1982

MANUAL

ON TAILINGS DAMS

AND DUMPS

BULLETIN 45

.1982

MANUAL

ON TAILINGS DAMS

AND DUMPS

BULLETIN 45

Report prepared by Messrs G.H.H. Legge, G. L 'Hériteau, A.D.M. Penman and W.A.

Wahler for the ICOLD Committee on Mine and Industrial Tailings Dams and approved

by the 50th Executive Meeting, Rio de Janeiro, April 1982 French translation by Ph. Mendes, revised by P. Bacave and G. L 'Hériteau.

2

NOTICE – DISCLAIMER :

The information, analyses and conclusions referred to herein are the soleresponsibility of the author(s) thereof.

The information, analyses and conclusions in this document have no legalforce and must not be considered as substituting for legally-enforceableofficial regulations. They are intended for the use of experiencedprofessionals who are alone equipped to judge their pertinence andapplicability and to apply accurately the recommendations to any particularcase.

This document has been drafted with the greatest care but, in view of thepace of change in science and technology, we cannot guarantee that itcovers all aspects of the topics discussed.

We decline all responsibility whatsoever for how the information herein isinterpreted and used and will accept no liability for any loss or damagearising therefrom.

Do not read on unless you accept this disclaimer without reservation.

3

CONTENTS

INTRODUCTION

1. LOCATION OF DAMS

2. SITE INVESTIGATION

3. DESIGN

4. CONSTRUCTION AND OPERATION

5. CLOSURE AND ABANDONMENT

GLOSSARY

SYMBOLS

TABLE OF CONTENTS

12/13 INTRODUCTION

1. LOCATION OF DAMS

1.1 . General 1.2. Dumps 1.3. lmpoundments

2. SITE INVESTIGATION

2.1. Site Selection

2.2. Properties and Characteristics of Mining and Industrial Wastes 2.2.1. Properties 2.2.2. Mechanical Characteristics

2.3. Properties and Characteristics of Foundations and Borrow Areas

2.4. Geotechnical Investigations

3. DESIGN

4

3.1. Introduction 3.1.1. General 3.1.2. Designer Qualifications

3.1.3. Tailings Dam Characte­ristics

3.1.4. Mine Refuse Disposai

3 .1.5. Basic Design Data Requi­rements

3.2. Hydrology and Hydraulics 3 .2.1. General 3.2.2. Input

3.2.2.1. Size 3.2.2.2. Hazard Potential 3.2.2.3. Design Data

3 .2.3. Storage 3.2.4. Outflow

3 .3. Conceptual Design 3 .3 .1. General 3.3.2. Tonnages Involved 3.3.3. Site Selection

3 .3 .4 . Selection of Type of Dam Embankment 3 .3.4 .1 . Upstream Method 3 .3 .4 .2 . Centreline Me-

thod 3 .3 .4 .3 . Downstream Me­

thod 3.3 .4 .4 . Borrow Material,

Water Retaining Type Dam

3 .3 .5 . Slurry Transport System

3 .3 .5 .1 . Gravity Flow

3 .3.5.2. Pumping 3 .3 .6 . Water Control Systems

3 .3 .6.1 . Vertical Seepage­General

3 .3 .6.2 . Water Diversion

3 .3 .6.3 . Pond Liners

3 .3 .6 .4 . Piezometers 3 .3.6.5 . Water Recovery

3 .3.7. Abandonment Plans 3 .3 .8. Construction Plans 3 .3 .9. Opera ting Plans 3 .3 .10 . Review of Design,

Construction and Opera­tion

3 .4 . Foundations 3 .4.1 . General 3 .4 .2 . Settlem en t

3 .4 .2 .1 . Rock F oundations

3 .4 .2 .2. Corn pressible F oundations

3.4.2 .3. Strength 3 .4 .3 . Seepage 3 .4 .4 . Seismicity 3 .4 .5 . Foundation and Abut­

ment Preparation

4 . CONSTRUCTION AND OPERATION

5

4 .1 . Introduction 4 .2 . Description of Tailings Deposi­

tion Techniques

6

4.2. 1 . General 4 . 2. 1 . 1 . Deposition by

Upstream Technique

4 . 2 .1.2. Deposition by Downstream Technique

4 . 2. 1.3. Deposition by Centerline Technique

4 .2 . 2. Wall Building by Cyclone Depositions 4.2. 2 . 1 . Cycloning

Objectives 4 . 2. 2. 2. Cyclone Depo­

sition Techniques and their Control

4 . 2 . 2.3. Cycloning by Upstream Me­thods

4 . 2. 2. 4 . Cycloning by Downstream Me­thods

4 .2.2.5. Cycloning by Successive Up­stream and Downstream Sequences

4.2.2.6. Cycloning by Centerline De­position

4 .2 . 2. 7 . Central Cyclone Stations Methods

4. 2.3. Spigot Discharge Wall Building Techniques

4.2.3.1. Main Delivery Line Left on the Ground Level

4 . 2 .3.2. Main Delivery Line Periodically Lifted

4.2 .3 .3 . Guidelines for Operation

4.2. 4. Paddock System

4 . 2 . 5 . Mechanized Placing of Tailings 4. 2 . 5 .1. Moisture Content

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4 .2 .5 .2. Mechanical Dumping Systems

4 .2 .5 .3. Mechanical Wall Building

4.2.6. Tailings Dam Walls Built with Earthfill or Rockfill

4.2.6. 1. Impervious Walls

4.2 .6.2. Pervious Walls

4 .2 .6 .3. Construction to Final Height

4.2.6.4. Stage Construc­tion of the Wail

4.2 .6.5. Placing of Tail­ings Behind Earthfill or Rock­fill Walls

4.2.6.6. Rock Buttresses

4.2.6.7. Starter Walls

4.2.7. Central Discharge of Thickened Tailings Slurry

4.2.8. Miscellaneous Techniques 4.2.8.1. Disposai into Sea

or Lake

4 .2 .8 .2. Underground Disposai

4.3. Toe Walls and Starter Dams 4.3. 1. Definitions 4.3.2. Toe Walls - Construction

Requirements 4.3.3. Starter Dams -- Cons­

truction Requirements 4.3.4. Ground Clearing 4.3.5. Materials of Construction

4.3.6. Height and Width 4.4. Filter Drainage

4.4.1. Location and Alignment

8

4.4.2. Select ion of Materials 4.4.3. Placing of Materials

4.4.4. Covering of Drains with Tailings

4.4.5. Maintenance of Drains 4.5. Effluent Systems

4.5.1. Definitions 4.5 .2. Construction Require­

ments 4.5 .3. Operating Requirements 4.5 .4. Pond and Beaeh Manage-

ment 4.5 .5. Maintenance

4.6. Monitoring of Tailings Dams Dur­ing Construction 4.6.1. Predeposition Remedial

Measures 4.6.2. Monitoring for Stability

and Seepage 4.6.2.1. Inspections 4.6.2.2. Seepage Surfaces

4.6.2.3. Seepage Through Liners

4.6.2.4. Profiles of Strength and Relative Density

4.6.2.5. Monitoring by Survey

4.6.3. Records 4.6.3.1. General 4.6.3.2. Delivered Ton­

nages 4.6.3.3. Volumetric/Level

Records

4.6.3.4. Effluent Records

4.6.3.5. Wall Building

4.6.4. General Maintenance 4.6.4. 1. Effluent Trenches

4.6.4.2. Runoff Paddocks

4.6.4.3. Effluent Control Systems

4.6.4.4. Access Roads 4.6.4.5. Pipelines

4.6.4.6. Valves 4.7. Remedial Measures

4.7. 1. General Problems 4.7 .2. Appraisal of the Safety of

Existing Tailings Dams 4.7 .2. 1. Inspection 4.7.2.2. Measurements 4.7.2.3. Analysis

4 .7 .3. Precautionary and Remedial Measures

5. CLOSURE AND ABANDONMENT

5 . 1. General 5 .2 . Regulations

5 .2 . 1. Regulations in Great Britain

5 .2.2. Regulations in Other Countries

GLOSSARY

SYMBOLS

9

NUMBER

1-1 1-2

3-1

3-2 3-3

3-4

3-5

4-1

4-2 4-3 4-4 4-5 4-6 4-7 4-8 4-9 4-10 4-11 4-12 4-13 4-14 4-15 4-16 4-17 4-18 4-19 4-20

4-21 4-22 4-23 4-24 4-25 4-26 4-27 4-28 4-29 4-30 4-31

LIST OF FIGURES

TITLE

Types of Refuse Disposa! Structures Cross-valley Impoundment with Two Impounding Dikes

Mine, Mill or Plant Refuse Embankment Construction Methods - Upstream Construction Method Mine, mill or plant refuse embankment construction methods - Surcharge Mine , Mill or Plant Refuse Embankment Construction Methods - Centreline Construction Method Mine, Mill or Plant Refuse Embankment Construction Methods - Downs­tream Construction Method Water Dam Embankment Type

Typical Cyclone & Tailings Dam Water Balance (Process Water Only)

Cycloning by upstream method Cycloning by downstream method Cycloning by successive upstream and downstream deposition Cycloning by centreline method Spigot method of construction Spigot system with extendable spigot lines Spigot system with extended spigot lines following dam profiles Spigot system with extendable branch line Spigot system : Lifting of main delivery line Paddock system : Downstream construction Paddock system : Centreline construction Paddock system : Upstream construction Paddock system : Outer wall inadequately compacted Paddock system : Full compaction of outer wall by machines Mechanical Placing : Contained cone of tailings Earth and Rockfill Walls : Impervious wall Earth and Rockfill Walls : Pervious wall Earth and Rockfill Walls : Stage construction of wall Earth and Rockfill Walls : Excess waste rock placement on wall

Earth and Rockfill Walls : Deposition of tailings against wall Standpipe piezometers suitable for installing in tailings deposits Typical Seepage Surface determined by means of Standpipe Piezometers Monitoring Walls installed around the perimeter of a Lined Residue Dam Remedial Measures : Reinstatement of major gulley or slide area Remedial Measures : Perforated Pipe Drains Remedial Measures : Addition of Under-Drained Tailings Buttress Remedial Measures: Addition of Filtered Rock Buttress Remedial Measures: Flattening slope by forming a Berru or Stepping Back Remedial Measures: Flattening slope by adding a Toe Weight Remedial Measures : Incorrect construction of Rockfill Buttress

11

INTRODUCTION

Tailings dams are dams constructed of: 1) mill tailings, 2) mine wastes, or 3) earth of rock fill; for the retention of tailings slurry or slurry water for reclamation. Tailings dumps are tailings disposai structures constructed by dry or hydraulic fill means but which do not impound significant quantities of water. Both types of structures have long been designed by empirical means with less than satisfactory performance.

The failure of the Aberfan Coal Waste Dump in Wales, England in the sixties and the Buffalo Creek Coal Waste Dam in West Virginia, USA in 1972 however focused the world's attention on the potential hazard created by some of these structures because each of these failures has been responsible for the death of more than 1 OO people who were not miners. Smaller scale failures killing only a few miners, or doing only environ­mental damage had previously been relatively common, but had usually been dismissed simply as an occupational hazard.

With the recognition of the potentially great hazard that could be created by these structures came the need for better design and construction to increase their safety. Recognition of water retention structures as "dams" regardless of the material or method of construction led to ICOLD's interest, and the formation in 1976 in Mexico City, Mexico, during the 44th Executive Meeting, of the ICOLD Committee on Mine and Industrial Tailings Dams which compiled this Manual. Dumps, while not the direct concern of ICOLD, are included in this Manual because much of the technology involv­ed in their design and construction is similar to, or an adaptation of that used in the design of the dams. Additionally many <lumps become dams at some time during their construction, and all dams become dumps when their impoundments are filled with tailings.

Tailings are the waste minerai remaining after the valued (values) minerais have been removed from the ore. Typically the ore (hardrock ore) is crushed to a fine sand consistency at a concentrator mill, and "values" are removed by floatation or chemical processes in the form of "concentrates". The valueless minerai remaining at the "tail" end of the process is referred to as tailings. Tailings are generally in slurry form at the end of the processing operation and for convenience and economy are generally transported to the tailings disposai structure as a slurry. It is the fonction of the tailings impoundment to retain the slurry during the process of sedimentation during which time the tailings go out of suspension and fall to the bottom of the impoundment. A decant system is used to remove (decant) the clarified slurry water from the impound­ment either for disposai or for re-use or "reclamation".

With coal which is mined from thin seams, it is necessary to remove the non combustive detritus from the material brought to the surface by a heavy media process to remove the coarse waste, and a washing process to remove the fine waste. The coarse waste material is generally used for dump or dam building, and the fine sludge is impounded for sedimentation and water clarification behind the tailings dam. Large amounts of water are used in both the milling and washing processes. This water cannot

13

be reused or disposed of in most streams without the removal of the suspended solids; therefore, there is a need for settlement ponds and retaining dams.

Tailings dams, or dams for the retention of tailings slurries have many features in common with water diversion or retention dams, therefore, it is natural to start with water dam technology for their design. However their operational needs are different, as are many of the schedule, construction techniques, and minerais characteristics considerations involved with minerais concentration mill and coal cleaning plant tailings disposai. For example, tailings dams are designed to be abandoned and not operated; their construction is usually simultaneous with their operation; where possible they do not cross streams, nor do they usually impound water for purposes other than sediment­ation and reclamation. Their ultimate purpose is for solid waste disposai, not water retention, which is only incidental to their operation. Therefore, the automatic applic­ation of water dam technology is often not recommendable and may be expensive. This manual therefore has been prepared to acquaint both the dam engineering fraternity and the mining industry with the application of civil engineering and geological engineer­ing technologies of climatology, hydrology, hydraulic structures, seismology, soil mechanics, and earth work engineering and construction applicable to tailings disposal. This is not a manual presenting the technology of these highly diverse and complex specialities, but rather it is a manual covering the application of these technologies to tail­ings disposal. It is not meant as a design handbook, nor is it addressed to senior specialists but intended as a reference book and guide.

Consequently this manual is only a modest start which may in future be updated and corrected as required. It is intended to remain a guiding tool to be gradually improv­ed by reports and publications inventoried and kept in an active and practical biblio­graphy, in other words a register of the art for reference and communication.

Tailings are not just a different type of earth material used in the construction of some exotic dams. They are one of the most abundent dam building materials in the world. They exist in some quantities in most countries, and in some countries they exist in huge quantities. Mine wastes are, in fact, the most commonly handled materials in the world. The volume or tonnage of materials handled throughout the world each year exceeds that of any other industry in the world including non-mine construction.

Consider just coal mining and non-ferrous metals. In 1977 world coal production was estimated at 3,8J5,000,000 tons while the 1974 world production of non-ferrous metals was estimated at 16,425,000 tons (of metal). Since that time the annual tonnage has increased by about half as much. The introduction of continuous mining machines by the Coal Mining Industry since the late 1940's, and the mining of thin layers of coal with shale or sandstone innerbedding has increased the amount of coal waste brought to the surface and has introduced the need for coal washing plants at the mine site in order to minirnize the haul costs and ash disposai at the point of use. The lowering of the quality of metal ore now being mined for example, in the copper industry, from about 3 or 4 % to 0.3 or 0.4 % has also increased the amount of waste produced. It is probable that mine waste, and mill and plant tailings production currently ( 1982) exceed, 5,000,000,000 tons annually therefore, tailings disposai is a big operation.

15

Tailings dams and disposai structures are individually large and therefore poten­tially significant engineering works. ln a 197 5 compilation by the US Department of the Interior, Bureau of Reclamation eight tailings dams were listed among the world 's major dams.1 The World Register of Mine and Industrial Tailings Dams (1982)2 with data on 28 countries, lists 8 tailings dams higher than 150 m, 22 higher than 100 m, and 115 higher than 50 m. In addition, six impoundments have a surface area greater than 100 km2, and a storage volume greater than 50 hm3•

The content of this manual comprises four main sections :

- Selection of dam locations and site investigation - Design - Construction and operation - Abandonment

Safety aspects are referred to within the text of the manual. It was considered more desirable to make pertinent references on the subject in the above four chapters.

Acknowledgements

On behalf of ICOLD, 1 wish to thank the members of the committee for their contribution and specially the authors, Messrs. G .H.H. Legge, G . L'Hériteau, A .D .M . Penman and W .A . Wahler and their confrères, for their most valued cooperation in the preparation of this document . We hope that the type of information it contains will contribute to raising the present standards in the industry to a higher level offered by present day technology .

1 also wish to express a special thank you to a dedicated and persevering colleague in our organisation, Philippe Mendes, for his tremendous help in coordinating the prepar­ation of this manual and for carrying out a tremendous effort in the translation into French. Also a note of acknowledgement to Pierre Bacave and G . L 'Hériteau for the review of the French translation.

*The Committee came to the end of its term in 1982 ••As from May 1982

BIBLIOGRAPHY

C .A . Dagenais

Chairman of the Committee (*) President, /COLD (**)

[1] The World 's Highest Dams, Largest Earth and Rock Dams, Greatest Man-Made Lakes, Largest Hydroelectric Plants, Major Dams. Bureau of Reclamation, U .S. Department of the Interior, 197 5.

[2] World Register of Tailings Dams and lndustrial Waste Embankments (1982), ICOLD, Central Office , Paris .

17

1. LOCATION OF DAMS

1.1 General

The dams generally s tudied by ICOLD are of the water retaining type and are built acros s or along a rive r . Tai lings embankments may be bui lt on any t opography f or the purpose of creat ing :

a dump if the tailings are laid and s t ockpiled in the dry ;

an impoundment if the tailings are lai d and s tockpiled with water.

As in the case of water ret aining dams the location of the dumps and the impoundments should satisfy the general requirements imposed by:

a ) economy b) the s tabi l i ty of the dispos al sys tem ( tailings dams or dumps ) c ) public safety and the environment d ) good funct ioning of the ope ration during its es t i mated lifet ime

and after abandonment .

S ize of catchment ups t ream of a dump or tailings dam is another important cons ideration in selecting locat ion ; the smaller the catchment , the less inf low that has to be by-pass ed around the dump or impoundment or handled through the impoundment and its ou tlet facilit ies .

The s i t ing of the mines is determined by the location of the corresponding geological depos it ; that of the process plant is m:::>s t often s elected s o as t o avoid t ranspo rtation costs of non-usable material such as mine or industrial was t e . The s i t ing of the tailings dams and impoundments depends on the s i ting of the mine or the plant and also on the ope rating condit ions and pollution risks .

As in the case of water retaining dams , s tabi lity is a funct ion of :

the field characteri s t ics and the local condit ions ( topography , geology , hydrology, climate, s eismicity, overall geotechnical conditions , earthquakes ) .

1 9

S t ability of the dumps or impoundments of mining and indus t rial tailings is also a function of :

the mod ifications to these characteri s t i cs by man made works ( mining works for ins t ance ) ,

the nature and part icular prope rt ies of the s tockpiled material and its by-product s ,

the evolut ion of those propert ies , by cons o lidat ion or alt eration in the case of me chanical and phy s ical ones , or by chemical reac t ion,

the s t ockpiling cond itions ( in the dry or in the wet ) , du ration and rates of s tockpiling ,

Public safety and res pect of the environment call for the same conditions as a water retai ning dam with added provisi ons against much great er pollut ion risks :

f rom the water , due to the pres ence of so luble s al t s in the ore or brought into the ore du ring p roces s ing , or the format ion of harmful subs tances res ult ing from alt erat ion or aging of the s t ockpi le mat erial . Dumping polluted water coming f rom the ope rat ion , or rain water flowing through the s tockpiles , into a river or into the ground water is to be avoide d ,

f rom the air by dust o r by gases f rom the ore , from i t s p roces s ing or from the chemi cal trans f ormations taking place during the aging of the s tockpi le .

These pollut ion risks must be avoided du ring the normal operat ion and also during whateve r incidents may occur during or fo llowing the termination of the operat i on ( development of leaks for ins tance ) . Chap t er S makes a review of precautions fo llowing abandonment .

In addition , certain s t ockpi les have sp ontaneous combus t ion risks ( for example , coal and pyrites ) , which may caus e fire or more f requent ly the product ion of noxious gases .

Even though all the necessary arrangements are f eas i ble and can therefore be implemented , the proper authorit ies may , in particular in the vi cini ty of populated zones or near tourist s i t e s , f orbid the construction of ponds , dumps or impoundment s , or require certain other protect ive measures , like plantat i ons , whi ch will increase the cos t of the operat ion.

A dump or an impoundment of mining or indus t rial was te mus t have suffi ci ent dimensions to al low the mine or p lant to ope rate during its es t imated lifetime .

A proj ect is implemented within certain technolog ical or economic condit ions whi ch may be modified du ring the lifet ime of the works . I t is therefore important to ensure from the ou tset that cons i s t ent with s ound civil engineering p ract i ce , poss ible changes in the main paramet ers affect ing operat ion are not overlooked in site s elect ion.

2 1

The preceding li s t ing is not res trictive . I t s purpose is to show that techniques generally used by conventional dam design engineers mus t be supplemented by those of mine and plant operating pers onnel.

The best s i te or s i t es f or a s tockpile or an impoundment of mining and indus trial was t es may be selected only in close collabo ration among the concerned parties : operating eng ineers , mine enginee rs , chemical enginee rs , geolog ists , geotechnical enginee rs , water retai ning dam des igners , and other specialists .

The choice of a dump or an impoundment or a combination of the two depends on the nature and condit ions of the contemplated ope ra­tion. This cho i ce is practically imposed on the civil engineer who intervenes ( or should intervene ) to take advantage techni cally and economi cally of such natural conditi ons as topography, geology , hydrology , climat e , soils , at the di f ferent pos s ible sites . The Engineer should also obt ain transportat ion and unloading cos ts per unit volume of the material stockpiled plus the cos ts of required remedial work (improvements and reinf orcements ) .

E ven with good initial des ign , this remedial work may become neces sary due to changes that occur in rate of output or the type of waste product during the long cons t ruction per iod of the dump . It is frequently needed on out s ide slopes cove ring a choice of materials , compaction, drainage to avoid pore pressures, etc . Less often , remedial work may be ca rried out on the bot tom, on the interior s lopes or on the <lumps by adding drainage and /or watert ightness facili ties . Ins ofar as pos s ible , it is aimed to :

in the cas e of <lumps , to reduce the surf ace area of the ou t s ide s lopes and to increase that of the natu ral s lopes ,

for impoundment s , to increase the ratio of the s tockpiled material to the volume of the retaining structures ; this ratio which is f airly low in f lat te rrain , may become much g reater in rugged terrai n .

Various types o f stockpiling are thus poss ible as shown on Fig. 1-1 {l).*

1 . 2 Dumps

1 . 2 . 1

This section des cribes five methods to dispose of mine and indus trial was t e .

The valley f ill type Al . This me thod is used in a natural valley ( or an artificial one in the case of an old open pit type operat ion) f i lled in by mining or industrial was t e . The embankment has an outside slope on one s i de only ; it may be made from bottom to top or by dumping from the top progres s ing along the s teeper s lopes of the valley . The hydraulic cond iti ons may require

* F rom paper R14, Q44, 1 2 th ICOLD Congres s

23

A 2 4 5

� � @; � �-----::::::::::

6 7 8 9 10 B

\,l�l•l•I� Fig. 1-1

Types of refuse disposai structures.

(A) Type of dump. (B) Type of impoundment.

( 1) Valley-fil/ (2) Cross-va/ley. ( 3) Side-hil/. (4) Ridge. (5) Pile.

(6) Cross-va//ey ( 7) Side-h il/. (8) Diked. (9) Incised.

( 10) Camp/ex: cross-va//ey, side-hill.

24

1 . 2 . 2

1 . 2 . 3

drainage of the valley bot tom and divers ion of stormwater from the cat chment surrounding the f i l l . The type of s tockpi led material , and the perme ability of the top slopes and of the bot t om, may cal l f or watertigh tness measures . This type of dumping is o f ten the mos t economical and is adop ted whenever poss ible .

Cross valley fill type A2 . This mine or industrial waste embank­ment has an outside s lope on the two s ides upst ream and downs t ream of the val ley . In general an impoundment wi th run-o f f water in the valley ups t ream of the embankment should be avoided and allowance for drainage unde r the stockpi le should be made .

Hills ide type A3 . The tailing stockpile has an ou ts ide slope on three s ides . I f it does not ext end down to the bottom of the valley the following mus t be ve rif ied :

Bll

the reduct ion of s tability of the natural slopes of the valley caused by the f i ll , s ince the stockpile is not a s tabilizing f actor over the natural slope as was the case for types Al and A2 ,

the poss i bility of leakage f rom behind the embankment into the natural slopes wi th the risk of causing a rise in pore pressures and pollution of the ground wate r .

FIG. 1-2

Retenue à deux digues en travers de la vallée Cross·val/ey impoundment with two impounding dikes.

1. Digue amont = Upstream Dike 2. Digue aval = Downstream Dike 3. Bassin de décantation = Decant Pond4. Tour de décantation = Decant Towers 5. Galerie de dérivation = Diversion Tunnel 6. Collecteur = Co/lector

2 5

1 . 2 . 4

1 . 2 . 5

1 . 3

1 . 3 . 1

1 . 3 . 2

1 . 3 . 3

1 . 3 . 4

1 . 3 . 5

1 . 3 . 6

Ridge type A4 . depos it height . apply.

The stockpi le is comparable to A3 , with a varying The comment s and observations made for A3 also

Pile type AS . The s t o ckpile has an out s i de s lope in all around its periphery . It is of ten the les s economi cal type of dump , although it is the only poss ible one f or f lat ground . It is used wherever t opography so dict ates .

Impoundments

Cross valley impoundment type B6 . The retaining dam has the same positioning as tai lings stockpi le A2 . But unlike to A2 , this dam is on a s ingle s ide of the impoundment . It must carry the load of partly liquid tai lings deposi t ed ups t ream. The stability and the risks of erosion of the valley slopes serving as abutments to the d ike should be verif ied . To avoid pollution by the impounded waters , the watertigh tnes s of all the upstream part of the val ley should be verified . As in the case Al , s tormwater diversion f acilit ies must be provided .

Hills ide type B7 . The impounding embankment has the same pos i­tioning as A3 . I t is on three s ides of the impoundment and carries the load of the impounded tailings . The stabi lity , the eros ion risks and the wate rt ightness of the valley abutment should be verified .

Impoundment surrounded by a s ingle dike , type B 8 . The dam surrounds the impoundment on all s id es . This case app l i es on f lat ground . The s tabi lity of the foundations of the embankment and the watert ightness of the bot t om of the impoundment should be veri f ied .

