&IR/LLVV- -

CODE OF PRACTICE ON THE MANAGEMENT.OF RADIOACTIVE WASTES FROM THE MINING-

AND MILLING OF RADIOACTIVE ORES (1982)

GUIDELINE

Tailings Impoundmentfor Uranium Mines

Date of adoption or revisionPrimary related ClauseOther related Clauses.

1983127(1,2,4).,15(1,,3)16(1,2)

CONTENTS

Paragraphs

INTRODUCTION l-l7

* Overview 4-17

SELECTION OF IMPOUNDMENT SITE 18-24

TYPES OF IMPOUNDMENTS 25-40

Valley Dam Impoundment 26-27* Ring Dyke Impoundment 28-29* Mine Pit Impoundment 30-35* Specially-Dug Pit Impoundment 36-38

Underground Mine Impoundment 39-40

DESIGN OF IMPOUNDMENT DAM STRUCTURES 41-57

* Seepage Management 48-57

CONSTRUCTION AND MONITORING OF IMPOUNDMENTS 58-66

Quality Control during Construction 59-61Monitoring of Impoundments 62-66

TAILINGS MANAGEMENT 67-93

* Comparison of Systems 69* Design Features 70-77* Saturated Tailings Management 72-77* Wet Tailings Management 78-84* Semi-Dry Tailings Management 85-89• Dry Tailings Management 90-93

REHABILITATION OF TAILINGS IMPOUNDMENTS 94-123

REVEGETATION OF TAILINGS IMPOUNDMENTS 124-127

SELECTED REFERENCES 128

FIGURES 1 TO 6

* zoning in Cover Structures* Rehabilitation - Open Pit Tailings Impoundments* Rehabilitation - Below Grade Impoundments* Rehabilitation - Trench Impoundment* Rehabilitation - Ring Dyke Impoundment

Rehabilitation - Valley Dam Impoundment

INTRODUCTION

1 The Code of Practice on the Management of RadioactiveWastes. from the Mining and Milling of Radioactive Ores(1982) requires the use of the best practicable technologyto ensure that the release of radioactive material isminimised during all stages of mining operations and afterthe completion of such operations. In addition, the Codespecifies that radioactive wastes shall be managed such thatexposure to radiation of employees and members of the publicis as low as reasonably achievable,, and below the relevantlimits prescribed in Schedules 1,2,3 and 4 of the Code ofPractice on Radiation Protection in the Mining and Millingof Radioactive Ores (1980).

2 This guideline is aimed at assisting users of the Codein siting, constructing, operating, decommissioning andrehabilitating uranium tailings impoundments. It presents arange of current options for impoundment of tailings fromuranium mills. In presenting this material, account hasbeen taken of the various influences which could determinethe best practicable technology applicable to a particularsite, and of the information required by the appropriateauthority in relation to approval for a tailings impoundmentsystem.

3 While the options presented imply a wide range ofpossible strategies, many will be inappropriate for aspecific site. Moreover, it is not intended that theguideline restrict or exclude consideration of innovativealternatives, given that the options in the guideline (andany alternatives) are interpreted in accordance with theCode. In all cases, however, the approval of theappropriate authority is required for all aspects of

..... tailings management and disposal to ensure that the bestpracticable technology is adopted and the provisions of theCode complied with.

Overview

4 Uranium mill tailings are the solid residues andassociated liquids remaining after uranium has beenextracted from an ore. The solids consist primarily of thefinely ground bulk material of the original ore, and alsocontain a variety of chemical substances precipitated fromthe tailings, liquids. The tailings solids and liquidscontain both radioactive and non-radioactive materialswhich, if released and dispersed in the environment inunacceptable amounts, may have some. detrimental impact onpersons and the e~nvironment.

5 Uranium mill tailings differ from most other metalmill tailings in their significant content of radioactivecomponents. The physical, and chemical properties of the

1

tailings will have a considerable effect on their structuralbehaviour and their leachability during the operationalphase of the project, as well as during decommissioning andin the long term. These properties in turn depend on thenature of the ore, its hardness and acidity, thedissemination of the ore minerals in the gangue materialsand the particular mineral form in which the uraniumoccurs. These factors will influence the fineness to whichthe ore is ground before leaching and whether an acidic(e.g. sulphuric acid) or basic (e.g. sodium carbonate) leachis used.

6 With a porous sandstone, the grind may be as coarseas '80% greater than 0.8 millimetres', or when there isconsiderable shale with the ore, as fine as '75% below 45micrometres'. The very fine material below 75 micrometresis referred to as "the slimes". Depending on the tailingsmanagement system used a high proportion of clay mineral in"the slimes" can lead to tailings which take a long time todewater and consolidate and therefore to stabilise.

7 During the extraction process 90% to 97% of theuranium is removed leaving essentially all the otherradionuclides of the uranium decay chain in the tailingsslurry. Some of these radionuclides are in solution insmall quantities. The slurry also contains undissolved anddissolved heavy metals in varying proportions plus certainamounts of mill chemicals. In general, most of theradionuclides are concentrated in the finer tailingsfraction; as much as 75% of the total activity can be foundin the minus 75 micrometre fraction.

8 The tailings from acid leach processes are normallyneutralised and then adjusted to pH 8 or higher with lime,to reduce . the concentration of soluble heavy metalcontaminants. If the ore contains pyrites or other. sulphideminerals they will normally be discharged with thetailings. These sulphide minerals, even at. lowconcentrations, can produce sulphuric acid by chemical andbacterial oxidation processes. Acid produced by thoseprocesses will initially react with any excess lime, andother basic materials present in the tailings. If theconcentration of basic materials, is insufficient to reactwith the acid produced, the pH will continue to decrease andheavy metals may redissolve. Movement of the acidicsolutions through the tailings will leach the heavy metalsand release them to the environment by seepage. Thepresence of pyrite or sulphide minerals in the ore and wasterock can thus have a very significant effect on the long-term behaviour of tailings and waste rock piles as potentialsources of pollution.

2

9 Radiological protection problems arise in the long-term management of mill tailings due to the presence of anumber of long-lived radionuclides of the U-238 decay chainwhich remain in the tailings following conventionalextraction processes. (These problems will be compounded ifthere are significant quantities of Th-232, with its decaychain, associated with the uranium orebody.). Theradionuclides of most concern are

Th-230: alpha emitter, 80,000 y half-life,Ra-226: alpha emitter, 1,600 y half-life,

(daughter of Th-230)Rn-222 plus daughters: alpha and gamma emitters,

Rn-222 has a 3.8 day half-life, (continually producedfrom Th-230 via Ra-226).

There will also be a small percentage of the originaluranium remaining in the tailings. For an orebody of, forexample 0.3% uranium, a 95% extraction efficiency wouldleave the tailings with 0.015% uranium. However for a richorebody of say 3% uranium a similar extraction efficiencywould leave 0.15% uranium in the tailings and in this casethe tailings would be classified as specified material asdefined in the Code.

10 The principal potential modes of radiologicalimpact of these radionuclides are via

Dust - which may be inhaled directly or depositedin the nearby environment,

Water release - particularly Ra-226 (and Th-230) inseepage leading to the contamination of surface andground waters and thence food chains,

Radon emanation - leading to lung exposure ofpeople in the vicinity, but also, at very. muchreduced levels, of those who may be quite remote,and

Gamma irradiation - arising from the short-liveddaughters of Rn-222 and a source of exposure onlyin the immediate vicinity of the tailings.

11 Although the uranium has been removed from the ore,thereby reducing the quantity of elemental radionuclides,,the milling process has increased the potential mobility ofthe remaining elements (both radioactive and non-radioactive) in 6he tailings. As such, careful disposal ofthe tailings is required to limit the transfer of mobileconstituents to people and the environment.

3

V- i

12 The basic design and operating objectives for theimpoundment of the uranium mill tailings in all cases shouldbe

to provide an impoundment which is both physicallyand chemically stable, and

to control the loss of radioactive elements to theenvironment to acceptable levels..

13 The important considerations that influence theeffectiveness of tailings impoundment relate to both siteselection and choice of the impoundment system. Eachpotential tailings impoundment site has inherent site-specific advantages and disadvantages. Site selectionshould make use of the site-specific advantages. The designand engineering should then account for *the site-specificdisadvantages.

14 Site selection and the design and engineering ofthe tailings impoundment should be considered in conjunctionwith the overall design and planning of the mining andmilling operation. Processes and methods used in both themine and mill will have a direct influence on therequirements, performance and cost of the impoundment.

15 The concepts of "storage" and "disposal" should becarefully distinguished when evaluating tailings managementsystems. The requirement of storage, that is, confinementof wastes over a limited period of time with the possibilityfor retrieval and eventual disposal of tailings, can be metby any properly sited and designed tailings impoundment.However, tailings disposal is the desired managementobjective. In this case there is no intent to retrieve and,more importantly, no reliance on continued surveillance andmaintenance to ensure integrity of the impoundment system.The time following decommissioning of the mill andrehabilitation of the tailings impoundment after whichsurveillance can be discontinued may vary considerably withthe impoundment design and, in particular, with certainsite-specific parameters.

16 There can be no guarantees that in the long termtailings deposited on or near the land surface, regardlessof the method used or the site selected, will not beeventually dispersed into the environment. However, thedesign goal for the impoundment system should be to disposeof tailings using the currently available best practicabletechnology, so as to minimise the possible future rates ofrelease of radiopuclides into the environment and the needfor long-term surveillance.

4

17 Detailed information should be supplied to theappropriate authority on all factors which will influencethe effectiveness of a proposed tailings impoundment system,including:

the characteristics of the orebody, and theprocesses and treatment to be used in. theproduction and handling of tailings,

the availability of sites for tailings impoundmentsand the basis of selection of the recommended site,

the type of impoundment proposed, the design of thestructures and their expected behaviour,

the type of tailings management to be utilised,

the long-term rehabilitation proposed for thetailings impoundment, and

the assessed environmental impact of theimpoundment during operations and for the longterm.

