Consolidation Modeling of Oil Sands Fine Tailings: History Matching

Transcription

1 GoEdmonton /GéoEdmonoton Consolidation Modling of Oil Sands Fin Tailings: History Matching S. Jravipoolvarn, R. J. Chalaturnyk and J. D. Scott Dpartmnt of Civil and Environmntal Enginring Univrsity of Albrta, Edmonton, AB, Canada ABSTRACT Th us of a finit strain consolidation thory to prdict comprssion bhaviour of oil sands fin tailings dos not provid a good agrmnt with xprimntal data. To sarch for a possibl xplanation of this discrpancy, history matching was prformd by thr approachs namly a pr-consolidation, a crp comprssion and a hydraulic conductivity layring consolidation. Rsults indicat that th pr-consolidation and crp mthods provid ngligibl improvmnt whil th layring approach givs th bst prdiction not only for intrfac prdiction but also for void ratio, xcss por prssur and ffctiv strss profils. Examination of modling paramtrs suggsts that th hydraulic conductivity incrass upward. This ld to a hypothsis that channling might caus th dviation of th conduction proprty of th slurry. For this oil sands fin tailings, it is concludd that th convntional approach can lad to a larg rror, considrations of a pr-consolidation and crp comprssion do not improv prdiction and a channling phnomna could b a major rason of th discrpancy obsrvd btwn th thory and th xprimnt. RÉSUMÉ L'utilisation d'un théori fini d consolidation d contraint d prévoir l comportmnt d comprssion ds produits d quu fins d sabls d'huil n fournit pas à un bonn concordanc ds donnés xpérimntals. Pour rchrchr un xplication possibl d ctt anomali, l'assortimnt d'histoir a été ffctué par trois approchs notammnt un pré-consolidation, un comprssion d fluag t un consolidation posant d conductivité hydrauliqu. Ls résultats indiqunt qu ls méthods d pré-consolidation t d fluag fournissnt l'amélioration négligabl tandis qu l'approch posant donn la millur prévision pas sulmnt pour la prévision d'intrfac mais égalmnt pour l rapport vid, prssion xcssiv d por t ds profils fficacs d'ffort. L'xamn d modlr ds paramètrs suggèr qu la conductivité hydrauliqu augmnt vrs l haut. Cci a mné à un hypothès qu crusr ds rigols pourrait causr la déviation d la propriété d conduction d la bou. Pour ctt huil ponc ls produits d quu fins, on l conclut qu l'approch convntionnll put mnr à un grand rrur, ds considérations d'un pré-consolidation t la comprssion d fluag n'améliornt pas la prévision t ls phénomèns crusants ds rigols pourraint êtr un raison important d l'anomali obsrvé ntr la théori t l'xpérinc. 1 INTRODUCTION In Northrn Albrta, Canada, oil sands industris produc larg volums of tailings. For Syncrud Ltd. alon, thr is currntly Mm 3 of fin tailings and this amount is growing. To manag th volums ffctivly, nginrs must b abl to prdict a rat of comprssion of ths tailings. Th gotchnical approach to prdict consolidation bhaviour of soft soils is to us a finit strain consolidation thory (Gibson t al. 197). Th thory is prformd by using two important constitutiv rlationships which ar ffctiv strss - void ratio and hydraulic conductivity - void ratio rlationships dirctly dtrmind from a larg strain consolidation tst. To vrify th thory, xprimntal rsults from a tn mtr high standpip tst filld with th fin tailings at th Univrsity of Albrta is usd. Th tst was startd in 19 and it has bn undr monitoring for mor than yars. This tst rsult provids a grat opportunity to modl long trm bhaviour of th tailings. Rsults of a straight implmntation of th thory and th constitutiv rlationships, howvr, do not provid a satisfying prdiction having a larg rror of mor than 