Mining Science and Technology, 3 (1985) 35-49Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

35

MATERIALCONSIDERATIONS IN THE DESIGN OFDOWNSTREAMEMBANKMENTSFOR TAILINGS

IMPOUNDMENTS

Jack A. Caldwell and Adrian Smith

Steffen, Robertson and Kirsten, Denver, CO (U.S.A.)

(Received January 1985; accepted March 1985)

ABSTRACT

Downstream embankments are constructed

to impound tailings when the tailings are fine-grained or toxic, or the environment is wet,seismic, or sensitive. This paper discusses thegeotechnical and hydrogeochemical elements ofdownstream embankments to impound tailings,and illustrates points made by describing twocase histories: the embankments for the CannonMine Project (Washington) and the GreensCreek Project (Alaska).

The major geotechnical elements describedare: the core, the filters, the drains, the shell,the foundations, the reservoir, and the decantfacilities. While many of these are common to

embankments to impound water, the need toimpound tailings imposes different design andconstruction constraints, such as the need forstaged construction, the use of other mine wastesin construction, less permeability (due to thesealing action of the tailings) and considerationof the response of construction materials to thechemicals in water seeping from the impound-ment.

Hydrogeochemical considerations such aschemical attack and chemical precipitation aredescribed and their influence on design consid-ered.

INTRODUCTION

Downstream embankments, while the mostexpensive way to impound tailings or residue,are the best solution when fine-grained,amorphous or toxic wastes are deposited inwet, seismic, or environmentally sensitiveareas. This paper discusses the reasons why a

downstream embankment should be built, ofcoarser tailings or borrowed or quarried soiland rock, in preference to an upstream orcenterline embankment. The basic elements ofan embankment to impound tailings and thehydrogeochemical factors relevant to designare discussed. Case histories of embankmentsto impound tailings are described.

- i _.. ... "".'...". ._.

36

CHOICE OF DOWNSTREAM METHOD

Embankments to impound tailings or wasteresidues may be constructed by the upstream,centerline or downstream method. If thedownstream embankment is constructed

primarily of the coarser tailings the cost is lessthan that of embankments built of borrowedsoil or quarried rock. As shown in Fig. 1, thecost of a downstream embankment comparedto that of an upstream embankment may benine to sixteen times as much, depending onwhether tailings or imported soil and rock areused. A downstream embankment may costthree to nine times as much as a centerlineembankment.

In spite of the greater cost, downstreamembankments are often built to impound tail-ings or mine and industrial residues. The mainreasons for selecting a downstream embank-ment if the coarser tailings themselves can beused in the construction of the embankmentare:

sensitivity of uncompacted tailings toliquefaction;

UNITVOLUME COSTRATIO RATIO

MAX.TOTALCOSTRATIO

UPSTREAM

~.- -.. - "":-:.=::~:-:~ lAILINGS- - - - 1.5 4.53

CENTERLINE

Q

2 126

4 24DOWNSTREAM EMBANKMENT

Fig. 1. Embankment comparison.

seepage control to preclude plpmg tS

required;rate of rise is too high for adequateconsolidation of the total tailings tooccur;seepage control requires an impermea-ble core.

The coarser tailings may not be suitable forembankment construction and a downstreamembankment may be required for these rea-sons:

tailings too fine to be incorporated inan embankment;tailings considered unsuitable to incor-porate in embankment (e.g., uraniumtailings);seismic risk too great to use compactedsandy tailings, if saturated;the impoundment will store largevolumes of water in addition to thetailings;logistics required an embankment builtbefore deposition begins (say of wasterock from open pit stripping).

From the above reasons, it follows that thedisadvantages of downstream embankmentsare the need to borrow soil or quarry rock, theneed for cyclone tailings and the need tobuild what are often complex embankmentcross-sections. The major disadvantage thatresults is higher costs.

Conversely, the advantages of downstreamembankments are that a well-supervised em-bankment may be built which controlsseepage, is stable under adverse loading (suchas those caused by earthquakes, i.e., seismicdisturbance), can impound water and finetoxic tailings or residues and which can bebuilt at once or subsequently according to theavailability of material, plant and money.

ELEMENTS OF THE EMBANKMENT

Civil engineering has a long history of de-signing and building embankments or dams

- - - --- -- ------..-------

to impound water. Those principles and prac-tice, developed from experience and theory,are applicable to the design and layout ofdownstream embankments to impound tail-ings. They are not repeated in this paper, thereader is referred to the standard works.

There are, however, important differencesbetween embankments to impound water andembankments to impound tailings. These dif-ferences are discussed in this paper by way ofa brief description of the essential elements ofa downstream embankment intended to im-pound tailings or residues.

RATE OF CONSTRUCTION OF THE EM-BANKMENT

A completely different philosophical ap-proach dictates the rate at which embank-ments to impound water and tailings are con-structed. A water-impounding embankment isusually built in one construction phase to theultimate crest elevation, and at that elevationa spillway is constructed. The purpose of theembankment is to impound as much water aspossible, and cash flow is best if the dam isfull.

