntd compltly, but it can b countractd by installing an nvlop matrial around th drainpip. Th nd of nvlop matrials around drainpips will dpnd on th physical and chmical proprtis of th soil, on th chmical composition of th watr to b draind and on th conditions undr which th pips ar installd. Howvr, whthr or not a soil prsnts problms is not asy to tll, bcaus it cannot asily b drivd from soil proprtis and conditions. Soil htrognity and th complicatd natur of th physical intractions btwn watr and soil nar drain opnings mak prdiction of th nd for drain nvlop matrials vry difficult. Attmpts hav bn mad to dfin and idntify soils that ar pron to caus minral clogging of drainpips. Although many soil typs hav bn idntifid as bing mor suscptibl to sdimntation than othrs, sound critria as to whthr drains rquir an nvlop or not hav not yt bn stablishd. With th currnt stat of knowldg, it is virtually impossibl to dtrmin univrsal critria and fixd paramtrs to prdict th tndncy of minral drain clogging for a givn soil and th associatd nd of an nvlop. Nvrthlss, th xprinc gaind during four dcads of invstigations and practic allows for a numbr of conclusions to b drawn. Ths ar xisting critria, usually basd on local xprinc and only valid for th rgions whr thy hav bn stablishd. Thy may thrfor not b dirctly transfrrd to othr rgions without vrification of thir applicability. Prmamtr xprimnts with soil sampls takn at dsign drain dpth may provid information on th nd of drain nvlops, by giving vidnc of th structural stability of a soil and th risk of soil particl invasion into drainpips. Prmamtr rsarch has bn prformd in th Unitd Stats (Willardson and Walkr, 1979; Samani and Willardson, 1981), th Nthrlands (Stuyt, 1992a), Blgium (Dirickx and Yüncüoglu, 1982), Franc (Lnnoz- Gratin and Zaïdi, 1987) and is currntly bing conductd in Egypt, Pakistan, and India. Prmamtr xprimnts on sampls of soils and potntially suitabl nvlop matrials ar carrid out with incrasing hydraulic gradints. If th soil rsists high gradints, a drain nvlop is not rquird. An application is th assssmnt of th hydraulic failur gradint of a soil (.g. Samani and Willardson, 1981). From comparison of prmamtr rsults with thos of fild drains, Lnnoz-Gratin t al. (1992) considr th prmamtr flow tst a rliabl mans to prdict minral clogging of drainpips. Th rsults of Stuyt (1992b), howvr, indicat that th association btwn laboratory data and fild data may b quit ambiguous. Apart from laboratory xprimnts, vry simpl fild obsrvations may giv clus to th nd to install nvlops in futur drainag projcts. Augr hols, intndd for th dtrmination of th hydraulic conductivity of th soil, may yild usful information in this rspct. If such

56 76 Guidlins to prdict whthr an nvlop is rquird hols collaps rapidly, so that a scrn must b usd, installation of an nvlop is vital to protct futur drains against minral clogging. Th occasional occurrnc of soil layrs or lnss of loos soil matrial at drain dpth in a soil profil whr drainpips do not normally rquir an nvlop may b a rason to wrap all drains with nvlops as a safty masur, in spit of th highr costs. In th following sctions th main soil proprtis rlatd to th risk of soil particl invasion into drainpips and th associatd nd to protct drainpips against siltation ar dscribd. In addition, th influnc of watr quality on soil chmical composition has bn considrd. Finally, som prdiction critria for th nd of drain nvlops hav bn dfind. PHYSICAL PROPERTIES OF THE SOIL Soil txtur A soil consists of a sklton of minral particls with voids or pors, which contain air and watr. Organic mattr may b prsnt as wll, particularly in shallow soil layrs. Minral particls of soils vary widly in shap, siz, minralogical composition, and surfac-chmical charactristics. Th particl siz distribution of a soil, oftn rfrrd to as soil txtur, is an important indicator for soil stability. It can b found by mchanical soil analysis. Soil particls FIGURE 40 Txtural classs (FAO, 1990)

57 Matrials for subsurfac land drainag systms 77 ar normally classifid as clay (< 2 µm), silt (2-50 µm) and sand ( µm). Th dry wight prcntags of sand, silt, and clay can b plottd in a triangular graph (Figur 40). Drawing ths prcntags on a lin paralll to th bas opposit to th indicatd cornr (which rprsnts 100 prcnt sand, silt, or clay) th txtural class can b found by th intrsction of th thr lins insid th triangl. Figur 40 shows that a soil with a clay fraction of 11 prcnt, a silt fraction of 27 prcnt and a sand fraction of 62 prcnt would b classifid as sandy loam. Th cumulativ particl siz distribution curv (Figur 41) givs information on th cumulativ prcntag of soil particls (on dry wight basis) that is smallr than a givn diamtr. For xampl, d 10 and d 50 ar th particl diamtrs for which rspctivly 10 and 50 prcnt of th soil particls (by dry wight) hav a smallr diamtr. A uniform soil has a stp particl siz distribution curv (curv `a` of Figur 41), whras a wll-gradd soil curv is lss stp (curv `b` of Figur 41). Th lattr has a d 10 of 1.7 and a d 50 of 105 µm. FIGURE 41 Particl siz distribution curvs Th cofficint of uniformity (C u ) of a soil is a masur of th bandwidth of th sizs of th soil particls that it contains. This cofficint, which is rflctd by th inclination or slop of its particl siz distribution curv, is givn by: C d d u = Th gratr th C u valu is, th lss uniform or th bttr gradd th soil will b. A uniform soil, with all particls of th sam siz, has C u = 1. Particl siz distribution and soil txtur classification can giv a first indication of th nd for a drain nvlop. For loos soils lik sands, th C u cofficint is oftn mployd to prdict th nd for drain nvlops. If th soil is cohsiv, th clay prcntag is a mor significant indicator. (42)