3 prcnt of th xprimntal valu. This disagrmnt btwn th thory and th xprimnt raiss a qustion on th prdiction capability of th thory. According to Krizk and Somogyi (19), thr ar two choics for nginrs to prform whn this problm occurs, on is to prform a trial and rror to find appropriat constitutiv rlationships for th problm and anothr is to rsarch for missing physics and implmnt it into a modl. Som consolidation rlatd phnomna that wr rportd and not originally includd in th finit strain consolidation thory ar sgrgation, channling, crp, pr-consolidation and non-darcian flow phnomna. Sgrgation is rfrrd to soil particl sorting; coarsr/havir particls slowly sink to th bottom of a sttling volum and lav smallr/lightr particls suspndd in th fluid. Sgrgation is not dsird as th slf-wight strsss ar not ffctivly utilizd. Whil sgrgation is important, th fin tailings contain approximatly 95% fins thrfor sgrgation ffcts ar potntially ngligibl. Comprssibility and hydraulic conductivity of th fin tailings ar shown in Figurs 1 and rspctivly. Figur 1 shows that th comprssibility of th fin tailings is 19

2 dpndnt on initial void ratio. This typ of bhaviour is similar to othr findings (Imai 191, Bn and Sills 191, Scully t al. 19). It can also b obsrvd that unlik normal slurry, comprssibility of this tailings show prconsolidation bhaviour at a low ffctiv strss of around 1 kpa. At highr ffctiv strss, all th comprssibilitis start to convrg around an ffctiv strss of kpa. It is notd that Scully t al. (19) also found that th comprssibility curvs showd an apparnt prconsolidation ffct in a rang of ffctiv strss of 1 kpa and abov for phosphatic tailings and Sridharan and Prakash (1999) showd that montmorillonit clay bhavs as an ovr consolidatd soil in th low ffctiv strss rang of.5 to. kpa. Similar comprssibility convrging bhaviour was shown by Myint t al. (3) who suggstd that th transition btwn ultra soft soils to Trzaghi soils is approximatly at an ffctiv strss of about kpa. Initial void ratio = 9.1 Initial void ratio = 7.3 Initial void ratio = Effctiv strss (kpa) Figur 1. Comprssibility of fin tailings (Suthakr 1995) Hydraulic conductivity (m/d) 1E+ 1E-1 1E- 1E-3 1E- 1E-5 1E- 1E-7 Pollock 19, Suthakr 1995, AGRA 1997 Figur. Hydraulic conductivity of fin tailings Anothr factor that affcts comprssibility is tim dpndnt bhaviour or crp. Sill (199) usd X-ray dnsity masurmnt coupld with por prssur data to valuat dnsity with ffctiv strss of various slurris and showd that comprssibility of slurry is not uniquly dtrmind. Latr Bartholomusn t al. (3) prsntd common ovrstimat prdictions of initial sttling rat by various consolidation modls and pointd out that this was du to tim or rat dpndnc of corrlation btwn ffctiv strss and void ratio. This is only partly tru for slurry as th diffrnc in comprssibility will not altr th initial sttling rat which is dominatd by th hydraulic conductivity - void ratio rlationship. Nvrthlss, th crp bhaviour affcts sttlmnt prdiction latr on in th consolidation procss spcifically aftr th linar sttling rgion. To b abl to captur crp bhaviour, it is suggstd that th most appropriat way is to includ tim in th constitutiv rlation in a form of void ratio, ffctiv strss and rat of comprssion (Bjrrum 197, Lrouil t al. 195, Kim and Lrouil 1, Bartholomusn 3). Th first implmntation of crp bhaviour in slurry consolidation modling was fulfilld by Bartholousn (3) who proposd a comprssibility surfac in a form of, σ and ė for slurry. This concpt was originatd from Bjrrum (197) and Lrouil (195) and th diffrnc is that a comprssibility surfac function and a fluid dynamic formulation ar usd