Conversely, an embankment to impoundtailings is required only to contain the tailingsalready produced by the mine. The best cashflow is achieved if no more of the embank-ment is constructed at any time than is re-quired to contain current tailings volumes. Asdescribed in the second case history, it iseconomic to build a number of spillways atthe crest of successive stages of the embank-ment.

THE CORE

A core mayor may not be required in adownstream embankment for tailings im-poundments. If the tailings or residues aretoxic or the impoundment is to hold water in

37

addition to the tailings or residues, a core isrequired. The core may be of natural or syn-thetic material, but must have a low enoughpermeability to preclude the passage of sig-nificant quantities of water. No core is everentirely impermeable; some seepage will al-ways occur. The core should be designed tolimit seepage to a value where flow quantitiesmay be controlled in seepage dams, returnedto the impoundment, or treated for release.

If the tailings are toxic, the specificationsfor the core may be more stringent than for awater dam. For the average dam, loss of waterthrough the core is satisfactory provided suchseepage is controlled to prevent distress to theembankment and provided the quantities lostdo not impair the storage potential of thedam. This may be different for a tailingsimpoundment. Cores are built not so much toprevent excessive loss from the reservoir (asfor a dam), as to preclude escape of poten-tially contaminated fluid to the environment.There are cases where the seepage from theimpoundment, through both the embankmentand the liner, of potentially contaminatedseepage are small. In those cases, it may bepossible to show that the impact on thegroundwater downstream of the impound-ment is small due to attenuation by thefoundation soils and rocks.

A well-designed impoundment usually pro-vides for deposition of the tailings in a waythat they augment the core (Fig. 2): thus, eventhough the geometry may be the same, theseepage gradient through the core of the tail-ings embankment will be less than thatthrough the core of an embankment impound-ing water. Fine tailings are usually of lowpermeability: if placed correctly upstream of

Fig. 2. Embankment with u/s tailings discharge.

Mining Science and Technology, 3 (1985) 35-49Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

35

MATERIALCONSIDERATIONS IN THE DESIGN OFDOWNSTREAMEMBANKMENTSFOR TAILINGS

IMPOUNDMENTS

Jack A. Caldwell and Adrian Smith

Steffen, Robertson and Kirsten, Denver, CO (U.S.A.)

(Received January 1985; accepted March 1985)

ABSTRACT

Downstream embankments are constructed

to impound tailings when the tailings are fine-grained or toxic, or the environment is wet,seismic, or sensitive. This paper discusses the

geotechnical and hydrogeochemical elements ofdownstream embankments to impound tailings,and illustrates points made by describing twocase histories: the embankments for the CannonMine Project (Washington) and the GreensCreek Project (Alaska).

The major geotechnical elements describedare: the core, the filters, the drains, the shell,the foundations, the reservoir, and the decantfacilities. While many of these are common to

embankments to impound water, the need to

impound tailings imposes different design andconstruction constraints, such as the need for

staged construction, the use of other mine wastesin construction, less permeability (due to thesealing action of the tailings) and considerationof the response of construction materials to thechemicals in water seeping from the impound-ment.

Hydrogeochemical considerations such aschemical attack and chemical precipitation aredescribed and their influence on design consid-ered.

INTRODUCTION

Downstream embankments, while the mostexpensive way to impound tailings or residue,are the best solution when fine-grained,amorphous or toxic wastes are deposited inwet, seismic, or environmentally sensitiveareas. This paper discusses the reasons why a

downstream embankment should be built, ofcoarser tailings or borrowed or quarried soiland rock, in preference to an upstream orcenterline embankment. The basic elements ofan embankment to impound tailings and thehydrogeochemical factors relevant to designare discussed. Case histories of embankmentsto impound tailings are described.

36

CHOICE OF DOWNSTREAM METHOD

Embankments to impound tailings or wasteresidues may be constructed by the upstream,centerline or downstream method. If thedownstream embankment is constructedprimarily of the coarser tailings the cost is lessthan that of embankments built of borrowedsoil or quarried rock. As shown in Fig. 1, thecost of a downstream embankment comparedto that of an upstream embankment may benine to sixteen times as much, depending onwhether tailings or imported soil and rock areused. A downstream embankment may costthree to nine times as much as a centerlineembankment.

In spite of the greater cost, downstreamembankments are often built to impound tail-ings or mine and industrial residues. The mainreasons for selecting a downstream embank-ment if the coarser tailings themselves can beused in the construction of the embankmentare:

sensitivity of uncompacted tailings toliquefaction;

UNITVOLUME COSTRATIO RATIO

MAX.TOTALCOSTRATIO

UPSTREAM

~.- ,,-::=::~:"::::: lhlLtNc.S- - - 3 1.5 4.5

CENTERLINE

6 2 12

~6 4 24DOWNSTREAM EMBANKMENT

Fig. 1. Embankment comparison.

---

seepage control to preclude pIpmg ISrequired;rate of rise is too high for adequateconsolidation of the total tailings tooccur;seepage control requires an impermea-ble core.

The coarser tailings may not be suitable forembankment construction and a downstreamembankment may be required for these rea-sons:

tailings too fine to be incorporated inan embankment;tailings considered unsuitable to incor-porate in embankment (e.g., uraniumtailings) ;seismic risk too great to use compactedsandy tailings, if saturated;the impoundment will store largevolumes of water in addition to thetailings;logistics required an embankment builtbefore deposition begins (say of wasterock from open pit stripping).