58 78 Guidlins to prdict whthr an nvlop is rquird In various rgions, critria basd on th clay contnt of a soil hav bn succssful as a mans of dtrmining whthr drain nvlops ar rquird. In Qubc, drainpips do not nd nvlops in soils with a clay contnt of at last 20 prcnt (CPVQ, 1989) whil in th Nthrlands, th clay contnt should b at last 25 prcnt (Van Zijts, 1992). In Egypt and in India, th clay contnt should b 30 prcnt or highr (Abdl-Daym, 1987; Rajad Projct Staff, 1995). Nvrthlss, som of ths soils still xhibitd minral clogging. This is causd by th fact that soil stability is not only dpnding on th physical, but also on th chmical composition of th soil (Sction Chmical proprtis of th soil). In fin cohsionlss sandy soils, drains normally rquir an nvlop. Howvr, in Qubc (CPVQ, 1989) no nvlop is rcommndd if th width of th prforations in th pip wall is smallr than 2 d 85 (th particl diamtr for which 85 prcnt of th soil particls by dry wight hav a smallr diamtr). Instad of 2, othr valus of this factor ranging from 0.5 to 10 hav bn accptd as wll. Attmpts to adapt th prforation width to a charactristic particl siz diamtr of th surrounding soil hav faild bcaus of th variability of both. Thrfor, in cohsionlss sandy soils, drain nvlops should b rcommndd undr all circumstancs. Although txtur alon is insufficint as a dcision paramtr for nvlop application, it is gnrally accptd that soils with d 50 btwn 50 and 150 µm ar mchanically quit unstabl and, as such, snsitiv to rosion (Dirickx and Lyman, 1991). Thy will thrfor rquir an nvlop. Givn th fact that soils with a grat bandwidth of particl sizs do not prsnt srious siltation problms, Olbrtz and Prss (1965) proposd th C u cofficint as an rosion liklihood paramtr: 1 < C u < 5 : vry uniform and vry snsitiv to rosion. 5 C u 15 : modratly uniform and snsitiv to rosion. C u > 15 : no dangr of rosion. Th ratio clay/silt prcntag of a soil is also important. According to Dilman and Trafford (FAO, 1976), th risk of minral pip clogging dcrass rapidly whn this ratio xcds 0.5, whr th particl siz of silt rangs from 2 to 20 µm. In any cas, soils with an important quantity of silt and a small amount of clay offr a grat risk for minral clogging of drains. A rang of particl siz distributions of such soils is prsntd in Figur 42. Any soil having a cumulativ particl siz distribution that lis compltly or largly in th shadd ara is likly to caus problms with drain clogging (Stuyt, 1982; Vldhuijzn van Zantn, 1986). Th rason is that ths soils hav particls which ar gnrally too big to b cohsiv yt not big nough to b stoppd from bing washd into drain opnings not protctd by an nvlop. Structural stability In th Nthrlands, fild data indicat that soils may diffr widly with rgard to th rat of minral clogging vn though thy hav a comparabl txtur (Stuyt, 1992a). It has bcom obvious, ovr th yars, that th structur of a soil is at last as important as its txtur. Howvr, it is rarly possibl to intrprt soil structur in trms of clogging risks, lt alon clogging rats. Soil structur rfrs to th way soil particls ar bound togthr into natural, mor or lss porous compounds or aggrgats. It is conditiond by th soil txtur, th prsnc of organic

59 Matrials for subsurfac land drainag systms 79 FIGURE 42 Rang of particl siz distribution of soils that may caus clogging of drains and othr cmnting substancs, and th ratios btwn various cations that ar prsnt in th soil. Soil aggrgats may b classifid dpnding on th strngth of th bonds btwn soil particls, which can rang from loos, wak, modrat to strong bonds. Soil structur consisting of loos, individual soil particls is typically associatd with sandy soils, yt th finr graind silts may also xhibit this typ of structur. Such soils ar structurlss and hav virtually no cohsion. Clay soils ar gnrally cohsiv and may b massiv or dvlop blocky and prismatic structurs. In som cass, howvr, thy los thir cohsion and gt disprsd (Sction on Chmical proprtis of th soil). Soil structur govrns, among othr things, watr flow toward drainpips. Th firmnss of th bonds btwn soil particls is calld cohsion. Soil consistncy rfrs to th bhaviour of a soil at various moistur contnts and largly dpnds on cohsion. Two wll-known consistncy limits ar th liquid limit and th plastic limit, which form th socalld Attrbrg limits. Th diffrnc btwn ths two limits givs th plasticity indx (I p ). Th I p indx is an indicator for th firmnss of th bonds btwn soil particls. Th structural stability of soil aggrgats is rlatd to th attracting forcs btwn th soil constitunts, and dtrmins th rsistanc of a soil to mchanical and physical-chmical dstructiv forcs. To a crtain xtnt, th structural stability of soil aggrgats is dtrmind by th amount of clay particls. Aggrgat stability is an important soil charactristic whn it coms to th assssmnt of th risk of minral clogging of drainpips, and it is known that drainpips installd in stabl structurd soil do not rquir nvlop matrials. In spit of th availability of various mthods to dtrmin aggrgat stability,.g. by wt siving, a straightforward, unambiguous procdur to classify th structural stability of soil aggrgats

60 80 Guidlins to prdict whthr an nvlop is rquird into significant figurs is not availabl. Th rason for that is that stability of aggrgats is not an intrinsic proprty of th soil but dpnds on various conditions such as moistur contnt and chmical proprtis. Slaking of dry soil aggrgats upon wtting is wll known. Howvr, if this soil rmains in th plastic stat at drain dpth, it will largly rsist slaking. Hnc th structural stability of a soil is not a vry rliabl indicator whn it coms to driv guidlins for th assssmnt of nvlop rquirmnt to prvnt minral clogging of drain lins. Th I p indx, mntiond abov, is usd to prdict th snsitivity of a soil to minral clogging of a drainpip. Dilman and Trafford (FAO, 1976) rport th following: I p < 6 : high tndncy to siltation. 6 I p 12 : limitd tndncy to siltation. I p > 12 : no tndncy to siltation. Thr ar various modifications of this approach, somtims in combination with othr critria (.g. Lagacé, 1983). Moistur contnt Undr gnral wtnss th structur of th soil is dtrimntally affctd whn a subsurfac drainag systm is installd. Putting drains undr wt conditions may dstroy th structur of a soil almost compltly and nhanc th risk of minral clogging of th pips. Thrfor, drains should not b installd undr too wt conditions. Unfortunatly, stopping th work during wt splls is oftn ignord for financial considrations. Morovr, drains must somtims b installd at locations whr th groundwatr tabl is prmanntly abov th nvisagd drain lvl. Th warning not to install drains, if possibl, during priods of xcss wtnss, or whn th watr tabl is quit shallow is not nw. Cavlaars (1966) was on of th first to mntion that th prformanc of a drain undr fild conditions is dtrmind to a far gratr xtnt by th actual condition of th soil around th drain, than by th typ of drain or nvlop matrial. His major conclusion was that installing drains undr wt conditions could hav a vry harmful ffct on th prformanc, spcially in soils of low structural stability. CHEMICAL PROPERTIES OF THE SOIL Structural stability of a soil is affctd by its salt and sodium contnt. In addition, cmnting agnts in sands and silts ar lim (CaCO 3 ) and ssquioxids (Al- and F-oxids). Lim prcipitats around th contact points btwn soil particls. Th binding capacity of F-oxids is ill-dfind, but Al-oxid is probably ffctiv. Apart from ths inorganic dposits, soil organisms and thir organic by-products may also kp soil particls togthr. Th chmical composition of a soil is also quit rlvant bcaus of potntial clogging of drainpips and/or nvlops du to iron, lim and sulphat compounds (Chaptr 5, Sction on Chmical and biochmical clogging). Although drain nvlops cannot prvnt chmical clogging, this phnomnon must b duly considrd in any nvlop slction procdur. Assssmnt of th risk of minral clogging of drainpips as a rsult of th chmical composition of th soil rquirs knowldg of th cation xchang capacity, and th salinity and sodicity of th soil. Cation Exchang Capacity Clay particls and humus hav adsorptiv proprtis. Clay particls ar colloids that ar so small that surfac ffcts ar dominant. Phnomna affctd by soil colloids ar disprsion,