Inside impoundment type B9 . The impoundment is partly buried . The dam affects only part of its periphery . This type applies on generally f l at ground . It is a combination of types B7 and B8 and the limit ing cas e of type B8 where the dam height and the dep th of backfill would vary . The commenta made on B7 and B8 apply.

Complex system, cross valley and s i cle hill type B l O . This is a combinat ion of types B6 and B 7 ; the comments made for B6 and B7 apply ,

Cross -valley impoundment with two impounding embankments B l l , as shown by f i g . 1 -2 . The f irs t embankment is on the downs tream s id e and the s econd one is on the upstream s id e . The t ai lings a r e impounded between the two embankments , which can b e built from borrow mat erial , earth or rock, or from the tai lings themselves i f they are coarse enough . The river i s dive rted by tunnels s ized to conduct the p roject design f lood . As the d is tance between the two dams may be arbi t rari ly long , this type offers big impoundment volume . It has the advantage that the volume of the dams is small with respect to the volume of the impoundment , i . e . les s coarse material is necessary to build a s table cons t ru ction ( 2 ) .

27

B IBLIOGRAPHY

1 . "Mine Refuse Impoundments in the United State s " , Wahle r , W . A . and Schli ck , D . P . , Proceedings 1 2t h Internat ional Congress on La rge Dams , Ques tion 44 , Report Rl 4 , Mexico City , Mexico , 1 9 7 6 .

2 . " Experience from the Construct ion o f Dams f o r Ta iling Dump s in the People ' s Republic o f Bulgaria " , Ilievs , S . Nikolov and G. Avramov , Proceedings 1 2th In ternational Congress on Large Dams , Ques tion 4 4 , Report 1 7 , Mexico City , Mexico , 197 6 .

3 . Carmet , Canada - Cente r for mineral and Energy Technology , " P i t S l ope Manual - Chap . 9 - Was te Embankments "

29

2. SITE INVESTIGATION

2 . 1 S i t e S elect ion

The s ite invest igation should provi de des igners with the knowledge of all necess ary elements to permi t the choi ce of the bes t type of impoundment (s ee Chapter 1 ) meet ing the crite ria imposed by e conomy , embankment s tabi l i ty , public safety and a sound operat ion. The extent of the s ite investigation is governed by :

dimensions of the dump and of the impounding embankment s , s i t e comp lexi ty , consequences of poss ible fai lure , risks of pollution , atmospheric or water resources .

I f the des ign height of a dump or impoundment dam is to exceed 10 m, the inves t igation should comp r i s e , in addi tion to testing and the assessment of charact eri s t i cs of the foundat ions , borrow areas and the mine and indust rial wastes , the same type of informat ion obt ained for a water retaining dam and cove ring :

topography wi th the locat ion of rivers , streams , dwellings , installations and underground workings ,

geology (nature , properties and arrangement of the d i fferent layers or forma t ions , rock out crops , geologi cal featu res , kars t , seismi city ) ,

hydrology (s t ream f lows , f loods , location and f luctuati ons of the phreat i c surface ) ,

climate ( rainfall di s tribut ion and ext remes ; t emperature pattern, evaporat ion, f ros t , f rost du rat ion and penetration , s nowfall ) ,

water usage , surface or underground .

Knowledge of regional geology is necessary to properly log the drill holes and to k now the extent of c lays , s i l t s , and gravels that might underlie the areas . In turn, the dri l l-hole logs help in conf irming the s i te geology . Rock type s , faults and mineral iza­t ion, that are neces s ary in ore del i neat ion are also helpful in choosing a tai l ing s i t e .

It should never b e assumed that n o economic mineraliz ation exists beneath a tailings s i t e . This should be ve rified by dri lling . Then, i f it i s relatively shallow , the overburden should be dril led and sampled down to bedrock or to a f irm base for the dam foundation.

3 1

Undis turbed samples should be taken, and the soil t es ted for such geotechnical p roperties as gradation, shear s t rength, cohesion, dens ity , mois ture and soil class if i cat ion . S tripping was tes , if to be used f or dam cons t ruct ion , should be tes ted for geotechni cal propert ies . Tes t pi ts should be excavated within the pond area and also sampled f or potent ial dam building materials . I t is important to know the kind and vo lume of various mat erials avai lable .

The tailing impoundment should be des igned to withs tand the maximum earthquake that could be expe cted at the s i t e . The magnitude , distance from the s ite , and dep th of the s t ronges t earthquake recorded should be noted , and from thi s , peak accelerat ion on bedrock , peak velocity and di s p lacement can be predicted .

The des ign engineer should know the weather in the area , using the nearest government meteorologi cal s ervi ces of f i ce unless it is s ome distance from the mine site . Then a weather station should be established at the s i te to get annual precipitat ion by months , evap orat ion, s treamf low , wind ve locity and direct ion , and temperature . Gold winter temperatures can affect the mode of tailing depos ition .

All dr ill holes should be carefully logged , regional and perched water tables noted , and water s ampled at dif ferent p laces within and below the embankment . Those ho les below the embankment should be kept for periodic water qual i ty tests bef ore the mine becomes operat iona l and during the ent i re li fe of the mine .

The informat ion to be obt ained in an inve s t i gat ion program, although comparable to that obtained for a water retaining dam, pres ents some di f ferences due to the mining or indus t rial operations under s tudy , as it may have an inf luence on the operation itself .

The topography becomes important in de termining :

the mos t economical type of dump that may be adop ted ,

the me thod of dumping (see Chap ter 4 ) ,

the embankment hei ghts to be allowed for in view of the disposa! space and the volume of tailings to be s tockpiled , and the permiss ible rates of rise .

33

2.2

2 . 2 . 1

The loca t ion of the s t reams and rive rs and the hydrological condi t i ons assume particular i mportance due to the water pollution r isks caused by the products of the ope ration unde r s tudy .

The ground permeability has a maj or import ance for the same reasons .

Earthquakes may cause liquefaction of the s tockpi led material and thus modify the eq uilibrium of the embankment , even though the embankment and its foundations may not themselves be liquefied .

The design should be such that catchment runof f does not mod i fy in any s igni f i cant way the freeboard or the water depth in the sediment at ion bas ins , these heights being set for saf ety and in order to obt ain the high degree of clarit.y des i red in the operat ion. L ikewis e , frost should not b ring about an unexpected s toppage of the operation , preventing the disposa! of run-of f or causing an unacceptable inc rease in pore p ressures .

The s ources and methods used to obtain the necessary information are the s ame as those used for the s tudy of water retaining dams . The difference lies essentially in the s tudy of the properties and characterist ics of the mining and industrial was tes and of thei r effect s .

A s to the topography, u s e is made of published maps and aerial photos . If not available , new pho to surveys can be carried out if necess ary . Preliminary s tud ies are generally made on 1 : 5000 s cale maps with contour lines every 2 , 50 m. Detailed studies are made at a larger s cale with closer contours. Maps should contain : contour lines , rock out crops , wooded areas , surf ace or underground drain­age , road access , all those elements whi ch may affect or be aff ec­ted by the s tockpi le or af fect the safety of the stockp i le , survey points , holes , boreholes and shaf ts ) .

F or the geology, use is made of maps and exi s t ing documents supple­me nted by the same comp lementary inve s t i gat ions by tes t pits , t renches , galleries , boreholes w ith s ample collection, and water and geophysical tes t ing. All natural anomali es , like artes ian phenomena, karst or swelling clays , or art ificial ones like mining work s , are searched for and ident ified. The equipment used , the technique and the collected samples ( remoulded or und i s turbed ) are those of civil engineering .

F o r hydrology and climatology the local recognized services and the methods in use in the s tudy of water retaining dams are used .

Properties and Characteris t i cs of Mining and Indus trial Was tes

Properties

Although soils and waters found in engineering water retaining dams are usually chemically neutral , such is of ten not the case for mining and industrial was tes . Their compos i t ion varies according

3 5

to t he natu re and cond i t i ons of the operat ion and they are prone t o change ove r a period of time . The was tes may contain soluble and toxic s a l t s .

In coal mi nes , was tes carry a high p ercentage o f schists and s i l t ­s tone or sands t one ; t h e schists an<l the s i l t s tone ma y be al t e re d i n the long t e rm. T he was t es cont ain coarse and f ine components which may be s tockpile d separately or toge ther . The coarse elements are used to cons t ruct re taining dams for the impoundment of f iner e le­me nts by wet me thods . In some cas e s the slurry is f i l t e re d and the cake is mixed with the coarser element s . The f i l te red mat erial is wet when i t leaves the plant and i t may harden during its t ransport .

A f resh dump of coal was tes is normally neut ral or s lighty alka­l i ne , but weathering may increase i t s acid i t y . A high degree of acidity and a high content in salts of manganes e , i ron or aluminum in par t i cular , can make the ef f luent toxic and affect vege tat ion .

I n asbes tos mines , the coarse and the f ine was t es may be dumped s eparatel y ; they are generally t rans p o r t ed in a d ry s t ate t oward a common dump . Thes e was tes contain clay elements which when expos ed to atmosphere and to water ( atmospheric or antidust sp rinkling wat e r ) may aff ect the s tabi lity of the dump .

I n copper , lea d , zinc and iron mines , the was tes may have a high su lphur cont e nt , par t i cularly iron. These are heavier than the s i licious elements and s e t t le at a fas t e r rat e . As they become oxid ized , they may form hard crusts preven t i ng or limi t i ng the drai nage .

A high content of sulphur may bring about igni t ion and spontaneous combu s t ion, t hus adding dangers to t hose due to water pollut ion . T he combu s t ion may be ext ingui s hed by cove ring or mi xing wi th was te s lurries but these may hamper the s t o ckpile s tabi l i t y .

In gold mi nes , the o re i s t reated w i t h sod ium cyanide whi ch i s highly taxi e . Fortunate ly , the cyanide oxides very rap i d ly on exposure to air to f o rm harmless compounds . When the gold is associated wi th sulphides ( pyri t es ) , some of which rema in in thet a i l ings , acidic cond i t i ons will develop as a result of oxidat ion .

I n uranium mines , acid is used to leach the ore ; the ef f luent has a very low pH and a high me t al conten t . E f f luents should be collec­ted , cont rolled and treated if nece s s ary , and the inf i l t rat ions reduced to a minimum. Radiat ion and radon gas are emi t t ed f rom uranium was tes and the i r ef f luent s . These emanat i ons should s imi larly be controlled if necessary .

In porphyry mines or quarrie s , was t es are ri ch in quartz and s i li­cat e . They have no cohes ion bu t they have a very high angle of shearing resis t ance . They may be used to build dams , but taking into account the i r high erodibi l i ty by wind and water , this implies reveget a t ion of the s lopes or blank e t i ng them with a layer of coars e non-erodible material .

37

2 . 2 . 2

I n potas s ium plants , the was tes vary in s iz e generally from 5 mm grains to the salt in the solut ion and have an allowable potas s ium chloride content (KCl ) with va riable contents of inso luble clays ( carbonates and s i l i cates ) . They are pumped back to bas ins in a saturated brine . The coarse component s set t le down and drain rapidly f orming stable depos its . The f ine part ic les s e t t le slowly bringing air wi th them and of ten producing foam at the bas in sur­face . Allowance should be made for a suff i cient sett lement bas in surface and for tranqui l i z i ng screens at the entrance to the bas in to s t op the f oam. D i f f i cult ies have been experienced when an oper­ation had only one sett lement bas in (br ine leakage , diss olut ion of brine by rainf all water ) . It is p referred to allow for natural bas ins to hold the po tass ium was tes , or to bulld retaining dams with the borrow mate rials , or to allow separated bas ins for the f inal sett lement of the clay po rt ion of the was t e . Brine fnf ilt ra­t i on into the ground should be l imited , and one should make sure that the soils at the bot t om of a dump have very low permeabi lity or otherwise provide a wat ert i ght membrane .

The preceding list ing is not res trictive . Invest igat ion methods require the use of techniques di f ferent from those in ci vil engi­neering , especially in the fields of chemi s t ry and mineralogy .

Mechanical Characteris t i cs

o Tailings

As in s o i l s , tailings have ident i f ication , s t rength anddeformat ion characteri s t i cs usually measured by soil me chanicstes t s . The was t es and the soi ls are a t riphase medium ( s olidgrains , liquid and gas in the pores ) . Howeve r , more frequent ly than in soil mechani c s , par t i cular cond iti ons are found in thewas tes , such as :

grains of an unusual nature , e . g . flaky particles or material having a thixotropic behavior like bentoni t e ,

pore f luid other than water o f neutral pH ,

inters t i t ial gas other than air , bringing about par t i cularly changeable cond i t ions (so luble salts whi ch in the long run cernent the depos ited materials ) .

The characteri s t ics of tailings are det ermined according to techniques well known to civil enginee ring . The f o llowing propert ies are part icula rly relevant :

i ) For ident i f i cation:

the part icle s ize dist ribut ion curve and the uniformity coef f i ci ent Cu = D60

D l O

39

Tailings may exhibit a large variat ion in granulometry, with grain sizes va rying from coarse sand to colloidal part icles ;

the Atterberg l imits : the l iquid l imit LL , the plastic l imit PL and the plastic index PI = LL - PL . Nomenc lature should comply w ith the lexicon pub l i shed in the magas ine Geotechnique Vol . 2 No 1 , pp . 8 4 - 8 6 , or any other more recently pub l i shed nomenc lature . For coal t a i l ings , the LL is gene r a l l y s i tuated between 2 0 and 6 0 % and the PI between 0 and 30 % .

- Water content w % .

the weight densi ty of the grains Y s the wet material Y the dry material Yd the satu rat ed material Y sat the inters t i t i al l iquid Yw

F or the was tes Ys generally l ies between 2 5 and 35 kN/m3 according to the mi neralogical compos ition. The vo i d rat io e or the porosity 11 may be established from the coeffi cients Y s and Yd • and the waste charact eris t ics may be compared to the maximum Standard P roctor maximum dens ity . the rate of variation of 't with depth ;

the relative dens i ty given by :

Dr = emax-e • 1 00% = emax emin

Y max • Y d - Y minY d Y max - y min

the permeability coef f icient k ( m/se c ) . The cond i t ions in whi ch the was tes are depos i t ed o f ten lead to a horiz ontal permeability that is 1 0 to 1 00 t imes greater than the vert i cal permeability .

the draining out or drying t ime of tailings sand ;

the weight averaged ratio weight/volume f or use in des i gn during tailing s i te fil ling and abandonment .

i i ) F or strength:

The shear strength parame t ers � ' and C ' may be obtained from di rect shear or triaxial compress ion tests made on undis turbed samp les . The undrained shear strength Cu may be obt ained from t riaxial comp ress i on tests or f rom in-s i tu tests such as the vane shear tes t . � ' represents the angle of shearing re s i s t ance in t erms of effective s t resses and C ' rep res ents the cohes ion int ercep t in terms of ef fect ive stresses .

F o r the was tes , one generally uses C ' = 0 and � ' comprised between 2 2 ° and 34 ° f or coal operations and between 30 ° and 36 ° for the other operat ions .

4 1

i i i ) For sett lement :

The measu rement made mos t of ten is by an oedome ter which gives :

a cu rve e = f (p ) of the variat ion of the vo id ratio e as a funct i on of the app lied p res sure p

the maximum pre-conso lidat ion p ressure Pc

the comp ress i on index Cc

the coef ici ent of consolidation Cv

the degree of cons olidat ion U •

F or more advanced s tudies ( f or instance , l iquefaction caused by an earthquake ) , the va riat ion of the def ormat ion moduli may be determined in terms of def ormation or applied stresses following the same technique as for soils .

o Depos i t ed Tai lings

The me chani cal characteri s t i cs of the depos i ted tailings dependon the way in whi ch they are lai d ; reference should be made toChap ter 4 , Sect ions 4 . 2 . 2 and 4 . 2 . 3 .

When the tailings are spi go t ted (see G lossary for des cription)over a non-submerged beach, the coarsest parti c les are depos itedf i rs t , fo llowed by a depos i t ion according to grain s iz e , wi ththe s l imes sett l ing in the pond . The separation of the materialdepends on its uniformi ty coefficient , as wel l as on the dens ityand quant i ty of the pu lp. I t is necessary to predict theseparation pat tern of the depos ited tailings as to coarsenessand grain s i z e , dens i t y , shear s t rength and permeabi lity . It ison this bas is that the me thod of cons truction of a tailings damis determined ( Ref . Chapter 4 ) . Analy t i cal (40 , 4 1 ) andempirical (42 ) me thods have been developed to predict thesecharacteris t i cs and separation pattern. Positive results canalso be obt ained by analogy with exis t ing works .

The natural separat ion of the tailings increases the s tabi l i ty of the tailings dam, s ince coarser material w ith a greater potential strength is depos ited in its outer zones , which assures the stabi l i ty .

The separat i on also has a favourable effect on the seepage ( 39 ) . The depos ited material in the outer zones is more permeable , whi le it is more impermeable in the inner z ones . The rat io of the permeabi l i ty coef f i ci ent at the nearby beach to that at i ts far end near the pond may vary f rom 1 0 0 t o 1 000 . This produces a lower phreatic surface than in the case of a homogeneous tailings dam.

43

I t is therefore important that the spigot ting be properly done so that a mo re s trongly rnarked separation can be ach ieved . In underwater spigotting separat ion is much less marked and may some t imes be di sregarded .

In the case of hy drocyc loned tailings the shear strength is greater, the angle of internal f rict ion reaching up t o 30 ° -40 ° . The shear strength depends , as does the dens i ty , on the mode of laying and on compact ion whi ch is poss ible only by t ransport machines or special compact ing equipment .

2 . 3 Properties and Characteri s t i c s o f Foundations and Barrow Areas

As indicated, the prope rties of the founda t ion and borrow areas and the means to obt ain them are i denti cal to those considered and used for the s tudy of a water retaining dam.

T he rne chanical characteri s t i cs to be determined ( ident if icat ion , s t rength and deformat i on ) are the same as those discussed above for was t es .

F urthermore , one should inves t i gate the mo dif ications which may be brought about by the was t es on the mineralogi cal , phys ical , chemi­cal and me chani cal characterist ics of the ground which supports them. The possible harmful effects of leakage water, even in smal l quant i ty , should always be cons i dered .

As for a water retaining dam , borrow areas rnay be selected wi th a view to ensuring d i f ferent or combined s tabi l i ty functions (high shear strength ) and watert ightness (pract ically zero permeabili ty ) or drainage (high permeability ) .

2 . 4 Geotechni cal Inves t igat ion

The geotechnical inves t igat ion required rnay be surnrnarized from the above as follows :

geologi cal and geotechnical reconnais sance on the site ,

in-s i tu tes t s ,

laborat ory tests ,

data from s imilar s i tes .

4 5

S i te inspections by a geologist and a geotechnical engineer permit them to def ine and set the inves tigation programs which in their opinion are necess ary by p it s , trenches , galleries , mechanical probes and geophys ical me thods ( as ind icated in paragraph 2 , 1 ) .

I n-situ tes ts are carried out , par t ly at leas t , during the dril ling program. They cover :

the ins tallat i on of piezometers to measure exis t ing pore pressures at di fferent t ime s of the year , bef ore and after rainfall s easons ;

in some materials , carrying out water t es ts at various dep ths during drilling to de termine soil and rock perme abilities by inj ection or preferably by extraction of water , with control piezomet ers located at va rious di stances away from the pumping wel l ;

in-s i tu tes ts i n boreholes ( S . P .T , , penetrometers or p ressure meters , vane tes t er ) t o give strength and def o rma t ion parameters of the natural soil ;

measurement of in-s i tu density (with sand or with membrane dens imeter or with a nucleodens ime t er ) ;

The drilling program should include deep bores to check on mi nera! depos its that might l ie below the proposed s i t e , the ext ract ion of which could affect future s tability .

All these t es ts are ident ical to thos e executed on a water retaining dam, I f poss ible , the s tudy of any exist ing was t e depos its i s adde d , including :

boreholes wit h , as for foundation t errain , the collection of undisturbed samples with s t rength and watertightness character­ist ics comparable in terras of the age of the dumps ,

tes ts in the boreholes { permeability , S . P .T . ) ,

i ns t rumentation (piezomet ers , pore pres sure measurement , de form­ation measurement , photography at regular intervals ) to moni tor the dump or depos i t during its life t ime ,

chemi cal analys is of eff luent wat ers .

These in-s itu tes ts perf o rmed on the was te depos its are usefully complemented by all the information provided by those who worked on the site and by dat a on cons truction me thods and equipment used , p revious observations , etc .

2 .5 Laboratory tests to measure the characteris tics , discuss ed in para­graph 2 . 2 should be made according to ASTM or other recognised national standards , They are identi cal to those for water retaining dams and well known to the des igners of these conventional dams .

47

In addit i on to tests made on f oundation and borrow areas , labora­t ory studies should cover the wastes themselves and determine their chemi cal and mineralogi cal modi fications along with variations in their characteris t ics , if pos sible , by measurements on samp les col­lected in exist ing depos i ts ( or <lumps ) . The se should be as old as poss ible , cons is tent wi th being composed of the same type of mater­ial to be put in the s t ructure being designed . Laboratory tests may also be perf ormed on samples previ ously subj ected to acce lerated pre-ageing. Such tests (wett ing-drying cy cles , f reez ing-thawing cycles , or acid at tacks or any other product or subs tance acting on the tailings ) do not give , in most cas e s , the expected f inal strength to be cons idered in the s tabi lity calculat i ons . They indicate , however, the alterat ion risks and provide the da ta needed to search for means to correct and delay the alterati on .

The accelerated ageing tests are not intended to subs t i tute deter­mi nat ion of the charact erist ics and behavi our of old waste emba nk­ment s . These embankments are always a useful complement to the accelerated tests .

The geotechnical invest igation should be followed , clas s if ied and presented by an experienced observer , capable of mod ifying the inves t igation programs to adapt them to the conditions found on the s i t e .

While there i s simi larity between certain aspects o f water retaining and tailings dams , the speci f ic des ign requi rements for the latter requires spe cialised knowledge of engineers experienced in this field .

49

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50

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5 1

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34- K. Terzagni e t R. Peck , "Mécanique des s o ls app liquée " fu nod - 1 9 5 7

3 5- J . Verdeyen , V . V o i s in , J . Nuyens , " La mé canique des sols " Duno<l - 1 9 68

36- F . C . Wa lker , J . W. Hi lf , W . W . Daehn , W . G . Ho l t z , A . A . Wagner e t H . I . G ibbs , "Earth Manual"

Bu reau of Rec lamati on - Denver - Co lorado

37- Ph. d. wayne , C. Teng , " Ca lcu l des f ondat ions et des mu rs de sou tènement s "

Eyr o l les - 1 9 6 6

3 8- Hans F . Winterkorn and Hsai -Yang Fang , " Foundations Engineering Handbook"

V . N . R . - N . Y . 1 9 7 5

3 9- Abadj iev C . B . ( 1 9 7 6 ) , Seepage Through Mi l l Tai ling Dams Douz i ème Congrès des G rands Barrages , Mexico , 1 9 7 6 , v o l . 1 ,pp 3 8 1 -3 94

40- Me lent iev V . A . , N . P . Ko lpashnikov and B .A . Vo lnin , "Hydrau lica lly f i l led hydrau lic s t ructures " , Ed . " Ene rgy " , bos cow , 1 9 7 3

4 1 - Provi s i onal ins t ru c t i ons f o r the cons t ruct ion techno logy of sp igot ted mi ll tai lings dams ( RSN 275 - 7 5 ) E d . Gos s t r oi , Kiev , 1 9 7 5 ( in Ru s s ian )

42- Ins tructions f o r the evaluat ion of the mechanical characteri s t ics of the materials in t ai lings dams , E d . MEHANOBR , Leningrad , 1 9 7 9 ( in Ru s s ian )

43- A . S . C . E . Proceedings of the Conf e rence on Geotechni ca l Prac t i ce f o r Disp osa! of Solid Was t e Materials - June 1 9 7 6 - 1 9 7 7

44- Y. At lan , M . Desurmont , JM Vagneron Stéri les et dé che ts miniers des digu es à s t é r i les B RGM - Rapp ort n° 7 8 SGN 570 GEG

4 5- F . Bague lin et JF Jézéqu e l Exp ansion de s ondes cylindriques dans les s o ls cohé rents

Bu l le t in de liaison du L . P . C . n° 5 1

52

46- Barres M. et a l . ( 1 9 7 8 ) � tude du comp ortement hydrodynamique e t de l ' évo lu t i on chimique des déche ts miniers f ins en décharge .

Journées d ' �tudes sur l ' Environnement , BRGM , 1 9 7 8

47- J . B iarez , M. Cas s a , M. Meuni e r , A. Tontoni Con t r i bu t ion à l ' é tude des pénétromè t res stat iques et dynamiques

Symp . Eu rop éen sur les essais de péné tration - S tockholm 1 9 7 4

48- L . Bj e rrun Fundamental cons ideration on the s t renght of s o i ls

Geote cnique - Vo l . II - 1 9 5 1

49- B rande la M . S t o ckage , stabi lisat i on e t va lorisation des rés i dus d e l ' indu s t r i e minérale

Industrie Minéra le

50- J . B rinch Hansen Vane tes ts in a novegian qui ck-c lay

Geotechnique - Vo l . 2 - 1 95 0

S l - Casagrande L . ( 1 9 7 4 ) Des i gn and Cons tru c t i on o f Tai lings Dams dans Stabi lity i n op en P i t Mining

B rawner C . O . e t Mili gan V. Edi t eu rs - pp . 1 8 1 -204

5 2- Chaf e t A . B . ( 1 9 7 5 ) , Guide lines for Des i gning , Cons truct ing and Op erating Tai lings Dams and Ponds

EM/J , décembre 1 9 7 5 , pp . 89-90

5 3- Department of Ene rgy , Mines and Resources ( 1 9 7 2 ) Tentat ive Design Gui de f or Mine Was t e Embankments in Canada

Techn i ca l B u l le t in TB 1 4 5 , Mines B ranch Mining Research Cent e r , Ottawa , 1 9 7 2

54- Dobry R. y Alvare z , L . ( 1 9 6 7 ) Seismi c Fai lu res of Chi lean Tai lings Dams

Jou rnal of the Soi l Mechani cs and Foundation Engineering , Divis ion, ASCE , v o l . 9 3 , n° SM6 , PP • 2 3 7-260

SS- Down C . G . e t S t o cks J. ( 1 9 7 6 ) The Environmental Problems o f tai lings disp osal a t metal mines

Dep artment of the Environment Res earch , Rep ort- nO 1 7 , Royaume Uni

56- F lorida Statutes , Ru les of the Dep a rtment o f Po llu t i on Cont ro l , chap i t re 1 7-9 ( 1 9 7 2 ) "Minimum Requi rements f o r Earthen Dams and Phosphate Mining and Process ing Op erations " - Tal lahas s e , F loride

57- Galp i n A . L . ( 1 9 7 1 ) The contr o l of water in tai lings ponds Se cond Interna t i ona l Conf e rence on St abi lity in Op en Pit Mining , Vancouver , Nov . 1 9 7 1 , pp . 1 7 3 - 1 9 6 .

