Performance of the oxygen limiting cover at the LTA site ... · PDF fileM.R. Julien, Golder Associates Ltd., Montreal, Quebec ABSTRACT Acid mine drainage (AMD) from mine wastes is

September/October 2006 1

Paper 20 Environment

INTRODUCTION

Mining operations produce large amounts ofwaste, such as waste rock and tailings, that must beproperly managed to protect the environment. In thisregard, more attention is required when they containsulphidic minerals such as pyrite and pyrrhotite. Theoxidation of sulphides exposed to atmospheric condi-tions tends to acidify waters which are then moreprone to mobilize metals contained in the rock; thesetwo phenomena generate an acidic leachate calledacid mine drainage (AMD; also called ARD for acidrock drainage; e.g. Ritcey, 1989; SRK, 1989; Aubertinet al., 2002).

A number of closure schemes have been pro-posed to inhibit or control the formation of AMDfrom sulphidic mine wastes. Many of the proposedoptions aim at limiting oxygen ingress through theuse of oxygen barriers to limit sulphide oxidation. Forinstance, it is possible to use a water cover to reducethe availability of oxygen to the underlying minewaste (e.g. Fraser and Robertson, 1994; Amyot andVézina, 1997; Simms et al., 2001). An oxygen barriercan also be created by placing a cover containing oxy-gen-consuming materials such as wood waste, strawmulch, or other organic residues (e.g. Tremblay, 1994;Tassé et al., 1997; Cabral et al., 2000). Another effec-

Performance of the oxygen limiting cover at theLTA site, Malartic, QuebecB. Bussière, A. Maqsoud, Université du Québec en Abitibi-Témiscamingue, Rouyn-Noranda, Quebec,M. Aubertin, École Polytechnique, Montreal, Quebec,J. Martschuk, J. McMullen, Barrick Gold Corp., Toronto, Ontario, and M.R. Julien, Golder Associates Ltd., Montreal, Quebec

ABSTRACT Acid mine drainage (AMD) from mine wastes is one of the most challenging environ-mental problems currently facing the mining industry. When mining wastes have the potential toproduce AMD, the effluents produced have to be managed to meet existing regulations on waterquality. The LTA impoundment (property of Barrick Gold Corp.) located near Malartic, Quebec, con-tains sulphide tailings that have the potential to generate an acidic leachate. The closure optionselected by Barrick was to construct a cover with capillary barrier effects (CCBE), in which the mois-ture-retaining layer is made of non-acid-generating tailings. Since its construction in 1996, the coverhas been monitored to evaluate its performance. For that purpose, various parameters have beenmeasured, including the volumetric water content and matric suction in the different layers of thecover. This paper presents some of the main results from the monitoring program that started eightyears ago. The results show that the cover is able to maintain a high degree of saturation in its mois-ture-retaining layer, acting as an oxygen barrier. The actual performance of the CCBE on the flat sur-face of the 60 ha site is better than expected at the design stage. The only concern with the LTAcover is the effect of slopes that can reduce the performance of the CCBE during dry periods. How-ever, these effects appear only for short periods of time, and are limited to very localized areas, sothat they do not significantly affect the overall performance of the CCBE and its ability to limit theproduction of AMD.

KEYWORDS Acid mine drainage, mine site rehabilitation, cover with capillary barrier effects, LTA site

tive way to limit oxygen migration where humid cli-matic conditions prevail is based on the use of a coverwith capillary barrier effects (CCBE). This type of bar-rier relies on high moisture retention in one of its mul-tiple layers to limit the oxygen flux (e.g. Rasmusonand Erikson, 1986; Collin, 1987; Nicholson et al.,1989; Yanful and St-Arnaud, 1991; Fayer et al., 1992;Aubertin et al., 1993, 1995; Aachib et al. 1993).

