Although the mining industry provides an essentiacl ontributiont o the economyo f several
provincesa crossC anada,i t is recognizedt hat mining operationsc an also be the sourceo f
various detrimental effects for the environment. In that regard, probably the most serious
problem associatedw ith mining activitiesi s acid mine drainage@ MD). SuchA MD can be
generatedw hen sulphuricm inerals( mainly iron sulphidess ucha s pyrite and pyrrhotite) are
oxidized in the presenceo f water. Acid waters may contain high levels of potentially toxic
elements,s ucha s lead, cadmium,m ercurya nda rsenic,a nd this constitutesa serioush azardf or
the local ecosystems.

The control of acidice ffluentsd uring anda fter miningo perationsis oftenv ery costly. Although
water treatmentis an efficient process,u sedw ith successb y minesf or decadesi,t can become
a heavy financial burden on any mining company faced with the prospect of having to control
water quality for tensi f not hundredso f years.

Onep ossiblea lternativeis to control thep roductiono f AMD. This approachis oftenc onsidered
when one wishes to reclaim the land and return it to a productive state. Among the few
techniquesa vailablefo r that purposeis the useo f covers( or caps)i nstalledo ver existingt ailings
ponds. It is also one of the most practical options. However, building such covers is very–
expensive,w ith most estimatesa bove2 00 000 dollars/hectareI.n order to reducet hesec osts,
the authorsh avep roposedt he use of tailings, free of acid generatingm inerals,t o built such
covers. This option would be advantageoufso r various reasons,in cludingt he fact that such
materialsa re often availablec loset o the siteb eingr ehabilitatedI.t is alsoa n interestingo ption
for mills that treat ores free of sulphidess, incet he resultingn on-acidg eneratingta ilingsc ould
be useda s a cover for the acid generatingta ilings. Another applicationi s related to recent
projects where mining companiesh ave useda separationte chniquet o produces ulphide-free
tailings, as these could also be used in a cover system.

If a tailings pond is to be reclaimed, it is advisable to stop production of AMD. It is often
consideredth at a cover that limits the flow of oxygena nd/or wateri s oneo f the mostp ractical
approachesfo r that purpose.A cover is called ccwet*w hen water is used to submerget he
tailings, thus reducing oxygen flux to negligible levels. However, such water covers may be
difficult to build and maintaino ver time, as topographya nd long term stability of the dams
becomek ey factorst o the successo f the project.
Onec ould also makeu se of geosynthetic(sg eomembraneass) an imperviousl ayer in a cover,
but costs and durability are major concerns.
Becauseth erei s a greatd ealo f experiencea vailablefr om the useo f so-called+ coversb uilt
from geologicaml aterials,m ostlyf or industriaal ndm unicipawl astest,h esea re oftenc onsidered
for reclamationp rojectso f acid generatingm illing wastesS. uchc oversa re not free of potential
problemse ither,b ut theyc anr epresentth em ostp racticals olutiona vailableto miningc ompanies
who are reclaiming their tailings impoundments.

To efficiently control the generationo f AMD, it is now generallya cceptedth at a multilayer
barrier system, with each layer having its own specific function, should be used. A schematic
representatioonf a multilayer cover is presentedo n Figure 1.1. The cover layers encountered,
starting with the uppermost, are as follows : a humid layer to support vegetation (layer A,
thicknesst 2 15 cm); a coarsem ateriall ayer containinga large portion of cobblest o prevent
biological intrusions from roots and animals (layer B, t 2 30 cm); a sandy material acting as
a drainagela yer (layer C, t 2 30 cm); a fine grain materiala ctingh erea s a moisturer etention
zone–.(layeDr , t = 50 to 150 cm); and a non-capillaryla yer (layer E, t 2 30 cm) to stop
capillary rise form the underlying reactive tailings (layer F) and to prevent significant moisture—
drainagef rom the fine materiall ayer above( layerD ). Eacha djacenlta yer of the cover should
satisfy filter criteria to prevent particles migration that could affect the integrity of the barrier.
In this multilayers tructure,t he two coarseg rain material ayers( layerC andE ) placeda djacent
to the capillary layer (D) play a double role. First, these materials (typically sands) provide a
flow path for the water to the drainage zones built around the site. Second, the grain size
contrast with the fine grainedm aterialp roducesa large differencei n suctionp ropertiesw hich
minimixesm oistured rainagea nd maintainsth e middlel ayer closet o saturation.I t is essential
that.a saturationr atio of close to 100 % be maintainedin this capillary layer to provide an
efficient barrier to oxygen transport ‘into the underlying reactive tailings.