5

SELECTION OF IMPOUNDMENT SITE

18 One of the most important aspects to be addressedin tailings management is the selection of a site for thetailings impoundment. The site-selection process consistsof collecting appropriate information regarding the naturalconditions, geology and geomorphology of the regionconcerned, and then choosing a site which has a favourablecombination of conditions with, respect to those factorswhich govern the rehabilitation strategy to be adopted andlong-term integrity of the impoundment.. The tailingsimpoundment site will be within the restricted release zone;approval of the zone is required under clause 5(1) of theCode.

19 The criteria that must be met by the selected siteinclude:

adequate storage capacity for the total quantity of

tailings to be produced,

away from population centres,

away from potentially active fault or seismicareas,

nearby availability of adequate quantities ofsuitable natural materials required for theconstruction of the impoundment and eventual coverstructures, and

no extraneous catchment area and not subject toflooding.

20 Desirable site characteristics are

low permeability strata in the impoundment area,

minor consequences of failure,

foundation properties sufficient to support thetailings and associated structures without highsettlements,

geological formations which will resist chemical(and physical) attack by the tailings liquid,

strata below the impoundment area which readilyabsorb radionuclides, and

that the geological and geomorphological processesknown to be operating in the area would improverather than prejudice continued containment of thetailings.

6

21 The site selection process cannot be carriedthrough to a final decision without considering, at leastconceptually, the impoundment design. The decision relatingto final site selection is frequently arrived atconcurrently with those pertaining to impoundment design andrehabilitation as discussed in this guideline and theguideline on Decommissioning and Rehabilitation of UraniumMine, Mill and Waste Disposal Sites.

22 In undertaking an evaluation of potentialimpoundment sites, the following information should normallybe obtained and made available to the appropriate authority:

Mining Operations

- characteristics of the orebody, includingmineralogy and grade,

- methods, plans and sequences of mining,

rates of production of barren wastes,contaminated wastes and ore, and

locations of the proposed impoundment sitesrelative to the mill site and the mine.

Milling Processes

expected production rate of the tailings,

processes and reagents to be used,

water requirements, and

chemical, physical and radioactive propertiesof the tailings.

Site Data

- topography and current land use,

- hydrology and current water use (surface andgroundwater),

- geology and surficial soils,.

- regional geomorphology,

- sei-6mology,

- meteorology,

- = demography,

7

cultural and recreational resources, and

ecology.

23 An evaluation of information on the topics listedabove should lead to the selection of a limited number ofthe more feasible sites which can then be evaluated ingreater detail to select the final intended site. The moredetailed evaluation requires compilation of additionalinformation for each site derived from exploration, drillingand testing, geophysical surveys etc., including

Geology

geological strata and surficial soils in theimpoundment area, and their engineeringproperties,.

foundation conditions at embankment andstructure locations,

lateral and vertical distribution andpermeability of. the strata underlying theimpoundment,

existence of faults, fractures and jointpatterns that may provide preferential seepagechannels, and

geochemical properties of soil and underlyingstrata, including their chemical exchangecapabilities..

Hydrogeology

depth to water table, including perched watertables if present,

magnitude and trends of annual water tablefluctuations,*

distance to nearest points of groundwater,spring water, or surface water usage (includeswell and spring inventories),

relation to recharge area of groundwater,

groundwater quality, and

possible chemical effects of pollutants inseepage water along potential flow paths.

8

24 On the basis of the evaluation studies referred toabove, a proposal for the location of the tailingsimpoundment site should then be submitted to the appropriateauthority for approval. Prior to construction, a reportshould be compiled and submitted to the appropriateauthority which details a range of environmental baselinedata for the approved site. This data would include:

the concentration of the radioactive and non-radioactive nuclides in soil, water, air andbiota in and around the site which would beexpected to be monitored and used asindicators of the integrity of the impoundmentand of the impact of the impoundment on theenvironment;

the external radiation exposure levels in theregion of the site; and

the radon exhalation rate(s) from the regionof the site.

9

TYPES OF IMPOUNDMENTS

25 Tailings impoundments generally consist of acombination of both natural and man-made features. Thetypes of impoundment systems discussed in this guideline arenot exhaustive, and innovative proposals for new facilitiesare encouraged, provided such proposals are adequatelysupported when submitted to the appropriate authority. Thefollowing types of impoundments considered are:

Valley dam impoundment,

• Ring dyke impoundment,

* Mine-pit impoundment,

• Specially-dug pit impoundment, and

• Underground mine impoundment.

The appropriate authority should be provided with a detailedassessment of why a certain type of impoundment has been.selected and why that type of impoundment is considered thebest practicable technology for the site.

Valley Dam Impoundment

26 The confinement basin is formed by constructing anembankment across a valley. The valley forms three sides ofthe impoundment and only a relatively short dam is generallyrequired. This type of impoundment can therefore be usedonly in hilly areas with adequate relief to achieve therequired storage with an acceptable dam height.

27 The following factors should be considered inassessing the location and use of a valley dam impoundment:

Valley impoundments are chosen to maximise the Useof natural topographical advantages of the area.The shape and locations of the impoundment basinare therefore determined by the availablility ofsuitable sites within a reasonable distance of themine. Because the characteristics of the site arefixed by nature there is usually little scope forvarying the site locations to avoid unfavourablefeatures.

Since the site locations are dictated by theavailable natural features, total distances fromthe ore.bodies to mines, stockpiles, mill plantsand impoundments may be substantial. Shorterdistances reduce the overall surface area that isaffected by the mining and milling operations, withattendant reduced risk of contaminating *theen'vironment by spillage.

10

Borrow material for the construction of the damwall should, if suitable, be obtained from withinthe storage area.

Rainwater runoff from areas adjacent to thetailings impoundment area should be diverted awayfrom the impoundment. Diversion drains to preventthe contamination of runoff water by the tailingsare required. For the long-term protection of thetailings impoundment these drains should bedesigned to carry the maximum probable flood fromtheir catchment. Ideally, such a tailingsimpoundment should be located at the head of anatural drainage area, allowing the diversiondrains to discharge to an adjacent catchment. Ifthis is not practicable, the diversion drainsshould be continued downstream of the dam for anadequate distance so that any scour in the areas ofsteep gradients will not progress back to thedam. Discharge on the contour into' natural sidewater courses is preferred.

Because of the shape of the valley, tailings depthsare shallow along the three natural sides andbecome progressively deeper towards the middle.The average tailings depth is therefore generallyfairly small and the final surface area relativelylarge. This tends to increase the area ofenvironmental impact, the surface area for seepageand wind erosion, and the volume of the coverrequired for stabilisation.

For a given volume of tailings the valley dam willhave to be relatively high with a subsequent steephydraulic gradient at the dam. Spillages andseepages from the dam area will rapidly join thesurface drainage system of the area.

The shape of the valley is often irregular.Complex shapes may render installation of a well-managed tailings facility difficult and costly tooperate. Where line discharges are used along thedam embankment, soft slimes zones usually formalong the outer basin, edges, creating placementdifficulties and differential settlement problemsfor the cap.

Valleys often occur as a result of some underlyingweak geo!logical feature, such as a fault or shearzone, which may also be a zone of higherpermeability. Deep alluvium deposits are. oftenasociated with valley floors. The horizontalpermeability of such deposits is often high. Deep

11

impermeable cut-offs will then be required withsubstantial grout curtains, through the highe~rpermeability rock to minimise the seepage at thedam area.

A valley normally restricts seepage to a knownseepage path to and along the valley floor towards.the dam. Monitoring and measures to return seepageflows to the impoundment during the operationalphase are therefore relatively simple.

Since the valley forms three sides of theimpoundment and only a relatively short dam wall isrequired to complete the impoundment, this type ofimpoundment is often cost-effective.

Ring Dyke Impoundment

28 Ring dyke impoundments are formed by constructing asingle self-closing embankment usually on relatively flatterrain. The head of a small valley is often used with partof the ring dyke following the catchment divides so that thetotal volume and cost of the required earthworks isminimised. Alternatively, where the terrain is moreundulating, the impoundment may be formed by a succession ofvalley embankments linked by low saddle embankments,constructed in single or multiple stages. However, avariety of shapes may be used, with perimeters ranging fromsquare or rectangular to curved or irregular. Moreover,internal embankments may be used to sub-divide theimpoundments into two or more compartments.

29 The following factors and characteristics should be,considered in assessing the location and use of a ring dykeimpoundment:

Ring dykes require generally flatter areas thanvalley dams, and thus there may be greaterflexibility in selection of a location close to themine and mill.

Close location to the mine may allow use of barrenmine waste in the construction of the embankmentand the containment may also be used for thedisposal of radioactive waste rock.

Borrow material for the construction of the damwall should, if suitable, be obtained from withinthe storage area.

V

The impoundment can also be used effectively forstorage and evaporation of. water if saturatedtailings management is used.

12

Ring dykes are commonly located near the crest ofridges where the depth to the water table isgreatest. This increases the time required forcontaminated seepages from the impoundment to reachwater courses and allows dilution withuncontaminated groundwater. Also, a greater timeperiod. may be available to implement remedialmeasures where large excursions of contaminatedgroundwater flow are found to occur.

The location of impoundments on the crest of ridgesexposes the tailings to conditions which favour theevaporation of tailings water. However, suchconditions are generally unfavourable for reducingwind erosion of dried-out tailings.

Ring dykes should be located away from streams andthe downstream toes of the embankments should bekept above maximum flood levels.

A regular shape and minimisation of the catchmentarea allows good control over the water depth, andthe tailings solids build-up, which are necessaryfor saturated as well as for semi-dry tailings.management.