From the above reasons, it follows that thedisadvantages of downstream embankmentsare the need to borrow soil or quarry rock, theneed for cyclone tailings and the need tobuild what are often complex embankmentcross-sections. The major disadvantage thatresults is higher costs.

Conversely, the advantages of downstreamembankments are that a well-supervised em-bankment may be built which controlsseepage, is stable under adverse loading (suchas those caused by earthquakes, i.e., seismicdisturbance), can impound water and finetoxic tailings or residues and which can bebuilt at once or subsequently according to theavailability of material, plant and money.

ELEMENTS OF THE EMBANKMENT

Civil engineering has a long history of de-signing and building embankments or dams

- --

to impound water. Those principles and prac-tice, developed from experience and theory,are applicable to the design and layout ofdownstream embankments to impound tail-ings. They are not repeated in this paper, thereader is referred to the standard works.

There are, however, important differencesbetween embankments to impound water andembankments to impound tailings. These dif-ferences are discussed in this paper by way ofa brief description of the essential elements ofa downstream embankment intended to im-

pound tailings or residues.

RATE OF CONSTRUCTION OF THE EM-BANKMENT

A completely different philosophical ap-proach dictates the rate at which embank-ments to impound water and tailings are con-structed. A water-impounding embankment isusually built in one construction phase to theultimate crest elevation, and at that elevationa spillway is constructed. The purpose of theembankment is to impound as much water aspossible, and cash flow is best if the dam isfull.

Conversely, an embankment to impoundtailings is required only to contain the tailingsalready produced by the mine. The best cashflow is achieved if no more of the embank-ment is constructed at any time than is re-quired to contain current tailings volumes. Asdescribed in the second case history, it iseconomic to build a number of spillways atthe crest of successive stages of the embank-ment.

THE CORE

A core mayor may not be required in adownstream embankment for tailings im-poundments. If the tailings or residues aretoxic or the impoundment is to hold water in

37

addition to the tailings or residues, a core isrequired. The core may be of natural or syn-thetic material, but must have a low enoughpermeability to preclude the passage of sig-nificant quantities of water. No core is everentirely impermeable; some seepage will al-ways occur. The core should be designed tolimit seepage to a value where flow quantitiesmay be controlled in seepage dams, returnedto the impoundment, or treated for release.

If the tailings are toxic, the specificationsfor the core may be more stringent than for awater dam. For the average dam, loss of waterthrough the core is satisfactory provided suchseepage is controlled to prevent distress to theembankment and provided the quantities lostdo not impair the storage potential of thedam. This may be different for a tailingsimpoundment. Cores are built not so much toprevent excessive loss from the reservoir (asfor a dam), as to preclude escape of poten-tially contaminated fluid to the environment.There are cases where the seepage from theimpoundment, through both the embankmentand the liner, of potentially contaminatedseepage are small. In those cases, it may bepossible to show that the impact on thegroundwater downstream of the impound-ment is small due to attenuation by thefoundation soils and rocks.

A well-designed impoundment usually pro-vides for deposition of the tailings in a waythat they augment the core (Fig. 2): thus, eventhough the geometry may be the same, theseepage gradient through the core of the tail-ings embankment will be less than thatthrough the core of an embankment impound-ing water. Fine tailings are usually of lowpermeability: if placed correctly upstream of

Fig. 2. Embankment with u/s tailings discharge.

38

the embankment or core they may act as abackup, or secondary lower-permeability ele-ment to the embankment proper. This is par-ticularly so if the pool can be kept some wayback from the embankment.

With time the tailings in the impoundmentwill consolidate and drain, and at closure orsome time thereafter the seepage gradientsmay be less than during the operation of theimpoundment. Thus, the requirement forlong-term integrity or performance of the coremay be less than in the case of a water dam,which, theoretically at least, has an unlimitedservice life.

In Example 2 discussed later, two alterna-tive cross-sections for the embankment aredescribed. One incorporates an HPDE lineras the core; the other incorporates a core ofcompacted glacial till. Cost and availability ofsuitable material affect the final choice ofcore. Because the tailings may be used toaugment the core and thus reduce pressuresand gradients and because the service periodof the core may be limited, synthetic linersmay be used in some downstream embank-ments. Data on liner properties are obtainablefrom manufacturers; the designer must seekto understand the product with which he dealsand provide features to deal with the follow-ing problems that could occur when liners areused (note possible solutions as given inbrackets) :

a tear or puncture of the liner (incorpo-rate liner within low permeability zones,and provide adequate drains);inadequate connection to foundationsand abutments (provide substantialanchor trenches);difficulty of connecting new section toold sections as the embankment is raised(reduce number of stages of construc-tion or provide for substantial overlaps).

If a core of natural material is specified,available quantities of material must be de-termined; soil characteristics such as Atter-berg Limits, gradation, strength, hydraulic

conductivity and compaction characteristicsmust be determined. From the hydraulic con-ductivity the designer may determine thethickness of the core that is required to limitseepage to acceptable amounts. From thestrength he may examine the effect of differ-ent core orientations on the stability of theembankment. From compaction characteris-tics the designer may specify the water con-tent of the soil and the amount of compactionrequired to achieve the desired strength andhydraulic conductivity.