by Bartholomusn (3). A prdiction of intrfac sttlmnt by th crp modl showd a good agrmnt with xprimntal data. In Figur, hydraulic conductivity masurmnts of th fin tailings from various sourcs indicat that this matrial is low in hydraulic conductivity and thr is a considrabl sprad of th data. Part of this is du to th non-darcian bhaviour of this matrial as th hydraulic conductivity is dpndnt on th hydraulic gradint; a highr hydraulic gradint rsults in a lowr hydraulic conductivity (Suthakr and Scott 199). Thrfor tst data with diffrnt hydraulic gradints giv ris to th discrpancy and it was suggstd by Suthakr and Scott (199) that for modling purposs hydraulic conductivity masurmnt should b prformd at th sam hydraulic gradint in th fild. Hydraulic conductivity valu can also b affctd by channling. Svral rsarchrs hav rportd flow channls (Bn and Sills 191, Caughill 199, Tang 1997, Edil and Fox ). Edil and Fox () discussd that this dwatring structur is a localizd macrostructural fatur rsulting from a prfrntial flow path and whn this is obsrvd th assumption of uniform por fluid migration may not b appropriat for consolidation thoris. Edil and Fox () also showd that subjctiv intrprtation of th dwatring structur indicats no obvious pattrn or distribution which suggsts a random phnomnon. On would xpct that por fluid has a high tndncy to flow through ths channls rathr than th soil matrix. Givn th complx phnomna and larg discrpancy btwn xprimnt and thortical prdiction on oil sands fin tailings, this articl prsnts historical consolidation analyzs of this tailings basd on diffrnt approachs. Emphasis of this study is on 191

3 modling by th convntional thory, pr-consolidation comprssibility, a crp comprssion and layring ffcts on hydraulic conductivity. Modling of th four approachs wr prformd sparatly and th rsults wr intrprtd to bring an insight viw of what might b th crucial mchanics controlling comprssion bhvaiour of this class of matrial. CONSOLIDATION MODELS This sction provids assumptions and xcutions of ach consolidation modling approach including th convntional finit strain consolidation analysis, prconsolidation bhaviour analysis, crp comprssion analyss and layring consolidation analysis..1 Convntional Finit Consolidation Analysis Two approachs ar prformd and thy ar th convntional finit strain consolidation analysis with and without adjusting constitutiv rlationships. Th two constitutiv rlationships ar ffctiv strss - void ratio and void ratio - hydraulic conductivity rlationships shown as Equations 1 and rspctivly. It is assumd that xprimntally dtrmind rlationships giv th bst rprsntation of th matrial consolidation bhaviour. Aσ B = [1] D k = C [] Whr σ is ffctiv strss, k is hydraulic conductivity, is void ratio and paramtrs A, B, C and D ar laboratory dtrmind paramtrs. With Equation 1 and, th finit strain consolidation quation (Somogyi 19) for quiscnt condition is xprssd as Equation 3. Whr u is xcss por prssur and z is matrial coordinat. u + t ( 1 B ) ( 1 B ) σ k u σ 1 + u + = [3] ABγ w 1 + z ABγ w z z. Pr-Consolidation Bhaviour Analysis According to considrabl vidnc (Suthakr 1995, Pollock 19) from larg strain consolidation tsts, th comprssibility of this matrial xhibits pr-consolidation bhaviour. This bhaviour maks th us of th powr law function [1] qustionabl and th us of a Wibull function [] is mployd for this study as this function can captur th pr-consolidation bhaviour and givs a bttr cofficint of dtrmination. For th hydraulic conductivity - void ratio rlationship, th powr law is still appropriat. k A B.xp[ E. σ ] F = [] In quation [], A, B, E and F ar laboratory