In this compositec over, thep ossibilitye xistso f usingv ariousm iningw astesfo r thec onstruction
of the different layers. For example, the tailings fine fraction (slimes), obtained by natural
segregationo r by hydrocyclonesc, ould be usedt o build the capillaryl ayer (layer D on Figure
1.1). The coarset ailings tractions( sands)c ould thenb e usedi n layersC and E, dependingo n
their availabilitya nd hydrogeologicapl roperties.L ayer B could includec obblesf ound in the
overburden or waste rock from the mine. Finally, humid layer A could be made with the
excavatedo verburdens oil, with the original topsoil (stackeda nd protected)u seda s the final
.v egetativela yer.

Becauseth e efficiency of such a cover systemd ependso n its effectivenessto reducew ater
infiltration and/or oxygenf lux, the most critical componenits the materialu sedf or layer D.
This experimentasl tudyc oncentrateodn finding lower cost materialsfo r this moisturer etention
layer. Tailingsw ith the correct hydro-geotechnicparl opertiesm ay be used.S amplesre covered
from various sites located in the province of Quebec have been studied as possible candidate
materials.

This report containss ix (6) ChaptersC. hapter1 summa&d above,p resentsa n introductiono n
the overall problem of AMD and the general principals behind the use of covers. Chapter 2 is
a state-of-the-arrte view on cover technologyt hat consideredn ot only mining relatedp rojects
but also other types of waste where covers have been installed, including landfills, industrial
refusep iles or contaminateds oils. Chapter3 reviewst he capillary barrier effectsc reatedi n
layeredc overs.M aterialp ropertiesi,n cludingm ineralogicacl ompositiong, rain size,c ompaction
curves;‘c onsolidationc haracteristicsh, ydraulicc onductivity,m oisturer etentionc urvesa nd the
effective diffusion coefficiento f oxygen, are presentedin Chapter4 . Chapter5 presentst hephysicala
nd numericalm odelingw ork, and the conclusionsfo llow in Chapter6 .

The reader is reminded that this report summa&es nine interim reports containing more than
600 pages already submitted to MEND, which include all the details of the testing program.
Theser eports are availablef rom the MEND Secretariaitn Ottawa.A lso, someo f the more
fnndamentapl ortionso f this researchw ere the subjecto f severalg raduatet hesisa nd internal
reports.

Cover systemsa re usedi n various wastes ite remediationp rojects, and may served ifferent
functions.T hey form an essentiacl omponenitn the overall managemenotf liquid andg asi n and
out of the disposal site. One major reason for building covers is to separate the wastes
(industrial, municipal, mining, etc.) from the surface environment, to limit water infiltration
and/or to control gaz flow from/to the wastes.S ite specificc haracteristicms ustb e considered
for coverd esignt o meett he requirementosf a project.H owever,t herea re someb asicp rinciples
that mustb e understoodb eforeu ndertakinga ny coverd esign.I n that regard, one shouldb e up
to dateo n the enormousa mounto f experiencea nd practicali nformationo n cover.a pplications
disseminateidn the literature,a nd summarizeidn Chapter2 . After presentingth e basicc oncepts
in the use of cover systems,t he authorsd escribed ifferent configurationsi,n cluding materials,
thicknesseasn d functions,o f cover systemsA. dvantageas nd limitationso f the different cover
systemsa re also given.

In compositec over systems,c apillary barrier effectsa re createdw hena coarseg rain material
is‘placedb elow a fine grain material.T he differencein moisturer etentionc urvesa nd hydraulic
conductivityf unctionsb etweenth esem aterialsc reatesc onditionst hat allow the fine materialt o
remainsp ractically saturateda t all time. In Chapter3 , this phenomenais explainedu sing
continuityc onditionsf or pressurea nd flux at the interfaceb etweentw o materials.T he analysis
showst hat capillary barrier effectsa rc favouredb y large contrastsin grain size betweent wo
adjacent materials.