The core may slope downstream or it maybe vertical as shown in Fig. 3. Downstreamsloping cores are preferable if the. embank-ment is to be constructed in a number ofstages. As shown in Fig. 3, the size of thefirst-stage embankment required for a verticalcore, which must be close to the line throughthe ultimate crest of the embankment, isgreater than the size of the first stage for anembankment with a downstream sloping core.

A downstream sloping core may, if thematerial of the core is weak, give rise topotential stability problems in the upstreamportion of the dam. Preferential failure pathsmay develop along the core. This problemmay be avoided by:

using a vertical coreflattening the upstream slope of the em-bankment

Fig. 3. D/S and centerline cores.

constructing the embankment in stagesso that successively placed tailings but-tress subsequent lifts of the embank-ment.

A downstream sloping core may be lesssusceptible to earthquake-induced cracking;as the embankment settles, the core movesdown and is generally placed under compres-sion. A vertical clay core may also be draggeddown as the adjacent shell settles. If the com-pression potential of the core is greater thanthat of the shell, however, the core may crackas it seeks to settle more than the adjacentshell. This is avoided with a downstream slop-mg core.

As, in general, the vertical core is closer tothe downstream toe of the embankment, itmay result in shorter drains and hence a costsaving as compared to a downstream slopingcore. Also, if low permeability materials areused in the upstream shell, the central, verti-cal core may be thinner than a downstreamsloping core. The low-permeability upstreamshell reduces the head on the core; some ofthe total head in the reservoir is taken up inflowing through the shell.

FILTERS

Filters are required both upstream anddownstream of the core in order to preventparticles from the core from washing out andto secure controlled collection of seepage. Thedesign of filters is discussed in many standardreferences. Because the life of an impound-ment is generally short in comparison with awater dam, geotextiles may, in appropriatecircumstances, be used to form the filters. Thegeotextile should be chosen to last for theservice life of the impoundment and performsatisfactory with regard to clogging, tearing,deforming, and filtering. If natural materialsare economically available, they are prefer-able, as in the long term they will be a naturalpart of what is, after all, a new topographic

39

form in the environment. If the core is of clayand cracking of the core is possible, the filtershould be designed to trap the fine flocs andaggregates that may pipe through such a crack.

DRAINS AND TRANSITION ZONES

Drains serve to remove water from theembankment and hence to control the phreaticline within the embankment. A continuousdrain is usually required downstream of thefilters. The orientation is thus similar to thatof the core.

The topography of the area beneath theembankment controls the layout of the drainson the ground. Blanket, strip, or finger drainsmay be used. In steep valleys it may be possi-ble to place a drain only down the center ofthe valley. In broad, flat valleys, drains maybe installed at suitable spacings up the sidesof the valleys. On flat ground, for long em-bankments, drains are placed at spacings de-termined by the capacity of the drain and theseepage quantities.

The design of drains is discussed in manyreferences and is not covered here. Generally,however, if at all possible, rounded stones andgravels should be used. Pipes within the drainsshould be avoided in seismic areas. They maybreak or clog and cannot in the long term berelied on to function as an harmonious partof the reclaimed impoundment. In non-seismicareas, if pipes are used, they should be sodesigned that they can be cleaned with aroto-rooter or equivalent. They must be corro-sion resistant.

THE SHELL

The shell of an embankment may be con-structed of coarser, usually cycloned tailings,of waste rock from the mine, or of borrowedsoil or quarried rock.

The first criterion in selecting the shell

40

material is that adequate quantities are eco-nomically available. Next, the material shouldbe of adequate strength to ensure that rea-sonable side slopes to the embankment can beused. If the site is seismically active, thematerial should not liquefy under the designearthquake; most sands can be suitably com-pacted to prevent liquefaction at moderatecost.

Compaction of the shell material mayormay not be required; the extent of compac-tion may differ depending on the zone of theembankment within which the material isplaced. The designer specifies compaction re-quirements from a knowledge of the materialproperties, the function of the zone within theembankment and the effect or consequence ofmovement as a result of a given degree ofcompaction.

Generally, the more free-draining materialsshould be placed on the downstream side ofthe embankment; less free-draining materialsshould be placed on the upstream side of theembankment. Less free-draining materialstend to reduce the seepage gradient throughthe core. The free-draining materials on thedownstream side promote stability in thatwater pressures do not build up within theembankment.

The upstream shell of the embankment willbe buttressed by the rising tailings. Rapiddrawdown conditions will not occur in a tail-ings impoundment under normal operatingconditions. For these reasons it may be possi-ble to construct the upstream shell steeperthan is conventional for water impoundingembankments.

The coarsest, least-erodible materials shouldbe placed on the outer faces of the embank-ment. This will be required on the upstreamface, where tailings are discharged, or watermay lap against the face. If water will standagainst the face of the embankment, the up-stream material should be designed as a rip-rap.