53

5 8- Goodman R . E . et Seed H . B . Earthquake Indu ced D i sp lacemnts in sand Embankments Journal of the S o i l Mechanics and F oundations Division, ASCE , Vo l . 9 2 , no SM2 , 1 2 5-146

5 9- Hamel V . J . e t Gunderson ( 1 9 7 3 ) Shear S t rength of Homestake S limes Tai lings

Journal of the Soi l Mechanics and Foundation Divis ion , ASCE , V o l , 99 , n° SMS , Mai 1 9 7 3 , pp . 4 27 -4 3 2 .

6 0- Hird C . C . e t Humphreys J . D ( 1 9 7 7 ) An e xp erimental s cheme f or the Disp osal of Micaceous Resi du es f rom the China C la y Indu s t ry

Quart i r ly - Journal of Engineering Geo logy , vo l . 1 0 , PP • 1 7 7- 1 94

6 1 - Holubec I . ( 1 9 7 6 ) Geotechnica l Asp ects o f Coal Was t e Embankments

Canadian Geot e chnical Journa l , Vo l . 1 3 , pp . 2 7-39

6 2 - Jez equel J . F . Les p énét romè tres s t at iques , i nf luence du mode d ' emp loi sur la rés i s tance en point e .

Bu lletin de Liai s on d e s LPC - n° 3 6 - Janvier 1 9 6 9

6 3 - Jezequ e l JF , Pine l M . , Ravi llay G . Pénét romèt r e é le c t rique à mesure continue

Bu lle t in de Liaison des LPC - n° 36 - Janvier 1 9 6 9

64- Kleiner D . E . ( 1 9 7 6 ) Des i gn and Cons truct ion of an Embankment Dam to Impound Gypsum Was t es

Dou z ième Congrès des G rands Barrages , Mexi co , Vol. 1 ,PP • 2 3 5-250

6 5- Klohn E . J . ( 1 97 2 ) Tai lings Dams in Brit ish C o lumbia

Geot echni cal Pract ice for S tabi li ty in Op en Pi t Mining B rawner C . O . e t V. Milli gan Edi t eu rs , pp . 1 51 - 1 7 2

66- Klohn E . G . et Maaetman C . H . ( 1 9 7 2 ) Cons tru ction of S ound Tai lings Dams by sp i go t t ing

Tai ling Disp o s a ! Today , L. Ap lin et G . O . Arga l l Ed i teurs , P P • 2 3 7

6 7- P . Laréal , G . Sang le rat , J . G i e l ly Comparaison des essais de péné t ration ef f e ctués avec d if f é rents pénét romè t res statiques e t dynamiques . Correlations between in s i tu p enetrometer tests and the comp ress ibi lity characteri s t i cs of soi ls - Londres 1969

68- L . C . P . C .

Symp . Europ . sur les Essais de péné t rat i on - Stockho lm 1 9 74

Essai s t a t ique de f ondations p rof ondes Dunod 1 9 7 0

54

essai p ressiomét rique norma l Du nod 1 9 7 1

Le tassomè t re p ou r la mesu re des t as sements Dunod 1 9 7 1

Essais d e p laque e t mécanique des chau s sées Bu l le t in de Liaison des LPC - Févri er 1 9 6 5

69- Leon J . L . ( 1 9 7 6 ) Seismi c Ana lys is of a Tai lings dam

Douzième Congrès des grands ba rrages , Mexi co , 1 9 6 , vo l . 1 ,pp . 2 1 1 -234

70- Londe P . , Guerber P . , Lap lace G . et Pou t o t G . ( 1 9 7 6 ) Barrage de B e rrien p ou r le stockage des stériles

Dou z ième Congrès International. des Grands Barrages , Mexico , 1 9 7 6 , Vo l 1 , pp . 1 9-32

7 1- L . Menard An app aratus f or measu ring the s t renght of s o i ls in p lace

Thèse - Univers i t é de l ' I l linois - 1956 Mesu re in s i tu des p rop riétés p hys iqu es des s o ls -

Annales des Ponts et Chau s sées - Mai 1 9 5 7 Ca lcu l d e la force p ortante des f ondat ions sur la base des résu ltats des es sais pressiomé t riques

S o l-Soi ls - Vo l . I I n° ( 1 963 )

72- G . G . Meyerhoff Pene t ra t ion tes t and bearing cap aci ty of cohes i onless soi ls

Proc. ASCE - Vo l . 82 - SMl et SM3 - 1 966

73- Mo rgenstern N . R . and V . E . Pr ice The anal ysis of the stab i l i t y o f general slep surfac e

Geo t echni que - Juin 1 96 5

?4- Newmark N . M. ( 1 965) Ef fects of Earthquake on Dams and Embankment s

Geo technique - Vo l . 1 5 , n ° 2 , pp . 1 3 9- 1 60

75- Pe t tibone H. et dankealy C. ( 1 97 1 ) Engineer ing Proper ties of Mi nes Ta il ing s

Jo urnal of the So i l Mechanics and Foundations Di vis ion , ASCE , Vo l . 97 n° SM9 , Se ptembre 1 9 7 1 , p p . 1 2 0 7 - 1 2 2 6

76- R. R. Pr oc tor The design and Const ruc t ion o f ro lled ea rth dams

Engineerings New Reco r d - Août 1 9 3 3

77- Ra f f ino t P . , Gi stau H . , Ca sali s J . A. et Audoli H. ( 1 9 7 2 ) Co nstruc t ion d e s aires d ' épandage des re jets d e s laveries de flottation

Revue de ! ' Indus trie Minérale/Mineralurgie , n° 3 , pp . 1 1 6- 1 3 4

78- Robinsky E . I . ( 1 9 7 5 ) Thickened Di scharge - A new approach t o ta il ings disposa!

CIM Bulle tin , Décembre 1 9 7 5 , pp . 4 7 -53

55

79- G. Sanglerat Péné tromè tre stati que e t dynam ique - Le péné tromètre sta t i que e t la prévi s ion des tassement s .

Séminaire CAST - INSA Lyon 1 97 1

80- G. Sanglerat et La real Sta te of the Art in Franc e

81- See d H . B .

Sym p . Europ . sur le s essais de péné tration - So tckholm 1 9 74

A m e thod fo r ear t hquake re s i s t an t de s i gn of earth dams Journal So i ls Mechanics and Foundat ions Div i s i o n - ASCM Ja nv i e r 1 96 6

82- Seed H . B . , Lee K . L . et Id r i s s I . M . Anal y s i s o f She f f i el d Dam Fa ilure

Journal of the So i l Mechanic s and Foundat ion eng ineering Divi s io n , ASCE , Vol . 9 5 , n° SM5 , pp . 1 1 9 9-1 2 1 8

83- Seed H . B . • , Lee K . L . , Id r i s s I . M. et Makd i s i F . ( 1 97 3 ) Analys i s o f the s l ides in the San Fe rnando Dams dur i ng the ear thquake of Feb . 9 , 1 9 7 1

Rappo rt n° EERC 7 3 - 2 , Univer s i té de Cali forni e , Berkeley

84- Shields D . H . ( 1 9 7 5 ) Innovation i n ta i l ing di sposa!

Canad ian Geo technical Journal , Vo l . 1 2 , p p . 320-325

85- Sode rberg R . L . et Busch R . A . ( 1 9 7 7 ) De s i gn Guide fo r me tal and no nme t a l ta i l i ng � d i s pôsâl

U. S . Bureau of Mines , IC 87 55

86- U . S . Depar tment of the In ter i o r , Bureau of Reclama tion ( 1 9 7 4 ) De sign of small dams

Second Edi tion

87- Wahler W . e t Schl ick D . P . ( 1 9 7 6 ) Mi ne refuse impoundmen t s in the Uni ted States

Douzième Congrès de s Grands Barrage s , Mex ico , Vo l . 1 ,pp . 2 7 9-320

88- Standard So i l s Te s t ing Me thods publi shed by :

ASTM ( Ame r ican Soc i e t y for Te s t ing and Ma terials) C . S .A . ( Canad i an Standards As soc iation)

89- Labora tory So i l s Te s t i ng ( 1 9 52 ) publ i s hed by the U . S . Army Corps o f Engineers

90- Tai l ings Di s po sa! Today , Proc eedings of the Fi r s t ( Tuc so n , Ar i zona , 1 9 7 2 ) and the Seco nd ( Denver , Colorado , 1 9 78 ) Interna t i onal Ta il i ng Sym po s i um .

9 1 - Comptes rendus d u ! 2ème Co ngrè s In te rnati onal de s Grands Barrage s - Mexico 1 9 76 - Vol . 1 - Que s t i o n 4 4 .

56

3. DESIGN

3 . 1

3 . 1 . 1

This chapter on tailings embankment design off ers the reader a brief overview of what must be cons idered by a des i gner bef ore the tailing s t ructure is bui l t . Numerous books and articles have been devot ed to the spe ci f ics of tailing des ign . Therefore , it is intended to highlight important aspe cts and lead the reader to more detailed literature through the footnotes .

Introduct ion

General

T ailing embankments , dumps or impoundments , are one of the major cost factors in the benefi ciation of low grade resources . Plant locat ions are chosen on a cos t analysis in which tai ling dispos a! can and has determined the locat ion of the benef i ciation plant . The environmental cons iderat ions , and other factors such as large land availabi lity, water and population centres are all used to determine the locat ion of the tailing embankments .

G reat di f ficult ies and high cos t of ten arise from the pract ice of designing tailings dams of a s ize based on investigat ion results gatered at a given l!K)ment . Usually , a few years later , new ore deposits are dis covered and it becomes expedient to enlarge the originally des igned dam. I f , in the early stages of the des ign , the second pos sible stage is not planned for, its implementat ion is either impossible or much l!K)re costly , while the tailings dams equipment (decant towers , collect ors, spillways , divers ion dikes and channels , roads , measuring ins t ruments ) must be doubled . That is why it is pertinent when designing a tai lings dam for a given s ite , to plan for the maximum poss ible utilization of the s ite , sucessive stages f i t ting eas i ly into the previous one s , without maj o r problems and cos t increases .

The type of tailing embankment used is generally de termined by s eismic activity, wat er clarification, tailings propert ies and s tability , foundation cond i tions , hydrologi cal condit ions , tailing distribution and environmental cons iderations .

Environmental cons ideration for a cost estimate would include such factors as dust control , seepage water , pollution control and t opography compat ibility .

59

3 . 1 . 2

3 . 1 . 3

Tailing t ransportat ion and dist ribution can det ermine the type of embankment or the type of embankment may de termine the type of tai ling d i s t ribution. The cost of tailing t ransportat ion and dis­t ribu t ion can be as sessed according to several me thods of transportat ion and depos ition .

I n cold regions added capi tal and operat ional cos ts are es timated to cover f reezing and thawing cond it ions .

It is qui te common , af ter the init ial cost estimat e , that a combi­nation of impoundment des i gns will prove the most economi ca l . It usually shows that the de laying of cap ital expenditu res will reduce the cos t . The cos t es timate of the tai ling transp ortat ion and dis­t ribut ion in conjunction with the embankment and environmental cos ts usully ind i cate the type of tailing impoundment to be built .

Having de cided on the type of tai ling embankment , the engineer must det ermi ne the s ize , height , operational pro cedure and detail des ign . The total embankment generally wi l l include seve ral cells , for f lexibi li ty and as an operational necessity . The t ai li ng embankment has other funct ions than the storage of tai lings . I t is also us ed f or water clarificat ion and s omet imes as a water reservoir for the ope ration of the plant .

Des i gner Quali f icat ion

F ew , if any countries , present ly specif ically certify tailings dam des igners as such . However, tailings p lans should be made by profess ional engineers ; the engineer should be knowledgeable in soil mechanics , hydrology , f oundation des i gn and have some knowledge of mining . Although des igning tai ling dams maybe no diff erent from designing any other s t ructure , there are maj or cons ide rat ions whi ch the designer should be aware of that make the tailings dam different from the water retaining dam.

Tailing Dam Characteris t i cs ( l )

The conventional dam is built of specif ied , cont rolled material t o impound water . A tailing dam , o n the other hand , is built o f was t e , borrow, mill proces s ed material or a combination o f any of these to impound solids and water . Mos t embankments , when shown to be s t ru cturally and e conomically f eas ible , use was te f rom the mining or mil ling process , making embankment and impoundment inter­twined . This is in the f orm of overburden or graded mill was te . Because the useful li f e of the tailing dam is cons tantly growing to a planned abandonment , the physi cal horiz ons in the embankment may vary with changing mine and mi l l processes . Mill tai ling , the was te produ ct of removing the ore from the body, is a crushed and screened material taking on the characteris t ics of clay , sand or s i l t , depending on the parent rock . The tailing ranges anywhere f rom 5 per cent solids by weight for dewatering treatment to 50 per

6 1

cent solids . This vast amount of water is collected , p referably at the back of the impoundment , to avoid freewater at the dam itself , and f or recirculat ion to the mill or evaporation . Because of low in-place dens ity of the tailing , liqueficat ion is pos sible and must be a maj or concern in a s eismic area . Tailing may also contain corros.ive materials , introduced through mill processes such as sulphide leaching , which affect drainage and impoundment systems .

There are also specific cons iderat ions the des igner mus t be aware of whi ch affe ct the tailing dam des ign based on the commodity mined . S pecif i c requi rements may apply in various count ries . P roperties of some of the maj or ores are lis ted below:

1 . Coal ( 2 ) :

a . Spontaneous combustibility .

b . Pyri tes and acid leach .

2 . Non-fe rrous me tals ( l ) ( coppe r , molybdenum, lead , zinc ) :

a . Size of tailing ope rat ions of ten reaching 26 square kilometers .

b . Up to 80 per cent minus 75 µ:n fract ion (or 0 . 07 5 mm) .

3 . Taconite l :

a . Size of tailing operations ranging up to 1 5 square kilo­met ers .

b. F ibres in tailing possible, making air plus water pollutiona serious cons ide ration.

4 . Uranium3 :

a . Radon and radiation dangers .

b . Zero dis charge of tailing, supernatant or run-off waters . In some countries dis charge and seepage are allowed ( 3 , 9 6 , 9 7 ) .

Other subs tances such as phosphate2 and shale oil p os s ess charac­teri s t i cs which must also be cons idered in t ai ling dam des i gn .

5 . Gold :

a. Up to 90% minus 7 5 µm (or 0 . 0 7 5 mm) .b . Low spread of particle s ize . c . Very of ten acidic .

6 . General - see paragraph 2 . 2 . 1 .

63

3 . 1 . 4

3 . 1 . 5

Mine Re fuse D i sposa l 4 , S , 6 :

A significant event wi th res pect to waste disposa! in the United States , that many mine pers onnel point t o , ocurred at Buffalo C reek6 . A Wes t Virginia rains torm, in 1 9 72 , preceded sudden fail­ure of a coal refuse impoundment , causing extens i ve damage and deaths . Up to that point , mine refuse disposa! was cons i dered a black art . Because of Buffalo C reek and the di saster that occurred at Aberfan , Wales , in 1 96 6 , regulations and regulatory agencies , relat ing to tai lings di spos a! have been set up in many countries of the wor l d . Thus , the Mine Safety and Health Administ rat ion of the Department of Labour , was incorporated in the United S tates . In Britain, cont rol was introduced through the Mines and Quarries (Tips ) Act 1 96 9 . (Further details are given in Chap ter 5 ) . Research and s cient i f i c endeavours have increased with advances in mine tailing disposa! techniques .

There are di f ferent types of mine waste depos its , each wi th varying characte r i s t i cs a des igner must consider , such as : tailing impoundments , overburden and was te rock s tockpiles , leach <lumps , preparation p lant was t es and c lean coal s tockp i les . The mine was te <lump can cont ain li t t le or no ore and is me rely the overburden or separat i ng layers in the case of sedimentary type depos its . Where ore is pres ent in low concent rat ions , the was te <lump may be leached and , in so doing , the s t ru ctural characteris t i cs may be altered .

Mi ll tailing impoundments use mine waste rock , borrow material , or tailing sand to build the embankment . Mill slurry is depos ited in various way s , such as the upst ream, downst ream, and cent reline methods exp la ined in the cons t ruct ion sect ion of this manual . I f back f i l l of the mine i s being p lanned, sandy material will be diverted from the slurry impoundment to the mine giving the underground s topes increased stabi l i ty to decrease ground subs i dence . But by doing so , one may de crease the stabi lity of the tailing building material .

Retent i on dams are bui lt around high water content tailings such as phosphate . These s lur ries could range in the 4 per cent pulp dens i ty making the tailing embankment a water retention dam.

Secondary uses of mine was t es should be cons ide red s ince a popular use of the material can eventually lead to dismant ling of the i mpounding dam. Sorne uses whi ch have so far proved e conomical are :

road aggregate , ceramics , agri cultural lime , soil nutrients and bricks .

Bas i c Design Data Requi rements

The designer must be concerned with aspects whi ch are not d irec t ly related to the dam structure itsel f , but none theless may heavily inf luence his decision. Daily tonnages of mined mat erial , the mineralogy of the material , and features of the mi l li ng operat ion ,

65

such as the mi ll f low sheet and s creen analys i s , all have a direct impact on the embankment .

S i t e select ion, also dealt with in Chap ter 2 , is a maj or undertaking f or the des igner . Topography must allow a cont rolled annual rise of tailing behind the embankment . The environmental impact of the embankment and impoundment must be cons idered as must the mass f low boundary--the physi cal and economic l imits of damage to lives and property , should the facility fai l .

A thorough foundat ion soil s tudy mus t be comp leted t o ensure the embankment stability . An increasingly seen p roblem is mining under an act ive or inactive tailing embankment • . Whether or not the s t rength of the soil between the impoundment and the mine worki ngs is suf f i ci ent to avoi d subs idence during act ivity is a maj o r concern .

An analy s is of annual and storm precipitation, s t ream f low, season­al runof f , and tempe rature charts mus t be accomp lished . Divers ion systems , sedimentat i on ponds , and eros ion control are no longer op­t ional and are called for in regulat ions . Careful s tudy of water f low through the impoundment s t ructure is requi red before the f aci l i ty is bui lt , during the life of the mining operat ion , and after eventual abandonment .

Regulat i ons most definitely must be a prime cons iderat ion for the des igne r . In the United S tates , for examp le , there are federal , state, and local laws to contend with. Depending on who owns the land and what state the mine is in , some of the agencies respon­s i ble f or mining operat ions and regulat i ons in the United S t ates are :

o Nuclear Regulatory Commissiono Bureau o f Indian Affairso Mine Saf ety and Health Adminis t ration o U . S . Geological Surveyo His torie P reservationo O f f i ce of Surf ace Miningo Nat ional Park S erviceo Environmental P rotection Service o Bureau of Land Managemento F i sh and Wildlifeo Forest Service

Permi ts , studies , and applicat i ons must be obt ained before a shovelful of dirt is moved and the des i gner must see that his embankment conforms to al l safety , health, environmental and e conomical rest rictions .

Other count ries may have their own specif ic regulations whi ch must be s tudied and adhered to .

67

3 . 2

3 . 2 . l

3 . 2 . 2

Hydrology and Hydraulics.

Gene ral

In addit ion to the impoundment of mill water and was te , the tail­ings dam mus t be designed so that it can be cons t ructed and opera­ted , generally , every day of its des ign li f e . The des igner must cons ider the ef fects of the normal climate on the proposed con­s truct ion and operation methods and also provide a s tructure cap­able of control ling t he design s torm. The highly erodible nature of the materials used in tailings dam cons t ruct ion makes it prob­able that over t opping will result in f ailure with the subsequent release of accumulated s to rm water , process water , and a subs tan­t ial quantity of tai lings . S ince some cat chment area, ranging from the pond area itself to the ent ire drainage bas in , wi ll always exi s t , the des igner mus t determine the quantity of runoff to be cont rolled and des i gn a means of control . In general , the bas ic hydrologie and hydraulic equat ion whi ch mus t be satisfied during operat ion of the impoundment is :

INFLOW = STORAGE + OUTFLOW

The des i gner ' s as sessment of the elements of these categories mus t be adequate both f or normal day t o day operation and for ext reme condi t ions . Elements of the inf low category are due to direct pre­cipitation onto the pond area and runoff from any cont ributing drainage area , ou tf low from any other ponds in the cont ribut ing drainage area, base f low from contributing s t reams , ground water s eepage , seepage recovery , mine wat e r disposa! , and mi ll wat e r and s lu rry . The impoundment a rea will provide most immediate s t o rage capacity; howeve r , the surface and underground propert ies of the cont ributing drainage area will provide some temporary s t orage capacity . The exis tence of decants and spillways , re-use of pond water by the mi l l , evaporat ion , and unrecovered seepage account for out f low, Local climatic condi t ions , regulatory res trict ions on the discharge of pond wate r , and the owner ' s need to recyc le p rocess water are prime determinants of the magni tude of the equat ion cate­gories . Regulat ory res t rict ions on the permiss i ble dis charge of the various contaminants , treatment of cont ami nants , and mi lling requirements are beyond the scope of this sect ion. Williams? provides a comprehens i ve di scus sion covering process wat e r .

Input

The hydrologie methods used to predict maximum flows into the pond area are present ed by Chow8 , S o i l Conservati on Service 9 2 Bureau of Reclamat ionl 0 , 1 1 , Mine Safety and Health Adminis t ration and theBrit ish F lood S tudies Report , to name only a f ew of the methods developed throughout the world , These publi cat ions , t aken to-

69

gether, p res ent a full range from very detai led in-depth s tudies to s tanda rdized me thods . Local me teorological of f icers should be approached f or weather informat ion and for data f rom a specif ic area .

In addition to balancing pond inf low with the plant req uirement s , the designer must provide for routing a design storm through the impoundment area . Many count ries have regulatory requirements based on different laws and the des igner should be f ami liar with the regulat ions governing the specif ic site location and type of operation . In addit ion to specif ic laws for the s i t e , ot�er regulat ions may af fect the des ign . The size and haz ard po tential of the s i te may demand that s torms of very large magnitude be pas­sed with saf ety . As an example , the following tables recommended by the U . S . Army Corps of Engineers l 2 and the Mine Safety andHealth Administ ration2 are sugges ted as minimum standard s .

3 . 2 . 2 . 1 S i z e

The clas s if ication f o r s iz e based o n the height o f a dam and st orage capaci ty should be in accordance with Table 3-1 . The height of the dam is es tablished with res pect to the maximum st orage potential measured from the natural bed of the st ream or watercourse at the downs tream toe of the barrier , or if it is not across a s t ream or watercourse, the height from the lowes t elevat ion of the outs ide limit of the barrier , to the maximum water s t o rage elevation. For the purpose of det ermining proj ect s iz e , the maximum storage elevation may be cons ide red equal to the t op of dam elevation . S ize clas s i f i cat ion may be det ermined by ei ther storage or height , whichever gives the larger size category . 1 2

Category

S mall

Table 3-1 - S iz e Clas s if i cation (U . S . Army Corps of Engineers )

Impoundment Storage Ac-F t

1 , 000 and 50

Height (F t )

40 and 2 5 ( 1 , 23 3 , 48 2 m3 and 6 1 , 6 7 4 m3 ) ( 1 2 . 1 9 m and 7 . 6 2 m )

I nt ermediate 1 , 000 and 50 , 000 40 and 1 00 ( 1 , 2 3 3 , 482 m3 and 6 1 , 6 7 4 m3 ) ( 1 2 . 1 9 m and 30 . 48

5 0 , 000 1 00 ( 6 1 , 6 7 4 . 09 2 m3 ) ( 30 . 48 m)

L arge

3 . 2 . 2 . 2 Hazard P otential

The clas s i f icat ion f or potential hazards should be in accordance with Table 3-2 . The haz ards pertain to potential los s of human

7 1

m )

life or p roperty damage in the area d owns t ream of the dam in event of failure or mi soperation of the dam or appurtenant faci lit ies . Dams conforming to criteria for the low hazard pot ent ial category generally wi ll be located in rural or agricultu ral areas where failure may damage farm buildings , limi ted agri cultural land , or t ownship and country roads .

S t ructures in the signif i cant haz ard potential category will be those located in predominantly rural or agricultural areas where failure may damage isolated homes, secondary highways or minor rai lroads , or cause interruption of use or service of relative ly important publ ic utilities . Dams in the high haz ard potential category wi ll be those located where f ailure may cause serious damage to homes , extens i ve agri cultural , indus t rial and commercial f aci l i t ies i important public utilities , main highways orrailroads . 2

Table 3-2 - Hazard Potential Clas s i f i cation

Cat egory

Low

S ignif icant

High

3 . 2 . 2 . 3 Des i gn Data

Los s of Life (Extent o f Development )

None expected (no per­manent structures for human habitation)

F ew ( no urban devel­opments and no more than a small number of inhabi t able s t ructures )

More than f ew

Economi e Loss (Extent o f Development )

Minimal (undeveloped t o occas ional structures or agriculture )

Appreciable ( notable agriculture , industry or s t ructu res )

Excessive (extensi ve community , industry or agriculture )

I n evaluat ing the safety of the dam , origi nal hydraulic and hydro­logie des ign assump t ions obtained from the proj e ct records should be as sessed to determine thei r accep tabi lity . All cons t raints on water control such as blocked ent rance s , res trictions on operation of spillway and outlet gates , inadequate energy dis s ipators or res t r i ctive channel cond i ti ons , s ignif i cant reduction in reservoir capaci ty by sedime nt depos its and other fact ors should be cons id­ered in evaluating the validity of dis charge rat ings , s torage capacity , hydrographs , rout ings and regulat ion plan . The di s ­charge capacity and /or s torage capaci ty should b e capable of saf ely handling the spillway des i gn f lood for the size and hazard potent ial c las s i f i cation of the dam as indicated in Table 3-3 .