Early in the experimentapl rogram, a generalt estingp rotocol was developedto evaluatet he
efficiency of different materialsa nd configurationsu sed in cover systems.I t includes the
evaluationo f hydro-geotechnicparl opertiesp, hysicaml odelinga ndn umericacl alculationsT. hese
componentsa re presentedin Chapters4 and 5. Resultsa re summarixedb elow.

At the beginning of the project, more than 30 different tailings sites in Quebec (most of which
beingl ocatedi n the Abitibi region)w eres ampledA. fter completinga serieso f prelimiting tests,
includingm ineralogicaal nalysis,g rain sizea nd At&berg limits, five sitesw here selecteda nd
further sampled for more detailed studies; The grain size curve of these tailings are shown in
Figure 4.1. Thesea re representiveo f averageg rain size curvesf or hard rock mine tailings.
Using the Unified Soil ClassificationS ystemt,h esem aterialsa re classifie&ss andys ilts or silty
sands with low plasticity.

Tailings sampledin bulk were homogenti and submittedt o variousl aboratoryt ests. Tables
4.1 to 4.3 presentss omeb asic propertieso f the tailings. Consolidationc haracteristicsw ere
obtainedu singa conventionaol edometear pparatusF. or theset ests,t he densificatione nergyf or
placement of the material was controlled to obtain an initial void ratio e that could be varied
from 0.5 to 1.1. The required densificatione nergy was determinedf rom compactiont ests.

Figure 4.3a shows some typical results. For the different tailings, the observed compression
index C!,v aried from about0 .05 to 0.15 and the coefficiento f consolidationG was foundt o be
between1 03 and la’ cm2/s( seeT able4 .4). The consolidationp ropertieso f the homogenized
tailings are well within the range of what is usually found for similar materials (i.e. sandy silts
or silty sands).

The hydraulicc onductivityk is one of the mosti mportantp ropertieso f any materialu sedi n a
cover system.T o evaluatek , threed ifferent testsw erec arriedo ut on the homogenizedta ilings.
They are : rigid wall permeameterte sts with constanth ead and falling head conditions;
permeabilityt estsi n the oedometecr ells with constanto tal stressa ndv arying water pressure;
and flexible wall permeability tests. These tests were carried out on tailings for different void
ratios. This alloweda n evaluationo f the effect of different factorso n the k value. Among the
dcistingr elationshipse stablishedto quantifyt he influenceo f thesef actors,i t wasf ound that the
Kozeny-Carmane quation( Eq. 4.1) describedt he observedb ehaviorf airly well. Figure 4.4a
showsa correlationb etweent he measureda nd calculatedv aluesf or one of the studiedt ailings.
other relationshipsh avea lso beenu sed.T he practicalityo f sucht ype of relationshipis that it
allowsa n approximatee valuationo f the hydraulicc onductivityo f homogenizetda ilings,a nd its
evolutiona s a, function of the void ratio and for other parametersa s showni n Eq.4.1. The
measureda nd calculatedk valuesa re given for total saturation(S , = 100 %). The k valuesa re
come&d for unsaturatedc onditions,u sing the moisturec haracteristic urve.

The moisturec haracteristicc urve of the homogenizedta ilings, which gives the relationship
between the volumetric moisture content 8 and the negative pore pressure (or suction) $ was
measuredu singa pressurep latea pparatuas nda modifiedT empec ell. Typical resultsa re shown
on Figure 4.6a. The results indicate that typical Air Entry Values (AEV) range from 1.5 to 3.5
m (about1 5 to 35 kPa). The resultsa re well describedb y the van Genuchtenm odel( E!q.4 .5).

The ability to controlo xygent ransporti s amongt he mostc ritical coverc haracteristictsh at play
an importantr ole in the efficiency of the system.I t is consideredth at oxygen flux is usually
controlledb y Fickian type diffusion (Eq. 4.6 to 4.8) andt hat pressurea nd temperatureg radient
effectsa re negligible.I n a Fickianf low, the oxygenf lux is largelyd ependenut pont he effective
diffusion coefficient of oxygen D, which in turn is related to grain size, porosity, tortuosity and
volumetric water content. This latter factor is very important, as the diffusion coefficient in
water is about 10 000 times lower than in air. Unfortunately,t he precisem easuremenotf D,,
asa functiono f the aboven otedp arametersi,s not simple.A speciasl etups howno n Figure4 .7
wasc reatedw ith the helpo f the NorandaT echnologyC enter( NTC). Thev alueso f D,, obtained
by comparing the evolution of oxygen concentration measured and calculated with the
POLIXJTE program, are shown on Figure 4.8a with predictive models (Eq. 4.9 to 4.11). The
resultsa re in fair agreemenwt ith the theoreticael stimates.