If environmental or reclamation considera-

--- --- - ----

Fig. 4. Downstream face reclamation.

tions dictate, the downstream face of the em-bankment may have to be dressed with soil inwhich vegetation may be established. Careshould be taken to specify slopes at whichsuch soil will remain on the embankment and

will not slide down. This angle will depend onthe soil type and strength in the downstreamslope. The authors' preference is that thesituation shown in Fig. 4 be used: flat bermson which vegetation may be established. Theinterberm slopes should be dressed with coarsenon-erodible rock placed at its natural angleof repose.

FOUNDATIONS

The foundations for a downstream em-bankment should be excavated to bedrock if

the soils are shallow. If this is not possible,then the embankment slope must be chosento provide for stability on the actual founda-tion soil. Cognizance should be taken of thefact that the chemistry of the seepage liquidmay change the geotechnical properties of thefoundation material; this is discussed later.

The core of the embankment should be

keyed into the foundation. If necessary, thefoundation should be grouted beneath thecore, in order to provide positive control ofseepage.

The settlement of the foundation, due tothe load from the embankment, must beestablished, and provision made in the designof the embankment for any settlements thatmay occur.

---

RESERVOIR

Tailings or residues deposited behind adownstream embankment may be placed wet,dry or semi-dry.

Generally, the tailings will be fine-grainedand will be placed wet. If at all possible, thetailings should be spigotted from the up-stream crest of the embankment. In this way,they form an extension to the embankmentwhich serves both to buttress the embank-ment and to reduce seepage through the em-bankment. Discharge from the embankmentenables the pool to be kept away from theembankment. This further reduces the poten-tial for seepage loss.

DECANT FACILITIES

Barges are the best way of decanting waterfrom a pool behind a downstream embank-ment. This precludes the need for decant pipespassing through the embankment, or for spill-ways which may have to be re-established asthe embankment is raised in successive stages.At reclamation, a spillway may be establishedto pass water from the surface of the re-claimed impoundment. If possible, the spill-way should pass over a natural saddle. If thisis not possible, the spillway should be cut intothe rock of the surrounding hillside. Only as alast resort should a spillway over the embank-ment be considered.

HYDROGEOCHEMICAL ASPECTS OFEMBANKMENT DESIGN

Four major areas where the hydrogeochem-istry of impounded tailings or waste materialmay affect embankment design are:

seepage of contaminants or toxic liquidsfrom the impoundment;chemical attack and modification of thegeotechnical properties of the materialsused to construct the embankment;

41

chemical precipitation in and hence re-duction of the capacity of the drains;chemical attack of concrete structuresin the facility.

As previously noted, the core is used tocontrol seepage from the impoundment. So-lute transport modeling enables the engineerto assess the potential for and the nature ofthe impact of seepage from the impoundmentthrough the core. A comprehensive analysisand a proper understanding of the nature andextent of seepage, obtained from hydrogeo-chemical modeling, may enable the engineerto avoid unnecessary complex or costlyseepage control measures.

Before construction, extensive and detailedtesting of the materials is done to establishtheir geotechnical properties. These propertiescan be adversely affected by chemical reac-tions with the tailings, the waste, or theirinterstitial liquids. Tailings and wastes thathave been shown to cause problems include:

acidic wastes, such as phosphogypsum,which have an inherently low pH;acid-generating materials, such assulfide-bearing tailings and calcine;alkaline wastes, such as high alkali flyash at pH 11-12;sulfate-bearing wastes or those thatgenerate sulfates;highly-saline wastes, such as brines andbitterns.

There are several types of reactions thatoccur when natural materials are in contactwith aggressive tailings solutions:

solution reactions, for example dissolu-tion of iron by an acid;compound formations, such as thechange of calcite to gypsum in contactwith sulfuric acid;redox reactions, e.g., reduction of ferriciron which increases the solubility ofthe iron;pH reactions: neutralization of an acidsolution by carbonate minerals in therock or soil.

42

Such reactions can lead to a loss of themineral phase of the soil or rock to the tail-ings liquid, or a change of state of the miner-als in the material. The material's geotechni-cal properties-for example permeability orbearing capacity-may be altered and theembankment may not perform as it was origi-nally designed for.

Another part of the embankment which isvulnerable to chemical reactions is thedrainage system, including the filters and thedrains. Precipitation or ex-solution of dis-solved species due to a change in temperatureor exposure to air can impair drain operation,particularly when flow velocities and quanti-ties are small. A blocked drain can detrimen-tally affect embankment stability.

The most common drain blocking processis precipitation of iron species. Iron, in solu-tion in its ferrous state, is oxidized to its lesssoluble trivalent ferric state when interstitialliquids come in contact with free oxygen inthe drain. The precipitates can and often doblock the drains.

Species whose solubility is controlled bytemperature may precipitate in the drains incold or high-altitude environments. The tail-ings and the associated liquids in the im-poundment tend to retain heat, having beendischarged above the current ambient temper-ature. Their temperature is higher than that inthe drains, particularly close to the drain exiton the downstream side of the embankment.If the hydrochemical system is saturated withrespect to that species at the waste tempera-ture, a drop in temperature in the drain re-sults in a decrease in the solubility product ofthat species, causing precipitation. An exam-ple of this process is the precipitation of silica(quartz), a common constituent of most minetailings, whose solubility is controlled by tem-perature.