The LTA site tailings impoundment contains sul-phides (about 6%, mainly pyrite) and is categorizedas potentially acid generating. Static tests performedon LTA tailings showed a net acid producing potentialof about 200 kg CaCO3/t (Ricard et al., 1997b). Theoxygen-consumption test on the LTA site before therehabilitation showed an oxygen consumptionbetween 228 and 836 moles of O2/m2/year (averagevalue of 517 moles of O2/m2/year; Golder Associates,1999); such high oxygen-consumption rates corre-spond to a reactive material prone to produce AMD ifno preventive measures are taken (e.g. Tibble andNicholson, 1997). To identify the best closure optionfor this site, extensive studies that included economi-cal and geochemical aspects were performed onceoperations were curtailed (SENES, 1995; McMullen etal., 1997; Ricard et al., 1997a, 1997b). Based onthese studies, the site owner (Barrick Gold Corp.)decided to construct a CCBE as the main component

of the rehabilitation approach for the LTA tailingsimpoundment. The geochemical study (SENES, 1995)stipulated that a degree of saturation (Sr) in the mois-ture-retaining layer of 85% should sufficiently reduceoxygen ingress in the long term to properly controlAMD production from the sulphide tailings. The geo-chemical modelling (based on effective oxygen diffu-sion coefficient of 10-8 m2/s) predicted that the finaleffluent would have a near neutral pH and a very lowconcentration of Cu and Zn (<1 mg/L) (SENES, 1995).More details on the selection of the best cost-effec-tive closure plan can be found in McMullen et al.(1997) and Ricard et al. (1997a).

The main objective of this paper is to presentdata that provides insight into the performance of theLTA multi-layered cover system, emphasising thewater distribution in the different layers of the CCBEsince its construction in 1995-1996. The cover con-structed at the LTA site is one of the first cover withcapillary barrier effects constructed in a humid climateon an acid-generating tailings impoundment.Another interesting aspect of the LTA case is the useof non acid-generating tailings as moisture-retaininglayer in the CCBE, a concept that was proposed 15years ago (Aubertin and Chapuis, 1991). To theauthors’ knowledge, this was the first attempt to re-use tailings as a construction material in a large-scaleengineered layered system. The first parts of thepaper present the main concepts behind the CCBEtechnology, followed by a brief description of the LTAcover and its instrumentation. The authors then pres-ent the hydraulic behaviour of the CCBE based onvolumetric water content (θ) and suction (ψ) meas-urements at strategic locations, after eight years of

monitoring. This is followed by a discussion and con-clusion on the performance of the CCBE and on itsability to limit AMD production at the LTA site.

COVER WITH CAPILLARY BARRIER EFFECTS:BASIC CONCEPTS

The main role of a CCBE constructed to reducethe production of AMD in a humid climate is to limitthe oxygen diffusion flux reaching the reactive miner-als. To do so, the cover must maintain a high degreeof saturation in one (or more) of its layers. The diffu-sion of gas through a nearly saturated porous media(such as a fine-grained soil or tailings) can be lowenough to limit the influx of oxygen from the atmos-phere to the sulphidic mine wastes, hence reducingthe rate of oxidation to a negligible value (e.g. Ras-muson and Erikson, 1986; Nicholson et al., 1989;Aachib et al., 1993).

The effectiveness of this type of cover is depend-ent upon a phenomenon called the capillary barriereffect. This effect is present when a fine-grainedmaterial is placed over a coarser one above the watertable. The two materials have different hydrogeologi-cal properties because of their different textures. Inthe desaturation stage following a water inflow, thefine-grained material layer will retain water more eas-ily than the coarse material layer because of itssmaller interstitial pores. As the coarse materialdrains, the presence of gas in its pore space reducesthe interconnectivity of the water-filled voids reducingits hydraulic conductivity (k). This reduction of k in thecoarse layer impedes vertical water flow from thefine-grained material above it, and it also helps to

control the magnitude of the suction(negative pore pressure) at the inter-face between the two materials for aperiod of time (capillary barrier effectsare transient phenomena that dependon materials properties). Conse-quently, the fine-grained material canremain almost fully saturated evenduring prolonged dry periods. Moredetails about capillary barrier effectsand CCBE can be found in the litera-ture (e.g. Rasmuson and Erikson,1986; Nicholson et al., 1989; Akin-dunni et al., 1991; Morel-Seytoux,1992; Aachib et al., 1993; Aubertin etal., 1995, 1999; Aachib, 1997; O’Kaneet al., 1998; Bussière, 1999; Bussièreet al., 2003).