61 Matrials for subsurfac land drainag systms 81 swlling, shrinkag, flocculation, cohsion, and plasticity of soils. Clay particls hav a ngativ charg and thus thy adsorb positivly chargd cations such as Na +, K +, H +, Ca 2+, and Mg 2+. Organic mattr has a stabilizing influnc on th physical and chmical proprtis of soils, dspit its gnrally modst quantity. It promots th dvlopmnt and th stability of soil structur. Th finr componnts of organic mattr ar convrtd into humus, as a rsult of thir dcomposition by micro-organisms. Lik clays, humus is also a colloidal matrial. Its capacity to hold ions xcds that of clay but clay is gnrally prsnt in largr amounts. Hnc, th contribution of clay to th chmical soil proprtis usually xcds that of humus, xcpt in vry sandy soils. If soil colloids contain a high proportion of Ca 2+ and othr divalnt ions, firm bonds ar formd btwn minral particls, lading to stabl soil structur. In soils rich in Na + -ions (sodic soils) th bonds ar unstabl, which rsults in a wak soil structur. Th total amount of cations that a soil can adsorb is dtrmind by th ngativly chargd soil colloids clay and humus. This amount is calld th Cation Exchang Capacity (CEC) of a soil and usually xprssd in mq/100g of dry soil. Soil salinity Soils may contain slightly solubl salts such as lim and gypsum and highly solubl salts such as sodium chlorid and sodium sulphat. Ths salts may b containd in th soil parnt matrial (primary salinization) or b transportd dissolvd in watr and dpositd aftr th soil has drid (scondary salinization). Th major sourcs of scondary salinization ar salts addd with th irrigation watr and through capillary ris of groundwatr, mainly if th groundwatr tabl is rchargd by spag. Salt containd in prcipitation is ngligibl in comparison with th salt contnt of th irrigation watr and th groundwatr. Th anions prdominantly prsnt in salty soils ar Cl - and SO 4 2-, yt som HCO 3 - at ph valus of 6-8 and CO 3 2- at ph valus highr than 8.5 may b found. Na +, Ca 2+ and Mg 2+ ar th prdominant cations. Th total dissolvd solids (TDS) can b assssd from masuring th lctrical conductivity (EC). Th EC-valu and TDS ar linarly rlatd (Richards, 1954), and givn by: TDS = 640 EC (43) whr TDS EC = total dissolvd solids (mg/l); and = lctrical conductivity (ds/m). Th lctrical conductivity of th soil xtract is usually dtrmind in a soil past saturatd with watr up to th liquid limit. This conductivity (EC ) is comparativly asy to masur. For most soils th EC of th soil solution at fild capacity (EC s ), som tim aftr a rain or irrigation, is about twic th EC -valu. Soil sodicity Th rlativ amount of adsorbd Na + -ions, compard to th total amount of cations that a soil can adsorb is calld th Exchangabl Sodium Prcntag (ESP): ESP (%) = (Na + ads /CEC) 100 (44)

62 82 Guidlins to prdict whthr an nvlop is rquird whr Na + ads is th quantity of adsorbd Na+ -ions (mq/100 g of dry soil). Th ESP xprsss th sodicity and hnc th disprsion tndncy of a soil. Information on th chmical proprtis of th soil adsorption complx can b obtaind from th soil solution sinc thr is quilibrium btwn th adsorbd cations and th dissolvd cations. Hnc, anothr masur for th sodicity is th Sodium Adsorption Ratio (SAR), drivd from th concntration of sodium, calcium, and magnsium in th soil solution. + Na SAR = ++ Ca + Mg 2 whr th cation concntration is xprssd in mq/l. ++ (45) Th SAR can b dtrmind mor asily than th ESP. Th ESP can howvr b calculatd asily from th SAR sinc thy ar rlatd as (Richards, 1954): 100( SAR) ESP(%) = (46) 1+ ( SAR) Within th rang 2-30, SAR and ESP valus ar almost qual, so SAR = ESP is a practical approximation. Outsid this rang, Eq. (46) must b usd. High ESP or SAR valus ar usually an indication of poor physical soil conditions and high ph. An asy fild mthod, thrfor, is tsting ph with th indicator phnolphthalin. If this turns pink (ph abov 8.5), th soil has probably a high ESP. Disprsion problms ar gnrally mor svr whn th ESP or SAR valus ar gratr. Disprsd matrial may b transportd by groundwatr and will ntr th drainpip. In gnral, undr arid climats, problms ar not xprincd in soils with ESP valus blow 15 prcnt. In India, th clay contnt of soils, for which no nvlops around drains ar rquird, is incrasd from 30 to 40 prcnt for soils with SAR xcding 13 (Rajad Projct Staff, 1995). As th salt concntration of th soil solution has an influnc on disprsion, th ESP of a soil cannot b usd as a singl indicator of soil stability. Soils having an ESP gratr than 15 prcnt will not disprs as long as th salt concntration in th soil solution is high. Whn this high salt concntration in th soil solution dcrass,.g. du to laching by rain or irrigation watr, disprsion problms may aris (Smdma and Rycroft, 1983). Th snsitivity of soils to disprsion also dpnds on th typ of clay minral (swlling or non-swlling typ of clay). Swlling clay typs ar mor suscptibl to disprsion problms than non-swlling clays. But vrtisols (strongly swlling and shrinking clay soils) in Gzira, Sudan and lswhr, ar xampls of soils which do not xhibit disprsion problms in spit of ESP-valus ranging from 20 to 25 prcnt (Smdma and Rycroft, 1983). In humid aras, whr laching by rain watr is dominant, difficultis with soil structur may alrady aris at ESP-valus as low as 5 prcnt, whras soils lachd by irrigation watr will usually tolrat 10 prcnt ESP (cf. Tabl 9).