The possibility of chemical attack on con-crete structures, such as spillways and decanttowers, in the tailings impoundment embank-ment, has been widely examined. Concrete is

susceptible to attack by high-sulfate solutions,brines, low pH wastes such as tars and creo-sote sludges, and high-alkali wastes, such assodium hydroxide stripping sludges. Preventa-tive measures can be employed to minimizethe adverse effects of these solutions on theconcrete, providing the problems are antic-ipated.

RECLAMATION

The objectives of reclamation of a tailingsimpoundment are primarily to create a newtopographic form which responds to the forcesof the environment sculpturing the landscapein a way that reduces, to the maximum extentpossible, the rate of release of contaminantsto the environment. The rate of release oftailings and the rate of erosion should be nomore than can be accommodated by naturalprocesses.

The water dam can be breached at the endof its useful life; but this is not possible witha tailings pile. Recontouring, covering withrip-rap, permanent diversion of streams, andestablishment of vegetation are the way opento the designer to create from the tailings pilea new topographic form that does not detri-mentally impact the environment.

EXAMPLE 1: CANNON MINE TAILINGSIMPOUNDMENT

The Cannon Mine is just southwest ofWenatchee, WA. A tailings impoundment tocontain between five and ten million tons of

tailings deposited over ten to fifteen years isrequired. The site chosen after a detailed siteselection is up Dry Gulch to the west of themme.

Dry Gulch is part of the valley and ridgesystem that rises from the flatter alluvial ter-race on which the mine and the town are

situated. The elevation of the gulch rises from

300 m at the alluvial terrace to about 480 m atthe impoundment site to about 1500 m at thecatchment boundary. The climate of the areais relatively mild and dry. Winters are char-acterized by light precipitation and cool tem-peratures while summers are hot and dry. Themean annual precipitation is 230 mm and theannual evaporation is 760 mm.

The principal geologic feature of the area isthe Chiwaukum graben. The oldest rocks arethe Swakane Biotite Gneiss. The Swauk con-sists of fluvial and lacustrine rocks depositedon the Gneiss. As the graben developed, fluvialand lacustrine sediments of the ChumstickFormation accumulated. Igneous activity oc-curred during this period. The WenatcheeFormation which underlies the area of theembankment of the impoundment was de-posited on top of the eroded surface of theChumstick Formation. Sometime after de-

position, the Wenatchee and older formations

43

were extensively folded and faulted. After thefaulting the Columbia River Basalt Groupwas deposited, probably covering the entirearea.

The Ancestral Columbia River, and its trib-utaries, breached the basalt as the entire areawas raised as part of the uplift of the CascadeMountains.

Figure 5 shows the general geology of thesite of the impoundment, and Fig. 6 shows across section down the valley. This layout wasdefined from an understanding of the regionalgeology and a site investigation which in-volved drilling eight boreholes, profilingtwenty test pits, and running eight refractionseismic lines. Borehole drilling was done witha conventional wireline core rig. NX-size corewas obtained. Packer testing was done in orderto measure the hydraulic conductivity of eachstratigraphic section. Figure 7 is a typicalborehole log.

D D'

SECTION D-D'

o 00 300m200

[]£] ChJmsh:kformation ~ Andesite

~ Weootchee formatIOn ~ Cross-bedded sandstone

~ ColluvIum D Sandstone

!:Q2iJAlluvium Approximate contact

~ Colluvialwedge _./ Approximate attitude of bedding

~ Artificiol fill tiOJ!Faullshowingrelative movement

Fig. 6. Geologic sections.

0 300m...

KEY

GIJ Artificial fill -'_/ - Approximate contact

I QTml Masswasting deposits Thrust fault ( barbson upper plate)

UO Andesite -L Strike anddip of beds

m:::J Basalt -- Strike and dip ofoverturnedbeds

Wenatcheeformation -t- Approximate aXIs ofsyncline

OU Chumstick formation

Fig. 5. Site Geology.

44

ROCK TYPE

15Sardstane, fresh, It. bluish gray to It.'::gray, med. grained, Clean, cross - :.:::bedded, Micaceous

Sandstone,whlte to It. gray, med. fOcoarse grained, Friable, Micaceous, '.:..:Carbonaceous, Massive :::

20

25

30

35

4a

45

5a

55

Sandstone, fresh, white, coarsegrained, Micaceous (Muscovite),Friable, cross- bedded,Carbonaceous

KEYLEGD

TC.R.

SPAC.

LEGEND

TOTALCORERECOVERY("!o)

SPACING (cm.>

RQ.D. - ROCK QUALITY DESIGNATION ("!o)

Fig. 7. Centerline borehole log.

The rocks at the site are generally massiveand constitute a strong and competent foun-dation. Bedrock hydraulic conductivity de-creasesfrom 10-6 mjsec at the surface to lessthan 10- to mjsec at about 50 m depth. Soilsand rocks available for construction of theembankment are: fine-grain colluvial and al-luvial sands and silts, waste rock from previ-ous operation of a sandstone quarry at thesite, decomposed basalt from a ridge to thenorth of the impoundment site, fresh basaltrock and processed alluvial sands from Col-umbia River terraces.