dtrmind paramtrs. Th modifid finit strain consolidation quation with Equation is xprssd as Equation 5. k( ) u z γ w (1 + ) z [5] k( ) u + + B. E. F.xp[ E. σ γ (1 + ) z w.3 Crp Comprssion Analysis F ]. σ ( F 1) u = t In this papr, crp is assumd to hav two mods which ar basd on two similar concpts. Th first on is calld crp typ 1 which is crp bhaviour that occurs indpndntly to consolidation. This typ of crp is basd on comprssion bhaviour obsrvation of th fin tailings in both fild and laboratory. Dtaild philosophy of this comprssion mod can b found lswhr (AGRA 1997). Thrfor total dformation is a combination of indpndnt crp comprssion and consolidation. A crp function is postulatd and it is xprssd as Equation. & = M. xp[. N] [] Whr M and N ar xprimntally dtrmind paramtrs. This crp function is fittd from xprimntal data of crp rats vrsus void ratio. A flow chart of crp typ 1 analysis is illustratd in Figur 3. Th analysis is prformd by xplicitly combining crp dformation ( cr ) to consolidation dformation ( con ) at th nd of ach tim stp ( t) of th numrical solution. As this will caus an incompatibility of comprssibility, it must b adjustd to follow th rsultd dformation. Th scond mod, crp typ, is basd on a concpt of strain rat prsntd by Lrouil, (195). Crp typ is prformd by assuming a constitutiv rlationship in a form of = f (σ, ė). A form of a function usd to combin rat of comprssion and comprssibility is slctd as Equation 7 whr G, H, I and J ar xprimntal dtrmind paramtrs and A is from Equation [1]. This rlationship is assumd to xist and availabl xprimntal data ar usd to obtain paramtrs in th rlationship. J A = G H.xp[ I. & ] [7] 19

4 Convntional finit strain consolidation modl Updat comprssibility Adjust comprssibility = con + cr No Crp function [] t End of analysis? Exit Ys Fox ). Concivably, th channls will act as a dwatring structur that drains off th xcss por prssur to th surfac of tailings. Thrfor th occurrnc of th channls would incras hydraulic conductivity locally. This lads to a scond assumption that hydraulic conductivity is varid vrtically du to channling. As slf-wight consolidation advancs upward for a singl drainag boundary condition, th dvlopmnt of ffctiv strss would likly shutoff th boiling procss and th subsquntly sttlmnt would sal off th drainag structur rsulting in th hydraulic conductivity incrasing upwards. Givn th larg variability of masurd hydraulic conductivity in Figur, a last assumption is givn that th hydraulic conductivity is lowst at th bottom and gradually incrass to highr valus at highr lvations. Boundaris of th hydraulic conductivity ar chosn from th availabl data in th following sction. Figur 3. A computational schm of crp typ 1 3 MODELING PARAMETERS Updat comprssibility Convntional finit strain consolidation modl t Calculat rat of comprssion ė = / t An implicit finit diffrnc mthod was chosn to solv all govrning quations numrically du to its stability. Numrical paramtrs usd in this papr ar z = z/, t =.1 for spatial and tmporal rsolutions. Initial conditions and soil paramtrs of th fin tailings in th vrification standpip ar an initial hight of m, an initial solids contnt of 3.