It is generally advisable that seepages from theimpoundment through the dam and the upperfoundations be collected and pumped back into thewater management system during the operationalphase and the construction activities associatedwith the rehabilitation phase of the project.

Where the embankment is elevated substantially forits full length above the surrounding naturalsurface, solutions for the long-term discharge ofrain runoff from a rehabilitated ring dykeimpoundment are costly.

High ring dykes, especially when located on ridges,have a significant visual impact.

The construction of ring dykes requires the use ofconsiderable quantities of earthfill and/orrockfill.

Additional tailings storage capacity can be readilyachieved by constructing additional stages oradditional dykes.

Mine-Pit Impoundment

30, Worked-out mine pits may be used for thecontainment of tailings. The procedure for backfilling the

13

pit with tailings vari•i-s considerably, depending on theclimate, the depth to and variability of groundwater, theproximity to streams, the susceptibility of the pit toflooding, the permeability of the wall rocks in the pit, themining program adopted, whether wet or dry tailingsmanagement is practised and the characteristics of thetailings.

31 In arid regions where evaporation considerablyexceeds precipitation and the depth to.groundwater is great,it may be feasible to backfill the worked-out minepit toabove the water table using general back-fill, then toinstall a liner followed by placement of the tailings in thepit. The risk of damage to the liner due to settlement ofthe fill should be assessed.

32 Where the groundwater table is close to 'the surfaceit is not possible to store tailings in a pit above thegroundwater level. If the permeability of the walls is lowor if the groundwater is not a resource, it may beacceptable to place the tailings in the bottom of the pit.However, the preferred upper level of the tailings shouldthen be kept below the lowest expected groundwater level andnot be located in the range of the active groundwaterfluctuation. Where permeable porous areas occur in the pitwalls these should be covered by a liner or grouted.

33 In areas where the net evaporation is low or wherethe inflow of water into the mine-pit cannot be controlled,then the surface of the tailings will remain wet andtherefore it may not be practicable to construct aneffective cover over the tailings. In such cases, it mayonly be possible to spigot a soil material over the top ofthe tailings, even though this may not provide adequatestabilisation and protection.

34 In some cases mine-pit impoundment may be carriedout in conjunction with mining in a different area of thepit. The protection required against radon exhalation andthe safety of the operation must be considered in detail.The procedures that would be used for backfi-lling of themine-pit with tailings are clearly site-specific and must beengineered in accordance with the prevailing conditions.

35 The following factors should be considered inassessing the use and construction of mine-pit impoundments:

Their use depends to a high degree on the miningprogram adopted. For example, they may be usedwhere th*a orebody is to be mined completely beforemilling operations commence or where there is anexisting worked-out mine pit. If the mining andmilling phases overlap -there would be a need toconstruct a temporary storage pond for the tailingsun~til the mine pit becomes available.

14

For a low waste-rock-to-ore ratio the tailings maybe too voluminous to be contained in the pit.

The greater depth of burial available provides

greater assurance of post-operation confinement.

Visual and air pollution impacts are low.

Instability of the pit wall may affect theintegrity of any liner.

Where lining of the pit is necessary, flatterexcavation of the pit slopes may be required tofacilitate liner installation. However, theresultant requirement for a wider pit could thenincrease the economic and environmentalconsequences.

Where a liner is used to retard seepage from thepit it would also retard water drainage into thepit through the walls, leading to possibleinstability and/or breach of the pit walls. Thisproblem is overcome where tailings are placed abovethe water table. Liners may also be subject todeformation and rupture due to differentialsettlement of the tailings.

A thorough knowledge of the climate, groundwaterfluctuations, rock mass structure and permeability,and transmissivity is necessary for, a mine-pitimpoundment.

The use of mined-out pits may complicate futuremining operations near the pit, particularly forunderground mining methods.

Uranium-enriched ore zones may be unevenlyscattered through the generally mineralisedregion. As planned pit boundaries are frequentlydefined by mining economics and not necessarily byore reserves, the tailings may cover potential orereserves. Re-mining of the area may be rendereddifficult, particularly if saturated or wetdisposal techniques are used, whereas containmentsof dry or semi-dry tailings would be more readilyre-mined.

S pecially-Dug Pit Impoundment

36 Excavation may be undertaken specifically for thepurpose of providing below-grade impoundment. The materialsexcavated to form the pit may be. used to form small

15

surrounding retaining embankments and for any otherstructures required at the site. With increasing depth Qfexcavation the procedure becomes essentially similar to themine-pit impoundment. If the top of the tailings reachesabove the natural surface the impoundment approaches a r-ingdyke impoundment condition. The technique is site-specificand its use depends on the geology and climate of the regionand on the nature of the mining operation. Various shapesand sizes of pits can be used, ranging from one large pit toa number of trenches.

37 The procedure in siting and constructing such animpoundment consists essentially of

selecting a geologically stable site, withfavourable soil and strength conditions to permit.easy deep excavation;

making an excavation of sufficient volume tocontain the anticipated quantities of tailings;.

forming surrounding embankments, if required forwater management purposes, with material excavatedfrom the pit;

providing liners or clay filled cut-offs to preventseepage from the pit through any near surfacepermeable soil layers, or maintaining the waterlevel in the pit below *the level of suchhorizons; and

constructing other impoundments which may proceedat the same time as, or subsequent to, the initialspecially-dug pit. Progressive rehabilitation willminimise the effects of radon exhalation from thetailings and provide experience in rehabilitationtechniques during operations.

38 The following factors should be considered inassessing the location and use of specially-dug pitimpoundments:

the pit should be in a location that is convenientto the mill;

the location should be selected such that.favourable topography (hence favourable drainage)exists. Geological and hydrogeological conditionsshould be such that the impoundment is positionedgenerall~y above the water table, over relativelyimpermeable horizons, and at locations free fromerosion or flooding.

16

The system involves considerable excavation, and isonly practicable in material that is easilyexcavated.

If "saturated" or "wet" management is practised,the tailings may be difficult to rehabilitate.Post-operational settlements of the tailings maylead to drainage problems on the cover.

Providing the site is carefully selected and managed, theimpoundment should be relatively free from the risk oferosion, flooding or failure.

Underground Mine Impoundment

39 Underground mine impoundment normally involvesseparation of the coarse fraction of the tailings from theslimes fraction and using the former as backfill within themine. In current practice, tailings used as backfill areplaced in underground mine areas to serve as floors formining equipment used in the mining operation, as well asfor stope fill. The proportion of tailings which may bereturned to the mine is a function of the ore mineralogy andmilling operation, which determine the proportion of coarsetailings to slimes. It is possible for 50% to 60% of thetailings to be returned to the mine in this manner. Theremainder of the tailings must be disposed of in other ways.

40 The following factors should be considered whenassessing the use of underground mine impoundment oftailings

Stabilisation of the placed tailings requires thatthey have a high permeability to permit rapiddrainage of excess water.

Excess water should be confined within the overallmine-mill water management system and not releasedto the environment without treatment.

A significant fraction of the radium, thorium andother contaminants are contained within the slimesfraction (approximately 75%" of the radium stays inthe slimes); thus, underground disposal of thecoarse fraction of the tailings does notappreciably reduce the total radioactivity in theremaining waste.

The surface management of the slimes provides acomplex .problem where they remain essentially asdense liquids and do not consolidate to anyappreciable degree.

17

The tailings placed underground will contribute toradon and radon daughter levels in the mine. Foractive mines, this may require additionalventilation, cement stabilisation of backfill andspecial precautions for handling the excess seepagefrom the tailings. Conversely, the backfilledtailings reduce the mine volume which may reduceventilation requirements.

Possible re-mining at a later date to recover lowergrade ore may be complicated by the presence of thebackfilled tailings.

18

DESIGN OF IMPOUNDMENT DAM STRUCTURES

41 Substantial retaining structures may be requiredfor tailings impoundments; for valley dam and ring dykeimpoundments these are generally in the form of earth orrock fill dams. Failure of such structures may have majorconsequences and great care is therefore required in theirdesign and construction. Consequently, methods developedand in general use for the design, construction, operationand maintenance of large water retaining dams, should beapplied to tailings dams, and the design of the dam shouldbe under the control of competent engineers experienced indam design.

42 The objectives for impoundment structures should beto produce a dam that will

satisfy all functional requirements,

* ensure integrity under any foreseeable conditions,

last indefinitely with little or no maintenance,and

minimise seepage through the structure and itsfoundations.

43 Safety considerations and good engineering practicedemand that the design of any such dam should satisfy thefollowing design criteria

The materials used and the construction of the dammust provide sufficient strength. along any,potential surface of sliding within the dam and/orfoundations to ensure that excessive deformation oractual sliding can never occur. This conditionmust be satisfied for any possible combination ofwater and tailings loads, dead loads, imposed liveloads and earthquake loads with proper accounttaken of the effects of internal pore pressures andtheir effect on shear resistance.

The dam and its foundations must be able towithstand internal damage (piping) by seepage.This requires that the materials used should becapable of stopping transport through them of anyfiner materials from an adjacent, less perviouszone, upstream.

The iinpmundment should be operated, so that at anytime it provides sufficient storage-volume belowthe crest of the dam to contain the estimatedinflow to the system resulting from an extremerainfall event (probable maximum precipitation)and, in addition, provides adequate freeboardagainst overtopping by wind-generated waves.

19

Adequate slope protection should be provided on thedam against wave action, erosion by wind and water,and weathering.

44 Extensive site investigation and testing ofmaterials will generally be required to determine thesuitability and properties of the foundations and of thematerials to be used for the construction of the dam. Suchinvestigations should be carried out under the control ofexperienced geotechnical engineers and geologists, and thefindings detailed in a comprehensive form in the designreport submitted to the appropriate authority. Theseinvestigations involve :

regional and detailed aerial geological photo-interpretation,

surface mapping of rock exposures in the

impoundment area,

* geophysical surveys,

detailed geological mapping of the impoundment areaand its immediate environs (using information fromboreholes, permeability testing, costeans, etc.),

test pits, and

laboratory testing of foundation and embankmentmaterials.