Covers with capillary barriereffects (CCBE) usually contain three tofive layers, made of different materials.Each layer has one (or more) func-tion(s). Figure 1 is a schematic illustra-tion of a CCBE. For a cover on tailings,the bottom layer is made of a fairlycoarse-grained material that acts asboth a mechanical support and a cap-illary break. The fine-grained material,

2 CIM Bulletin ■ Vol. 99, N° 1096

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Fig. 1. Typical configuration of a CCBE used to limit the production of AMD (Aubertin et al., 1995).

utilized as the moisture-retaining layer, is placed uponthe first layer to create the capillary barrier effect. Thismaterial should have a relatively low saturatedhydraulic conductivity ksat compared to the capillarybreak layer (i.e. ksat two to three orders of magnitudelower than the material below it) and a significantwater retention capacity. Another coarse-grainedmaterial is placed upon the moisture-retaining layerto prevent water loss by evaporation and to promotelateral drainage. The other two layers (protection andsurface layers) are protective layers against erosionand bio-intrusion for the CCBE. The number of layerscan vary depending on the situation; some layers canbe combined and play more than one function.

A number of studies performed in Canada overthe last decade or so, in the laboratory (e.g. Aubertinet al., 1993, 1995; Aachib et al., 1993, 1994, 1998;Yanful and Aubé, 1993, Bussière et al., 1997a,1997b, 1998) and at an intermediatescale in the field (Yanful and St-Arnaud, 1991; Yanful et al., 1993;Aubertin et al., 1997b, 1999; Bus-sière and Aubertin, 1999; see alsoMEND, 2001), have confirmed theability of a well-designed CCBE torestrict oxygen flux and limit the pro-duction of AMD. However, therehave been very few applications of the CCBE tech-nology on a large scale; the LTA site was one of thefirst potentially AMD-generating sites to be rehabili-tated with that type of oxygen barrier cover.

THE LTA COVER CHARACTERISTICS

LOCATION AND CONFIGURATION

The LTA tailings pond is located in Fournière andDubuisson townships, approximately 8.5 km south-east of Malartic, Québec. The LTA tailings site coversan area of approximately 60 ha andcontains about 12 m of sulphide tail-ings placed over 5 m of non-acid-gen-erating tailings (McMullen et al., 1997;Ricard et al., 1997a). This site is con-fined by the natural topograpgy in thesouth and by three tailings dams (Fig.2). The site was rehabilitated in 1995-1996. The rehabilitation scenario forthe tailings impoundment consisted inthe construction of a multi-layeredcover, designed to act as a CCBE toreduce the oxygen flux to the reactivetailings and, consequently, the produc-tion of AMD.

The LTA cover is made of three lay-ers. From the bottom to the top (Fig. 2):• a 0.5 m layer of sand used as sup-

port and a capillary break layer;• a moisture-retaining layer with a

thickness of 0.8 m, made of a non-acid-generating tailings (MRN tail-ings) taken from the nearby

Malartic Goldfield property (a site that belongs tothe Quebec Natural Resources Ministry); and

• a 0.3 m layer of sand and gravel on top to createthe drainage and protective layer against erosion,evaporation, and bio-intrusions.

MATERIALS PROPERTIES

The geotechnical properties of the MRN tailingsand of the sand used in the covers were obtained fol-lowing an extensive lab testing program performed atÉcole Polytechnique de Montréal. The main propertiesare summarized in Table 1 (more details are includedin Golder Associates, 1996; Firlotte, 1996). Propertiesof the sand and gravel are almost similar to those ofthe sand (Ricard et al., 1997a). The MRN tailings havea grain-size distribution typical of hard rock tailings(Aubertin et al., 1995, 1996, 1998) with a percentage

passing 0.08 mm between 65% and 90% and a D10(diameter of particles at 10% passing) of about0.005 mm. The sand has a much lower percentage offine particles (less than 5% smaller than 0.08 mm),with few cobbles (no particles with a diameter greaterthan 15 cm). The saturated hydraulic conductivitiesksat of these materials are estimated to be 5x10-5 cm/sfor the MRN tailings and 1.2x10-1 cm/s for the sand,which can be considered as typical for such grain size(e.g. Mbonimpa et al., 2002a). The porosity, n, of themoisture-retaining layer (MRN tailings) measured inthe field varied between 0.36 and 0.50. This variabil-

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Fig. 2. Aerial picture of the LTA site before construction of the CCBE and cover configuration.