73

The hydraulic and hydrologie determinations for des ign as obtained from p roj e ct records wi ll be acceptable if conventional techniques s imi lar to the procedures out lined in paragraph 3 . 3 were used in obtaining the dat a . When the p roj e ct des i gn f lood actually used exceeds the recommended spillway des i gn flood , from T able 3-3 , the proj ect desi�n f lood will be acceptable in evaluating the safetyof the dam. l ·

Hazard

Law

Table 3-3 - Hydrolog i e Evaluation Guidelines RECOMME NDED SPILLWAY DES IGN FLOODS

Size *SEillwal Des ign F lood( SDF )

S mall 5 0 t o 1 00-yr return period Intermediate 1 00-yr to 1 / 2 PMF Large 1 /2 PMF to PMF

S ignificant Small 1 00-yr t o 1 /2 PMF Intermediate 1 /2 PMF to PMF Large PMF

High Small 1 /2 PMF to PMF Intermediate PMF Largè PMF

*The recommended design f loods in this column represent the magni­tude of the spillway des i gn f lood (SDF ) , which is intended to re­pres ent the largest f lood that need be cons i de red in the evalu­ation of a given proj e ct , regard less of whether a spillway is pro­vided ; i . e . , a given proj e ct should have s t ructures capable of saf ely pass ing the appropriate SDF . Where a range of SDF is indicated , the magni tude that most closely relates to the risk involved should be selected .

1 00-yr = 1 00-Year Exceedance I nterval . The flood magnitude expected to be exceeded on the average of once in 1 00 years . I t may also be expressed as an exceedance f requency wi th a one per cent chance of being exceeded in any given year.

PMF Probable Maximum F lood . The f lood that may be expected f rom the most severe combinat ion of critical meteorologic and hydrologie condit ions that are reasonably pos s ible in the region . The PMF is derived from p robable maximum precipitation (PMP ) ; this inf ormat ion is generally avail­able f rom the nat ional weather service . Reductions to the PMP may be app lied because rainfall isohyetals are unlikely to conform to the exact shape of the drainage bas in and /or the s torm is not likely to centre exactly over the drainage bas i n . In some cases lo cal topography wil l cause changes f rom the generalized PMP values ; therefore , it may be advisable to contact local cons­t ruct ion agencies to obtain the prevailing practice in specific areas .

75

T able 3-4 Recommended Minimum Des i gn S to rm C r i teria for Long-Term Refu s e Disposal Impoundments7

A . Impoundment Size Class i f icat i on

Categery

Small

Maximum Volume of S tored Water During

S t o rm (Ac-ft )

50 ( 6 1 , 6 7 4 m3 )

and

Maximum Depth of Water During Des ign S torm

(F t )

20 ( 6 . 1 0 m )

Intermediate 5 0 and 1 , 000 or ( 6 1 , 6 7 4 m3 and 1 , 23 3 , 48 2 m3 )

20 and 4 0

Large 1 , 000 ( 1 , 2 3 3 , 482 m3 )

or

( 6 . 1 0 m and 1 2 . 1 9 m)

40

B . Hazard Potent ial Clas s i fi cat ion2

CATEGORY DESCRIPTION

a. Low Hazard Pot enti al Faci l i t ies in rural or agricultural areas where fai lure would cause only s li ght damage , such as to farm bui ldings , fores t , agri cultura land or minor roads .

b . Mode rate Hazard F aci lit ies in predominantly rural Potent ial areas where failure may damage

i so lated homes , main highways or minor rai lroads , disrupting services , o r relat i vely important facilit ies 6

c . High Hazard Pot ential F acilit ies where a failure could reasonably be expected to cause loss of lif e , serious damage to houses , industrial and commercial buildings , important ut ilit ies , h ighways and railroads .

77

C . Recommended Des ign Storm for Long-Term Conditions ?

IMPOUNDMENT HAZARD MI NIMUM DESIGN STORM ADDIT IONAL SIZE POTENT IAL BASED ON PRECIPITAT ION CRI TERI ON

Small a . Low DPP The ind i cated s torm b . Mode rat e 1 /2 PMP is appropriate only c . High PMP i f the combinat ion

of spillways and Intermediate a . Low DPP decants for the

b . Moderate 1 /2 PMP f aci lity can evacu-c . High PMP ate 9 0 % o f the max-

imum volume of Large a . Low 1 /2 PMP stored s torm water

b . Moderate PMP within 1 0 days . c . High PMP

I t should be not ed that the maximum volumes under impoundment size include st ored tailings , s lime s , and sediments in addi t ion to water under the des ign s torm condition. F urther clarificat ion of the MSHA s tandards is neces sary because of the wide variety of t ai li ng dam conf igurations . General s torms of s ix-hour duration and the thunderst orms of one-hour duration are used to establish peak flows but are inadequate to es tablish total volumes which may have to be handled due to antecedent s torms , s now me l t , or general s to rms of longer duration . In general ,

( a ) Where no decants exi s t and the ope rator can di s charge without res triction, the pond is assumed full and a spillway capable of dis charging the peak f low es tabli shed by the tabulated s t orm is required .

( b ) Where decant sys tems having adequate capacity to f ix the pond elevat ion at a lower level than the cres t and also carry the calculated f low whi ch will exist aft er sixhours of an ext ended general storm, the volume available between the decant inlet and the crest elevation, minus f reeboard can be used to s tore the s torm provided 90 per cent of the volume can be evacuated within ten days . A smaller spi llway can be used to carry parto f the peak flow .

( c ) Where other regulatory res trict ions on dis charge exi s t and a s t o rm volume to satisfy the dis charge rest riction has been es tablished , a spi llway and /or decant system will be required to pass the t abulated s torm in a way that will sat i s fy condit ions ( a ) or ( b ) , above , using the maximum allowable e levat i on of the dis charge-regulated pool as the starting e levation of the tabulated storm rout ing .

79

3 . 2 . 3

3 . 2 . 4

3.3

3.3 . 1

Storage

The amount of storage available is cont rolled by the s ite conf ig­urat ion, the elevat ion of the normal operating pool or tai ling lev�ls , and any requirements for minimum freeboard between the maximum pool e levation expected during the design s torm and the minimum crest elevation.

A minimum freeboard is provided for prot ect ion agains t wave action, the possibility of s l ightly dif ferent s torm conditions from those expected, pos s ible sett lement of the dam , or some malfunct ion of the out let work s . Bu reau of Reclamat ion5 providesa di s cussion together wi th inf ormat ion on the use of impoundment length and wind velocit ies to establish minimum f reeboard values .

Out flow

During normal operations outf low due to seepage , evaporat ion , recovery of process wat e r , or the use of decant systems for free dis charge represent a substant ial proportion of the pond water balance. Seepage , evaporation and water recovery are insignif­i cant during storm rout ing operation s . Hydrauli c sys tems capable of controlling the inf low from the design s torm are needed t o route the storm through the impoundment . MSHA2 , Bureau o fReclamation l l , Chow8 , 1 3 , and Brater and King l 4 all provide methodso f reservoir rout ing and design of spillways , decants , and otheroutlet works . The designer should consider the need for ant i­f lotation col lars , thrust blocks , ant i-s eepage collars , ar t i cu­lating j oints , and j oint pro tection against int erna! water pres­sure whenever pipe is used as par t of the outlet system , Inlets t o p ipe systems should be equipped with trashracks t o provide agains t accidenta! blockage. Spillways should be des igned to avoid eros ion under the expected veloci ties .

Conceptual Des ign

General

S omet imes we can change things in this world , but one of the un­changeables is the locat i on of an ore body . However , there may be some choice in where the shaf t wil l be located or how the open pit will be mined, whi ch di ctates where the mi ll or concentrator wi ll be located ; yet its location is probably more cont rolled by the ore body than by the tailings d isposa! area . Tailing disposa! looms much larger in the overall mining plan than in the pas t because of larger t onnage s , environmental res t raint s , laws and regulat ions .

8 1

3 . 3 . 2

I f there is a choice of several good s i tes for tailing dispos a! , the f irst cons iderat i on is to balance the cost between cap i t al and operat ing cos t wi th the balance in favour of easy , safe and bes t operation , p lus more favourable opportunit ies t o spread capital cos t in t ime , by staged development . The soil type in the f oundat i on and the s ize of the area in relat ion to the daily t otal tonnage to be mined , and a s ite downs tream from the concentrator are of next importance . If a tailing s ite cannot be seen from main t ravel le d roads , or can be hidden by trees , it should be p referred to a s ite not so hidden .

Protect ing the envi ronment f rom run-o f f , seepage and dus t , and making the impoundment aes the t i cally accep table , can generally be des igned into nearly any s i t e . In areas near popu lat ion centre s , disposa! s i tes of ten need t o b e reclaime d f o r u s e other than return to the f ormer natural s tate . This may not always be compat i ble with embankment s tabi lity , whi ch mus t have prior i t y .

B e cause each mine is s it e speci f i c , the des i gn engineer needs t o be innova ti ve to take every advantage of the terrain, the material t o be impounded , volume , s tripping was te (open pit ) , surface soi l , and any other factors t o get the most effi cient and best operat ion. He must also s trike a balance between capital and operat ing cost to get the best operation at the least cos t .

Tonnages Involved

Select ing a s ite f or tailing d isposa! for a new operat ion is more crit i cal than later select ions because the ini t ial area must dis­pose of all tailings without delaying mining or milling . T here­f ore , a large tonnage mine needs two areas that can be used alter­nate ly unless local cond i t ions or methods dictate otherwis e . Later impoundment s should be prepared well in advance o f the t ime f or accepting the ent i re t onnage , but at all t imes there must be a backup area for emergencies .

The daily tonnage , annual tonnage , expected increases in tonnage , gradation, and annual maximum t onnage to be i mpounded mus t be known . I f a port ion of the coarse sand i s to be used for under­ground f i l l , it will affect the des i gn because less of the sand will be available for dam bui lding and les s total sand wil l have to be impounded . I f a large port ion is used underground or the grind is ext remely fine , it may make berm building with sand more d i ff icult or impos sible . Use of sand for underground f i ll , or a f ine grind of sand , may indicate that cyclones should be used .

83

3 . 3 . 3

Small amounts o f cyanide are used i n many mi l l circuits , but most usually oxidize in the tailings pond f ast enough so that they are no problem even if the tailings water is di s charged . Mos t tailings wat er is in a c losed circuit with the mi l l , precluding this problem except for seep age that must be cons idered in the design .

I f poss i ble, a s i te should contain the tailings f o r the en;ire l i f e of the mine , or should be the maximum feas ible size for saf ety and economy of cap ital and operat ing cos t .

S i t e Selection

As soon as p rospect ing indi cates a potent ial mine , concent rato r and tailings sites should be cons i dered . B y the time the extent of the o re body is known , the mining method should be determined . Whether the mine wil l be open pit or unde rground , dai ly tonnage , approximate screen analys i s , and o ther key factors should be known . The concentrat o r site should be firm bef ore a f inal decis i on is made for the tailings s ite or s it es .

Preliminary inves tigations should caver all possible s i tes near or f ar from the mi l l , preferably at a lower elevat ion . The size of the s ite in relati on to the daily t onnage varies with the terrain and whe ther it is in a wide o r narrow mountain valley or in an open and relat ively f l at plain , but a r ough figure o f 1 4 . 0 hec­t ares for each 1 , 000 tonnes of dai ly capacity is required . In a mountain val l ey where the annual rise will be rapid for the f i rs t 5 t o 1 0 y ears , careful planning is required to ensure that the tailings dam can be raised suffi ciently quick1y to accept ail the discharged tailings . In such areas there are two pos s ibilities . The first is to have two separate s i tes where each would take the ent i re production for a time , say 6 months or long enough to raise the berms , move the pipe s , and be ready to accept tailing agai n . This can b e used wi th the upstream method b y spigot ing around the periphery . The second possibility is continuous tailing depos it ion not subj ect to shut down for berm bui lding . In this s ituation, the header pipe and cyclones are supported between twin ve rtica l pipes . Hydraulic jacks raise the ent i re line , every day if necess ary , whi le discharging tai li ngs sand in the cent reline method of dam building . The site and impounding me thod must not subj ect the concent rator to shut ting down for lack of tai lings room.

85

3 . 3 . 4

When choos ing a s i te , the cons equences should be assessed . I nundation maps should be drawn showing how far a f ailure would cause the tailings s lurry to flow , how high on the valley walls it would reach, and how many peop le live in its path .

The s eepage wat er from the tailings pond should go back to the mil l by whatever me thods are available . Where an impervious zone or bedrock is close to the surface , an i nt ercepter t rench can cat ch the seepage , or if the alluvium is dee p , int e rceptor wells can cat ch the s eepage . This can be checked by moni tor wells . Where necess ary , water treatment plants can clean the excess water befo re it is re leas e d .

F or ins tance , t h e U S Nuclear Regulatory Commis s ion now requ i res zero dis charge from al l nuclear tailings ponds ; this means an impervious clay or plas t ic line r . By apparent ly s o lving the immediate problem of seepage into groundwater , seve ral other problems are created . The first is drainage and stability and the s econd is long-t e rra seepage control .

I n some coun t ries dis charge and s eepage from uranium tailings ponds are allowed ( 3 , 96 , 9 7 ) . The e conomics of tai lings disposal is one of the primary cons iderations in choos ing a site . The others are s af e ty and ease of operations .

During s eas ons of ext reme heat and cold , labour becomes ineffic­ient , so on-s ite labor should be minimi z ed . A site having an eas y , f oolproof operat ion with a minimum of daily physi cal on-s ite work , and where most of the work is conf ined to monitoring the water and adj ust ing the dis charge into the pond , is a big asse t . This reduces ope rating cos ts cons ide rably , al though there may be some increase in capital cos t .

S i te s e lection is fu lly covered in Chapter 1 .

Selection of Type o f Dam Embankment

Design methodology and criteria leading to se lect ion of the mos t appropriate cros s-s e ct ion and type of embankment have been the subj ect of reports cont ributed by many count ries . ( S ee Bibliography ) . The main cons t ruction me thods are des cribe d hereaf t e r . Chapt e r 4 presents t h e select ion of a n embankment f rom a cons t ruct ion and operation viewpoint .

3 . 3 . 4 . l Ups t ream Method ( F ig . 3 - 1 )

The mo s t common tailings pond is the ups t ream type wi th s pigot ing . I t is used if the s and is suitable for dam building , if the seismi ci ty risk is relat ively low , and if the dams wi ll not be exceedingly high . An excep t ion is when an extremely fine grind , not generally suitable for dam buildings , is used i n

8 7

FIG. 3-1

Mine, mill or plant refuse embankment construction methods.

(A) Upstream construction method. ( 1) Mechanically constructed dams. (2) Hydraulically constructed dams.

88

conj unction wi th cyclone separat ion prior to depos i t ion; in these circums tances the width of the valley would be critical to the adoption of the upst ream method and a narrow valley wi th steep abutments could be the only possibi lity .

Tailings dams of this type are the most economical ones . The bas ic structures , the s tarter dam and the decant works are the fas test bui lt and thus are the f as test in operat ion . That is why they are to be always preferred , if the necessary condi t ions for their adopt i on are present . In order to obtain s table supp orting components from a ma terial of suf f icient shear characteris t i cs , i t is neces sary to perform proper decent ral ised spigo t t ing f rom the s tarter dam and to maint ain long , non-submerged beaches . Thus , natural f low separation of the deposi ted mat erial ( t ailings ) takes place along the beach wi th coarser sand material disposed in the outer zone s , where the supporting parts are f orme d . F iner silty and clay mat ter is deposited towards the decant pond . I f the original ma terial contains a sufficient amount of sand fractions and the beach is long, a stable supp ort of the ne ces sary size is eas ily forme d . A d i sadvantage of this type of tailings dams is that the sect ions of the support component cons t ructed at later stages wil l li e on fine r granular ma terial , laid during the preceding s tages . This mat erial is of lower s t rength chara cteris­t ics and cons olidates slowly . Thus , the s tabi l i ty of this type of embankment is inversely proportional to an increase in height .

FIG. 3-2

Méthodes de construction des remblais de stériles miniers ou industriels - recharge Mine, mill or plant refuse embankment construction methods - surcharge

1 . Recharge = Supporting Part (shell) 2. Longueur de la zone de recharge = Length of the Supporting Part 3. Bassin de décantation = Decant Pond 4. Longueur du bassin de décantation = Length of the Decant Pond 5. Boues = Slimes 6. Remblai d 'amorce = Starter Dam 7. Couche drainante = Drain Layer 8. Pente aval = Downstream Slope 9. Plage = Beach

10. Tour de décantation = Decant Tower 1 1 . Terrain naturel (TN) = Original Ground (0.4)

89

This leads to the necessity of limi ting the height of this type of tai lings dam,

When the foundat i on is inclined, stability is secured with even great er di f f i cult y . In this case , during the init ial stages the beaches are shorter , and f ine material is depos ited close to the s tarter dam. In the beginning there can even be unde r-water spi­got t ing whi ch causes almost no s t ratifi cat ion of the mate rial , and fine , slow-to-cons olidate ma terial remains in the proximity of the s tarter dam, This further reduces the height to whi ch the tail­ings dam is stable . The s ite condit ions are the mos t de t erminant crite ria to deci de on the suitabili ty of this type of tai lings dams l 5 , For cross-valley type of s t ructures B-6 f rom F ig . 1 - 1 anda narrow valley at the s tarter dam with a small spigott ing front and a long widening res ervo i r , the relation of the surface of depos ition to the length of the spigot ting front is large . In this case the spigot ted ups t ream tailings dam can be cons tructed even if the original tailings are ve ry small ( 90% 0 , 07 4 mm) .

Conversely , in the cas e of wide val leys wi th a long spigo t t ing f ront and a short reservoi r , or diked f rom all s icles ( s t adium) type B-8 f rom F ig . 1 -1 no spigot ted ups t ream tailings dam can be built even in the p resence of cons ide rably coarse tailings ( 60% 0 , 0 7 4 mm) , since the sand fract ions are not sufficient for the forma t ion of stable support ing parts along the ent ire spigot ti ng f ront . Sorne relat ionships between the length of depos it ion of the tailings and the pos s ible height of the tailings dam are given inl 5 , Higher tai lings dams of this type on aninclined foundat ion are pos s ible wi th the scheme shown on F i g . 3-2 1 6 , If the s tarter dam is permeable, it may be extendedby a blanket slightly inclined towards the pond , The length of this drain blanket is such as to assure a suf f icient size of the supporting part . This results in the pond and zones of f iner mate rial being farther from the s tarter dam. At the same time it lowers the groundwater curve and drains the depos ited tailings , thus increas i ng the static and dynamic stabi lity of the tai lings dam. If the s tarter dam is built of a material more impermeable than the tailings , this blanket is of a greater length. When the foundation slope is large in the pond and over the length of the drain blanket , some earthwork may be clone ( F i g . 3-2 ) and the dug out material may be used like borrow material for the s tarter dam. The height of tailings dams of this type may also be increas e d , af ter the crit i cal height has been reached by overloading the downs t ream s lope with borrow earth and ro clc f i l l .

After the tailings dam has been built t o a certain height and the length over which the ma terial is depos ited has increas ed , the balance of the sand f ractions requi red for s tabi lity with respect

9 1

to the f ines becomes pos it ive and then cycloned sand can be used f or the overloading . Usually this solut ion is adopted if at a later stage the init ial height of the tai lings dam has to be increased or if the s t rength characteris t i cs of the tailings decrease as a result of changes in the mi lling process or in the characteristics of the ore itself .

3 . 3 . 4 . 2 Centreline Method ( F ig . 3 - 3 )

The centreline method is a compromise between the upst ream and downs tream me thods because seismically it is bet t er than the upst ream, and requi res much less cyclone underf low than the down­s tream. I t can use f iner grind than the upst ream me thod , but on occas ion might need supp lemental waste rock f or a butt ress on the downs t ream face for extra stabi lity . I t can be de s i gned for easy operat i on with no interrup t i on in the dis charge of t ai ling, pro­vided the area is large enough and the cyclones produce a sand that has suf f i ci ent permeabil i ty that the phreatic line is kept wel l below the surface . This could become a popular method , especially with open-pit mines where amp le was te is available f or the but tress . It does have the added cos t of cyclones , and of pump ing cos ts if gravity dis charge is not possible ; but should cal l for les s labour than other method s . The advantage of no interrupt i on in dis charge is a real p lus , but this should not be used as an excuse for having the area too small so that the annual rise is t oo great . The cyclone underf low is s aturated when discharged and must have t ime to drain to keep the phreatic line low enough for stabi l i ty . The permeabil i ty of the cyclone underf low must be such that water has t ime to drain as the cyclone underf low is moved along the face of the dam.

3 . 3 . 4 . 3 'Downs t ream Method ( f i g . 3-4 )

This method would only be used in extremely sens it i ve seismi c areas where it might b e the only method that would make a saf e embankment . I t is somewhat limi ted i n height because of the vast amount of sand needed f or each incremental increase . It is more cos t ly because of the me chani cal equipment needed to make the flat downst ream s lope. The operat ion is more awkward because access to the header pipe is blocked of f by the cyclone underf low i f the access road is downst ream, and blocked off by the cyclone overf low pipe if ups t ream. It is not one of the better operat ing methods .

This type of embankment offers the greates t stability . In the initial period, it has the disadvantage that the balance between the larger fract ions of the support ing part and the small f ractions retained by the supporting part is negative , i . e . the

93

FIG. 3-4

Mine, mil/ or plant refuse embankment construction methods. (CJ Downstream construction methods. (IJ Inclined method. (2) Horizontal method.

larger fract ions that are to shape the support ing part are insufficient . This requires a higher s tarter dam to supply volume for the small fract ions .

The cross-valley type tailings dams built by the downs tream method has , from a certain height upwards , has a positive balance with respect to the coarser fractions , s ince the res ervoir volume increases very fast with height . This permi ts two operation decis ions to prevent , if required , the portion of the cyclone underf low to advance ahead of the s limes ( 1 7 ) :

T o increase the s plit diameter of hydrocy cloning . This will result in coarser cycloned material and coarser cyclone overf low which splits better and helps maintain longer beaches . The p ond moves further away from the outer zones , seepage is weaker and the groundline curve lower .

To dis continue the hydrocycloning from time to t ime , and t o s pigot wi th the total amount o f tailings . Layers o f coarser fractions are formed in the part cons isting of the cyclone overflow. These layers accelerate the proce s s of cons olidat ion and lower the groundwater curve .

95

Both s olut ions produce a favourable effect , the beach expands , cons o lidation is accelerated , the ground wa ter curve is lowered and t he shear s t rength of the inters t it ial z on e , near the cyc loned part , is increas e d . As a result , the s tatic and dynami c stabi l i t y becomes great e r .

I f the beach length in the f inal s tage i s great , the sand fract ion is suf f i cient t o permit coarser mat erial to be depos i t ed in the s upport ing part , by spigo t t i ng wit hout cyc lones , wi th subs t an t ial e conomy . The moment suitable for swi t ch i ng f rom cycloning to normal spigo t t ing can be de t errnined by tes t ing the depos ited mate r i al du ring the periodic spigo t t i ng without cy clones .

3 . 3 . 4 . 4 Bo rrow Material , Water Retaining Type Dam

Borrow types are adopted when :

a ) t a i lings are of fine gri nd ;

b ) the rate of rise of the t a i l i ngs level is excess i ve with respect to the f il ling t irne of each succe s s i ve s tage in any of the p revious cons t ruct ion methods 3 . 3 . 4 . 1 t hrough 3 . 3 . 4 . 3 ; or

c ) f or o ther p ract i cal reasons .

Borrow types may be z oned ( F i g . 3 - 5 ) homogeneous or rnodi f i e dhornogeneous ( fo r ins tance , wit h a f i l t e r drain ) .

A precons tructed dam i s safer and more convenient but usually mo re expensi ve than coincidenta l ly bui l t and operated impoundrnent s .

3 . 3 . 5 S lurry T rans port System

3 . 3 . 5 . 1 G ravi ty F low

To avoid cos ts and p roblems of pumping, the i deal s i tuation is t o have gravi ty flow from t h e mi l l thickeners to t h e t ai ling pond for the ent i re life of the pond . The grade for the l i ne will dif fer for a coars e grind at 48 t o 50 per cent solids . This Im.lSt be determined f or each mine . A rough f i gure is 0 . 5 feet p e r 1 00 f eet . Drop boxes are ge nerally emp loyed to take care of exces s grade above 0 . 5 percen t , and these drop boxes are by-passed t o keep the proper pres sure to the spigo t s o r cyc lone a s the e levation of the pond rises . To p revent exce s s i ve wear i t is important that the ve locity be not too high and t o preclude p lugging of the l ine the velocity should not be too low. The f low should be be tween 1 , 2 t o 1 , 8 m per se cond for non-fe rrous tails and as high as 3 , 6 to 4 , 2 m per second for taconit e .

3 . 3 . 5 . 2 Pumpi ng

When pumping t ai l ing s lurry , the velo c i ty should be the sarne as for gravi ty flow. The tonnage , materi a l , and t o t a l head determine the pump t o be used .

97

Barge & Pumps for Recl a i m of Dccan t e d Wa t e r

Tai l i n��

Imperv i o us Core

Grou t Cou r t a i n

� Bedrock

Rubber-lined cent rifuga! pumps are common for low-head demands in concentrators where seal water is available and eas ily controlled , but can be used for pumping to a tailing pond or to underground f i ll s topes . The difficulty w ith this type of pump during cold winter ope rations is to assure that the seal water is always available and at a p ressure above the pump discharge pressure . Where several pump s tations are required in the pump line , a <lump valve should be just ahead of each pump s tation, with a catch bas in to hold the entire contents of the line in case of a power f ailure .

Centrifuga! pumps of abrasive res istant (AR) s t eel or abras i ve res istant plas tic liner are much s impler to operate and are probably the mo st popular f or tailing s lurry pumping . Sorne new plas t i cs are abrasive res istant as are many of the new alloy s teel s .

A posi tive disp lacement (PD ) pump is p robably the best for high pressures such as coal slurry lines many kilometers long . Coal s lurry is much less abrasive than mill t ai ling , but PD pumps are also used on mi ll tailing lines , especially where a high pressure pump is requi red and it is difficu lt to inst all an intermediate pump s tation . Each j o b must be engineered for the condi tions that exi s t . Pump manufacture rs are helpful as they have the experience and equipment .

99

3 . 3 . 6 Water Cont rol S ys tems

3 . 3 . 6 . 1 Vert ical S eepage - General ( 1 6 )

The seepage of tailing pond water through the bottom of the pond and . into the groundwater is becoming an increas ing concern for water quality. Meta! and non-me tal tailings can carry me tallic ions in s olut ion that may be harmful if the pH is low or if sul­phides oxidize and continually dis s olve me tals which go into the groundwater. Most mi l! circuits have a high pH , and if pyrite is p res ent , can soon become aci d i c , but if the soil is bas ic the me tals would precipitate and become harmles s . An unlined tai ling pond wi ll eventually be come almos t impe rvious and have a permea­bility (k ) of 1 0-6 or 1 0-7 cm/s f r om the consolidat ion anddrainage of the s limes on the bot tom. Of course unt i l th� pond i s complet ely covered and consolidation takes p lace , the s eepage through the bot tom can be qui t e high if the natural soi! pe rmea­bility is high . One way to reduce su ch permeabil ity is to compact the top foot or mo re to a dens ity of 90-95 per cent of S tandard oreven Modified Proctor ( 1 8 , 1 9 , 20 , 2 1 ) . This would be espe cially eff e ct ive if a small amount of clay were pres ent in the natural s o i l . This would be far less cos t ly than importing clay for a line r .