The tailings are described by Caldwell et al.[1]. The gold at the Cannon Project occurs asa hydrothermal deposit in the siltstones of theSwauk Formation. The ore will be groundand crushed to 90% less than 200 mesh and

---

S.CR - SOUD CORE RECOVERY ("!o)

STRGTH - STRENGTH (MPa)

HYD.CON.- HYDRAULIC CONDUCTIVITY (cm./sec.)

ELEV - ELEVATION(metres)

the gold will be removed by flotation, pres-sure oxidation, and leaching. Ninety toninety-five percent of the flotation end prod-uct will be tailings that is not treated anyfurther before discharge to the impoundment.Five to ten percent, called the concentrate,will be pressure-oxidized, leached, and passedthrough a carbon-in-pulp system. The wasteslurry will be neutralized and the residualconcentrate tailings will be mixed with theflotation tailings prior to discharge to theimpounded.

Table 1 gives the chemistry of the flotationtailings and of the mixed tailings. Also givenare the tailings chemistry after they have beenmixed with a 10% sodium bisulfate solution.This represents the likely tailings chemistryafter hydrogeochemical reduction of the tail-

----

R3420

415

RI

41a

R2

RI405

R24aa

395

51.1

I -I 390

29.9 III -i385'"

IQ.;

29.7I -I 38a

I

45

TABLE 1

Chemistry of "mixed" tailings, flotation tailings liquids, and reaction products chemistry of "mixed" tailings

ings supernatant in the impoundment. Inorder to assess the potential modification ofthe tailings liquid chemistry that might occuron the top of the tailings and in the pool,mixed tailings were equilibrated with simu-lated rainwater (distilled water mixed withhydrochloric acid to a pH of 5). The results ofthis are given in Table l.

As shown in Table 1 the bulk of the resid-ual cyanide is chemically complexed, hencechemically unreactive and non-toxic. Dilutionwith rainwater increases the level of free

cyanide and iron slightly, but the overallchemistry of the liquid is better than that ofthe initial tailings slurry. Hydrogeochemicalreduction increases the soluble iron, and de-creases both total and free cyanide and copperto below analytical detection limits.

Figure 8 shows a cross section of the em-bankment intended for the site. The embank-ment incorporates the following zones:

a low-permeability core which slopesupstream in order to provide for stage

construction, and facilitate proper con-tact with bedrock;a filter to preclude movement of par-ticles from the core due to seepage ofwater from the reservoir;drains which remove water from theembankment and control the positionof the phreatic line within the embank-ment;shells of free-draining, compacted soils

LEGEND

CORE CLAYEYSILT

SHELL RANDOMBASALT

ZA SMELL COLlUV IUM

FILTER

DRAIN

ROCKFILL

'T"AILJNGS

Fig. 8. Cannon tailings embankment cross-section.

... ... ". ---...

Parameter * Mixed tailings Floatation tailings Mixed tailings Mixed tailingssupernatant supernatant reduced with Na2S203 leached with rainwater

pH value (units) 7.17 7.30 5.17 4.80Total dissolved solids 4320 440 N/A 2750Sulfate 1660 170 N/A 1587Chloride 1040 15.5 3750 202Total cyanide 284 < 0.05 < 0.05 57Free cyanide 0.35 < 0.05 < 0.05 2.58Sodium 480 90 N/A 87Iron 10 < 0.05 1100 18Arsenic 0.07 0.01 < 0.01 0.08Cadmium < 0.01 < 0.01 < 0.01 < 0.01Cobalt 0.33 < 0.01 0.55 0.08Copper 0.03 < 0.01 < 0.01 < 0.01Lead < 0.01 < 0.01 < 0.01 < 0.01Mercury 0.0024 < 0.0003 0.0062 0.0011Nickel < 0.05 < 0.05 5.4 < 0.05Selenium < 0.01 < 0.01 < 0.01 < 0.01Silver < 0.01 < 0.01 < 0.01 < 0.01

* All values in mg/I, except pH.

46

and rock to provide the required stabil-ity;an upstream blanket, which is an exten-sion of the core; this is installed inorder to control seepage and maintainlow hydraulic gradients at the core tofoundation contact;a grout curtain which is intended topreclude seepage through significantjoints or other geological discontinui-ties.

The embankment will be constructed III

stages to accommodate the rising tailings.

EXAMPLE 2: GREENS CREEK EMBANK-MENTS

The Greens Creek Impoundment is on Ad-miralty Island in southeast Alaska. The im-poundment is designed to receive up to fourmillion tons of tailings at about six hundredtons per day for a design life of ten to seven-teen years.

The site chosen is a flat muskeq-coveredvalley bounded on the north by a low saddle;on the east is a steep ridge rising to a series ofmountains; on the west is a series of low,nearly parallel, discontinuous ridges betweenwhich are sediment-filled gaps. The site isdrained by a small creek which flows south.

During glaciation, the layered metasedi-ments were scoured to form a series of deepV-shaped valleys. The valleys were filled withsequences of sand, silt and clay as the glaciersretreated. Figure 9 shows the layout of thesite and the embankment, and Fig. 10 showsthe bedrock and soils along the centerline ofthe proposed embankment.