% ( = 5.17, w = 7%) and a spcific gravity of.. Modling paramtrs of approachs.1 to.3 ar listd in Tabl 1. Rdfin comprssibility from a strain rat function [7] No Figur. A computational schm of crp typ A flow chart of crp typ analysis is shown in Figur. Th analysis is prformd by calculating ė and subsquntly rdfining soil comprssibility basd on ė data. As a nw comprssibility is obtaind basd on th void ratio rat corrlation, a nxt tim stp is prformd until th nd of a spcifid simulation priod.. Layring Consolidation Analysis End of analysis? Exit Ys A channling or dwatring structur is obsrvd as discussd in Sction 1. Obsrvations suggst that channling occurs du to boiling; a condition whr th ffctiv strss is lowr than th spag strss. As this is localizd and randomly occurrd it is not currntly possibl to quantify this ffct. In ordr to continu this invstigation, a first assumption is drawn that channling occurs homognously in a horizontal plan and thrfor it is a on dimnsional (vrtical) ffct. This is prsuasiv according to 3D rconstruction of channling (Edil and Tabl 1. Modling paramtrs (units in kpa, m/d) Paramtrs Convntional Prconsolidation Crp typ 1 Crp typ A B C D E F G H I J M N Th convntional modling paramtrs ar obtaind by using a powr law on comprssibility and hydraulic conductivity in Figurs 5 and. Th pr-consolidation modl uss a Wibull function instad of a powr law for comprssibility (Figur 5). Crp modls us th sam comprssibility and hydraulic conductivity to th convntional modl with additional crp functions. Crp typ 1 paramtrs (M, N) ar obtaind by bst fittd on xprimntal crp data (Figur 7) by Equation and 193

5 crp typ paramtrs (G, H, I, J) ar arbitrary chosn basd on comprssibility and strain rat rlations (Figur ) of th fin tailings givn by Jravipoolvarn (5) Larg strain consoldiation tst data Convntional (Pow r law ) Pr-consolidation A.5. A=G-H xp[-i ė J ]. =A-B xp[-e σ' F ], R = / t (D -1 ). Figur. Crp rat vs. comprssibility paramtr A =A σ' B, R = Effctiv strss (kpa) Figur 5. Comprssibility of fin tailings Hydraulic conductivity (m/d) 1E-1 1E- 1E-3 1E- 1E-5 1E- 1E-7 k=c D, R =.7 UofA data (Suthakr 1995, Pollock 19) Othr studis (AGRA 1997) Powr law (UofA data) Uppr limit Lowr limit Figur. Hydraulic conductivity of fin tailings / t (D -1 ) 1E+1 1E+ 1E-1 1E- 1E-3 m standpip data m standpip data (Suthakr 1995).m standpip data (Suthakr 1995) Consolidation data (Suthakr 1995) Crp typ 1 ė = M. xp[.n ] 1E- 1 Figur 7. Crp rat vs. void ratio of fin tailings For th layring consolidation analysis, th hydraulic conductivity paramtr D of th soil layrs is assumd to rang btwn th uppr and lowr limits of th xprimntal data in Figur. Th uppr and lowr limits ar assumd to rprsnt maximum channld systm and homognous systm rspctivly. An arbitrary distribution of th hydraulic conductivity paramtr D is slctd for optimum history matching and th distribution is plottd against normalizd hight, H/Ho, in Figur 9. H/H D Figur 9. Hydraulic conductivity paramtr D vs. H/Ho MODELING RESULTS Tailings-watr intrfac masurmnt is shown with all simulation rsults in Figur 9. Comparisons of xprimntal masurmnt of void ratio profils, xcss por watr prssur profil and ffctiv strss profils with numrical prdictions ar shown in Figurs, 11 and 1 rspctivly. Elapsd tims of days, yars and yars ar chosn for th profil comparisons whil xprimntal data and prdictions for othr lapsd tim fall btwn ths numbrs. 19

6 Tailings-watr Intrfac sttlmnt (m) Exprimnt Convntional Pr-consolidation Crp typ 1 Crp typ Layring 1 1 Elapsd tim (yars) It is shown by th intrfac sttlmnt comparison (Figur ) that th convntional finit strain consolidation thory with assignd powr law functions to comprssibility and hydraulic conductivity is abl to captur th intrfac sttlmnt up to about yars but fail to captur furthr intrfac sttlmnt data of th fin tailings with an ovrstimation rror of 33% at 1 yars. Th profil comparisons of this modl ar shown in Figurs 11(a), 1(a) and 13(a). Th rsults indicat that non of th profils ar in good agrmnt. Ths disagrmnts ld to xtnsiv invstigations of othr possibl mchanisms