63 Matrials for subsurfac land drainag systms 83 WATER QUALITY Th chmical composition of a soil largly dpnds on th quality of th irrigation watr, th amount of rainfall and on th chmical composition of th groundwatr. Th lattr may b rchargd by irrigation watr, rainfall or spag, causing th watr tabl to ris far nough to influnc th soil. Irrigation watr Th stability of th soil structur in th arabl layr and th root zon dpnds in th long run on salts addd with th irrigation watr. In th long run, th EC and SAR of th soil solution at fild capacity (EC s and SAR s ) dpnd on th EC and SAR of th irrigation watr (EC iw and SAR iw ) with which th soil has bn irrigatd: and EC s = n EC iw (47) whr n = factor of concntration of th irrigation watr in th soil. It dpnds on th laching fraction (th fraction of irrigation watr draind). For high laching fractions (LF 0.3) th n-valu is approximatly 2. If th EC and SAR ar xprssd in trms of th saturatd past EC EC iw and SAR SAR iw (Ayrs and Wstcot, FAO, 1985). For mdium laching fractions (LF ranging btwn 0.15 to 0.20) EC 1.5 EC iw and SAR 1.22 SAR iw. Th ffct of th quality of irrigation watr on th stability of soil structur may b diagnosd on th basis of its EC iw and SAR iw -valus. Guidlins to valuat th impact of th chmical composition of irrigation watr on th infiltration rat of watr into th soil wr givn by Ayrs and Wstcot (FAO, 1985). Ths guidlins, which ar summarizd in Tabl 9, may b usd to assss th ffct of th quality of th irrigation watr on soil stability in th arabl layr and th root zon. TABLE 9 Problms with th infiltration rat of watr into a soil as rlatd to SAR iw and EC iw of irrigation watr (aftr Ayrs and Wstcot, FAO, 1985) SARiw SAR s = n SAR iw ECiw (ds/m) No problms Modrat problms Svr problms > < 0.2 > < 0.3 > < 0.5 > < 1.3 > < 2.9 (48) Irrigation with watr of low salinity will dcras soil stability if th salt concntration of th soil solution is substantial. Rainwatr diluts th soil solution and may caus gratr disprsion than most irrigation watrs. Groundwatr Salinity problms and disprsion of clays, as ncountrd in irrigatd agricultur, ar vry frquntly associatd with an uncontrolld watr tabl within on to two mtrs blow th ground surfac. If th groundwatr is too clos to th surfac, it riss by capillary action in dry

64 84 Guidlins to prdict whthr an nvlop is rquird priods and salinizs th soil surfac. If th groundwatr contains salts, a continuous load of salt accumulats into th root zon. Th combination of high groundwatr with salts spcially ariss in placs whr upward spag occurs. Unlss th xcss groundwatr is rmovd by an adquat drainag systm its lvl must b kpt blow th critical dpth. This is th dpth blow which capillary ris can b nglctd: about 1 m in sands (bcaus of low capillary ris), about 2 m in most clays (whr th vlocity is limiting), and 3 m or mor in silt loams (with high capillary ris and sufficint vlocity). If th groundwatr tabl is controlld by a subsurfac drainag systm, both th EC and SAR of th groundwatr (EC gw and SAR gw ) may hav a profound ffct on th structural stability of th soil at drain lvl. This is bcaus th EC and th SAR of th soil solution will b similar to th EC gw and th SAR gw if th soil at drain lvl is prmanntly saturatd. Howvr, th EC and th SAR of th soil solution may b substantially highr if th soil at drain lvl is unsaturatd, and salt accumulats du to capillary ris. Effctiv salinity control must thrfor includ not only adquat drainag to control and stabiliz th watr tabl and to prvnt salt accumulation in th shallow soil layrs, but also a nt downward movmnt of watr to prvnt salinization by capillary ris. PREDICTION CRITERIA Th prdiction critria dfind in th abov sctions ar summarizd blow. Ths ruls ar mrly guidlins or rcommndations that do not guarant 100 prcnt crtainty. If at drain dpth, augr hols can b mad only with th us of a scrn, bcaus thir walls collaps rapidly, installation of an nvlop is vital to protct futur drains against minral clogging. In cohsionlss sandy soils drain nvlops should b rcommndd undr all circumstancs. Any soil having a cumulativ particl siz distribution that lis compltly or largly in th shadd ara of Figur 42, is likly to caus problms with clogging of drains without nvlops. In tmprat aras, drainpips do not usually nd nvlops in soils with a clay contnt of at last prcnt, providing that drains ar not installd undr gnral wtnss. Soils with a plasticity indx of at last 12 show no tndncy to siltation. In irrigatd aras, drainpips installd in soils with a clay contnt xcding 40 prcnt do not nd an nvlop, rgardlss th SAR of th soil solution. Th nd for an nvlop in soils with a clay contnt ranging from 20 to 40 prcnt dpnds on th ESP, which is approximatly qual to th SAR of th soil solution (or somwhat highr). This SAR is gratly influncd by th quality of th irrigation watr and somtims by th groundwatr composition (th lattr in cas of dominant capillary ris). Gnrally, no nvlop is rquird in all cass whr SAR iw and EC iw appar to xclud soil stability problms, following th guidlins spcifid in Tabl 9. In cass, whr SAR and EC of th irrigation watr and/or groundwatr will prsumably invok soil stability problms, an nvlop is rcommndd. If thr is nt upward movmnt of salin groundwatr thr will b problms with salinization and disprsion of clays. Maintaining a nt downward watr movmnt is th ky masur to avoid such problms in soils with or without drainag systms.

65 Matrials for subsurfac land drainag systms 85 Chaptr 7 Guidlins for installation and maintnanc of drainag matrials INSTALLATION OF SUBSURFACE DRAINAGE MATERIALS Installation procdurs Drainag machinry Th succss of a drainag systm dos not only dpnd on th dsign and th proprtis of th soil and th nvlop. It is also dtrmind by soil wtnss during installation, trnch backfilling and th gnral quality of th work. Manual installation of drains and installation with backho machins ar a valid option for small drainag projcts. Backhos mak widr trnchs than drainag machins commonly usd in larg projcts. Thy ar also usd for wid and dp xcavations for larg collctors. Drainag machins ithr mak narrow trnchs in which th drains ar laid (trnch mthod) or thy put th drain dirctly into th ground (trnchlss mthod). Trnching machins ar ithr whl or chain trnchrs. Thy ar appropriat for a wid rang of working dpths and widths. Trnchlss machins can b classifid in ithr vrtical or V-ploughs. Th trnchlss installation mthod, howvr, has som practical limitations with rspct to drain typs, drain sizs, gravl application and installation dpth. Thrfor, trnchlss drainag has not yt bn widly implmntd in irrigatd aras (Zijlstra, 1987). Installing drains by manual labour or with classic xcavators rquirs a sris of succssiv oprations: xcavating th trnch, installing th pip, applying th nvlop matrial and backfilling th trnch. Ths oprations ar don simultanously by trnching machins. Somtims, backfilling is don by a sparat augr or blad on a tractor. Backfilling can also b don by an implmnt, attachd on th drainag machin whn driving backward to bgin xcavating a nw trnch (Ochs and Bishay, 1992). Contmporary drainag machins ar quippd with lasr grad control, which has significantly contributd to th fficincy and accuracy in th installation of subsurfac drains. Th maximum digging spd, howvr, should b adjustd to th spd of th hydraulic systm that is usd for automatic dpth rgulation, othrwis th installation accuracy will b poor. Although a crtain dviation from th dsign grad can b tolratd, it should not xcd half th pip diamtr. Largr dviations promot air locks in high and sdimntation in low placs, which obstruct watr movmnt through th drain. Similarly, drain sctions with a rvrs grad cannot b tolratd. Blinding Sinc th risk of sdimntation is largst during installation and in th immdiat subsqunt priod as long as th backfill has not sttld and stabilizd, drains ar normally covrd with