I n mountain valleys that have a high water table , the cross val ley dam must cope with the s t ream as well as s e epage . B lanket, s t r i p , or pipe drains should be used t o carry the seepage water out to the downs tream f ace of the dam where it can be retu rned to the mi l l , released downs tream, or treated and releas e d . These drains are a neces s i ty to keep the phreatic line low so that no seepage wat er reaches the downs t ream face of the s tart er dam. The total s eepage should be calculated and the d rains des igned for a safety f actor of 1 0 . T his is usually done by flow nets ( 22 ) .

The rock and grave! filters for the drains should be compat ible wi th the tailing wat e r . If the water at any time be comes acid i c , lime s tone should not be used. T h e filters should consi s t of sound rock part i cles so that it will not crush under the weight of the ult imate tailing dam. The filt ers surrounding the coarse rock d rain should be s i z ed to accep table f ilter crite ria ( 2 3 , 24 ) . I t i s important that the tailing material should not penetrate this fi lter and es cape through the coarse rock d rain, or clog it up in time .

3 . 3 . 6 . 2 Water D ivers ion

T he s t ream in a valley where a tailing dam is to be cons tructed can be handled in three diffe rent way s . F i r s t , it can be by-pas sed around the dam in a dive rs ion canal which mus t be sized to contain the des ign s t o rm and mus t be cons t ructed to resist f ai lure unde r flooding . Second , the decant system and barge pumps

1 0 1

can be s ized to remove the water , but the f reeboard must be great enough to s tore a portion of the s to rmwater if the pumps fai l . Third , s t ormwater can b e by-pas s ed beneath the pond in a reinf O C'ced concrete condui t siz ed for the flow . And las t , a s p i liway may be p os s i ble and be maintenance f ree af ter abandonment . None of the s e options is perma nent , but they are the best acceptable at the p res ent time .

One option, and probably the best for the long run , is to cons t ruct a spillway af ter the pond is abandoned to channe� all water f rom the creek directly on top of the p ond and over the spi l lway . If the creek is pe rennial a lake wi ll be formed , the s ize depending on the height of the inve rt of the spillway above the top of the pond . An annual s t ream would leave a lake during the s p ring run-o ff until it d ri ed up through evapo ration and s eepage . The top of the completed tai ling pond could be filled so that no lake should be f ormed but special ar rangements wou ld have to be made to avo id any loss of tail ings over the spillway during f lood . Not all p onds of this type are s i tuated so that a rock spillway is pos s ible or even nece ssary .

3, 3 . 6 . 3 Pond Liners

Artificial or specially prepared clay linings are quite common i n uranium tai ling dams . I f sod ium i o n exchange d oes not take p lace , the clay lining remains impervious . Before us ing a clay liner where acid wi ll be p resent , test the clay for s tabi lity. If the clay and tailing water are compa t ible , permeability should remain const ant or decrease with t ime .

Artifi cial line rs a re imp ermeable , but they eventually deterior­ate . They are expens ive and though they stop seepage they create another p ro blem, Tailing fines that cannot drain conso lidate s lowly s o they occupy more space than if allowed to drain at the bot tom . Ponds wi th p las t ic or clay line rs should have a permeable drainage blanket , s trip , or pipe drain on top of the liner for removing d rain water to the downs t ream s i de to a cat chment basi n f o r recycli ng . T h i s wou ld increase t h e dens ity of the material i n the pond , and when the pond was abandoned , most of the water would drain from the sand and s lime area , making it eas ier to C'eclaim. More than that , if there were no d rain and the liner failed , all the trapped water would es cape and go down the drainage .

1 03

3 . 3 . 6 . 4 Piezometers

T ailing dams shou ld have piezometers to cons t antly monitor the s eepage line in the embankment . These inst ruments should be ins talled within the impoundment if artes i an water is suspected. The p reviou s ly mentioned drains should be used where artes ian water may be found or suspecte d . S imple springs , if conf ined by tailing, can cause uplift beneath the impoundment . This i s especi ally serious beneath the starter dam and mus t be prevented . The open well piezometer is good in s and where water movement i s rapi d enough to register water r i s e and f all in the s tandpipe within hours , but piezomet e rs with a more rapid resp onse should be used in dense clay s . Vibrat ing wire and s train-gauged diaghragm p i ez ometers are equally acceptab le .

Piezometers should be p ro te ct ed where p ore water is highly acid i c .

3 . 3 . 6 . 5 Wat er Recovery

Decant

In the p as t , decants have been the mos t common method of water cont rol in the pond . These can be anything from a 200 mm thin wal l steel p i pe on a small operat ion to twi n , 1 , 000 m s t eel p ipes encased in reinforced concrete to withstand 70 Mpa pressure . Each must be laid on a f i rm f oundat i on , wi ths t and the p ressure expected , have seep rings through the s tarter dam , and have water inlets to control the water elevat ion in the pond . All must last t�e lif e of the mine or longer i f used for flood control after abandonment .

Advant ages :

1 . Pos i t i ve water cont r o l . 2 . Uses n o power so w i l l b e effective even during power f a i lure .

Disadvantages :

1 . More cos t ly than barge pump s . 2 . Acces s to decant towers may require cos t ly long walkway s . 3 . Damaged decant l ines may be d i f f i cult or i mp os s ib le to repair .4 . Higher pumping cos ts because of increased head as dam rises . S . Danger of maj or damage to t ower and culvert pas s ing under dam .

Barge Pumps

Barge pumps a re becomi ng more popular wherever the terrain per­mi t s . S t eep terrain in large ponds i s ideal because the pump is in fai r ly deep water and out of the mud . S l imes are a p roblem when the maximum slope of the land is only 1 -2 per cent .

1 0 5

3 . 3 . 7

Advantages :

l , Much les s cos t ly cons t ruction and lower pumping cos ts becauseof reduced head as the embankment ris es .

2 . Good water cont rol . 3 , Good access to pump s . 4 . I n seismic regions , les s e r risk of failure than decant towers

or pipes .

Dis advantages :

l , No way to get rid of water should a power failure occur duringa flood .

2 , Higher f reeboard requi red to protect agai ns t f lood . 3 . F reezing during ex treme winter weather (area around pump can be

kept i ce f ree by dis charge of low p ressure air around barge ) .4 . Can be used in deep pools only.

Siphons

S iphons in tailing ponds should not be used except wi th a water­type dam. Dams bui lt of borrow or--t ailing , j us t dumped in p lace without prior de sign and with no compact ion , should not have the water against the dam. Each success ive lift is built over the wors t pos s ible slime lay e r , and stability wil l become a serious problem. Siphons have a low cap ital and operating cos t , bu t cannot take advantage of pond elevation as barge pumps can . Underwater p ropelle rs can be used t o bring warmer water t o the surface to prevent icing of the s iphon intake .

Abandonment P lans

Abandonment plans ( s e e chap ter 5 ) should be made at the same time as cons t ruction plans , and s ome abandonment cons t ruct ion performed during the init ial cons t ruct ion phas e . Topsoil can be s tockpiled f or revege tat ion and can be p laced on the downs t ream s lope of the s tarter dike and planted even bef ore production begins . Gare should be t aken not to cover the surface with less permeable s oil than is in the s tarter dam if it is to be a permeable dam built to let water p ass f reely through it . Nor should tops oil be placed ove r grave! drains to impede drainage . The embankment slopes and benches s hould be planned so they can be p lanted . During cons t ruction, the benches should be s loped to the ins i de to prevent accumulated water from eroding the embankment to the next lower bench. On s ome s i tes precipitation is caught on each bench

107

3 . 3 . 8

and piped down t o the next lower bench , Wind and water eros ion and water control are the biggest p roblems after abandonment . In areas , wi th 40-1 00 cm of preci p itation and non-acidic tailing and t op s oil , grass and trees can be eas i ly grown, E ven without t opsoil , plant ing with a mulch and fert iliz e r can produce fairly good growth af ter a f ew year s , High acidic tai lings with a thick layer of topsoil ( 30 cm) can produce vege tation , but it would be limi t ed to shallow rooted p lant s . Open pit s t rip waste , even in desert areas , can grow native grass and plants , yet it needs supp lemental wat er and fertilizer for a f ew year s . S calped mine was te rock ( 7 cm) can be used as a caver for highly acid tailing to suppress dust both on the t op and on the benches of s lopes if no topsoil is available . A spillway is needed af ter abandonment even if the re is a large drainage area upst ream with intermittent s t reamflow.

F low from the drains of an abandoned tai lings pond would need monitoring f or a f ew years , and if necessary the water should be t reated bef ore di s charge . How long this would have to be done would depend on the sulphide cont ent and annual p recipitat ion. I n a few years the drain water could be pure enough t o be dis charged without treatment , and the area comp letely abandoned wi thout any further expens e , Each tailings embankment would have a different pos t-operat ing plan depending on the metallic content of the tails , and the terrai n , elevat ion , size and height of embankment and climate .

Cons t ruction P lan

If s i te invest igat ion and des ign are well done, cons t ruct ion p lans can be speci f i c wi th de tails of volumes for topsoil s tripping ands t orage , dep th of excavation for the base of the s tarter dam, and s tarter dam volume . The sou r ce of sup p ly , s iz e , and volume of coarse rock f or drains mus t be specified. Detail size and shape of each drain and thicknes s of filt ers above and below coarse rock drains . If pipe is to be used in the drai ns , the designer should calculate the perforation s iz e , the pipe diameter , shape and pos i ­t i o n . All s t eel p ip es must have a n asphalt coat ing t o protect agains t ru s t . I f a decant line is to be cons t ructed on soil , below-grade excavation and compact ion may be necessary. In-p lace dens ity samples may be necessary along the pipeline rou t e to

1 09

3 . 3 . 9

determine where ext ra compaction i s needed ( 1 8 , 1 9 ) , Borrow would be scheduled to go to speci f ic areas in the starter dam, depending on gradation and permeabilit ies at the specific dens i t y . In-place moi s ture and dens ity tes ts would be required during cons t ru ct ion . Rand · compaction is necessary around the seep rings on the decant and drain lines .

Wi th good cons t ruct ion plans and specificat ions , tailing area con­s truction can eas i ly be put out for bids or done in-house with the cos ts eas i ly es tima ted .

Operating P lans

The des ign engineer and the mine ope rator de termine the overall operat ing method , whether upst ream, downs t ream, cent reline, borrow or water-type dam. Ease of operation , safety , low manpower requi rement s , and low operating costs must be balanced with capi­tal cos t s . Sometimes condit ions are such that , wi th a li t t le mo re cap ital cos t , operating costs can be reduced subs tant ially. F ew areas have such condi t ions , and cap ital and ope rat ing cos ts are about equal .

To eut costs in the upst ream method , using spigot s , there should be enough head so that at leas t a 30-foot li f t can be made between moves of the header pipe . Spigot changes should be made as required and only then to cont rol the location of the water pool around the decant by a turn of a valv e . Moving the s pigots often may be req uired to reduce dust occas ionally. If water from the beach rai s es the phreatic line quite rapidly, arrange open wells or piezome t ers to monitor the rise so that spigoting can be s topped befo re the line reaches a critical height , A cross s ect ion of the embankment showing the maximum allowable phreat ic line that is the critical height for a 1 , 5 f actor of safety should be readily available to compare wi th f ield measurements ( 25 , 26 , 2 7 ) .

Permanent points can be placed on the abandoned benches to measurethe hor iz ontal and ve rtical movements of the embankment . Slope indicators can also be used to measure movement .

All decant and d rain outf low l ines should be eas i ly acces s ible f o r inspect ion . T h e ope rator o n each shift should ins pect the downs t ream face f or seepage , s lough s , or any evidence o f

I l l

3 . 3 . 1 0

ins tability as wel l as s ink ho les i n the pond which would indicate piping . By regulating the f low into the decan t , decant water can be monit ored so that no slime ent e rs the line . Sand in the decant dis charge would ind i cate a break in the l ine whi ch is difficu lt or impos s ib le to repair . During high winds the pond water eleva t ion may have to be rais ed to ensure a c lean overf low, as the wave act ion in shal low water wi l l pick up the s lime from the bot tom of t he pond and keep it in cons tant agitation.

Raising the berm with sand in the ups t ream method causes a great deal of downtime for the ope rat ion . F or this reas on , two separate ponds a re required f or a new mine or an area large enough so that half can be shut down for 6 months or a year.

Depending on grain size and weather , i t may take several months before a bul ldozer can get on the beach to push up sand for the embankment or a dragline can go to the beach for the same purpose . Amp le time mus t be al lowed for this part of the operat ion , which is of ten one of the des ign unknowns .

One new system has been devised for the cent reline method to over­come this delay . By having the header p ipe and cyclones mounted between two upright pipes , the entire pipeline can be raised by hyd raulic cylinders whi le it is in oper at ion.

Review o f Design , Cons t ruct ion and Operation

P lans for a tailing embankment should be reviewed by a competent geotechnical engineer specializing in tailing embankment des ign . The results of the physi cal p roper ty t ests of the foundat ion soils , embankment soils , and tailing cons t ru ct ion ma terial should be thoroughly s tudied .

The geotechnical review engineer will not usually require any tests unless there is an obvious quest ion on some material that appears wrong. But he should che ck the calcu lat ions for the factor of safety us ing the embankment geome t ry and ma t erial s trength ( 2 8 , 29 , 30 ) , The volume of water expe cted in the drain f rom the horizontal and ve rtical permeabi l i ty of the sand is especially critica l , and is not too difficu lt to calcu late using flow nets ( 2 2 , 3 1 , 3 2 ) , Because it is crit ical , a safety factor of 1 0 for the drain is common .

The cons t ruction review should caver everything f rom the position of the topsoil s tockpi le to pipe ins tallat ion . O f special impor­tance is the placement of drains and f i l t e rs and the p lacement of the material on the s tart er dam itself so that the soil , sand , or

1 1 3

3 . 4

3 . 4 . 1

grave! f rom each borrow pit goes to the right p lace in the dam. The compact ion speci f ied for the starter dam is al so critical ( 1 8 , 1 9 ) .

The operat ing procedure fo r a tailing impoundment is fai rly s tandard for each method of deposition, with lo cal variat ions because of tonnage , grain size , and local preference . The engineer reviewing the operating method can be mos t helpful if he knows of a be tter me thod of deposi tion , an innovation , or anything that can inf luence the overall p lan . It is the ref ore imperative that the des ign engineer know the lat es t t rends in iœ thods of operat ion , plus having his p lan concept reviewed bef ore any des i gn is started . The de cisions made at the s tart of des i gn de t ermine the entire plan f rom then on. That is why this phase is s o important . Of cours e , a f inal review of al! plans should be made before cons t ruction s t arts .

Foundations

General

The successful des i gn of a tai lings dam is largely dependent on a thorough evaluation of the foundation . Disposa! of mine and mil! was te , using the water from the mi l ling process , demands that the foundat ion evaluation satisfy both s t ructural and pollution requirements . Normal structural concerns may be compounded by having to locate the dam and impoundment over mine workings or ore bodies whi ch have yet to be mined . Assessment of the permeabi lity and seepage may have an added dimension of determining possible cont aminants and maint aining any release wi thin limi ts es tabli shed by regulatory agencies . Williams ? discusses the general nature , disposa! problems , technology , and eff luent limi tation of many segments of the mining indus t ry . The volume of was te available can allow inexpensive so lut ions to some foundat ion problems ; however, the rest raints i mposed by the available land , the need for short trans portat ion dis tance , and the charact eristics of the waste and effluent may force foundation treatments whi ch might be avoided in a conventional embankment dam.

The foundation nrust have adequate strength to support the loads of the embankment and any hydraulic s t ru cture included in the embankment . The foundation nrust als o be sufficient ly impermeable to control s eepage and satisfy s t ructural and p ollut ion requirements .

1 1 S

3 . 4 . 2 Sett lement

3 . 4 . 2 . 1 Rock F oundations

In the absence of mining or other underground cavi ties , the s e t t lement of a sound rock foundat ion is gene rally not a p roblem in the des ign of an embankment dam. Unde r some dams founded on rock, f oundat i on s e t t lements of more than 305 mm are reported by Sherard33 and this des ign cons ideration cannot be ignored . Asnoted by Jansen3 4 in h is discuss ion on earth dams , "differentials e t t lement at irregula r rock surfaces has not been an uncommon problem" .

Tail ings dams will generally not receive the same quality ,of cons t ruction cont rol as a convent ional dam. A large number of tailings dams is bui lt from non-cohes ive mat eria ls , and are therefore not subj e ct to cracking . Part icular attent ion should be given however to rock contacts in both starter dams and tailings dams to avo id cracking of pos s ible cohes ive soil zones in the embankment caused by different ial sett lement between a compress ible valley bot tom and rock abutments or out crops . Mechanics of cracking is p res ent ed by Sherard33 .

The exi s t ence of abandoned , act ive or proposed mine workings under the dam or impoundment is not an uncommon occurrence . The s i z e and spacing o f the underground openings , together wi th the dep th and quality of t he overburden will affect the design of the dam and the mine . F oundat ion failure can result in:

excess seepage with result ing pumping cos ts , • r apid inundat i on with haz ard to underground pers onnel ,

los s of the mine , • loss of the dam impoundment .

Engine e rs I nt e rnational Inc. 3 5 , Skelly and Loy36 , and K . Wardelland Partners 37 under gove rnment contract , have provided comp re­hensive s tudies , toge ther with an extensive bibliography, and analytical me thods for de termining the surf ace effects of under­ground mines .

The pres ence of s olut ion cavities or foundat ion materials which can be att acked by wat er such as water sens it ive shales , limes tone and gypsum should be dete rmined early enough to be accommodated in the des ign . In addit ion , the mi ll eff luent may carry chemi cals whi ch can att ack the f oundat ion and where this possibility exis t s , the services o f a geochemi s t may ident ify problems which can be avoided by changing s i t es or treating mill water bef ore release int o the pond .

1 1 7

3 . 4 . 2 . 2 Compres s i ble F oundat ions

Cons olidation theory and analyt ical me thods to predict the expected amount of sett lement are p resented in many s oil mechanics text

.s ( 38 , 2 1 , 3 9 ) . Sett lement of the foundation can lead to

cracking of the embankment , excess foundation pore p ressu res and both ve rtical and ho riz ontal distort ion of the embedded conduits whi ch are often us ed in the decant systems of tailings dams . Rutledge and Gould40 report on a s tudy of the horizontal andvertical movements of concrete-p ipe condu its and provide a means of pipe j o int openings . Blinde 41 point s out the di fficulty inanalyz ing the f or ces on condu its through tailings dams . T aylor and D ' Appolonia42 and Walker4 3 p rovide data on sett lement problems encounte red with comp ress ible foundations .

The starter dam concept is of ten used to t ake advantage of the natural topography by cons t ructing a s e ries of s tarter dams between t he hi llocks of natural s oil and/or rock outcropping s . Subs equent tai lings placement ut ilizes the cres t o f the s tarter dam and the natural ground as a f oundat ion for the s t ru ctural portion of the tailings dam which provides both s torage for the s limes and control of f lood wat e r . The design should consider differential settlement between the starter dam and the natural ground and the ef f e c ts of the tailings water on this combination o f foundation material. Large parts of the arid and semi-aridregions are covered wi th soils of low dens ity and high dry s t rength. These soils are p rimarily wind deposited ( loess ) but may have other ge ologic origins . With the add i t ion of water from the tailings pond, these soils will suf fer f rom a subs tantial decrease in volume and may result in total collaps e of the foundation ( 4 3 , 3 3 , 45 ) , The U . S . Bureau of Reclamation reports succe s s ful p re-we t ting and provides sample specificat ions 44 ; howeve r , Sherard3 3 points out the risk of liquefaction if thefoundat ion is pre-we tted but not collapsed ,

3 . 4 . 2 . 3 S tr ength

Rock F oundat ions

The type , cond i t ion and homogeneity of a rock foundat ion mus t be evaluated in order to determine a des ign s t rength, While rock s trength is not normally a problem in an embankment dam , the height of a tailings dam is virtually unlimi t ed and p lanned ult imate hei ghts may of ten be increase d at some futu re dat e because o f changes i n the e conomic value o f the o re . Most sound rock wi ll provide adequate s trength . Sherard3 3 and Jansen33

discuss the evaluat i on of hazards f rom weaker shales and tuf f s , j oi nt s , foliat ion and fault s . The pres ence of planes of weakness within the rock mass mus t be determined to avoid the poss ibility

1 1 9

3 . 4 . 3

o f sliding o f the foundation . Tailings dams are of ten bui l t onhills ides where the combined effect of s lurry, water and embankment weight may produce lands lides . F ang and B rown3 9 andthe .U . S . Army Corps of E ngineers 1 3 pro vi de methods of analyzingtrans lat ional and rotational s lides .

S o i l F oundat ions

T he select ion of soil s t rength to be used in the analy s i s can be a d i f f icult problem and must consi der the cond i t ions during cons t ruction and ope rat ion of the s ite . The nature of was te di sposa! is such that a very large quant i ty of was te can be p laced in a relat ively sho rt t ime and the need for this rapi d placement is a funct i on of the mining and mil ling requi rements . F oundation ma terials mus t be al lowed adequate time to consolidate to attain the design s t rengths . It s hould be noted that the upst ream method of embankment cons t ruct ion requires that the s t ructural part of the embankment be rai s ed over previous ly placed s l imes whi ch act as the foundation for the next dike . F ang39 , U . S . Army Corps ofEngineers46 , Sherard3 3 , and Lambe2 1 p rovide discus s i ons regardingthe interpretation of strength tes t data and the app l i cation of total and effect ive s t ress analys i s . T ay lor and D ' Appolonia42 andWalker43 report test and f ield data on the perf ormance of two verysoft f oundations . Earthquake induced l iquefact ion is d i s cussed i n a following seismi c section; however , the des i gner should cons ider t he poss i bi l i ty of l iquefact ion due to interna! movement or s t rain induced liquefaction whenever loose , satu rat ed sands affect the s t ructur e .

Seepage

S ince the amount and quality of seepage can have a maj o r ef fect on the method used f or waste dispos a! , the pollutive and s t ru ctural effects must be cons idered early i n the des ign . Theory and use of f l ownets to determine the quant i t ies , locat ion, f or ces and pressures due to seepage are presented by Cedergren22 andCasagrande4 7 , 4 8 , An app l i cat ion of f inite element analysis tos eepage problems is given by Kealy and Williams4 9 and Kealy andBusch50 , F oundati on res i s tance to at tack by both water and leachate should be es tablished . Seepage through j oints and cracks in rock may diss olve the rock, at tack the bonding agent , or increase the size of openings by erosi on , and can result in s i nkholes , t ransportation of fine grained particles through the larger openings and piping failures , or serious upli f t pressures . The existence of lenses or s eams of gap graded materials can have the same effect . As not ed earlier , the presence of collaps ible s o i ls mus t be considered early in the design.

1 2 1

3 .4 . 4

The cont rol of f oundation s eepage can follow one of the three bas ic me thods : virtual elimination reduct ion , or accep tance of the s eepage with provisions f or control , S eepage control usually involves one or a combination of cutoff trenches or walls , grouting, upst ream earth blankets , drains and relief wells . Des ign cons iderations and methods are exp lained by Sherard33 , Jansen3 4 , USBR44 , Cedergren2 2 , Thomas 5 1 , and Koerner and Welsh52 ,

The s eepage through tailings dams cons t ructed by spigott ing cons ide rably di f fers from the seepage through conventional dams , cons t ructed of compacted s o i l . The difference is due to the following factors which are inherent in a tai lings dam and affect the ph reatic surface:

- the surface is lowe red by non homogeneity in the horiz ontal direction; by subs tant ial anisotropy at the commencement of seepage f low; by a near horizontal s lope to the upst ream beach, particularly a long one .

- t he surf ace is raised by non homogeneity in the ve rt i cal direction; by subs tantial anis o t ropy during the latter phases of s eepage flow ; and by inf iltration from the beach .

As a result of the somewhat compens ating act ion of all these factors, the phreat ic surf ace is f rom 20 to 50 percent lower than it is wi th convent ional earth dams .

Another peculiarity of the tai lings dams is that the coarse tai lings are cohes ionless and very suscep t i ble to the erosion act ion of the water and seepage flow . This is why great attent ion must be paid to the p rotection of the t ailings by f i l t e rs at any p lace where eros ion and pip ing could appea r .

Seismi ci ty

Increas ing attent ion is being given to the effects of vibrat ion caused by earthquakes or other causes such as explos ives . The use of pseudo-s tatic me thods of analysis is generally acceptable in areas of relatively low earthquake activity. The use of the more recent ly developed dynamic methods have , largely because of cos t ,

1 23

been rest ri cted to zones of high earthquake activi ty . B oth the types of product and the me thod of disposal of many types of tai lings force additional concern over the e f fect of s eismic events . T he U . S . Corps of Engineers 5 3 p rovides a seismi c zone mapand a · discuss i on of t he use of s eismic coeffi cient s . Seismic maps which . cons ider frequency of occurrence have also been published ( 5 4 , 5 5 ) , In addition, seismic map for the world have beenpublis hed in B ri tain and other countries . A summary of recorded earthquakes and des criptions within a specifi ed radius of a s i te location identi f ied by coordinates can be obtained from NOAA56 . The select i on of seismi c coeffi cient s , limitati ons of pseudo­s tatic me thods , and dynamic response analy s is is presented by Seed 39 , 4 0 . The potent ial for liquefaction of foundat ion soils , and particularly the upst ream met ho d where each success ive embank­ment is raised using previously depos ited, wet , fine grained ma terials as a foundat io n , mus t be de t ermi ned . Seed and Idris5 7

and Castro 5 8 provide correlations between s tandard pene t rationtes t results and liquefac t ion potent ial . S elected papers by several authors5 9 provide discussions of current test ing and evaluat ion me thod s . Ellison and C ho60 exami ne liquefactionpotential of coal mine waste s lurries . Earthquake Engineering and Soil Dynamics6 1 contains ext ens ive discussion of earthquakeproblems .