45 A design report, prepared by engineers experienced,in the design and operation of impoundments, on the proposedtailings impoundment should be submitted to the appropriateauthority detailing

the intended operation of the impoundment,

• the expected properties of the tailings,

foundation conditions,

availability of construction materials,

geotechnical design parameters,

* embankment design,

estimateI seepage flow-rates from various parts ofthe impoundment (e.g. through the embankment, theupper parts and the deeper parts of the damfoundation) and a prediction of the seepage loss(including contaminant load) during all stages ofana after cessation of the operation.

20

construction factors, and

proposals for monitoring and surveillance.

46 For the embankment stability analyses, conservativevalues of the shear strength and density of the compactedembankment materials must be used with conservativeestimates of the pore pressures within the embankment for

all loading cases considered. For potential failuresurfaces the minimum factors of safety, that is,the ratio

of (total shear resistance) along the(total force contributing to sliding)

failure surface, should be as follows

Minimum Factor

Loading Condition of Safety_

End of construction

Partial pool with steady seepage

Maximum pool with steady seepage

Earthquake (in combination with

above conditions)

1.3

1.5

1.5

1.0

The magnitude of earthquake loading should be determinedfrom the probable maximum earthquake acceleration for thesite area, together with liquefaction of the tailings ifthis is considered possible.

47 If the foundations under the embankment consist oflayers of varying thickness of compressible soils or softrock or if the bedrock is very irregular, differential;ettlements would occur resulting in cracks in the dam; in,hich case the dam should be designed to absorb the.nticipated differential settlements. If settlements areKpected to be large, the dam should be built higher tolow for these settlements.

epage Management

A basic performance criteria of the impoundment isminimisation of the loss of radioactive and non-

ioactive nuclides by seepage. The site of therundment and the design of the dam structure are the two)onents which together ensure that seepage is minimisedughout the mining and milling operation and after itsition. Generally, the most important step is the choicehe impoundment site. An ideal site would have low-ability strata under the floor of the impoundment

21

(i..

construction factors, and

proposals for monitoring and surveillance.

46 For the embankment stability analyses, conservativevalues of the shear strength and density of the compactedembankment materials must be used with conservativeestimates of the pore pressures within the embankment forall loading cases considered. For potential failuresurfaces the minimum factors of safety, that is,the ratio

of (total shear resistance)(total force contributing to sliding)

failure surface, should be as follows

Minimum Factor

Loading Condition of Safety

End of construction 1.3

Partial pool with steady seepage 1.5

Maximum pool with steady seepage 1.5

Earthquake (in combination with 1.0

above conditions)

The magnitude of earthquake loading should be determinedfrom the probable maximum earthquake acceleration for thesite area, together with liquefaction of the tailings ifthis is considered possible.

47 If the foundations under the embankment consist oflayers of varying thickness of compressible soils or softrock or if the bedrock is very irregular, differentialsettlements would occur resulting in cracks in the dam; inwhich case the dam should be designed to absorb theanticipated differential settlements. If settlements areexpected to be large, the dam should be built higher toallow for these settlements.

Seepage Management

48 A basic performance criteria of the impoundment isthe minimisation of the loss of radioactive and non-radioactive nuclides by seepage. The site of theimpoundment and the design of the dam structure are the twocomponents which together ensure that seepage is minimisedthroughout the mining and milling operation and after itscessation. Generally, the most important step is the choiceof the impoundment site. An ideal site would have low-permeability strata under the floor of the impoundment

21

coupled with the ready availablility of low-permeabilitysoils for the construction of impervious zones in theembankments. Alternatively, such a site could also featurea long seepage path to the point of discharge into theenvironment. Where the ideal site is not available, thequestion arises as to the best practicable technology to be.applied to and complement the inherent advantages of theimpoundment site. A range of technologies is discussed inthe following paragraphs.

49 Liners - If natural low-permeability strata are notpresent, a blanket of low-permeability material (liner)should be constructed to control the seepage from theimpoundment. If clay soils or soft weathered rocks areavailable, this material can be spread and compacted insuitable layers to form a blanket of low permeability; suchnatural liners should. generally have preference over anytype of synthetic liner. Liners may be restricted to..partsof the impoundment basin to seal potential seepage paths dueto the existence of faults, shear zones, porous rocks,sands, or gravels. Clay liners not only reduce seepageflows from the basin but also, as a result of their cationexchange, adsorption and neutralising capacities, serve toretain or at least retard the migration of radionuclides, aswell as some of the non-radioactive contaminants. Clayliners from site-derived clays or from imported materials(e.g. bentonite) can be constructed to permeabilities of

10-6 to 10-7 cm/sec or lower.

50 Long-term retention of these low-permeabilitycharacteristics may be affected by a high cation content andby a low pH seepage, but it is accepted that such a linerwill not entirely disintegrate. Clay liners are generallyconstructed in a number of layers, each approximately 200 mmthick, to a total -thickness of around 1 metre. Surfacecracking occurs readily when drying out is allowed tooccur. Such cracking can often be prevented by spreading a300 mm thick layer of loose soil over the finished compactedsurface.

51 Synthetic membranes of plastics or elastomers (0.5to 1.5 mm thick) may be useful liners for the short terme.g. during the operational life of the mine. However,their long-term stability over thousands of years whensubjected to the chemical and physical environment of thetailings impoundment is, to say the least, questionable.The same uncertainty applies to other types of linermaterials in common use in water-retaining structures. Forexample, concrete, asphalt, etc., which may be veryeffectively used vfor short-term purposes as temporary linersin tailings- impoundments, for storage of sludges andcontaminated water, should. not be considered as providingadequate long-term seals when disposing of uranium mill

22

tailings.

52 Grouting - Jointing, shearing and faulting mayincrease the permeability of rock strata. Cement groutingis generally used under the core of water-retaining dams,down to less permeable strata, in an attempt to seal theseepage pathways from the impoundment. Sulphate resistantgrout should be used, where the seepage is high in sulphatese.g. Type D cement or a mixture of cement and ground blastfurnace slag. Though grouting is recognised as an effectiveway of sealing open-jointed rock, the permeability in thenarrow band of grouted rock can only be reduced to around10-5 cm/sec. Its long-term effectiveness is also doubtful,especially where the pH of seepage water is expected togradually reduce after rehabilitation.

53 Placement - If the tailings are deposited to ensurean even distribution of the coarse and fine fractions incontinuous layers, after consolidation seepage from theoverlying water will be restricted.

54 Seepage Collection Measures - For dams with thehighest operating level more than 3 metres above theadjacent natural surface, the impoundment design shouldincorporate a seepage collection system whereby thecollected seepages would be returned to the water managementsystem. In this situation and others where seepage flowsfrom an impoundment are collected during the operationalperiod, care should be taken in the design of theimpoundment system to prevent the formation of contaminatedsprings after the rehabilitation is completed due to theconcentration of such seepage flows..

55 Although mainly effective during the operationalperiod a number of seepage collection measures include:

Seepage collection in a toe drain downstream of theembankment. Seepages through the embankment andthrough the near surface zone of the dam foundationare collected and pumped back into the impoundment.

Underdrainage on the floor of the impoundment andpumping back of the collected seepages. This canconsist of either a continuous blanket of perviousmaterial, such as sand, or of a system ofinterconnected pipe drains surrounded by perviousfilter materials. Such underdrainage can belocated above the impervious strata in conjunctionwith semi-dry tailings management or under aconstructed liner. These underdrainage systemshave advantages, in that they can de-water thetailings, thereby reducing the total wateravailable for seepage and the pore water pressuresin the tailings, with consequent rapid

23

consolidation and stabilisation of the tailings.However, underdrainage systems are liable to clogup by the movement into the system of fineparticles from adjacent material, by chemicalprecipitation and bacterial activity. Specialprecautions may be required when the tailingscontain a high percentage of pyrites.

56 Hydraulic Barriers - These include theconstruction of water storages or evaporation pondscontiguous to the tailings impoundment.

57 The Long Term -. Seepage minimisation during theoperation phase is dependent on both active and passivesystems. Following decommissioning and rehabilitation ofthe impoundment most of the active measures arediscontinued; subsequently, in the long term, reliance onany active system is undesirable. Accordingly, theimpoundment design should incorporate estimates of theseepage rates for the long term' and an assessment of theimpact of such seepage on the environment.

24

CONSTRUCTION AND MONITORING OF IMPOUNDMENTS

58 The construction of the impoundment should beundertaken in accordance with the approved design and theuse of good engineering practice. Quality assuranceprograms should be implemented so as to ensure that bestpracticable technology is achieved. Following commissioningof the impoundment monitoring programs approved by theappropriate authority, should be implemented to assessperformance of the structures.

Quality Control during Construction

59 Quality assurance programs should take into accountthe dam construction materials being used, foundationconditions being encountered, and the methods and schedulingof dam construction. Safety factors determined, at thedesign stage are based on the physical properties of theanticipated construction materials, and the anticipatedfoundation conditions. If, during construction of theimpoundment structures, the foundation conditions and thephysical properties of the construction materials are foundto differ from the assumptions made in formulating thespecifications then' changes to the specifications may berequired to ensure compliance with the principles of thedesign. Such changes to the specifications should beapproved by the appropriate authority.

60 The construction of impoundment structures shouldbe approved at key stages by the appropriate authority toensure compliance both with the specifications and with theprinciples embodied within the approved design. Tofacilitate decision making a comitttee consisting oftechnical experts should be established comprisingrepresentatives from the mining company, the- appropriateauthority and their respective tehnical consultants. Thecommittee should consider the key.stage decisions, includingthe review of results of quality control testing and ofinspections and make recommendations, as appropriate, to themining company and the appropriate authority.