66 86 Guidlins for installation and maintnanc of drainag matrials friabl topsoil to crat a stabl and highly prmabl soil surround, and to prsrv th alignmnt. Thrfor trnching machins ar quippd with cuttrs to bring a layr of topsoil or soil from anothr suitabl layr from th sids of th trnch on top of th drain. Its thicknss should b at last 100 to 250 mm, dpnding on th drain diamtr. Granular nvlop matrial (lik gravl) can also b usd to achiv a highly prmabl drain surround and to prvnt vrtical and horizontal displacmnt onc th pip is installd. Any nvlop matrial to b usd must b in plac around th pip bfor blinding is don. Blinding, th initial covring of th drain with topsoil, is not rcommndd whn organic nvlops ar usd, bcaus topsoil with organic mattr and intnsiv microbiological activity nhancs th risk of microbiological dcomposition of ths nvlops. In such cass, soil from anothr suitabl layr, with low organic mattr, can b usd for blinding. Furthr backfilling of th trnch should b don as soon as possibl and, at th latst, at th nd of ach day if thr is a risk of surfac watr ntring th trnch. Soil conditions Sinc soil cohsion is strongly corrlatd with its watr contnt, installation of th drainag systm should prfrably b don in unsaturatd soil conditions with th watr tabl blow installation dpth and outsid priods of gnral wtnss. In addition, th backfill should hav sttld bfor havy rain or irrigation. In som situations, howvr, ths conditions ar not, or cannot b fulfilld. Drainag installation in wt conditions is discouragd, yt it is not always possibl to drain undr favourabl or idal circumstancs. Whn cohsionlss soils ar draind in saturatd conditions, an nvlop must b wrappd immdiatly around th drain and th drain covrd with backfill matrial bfor th liquid sand flows into th trnch. Caving of th trnch wall, which oftn occurs in cohsionlss or low cohsiv soils, may damag and/or displac th drain. In vry cas, th drain and th nvlop should b in plac bfor th trnch box has passd. Possibly, a longr trnch shild may b usd to protct a gratr lngth of th trnch. Th drain should b blindd immdiatly. Simultanous and instantanous backfilling will hlp to prvnt trnch wall failur. Howvr, th trnch may collaps as soon as th trnch box has passd and, thrfor, a chut should b providd at th nd of th trnch box to convy th caving soil down to th top of th drain in ordr to avoid damag by falling clods and stons. In cohsionlss soils, drainag machins should b kpt moving at all tims. If not, fluid sand is likly to ntr th trnch box and caus problms with sdimntation as wll as with alignmnt and grad of drains (Ochs and Bishay, 1992). Many problms, ncountrd with trnchrs or backho xcavators in saturatd cohsionlss soils, can b avoidd by trnchlss drainag installation. Drainag of physically stabl, wll-structurd soils undr gnral wtnss may dstroy th soil structur during xcavation and crat a lss prmabl trnch backfill (Stuyt, 1992a). Morovr, such conditions also promot minral clogging of pip and nvlop. In any cas, th us of an nvlop cannot compnsat for th advrsly affctd soil conditions. Evry ffort should b mad to prsrv th xisting soil structur and to protct th drain from soil failur. Adjusting th forward spd of th machin can b don to limit th dstruction of th soil structur. Obsrvation of th condition of th xcavatd soil can b a guid to th propr machin spd. Th machin should mov fast nough to prsrv th structur of th soil and not turn th xcavatd soil into slurry (Stuyt and Willardson, 1999).

67 Matrials for subsurfac land drainag systms 87 Structural dtrioration of an originally stabl, wll-structurd soil can b avoidd with trnchlss drainag installation. Th functioning of drains installd with th trnchlss tchniqu dpnds vry much on th changs in soil structur brought about by th passing of th blad (Zijlstra, 1987). This dpnds on th soil, th circumstancs (not wt) and th dpth (not ovr approximatly 1.5 m). Drainag of clay soils in wt conditions will unavoidably rsult in smaring and rduction of th hydraulic conductivity whr th machin has physical contact with th soil. Drainag of cohsiv soils in wt conditions must b avoidd, rgardlss of th availabl drainag machin. Th installation conditions for latrals of a composit drainag systm in saturatd soil ar improvd if th tim span btwn th installation of prmabl collctors and installation of th latrals is long nough. This is bcaus much of th local groundwatr has th opportunity to drain out bfor th latrals ar installd. In svr cass, whr th construction of collctors is difficult bcaus of quicksand, a tmporary drain (at gratr dpth) may b hlpful. It is usually far chapr than using wll-points. Backfilling Backfilling and finishing of trnchs should nsur a minimum of latr land subsidnc and prclud th occurrnc of piping. Th piping phnomnon may occur as a rsult of intrnal rosion of trnch backfill by watr flowing from th soil surfac dirctly to th drains through th loos backfill matrial (Van Zijts and Zijlstra, 1990). This is crucial in irrigatd lands, whr irrigation watr that can flow frly through th trnch or drain plough fissurs into th drainpip, will dramatically lowr th irrigation fficincy. Furthrmor, soil piping may caus soil matrial to b carrid by th flowing watr into th drain, crating sinkhols at th soil surfac and/or minral clogging of drains and nvlops, if prsnt. Propr backfilling of th trnch or plough fissurs is thrfor ssntial. It is asir to backfill and compact V-plough fissurs than trnchs. Fissurs, cratd by vrtical ploughs caus th most problms (Van Zijts and Naarding, 1990). Nithr havy loads, nor significant flooding should b imposd on nwly installd drains until th soil in th trnch is consolidatd. Th loos backfill matrial will sttl naturally with tim. Sinc backfilling is usually don with a tractor quippd with a dozr blad, passag of th tractor whl ovr th backfilld trnch, filling it up, and running ovr it again will spd up th procss, yt car must b takn to avoid crushing th pip. This procdur nsurs that only th top part of th trnch backfill is compactd, and that th dpr part of th backfill rtains a good prmability and a low ntranc rsistanc. In cas of trnchlss drain installation with a vrtical plough, compaction of th uppr part of th disturbd soil is qually important. A common procdur is that on track of th drainag machin runs ovr th drain lin on its way back to th outlt drain to bgin installing th nxt latral. In dry soil, th rat of compaction following this procdur may not b sufficint. Application of irrigation watr to unconsolidatd matrial in trnchs to sttl th backfill is a practic that should b don vry cautiously, howvr. If a fild is to b flood irrigatd bfor th trnch backfill is consolidatd, dirct ntry of uncontrolld surfac watr into th trnch should b avoidd by raising tmporary ridgs along both sids of th trnch (Stuyt and Willardson, 1999). Guidlins with rspct to drainpips Trnching machins can install clay, concrt, or plastic pips. Clay and concrt pips ar manually placd on a chut that convys th tils down into th trnch shild whr thy automatically mov into th right position on th bottom of th trnch. Th tils should b installd