Table 1. Main properties of the materials used in the LTA cover

n D10 (mm) D60 (mm) D50 (mm) ψa (cm of water) ksat (cm/s)

MRN tailings 0.36 to 0.5 0.003 to 0.0075 0.037 to 0.085 0.030 to 0.065 200 to 275 5.0x10-5

(n = 0.44)

Sand 0.34 to 0.36 0.120 to 0.600 1 to 15 0.7 to 8 20 to 40 1.2x10-1

(n = 0.35)


ity of in situ posority measurements is mainly due todifferent factors such as the variability of the grain-size distribution of the MRN tailings and their inialwater content, and also the varying construction con-ditions during the winter time (Firlotte, 1996; Ricardet al., 1997a, 1997b, 1999). Representative water-retention curves measured in the laboratory for thesand and the MRN tailings are presented in Figure 3;these are typical of those measured on similar media(e.g. Aubertin et al., 1998, 2003). The air entry valueψa obtained from the water retention curves meas-ured in the lab is between 200 to 275 cm of water forthe MRN tailings (about 20 to 27 kPa), depending onthe porosity and the grain size (McMullen et al.,1997; Ricard et al., 1997a). The ψa of the sand meas-ured in the lab is between 20 to 40 cm of water(Ricard et al., 1997a).

The MRN tailings contain a small quantity of sul-phur (on average 1.82%) but are not considered acidgenerating based on their high neutralization poten-tial; the net neutralization potential is estimated at100 kg CaCO3/t (SENES, 1995; Ricard et al., 1997a).

A natural silt available near the site with higherwater retention properties was also used as the mois-ture-retaining layer in a portion of the slope of theWest dyke. The main reason for using this materialwas to reduce construction costs (the transportationdistance was shorter).

In the following, only the results obtained onareas covered with the MRN tailings are presentedand discussed. For more details about materials prop-erties, and the design and construction of the CCBE,the reader is referred to McMullen et al., (1997),Ricard et al., (1997a, 1999), Golder Associates(1999), and Aubertin et al., (2002, Chapter 13).

INSTRUMENTATION

The two main parameters monitored for the LTACCBE were volumetric water content and suction(negative pore pressure). The time domain reflectom-

etry (TDR) technique (e.g. Topp et al., 1980) wasretained to measure the volumetric water con-tent; the Trase System I of Soilmoisture Equip-ment Corp. was used. This type of TDR has alsobeen successfully used for laboratory measure-ments (e.g. Aubertin et al., 1995, 1999; Aachibet al., 1998) and for field measurements to eval-uate cover performance on different projects(e.g. Yanful and St-Arnault, 1991; Aubertin etal., 1997a; Benson and Bosscher, 1999; Dage-nais et al., 2001; Bussière et al., 2004). Themeasurement error on θ given by the manufac-turer is typically lower than 0.025. The cover isalso equipped with Watermark sensors to meas-ure matric suction in the moisture-retaininglayer. The Watermark sensor (granular matrixsensor) used here is based on an indirect, cali-brated method of measuring negative pore pres-sure in particulate media. It is an electricalresistance sensor, read by a hand-held meterwhich converts the electric resistance reading to