affcting th comprssion bhaviour of th fin tails (Banas 1991, Suthakr 1995, Tang t al. 1997, AGRA 1997, Jravipoolvarn 5). Figur. Tailings watr intrfac sttlmnt comparison days-e yrs -E yrs-e days-p yrs -P yrs-p days-e yrs -E yrs-e days-p yrs -P yrs-p days-e yrs -E yrs-e days-p yrs -P yrs-p days-e yrs -E yrs-e days-p yrs -P yrs-p days-e yrs -E yrs-e days-p yrs -P yrs-p (a) (b) (c) (d) () Figur 11. comparison (a) convntional (b) pr-consolidation (c) crp typ 1 (d) crp typ () layring days-e yrs -E yrs-e days-p yrs -P yrs-p days-e yrs -E yrs-e days-p yrs -P yrs-p days-e yrs -E yrs-e days-p yrs -P yrs-p days-e yrs -E yrs-e days-p yrs -P yrs-p days-e yrs -E yrs-e days-p yrs -P yrs-p (a) (b) (c) (d) () Figur 1. Excss por watr prssur comparison (a) convntional (b) pr-consolidation (c) crp typ 1 (d) crp typ () layring days-e yrs -E yrs-e days-p yrs -P yrs-p days-e yrs -E yrs-e days-p yrs -P yrs-p days-e yrs -E yrs-e days-p yrs -P yrs-p days-e yrs -E yrs-e days-p yrs -P yrs-p days-e yrs -E yrs-e days-p yrs -P yrs-p (a) (b) (c) (d) () Figur 13. Effctiv strss comparison (a) convntional (b) pr-consolidation (c) crp typ 1 (d) crp typ () layring 195

7 Blivd to b causd by a high thixotropy gain in strngth of th fin tailings, th pr-consolidation comprssibility approach vn though it contains a considrably bttr comprssibility function (highr cofficint of dtrmination) only givs a slight improvmnt of th intrfac sttlmnt prdiction (Figur ). Figurs 11(b), 1(b) and 13(b) show that th prconsolidation approach is not significantly diffrnt to th convntional approach. Givn that th initial void ratio is 5.17, thr crtainly is a small ovr-consolidation prssur to ovrcom by slf-wight strsss (Figur 1). Th crp typ 1 intrfac prdiction givs a larg dviation from th xprimntal data. Th void ratio profils (Figur 11(c)) giv an intriguing rduction in void ratio whr thr is no consolidation similar to xprimntal obsrvations. Ths ar howvr not unxpctd as th additional strain was introducd by crp Equation. Nvrthlss, crp typ 1 is unabl to captur xprimntal data in all aspcts giving that th hydraulic conductivity is unchangd. According to AGRA (1997), an improvd prdiction can b obtaind whn a lowr bound hydraulic conductivity is implmntd with crp typ 1. Rat dpndnt comprssibility, th crp typ, givs a good agrmnt of intrfac prdiction to about yars similar to th convntional approach whil th prdiction starts to divrg gratr than th convntional mthod aftr yars (Figur ). Th ffctiv strss and xcss por prssur rsponss at th yar prdiction appar to b closr to xprimntal data howvr this is not to b dciving as th intrfac rror is largr than th convntional approach. Considring a crp mchanism, th comprssibility of soils is likly gratr du to th viscous rspons as th duration of consolidation incrass. This conflicts with th philosophy of thixotropy gain in strngth bhaviour whr it is assumd that th pr-consolidation is causd by incrasing undraind shar strngth du to th dvlopmnt of gl structur with tim. Sills (199) and Barthomusn (3) showd masurmnt of ffctiv strss rspons with tim for various slurris and from thir xprimntal data it can b concludd that viscous rspons significantly maks slurry mor comprssibl with tim. According to Lrouil (195), th magnitud of pr-consolidation prssur is a consqunc of comprssion rat. This is consistnt with th comprssibility data of th fin tailings in Figur 1. As a highr initial void ratio with th sam sampl thicknss implis a rlativly highr rat of comprssion compard to a lowr void ratio, th highr pr-consolidation prssur blongs to th highr initial void ratio. Thrfor, it is concludd that for oil sands fin