61 To ensure that the construction of the impoundmentstructures conforms with the design specifications themining company should, as directed by the appropriateauthority, demonstrate. that the materials being used complywith and are placed in accordance with the specifications.This would be achieved by utilising supervising staffexperienced in the construction of water management andhaving the materials tested in accordance with appropriateAustralian standards by testing laboratories which areappropriately registered by the National Association ofTesting Authorities (NATA).

25

Monitoring of Impoundments

62 Monitoring, during and after construction, of thestructures is undertaken with the objectives of:

determining the actual performance of the facility,specifically to determine the integrity of thestructure and the available safety factors,

determining compliance with approved objectives,and

determining the validity of the models being usedto predict the future performance of the facilityand the adequacy of the rehabilitation measuresproposed.

63 Accordingly, the design and construction of theimpoundment should include provision for monitoring deviceswhich enable assessments of the stability, settlements,erosion and seepage associated with those structures to becarried out. These assessments form part of the overallmonitoring program approved by the appropriate authority.

64 The monitoring program should include monitoringfor the existence and extent of seepage from theimpoundment. This would include the establishment of anetwork of monitoring wells and devices around and near theimpoundment. (The location and depths of the wells and theparameters to be monitored should be determined inconsultation with the appropriate authority.) Theinformation thus obtained will allow the effectiveness ofthe seepage control measures to be evaluated, the need forremedial actions quickly determined, and the appropriateactions implemented promptly and effectively.

65 The manager should compile, and make available asrequired by the appropriate authority, a record of dataobtained from the monitoring program including:

observations on the conditions of impoundmentstructures,

. settlements and other movements,

* details of any repairs carried out,

* water levels in piezometers and wells,

quality iof water from piezometers and wells,

findings of resistivity and other geophysicalsurveys,

26

quality and quantity of waters collected fromseepage collectors,

dry densities of the tailings at various depths,and

depths of tailings.

66 The manager should, ensure that, during theoperational, decommissioning and rehabilitation phases ofthe mining and milling operation, periodic surveillanceinspections are carried out in conjuction with theappropriate authority. The personnel undertaking theseinspections should include an engineer experienced in moderndam technology and hydrology and a suitably qualifiedhydrogeologist. The inspections should be undertaken atleast annually or more frequently, as- required by theappropriate authority. From, these inspections and otherassessments, the manager should, where required, propose forapproval by the appropriate authority, any remedial workconsidered necessary to ensure the integrity of theimpoundment structure is maintained within the approvedobjectives.

27

TAILINGS MANAGEMENT

67 Tailings management includes methods used to placethe tailings in the impoundment area and the conditionsunder which such placement occurs. Tailings management isclosely related to water management for the mine and millsince the volume of excess water stored in the tailingsimpoundment influences decisions regarding the recirculationof tailings water back to the mill, which in turn influencesother aspects of the water management system.

68 Selection of the type of. system for tailingsmanagement is very site-specific. For example, in aridregions systems which allow the tailings surface to dry out,the resultant radon exhalation and wind erosion of particlesmay, if not controlled, distribute contaminants to theenvironment beyond the limits of the impoundment. In wetclimates sufficient excess water will usually be availableto ensure that a water cover of required minimum depth canbe maintained at all times. This cover will reduce radonand particulate dispersion but will provide a steeperhydraulic gradient for loss of solutions by seepage to thegroundwater system. Nevertheless, it is generally desirableto maintain a water cover or a high moisture content in thetailings.

Comparison of Systems

69 The appropriate authority should be provided with areport comparing the use of various tailings managementsystems for the project, and why it is considered that *theselected tailings management system is considered to be thebest practicable. technology for the site and themining/milling operation.

Design Features

70 Tailings management systems should be designed toinclude the following features

Control of distribution of tailings within theimpoundment system - Where distribution is effectedbeneath the water surface by discharge from afloating movable point (barge), considerablecontrol can be exercised on the location anddistribution . of both the coarse and finefractions. Where distribution is from the sides ofthe embankment,. control can be exercised over thelocation and geometry of the beach, and thelocatior* and size of the pond.

Control of excess water volume in the impoundmentduring the mill operating period - Where a watercover, is maintained over the tailings, this water

28

adds significantly to the hydraulic gradient andhence to the potential seepage losses. Afterrehabilitation of tailings impoundments, pondedwater will no longer exist and the hydraulicgradient may be reduced considerably, as will therate of seepage from the system.

Control of freeboard in the impoundment - Provisionshould be made in the water management system toprovide for control of the water level in theimpoundment, so that the freeboard on theembankment will not be reduced to a level criticalto the safety of the embankment. Recirculation ofwater to the mill, transfer to retention orcontrolled evaporation ponds, or release of waterin accordance with approved standards, are optionsavailable for this control. The introduction ofwater into the restricted release zone should bekept to the minimum level appropriate to the needsof the milling process, so as to preventunnecessary accumulation of contaminated water.

Control of density, compressibility, permeabilityand shear strength of the tailings within variousimpoundment zones - Where tailings are dischargedbelow the water surface from a pipeline attached toa barge, it is possible to ensure that the fine andcoarse fractions are intermixed, thereby developinga deposit which is of uniform permeability andcompressibility. Where tailings are dischargedfrom a point source on the beach, segregation ofthe fine and coarse fractions occurs. Thissegregation may result in relatively densepermeable deposits in the discharge area, andhighly compressible deposits of low permeabilityslimes away from this area. Segregation of theslimes and coarse fractions into separate areas mayrender the task of tailings stabilisation andrehabilitation more difficult.

71 Important variations in tailings management systemsdepend on the moisture content and the physical state of thetailings emplaced, and the method of tailings placement.These are discussed in the following sub-sections.

Saturated Tailings Management

72 'Saturated management' is used here to indicateplacement methods by which the tailings are transported,distributed and maintained in a saturated state during theoperational life of the project. With this method thetailings are pumped as a slurry from the mill to theimpoundment and discharged into a water cover from afloating pipeline. The end of the pipeline is moved

29

regularly by winch ropes so that the tailings solids aredeposited in horizontal layers with minimum segregation.This method of management can result in an even distributionof the coarse and fine fractions throughout the deposit anddevelop a low-permeability deposit which significantlyrestricts seepage. During these operations a water cover ismaintained over all of the tailings surface, thuseliminating wind dusting and restricting radon emissions.

73 Though only a small depth of water is required overthe tailings to prevent wind erosion and reduce radonemission to an acceptable level, a depth of around 2 metresis generally required for efficient discharge. As depthsgreater than 2 metres can generally be accommodated, thetailings impoundment for saturated water management can beused as the major contaminated water storage and evaporationpond on the project site. This will- reduce the arearequired for water management ponds and the cost of theirconstruction and operation.

74 The large water-storage over the tailings reducesthe necessity of other site water-storages. Tailings watercan be recycled as process water in the plant for up toapproximately 60% of the total plant requirement. However,the free water over the tailings makes large volumes ofwater available for seepage and enhances seepage due to theincreased hydraulic head.

75 Saturated tailings management is more suitable forwet climates, where excess water in the water managementsystem will generally occur, than for dry climates, whereexcess water is less likely to occur and a large evaporationsurface is not required. During decommissioning the watercover is removed to a storage pond of contaminated water orto a treatment plant for eventual discharge from the site byevaporation or release, within approved standards. The'tailings surface is then allowed to dry out so that accessis possible for the construction of a cover. During thedrying out period radon emissions and wind dusting willbecome of increasing concern. The tailings will be"normally consolidated" and the underwater dry densities ofthe tailings will be low. The dry densities obtained willgreatly depend on the type of material, particle shape, andthe percentage of fines in the tailings.

76 When these tailings dry out and are surcharged witha cover structure, large settlements can occur. The covermust accommodate these. settlements without loss ofintegrity. These subsidences, small along the edges andlargest over thQ deepest area of the impoundment, willchange the drainage profile of the cover structure. Acareful assessment will have to be made of the total long-term settlements which may occur, so that adequate allowancecan be made in the constructed drainage surface and future

30

ponding kept to a minimum. In valley dams with the greaterrange of tailings depths, large settlement differentialswill occur. Also, the often irregular shapes of valleys maymake it difficult to discharge the tailings at alllocations. In such inaccessible areas, slimes willpredominate and may be verydifficult to cover.

77 High water-content and low dry-tailings densitymakes liquefaction of the tailings mass possible e.g. duringearthquakes. A well-designed dam can cope with such acondition. After rehabilitation the liquefaction potentialwill gradually disappear as tailings dry out, andconsolidate. In localities with regular rainfall it may bevery difficult to obtain a dried-out tailings surface toallow access for equipment required for the construction ofthe tailings cover.

Wet Tailings Management

78 The term 'wet management' is used here to indicateplacement methods . resulting in tailings that havesubstantial saturated zones, even though the total tailingsimpoundment is not saturated throughout the operational lifeof the project.

79 The method covers the discharge of a tailingsslurry onto a beach above the water table. This is aconventional management method in which the coarserparticles settle out close to the discharge point as thetailings flow over the beach. Excess water and slimes arecollected in a pond at the end of the beach. The slimesthen settle and the partially clarified surface waterevaporates or is decanted and recycled to the plant asprocess water, or to other ponds for storage or evaporation.

80 Using a variable location single point or linedischarge allows some control over the size and location ofthe pond and beach. If the tailings dam is also used forwater storage,. the pond may be large.