68 88 Guidlins for installation and maintnanc of drainag matrials in th trnch in such a way that a prfct junction btwn drains is obtaind. For drains of largr sizs, an inspctor, standing or sitting in th shild, chcks for corrct laying. Th maximum gap btwn drains may not b mor than 3 mm xcpt for sandy soils or soils with a sandy layr on drain dpth whr it should b not mor than 2d 85. Clay and concrt tils without gravl or appropriat synthtic nvlops ar not rcommndd in cohsionlss fin sand (CPVQ, 1989). Plastic drains ar normally fd through a conducting pip, mountd just bhind (whl trnchr) or abov (chain trnchr) th digging mchanism of th trnchr. Trnchlss machins hav bn dvlopd to install only corrugatd drains of not too larg a diamtr. Thy should not b installd with a curvatur radius lss than fiv tims th pip diamtr, particularly if th pip is wrappd with an nvlop. For machin installation, th quality of drainpips is of utmost importanc. Drainpips with fissurs, cracks or othr visibl shortcomings and badly formd pips or torn nvlop matrial, which do not allow a propr installation or assur a rliabl prformanc, should not b usd. Furthrmor, all drains and collctors must b closd at th upward nd to avoid soil invasion (s Chaptr 2, Sction End caps). Failurs that may occur during installation of corrugatd drains ar crushd or collapsd pips, twistd pip sctions, couplings pulld apart and snappdoff pips (Van Zijts and Zijlstra, 1990). In such cass, th discharg is obstructd. Although th watr may finally find its way through th soil to a proprly functioning downstram part of th drain and to nighbouring drains, stagnation occurs. Upstram th blockag, watr may stand abov th drain and a highr groundwatr tabl will rsult. Coils of smallr diamtr pips ar usually carrid on a rl on ithr trnching or trnchlss machin and wound off as installation procds. Largr diamtr pips ar usually laid out on th fild bforhand, and thn guidd through th trnching machin. Excssiv pulling can rsult in connctions bcoming loos or pips braking off. During th uncoiling of th pip, pip brakag can b asily ovrlookd, yt th missing pic of drain will caus local wtnss. Thrfor, trnchlss drainag machins must b quippd with guids to facilitat smooth ntranc of th drainpip into th fdr tub. Gravl nvlop application can ntail substantial, undsirabl longation of th drainpip if th gravl dos not flow smoothly downward through th supply tub. Whil claning corrugatd PVC drains by jtting (Sction Maintnanc of drainpips), it is somtims obsrvd that drains wr not laid in a straight lin, but spiralld slightly. This phnomnon is attributd to th tnsion in th pip matrial gnratd in th unwinding of th rolls at installation (Van Zijts, 1987), and may nhanc th dvlopmnt of unwantd airlocks insid th drain. PVC pips should not b installd at tmpraturs blow 3 C bcaus of thir brittlnss at low tmpraturs. Storag at tmpraturs xcding 40 C for PE and 80 C for PVC pips, as wll as installation at tmpraturs abov 40 o C should b avoidd in ordr to prvnt pip dformation as a rsult of load and longitudinal strss. Exposur to UV rays of solar radiation also affcts th strngth proprtis of corrugatd plastic pips (Dsmond and Schwab, 1986; Dirickx, 1998a). Stord pips should thrfor b protctd from th influnc of dirct sunlight if not installd within on wk (tropical climats) or on month (tmprat climats) aftr dlivry (s Chaptr 2, Sction Plastic drainpips).

69 Matrials for subsurfac land drainag systms 89 Guidlins with rspct to nvlops Whatvr nvlop matrial is usd, and by whatvr mthod it is installd, nvlops must fully surround a drainpip, unlss th drain is installd on an imprvious layr. An nvlop mrly on top of a drain dos not suffic bcaus minral clogging also occurs from undrnath if watr ntrs th drain from all around. Bulky nvlops can b sprad out by hand in th bottom of th trnch bfor th pip is placd, but this is only possibl in stabl soil whr trnch walls do not collaps. If drains ar laid by hand and a layr of th bulky nvlop should surround th drain, th nvlop is placd on th bottom of th trnch and lvlld first. Nxt, th drain is installd and covrd furthr with bulky nvlop to th rquird hight. This also holds for machin installation of drains with a bulky nvlop. Envlop strips, dlivrd on rolls, should b applid blow and on top of th drain. Th matrial at th bottom nds not ncssarily b th sam as th matrial on th top. Prwrappd drains, howvr, ar prfrrd sinc thy protct drains from all sids, and offr a gratr safty than bulky nvlops or nvlop strips can do. Envlops that ar good and rliabl, howvr, will only b succssful if proprly installd undr favourabl physical soil and wathr conditions. Slurry in th bottom of a trnch will caus immdiat and complt failur of th nvlop matrial and hnc of th drain. Th gnral us of gravl nvlops has dcrasd continuously in spit of all fforts to mchaniz and prfct installation by.g. introducing a gravl augr at th nd of th trnch box. This gravl augr rducs pip strtch but gravl-fding problms ar still not compltly solvd (Vlotman t al., in prss). Thortically, it is also possibl to apply gravl with th vrtical drain plough as wll as with th V-plough. Howvr, th risk of stagnation of gravl in th supply tub of th machins maks th trnchlss tchniqu lss suitabl for gravl installation. Th installation of gravl rmains a difficult and labour-intnsiv opration. Practical xprinc shows shortcomings causing bas soil intrusion and pip siltation. Th major shortcomings ar (Dirickx, 1993): sgrgation during transportation and installation; flow problms in th supply tub; unqual distribution around th drainpip; and accidntal incorporation of soil into th gravl on th bottom of th stockpil. Coars, wll-gradd sand can also b usd as a drain nvlop. Howvr, th shar rsistanc of sand, spcially if it is not compltly dry, will hampr mchanical installation vn mor sriously than gravl dos. Organic and synthtic nvlops, pr-wrappd around corrugatd drainpips can b installd adquatly with both trnching and trnchlss machins. Thy ar howvr pron to damag, causd by transport and/or rapid machin installation, spcially whn matrials of infrior quality ar usd or whn th pip is not carfully wrappd. In ordr to avoid local spots of soil particl invasion, prwrappd nvlops covr th ntir drain circumfrnc. Furthrmor, thy should not b damagd during handling and installation. Thrfor, th layr of loos matrial bfor wrapping should b sufficintly thick and as uniform as possibl to avoid opn spots. Gotxtils that ar usd for th wrapping of drainpips ar usually supplid on rolls. Th shts should b wid nough to facilitat adquat ovrlap so that th pips ar compltly wrappd, without opn joints. If both longitudinal dgs of a gotxtil sht ar swn, th sht should b wid nough to facilitat this. If a gotxtil sock is pulld manually ovr th drain laid out on th fild, both th gotxtil and th sam, if any, should b strong nough to rsist this