tailings, th caus of th apparnt pr-consolidation prssur rmains unknown but th magnitud of it is potntially rlatd to th rat of comprssion. Th influnc of th pr-consolidation on comprssion bhaviour is small and by considring a crp mchanism, it is concivably ngligibl. Th layring approach intrfac prdiction shown in Figur indicats good history matching rsults. Shap and location of void ratio masurmnts ar in good agrmnt with prdictd profils (Figur 11()). Excss por prssur and ffctiv strss prdictions ar also in a good agrmnt with xprimntal data (Figurs 1() and 13()). Considring two thirds of th ffctiv strss masurmnt data from th top of th comprssing volum, this history matching slightly ovrstimats th ffctiv strss data. At this location, thr appars to b a rduction in void ratio without considrabl ffctiv strss dvlopmnt. As th layring approach nglcts crp comprssion, a slight disagrmnt is xpctd. Th diffrnt is small in a rang of 1 to kpa. Th approachs in this papr can b dividd into two main parts which ar comprssibility approachs (Sctions. and.3) and hydraulic conductivity approach (Sction.). It is arguabl that both factors coxist during comprssion of slurry. Whil this is tru, it must b strssd that th comprssibility function dos not hav a larg influnc on th sttlmnt prdiction spcially during th arly stag (high void ratio stag) bcaus of th low ffctiv strsss in th slurry. Th outcoms of all numrical simulations rinforc th grat importanc of hydraulic conductivity on th oil sands fin tailings comprssion bhaviour. Th discrpancy btwn xprimntal data and thortical prdiction likly involvs mchanisms that affct hydraulic conductivity mor than affcting th comprssibility. 5 DISCUSSION AND CONCLUSIONS It should b mphasizd that th obsrvd bhaviour of both channling and rduction in void ratio without gain in ffctiv strss is not common only to th oil sands fin tailings but also appar in othr typs of slurris and tailings. History analyss wr prformd in this study giving intriguing hypothss that might improv our undrstanding of slurry comprssion. Two main simulation approachs wr prsntd; comprssibility and hydraulic conductivity approachs. Th comprssibility approach includs pr-consolidation and crp comprssion. Th hydraulic conductivity approach rfrs to th layring mchanism that causs dviations in vrtical hydraulic conductivity. Th pr-consolidation bhaviour found in th comprssibility of th oil sands tailings is blivd to b causd by thixotropic gain in strngth. In this study th pr-consolidation bhaviour is modld by a Wibull comprssibility function which is found to b a mor appropriat form of comprssibility for th tailings compard to th convntional powr law. Rsults from this approach giv a slight improvmnt of intrfac sttlmnt prdiction., ffctiv strss and xcss por prssur profil prdictions howvr show a ngligibl advantag of this mthod. 19

8 For th crp comprssion approachs, both crp typ 1 and typ incras dformations with tim du to a viscous ffct. As a rsult, th intrfac prdiction with crp furthr ovrstimats th masurd intrfac sttlmnt data. Crp typ 1 is found to b abl to produc a similar bhaviour as in th xprimntal data whr thr is a rduction of void ratio with no ffctiv strss dvlopmnt. Howvr givn that th intrfac prdiction has a larg rror and th additional dformation is unjustifid as som of it is alrady mbddd in th comprssibility, this mthod rquirs furthr invstigation. It is concludd that without any considration of changs in hydraulic conductivity, all comprssibility approachs ar not abl to provid bttr prdictions compard to th convntional approach. Whil th comprssibility approachs ar not promising, th