81 Underdrainage systems are not usually provided aspart of this management method. The area below the pondsurface elevation remains wet and saturated and has lowshear strength. For this reason, discharge in the vicinityof the dam wall is common. If large beach areas aremaintained to limit pond size and water pressure heads toreduce seepage, the beaches may dry out and be subject towind erosion. Only a limited capability exists for keepingbeaches wet by additional discharges from the dischargepoints. The pond and soft slurries are located adjacent tothe basin sides remote from the discharge. Large areas ofsoft, permanently wet slimes result, and these may bedifficult to effectively cover using mechanical equipment atthe time of tailings pile stabilisation. As the pond

31

elevation rises and water is distributed over new areas,seepage losses can increase. With the comparatively larg.esize of the pond, which is typical for this system, largeareas of the dam are wet and saturated for long timeperiods.

82 When the tailings deposit is ready forstabilisation the major portion is saturated. The depositconsists of incompletely consolidated tailings with a highwater table, and it has large zones of under-consolidatedslimes with very low strength. The slimes may have voidvolumes greater than the volumes of the solids. The highwater table provides high pressure heads that tend to induceseepage.

83 Significant excess pore pressures can exist in theslimes creating unstable conditions under imposed loading.The excess pressure head is dependent on. the permeability(which is dependent on other factors, e.g. the grain size ofthe material in which it occurs). The time required fordissipation of this excess pressure head after shut-down ofthe mill will range from nearly zero for the very coarsebeach area to hundreds of years. for the very fine slimezones. In time, the excess pore pressure will dissipate,resulting in an effective stress increase within thetailings deposit. Similarly, as the water table in theimpoundment slowly reduces to the base of the dam, theeffective stresses in all the tailings increase. Thesestress increases can be large and result in largesubsidence. Lesser subsidence occurs in the zone withcoarser-grained tailings. The large differences in settlingrates and amounts in the various areas of the tailings pilescan be anticipated to damage the integrity of a cap andchange a contoured drainage pattern on the impoundment.

84 The main characteristics of a wet disposal systemare

high water table and pressure heads, hence highseepage gradients at the liners;

large volumes of contaminated water available forloss by seepage;

appreciable areas of soft slimes with low shearstrength that are difficult to cover;

high tailings compressiblity combined with higheffective stress changes., resulting in high surfacesettling and differential settling;

high water content and low tailings density ensuresa liquefaction potential for the tailings mass,thus increasing the risk and consequence of damfailure; and

32

lower operating costs and the need for a singletailings and water impoundment.

Semi-Dry Tailings Management

85 The term 'semi-dry' refers to a method of tailingsplacement which relies on a rapid systematic reduction, byevaporation and bleeding, of the water content in arelatively thin layer of newly placed tailings. Theconsequential consolidation ensures that the body of thetailings generally consists of over-consolidated tailingswith a relatively high shear strength. Development iscontinuing on a number of methods which have been used toobtain this condition.

86 Generally, tailings from the mill are firstthickened to a solids content which still allows efficienttransport by pipeline. The solids content can vary frombelow 40%, to 50% and over, depending on the tailingsproperties, such as their particle gradings and shapes. Thethickened tailings are placed for a short depositionalperiod, so that they are distributed in a layer of equalthickness, over a selected area of suitable slope. When.placement in that area ceases, the solids settle out andsome of the water bleeds to the top and runs down the slopewhere it is collected and transferred to a water storagepond. Some of the water may also be absorbed into theunderlying unsaturated tailings. Evaporation is allowed toremove further water from the fresh tailings layer until anunsaturated, consolidated (semi-dry) layer of tailings isobtained.

87 An underdrainage system is required under thetailings disposal site to collect and remove any, seepagewater moving down through the tailings. Bleed water fromthe tailings and rainfall runoff are collected and placed inseparate water storage ponds for re-use in the mill process,for evaporation, or directed through a treatment plant fordischarge from the site.

88 Evaporation of water from the thin layer oftailings is controlled so the tailings are no longersaturated but are at negative pore pressures which induceeffective stresses with attendant consolidation. Moisturecontents. of around 85% saturation should achieve thisobjective while still minimising the dispersion of radon andthe tendency to desiccate the surface to dust. Increasingdesiccation allows wind erosion to occur; to prevent this,fresh tailings 4re placed over the previous layer. Itshould be noted however that the fines tend to beconcentrated near the upper surface of each successive layerand on drying tend to form a slightly cohesive delicatecrust. This crust resists wind erosion until damaged and is

33

renewed with each successive layer. Where dusting doesoccur, beaches can be dampened by discharging water or moretailings, or by applying a chemical crusting agent.

89 This method of tailings management allows coverconstruction to be carried out immediately after millingceases, with good stable drainage grades promoting runoffwithout erosion. Two techniques to.achieve effective semi-dry tailings management are

Coning, where the thickened tailings are dischargedfrom an elevated position near the centre of theimpoundment area.:

The tailings solids settle out on a graduallydecreasing slope to form a cone. The excesswater evaporates or runs down the slope to aperipheral catch drain at the toe of the cone,from where it is directed to a storage pond.Experience has shown that normally more than95 percent of all solids including those ofsub-micron sizes remain, within the slopingdeposit. Rain runoff from the tailingssurface only erodes a small fraction of thetailings fines which then can settle out inthe storage pond. The surface of the tailingsarea is wetted continuously by new slurry, sowind dusting and radon emission are reduced.Resultant slopes are of 5% to 8%. depending onsuch factors as the grading of the tailings,the percentage solids of the thickenedtailings, the temperature of the tailings, pH,and climatic conditions at the site.

Sub-aerial tailings deposition, where a thickenedtailings slurry is discharged sequentially ontosections of a tailings beach from multiple pointsto form a layer of approximately 100 mm thickness:

The discharge is. moved to a new section ofbeach when this thickness is obtained, whilethe solids in the first section are allowed tosettle out and excess water evaporates ordrains off to the bottom of the slopingtailings surface, from where it is collectedand recovered. The slope of the beach isapproximately 1%. A considerable proportionof the slimes in the tailings concentrates inthe top of the settled tailings layer.Suocessive layers of tailings form a highlystratified deposit with moderate permeabilityparallel to the layer interfaces and lowpermeability normal to. the layers. This lowvertical permeability through the top of each

34

layer prevents the entry of substantialquantities of water during the placing Qflater slurry layers and results in a partlysaturated deposit. Layers are allowed to dryto around 85% saturation before the next layeris placed. The dried-out slimes form asealing layer which affords some protectionagainst wind dusting and is believed to reduceradon emissions. The depositional sequenceand permissible rate of rise depends on theclimatic conditions of the site.

A good underdrainage system is required toensure that a gradual increase of thesaturation level in the lower levels of thedeposit does not occur. Well-controlled andspecific tailings depositional techniques arerequired to prevent underdrainage clogging andbinding. An effective system to rapidlyremove bleed water from the tailings andrainfall runoff from the tailings surface hasto be incorporated into this system. Seepageof contaminated water into the underlyingground will be small compared with that whichoccurs from semi-wet and saturated tailingsmanagement structures. Structures to containthe tailings may not have to be designed aswater retaining structures. Long-termsettlements of the tailings mass will besmall. Long-term seepages from theimpoundment will depend on the effectivenessof the cover structure in preventing thepercolation of rainwater into the tailings.

During sun-drying of the beach areas, negativepore water pressures are induced that producehigh effective stresses in these beach areasof tailings. These stresses tend toconsolidate and reduce the tailings voidratio. In this manner changes in void ratio,i.e. from greater than 1.5 to less than 0.8are effected. The lower void ratio implies

lower moisture content at saturation,

considerably lower settlement potentialunder increases in effective stress,.

lower total tailings volume,

considerably increased shear strength, and

low liquefaction potential.

35

Dry Tailings Management

90 This management technique involves the drying oftailings to a moisture content that will permit transportinto an impoundment other than as a slurry. Transport canbe by conventional earth-moving equipment or by conveyors.Heap leach waste can be considered as dry tailings.

91 Dry tailings can be produced by evaporative dryingin layers which can then be transported to the eventualtailings impoundment. Tailings "drying" by belt filtrationin the milling process is currently receiving considerableattention. Tailings resulting from this process generallystill have a moisture content above the liquid limit and aredifficult to handle without some further reduction of themoisture content. Some mills currently using beltfiltration revert to a form of wet tailings management byrewetting the tailings to a thick slurry so that it can bepumped into the impoundment.

92 Dry tailings with a moisture content below theliquid limit can be trucked, spread and compacted as normalearthfill to a suitable shape for subsequent cover bysuitable barren materials. Tailings with a moisture contentjust above the liquid limit can be spread from conveyors intrenches for subsequent cover when moisture reduction byevaporation occurs in the upper layers.

93 Cover construction should be progressive with aminimal surface of tailings exposed to the elements at anytime, so that drying out with the potential for wind erosionand erosion by rainwater runoff does not become a problem.

36

REHABILITATION OF TAILINGS IMPOUNDMENTS

94 After the active operation of the mill plant andits associated tailings impoundment ceases, a program mustbe carried out to contain the tailings for the indefinitefuture so as to meet the requirements of health, safety andenvironmental protection. A protective cover placed overthe tailings impoundment is usually necessary to protect thetailings from natural erosion forces and to restore the areato a suitable land-use capability. The protective covermust also serve the important functions of controlling theexhalation of radon gas and the emission of gamma radiationto acceptable levels.

95 An engineered cover, to a standard meeting both the"disposal" and "rehabilitation" requirements has yet to beconstructed in Australia and few examples, if any, existelsewhere in the world. It can be expected that presentlyheld ideas on cover requirements and on best practicabletechnology for rehabilitation will be subject to change.The following comments therefore should not be regarded asdefinitive but as a guide to the present. state of the artconcerning rehabilitation structures. The guideline onDecommissioning and Rehabilitation of Uranium Mine, Mill andWaste Disposal Sites should also be consulted.-

96 Prior to commencement of any mining operations, adetailed proposal outlining measures for rehabilitation ofthe tailings impoundments should be submitted to theappropriate authority for approval. A program of monitoringand testing of the tailings during milling operations toobtain. data required for the final design of therehabilitation measures proposed is also to be provided tothat authority.