70 90 Guidlins for installation and maintnanc of drainag matrials handling without damag. Gotxtils usually hav adquat mchanical strngth to rsist mchanical loads during installation. Machin installation rquirs adquat drainag matrials to assur a straightforward installation and a propr drainag prformanc. Thrfor high-quality matrials ar rquird and thir proprtis nd b chckd prior to installation according to wll-considrd standard spcifications. Quality standards of drainpips and drain nvlops ar thrfor of paramount importanc (Chaptr 9). Nithr PLMs nor prwrappd gotxtils show particular problms during installation with both trnching and trnchlss machins. Thir light wight maks thm suitabl in soft soils whr th us of gravl crats problms bcaus of th wight of th gravl. MAINTENANCE OF DRAIN PIPES Jt flushing Maintnanc is obvious whn thr is svr clogging. If don rgularly it may xtnd th srvic lif of th systm and nhanc its prformanc. In cas of light obstructions in pips (lik frsh ochr) dry rodding may b hlpful: a long sris of coupld rods, with a scratchr at th nd, is pushd into th drain and rmovd latr. If don during a priod of considrabl discharg, th loosnd matrials will b dischargd. For mor srious forms of clogging, jt flushing has to b usd. Jt flushing is a tchniqu usd to rmov clogging and prcipitating agnts (.g. soil particls and microbiological dposits, including iron ochr) from drainpips through th impact of watr jts. Mor particularly, th functions of jt flushing ar: lifting of blockags insid th pip drain; rmoval of dposits from th innr wall surfac of th drain; claning of cloggd prforations; rmoval of loos smallr roots of agricultural crops and wds; and supply of sufficint watr to carry th loosnd agnts, including sand and clay particls towards th drain outlt. Idally, th watr that dischargs from th drain vacuats th major part of th clogging agnts. Particls, largr than approximatly 75 µm may b dislodgd, yt ar gnrally too havy to b rmovd from th drain (Bussr and Scholtn, 1979). It is not clar to what xtnt pip prforations can b cland fficintly and non-dstructivly. It is assumd that jt flushing has a ngligibl ffct on cloggd nvlops. A typical jtting dvic is opratd from th powr takoff of an agricultural tractor. It consists of a pump, a suction pump inlt, and a rl with a m long prssur hos fittd with a nozzl, as shown in Figur 43. Th nozzl is fd into th pip drain from th downstram nd. Thrfor, th prssur hos is pointd to th drain outlt with th hlp of an adjustabl hos guid. Accss of th outlts of latrals is asy if thy discharg into opn collctor ditchs. Contrary to ths singular drainag systms, as common in humid tmprat zons, drainag systms in smi-arid countris oftn hav a composit layout, whrby latrals discharg into pip collctors instad of opn collctors. If th junctions btwn latrals and collctors ar locatd at manhols, ths can b usd to accpt a jtting hos, providd that th diamtr of th manhol is at last 0.3 m. In som countris,.g. Egypt, latrals ar accssibl at thir upstram nd (Figur 14).

71 Matrials for subsurfac land drainag systms 91 FIGURE 43 Jt flushing with a mdium prssur unit (aftr Bons and Van Zijts, 1991) On avrag, jtting rquirs 1-2 m 3 of watr pr 100 m of drain. Th watr can b pumpd from a drainag ditch, an irrigation supply canal, or a tankr must supply it. Salin watr is a harsh and corrosiv nvironmnt for flushing machins. If salin watr must b usd, th flushing machin should b mad of high quality salt rsistant machin parts. Th us of salt watr for flushing must b avoidd: it damags th soil structur around th drain and it is harmful for th machin. During th jtting procdur, th nozzl must b insrtd into th pip as fast as possibl. Th pulsating action of th piston pump nhancs th forward movmnt of th nozzl. Aftr th

72 92 Guidlins for installation and maintnanc of drainag matrials nozzl has rachd th upstram nd of th drain, th hos is rtratd by rling, at a stady pac of approximatly 0.3 m/s whil pumping continus (Van Zijts and Bons, 1993). Th claning action is influncd by th claning forc, th angl of attack of th watr jts, th duration of claning, th watr tmpratur and th us of chmicals (Hrs t al., 1985). Th claning forc is proportional to th flow rat tims th squar root of th watr prssur at th nozzl (Lchlr, 1980). Environmntal rstrictions as wll as cost considrations gnrally prclud th us of chmicals whil jtting. A balanc must b found btwn th prssur and th flow vlocity of th watr jts coming from th nozzl, prfrably on sit. Th optimum ratio is likly to dpnd on th insid diamtr of th drains; howvr, no data ar availabl to support this assumption. On many commrcial jt flushing units, th ratio btwn flow rat and prssur can b adjustd. Flow rats ar adjustd by changing th pumping spd. Th watr prssur is adjustd by slcting an appropriat nozzl (numbr, siz and orintation of hols). Jt flushing will tmporarily incras th watr prssur in th drainpip and thus in th surrounding soil, possibly affcting soil stability around th drain. Th incrasd watr prssur causs a rduction of cohsiv forcs btwn soil particls, which may lad to instant and hazardous quicksand conditions. Notably in wakly cohsiv soils, thr is a risk of th dvlopmnt of quicksand. Aftr th nozzl has passd, structurlss soil matrial may flow into th pip. In addition, th hydraulic conductivity of th soil may b advrsly affctd. Rgardlss of th discharg from th nozzl, dislodgd substancs ar mor asily vacuatd from small than larg diamtr drains du to th highr flow vlocitis in th smallr diamtr pips. As far as th watr prssur is concrnd, thr catgoris of jt flushing units ar bing manufacturd: high prssur quipmnt : > 100 bar at th pump; mdium prssur quipmnt : bar at th pump; low prssur quipmnt : < 20 bar at th pump. High-prssur units cannot b rcommndd, bcaus mpirical xprinc vidncd that this typ of flushing machin dstabilizs th soil around th drain and dstroys its structur. Watr prssur at th nozzl is approximatly 50 prcnt of th prssur at th pump. Hydraulic data of nozzl, pump prssur, and flow rats providd by a commrcial flushing unit manufacturr for a flxibl hos with an insid diamtr of 20 mm and a lngth of 300 m, ar givn in Tabl 10 (Bons and Van Zijts, 1991). Th highlightd lin contains rcommndd figurs (i.. prssurs and dischargs). TABLE 10 Rlation btwn pump prssur, nozzl prssur and discharg for a flxibl hos with an insid diamtr of 20 mm and a lngth of 300 m (aftr Bons and Van Zijts, 1991) Pump Prssur Nozzl with 2-mm hols Nozzl with 1.5-mm hols (bar) Prssur at nozzl (bar) Discharg (l/min) Prssur at nozzl (bar) Discharg (l/min)