S t ability of tailings dams can be increased and risk of l ique­f act ion decreased as fo llows :

- lowering the phreat i c surf ace . This is achieved by al l pos s i ble means such as drainage , pervious s tarter dams , drain liners , d rain blanket s .

by compacting the tailings ; i f the tailings are well-compacted to a relative dens i ty greater than 0 , 6 , the danger of liquefaction is very smal l ; with relative dens i ty greater than 0 , 6 5 , there is practically no danger of liquefac t ion,

- overloading drain material ( for examp le , open pit s t rip mat erial ) ,

- i nt ermediate lay e rs of coars er materials ( drain blanket s ) , which separate the f i ner damp material i nto layers ; then the material above the drains serves as load material for that below the drain and improves s tabi l i ty against l iq uefaction,

- acce lerat ing the cons olidation of the tailings ; cons olidat ion makes t he mate rial more compact and reduces mois ture , two f actors that make it les s suscepti ble towards liquefaction; conso lidation may be i ncreased by the s ame measures : all sorts of draining , drain blankets , loading and press in g .

1 25

3 . 4 . 5

B y using al l avai lable materials and j udiciously dis t ribut ing them within the cross-section of the tailings dam , ( the permeable materials draining the les s permeable , by alternate cycloning and spigott ing ) and wi th maintenance of long beaches , we can assure the dynamic s tability of the tailings dams qui te often without the need f or addi t ional compact ion .

F oundation and Abutment P reparation

Minimal f oundation preparations would cons ist of clearing and grubbing under those parts of the dam whe re voids , created by rot t ing material, or movement could not be tolerated . G iven a large amount of was te , the use of flatter slopes and wider dams may avoid s tripping of less desirable f oundation material . I t would be advisable that this ini tial cons truct ion phase b e under the cont rol of peop le fami liar with the des i gn assumptions . Most des igns do not pe rmi t a se cond opportunity and the cos t of remedial work wi ll be more expensive and less satisfactory than the original des ign . The many pos s ible types of foundat ion and abutment p reparation and design consideration are covered by many authors . USBR44 , Thomas S l Sherard33 , Cede rgren22 , D ' Appolonia2 , Jansen34 , and Halliburton6 � all provide insight into the various problems and solut ions .

1 27

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8 1 . Li , C . Y . Cons truction Pore Pressures in an Earth Dam. J . So i l Mechanics and Foundation Div . , ASCE , v . 8 5 , No . SM5 , October 1 9 5 9 , PP • 43-59 .

1 34

8 2 . Maclver , B . N . How the Soi ls Engine e r Can He lp the Mi l l Man i n Cons t ruction of Prop e r Tai lings Dams . Eng . and Min . J . , v . 1 6 2 , No . 5 , May 1 9 6 1 , pp . 85-9 0 .

8 3 . Me lentiev V . A . , N . P . Ko lp as hnikov and B . A . Volnin , Hydrau li ca l ly f i lled hydrau lic s t ru ctures , Ed . "Ene rgy " , Mos cow , 1 9 7 3 ( in Rus s i an ) .

84 . Parcher , J . and R. Means . Soi l Mechanics and Foundati ons . Charle s E . Merri l l Publishing C o . , Co lumbus , Ohio , 1 96 8 , 543 pp .

8 5 . Rainf a l l F requency - Du ration At lase s , Vari ou s Pub licatioh dates for dif f erent areas and storms . U . S . Dep artment of Commerce , Weathe r Bureau .

86 . U . S . Army Corp s of Engineers - Runof f from Snowme l t . EM 1 1 1 0-2-1 406 in 1 9 60 Engineering and Design , 1 960 , 6 1 pp .

87 . Seed , H .B . A Me thod of Earthquake Res i s t ant Design of Earth Dams . J. Soi l Mechanics and Foundation Div . , ASCE , v . 9 , No . SMl , January 1 96 6 , pp . 1 3-4 1 .

88. Seed , H . and K . L . Lee . Liquef act i on of Saturated Sand s During Cyclic Loading . J . Soi l Mechanics and F oundat ion D i v . , ASCE , v . 9 2 , No . SM6 , November 1 9 6 6 , pp . 1 0 5- 1 34 .

89 . Smi t h , E . S . Tai lings Disposal and Liquef action . Trans . AIME , v . 244 , 1 9 6 9 , pp . 1 7 9-1 8 7 .

90. Tay lor , R . L . and C . B . Brown , Darcy ' s Flow So lu t ions wi th a free Surf ace . ASCF , Hydrau lic s Div . , v. 93 , No . 4 2 , March 1 9 6 7 , pp . 2 5-33 .

9 1 . Terzaghi , K. Theoretical Soi l Mechani cs . John Wi ley & Sons , Inc . , New York , 1 9 6 3 , pp . 1 - 1 82 .

92 . Terzaghi , K. and R . B . Peck . Soi l Mechanics i n Enginee ring Prac t i ce , 2nd Ed . ; John Wi ley & Sons .

93. U . S . Department of the Interior - Bureau of Rec lamation. Des i gn of Sma l l Dams , 2nd E d . Supe rint endent of Documents , U . S . Government Print ing Of f ice , Washington , D . C . 2040 2 .

94 . U . S . Army Co rps of Engineers . Des i gn of Fini t e Re lief We l l Sys tems . EM 1 1 1 0-2- 1 905 , 1 9 6 3 , 4 7 PP •

1 3 5

9 5 . Wah le r , W . A . and As sociates , Eva luation of Mi l l Tai lings Disposal Practices and Potent i a l Dam Stabi lity Prob lems i n Sou thwe s te rn Uni ted States , U . S . Department of the Interior , Bureau of Mines , Washington , D . C .

9 6 . Tay lor , M . J . and Antommaria , P . E . " Immobi lization of Radionu c li des at Uranium Tai lings Disposal S i tes " - Uranium Mi l l Tai lings Management - Co lorado State Unive rs ity - Vo l. II ( 1 9 7 1 ) .

9 7 . Guarnas che l li , C and Shie lds , D . M . - Comments on Physical-Chemical App roach to Uranium Mi l l Tai lings Disp o s a l " - Uranium Mi ll Tai lings Management - Co lorado S tate Unive rsity - Vo l . I I ( 1 9 7 8 )

1 3 6

4. CONSTRUCTION AND OPERATION

4 . 1 Int roduct ion

4 . 2

4 . 2 . 1

These notes do not attempt to set out detailed techniques for the ins tallat ion , handling and ope ration of equipment and the actual placing of tailings f or the many diffe rent tailings dam construc­tion me thods which vary cons iderably at individual locations and f rom country to count ry . Only general aspects of tailings dam cons truct ion wi ll be di scussed. Thereaf ter , cons t ruct ion require­ments for various pre-deposit ion works will be des cri bed , followed by a dis cussion on depos it ion obj ect ives and pitfalls , and depos i­tion moni toring and maintenance requirement s . F inally, some notes on remedial measures are present e d .

Description of Tailings Deposit ion Techniques

General

The bas i c obj ect ive in cons truction of any t ailings dam is to dis­pose of the was te tailings in the most efficient , safe and e conom­ical way for the cond i t ions applying .

A tailings dam generally involves the placement of tailings mater­ial in an orderly and planned fashion so as to form a s table de­pos it of the tailings in the long term. As it is generally a cont inuous 2 4-hour day operation it is necessary to p lan ahead and t o have a specific plan of ope ration which is cons istent with the design requi rements of the dam. The philosophy of design must be fully unders tood and the depos i tion planning mus t be such as to anti cipate the design requirements in the future and to ensure that thes e are achievable during the depos i t ion phase .

4 . 2 . 1 . l Depos it ion by Ups t ream T e chnique

This method is one in which the cres t of the dam moves progres­s ively upst ream as the impoundment is raised . If the proport ion of coarse sand s izes in the tailings is small it results in a rather insubs tantial outer shell of high st rength material for the impoundment .

4 . 2 . 1 . 2 Depos i t ion by Downs tream T e chnique

In this technique the cres t of the dam wall moves progress ively downs tream as the impoundment is rais ed thus providing a subs tan-

! 39

t ial "full wedge " impoundment of high strength mate rial and is achieved by start ing depos it ion at a starter wall whi ch is a dis­tance ins ide the perime ter toe wall of the dam and by depos it ing material f rom the s tarter wall posit ion success ively upwards and outwards towards the toe wal l . The centre line of the impoundment thus moves outwards towards the outer toe wall of the dam as it ris es .

4 . 2 . 1 . 3 Depos it ion by Cent reline Te chnique

This me thod is one in which the crest of the dam remains in a cons tant posit ion in plan with respect to the toe wall as the level of the impoundment rises .

4 . 2 . 2 Wall Building by Cyclone D epos it ion (Des cription o f techniques )

4 . 2 . 2 . 1 Cycloning Obj e ct ives

A hydrocyclone spli ts incoming tailings s lurry int o two component s , namely:

- the cyclone underf low whi ch cont ains the coarser particles and a reduced water content ; and

- t he cy clone ove rflow which contains the f iner part icles and mos t of the wat e r .

A defini t i on sketch and water/soli ds balance f o r a cyclone is shown in Fig . 4 - 1 .

The cyclone underf low product generally has good shear strength p roperti e s , is relatively f ree d raining and will form a cone upon discharge from the cyclone , providing the cy clone perf ormance is carefully designed and monit ored .

The cyclone overf low product is a wet s lurry whi ch has a low permeabi lity and good flow characteris tics .

The obj e ct ive in using a hydrocyclone for tailings dam cons t ruction is to place the underf low p roduct in such a way as to create an impoundment for the containment of the overf low product at all stages of the life of the d isposa! area.

4 . 2 . 2 . 2 Cyclone Depos i t i on Techniques and Their Control

The bui lding of the outer wall of a tailings dam by cy clone deposit ion technique involves a logical progres s ion of placement of the coarse cyclone on the outer periphe ry of the tailings dam to form the impoundment . The f ine overflow is discharged into the pool area.

During the design phase a p rocedure f or deposit ion would have been evolved which would ensure that the growth of the wall wi ll at all t imes keep ahead of the growth of the fines beach inf ill behind the wal l . This assumes that suf f i ci ent unde rf low material wi ll be available to bui ld the wall and that the geote chni cal properties of this underflow material wi ll be structurally adequate . The

1 4 1

Feed Pipe

Slurry Feed ( M s + Mw )

Rock Toe

In l e t Head

Under flow ( 0, 25 Ms + 0, 08

n Decan t PipeFi l t er Dra i n

+ 0 , 92 Mw)

Finder

Fi nes Beach 0 , 7 5 Ms

Decan t Pond

Decan t Ou t l e t

tailings are depos ited i n a ce rtain s equence using either the downs t ream, upst ream, cent reline or upstream/downs tream placement technique s . I t nru s t be ensured that thes e sequences are adhered to unless records show that a variation from the design p lan is requi red to meet conditions applying on s i t e . I t is essential that regular records of the cyclone performance are t aken during the deposit ion phase of the tailings dam, the mos t important of which are:

a ) Split of mat e rial to the f low.

b) Quality of the underflow p roduct with respect to s ize s epara­t ion and in particular the D l O s iz e which affects the pe rmea­bility of this mat erial . It is essential that the underf lowmat erial be permeable and the amount of D l O material have a direct relati onship to the permeabi lity of the material . I tmus t form a suitable filter f o r the slurry .

c ) D ens ity of the underf low slurry . Unless a cons istent dens ity is maintained, the underf low material will not form a wall at the required slopes . If it is too wet , a wet sloppy heap wi ll form and if it is t oo dry a s teep, uns table pile of material will build up .

d ) The in-situ dens ity of the cyclone product should be sampled and measured to ensure that original design tonnage/ volume assumptions were correct .

1 43

e ) Simi larly , in-s itu permeability tes ts should be undertaken to ensure that the material has the correct propert ies .

Cyclone performance , as monit ored by these t es t s , is an essential part of the succes s of this wall bui lding technique . Variations of cyclone performance are usually due to e ither inadequate feed pressure or worn vo rtex f inders or apexes of the cy clone , Careful maintenance procedures must be f ollowed to ensure that these are regularly replaced and maintaine d ,

Visual ins pection o f the cyclone unde rf low product must take place hourly to ensure that gross malfunctioning of the cyclone is not taking place . ln a short time it becomes pos s ible to recognise any variations f rom the correct cyclone underf low product . Tao wet a product will cause eros i on of previously depos i ted material and t oo dry a product will cause an unacceptably s t e ep depos i ­t ion. Generally , the cyclone underf low should be ope rated wi th a spray dis charge ( as opposed to a rope discharge ) as better cy clone efficiency and split is achieved in this mode of ope rat ion. The cyclone depos it ion technique generally i nvolves l i f t i ng and re­pos i ti oning the feed pipelines from t ime to t ime as the wal l grows in height . This must generally be done in sect ions whi le other areas of the dam cont i nue to operate and receive tailings . Alt e r­native ly the p ipe moves can be made whi le material is discharged into the storage bas in of the dam by open-ended procedures . I f such a move is made , care mus t be taken so as not to unduly f i ll up the storage created by the coarse wall impoundment .

It is s t res s ed that the rate of rise of the beach infill with res­pect to the rate of rise of the coarse wall mu s t be monitored to ensure that the coarse wall always stays cons istent ly above the beach leve l . Should it be found that the beach rise is too fas t wi th respect to the wall growth, a re-ass essment of the des ign assumptions mus t be made . As an example , this could be caused by a smaller split to the underf l ow being achieved f rom the cyclones or by the beach in-s i tu dens ity being lower than assumed in the design phas e .

In general , the cyclone depo s i t ion technique rel i es heavi ly on cons i s t ent cyclone pe rformance and on the logi cal and planned movement of cyclones so as to achieve depos i tion of tai lings in the des ired manne r , always bearing in mind the main obj ect ive i s t o keep the coarse wall level ahead o f the beach level whi le , at the same time , maintaining a s table structure .

4 . 2 . 2 . 3 Cycloning by Upstream Method

ln this technique the slurry delive ry pipe would initially be placed on the t oe wall of the dam and the cyclone underf low depos i t s placed ins ide the toe wal l ( i . e . upst ream from the delivery p ipe ) and part ially on t op of the fines beach which develops at the same t ime . F ig . 4 - 2 i l lus trates this rne thod ,

145 10

Dam Cre s t Fines Beach (C cl o n e ver f l ow)

Toe Drai n

lToe Wal l

<D a> @ - - - - -SV

( i e . Cyclone Unde r f l ow)

= n Successi ve Fced Pipe Pos i t i o ns

e Succes s i v e C c l o ne Pos i t i o ns

FIG. 4-2

Cyclonin by upstream method

146

Dam Cre st

Toe Wal l

l 1 [Beach

(C cl o ne Under f l ow) Di v i d ing Wal l

<D (2) @ = s Successi ve Slurry Feed Pipe Posi t i o n s

_ _ _ _ _ .... e Success i v e Cyclone Pos i t i o ns

FIG. 4-3

Cycloning by downstream method

4 , 2 , 2 , 4 Cycloning by Downs tream Method

Here the slurry de live ry pipeline is placed on the interna! divid­ing wall, and cyclone underf low material is placed in the area be­tween the starter and toe walls ( i . e . downs tream from the de live ry pipe ) , The starter wall initially acts as a storage dam f o r f ines , thus allowing the coarse tailings depos its t o develop t o a s tage where they can f orm an adequate impoundment f or confinement of the fines beach and pool,

F i g _ 4-3 illus trates this method .

4 . 2 . 2 . S Cycloning by Successive Ups tream and Downs tream S equences

This technique is one in wh ich a satis f actory ou ter impoundment shell of coarse high s t rength tailings is depos i t ed in a seri es of successive ups tream and downs tream sequences , so planned as to make use of a limited amount of coarse material being dis charged f rom the cyclone underflow , i . e , in the cas e where there is insuf­fici ent material f or a full downs t ream cons t ruction s equence but whe re the re is suf f icient to create a partial wedge of coarse mat er ial , rather than by employing the rather uns atis f actory ups t ream technique . This method is illus trated in F igure 4-4 .

4 , 2 , 2 . 6 Cycloning by Cent re Line D eposition

In this technique the s lurry delivery p ipe and cyclones are posi­t ioned on the cent r e line of the ultimate impoundment wall and they remain in the same relative posit ion throughout cons t ruct ion by rais ing them ve rtically . This can usually only be achieved by the use of gantries to support the tailings delivery pipe and the

147

Cyc l o ned Coarse Ta i l ings Dcpos i t cd by Ups t ream Mcthod Cyc l o nes Coarse Tai l ings

Deposi ted by Down s t ream , lcthod

Acccss Road

'foc Dra in l Ground Level

ç,;:z:;c;;;

e Ear t h f i l l Star ter Wal lApproximatc Fine Coarse

Tai l i ngs In t e r face Ear t hfi l l Toe Wa l l

CD ® @ _ _ _ _ _ ...,

Success i v e Fccd Pipe Pos i t i o ns

e Successi v e C y c l o ne Pos i t i o ns

FIG. 4-4

Cycloning by successive upstream & downstream deposition

1 48

g r o s s i e r s Coarse lmpoundment

Toe Wal l

1 � !

Fccd Pipe Gan t ry

r- P l age d ' él éme n t sFines Beach

......_ ____ _

Di v i d i n g Wa 1 1

FlG. 4-5

Cydoning by centreline method

cyclones and to al low the i r movement ve rt ically upwards without burying them in the depo s i t ed material .

F i g . 4 - 5 i llus t rates this method .

4 . 2 . 2 . 7 Cent ral Cyclone S tations Methods

4 . 2 . 3

This t echnique involves the separation of the tailings s lurry feed into coarse and fine fract ion at a cent ralised cyclone station in the vi cini ty of the tailings dam, as opposed to the previous tech­niques whi ch do the separat ion proces s on the tailings wall as it grows . The coarse mate rial would then be placed hydrauli cally or mechani cally .

Spigot D i s charge Wall Bui lding Techniques

S pigo t s are multiple ou t lets along a delive ry line and are used when it is both pos s ible and desirable to cause a grading split between the coarse and f ine fract ions of the tailings product . S pigots break up the tailings dis charge s t ream into smaller volumed streams , thus allowing a rap id drop in stream velocity . This velocity drop enables the coarser f ractions to sett le out swi f t ly . Part icle sizes de crease wi th distance from the spigot , the finer f raction reaching the pond area . Suffi cient spigots are opened at any one t ime to cater for the full delive ry s t ream.

C ompared with the cyclone techniques which permi t an immediat e , more effici ent separat ion of the coarse f ract ion of the tailings , where justified from an operational viewp oint , the spigot me thod is used where its more economical, natural g rading process is accept able .

149

The spigot sys tem of impoundment wall building is li mi ted in the main to upst ream methods of dam cons t ruction, al though centre line and downs tream cons truct ion me thods can be used wi th favourable product s . After each sp igot cycle a new cont ainment wall has to be built . This wall provides both the dam to impound the next s pigot depos ition and provide vertical f reeboard to contain the pond wat e r .

Wall bu ilding is a me chanical o r manual process requiring that sect ions of the spigot lines be moved out of the way , while allow­ing unint er rupted depos it ion to continue on other sect ions . Bull­dozers , front-end loade rs , backactors and sc rape r-loaders are used to build these walls mechanically, while shove ls wielded by manual labour are also use d .

I n addi t ion t o the ve rt ical freeboard provided by the wal l con­s truct ion, the spigotted beach develops beach freeboard . The coars er fract ions depos it at a steeper slope than the f iner frac­tions . F igure 4-6 illust rates the general spigot dis charge depos ition me thod .

The storage volume thus created can materially as s is t pond manage­ment by keeping the water well away f rom the edge of the tailings dam.

It should be not ed that while spigotted dams often involve lower cap ital and operat ional costs than cycloned dams , the particle s eparation achieved is not nearly as cont rol lable . A degree of horizontal layering is inevit able and this reduces vertical per­meabi li ty and encourages ho rizontal seepage f low in the tail ings .

4 . 2 . 3 . 1 Main Delive ry Line L e f t on the G round L evel

It is obviously an advantage if the main de live ry line can be le f t i n i t s original posit ion a t ground leve l . Maintenance and inspec­t ions are then eas ily ca rried out . This sys tem relies on ext ens ions to the spigot out let pipes or branch lines .

a ) Extendable Spigot L ines

As the dam grows in heigh t , the spigot lines are ext ended while the main line is lef t behind . These spigot lines , of smaller diame t er than the main delivery line , are easy to ext end ( see F ig . 4 - 7 ) . However and part icularly if these spigot pipes have to follow the dam prof i le as shown in Fig . 4 - 8 , high frict ion heads can be developed leading to pumping problems or burst pipes .

1 51

Con s t ruc ted Wa l l

Delivery Line

Val ve ___ ,

Fine

FIG. 4-6 Spigot metlwd uf construction

Lance s Extcndablc Spi got L i n es

��.; '"' L���: f Fines t

FIG. 4-7

Spigvt system with extendable spigot lines

Wa l l Ve r t i ca l Free Board e Beach Free Bonrd

Frac t i ons

1 52

FIG. 4-8

Spigot system with extended spigot Unes jollowing dam profiles

b ) Extendable B ranch L ines

Branch lines are led from the main delivery line to the t op of the dam. Here the line is led into a secondary line ins ide and parallel to the crest of the dam, whi ch has controllable spigot holes in it at suitable intervals . While requiring two sets of pipes , the second s et is o f ten of a lower p ressure rating and smaller diame t er ( see Fiq . 4 - 9 ) .

4 . 2 . 3 . 2 Main Delivery Line P eriodically Lif ted

When the spigot Unes shown in F i a . 4 - 7 and 4-8 become too long, the main delivery l ine should be lif ted to a suitable new bas e elevat ion . To accomplish this lif t , suitable s tepbacks are requi red in the dam p rof ile , as shown in Fiq . 4 - 1 0 .

1 53

Extendab l e Branch Line (Spaced a t Di s tan t ln tcrva l s )

Del i very Line

FIG. 4-9

Spi ot system with extendable branch line

1 54

1 ·

Be r111e Stcp Back

New Posi t i o n o f De l i very Line

FIG. 4-10

Spigot system : Lifting of main delivery Une

There is still ano ther technology o f spigo t t ing , when berms on the downstream s lope (or s econdary dike s ) are constructed at intervals of 2 t o 3 �. and the main del ivery l ine o f each s econdary d ike is shifted. In that cas e , the short spigo t pipe deviations point downwards only . The advantages o f this type consist in not let­ting off pulp on the downs t ream s lope when spigot pipe deviations are cleaned and in cases o f fai lures with the delivery l ine , the deviations and the t aps . This technology is suitable for large s ites , or during their later stages when the annual rise in height i s 1 to 3 m .

4. 2 . 3 . 3 Guidelines for Operation

Many pit falls may exist in the spigot system o f tailings disposal , particularly in the cons truction of the impoundment wal l . Ga re should be taken not t o build this tailings wall too high without adequate regard t o s t andard dam wall requirements such as comp ac­tion and angle of outer f ac e . Cas es of severe piping erosion t hrough inadequately cons tructed walls are legend. In addit ion care should be taken that the slimes placed in such walls is suf­f iciently impervious . Cl ods and lump s of tailings should be broken down and compacte d . It may be necessary to carry out con­t inuous maintenance of such walls as deposi tion proceeds , such as wet ting the wall to close shrinkage cracks , and filling e rosion gulley s .

Correct s pigo t t ing is achieved by manoeuv ering the valves o f the spigott ing outlets so that the pulp s tream i s always normal to the s pigo tt ing f ront . Only t hus can the separation of the fract ions take place uniformly and gradual ly , without discontinuities which would cause increas ing s eepage and endanger the stability.

1 55

4. 2. 4 Paddock Sy s t em

In the paddock sys tem tailings are depo s ited in a paddock con­s t ructed by forming a walled enclosed area or pond. 1 00 mm to 1 50 inm depth of tailings is depos i ted in the paddock and , after settling, the supernatant water i s led off t o the poo l . Af ter a period of drying the surrounding wal ls are raised again by hand or by us ing mechanical aids . The method relies heavily on evapora­t ive drying and is only suitable for arid and s emi-ar id c l imatic cond i t ions .

A grid of paddocks i s cons t ructed around the dam perime ter or along the l ine of an impoundment . Each paddock i s used in turn for deposi tion, the balance of the material being depos ited in the body of the impoundment. The depo s i tion cycle depends on the rate of rise of the dam and is kept as long as possi ble to allow the previous layer of depo s i tion t o d ry out by d ra ining and by evaporation before the next layer i s depo s ited .

The paddock sys tem of cons truction can be used with e i ther down-s tream , upstream or centre-line cons t ruct ion methods as ind icated in Fig . 4 - 1 1 , 4- 1 2 and 4 - 1 3. It is particularly suitable for s ingle s ized products . When used with graded products , gravitational sort ing of the particle s i ze s results in the format ion of a series of f ine hori zontal impervious layers which have the e ffect of increas ing the rat i o of horizontal permeability to vertical permeability . The result may be a deposit with highly ani s otropie prope r t ie s , a high seepage surface and consequent problems with s lope stability .

Although the paddock sys tem as describe d is very commonly used for gold mining tail ings or slime s , i t has been adapted to o ther tailings on a much larger scal e . Containment walls of up to 2 m depth a re bui l t round the paddock by mechanical means . The paddock is then f i lled by open-ending into i t . Up to one metre o f tailings i s depos i t ed a t a t ime . Wall building proceeds by s craping material up from the deposit ion area and placing i t on the outer face of t he dam by means of f ront-end loaders , bull­dozers or scrapers , t o form the containment wall for future deposit ions . This operation i s therefore carried out f rom i nside the dam and can result in a loose uncompacted wal l . It is gen­e rally not necesary to introduce special i ze d compac t i on equipment on tailings dams as const ruction traf fic produces adequate compac­t ion. To achieve this the outer wall must be wide enough to safely carry the construction plant , i . e . the width of the top o f the wal l should be a t leas t 1 , 5 t imes the width of track o f the plant being use d . Suf f icient pas ses should be made to ensure that the material i s broken up and compac ted , as shown in Fia . 4 - 1 4and 4- 1 5. S orne tailings contain cementing agents which s e t the deposi ted tailings i nt o a compact mas s which has t o be broken up f o r placement on the wal l . If the larger fragments are not broken up during the scraping , placement and compact ion process , the voids between them form pas sages leading to piping f lows through t he wall .

1 57

En c l os ty p i ques Typ 1 ca l Paddock Wa l l s

<D-@-® Success i ve Pos i t i ons o f Ccn t rc -Line

FIG. 4-1 1

Paddock system : Downstream construction

Fixed Cen trc -Line

FIG. 4-12

Paddock system: Centreline construction

1 58

4. 2 . 5

Successi ve Pos i t i o ns

o f Cen t r c -L i ne

, ,,-.... �

/ .......... / '\. / ...........

�' / ............

,, "\.. "-!

l l

FIG. 4-1 3

Paddock system : Upstream constrnction

Me chanized Placing of Tail ings

1/ ............ 1 / ...............