97 Prior to commencement of rehabilitation work for atailings impoundment, or when directed by the appropriateauthority, a proposal should be submitted to the appropriateauthority for approval, giving full details of therehabilitation measures to be taken.

98 A number of requirements are currently recognisedas necessary for an acceptable tailings cover they aredetailed below:

The cover placed on the tailings should remaineffective over a very long time span to prevent theexposure of tailings to erosion by wind and water.

It is not practicable at present to determine adefinita life for a cover, as the future effects ofwind and water erosion on the cover cannot beaccurately assessed. For . example, if localexposures of tailings occur, e.g. by gullying, arapid dispersal of all tailings into the

37

surrounding area may not necessarily occur.Furthermore, by the time a well-constructedimpoundment cover loses its effectiveness, thelevel, of contamination potential of. the tailingsmay have decreased.

The radon exhalation and gamma emission from thetailings impoundments should. be reduced by thecover structure to acceptable levels. These levelsshould be such that the exposure to radiation ofmembers of the public is as low as reasonablyachievable and below the relevant limits prescribedin Schedules 2 and 4 of the Radiation Protection(Mining and Milling) Code 1980. Furthermore, theappropriate authority may require that the radonexhalation from the covered tailings should belimited to a rate consistent with future land usesand pre-operational background levels. As anexample the radon flux from uncovered tailings wasreduced by a factor of over 100 from 8 Bq m- 2 s- 1 to0.07 Bq m- 2 s-1 by the use of a cover consisting of610mm clay, 1810mm overburden and 300mm topsoil

The seepage through the tailings should beminimised so that dispersal of contaminants bygroundwater remains at a low rate. As allrainwater which percolates through the cover intothe tailings will ultimately be discharged out ofthe tailings, the cover should be designed toprevent rainwater, percolation as far as ispracticable.

Where the climate allows, a self-sustainingvegetation should be established on the cover. Notonly will this assist in blending the impoundmentinto the landscape but also greatly reduce theerosion due to wind and water, and reduce, bytranspiration, the water available for percolationthrough the cover.

99 To satisfy the above requirements the thickness ofthe tailings cover structure may have to exceed 3 metres. Acombination of cover materials usually provides a betterprotection than any one material used alone and a number ofzones and layers of natural soils and rock are .generallyused to form a complete impoundment cover. . The materialselected for these zones will greatly depend on the economicavailability of suitable materials at or near the site, onthe climate of the area, on the type of impoundment used,and on the tailigs management system employed for placingthe tailings in the impoundment.

100 Zones of the cover structures may consist of layersof clay soil, rock, soil and rock, and topsoil and

38

vegetation. These zones are discussed in the followingparagraphs.

101 Clay soil - This material is selected for its lowpermeability property when compacted.. A clay soil zone willminimise the infiltration of rainwater into the tailings.Such a zone is also effective in substantially reducing theradon exhalation, especially when the zone retains a highmoisture content. To maximise its effectiveness it shouldbe placed in layers of approximately 200mm thickness andcompacted under moisture control to a design density of notless than 95% standard dry density. The total thickness ofsuch a clay zone may be expected to range upwards from0.6m. If an adequate quantity of low-permeability clay soilis not obtainable from near the site, the permeability ofother soils can be reduced by mixing with bentonite or usingother admixtures.

102 This is the main protective zone of the cover.Other cover zones are provided mainly as a physicalprotection of the clay zone, to keep the clay zone moist(which prevents cracking and minimises radon diffusion), toreduce root penetration and animal burrowing and to increasethe range of potential land uses of the impoundment area. A1 metre zone of moist clay will usually be more thansufficient to comply with the radon exhalation requirementsand such a zone together with other protective zones willeffectively reduce the gamma radiation to the pre-operational baseline level.

103 Rock - This may consist of barren waste rock fromthe mine or of specially quarried rock or gravels. Thematerial should be resistant to weathering so that itretains its properties over a long time. Such rock may beresistant to wind and water erosion but ineffective inreducing radon exhalation. When used at the surface, rockwill tend to collect wind-blown soil particles which mayform a favourable habitat for vegetation growth between therocks. Rock will resist invasion of burrowing animals, forexample, rabbits and wombats.

104 An important use of rock is as a pore breakingzone, preventing the capillary rise of contaminated tailingswater to the upper cover layers, where it can interfere withplant growth and be dispersed to the environment by wind andrainwater. Where such a pore breaking zone is overlain bysoil with small particle sizes; a filter should beconstructed between the zones toprevent movement of fineparticles into the pore breaking zone. Such movement wouldgradually render-the pore breaking zone ineffective as wellas destroying the integrity of the overlaying zone.

105 Soil and rock - Mixtures of soil;and rock can beeffectively used to improve the properties of each

39

material. Such a zone will be more, resistant to erosionthan soil alone, will have a low permeability compared withrock alone and will be able to sustain vegetation. It wiMlalso be effective in reducing radon exhalation if suitablycompacted and kept moist.

106 Topsoil and vegetation - Topsoil generally formsthe upper layer of the cover structure, and should bespecially selected for the establishment of a self-sustaining vegetation cover. The choice of particular plantspecies must take account of their adaptability to theparticular conditions in the area of the impoundment sitesuch as climate and the water retention capacity of thecover. The topsoil should overlie a soil or soil and rockzone which can, store adequate water to sustain thevegetation through dry weather periods.

107 In nature, a soil profile overlaying rock usuallyshows a gradually reducing permeability with depth. Wherezoning is used contrary to such a'profile, the tendency willbe for fines from the higher areas to migrate into the moreopen structures of th. lower zones. Changes will also occurto the level of soluble contaminants held in the uppertailings layers and the need for a pore breaking zone may bythen have disappeared.

108 Pipe drains may be used to collect and dischargerainwater which seeps through one or more of the coverzones. Whilst it is doubtful if these pipes will remaineffective in the long term, they may be very useful forshort-term purposes.

109 In the longterm at most locations, the uppersurfaces of the cover will gradually erode so that lowerzones become exposed and have to take over some of thefunctions of the eroded zones, such as the maintenance of'the vegetation cover.

110 The main protection zone, the low-permeability claysoil zone, is therefore best located as close to thetailings as practicable. Where tailings below the cover canbe expected to be saturated, frequently, where high osmoticpressures can be expected in the tailings, and probably alsofor tailings with a high pyrite content, a pore breakingzone should be placed between the clay soil zone and thetailings. Where saturated or wet tailings management wasused it may be advisable to use a pore breaking zone as afirst zone over the tailings, to, allow for temporary upwardmovement of tailings watec.

ill The surface of any impoundment cover structureshould be graded to ensure that rainfall, in excess of thatrequired to maintain vegetation, runs off thereby minimisingthe seepage of rainwater into the tailings. To ensure long-

40

term positive drainage grades, due allowance must be madefor the settlements which will occur in the tailings whensurcharged by the cover structure and for water loss fromthe tailings by seepage. The quantity of cover fillrequired can be reduced by grading the surface of thetailings and by using small drainage grades. Grades of thefinal tailings surface can often be controlled by theplacement of the tailings e.g. by suitable beachingmethods. Final drainage grades should be between 0.5% and3.0%.

112 For impoundments with a finished surface onlyslightly above grade, the drainage grades should be directedto one or more of the sides. Where the cover surface iswell above the adjacent natural surface, such as with a ringdyke, a surface grading to one or more spillways isindicated. Such spillways are preferrably located overlower sections of the outer embankment as wide structures ofuniformly graded rock fill, allowing the normal discharge ofrunoff water to flow through the rock fill so that the flowvelocities are kept small.

113 In very dry areas it may be. practical to shape thesurface of a ring dyke impoundment to an internal drainagesystem with drainage grades from the outer edges to thecentre:' After infrequent heavy rain storms a temporary pondmay be formed, from where most of the collected surfacewater can evaporate. In such a case, an emergency spillwayat the location of the lowest embankment height should stillbe provided. If the climate is so dry that it is unlikelythat any cover vegetation can be sustained, a zone of gravelor broken rock over a low-permeability clay soil zone maygive a good protection. Where some vegetation can beexpected a soil/rock upper zone would be better than rockalone.

114 Zoning for a cover structure is greatly dependenton site conditions such as economic availability ofmaterials, total climate, vegetation requirements andlimits, type of tailings management used, and moisturecontent in upper tailings. Figure .1 shows some zoningvariations corresponding to different rainfall and tailingsconditions.

115 Figure 2 shows a rehabilitation option for an openmine pit tailings impoundment. In this case, below oregrade uranium material, wastes from plant and watermanagement structures and all waste rock can also be placedin the pit resulting in a small hill covering all theproject wastes in one location. Where saturated or wettailings management in a pit was used the future settlementscould be quite large. The constructed shape of theimpermeable compacted clay soil layer should allow for thesesettlements to occur without the formation of areas of

41

negative curvature where seepage 'rainwater could be ponded,resulting in unnecessary seepages into the pit area as wellas the probability of crack formation in the clay soil.

116 Figure 3 shows a possible rehabilitation system fora below grade excavated tailings pit. For this example, ithas been assumed that saturated tailings management was usedwith water storage over the tailings in an area with highevaporation, so that the upper layer of tailings could beeffectively dried out. before the clay soil layer isconstructed.

117 Figure 4 shows a trench tailings disposal systemwhich is one type of specially-dug pit impoundment withprogressive construction of the trenches and progressiverehabilitation of the filled trenches resulting in only asmall area of tailings exposed to the elements at anytime. Dry tailings management was assumed, as this allowsconstruction of the cover as soon as a section of trenchfilled with tailings is completed. The trench coverconsists of all the excavated material from the adjacenttrench and would generally result in a cover thickness ofmore than 3. metres.. For up to moderate rainfall climateszoning would probably not be required except that thetopsoil should be separately stripped and placed over thesoil cover to obtain the best revegetation conditions.