73 Matrials for subsurfac land drainag systms 93 Th maximum flow of watr that can b mployd dpnds on th cross sction of th drain. Empirically it was found that a discharg of approximatly 70 l/min is satisfactory for 50 to 70 mm pip diamtrs. Such dischargs ar indd ralizd with th highly popular mdium prssur units. Highr dischargs may forc too much watr through th pip prforations, which is hazardous for th nvlop and th structur of th abutting soil. Th cost/bnfit ffcts of rgular maintnanc of drains by jt flushing ar hard to quantify. Still, som figurs may b informativ. Th cost of jt flushing in Th Nthrlands, at mdium prssur, is approximatly US $0.15 pr m of drain which is 12 prcnt of th installation cost of $1.25 pr m. With a typical drain lngth of 800 m pr hctar and a flushing frquncy of onc in vry thr yars, th annual cost amounts to $40 pr hctar pr yar. Th avrag annual gross yild of arabl land is approximatly $2500 pr hctar. Th calculatd maintnanc cost is thrfor lss than 2 prcnt of th annual gross yild. Empirical xprinc with jtting in northwstrn Europ Dry rodding and jtting of drains ar usful for rmoving ochrous substancs but gnrally not for rmoving roots from drains, with th xcption of loos, tiny ons (agricultural crops, som wds). Bfor jtting, som drains should b xamind intrnally first,.g. with a miniatur vido camra, in ordr to chck th kind of clogging and to assss th jtting fficincy. In cas of ochrous substancs, prvntiv jtting may b usful in ordr to prvnt total blocking of pip prforations. Ochr is a soft substanc whn prcipitating, but bcoms dns and sticky with tim, making it difficult to rmov (Cstr and Houot, 1984). Jtting cannot gnrally ropn pip prforations that wr cloggd with ncrustd ochrous substancs. Ochr dposits should thrfor b rmovd bfor drying out by frqunt flushing with mdium prssur (Von Schffr, 1982). Basd on rcntly acquird xprinc in Th Nthrlands, this rcommndation is nowadays rlaxd somwhat in th sns that flushing is rcommndd only if th ochr dposits do noticably impd propr functioning of th drain. This rcommndation also holds for othr kinds of microbiological dposits insid drains. Th following conditions may nhanc th risk of drain sdimntation through jtting: th us of high prssur quipmnt; jtting shortly aftr drain installation (soil not yt sttld nor stabilizd); damagd pips and/or dcomposd nvlops; non-cohsiv and wakly-cohsiv soils; and slow pac of movmnt or (tmporary) blockag of th nozzl. In Th Nthrlands, approximatly hctars of agricultural lands ar providd with a subsurfac drainag systm. No prcis data about th ara priodically flushd is availabl. In 1998, th numbr of flushing units in opration was stimatd at svral thousands, so a considrabl ara is rgularly maintaind. Th mdium prssur unit (35 bar at th pump and 10 to 15 bar at th nozzl, highlightd in Tabl 10) is by far th most widly usd. In th past, jt flushing has bn rportd to hav a positiv ffct on drain prformanc in a pilot ara, whr drains wr pron to xcssiv biochmical clogging du to intns upward spag of frrous groundwatr (Vn, 1986). As long as th drains wr jttd priodically, th drainag systm mt th dsign critria in trms of drawdown of groundwatr and discharg. Aftr jtting was discontinud, th plots suffrd from watrlogging. Van Hoorn and Bouma (1981) invstigatd th ffct of jtting on drains, installd in clay soils, which had bn submrgd rgularly and cloggd by minral particls and biochmical substancs. Th ffct was quit

74 94 Guidlins for installation and maintnanc of drainag matrials positiv. At anothr pilot ara in Th Nthrlands with comparabl conditions, howvr, Huinink (1991) stablishd that drain prformanc could not b rstord, dspit th implmntation of an xtnsiv jtting projct. Exprincs with high-prssur quipmnt in northwstrn Europ ar unfavourabl, whil substantial pip sdimntation is occasionally rportd with intrmdiat prssur quipmnt (Brinkhorst t. al, 1983). Practical xprinc of farmrs and contractors larnd that flushing with high prssurs nhancs sdimntation rats. Th nxt flushing had to b don soonr than in cas mdium or low prssur was usd. Around 1980, thrfor, th us of high-prssur quipmnt was gradually discontinud. During th nintis, th frquncy of jt flushing as advisd to th farmr varid from annually to onc in vry fiv yars. During this dcad, farmrs hav gradually bcom somwhat suspicious towards jtting of drains. Intns monitoring of drain prformanc in various pilot aras rvald that th assumd bnficial ffcts wr not so obvious as was assumd for a long tim (Huinink, 1991). If any improvmnt in drain prformanc could b noticd at all, it would gnrally last for a vry short tim. This fact has inducd som rluctanc towards prvntiv jtting of drains. Drainag xprts nowadays giv th following advic to th farmrs: do not jt any drain as a form of prvntiv maintnanc, unlss thr is a substantial risk of ochr clogging. On th othr hand, jtting is usful if th prformanc of drains has significantly dtrioratd, as obsrvd by th farmr. Drains, prwrappd with suitabl and lasting nvlops should howvr b practically maintnanc fr (Dirickx, 1993). A likwis obsrvation was mad in th Unitd Stats som 20 yars arlir (Wingr, 1973). Bcaus of this dvlopmnt, th numbr of Dutch manufacturrs of high and mdium prssur quipmnt wnt down from six in 1991 to two in Comparativly simpl low prssur jtting quipmnt is howvr manufacturd at various locations. Guidlins for jtting In summary, th following guidlins for jtting wr mpirically dvlopd in Dnmark, Grmany and Th Nthrlands for various typs of drainpips with diamtrs ranging from 40 to 90 mm:

75 Matrials for subsurfac land drainag systms Jtting must prfrably b don whn th groundwatr tabl is at or abov drain lvl. This is bcaus wt sdimnt is asir to rmov, and bcaus a wt soil will rstrict th undsirabl pntration of th jttd watr into nvlops and soils. 2. Satisfactory rsults wr achivd with th following machin spcifications and sttings: a middl prssur pump (35 bar at th pump and 12 to 15 bar at th nozzl); a standard nozzl with on hol forward and 12 hols backward; a flow rat of 50 to 70 l/min; an advanc (pntration) rat of 0.5 m/s; and a withdrawal rat of 0.3 m/s. 3. Whn th movmnt of th nozzl is obstructd, th pump should b stoppd immdiatly to prvnt local physical damag to th drain, nvlop, and to th soil structur. 4. Nglctd drains that contain hardnd clay and silt dposits should b jttd with a spcial nozzl with lss yt largr diamtr hols (.g. on forward and four to th rar). Th high impact watr jts will cut groovs in th sdimnts, braking thm up into pics, which facilitats thir rmoval. 5. Sdimnts consisting of fin sands must b rmovd with a nozzl with smallr jt angls,.g. 30 o. Wt sand can b loosnd rlativly asy, but is mor difficult to rmov from th pip than dposits that consist of finr particls lik silts and clays. Th sand must b kpt moving by larg quantitis of watr. 6. Drains that ar svrly cloggd should b cland in stags with an intrval of svral wks. Ths intrvals ar rquird to allow th soil around th drains to stabiliz aftr jtting. 7. If th rat of minral clogging of drains is so high that installation of nw drains must b considrd, a last, drastic attmpt may b mad to rstor thm. In such cass, th drain must b jttd by rpatdly insrting and pulling back th nozzl, ach tim a fw mtrs furthr, whrby application of high prssurs may b considrd. In ordr to minimiz th risk of dstabilizing th surrounding soil, th spd of insrtion of th nozzl into th drain should b maximum with low watr flow, whras th pac of withdrawal and th pumping rat should b such that th sand is kpt in front of th jt sprays. It is crucial to stablish and maintain a substantial discharg vlocity in th drain.

76 96 Guidlins for installation and maintnanc of drainag matrials