a calibrated reading of suction. The main advantagesof this sensor include: no periodic maintenance; notsubject to damage by freezing temperatures; and thematrix does not dissolve in the soil water (unlike gyp-sum block). The Watermark sensor gives reliableresults typically for suction between 20 and 800 cmof water (2 to 80 kPa; e.g. Shock et al., 1999, 2002;Aubertin et al., 2002). Each Watermark sensor waslocated close to a TDR probe to evaluate in situ waterretention response. These sensors were also used inother studies to evaluate hydraulic behaviour of CCBE(e.g. Aubertin et al., 1999; Dagenais et al., 2001; Bus-sière et al., 2004). The location of the TDR probes andWatermark sensors is presented in Figure 4. TheWatermark sensor W2 and the TDR probe T3 arelocated at the same elevation (~15 cm in the mois-ture-retaining layer above the interface of the mois-ture-retaining layer and the bottom sand layer), whilethe watermark sensor W4 and the TDR probe T5 are

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Fig. 3. Typical water retention curves measured in the laboratory for the sand and the MRN tailings at differentporosity n (modified from Golder Associates, 1996).

Fig. 4. Location of TDR probes and Watermark sensors in the differentmonitoring stations.


installed in the moisture-retaining layer at 15 cmbelow the interface between the moisture-retainingand top layers. A TDR probe was also installed in thebottom sand layer, at 15 cm from the interface withthe moisture-retaining layer. More details about theinstrumentation can be found in Golder Associates(1996).

Various instrumented monitoring stations wereinstalled on the site to evaluate the hydraulic behav-iour of the CCBE; emphasis was placed on assessingthe response of the moisture-retaining layer. Somestations are equipped with TDR probes and Water-mark sensors while others are only equipped withTDR probes. The first series of instruments wereinstalled in the cover in 1996 (Stations CS 96 and PS96 in Figure 5). A few other instruments were alsoinstalled elsewhere on the site after 1996. In the pres-ent study, results of only a few representative stationsare presented. For more details, the interested readeris referred to MEND Reports 2.22.4a and 2.22.4b(Golder Associates, 1996, 1999).

MONITORING RESULTS

This section presents representative resultsrelated to the CCBE hydraulic behaviour at the LTAsite. The evolution of volumetric water content meas-urements in the CCBE, for representative stations, isshown together with the corresponding suction val-ues, when applicable [more detailed results can befound in Golder Associates (1996, 1999), and in otherunpublished reports]. These results are shown for thestations located on both the nearly flat and slopingareas of the site. It can be recalled here that thedesign objective was to reduce oxygen migrationthrough the CCBE based on a degree of saturationobjective Sr ≥ 85% (for a porosity n of 0.44, this cor-responds to a volumetric water content θ of about37%) in the moisture-retaining layer.

RESULTS OBTAINED ON THE FLAT AREA

Figure 6 shows the evolution ofvolumetric water content measure-ments since 1996 at Station CS 96-1(see Figure 5 for the location). In thisfigure, CS 96-1-T1 corresponds to thevolumetric water content in the bottomsand (capillary break) layer while CS 96-1-T3 and CS 96-1-T5 correspondto volumetric water content measuredat the bottom and at the top of themoisture-retaining layer, respectively.The θ values measured by TDR probesCS 96-1-T3 and CS 96-1-T5 are usuallygreater than 37% (corresponding to Srvalues greater than 85% for an averageporosity n measured on the site of 0.44)while the ones measured by CS 96-1-T1are typically between 12% and 18%(corresponding to Sr values between34% and 51% for a porosity n of 0.35).

The fact that the degrees of saturation in the bottomsand layer (CS 96-1-T1) are much lower than the onesin the moisture-retaining layer clearly indicates thatthe desired capillary barrier effect is well developed inthe cover. Results from other stations on the flat area(such as Station CS 96-3, not shown here) show awater distribution similar to the one presented in Fig-ure 6. Since 1996, the θ measurements in the capil-lary break layer generally vary between 15% and25%, while measurements in the moisture-retaininglayer vary between 37% and 45% for the bottom

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Fig. 5. Schematic plane view of the LTA site with the main monitoringstations.

Fig. 6. Evolution of volumetric water content measured at CS 96-1 station (T1 placed in the capillary break layer, T3 and T5are in the MRN tailings).


portion of the layer (TDR probe CS 96-3-T3) andbetween 33% and 39% in the upper portion of thelayer (TDR probe CS 96-3-T5).