Seismic activity at the site is significant: thedesign bedrock acceleration is 0.3g.

Annual average precipitation is 1500 mmand evaporation is 500 mm. Mean annualsnowfall depth is 2700 mm (about 410 mmwater equivalent). Annual average tempera-ture is 4°C, ranging from -1°C in January to18° in July.

Fig. 9. Greens Creek embankment layout.

Figures 11 and 12 show alternative cross-sections proposed for the embankments. Theouter dimensions and side slopes of the em-bankment are the same for both alternatives.

The first, incorporating the till core, is the

EAST WESTEAST BEDROCK

VALLEYWE ST BEDROCK

VALLEY

.~~: :'~ ~1/1/1

EASTAT DOWNSTREAM TOE

OF PROPOSED EMBANKMENT WEST

EASTALONG CENTERLINE OFPROPOSED EMBANKMENT WEST

o VEGETATION 8 PEAT

E!Z8 CLAYEY SILT

ti=L] SANDY SILTV GRAVEL

[IJJ SILTY SAND

ON UPSTREAM SIDEOF PROPOSEDEMBANKMENT

j

:'0'0o

Fig. 10. Tailings impoundment site geological cross-sec-tions.

/',850

1\

\\, {\l,

UPSTREAMmetres

50

10

o

WEST VALLEYSECTION

47

DOWNSTREAM

~I2

EAST VALLEYSECTION

LEGEND

CD Impervious Core

@ Filter

<ID Transition Zone

@ Inner Shell

@ Filler

@ Outer Shell

@) Foundat ion

GlacIal Till

Concrete Sand

Fine Gravel

Sand 6 Gravel (Millsite)

Sand 6 Gravel (Selected Millsite)

Rockfi II (Quarries)

([) Berm

@) Tai lings

Excavated Sol I

Electro-Osmotically TreatedSilly Clay (In -situ)

Fig. 11. Embankment cross sections alternative I-till core.

more conventional. There is, however, someconcern that sufficient quantities of suitabletill are available. Also, difficulties may beencountered in placing a till core in the wetclimate at the site. Accordingly, the secondalternative incorporates a synthetic (high-den-sity polyethylene) liner. The liner is flexibleand can strain in tension eight times its origi-nal length before breaking. These properties

make it well suited for use where settlement

of the foundations and earthquake-induceddeformations could occur.

Soils in the east bedrock valley are sandsand silts. They could liquefy in the event ofthe design earthquake, and seepage may occurthrough them. Because the depth to bedrockbeneath the embankment in the east bedrockvalley is limited, an excavation to bedrock

metres50

40

30I

20'2

10

0

48

UPSTREAM

metres

DOWNSTREAM

WEST VALLEYSECTIONS

EAST VALLEYSECTIONS

Fig. 12. Embankment cross sections alternative 2-synthetic liner.

will be formed under most of the embank-ment.

The silty clays in the west bedrock valleyare a glacially-derived rock flour of low plas-ticity. The upper 6 m are medium stiff andover-consolidated. Below that, they are softand sensitive. When remolded to a reasonable

degree in the hand, they become rather sticky,

flowing materials. Depth to bedrock andproblems of disposing of material precludecomplete excavation to bedrock. Accordingly,where the silty clays are left in place as afoundation for the tailings embankment, theywill be stabilized by electro-osmosis.

A layer of sand beneath the silty clay, andthe limited bedrock depth lead to a decision

30

20

10

0

metres50

40

30 . Ie:220

10

0

LEGEND

CD Impervious Glacial Till

@ Core Synthetic Liner

(g) Filter Concrete Sand

@) Drain Fine Gravel

@ Inner Shell Sand 8 Gravel (Millsite)

@ Filter Sand 8 Gravel (Selected MiIIsite)

@ Outer Shell Rocklill Quarries

@ Foundation Electro- Osmotically TreatedSilty Clay (in-situ)

C1> Berm Excavated Soi I

@ Tailings

to excavate beneath the downstream part ofthe embankment in the west bedrock valley.

CONCLUSIONS

This paper has described why a down-stream embankment may be chosen in prefer-ence to a centerline or upstream embankment.The advantages and disadvantages of thedownstream embankment have been consid-ered; while a downstream embankment is themost costly way to impound tailings or re-sidues, it provides the best method of contain-ing fine-grained or toxic tailings in seismic,wet or sensitive areas.

The various components of embankmentbuilt by the downstream method have beendescribed in order to highlight the differences

49

between embankments built to impound tail-ings or residues and embankments built tosotre water.

The two case histories described have il-lustrated the basic features of downstreamembankments, and have displayed the extentto which the downstream embankment is sui-

table for use in a wide diversity of areas, tocontain a wide variety of waste products.

REFERENCES

J.A. Caldwell, D. Moore and A.C.S.S. Smith, Seepageand groundwater impact of tailings disposal: CannonMine project, Proc. Conf. on The Impact of Miningon Groundwater, National Water Well Association,Denver, CO, Aug. 28 and 29, 1984.

2 Caldwell and DeDycker, "Site Selection of a TailingsImpoundment using Utility Functions". Submittedfor publication.