113 Figure 5 shows a ring dyke impoundment. Therehabilitation uses an internal drainage system directed toone "through-flow rock" spillway through which rainwaterrun-off is discharged to the natural surface. The requireddrainage pattern may be difficult to retain permanently dueto larger settlements in the areas of deepest tailings.

119 Figure 6 shows a possible rehabilitation system fora valley dam tailings impoundment. Site conditions for suchan impoundment will greatly influence the selection of asuitable drainage arrangement for the impoundment and itscontiguous area.

120 Outer slopes of embankments of tailingsimpoundments should be relatively flat when therehabilitation work. is completed to minimise the erosionpotential. The low permeability clay zones in theembankments may be affected by chemical reactions withconstituents of the tailings seepage water. This alsoapplies to coarse materials in the embankments which willalso be subject to natural weathering.

121 Conservative values are required for the factors ofsafety for the long-term embankment stability, usuallyslopes of 1 vertical to 10 horizontal should be used wherethe flattening of the embankment slopes is carried out usingmainly soil material and not steeper than 1 vertical to 5

42

horizontal where the material used to flatten the slopes ismainly rock. Where steeper slopes are proposed anassessment should be given as to why flatter slopes areconsidered impracticable and the compensating factors andconditions identified which could make such steeper slopesacceptable.

122 The final rehabilitation proposal should includebut not necessarily be limited to

a time schedule for all intended work up to theproposed date of termination of the owner's,operator's and manager's responsibility,

the properties and quantities of materials to beused for the rehabilitation structures, and thelocations from where such materials will beobtained,

details of zoning of the cover structures and therationale for each zone,

expected settlements of the tailings,

proposed drainage profile for the surface of thecover structures and how allowance will be made forexpected settlements,

methods and structures proposed for discharge ofrain water run off,

expected location of the phreatic surface in theimpoundment and seepage flows from the impoundmentprior to rehabilitation work, during thetransitional period and over the long term,

the estimated range of radon exhalation and gamma-ray emission rates over the surface of therehabilitated tailings impoundment for the expected.range of moisture contents in the tailings and thevarious zones of the cover structure,

proposed surface revegetation procedures,

estimated long-term life of the cover structure andbasis for such a prediction, and

monitoring and inspection program to be undertakenby the owner, operator or manager. This programwould address such matters as:

the behaviour of the tailings,

seepages from the impoundment,

43

pore water pressures in the tailings,

- rainwater infiltration,

moisture content and water quality in variouszones of the cover structure,

radon exhalation and gamma-ray emmission fromthe tailings,

- rainwater runoff (quality and quantity),

- settlements of rehabilitated surfaces, and

- vegetation growth and fauna re-establishment.

123 Rehabilitation is a necessary final step aftertailings impoundment. But what is done during impoundmentcan have major consequences in terms of the cost, ease andeffectiveness of rehabilitation. Rehabilitationrequirements may also place constraints on the methods usedfor tailings impoundment. It is necessary, therefore, thatthis guideline be followed in conjunction with the guidelineon Decommissioning and Rehabilitation of Uranium Mine, Milland Waste Disposal Sites.

44

REVEGETATION OF TAILINGS IMPOUNDMENTS

124 In designing the cover for the impoundment theappropriate authority should be consulted to determinewhether or not the cover is to be vegetated. The lattercase would normally only be considered in those areas wherearid conditions prevail and the existing vegetative cover issparse, non-existent or dependent on deep root systems.

125 Where vegetation is to be established its principalpurpose is to provide additional protection against erosioneither by wind or water while at the same time improving theaesthestic appearance of the area. However, in consideringthe establishment of vegetation it should be recognised thatthe cover is not a natural system. Therefore, a growingzone will need to be established which approximates anatural progression through horizons providing for runoff ofexcess rainfall, retention of moisture in the zone for plantgrowth and sufficient storage to sustain transpirationdemands of vegetation through the variations in weather. Inthis regard, the initial site preparation of the tailingsimpoundment area should involve the stripping andstockpiling of the topsoil for later re-use. This topsoilwill form the basis for developing the growing zone. Thedesign of this growing zone should be undertaken by soilconservationists, in consultation with the engineersresponsible for the cover design in order to ensure that therevegetation will not compromise the integrity of the cover.

126 At an early stage in the operation of the mill atrial impoundment which reflects all the conditions of thefinal impoundment should be constructed. This will allowthe selection of the most suitable species and management.requirements to achieve a self-sustaining vegetativecover. The selection of a vegetation type for theartificially formed cover structure may requireconsideration of plant types which are not natural to thesite prior to disturbance.

127 For a detailed discussion of this topic refer tothe guideline on Decommissioning and Rehabilitation ofUranium Mine, Mill and Waste Disposal Sites.

45

SELECTED REFERENCES

128 These references have been selected to provide abroad overview of methods and technologies which are, or maybe applicable to the design, construction, operation andevaluation of tailings impoundments. It should be notedthat new impoundment technologies are being continouslydeveloped and introduced.

1. Australian National Committee on Large Dams. 'CurrentTechnical Practices for Design, Construction,- Operationand Maintenance of Large Dams in Australia'. June,1969.

2. Australian National Committee on Large Dams. 'Guidelinesfor Operation, Maintenance and Surveillance of Dams'.February, 1976.

3. Bishop, A.W. 'The Use of the Slip Circle in theStability Analysis of Slopes'. Geotechnique, 1955.

4. Cedergren H.R. 'Seepage, Drainage, and Flow Nets'.Wiley, New York, 1967.

5. Department Of Energy, Mines And Resources, Mines Branch.'Tentative Design Guide for Mines Waste Embankments inCanada.' Ottawa. March, 1972.

6. International Atomic Energy Agency. 'Current Practicesand Options for Confinement of Uranium Mill Tailings'.Technical Reports Series No. 209. Vienna, 1981.

7. Karpoff, K.P. . 'The Use of Laboratory. Tests to Develop.Design Criteria for Protective Filters'. Proc. -ASTMVol. 55, 1955.

8. Nuclear Energy Agency. 'NEA Study on the Long-TermAspects of the Management of Wastes from Uranium Miningand Milling'. OECD, NEA, Paris. In Preparation.

9. Sherard, J.L., Woodword, R.J., Gizienskis, S.F., andClevenger, W.A. 'Earth and Earth-Rock Dams'. Wiley,New York, 1963.

10. United States Corps Of Engineers. 'Engineering andDesign - Stability of Earth and Rockfill Dams'. ManualEM 1110-2-1902, 1970.

11. U.S. Nuclear Regulatory Commission. 'Final GenericEnvironmental- Impact Statement on Uranium Milling'.Office of Nuclear Material Safety and Safeguards, April1979.

46

12. Wilson, S.D. and Marsal, R. 'Current Trends in Designand Construction of Embankment Dams'. American Societyof Civil Engineering. New York. 1979.

12. Wittingham H.E. 'Extreme Wind Gusts in Australia'.Bureau of Meteorology, Bulletin No. 46, 1964.

47

DO 01

Rock Fill Very Dry Climate

UnsaturatedTailings

Compacted Clay No Vegetation

Tailings

Soil I Rock Dry Climate

UnsaturatedTailings

Sparse. Vegetation2

Compacted Clay

Tailings

4

Top Soil

Random SoilFilterPore Breaking Zone

Compacted Clay

-Tailings .. -

Moderate Climate

UnsaturatedTailings

Fair VegetationCover

5

Top Soil

Randomý Soil Rock

Compacted Clay

Filter

Pore Breaking Zone

Tailings

Wet ClimateSaturated Tailings

Good Vegetation

ZONING 1N COVER STRUCTURES

4,

.t

Vegetation

Clay SoiLiner

Pit Outline

* . . Settled, Profile oTailings

... '.Tailings fromSaturated TailingManagment

SECTION THROUGH PIT

REHABILITATION BACKF

Bogumr

Wastes from plant E watermanagment structures

® Waste Rock

® Gravel Soil Filter- O-5m

REHABILITATION FOR OPEF

ILL ® Clay Soil Ca'pping - 1-OrmBarren Waste Rock

®Gravelly Soil.- O-5m

Soil &•Top Soil - 1-O m

Zone of pervious soils &open jointed rock

P. PIT TAILINGS IMPOUNDMENT

C

0

.

Diversion Drain

- /

PLANTopsoil

• .•":: ::..Tailirngs...

Dam cut down tosuit rehabilitatedsurface.

SECTION A

Zone of pervioussoils and openjointed rock.

REHABILITATIONBELOW GRADE' IMPOUN DMENT --

Fig3

. •:,•. • :':•',.;.:•,:-, .•':•o'• ':-:4 .-,-.;.; •_,•

'AAStock pil e -

I TenhProgressivDevelopment

PLAN

Cover Cap\CoverCap \14,Dry Tailings Deposition

Zone of pervious

soils and open SECTION A-Ajointed rock.

Tailings.\ Cover CapNatural Surface Tai li n... - I

............. ... :..'..:..:...:....'.,'

SECTION B-B

REHABILITATIONTRENCH IIMPOUNDMENT....

( Dry Tailings Management)S-. Fig.4

.Waste Rocko Cover

Embankment * Tailings.-

SECTION B-B

Cover

,-TailingsFilter Selected Rock Fill

for through-flowdischarge

LRandom Fill,CompactedSECTION A-A

REHABILITATION ---O-RING DYKE IMPOUNDMENT

Fig 5

F, -

Open Channel Cut

PLAN

ut Off Drain

SECTION A-A

.REHABILITATIONVALLEY DAM -IMPOUNDMENT. .. i'---•.•