,, ........... 1

Al though hydrauli c trans port and placement of tailings i s usually the mos t e f f icient sys tem for tailings disposa ! , there are occa­s ions when it is advantageous to use mechanical transport and placement or a combination o f hyd raulic transport and mechanical p lacement. 'Th. is s ec t ion excludes the us e of mechanical plant for wall building on s pigo tted and paddocked dams which have already been discus sed under sect ions 4 . 2 . 3 and 4 . 2 . 4 .

The maj or use of mechanized placement is where a dry dam i s wanted , i . e . wher e there will no t be large quantities o f tailings e ffluent requir ing decant systems .

1 59

- ""' --- - ....... " 0 ' ' ..... , x'-"

Maté r i au x��"'� ' '-..... ............. """ � / ....,_"'-.,. '- � ::::-.::::::: Uncompac t c .............. ........_ ""-�

Ma t c r i a l p ��-:::.:::::: � ...;::::::: ""':::::::_-/ Compac tcd by Bul l dozcr � -=

Tracks =::::--::::-

FIG. 4-14

Paddock system : Outer wal/ inadequately compacted

FIG. 4-15

Paddock system : Full compact ion of outer wal/ by machines.

1 60

Such dams or <lump s can be placed within an impoundment , whi ch will control the surface run-o f f f rom the dump and the small amount of seepage which may occur . In this way a relat ively high <lump can be formed within low impoundment wal ls , as shown in Figure 4- 1 6.

Furthermore a conical dump gives a very good relationship between surf ace area and volume of mat erial stored. Dry dams may be required where the tailings l iquor is part icularly noxious and it is advisable t o remove it before the tailings are deposited or where the settling and consolidation characte ris tics of the tailings are poo r , leading to a large volume of very sens i t ive material if normal hydraulic depo s i t ion is used. A further consideration which would necess itate mechanical placement is climatic conditions which make hydraul ic deposit ion hazardous or impract icable .

Mechanized placing o f tailings i s o f ten used for smaller plants where the output does not warrant a large dam suitable f or hyd rau­lic deposi tion, or where hydrauli c deposition would no t produce

Imoundmen t Wa I l

Runoff Col l e c t e d for l um ing Decan t ing

Tai l i nll Cone

FIG. 4-16

Mechanica/ p/acing . Contained cane of tailings

1 6 1

11

enough ma terial of the q uality required for wal l building. It also has advantages where the plant produces dif f erent tailings products which need to be d eposited in separate areas , as mechan­ical dumping gives a great measure of control as to the exact area where a certain tail ings p roduct is to be p laced . Mechanical placing can also be used as an adj unct to a hydrauli c disposa! system, part icularly in the early stages of cons t ruct ion bef ore the hydraulic system can corne into full operation and in dealing with the topographie features which present dif ficulties to the main construct ion program.

4 . 2 . 5 . 1 Moi sture Content

Mechani zed handling and pl acement requires that the material shall at least be in a plas t i c or semi-solid s tate. If the tail ings product is in this s tate when dis charged from the extract ion pro­ces s , e . g . filter cake , it can be t ransported mechanical ly by t ruck or conveyor sys tem to t he di sposa! s i t e . Where the proces­sed product has a high moi sture content or has been t ransported hydraul i cally t o the deposition s i t e , the mois ture content will be too high f or d i rect mechanical handling and some t reatment will be required to reduce the moi sture content . This may be done by means of filters , mechanical driers or hydrocyclones . Where the p laced material has to carry the weight of t rucks or mechanical equipment used to p lace and spread i t , the moi sture content of the placed material mus t be low enough to allow the cons t ruction t raf­f i c to pass over it without becourl.ng bogged down . Where c l imat i c conditions are suitable , a two s tage proces s can b e used where the dump ed material is allowed to dry out by natural d rainage and evaporation before being moved and p laced at i t s f inal locat ion .

It should be noted that in a mechanically deposited <lump , the material is dumped at the mois ture content at which it is del iver­ed to the d umping apparatus and the mound bui lds up cont inuously without specific drying periods as in the case o f hyd raul ically deposited materia l . The drying which occurs in the dumped material is therefore due to the p revailing c limatic conditions and the rate o f dumping. The rate of dumping i s normally relat ively f as t to obtain maximum e ff i c iency f rom the apparatus and it is not uncommon to find that the mound is virtually saturat e d , resulting in a high water table wi thin i t . This should be borne in mind when cons idering drainage and the s tability o f the mound and its environs .

The position of the phreat i c l ine in the dams can be obtained from piezome t er readings and should be moni tored at all t imes .

4 . 2 . 5 . 2 Mechanical Dumping Systems

Mechanical dumping applies to the mass placement of tai lings where there are of ten no specific requirements for compaction or s t rength , except those which are inherent t o the sys tem that is

1 63

use d . Various mechanical sys tems are available and the mos t appropriate sys tem or comb inations of sys t ems will have to be selected t o mee t the specific needs of each tail ings operation .

On-road or of f-road trucks can be used d i rectly f rom the plant or f rom a dry ing s tat ion at the d isposal s i t e . The great est mer i t o f t h i s sys tem is t h e f lexibility i t gives in plac ing each load of t ai lings in t he exact pl ace where i t i s required. The dump may be buil t up by end-tipping , but this i s no t recommended as it leads t o a loose unconso l idated outer surface and a weak t op sur f ace which has to carry c ons t ruction traf fic for the next placement . A much mo re s t ab le <lump will be formed if the tail ings are placed in layers and compacted by the cons t ruct ion traf fic. Graders o r bul ldoz ers may b e used t o ass i s t in s preading the laye r s . The leas t flexible mechanical sys tem is the f ixed conveyor bel t which deposits the ma terial at one point to form a cone . Greater f lexibility can be given t o a conveyor bel t by providing one or more movable extensions s o that cones can be f ormed at several places . Such a sys tem mus t take into account the effort required to move the extension and the f requency with which it will have to be moved .

Care mus t be exe rcised i n locat ing the d i f ferent cones t o ensure that they do not imp ede the drainage of the surf ace runo f f and form cat chment basins within the d ump . Spreade r , f l inger o r s tacker equipment can b e used in conjunction w i t h t he conveyor belt d ischarge t o d i s t r ibute the tailings over a larger are a . Thi s will result in the format ion of a mound wit h s ide s lopes which will not exceed the natural angle of repose of the mate r­i al . It should be noted t hat in contrast t o the me thods p revious­ly discus s e d , depo s i tion mounds wil l not be layered but may be pat chy .

Overhead t ramming allows material to be deposi ted at any point along its t ravers e , thus a series o f cones may be deposi ted to f orm a r idge and i f t he overhead sys tem can be shi f t ed lat erally , the whole depo s i t io n area can be covere d . Thi s sys tem can be used very successfully whe re the tailings can be separated into dis­t inct coarse and f ine product s : the coarse product i s d umped by overhead tramming round the periphery t o form the impoundment wal ls while the f ines are depo s i t ed hydrauli cally in t he cent ral area .

In the two s tage process the dumped material may be spread by bul ldoz er, scrape r , grader or f ront-end loader and may even be re-distributed wi thin the deposit ion area by t ruck. Not only do these opera tions make i t easier t o cont rol the shape of the dump but the cons t ruction traf f i c compacts the tailings .

Where dry l umpy material i s place d , care mus t be taken to ensure that it is broken up and we l l compac ted o t herwise surface water

1 6 5

will f low down through the voids caus ing internal eros ion which can lead to p ip ing f lows and the formation of lo cal depressions .

4 . 2 . 5 . 3 Me chanical Wal l Build ing

4. 2. 6

The p lacing r equirement s on mechanically p l aced dams are the same as for s pigotted and paddocked dams as des cribed in sect ions 4 . 2 . 3 and 4 . 2 . 4 .

Tailings Dam Walls Bui lt with Ea rthfi l l or Ro ckfill

Where the engineer ing prope r t ies o f t he tail ings are unsuitable for wall building , and / o r the nature of the terrain and/ o r the climate preclude the use of the tailings f or wall bui lding, im­ported earth o r rockf ill are used for construct ing the impoundment walls . Economie factors also play a role and in s ome cas es over­all bene fit is derived by the disposal o f any was te rock or over­burden in the f orm of the tailings dam wall . Valley impoundments are mos t suited for this type of cons t ruct ion particularly in the early s tages of deposi tion where rap id r i s e of the wal l is requir­ed to increase the depo s i t ion volume fast enough to contain the volume o f tailings that is being p roduced. The s e s t ructures may be subdivided into impervious and p ervious walls and into thos e which are cons t ructed to f inal height and shape in one relatively fast operation virtually bef ore impoundment starts and those whi ch are raised continuous ly o r in s tage cons t ruct ion , keeping the necessary f reeboard above the tailings and water in the impound­ment area.

4 . 2 . 6. 1 Impervious Wal l s

Where t h e tai l ings cont ain mate rials which c a n give rise to nox­ious e f f luent s , local pollut ion regulations may require that s uch containment areas shal l be waterproof to prevent any s eepage o f t h e eff luent through t h e walls o r i n t o the groundwate r . The con­t ainment wal l s mus t therefore be impervious . In such cases the walls are cons t ructed using the normal procedures for the con­s truct ion of water retaining dams o f earth or rock f i l l cons truc­tion with an impervious lining of clay or synthetic mat erial on the upstream or inside face , which i s connected to an imp e rvious eut-o f f o r floor l ining. Thi s is shown i n Fig . 4 - 1 7

4 . 2 . 6 . 2 Pervious Walls

Where earthf ill and mor e part icularly rock f ill i s used s imply to provide s tability to the outer wal l , s eepage through the wal l is permi tted. Again the walls are constructed us ing accepted dam­bui ldiug techniques but iustead of an impervous line r , the up­s tream face mus t be covered by a suitable f ilter blanket to pre­vent tailings being eroded into and through the earth or rockf i l l wal l . Provis ion nnJ S t then be made f o r collecting t he seepage water and cont rolling i t s discharge . Par ticular care is required

1 67

in placing tailings against the ups t ream face to prevent damage to t he f ilter layers and i t i s advisable t o place a protec t ive laye r o f earth o r rockf ill over the f i lters to protect them agains t scour eros ion by t a iling s depos i t i on or wave action, Fig . 4 - 1 8

illustrates thi s .

4 . 2 . 6 . 3 Construction t o Final He ight

Earth and rockfill dams wi ll normally be t rapezoidal in cros s-sec­tion with a base width a s much a s s ix t imes the f inal height o f the wall . Thus on a level s i t e approxima tely s i x t imes as much material will be required to cons t ruct one me tre height o f wall at i ts base than will be required to raise i t the same height near the cres t . In a valley locat ion the initial length of wal l will be short but as the wall r i s e s into the broader sect ions o f the valley , the length o f wall will increa s e , The topography o f the s it e wi ll t he refore determine the amount o f material t ha t will be required to reach any s tage o f cons truc tion. If enough material i s readi ly avai lable to permi t cons t ruct ion o f the wall t o f inal height , then cons t ruction of t he whole wall can be undertaken in one operation, The f oundation area should be cleared and the wal l should b e buil t by placing hori zontal layer s o f s o i l or rock and compact ing them. Where the engineering characteristic s of dumped rock are sufficient to form an effective wal l , end-tipp ing may be used provided that the s lopes are trimmed t o the correct des ign s lope .

4 . 2 . 6 . 4 Stage Cons t ruct ion of the Wal l

A t many mines the production o f was t e rock increases as the mine d evelops and not only is t here insuf f icient rock f i l l available initially to build the whole wal l but o f t en there is insuff icient mat erial to bui ld the full width o f the base of the wal l , There­fore , the f i r s t s tage of the wal l mus t be constructed to utiliz e the was t e rock produced dur ing that s t age t o contain t he tailings produced during the same period . If an adequate cross section cannot be const ructed wi th the ava ilable mat erial , addit i onal material will have to be i mported. Each succeeding s tage is con­s t ructed using t he available mat erial to provide a cross sect ion to contain the t ailing s by extending the wall by the downstream cons truction method unt i l t he f inal cro s s sect ion at f inal height is r eached . Here again layered compacted construc t ion i s preferred but s id e-tipping may b e used provided that the slope i s trimmed t o the correct des ign angle. Thi s i s s hown i n Fig . 4 - 1 9 • Cons truction b y s tages may b e d i c tated b y economics .

If the tailings area i s also the was te r ock d i s posa! area and the p ro duct ion o f was t e rock i s g reater than required for wall build­ing, the exce s s material may be placed on the inner s ide o f the wall provided that the impervious membrane or filter layer i s continuous and undamaged , a s s hown i n F i g . 4 - 2 0 .

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Compac t cd Ear thfi 1 1 or Rock fi l l ·

No t to �ci l l e

FIG. 4-17

Pro t c c t i v e Soi f Layer

Impcn· i o us L i n ing

F lôôi Lifi in g t

Earth and rockfill walls : Impervious wall

E n roch emen t s Rock f i 1 1

Captage de perco l a t i oln Seepagc Co l l ec t i on Tren ch

F i l t re F i l ter

p a l n t s

Couche de p rotec t i on Pro t cc t i ve Layer

G. 4-18

Earth and rockfi wa s: Pervious wa

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Final Crest

FIG. 4-19

Earth and rock fil/ walls : Stage constr ction of wall

Imoundment Area

4 . 2 . 6 . 5 Placing of Tail ings Behind Earth or Rockfill Walls

The filling of the basin behind an earth or rockfill wal l requires careful operat ion and management . If the wal l has been des igned to resist the forces exerted on i t by the exces s water , serious overstressing can deve lop if a large depth o f unconso lidated tail­ings slurry , with i t s higher dens ity , builds up agains t the wall . It is therefore essent ial that some of the first f i lling should be placed on and agains t the wal l . This may b e done b y careful spigo t ting or cycloning sa that a layer of the coarser tailings can be built up next ta the wal l . The f i r s t reason i s ta provide addi tional protect ion for the f ilter or impervious layers in the wall and the second reason i s that depo s i t ion o f stable material agains t the wal l increases its effective thickness whereas later depo sit ion into a depth of water may lead ta overstress ing the wall. This is s hown in Fia . 4 - 2 1 .

The f i l ter can quickly and effi ciently be protected by woven or non-woven synthetic f ilter fabrics .

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�.,. , Ear th o r Rock fi l l

D i s c hargc Poi n t for Tai l ings

Bu i l d u p o f carefu l l y p l accd Ta i l ings

FIG. 4-21

Earth and rock fi walls : Deposition of tailings against wall

4 . 2 . 6 . 6 Rock But tresses

The rock buttress is essentially a remedial measure but some mine s d is pose of was te rock by plac ing i t round t he tailings dam in the form of rock buttresses .

4 . 2 . 6 . 7 Starter Walls

4. 2 . 7

The use of earth and rockfill cons t ruct ion as starter walls i s d iscussed i n sect ion 4 . 3 .

Cent ral Discharge of Thickened Tailings Slurry

This method cons ists of t he cons truction of an approximately cir­cular outer toe wall for the dam and discharging the slurry int o this area through a pipe located at the centre point of the dam. The pipe discharges vertically upwards and extends vert ically up­wards as the level o f t he deposited s lurry grows in a heap . lt i s necessary t o discharge t h e s lurry at ext remely high s lurry dens i­t ies approaching 70% density by weight ( The s lurry dens ity i s def ined as the mass o f solids divided b y the mass o f the slurry , mul t iplied by 1 00 ) . A conical heap o f tai lings is deve loped in t his way with excess water running down t he slopes to be collected at the perimeter toe wal l . The high point of the dam is , t herefore , always at i t s centre.

4 . 2. 8 Mi scellaneous Techniques

4 . 2 . 8 . 1 Disposal into Sea or Lake

Di sposal of t ail ings slurry under water has , in the pas t , been an accepted method of tailings deposit ion. However , the worldwide swing towards environmental cont rol is making this practice less at tractive .

. J 73

4 . 2 . 8 . 2 Underground Disposal

4. 3

4. 3. 1

Mine waste has for many years been d isposed o f underground where i t is used to fill mined-out areas . The obj ec tives of underground f i lling may be one or a combination of the fol lowing :

1 . to save s torage volume and area at the surfac e ; 2 . to provide a working plat form f o r t he mining of wide or mass ive

ore bodies ; 3 . to p rovide lateral suppor t for underground pi l lars thus

increasing their load-bearing capacity ; or 4 . to s upport the hanging in deep mines and to act as a means o f

absorbing s t rain energy released from the mining excavations .

Depending on the mining s i tuat ion , the tailings may be pumped into place or simply allowed to gravi tate down dip into the area of deposition, or the fi lling may be a carefully controlled engineer­ing operation . In this case the f il l , possibly wi th the addit ion of controlled quantities of Portland cernent , is placed at a carefully controlled water content , t o form a designed pattern o f support ing ribs . Underground f i lling has also been used to permi t the complete extraction of an ore body . The mining is carried out in two stages . A series of parallel slots are first mined out , t hese are then filled with tail ings , following which the intervening pillars are remove d , the hanging being supported on the p revious ly placed f i l l .

Toe Walls and St arter Dams

Definitions

a) Toe Wal l

The t oe wall o f t h e dam i s usually an earth or rockf ill wal lwhich i s placed t o provide a neat and orderly ext remity t o theimpoundment . In certain cases , t he t oe wall will have s uf fic­ient height and storage volume to prevent excessive initialrates o f rise where paddock o r s pigot techniques are beingused .

b ) S t arter Wall

A starter wall is an earth wall formed at some intermediate pos i t i on within the area of the dam. This wall is often ne ces ­s ary in order to es t ablish an init ia l build-up of coars e tail­ings in cyclone techniques . I t will have sufficient height and s torage volume behind it to enable fine tailings depos i tion to t ake place in the ini t i al stages of const ruction . In the ups t ream method of cons t ruct ion the s tarte r wall is als o the toe wal l .

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4 . 3 . 2

4 . 3 . 3

4 . 3 . 4

Toe Walls - Cons t ruction Requirements

A t oe wall as def ined in 4 . 3 above does not normally have a s t ruc­tural funct ion , although there may be cases where this is neces­sary . In either case the t oe wall should be cons t ructed to a relat ively high standard as it wi ll always be exposed to external weathering , as well as to possible seepage f rom the s tored tailings material .

The toe wall may be of impervious cons truct ion as it is conven­t ional to �rovide f i lter drain out lets under the wal l . In certain cas es a filter blanket agains t the upst ream face of the wall is also provided but , again, with its own outlets p laced under the wal l . Should rockfill be used for the toe wall cons truct ion it would be ne ces s ary to place a reve rse f ilter blanket against the rockfill to prevent migration of the s tored tailings material through the rock ( F i g . 4-1 8 ) . The permanently exposed f ace of the toe wall mus t be protect ed agains t de t eriorat ion by rock cove r , grass ing o r other suitable means .

Starter Dams - Cons t ruct i on Requi rements

A s tarter dam is a temporary facility, but must be bui lt to ade­quate standards of strength and s tabi lity to ensure that it per­forms its temporary funct i on of sto ring tailings wi thout excess ive s eepage . Within limi t s a lower s tandard of cons truct ion is , therefore, acceptable than would be requi red for an embankment s toring water , but it is necessary to ensure that it wi l l form a st ructurally adequate component of the comp leted tailings dam.

While serving i ts purpose as a s tarter dam it must perform as a barrier agains t migration of tailings through i t ; it must be an effective barrier against undue s eepage and it must be capable o f res i s t ing the full pres sure o f super-s aturated tailings from up­st ream. The sophis t i cat ion of cons t ruct ion and compaction effort required for the cons truct ion of the starter wall is lower than that requ ired f or a t oe wall , as it is only a t emporary s t ructure . Its height is determined so as to prevent failure by overtopping during f lood s .

Ground Clearing

To ensure that vegetation growth or vo ids from organic decay wi l l not o ccur in the st ructural impoundment of any dam, it i s necess ary to clear al l vegetation , including i f pos s ible roots and s tump s , from the surface contact areas beneath toe walls , main tailings impounding walls and s tarter wal ls . If cons ide red neces­sary in high vegetat i on growth areas , weed killer can be sprayed after clearing . This is part icularly important unde r f il t er drains and linings .

1 7 7

4 . 3 . 5

4 . 3 . 6

Materials o f Cons t ruct ion

Most rock or s oil f i ll mate rials would be suit able for use in the formation of toe and s tarter dam walls . Obviously there are limi­tations to this s tatement . For ins tance, if coarse rockfill is used to form the wall this would be extremely porous and could allow migration of tailings and water through the wall . If this material is use d , it would be nece s sary to place a layer of rela­tively impermeable material or an adequately designed fi lter on the upst ream face to prevent seepage and migration of tailings . ( Se e 4 . 2 . 6 . 2 ) S imilarly, clays may be unsuitable if an ext reme effort is required to place and compact them. Mos t materials between these two extremes would otherwise be suitable . Compaction of the placed materials is required in order to provide some degree of impermeabi l i ty and s t ructural s tabi l i ty when the wal l is loaded for the short period of i.ts structural li f e . However , reduced degrees o f compact ion over those normally app lied t o say a maj o r dam for water supplies , could be applied . For i ns tance , in the case of a pure rockf ill wall very l i t t le compact ion of the rockfill need be app lied .

Where it is necessary to minimi z e pos t cons t ruct ion sett lement the p laced rock f i ll should be well s lu iced with water . Depending o n the nature o f other materials that may have to be used , di f ferent degrees and types of compac tion effort would be required to provide the required dens ity .

Height and Width

The height of a toe or s tarter dam wal l is generally governed by the design requi rements of that wall , whi ch will include such con­s iderat ions as stormwater run-of f containment or storage to limi t the rate of r ise of initial tailings p lacement . This part icular subj ect has been dealt wi th in the design sect ion of the manual .

The top width is dependent upon the width required for saf e move­ment of the part icular earth building machinery be ing used and on operational access requirement s . This width , and the des igned side s lopes , would determine the f i nal base width of the walls . The s ide slope is governed by design requirements , taking cognisance of the material being used . Again, cross reference t o t h e des ign sect ion is required .

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4 . 4

4 . 4 . 1

4 . 4 . 2

4 . 4 .3

4 . 4 .4

F ilter D rai nage

Locat ion and Alignment

F ilter drains will have been located on the cons truct i on drawings on the bas is of i nf ormation gleaned f rom aerial photographs or site surveys . It is some t imes the case that when cons truct ion commences these locations must be amended to suit local cond i t ions not revealed by the surveys . For ins tance , if there is a dep res­s ion in the ground whi ch would p revent a gravi ty f low of water in the trench or if there is rock outcropping which may prevent t renching f or the f i lt er drain, the drain would requi re realign ­ment on s it e . It is always es sent ial to record such deviat ions on the cons truction drawing s .

Selection of Mat e rials

The success of a filter drain depends largely on the correct use and selection of materials for the cons t ruction of the drai n .

Part icle size grading o f the materials is ex tremely important and care must be t aken to ensure that the materials used f all well within the des ign enve lopes of grading .

Care mus t be taken not to use ma terials which wi l l degrade due to f low of water through the drain, or which will change compos i t i on due to chemical attack from any ef f luents that may be in the wat er . The materials used must also have an adequate compres sive strength to res i s t their breakup under the loading when the tailings are placed over them.

P lacing of Materials

To ensu re the ultimate successful performance of the drain, the placing of filter drainage mat erials must be carefully cont rolled on s i t e . Care must be t aken to avoid contamination of the drain­age media during placing which could lead to eventual c logging of the drain. In order to ensure that exces s i ve erosi on does not t ak e place and that further cons olida tion of the drain does not take place after loading , it is good p ract ice to compact the f i lter materials .

Placing may be done by hand , earthmoving equipment or special place rs and care must be exercised to avoid segregation .

Covering o f D rains with Tailings

The most import ant procedure for the succes s f ul long term funct ioning of a f il t er drain in a tailings dam, is the init ial covering of the drain with tailings material. If this is careless or uncontrolle d , thus causing eros ion of the drain or allowing f i ne material to initia lly corne into conta ct with the drain , c logging is likely to occur.

1 8 1

4 . 4. 5

It i s recolfilllended that plans be raade to ensure that the d rains are covered with tail ings at an early s tage . This wil l prevent eros ion f rom weathering and stormwat er and wi ll ensure tha t the correct material is placed dire c t ly on them in the first ins tance .

Where drains prot rude above the g round , s to rmwater protection berms 1nus t be provided to p revent damage .

Maintenance o f Drains

F i lter drains require cons tant and careful maintenance in the per­iod between the i r initial construction and the covering o f the d rain wi th tailings mat e rial . If poss ible , period s of long expo­sure should be avoided by const ruct ing f il t e r drains in s tage s , j us t ahead of requirement for s e rvice.

Damage can b e cause d by various rnechanisms , a s follow s :

1 ) Erosion f rorn d i re c t surface run-of f o r wind .

2 ) Eros ion by channel ling of s torrnwate r i nto a subsurface f ilter d rain t rench , o r along the drain/ground contact i n the case of a surface f ilter d rai n.

3 ) Erosion by overtopp ing o f stormwater run-o f f in the case of a surf ace drain.

4 ) Deposi tion of fine s i l t onto the surface of the drain by wind , s tormwa ter o r poo l water.

5) Me chanical damage by vehicles o r animals .

6 ) Vegetation growth.

In all cases o f damage , maintenance and repair should be comp leted before the drain i s covered with tailing s . Temporary works mus t b e cons t ructed t o protect f i lters . The s e wil l t ake the forrn o f weir cro s s ings over drains for s tormwater and s tormwater p rotection or d ivers ion channels and bund walls . As previous ly pointed out ( 4 . 2 . 6 . 5 ) f i l ters can be p rotected by the use o f non-woven sythetic f i l te r f abrics .

F ine s i l t deposits on the surface o f the filter d rain which would p revent the ingress of water into the d rain mus t be sk.immed off , and the f i l te r layer reins tated. Erosion damage mus t be repaired using correctly graded f ilter material.

All vegetation must be removed, including roots . If necessary , weed killer mus t be applied.

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4. 5

4. 5. 1

Ef fluent Sys t ems

Def initions

a) S pillways

A spillway is a p rovis ion made in the wall or on natural g roundat the flank of the dam at suitable levels , so t hat as the damrises , t he g ravi t y d i scharge of excess wate r out of the pondedarea o f the dam is al lowe d .

b) Decant Towers

A decant tower cons i s t s of a vert ical tower placed in such a pos i t ion as to always be in contact wi th the wat er pond. Itis connected to a hor i zontal outlet pipe running beneath thed am to allow d i s charge to a recove ry pumps ta tion o r to a natural s t ream below the dam. The decant tower i s extendedvert ically , in t ime wi t h the ri s e of pond leve l , as t he damis filled. The hori zontal outlet pipe is liable to damage byd i f ferential set t lement and tens ile s train. Check on s pe cialprovis ion t hat may be called for in the d e s ign.