Results from Stations CS 96-10, CS 96-8, CS 96-7, CS 96-6, and CS 96-5 indicate that there isa high water content in the moisture-retaining layerand in the capillary break layer since the constructionof the cover in 1996. An example of measured results(θ vs time) for these stations is presented in Figure 7for Station CS 96-5 (see Figure 5 for the location). Inthis figure, CS 96-5-T1 corresponds to the measure-ments in the capillary break layer (bottom sand layer)and CS 96-5-T3 is associated to θ measurements nearthe base of the moisture-retaining layer. Figure 7shows a slightly lower θ in the moisture-retaininglayer during the first year but the measurementsstayed near 40% (for the average in situ porosity n of0.44, this θ value corresponds to a degree of satura-

tion Sr of 90%). The somewhat smaller θ valuesobserved during the first year (on almost the entiresite) can be attributed to the fact that the degree ofsaturation was relatively low during material place-ment. It took a year (with a spring snow melt) torecharge the layers. It is also possible that some of thegeotechnical properties of the moisture-retaininglayer changed during the first season. Nonetheless,the results indicate that the oxygen flux towards thereactive tailings is reduced significantly for all the sta-tions on the flat area, due to the high degree of sat-uration in part(s) of the cover and, in some cases, inthe sulphidic tailings (when the phreatic surface isnear the surface).

At Station CS 96-5, the ψ values measured withthe Watermark sensor, since 1996, were between 0and 130 cm of water (or 0 to 13 kPa) near probe CS 96-5-T3. Thus, these suctions remain well under

the air entry value (ψa between 200and 275 cm of water; see Table 1and Figure 3) of the MRN tailings.This explains the high θ values meas-ured in this material. A portion ofthe in situ water retention curve(WRC) is obtained by plotting simul-taneous measurements of ψ and θ,as shown in Figure 8; it also con-firms that there was no evidence ofdesaturation for the moisture-retaining layer (Fig. 8).

In summary, two differenthydraulic behaviours were observedon the nearly flat area of the LTAsite: 1) capillary barrier effects areactive in the cover when the watertable is deep; and 2) the cover isnearly saturated by capillary risewhen the phreatic surface is close toor in the CCBE. In both situations,the volumetric water content meas-urements in the moisture-retaininglayer are close to those at saturation(and usually greater than the designobjective of 37%, based on a poros-ity of 0.44).

RESULTS OBTAINED ON THESLOPE

A small aerial portion of thecover constructed on the LTA site islocated on the sloping side of thetailings dykes. Previous studiesshowed that slopes influence thehydraulic behaviour of CCBE (e.g.Ross, 1990; Miyazaki, 1993; Stor-mont, 1996; Aubertin et al., 1997a;Bussière, 1999; Bussière et al.,2003). To evaluate the effect of theslopes on the behaviour and per-formance of the LTA cover, monitor-ing stations were installed along the


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Fig. 8. Measured values of the in situ volumetric water content and suction in the MRN tailings cover layer (CS 96-5 station).

Fig. 7. Evolution of volumetric water content measured at CS 96-5 station (T1 placed in the capillary break layer and T3 in themoisture-retaining layer).

inclined areas (Fig. 5). To illustratehow slope influences this behaviour,two stations are chosen here—thefirst one is located near the top ofthe southeast slope (PS 96-1) andthe second is located downslope (PS96-3).

STATION PS 96-1 LOCATED NEARTHE TOP

Station PS 96-1 is located nearthe upper part of the southeast dyke(Fig. 5). The PS 96-1-T3 TDR probewas defective and only the resultsfrom the PS 96-1-T5 probe locatedin the moisture-retaining layer, nearthe interface with the upper sandand gravel layer, is shown here. Ascan be seen in Figure 9, the θ valuesusually vary between 30% and 40%since 1996, except for the measure-ments done during August and earlySeptember of 2002 when θ droppedto approximately 24.5%. This low θvalue is attributed to the longdrought period observed in theAbitibi-Témiscamingue region atthat time. However, this situationquickly normalized after it rained inSeptember and the θ measurementsreached approximately 30% beforethe winter. Somewhat similarhydraulic behaviour was equallyobserved at Stations PS 96-6 and PS96-7.