layring approach or hydraulic conductivity approach givs rasonabl history matching in all aspcts. Th important fatur of rduction in void ratio undr low ffctiv strsss is also capturd similar to crp typ 1. Although th slctd hydraulic conductivity paramtrs fall within th masurd hydraulic conductivity data, a critical assumption drawn for this approach, that th dwatring structur incrass hydraulic conductivity upward, is yt to b validatd. Othr possibl mchanisms that affct micro and macro structurs and thrfor hydraulic conductivity of th slurry, such as thixotropy and non-darcian flow bhaviour, ar also possibl xplanations. Furthr invstigation on such phnomna is rquird to xtnd th currnt stag of undrstanding. ACKNOWLEDGEMENTS Th authors ar gratful for th financial support from th Univrsity of Albrta and Syncrud Canada Ltd. and discussions and suggstions from Mr. Bill Shaw, Dr. W. D. Carrir III, Dr. E. C. McRobrts and Dr. G. McKnna. REFERENCES AGRA Progrss on MFT Modling. Rport to Syncrud Canada Ltd, Enginring rport, AGRA Earth & Environmntal limitd, Edmonton, Albrta. Banas, L Thixotropic Bhaviour of oil sands tailings sludg, MS.c. Thsis, Univrsity of Albrta, Edmonton, Canada. Bartholomusn, G. 3. Compound shock wavs and crp bhaviour in sdimnt bds. Ph.D. Thsis, Oxford Univrsity. Bn, K. and Sills, G.C Slf-wight consolidation of soft soils. Géotchniqu, 31: Caughill, D.L Gotchnics of non-sgrgating oil sand tailings, MS.c. Thsis, Univrsity of Albrta. Edil, T. B. and Fox, P.J.. Gotchnics of High Watr Contnt Matrials, ASTM intrnational Publishr. Gibson, R.E., England, G.L., and Hussy, M.J.L Th Thory of On-Dimnsional Consolidation of Saturatd Clays I, Finit Non-Linar Consolidation of Thin Homognous Layrs. Géotchniqu, 17:1-73. Jravipoolvarn, S. 5. Comprssion Bhaviour of Thixotropic Oil Sands Tailings. MS.c. Thsis, Univrsity of Albrta. Kim, Y.T. and Lrouil, S. 1.Modling th viscoplastic bhaviour of clay during consolidation: application to Brthirvill clay in both laboratory and fild conditions, Canadian Gotchnical Journal, 3:- 97. Lrouil, S., Kabbaj, M., Tavnas, F., and Bouchard, R Strss-strain-strain rat rlation for th comprssibility of snsitiv natural clays. Géotchniqu,35( ): Myint, W.B., Wong, K.S., Victor, C., Th, C.I. 3. Comprssion Tsts of Ultra-Soft Soil Using an Hydraulic Consolidation Cll. Gotchnical Tsting Journal, (3):3-319 Pan, V. and Schiffman, R.L A not on sdimntation and consolidation. Géotchniqu, 35(1):9-7. Pollock, G., 19. Larg Strain Consolidation of Oil Sand Tailings Sludg, MS.c. Thsis, Univrsity of Albrta, Edmonton, Canada. Scully, R.W, Schiffman, R.L., Olsn, H.W. and Ko, H.Y. 19. Validation of consolidation proprtis of phosphatic clay at vry high void ratios, Sdimntation consolidation modls: prdictions and validation: procdings of a symposium, R.N. Yong and F.C. Townsnd Eds, ASCE, Nwyork, Sills, G., 199. Dvlopmnt of structur in sdimnting soils, Philosophical Transactions of th Royal Socity A, Vol. 35, pp Somogyi, F., 19. Larg Strain Consolidation of Fin Graind Slurris. Prsntd at th Canadian Socity for Civil Enginring, Winnipg, Manitoba, May 9-3. Sridharan, A. and Prakash, K A consolidation and prmability bhavior of sgrgatd and homognous sdimnts, Gotchnical Tsting Journal, : 9 1 Suthakr, N. N., Gotchnics of Oil Sand Fin Tailings, Ph.D. Thsis, Univrsity of Albrta, Edmonton, Canada. Tang, J., Biggar, K.W., Scott, J.D. and Sgo, D.C Examination of matur fin tailings using a scanning lctron microscop, Canadian Gotchnical Confrnc, Ottawa, Ontario, Toorman, E.A Sdimntation and slf-wight consolidation: constitutiv quations and numrical modling. Géotchniqu, 9():