The behavioro f the homogenizdt ailingsm aterialsin cover systemsh asa lsob eeni nvestigated
by usingp hysicala nd numericalm odels.T he hydraulicc onditionsin layereds ystemsw as first
studiedu sing a plexiglassd rainagec olumnw ith an internald iametero f 15.5 cm and a height
of 110c m (Figure5 .1). The columni s instrumentedw ith tensiometear ndT DR (“time domain
reflectometry”)p robest o measures uctiona ndv olumetricw aterc ontent,r espectivelya, longi ts
length The column design was based on the ones used at NTC and University of Waterloo for
other cover projects.R esultso f the drainagec olumnt estsa re comparedto resultso btainedo n
individual materialsi n capillary testsa nd to numericalc alculations( Figure5 .3b). The results
are in accordancew ith the project assumptionsa,n d showt hat the fine layer will remainc lose
to saturatione vena fter long periodso f drought.

The efficiencyo f differentc over systemsp lacedo ver sulphideta ilingsw asa lsoe valuatedu sing
plexiglass columns of 1.7m in height. Duplicate columns were prepared for each system, the
first instrumentedw ith TDR probesa ndt hermocouple(sF igure5 .2) andt he secondf ree of any
instruments.T he cover layersp lacedo ver a layer of sulphidec ontainingt ailings (about20 %—-
of iron sulphide) include a sand layer (30 cm in thickness), a fine tailings layer (60 cm in
thickness)a nd a top layer of sand( 10 to 20 cm in thickness).C oncretes andw as usedi n the
covers. The capillary layers consisted of three different sulphide free tailings. The last two
columns had tailings with a small amount of pyrite in the capillary layer. Two smaller columns
(calledr eferencec olumns)w ere alsob uilt with reactivet ailingsw ithout a covera nda re useda s
controlst o evaluateth e relativee ffectivenesosf the covers.I n all the columns,w ater is added
from the top periodically, and the percolating water sampled at the bottom is analyzed for
electric conductivity,p H, sulphatea nd metalc ontents.T heser esultsp rovidei ndicationso f the
possibler eactionsh appeningin the systemT. emperaturem easuremenitns thec olumnsa lsos erve
as indirect evidenceo f chemicalr eactions.T he effectivenesosf the cover systemsis illustrated
by comparingt he parameterfso r the differentc olumns( e.g. Figures5 .4a and5 .4b for pH, and
Figures 5.5a and 5.5b for sulphates)A. lthough somec olumnsh ave showns ome abnormal
behavior,u suallya s a result- of experimentapl roblems( leaks,p reoxydatione, tc.), it is clearly
shownt hat the coversc an effectively preventa cid generationa nd the oxidation of sulphidic
minerals. While looking at the column tests results, the reader should also keep in mind that the
coverc onfigurationsu sedi n the controlc olumnsw eren ot selectedfo r optimizingt he efficiency,
but rathert o verify thep redictivec apabilitieso f thee xperimentaal ndn umericatl oolsd eveloped.

Using the availablei nformation, the,e fficiency of various covers was finally calculated. by
comparing the reduction in oxygen flux, and the results are shown in Figure 5.11. This shows
that if high saturation( Sr 2 90 %) can be maintainedin the cover throughc apillary barrier
effects, then a.one meter layer of tie material sandwichedb etweent wo sand layers will
effectively reduce the oxygen flux to the reactive tailings material by a factor of about 1000 or
more. The resultsa re in accordancew ith calculationsm adeb y other authorsf or naturals oils,
thus showing that tailings can be used effectively as the fine material layer in cover systems.

The resultsp resentedin this report are very encouraginga nd warrant the continuationo f the
researchp rogramu sing more representativceo nditions.F or that purpose,f ield test plots have
been constructedd uring the summero f 1995a nd new column tests were started to further
analyzet he practicalu seo f non reactivet ailingsi n layeredc over systemsto control AMD.