The hydraulic behaviour of thecover at Station PS 96-1 can also berepresented by a portion of the insitu WRC. Figure 10 shows that thedrop of θ is associated to an increasein suction in the moisture-retaininglayer. The higher suctions measuredexceeded 300 cm of water (valuesgreater than the laboratory ψa forthe MRN Tailings); such suctions would thus induce apartial drainage of the moisture-retaining layer. Thesuction and volumetric water content measurementswere fitted with the well known van Genuchten(1980) equation. According to this best fitted curve,the in situ ψa is close to about 100 and 150 cm ofwater, which is lower than the laboratory ψa; Thislower ψa could be due to different properties (coarsergrain size distribution and/or greater porosity) of theMRN tailings at this location.

These results (and others not shown here) confirmthat cover inclination (and difference in elevation) caninduce a temporary desaturation of the moisture-retaining layer at some locations, particularly near thetop of the slope. This behaviour was expected sincethe design stage, as the numerical analysis performedon the LTA cover had predicted a temporary uphill

desaturation. However, in this particular case, thedesaturation near the top of the slope is limited to asmall portion of the cover (estimated at less than 5%of the total covered area) and it appears only for shortperiods of time. Since the construction of the cover,slope effect has not been deemed detrimental to theoverall LTA cover performance.

STATION PS 96-3 LOCATED DOWNSLOPE

Station PS 96-3 is located close to the bottom ofthe southeast dyke (Fig. 5) along the same line as stationPS 96-1. The evolution of θ measurements since 1996given by the TDR probes located near the base of themoisture-retaining layer is shown in Figure 11. The volu-metric water content is usually above 40%, which cor-responds to a Sr value of more than 90% (greater than

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Fig. 9. Evolution of of the volumetric moisture content near the top of the slope in the water-retaining layer for PS 96-1 station.


Fig. 10. In situ measurements and fitted [using the van Genuchten (1980)] model water retention curves for the MRN tailings atStation PS 96-1.

the design criteria of 85%). Even the dry periodobserved in 2002 did not seem to affect the θ valuesmeasured in the moisture-retaining layer at this location.

Contrary to Station PS 96-1, the in situ portion ofthe WRC (Fig. 12) does not show any significantdesaturation of the moisture-retaining layer. This islargely due to the lower suctions measured in theMRN tailings, which were usually lower than 100 cmof water. These values are below the ψa of the MRNtailings measured in the lab and in the field.

SUMMARY AND CONCLUSION

The LTA site has been continuously monitoredsince its construction in 1996. One of the key featuresof the program has been the evaluation of the CCBEhydraulic behaviour by measuring the volumetricwater content and soil matric suction. The mainresults from the eight-year monitoring program are:• Overall, the cover is performing above the design

criteria. For instance, measurements indicate thatthe degree of saturation in the moisture-retaininglayer remains above 85%.

• The non acid-generating tailings from the nearbyMalartic Goldfield property (MRN tailings) have theappropriate hydrogeotechnical properties forbeing used as a moisture-retaining layer in a CCBE.

• It is more difficult to maintain the desired volumetricwater content at all times in the moisture-retaininglayer along the upper parts of a slope, and this pos-sible desaturation can influence the performance ofthe cover. These upper parts are more affected bythe desaturation process than the lower portion,especially during prolonged dry periods; however,the volumetric water content increases rapidly whenprecipitation occurs. In the case of the LTA site, it isnot necessary to modify the cover as its overall per-formance to achieve the original objectives are met.

• In the south section of the site, the phreatic sur-face is located close to or in the cover, whichexplains the high θ values in the capillary breaklayer and moisture-retaining layer.

• In the areas where the phreatic surface is locatedwell below the cover, the capillary barrier effectsare active.

• This case study shows the importance of analyzingseparately, at the design stage, theflat and inclined portions of a CCBE(as was done for the LTA coverdesign). The inclination influenceswater distribution in the CCBE,which can affect its ability to limitoxygen migration. Ignoring thiseffect could lead to misleadingassumptions and inadequate coverconfiguration.

The LTA site is one of the firstmine tailings impoundment locatedin a humid climate that has beenrehabilitated using a cover withcapillary barrier effects aimed atlimiting oxygen migration. Resultsshown here confirm that the overallperformance of the CCBE on the 60ha site exceeds expectations fromthe design stage. More work ispresently being done to evaluatethe performance of the LTA cover byintegrating the influence of theslightly reactive MRN tailings whichmay have a positive impact on itsability to limit gas diffusion (Bus-sière et al., 2002; Mbonimpa et al.,2003). Other works are also beingperformed using the LTA site as ref-erence case, such as long-term oxy-gen consumption tests (Mbonimpaet al., 2002b), in situ tests to evalu-ate the impact of hydraulic break tolimit slope desaturation (Bussière etal., 2000), and the use of numericalhydro-geochemical models to pre-dict the performance of CCBEplaced on reactive tailings (Molsonet al., 2004).

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Fig. 12. In situ measurements of the water content and suction, in the MRN tailings layer, at station PS 96-3.

Fig. 11. Evolution of the volumetric moisture content near the bottom of slope, measured the water-retaining layer of the PS 96-3 station.



ACKNOWLEDGMENT

The authors would like to thank Golder Associ-ates and Barrick Gold Corp. for their collaborationduring the project. The authors also thankfullyacknowledge the financial support of the participantsof the industrial NSERC Polytechnique-UQAT Chair onEnvironment and Mine Wastes Management.

Paper reviewed and approved for publication by theEnvironmental Society of CIM.

Bruno Bussière is co-holder of the Industrial NSERCPolytechnique-UQAT Chair in Environment and Mine WastesManagement and the holder of the Canada Research Chair onRehabilitation of Abandoned Mine Sites. His teaching andresearch activities at UQAT focus on constitutive andnumerical modelling of unsaturated flow, tailingscharacterization and backfill behaviour, environmentaldesulphurization, and reclamation methods for acid-generating mine sites.

Abdelkabir Maqsoud is a research assistant for the IndustrialNSERC Polytechnique-UQAT Chair in Environment and MineWastes Management. His research activities at UQAT are mainlyon unsaturated water flow through porous media includinghysteresis effects, monitoring and evaluation of covers used to

limit AMD, and performance improvement of inclined coversusing suction breaks.

Michel Aubertin is a professor in the Department of Civil,Geological, and Mining Engineering at École Polytechnique. Healso holds the Industrial NSERC Polytechnique-UQAT Chair onEnvironment and Mine Wastes Management. Dr. Aubertin is aFellow of the Engineering Institute of Canada (FEIC) and of theCanadian Academy of Engineers (FCAE). He is currently chairingthe Geotechnical Research Board of the Canadian GeotechnicalSociety. His main expertise lies in the area of geotechnical miningand environmental waste management.

John Martschuk is the director, environmental services, forBarrick Gold Corporation.

Jacques McMullen is the corporate head, metallurgy andprocess development, Barrick Gold, based in Toronto. In thiscapacity, with his team, technical support is provided to all ofBarrick’s operating facilities and to the business development fromdue diligence to the design and developments of new processingfacilities. Although metallurgy is the main area of expertise,environmental performance within the entire life cycle of theprojects also draws significant focus.

Michel R. Julien is a principal in Golder Associates Ltd. He isalso currently regional director on the board of directors of theCanadian company. Professionally, Dr. Julien has been mostlyinvolved on geo-environmental aspects of mine wastemanagement for projects in Canada and overseas. He iscurrently the senior leader of the geo-engineering group in theMontreal office.

Environment

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Environment

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Documents

Performance of the oxygen limiting cover at the LTA site ... · PDF fileM.R. Julien, Golder Associates Ltd., Montreal, Quebec ABSTRACT Acid mine drainage (AMD) from mine wastes is