Gare mus t be taken t o ensure that hor i zontal decant p i pe s aref ounded on comp e t ent mat erial and that differential s e t t lementswill not give rise to failure s . The decant tower bas e mus t be f ounded on mat erial which can withs tand the ext remely highdownward f o rce on the tower developed by down d rag of consoli­dating tailings . It i s also mos t imp ortant t o ensure t hat thep ipe has a t ens ile s t rength suffici ent to resist any horizontalf orces t hat may be imposed upon it due to the nature andconfiguration o f the consolidating tailings as the dam fills , and t o seismic e f f ec t s .

c ) Hi llside Ri ser Decan t

This is s imilar to a decant tower in conce p t , b u t ins tead o f s t anding vertically i t lies i n a s loping position against the natural ground. It i s extended upgrade a s the d am l evel rises.

d ) Float ing Pump Barge

This i s , a s i t s name describe s , a f loating pump station which i s located on t he tailings dam pond and i s used t o remove exce s s water f rom the pond by pump ing . The discharge p ipelines would e ither be located on a pipe gantry which is extendible vertically a s t he pond rise s , or on a s e ries of f loats extend­ing acros s the pond and beach . The p ump barge i s anchored o r moored. The dis charge main i s f i t t d with f lexible sec t ions so that i t can adj us t t o variat ions of water level in the dam.

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4 . 5 . 2

4 . 5 . 3

e ) Syphon Pipes

A syphon pipe is a sys tem for removing excess water f rom the tailings dam pond by syphoning action. The inlet end o f the p ipe is located in the water pond on f loats and the pipe rises up across the beach on the dam and over the tailings wal l t o dis charge to the collection pond or st ream below the dam wall. Such a pipe is conventionally serviced with a vacuum pump a t i t s crest and would have a contro l valve o n the discharge leg of the line .

Construct ion Requirements

Provisions for extraction o f excess water are ext remely importan t in any tailings dam. Their failure could give ri se to storage levels in the dam which , in turn , could cause a total plant shutdown. It is therefore neces sary to cons truct these facilities to the highest standard and exactly in accordance with the design requirements .

Spi llways are prone to erosion damage under high f lood f lows and the cons truction mus t be of the highest standard part icularly where the spillway is located on or near the highly erodable tail­ings materia l .

When extending decant towers with concrete rings o r wei r planks o r wha tever means are used , care must b e taken t o comply with the des ign specif ication s for that part icular tower in order to ensure that failures will not occur due to poor cons truct ion.

For example , inadequate seal ing between succes s ive precas t ele­ment s . It i s again s t res sed that once a decant facility is bur ied beneath tai lings ma terial there wil l be no easy acces s to it for repairs and the necessity for high standards o f cons truction i s emphasized.

The same would apply t o hills ide riser type decants where the in­lets are s ealed by wedged planks as the water level rises . Here again great care 11n.1st be taken to ensure that the planks are of adequate const ruct ion, made s t rictly in accordance with the s pecification and that they are instal led competently .

Op erat ing Requirements

A spillway type outlet 11n.1st be carefully maintained during the operational phas e . Generally this type of decant will have to be relocated with t ime as the water level rises . Ea ch new spillway

1 87

4 . 5 . 4

4 . s . s

must b e buil t t o the same standards a s the original and old spill­ways mus t be closed down in accordance with the recommended pro ce­dure s . These procedures must be planned in such a way that they will leave no weaknesses in the dam wall.

The gravity decant is generally a ve ry simple ope ration but never­theless requi res careful attention du ring the operational phase so as to ensu re its continued successful operation. Care must be t aken to comply with the design specif i cation in every respect during the operational phas e .

I t mus t be emphas ised again that the decant faci lity i n any tail­ings dam is an essential f eature and its f ailu re could give rise to closure of the tailings dam and the process plant feeding the tailings dam. Operat ional p rocedures for decant f aci lities mus t be of the highes t standard.

Pumped decant faci li ties must be designed , cons t ructed and main­t ained to the highest standards to ensure that they operate with the minimum of downtime and to ensure that a continuous and reliable means of removing water from the dam is always available.

Pond and Beach Management

Unless specific provision is made at the des i gn s tage , a tai lings dam should not be used for the storage of water . The pond level should be maintained at a minimum cons is t ent with the following obj e ct ives :

( I )

( I I )

( I I I )

( IV)

(V)

Decant fac i l i ty inlet mus t always be submerged .

Water flowing into decant to be free of suspended solids

Minimum s tormwater f reeboard between pond level and head of beach to be maintaine d , as s t ipulated by the design

Evaporat i on loss requirements to be met if this is es s ential for the plant water balance

Cont rol of downs t ream s eepage and interna! water table t o b e maintained , The higher the pond leve l , the greater wi ll be the head causing seepage f rom the toe of the dam and also reduct ion of the s tability safety factor .

Maintenance

The need for a high level of maint enance of all components of the effluent system must be s t ressed .

! �

4 . 6

4 . 6 . 1

4 . 6 . 2

Monitoring of Tailings Dams During Cons t ruction

P redepos it i on Remedial Measures

Site exp lorat i on of a potent ial tailings dam is convent ionally made at the commencement of the des ign s tage of the dam. Based on the information gleaned during this investigat ion certain design assumptions are made and the des ign proceeds according to these as sump tions . During the cons t ruction phase it is often the cas e that the cons truction sequence exposes cert ain additional inf o rmat ion which may have an eff ect on the design assumptions . In these cas es , decis ions must be taken by the cons t ruct ion team to amend the des ign de tai ls . These de cis ions must always be taken with a knowledge and understanding of the intended function of the item in ques tion. Sections 4 . 3 . 5 and 4 . 4 . 1 discuss this point in more detai l .

Monitoring f or S tabil i ty and Seepage

The s tability of a tailings dam cons t ructed ent i rely with tailings depends on certain assumpt ions made during its des ign . The mos t important of these assumptions re late to the level and pos it ion o f t h e phreatic or seepage surface within the dam, to the funct ioning of the under-drainage system and the l iner ( if p resent ) and to the prof ile of shear strength wi th depth .

4 . 6 . 2 . 1 Ins pect ions

Regular ins pect ions of the condi t ion of the slopes of tailings dams f orm an essent ial part of any monitoring p rogram. During these visual ins pe ct ions part icular attention should be paid to the f o llowing :

a ) The p resence of cracks parallel to or t ransverse to the crest o f the slopes or the presence of cracks on the slopesthemselves ;

b ) The p resence of cracks in the s o il at the t oe of the s lope and any visible displacement ( either ho rizontally or ve rt ically ) of s olut i on or drainage trenches at the t oes of the s lope s ;

c ) Any visible sagging o f the crest o f the s lope or bulging a t the toe of the slope ;

d ) The vis i ble emergence of seepage at the toe of the slope . This would be indicated by wetness of the surface , local concent rations of plant growth or exces sive eros ion of the s lope surface .

The appearance of any of these warning s igns is a s t rong indication that the slope may be uns table and that remedial measures may be ne ces sary .

1 9 1

4 . 6 . 2 . 2 S eepage Surfaces

The height and pos i t i on of seepage surfaces must be observed by means of piezometers , such as those shown in F igure 4-2 2 . It is usually convenient to install stand-pipe piezometers once the dam has reached a certain height . The s tand-pipes can then be extended if necess ary by adding sect i ons of tubing at the surf ace to increase their height . Stand-pipes are usually ins talled along a section normal to the s lope of the dam at cri t i cal points for s tabi lity. A suff icient number of stand-pipes mus t be ins talled to enable the seepage surface to be det ermined accurately , as i l lustrated in Figure 4 - 2 3 , and to pick up certain crit ical points on the surface such as the height of the seepage surface adj acent to under-drains , etc. St and-p ipes should be installed in pos i ti ons where they will be readily access ible at all t imes . Sui t able locat ions would be on dividing walls or s imi lar prominences .

As an alternat ive to stand-pipe piezometers , several o ther types are ava ilable . Fo r example , twin-tube hydraul i c piezome t ers , pneumatic pie zome ters , vibrat ing wire and strain-gauged diaphragm piezometers may be more suit able for spec ific applicat ions . They use horizontal connections and avoid dif f icul t ies of maintaining vert ical tubes during cons truction.

4 . 6 . 2 . 3 Seepage through Liners

The f unctioning o f the under-drainage of a dam, or collector drains ins talled beneath a l iner can be monitored by measuring the rate of discharge which should agree reasonably well with the cal­culated quant ity of discharge. Monitor wells around the perimeter of a l ined dam, as i llus trated in F i q . 4 - 2 4 , penetrating the water table can also be used to moni tor the effectiveness of a l iner. The effluent from col lector drains and samples t aken f rom moni tor wells should both be checked to ensure that the quality o f the groundwat er is n o t being unduly a f fected by the pre sence o f the res idue depos i t . I f for some reason the monitoring procedure shows that the liner is not ef fect ive , it may be necessary to const ruct a ver tical cut-of f or else a relief wall drain around the tailings dam. The ins t allation of such measures will require the complete reappraisal of the stability of the dam , as , particularly in t he case of a cutoff wall, the water levels in t he f oundat ion s trata will be af fected.

It i s sometimes necessary to ensure that there is no seepage what­ever f rom the tail ings dam into the groundwater sys tem. In such cases it is essential to prevent seepage by depo s i t ing the tail­i ngs on a prepared impervious l ine r of compacted clay or of an impervious synthetic sheeting such as butyl rubber or some other e ffective sealer.

4 . 6 . 2 . 4 Prof iles o f St rength and Relat ive Dens ity

In this context "pro file" s igni f ies the values o f s trength or relative dens ity over a phys ical range def ined by the designers as a crit ical spectrum t o ensure the stabi l i ty o f the s truc ture .

1 9 3

� Cap u chonsScrew Caps t o Sea/ Tubes

., - i::: _,, c: - � V1E E - e E O OO ON <"'<

· ::

-----1··

-----1:::· · ,\ .,. . . . . . , . . . .

• .: :, . . . · ....

Rcs i d ue Tamped i n t o P l ace

R i g i d PW:: W ater Pipe

S t a ndard Ceram i c Wa ter Fi l ter Candi e

See Note Q)

_--i-l�.----1'.�fg{#d' -1 � ·:s{��;�:)�):

. Funne l fi l l e d wi th sand before l o wering into hole.

FIG. 4·22

Stand.pipe piezometers suitable for installing in tai/ings deposits

1 94

Wa t e r Level s Recorded in Stand-Pipes

Secpage Emerg i n g hcre

FIG. 4-23

Typica/ seepage surface determined by means of stand-pipe piezometers

· · · · · . .... . .. .

L iner

ReA ional Wa ter Tabl e Mon i tor Wel l s

FIG. 4-24 Monitoring wells installed around the perimeter of a lined residue dam

1 9 5

The s trength pro f i le of the mat erial con s t i tuting a dam wal l i s bes t moni tored by performing i n-s itu shear tests a f ter three to f ive met res o f ma terial have accumu lated i n the depo s i t . At the same time tests can be performed to es tablish relative dens ity p ro f i les . The results of these i n-s itu tests can then be compared with condit ions as sumed for the design and the s lope of the t a i lings dam can, if ne cessary, be amended accordi ngly .

Seve ra! methods are avai lable f or in-s itu shear test i ng . The shear vane , piezometer cone penetrome t e r , pres sure meter and standard penet rat ion test have all been used successfully to es tabl ish prof iles of st rengt h . The cone penet rometer and stand­ard penetration test can be used to establ ish relative dens ities . Whatever tests are chosen should be perf ormed unde r the careful supervision of the geotechnical engineer who will ult imately interpret the results and check the s tabi lity of the dam.

4 . 6 . 2 . 5 Moni to ring by Survey

Survey methods may al so be used to monitor tai lings dams . The s implest f orm of survey system cons ists of a l ine of beacons along the toe of the s lope . These beacons are aligned on ins tal lat ion and also levelled. The reaf ter, periodic observati ons for line and level are made . The upward or outward movement of any of the line of beacons would indi cate the development of shear i nstabi l i ty i n the slope . A simi lar line o f beacons could be ins tal led o n be rms part way up the s lopes of a res idue depos i t .

More complex ins t rumentation, e . g . s lope indicators , may also be ins talled to monitor the movement of slopes .

4 . 6 . 3 Re cords

4 . 6 . 3 . 1 General

It is essentiel that accurate records be kept du ring the cons truc­tion of a tailings dam. They will be used for back-up monitoring of the dam and wil l provide vi tal evidence of the pe rformance of the dam. If a failu re does occur, whether it be the dam itsel f , the line r , other po l lution control measures o r whateve r , the accuracy of the record keeping will be tes ted to the fu l l , lead i ng probably to the correct assessment of factors leading to failur e .

A comp le t e s e t o f "as bui l t " working drawings and the pre-deposi­tion and tailings placement des ign drawings should be kept on hand at all times .

197

4 . 6 . 3 . 2 Delive red T onnages

An accurate measurement and recording of measu red tonnages wi ll allow good fo recas ts of dam life and wall growth to be made . Such records are also essential to cal culate payment when a cont rac tor is cons truct ing the dam. In many cas e s , the operator should keep an hourly record of where the tailings have been de live red .

4 . 6 . 3 . 3 Volume t ric/Level Records

Volumetric and level de terminations from survey data should be recorded and checked agai ns t des i gn predictions . In some count ries , a yearly survey of the tailings dam has to be made and presented to the relevant au thority, together with future predict ions of heights .

4 . 6 . 3 . 4 E f f luent Records

E f f luent records may again be a statutory requi rement or only used as empir ical back-up .

a ) Drains

Records of drain outf low performance is a good indication of the ef f iciency of the unde rdrainage sys tem. It wi ll also give vital information on problems developing if moni tored correctly.

b ) P ond Return Water

Whe rever pos s ible , measurements of pond water returned or disposed of should be recorded . Records form an important management and des ign too l .

c ) P ol lut ion

When pollution of any of the air , surface water or groundwater is poss ible , measurement of this pollut ion must be taken and records kep t . Minimum standards of measurement are often legis lated f o r .

d ) Phreat i c Surface

Of all the records that should be kep t , the piezometric records are the mos t impo rtant . They indicate the level of the phreatic surf ace, whi ch bears directly on the s tabi lity of the tailings dam. Readings should be taken at regular intervals and then plotted on s cale sections of the dam . Any increases beyond or approaching design limi ts should be inves t igated . See sect ion 4 . 6 . 1 . 2 .

199

e ) Other Re cords

Specif ic wall building techniques may require other records to be kep t ( e . g . cyclone split and performance ) .

4 . 6 . 3 . 5 Wall Bui lding

4 . 6. 4

In certain cons truct ion methods used for wall building , sophisti­cated measurement techniques may be requi red. Rates of cons t ruc­t ion wil l be cont rolled by reference to these records ( e . g . piezo­meters ) .

Gene ral Maintenance

Maintenance of any f acility is a very necessary bu t cos tly i t em, Tail ings dams should be des igned for the minimum of maintenance . Not only are they usually s i tuated at a s ite remo te from the plant , they are open to environmental conditions and f eature low on the p lant manager ' s priorities.

4 . 6. 4 . 1 Ef f luent Trenches

E f f luent trenches should at all times be kept f ree of weed or reed growth that inhibits f low . Similarly , erosion sediments should be per iodically removed .

Where these trenches are lined , maintenance would generally be o f a f a r higher orde r , to prevent leakage and exces sive damage o ccurring .

4 . 6 . 4 . 2 Run-of f Paddocks

In areas of high rainfall, run-of f paddocks are provided around the toe of the impoundment embankment to catch both the eroded tailings for sett lement and the run-o f f water. As they f i l l , the containment walls will require heightening , while decant s for the water may also require heightening.

4 . 6 . 4 . 3 Ef f luent Co ntrol Syst ems

Much of the eff luent from a tailings dam may carry a percentage of suspended solids , deleterious in nature t o ret urn pump faci lities or the s tream. Any effluent control sys t em built t o settle out s uch f ines will need t o be cleaned out periodically .

4 . 6 . 4 . 4 Access Roads

Problems on a tailings dam invariably occur in the wor s t weather . It is thus an imp or tant requirement t o maintain access roads , in fact t o build them in the f i r s t plac e , in good cond i t io n .

201

4. 6 . 4 . 5 Pipelines

Regular inspections of the tailings pipelines are essential t o prevent. pollution from breakages and allow t imely replacements to be made .

In order to reduce maintenance cos t s , higher capital costs may prove advantageous : a change to rubber or polyurethane lined s teel pipes or a change t o a plas t i c p i p e .

4 . 6 . 4 . 6 Valves

4 . 7

4 . 7 . 1

Tailings disposal i s controlled by valves , which tend t o be the bane of the operat or ' s l i fe , Once the mos t sui table valve is found , regular ins pect ions and maint enance is es sential to ens ure control .

Remedial Measures

General Pro blems

Certain maj or p roblems can arise on a tailings dam whi ch requi re speci f i c remedial measures as di s t inct from rou t ine ma int enance . The mos t serious may be grouped together as f o l lows :

a ) Overtopp ing

S t raightforward over topping occurs where the pond level rises above the wall level and water flows over the cre s t of the wal l . This should never occur if proper operational p ro cedures have been maintained to ensure adequate freeboard , even for abnormal rainfall , and an effect ive pens tock or de cant system t o remove excess water is in use . Howeve r , if this should have o ccurred , the material will normally be eroded to f orm a gul l ey the s iz e of which wil l depend on the degree of overtopping and t he erodibility of the mat erial .

b) Surf ace Erosi on

The outer face of a tailings dam p resents a consi derablecat chment area for rainf all and the resultant run-of f tends toerode gullies on the exposed fa ces of the wal l s . Gu llies may also be eroded as a result of spi l lage of tailings ove r the topof the wal l . Where the tailings material in the outer wall isrelatively res istant to eros i on the gul leying wil l be minor and is accept able . Maj or gul l i es wi l l , however , aff e ct the overalls tabi l i ty of the dam and remedial act ion arust be taken ,

103

c ) Seepage E ros ion

Where there is inadequate unde rdrainage , the seepage water exits at the t oe of the wall creating a wet weak area , very often accompanied by erosion , leading to the unde rcutt ing of the outer walls . This se rious cond i tion, caused by allowing the phreatic surf ace to reach an unacceptably high level in the downst ream part of the dam, must be remedied immediately to avoid progres s ive failu r e . Remedial measures are described in Section 4 . 7 , 3 ( c ) .

d ) P iping

A part icular form of seepage eros ion is pip ing eros ion ; s eepage f low is concent rated in a preferred channel leading to interna! eros ion and the formation of a pipe or tunnel which can lead to the breaching of the wall if not controlled , Pip ing does not neces sarily exi t at the toe , but may appear anywhere on the f ace of a dam. Remedial measures are described in Section 4 . 7 . 3 ( d ) .

e ) S lope I ns tabi lity

A combinat ion of inadequate strength in the tailings and /or the f oundat i on soi! can lead to the threat of a s lide f ai lure or breakaway in the outer wall . Visual signs of serious problems are tens i on cracks on the upper surf aces of the wall near the outer edges and toe heave . In thes e cases the probabi lity of a failu re is very great and earlier warnings of problems should be obtained from the s tudy of ins t rumentation records and stability analyses using the actual s t rengths measured in the dam.

f ) Ins tability or Untightness of the Decant Towers or S ide Hill Ris e r Decant s . The instability of the decant t owers is due t o weaknesses i n their foundations and connect ions to the collect ors . It may also be due to the unreliable clos ing of the spillway openings . Tailings may flow through cracks and unt ightly closed openings into the collector . Large craters may form, endangering the security of the whole dam. These defects cannot be repaired , s ince the affected comp onents lie unde r water-satured t ai lings . In order to avoid such fai lures , the towers must be st rong and reliable, erected on sound f oundations separately from the collector . The spillway openings should be clos ed and very carefully sealed . The spillway openings of the s ide hill riser decants should also be very carefully closed .

205

4 . 7 . 2

g ) Ins tability and Leakage Defects of the Collectors

Failures may occur if the collect ors are not properly designed and manufactu red . Tailings pulp begins to seep through crack s ; these cracks enlarge gradually, a large amount of tailings are washed out and big craters may form in the impoundment . If a crater is f ormed , the washing of tailings through the collector may be s topped by dumping large amount s of grave! and s traw into the crater . If the ruptured collector is vis ible in the crater , cernent bags mixed wi th s and should be dumped into it and again f o llowed by grave! , s and and tai lings , so that the hole is properly f il led in and the water-t ightnes s res to red .

Collectors and decant towers are of ten calculated for a certain init ial impoundment depth. Later on it may be decided that the impoundment depth should be greate r . In this case the collectors and the towers become overloaded . That is why they should be des i gned for the f inal s tage .

F ai lures in collectors can also be attributed to leaching of the concrete , due to pulp chemi cal agents and reagent s left in the tailings during the oredress ing proces s at the factory. If the pulp cont ai ns chemi cals whi ch attach the concret e , the collectors and towers mus t be chemi cally res i s tant or should be otherwi se protected . The s ame app lies to s teel , where thin wal led steel tubes are used in the cons truct ion .

Appraisal of the S afety of Exis t ing T ai lings Dams

I t is of ten neces s ary to carry out an app raisal of the s afety of an exis t ing tailings dam. This may arise either because the dam was not ori ginally adequately des igned and cons t ructed or because a problem has aris en f rom operational procedu res . A change in the usage of the depos it or the characteris t i c of the res idue wou ld also requi re reappraisal as would a contemp lated change in the me thod of management or of cons truct ion of the dam. In fact such appraisals should be carried out at regular intervals , s ay once a year , to mak e absolutely ce rt ain that no incipient problems have been overlooked in the day-to-day operation of the dam.

4 . 7 . 2 . 1 Inspect ion

In the case of such reappraisal , the f i rs t s t ep should be to carry out a careful ins pect ion of the dam in accordance wi th the guidelines s et out in Section 4 . 6 . 1 . If approp riat e , reference to s tabi lity charts may be made to obtain a prel iminary es t imate of the s t abi lity of the s lopes from the knowledge of their height , angle , rate of rise and the cons olidat ion and strength character­i s t ics of the tailings . If this p re liminary s tabil i ty survey does not prove reas su ring , further measures to inve s tigate the s t abi lity should be taken.

207

4 . 7 . 2 . 2 Measurements

The f i rst essentials are to locate the s eepage surface in the dam and to assess the shear strength of the tailings . Measu rement of in-situ shear st rength profiles for the dam may be combined with the operation of ins talling piezome ters . S trength prof iles are f irst measured at s t rategic points using a vane shear apparatus, a piezome ter cone penetrome ter or a press iome ter , af ter which stand-pipe piezometers may be installed in the shear s t rength holes . The strength prof iles should extend through the full height of the tailings deposit and as far as possible into the founda tion s tratum. S trength prof iles should also be measured and p iezomet ers ins t alled in the foundation s t ratum at the toe of the s lope .

4 . 7 . 2 . 3 Analysis

4 . 7 . 3

Once the results of these measurements have become available a full s tabi lity analy s is of the dam in its pres ent cond it ion can be performed and the effect of changes in future use of the dam, such as extensions of its height , investigated and accommod ated . I n order t o effect such extensions and changes and overcome ce rt ain p roblems with safety it may be necessary to undertake precaut ionary and remedial measures .

Pre cautionary and Remedial Measures

Once a problem has been identif ied , its cause must be det ermined . Wherever poss ible operational p rocedu res mus t be modif ied to e lim­inate the cause of the problem. In mos t cases , however , additional precautionary and remedial measures will be requi red , s ome of which are out lined in this sect ion .

a ) Gully Eros ion

The protect ion agains t surface eros ion by vegetat ing slopes or other p rotective t reatment is a subj e ct on its own and is not t reated here . Where gu llies have formed and threaten to become serious , they can be f i lled with grass sods where the fibrous root sys tem wil l ho ld the soil together and prevent further erosion.

b ) Breaks in Wall A breach in the outer wall caused by a maj or eros ion gully or a slope f ai lure or breakaway requires both immediate and long­term act ion . The immediate action, if that section of the dam is to continue to operate , is to step back the ou ter face of the wall to a safe dis tance behind the head of the break . This has two dis advantages in that the outer wall in this weak sector is carried on weak unconsolidated material and that a cons i de rable depos ition area is les t .

209

: 1 • • • • ; • • :

The longer term obj ective is to recover and reins tate the aff ected area. The sector should be cleared out to a good foundat ion at the toe of the wall and an underdrained wall should be cons t ructed in the break to allow the area to be f i l led as rapidly as safety pe rmi t s to reins tate the area and cat ch up wi th the t op of the wall . This is illust rated in Fig . 4 - 2 5 .

c ) S eepage P roblems

The obvious answer to seepage problems at the toe is to improve the underdrainage and this may be done in seve ral ways :

( i ) Pushing o r j e tting perforated pipe drains into the existing wall , as shown in F ig . 4 - 2 6 .

( i i ) Cons t ruct ing an underdrained tai lings butt ress agains t t he affected area . This in effect means adding a new underdrained t oe to the wal l . Normally such a but t ress will be 15 to 2 0 m wide to provide an adequate underdrain, and its outer s lope shall be the same as the s lope of the exi s t ing wall or les s , as illustrated in Fig . 4 - 2 7 .

Depending on the height of the exi s ting wall and the level of the pond , there wi ll be a minimum height of but t res s ; otherwise the seepage may reappear above the but tres s .

Or i g inal Seepage Surface

:·::.�· ::: ·>·· · ·. ·: ".: · .; . . ': ... . . : · . . . . ·; . .

Seepage Sur face a ft e r In s t a l l ing Pipe Drains

Perforated Pipes Inst a l l e d at Sl i g h t upward Incl i n a t i o n in Pre-]e t t e d Ha les

FIG. 4-26

Remedial meas res : Perforated pipe

drai s

2 1 1

( i i i ) P lacing a filtered rock bu ttress aga ins t the af fected area . The aim of this treatment is to allow the seepage water to exi t from the tailings face without eroding tailings mat erial. Tqe f i lter layers are p laced agains t the tailings face to prevent eros ion of the tailings and the free-draining rock is dumped to form a butt ress to hold the filter in place . The but t ress should have a width of about 4 m to permit t rucks to travel on it to dump the rock , and it should have an ou ter slope equal to or less than the exi sting tailings s lope . It should ext end to slightly above the height of the ant icipated seepage exit z one as shown in Fig . 4 - 2 8 . A problem that arises wi th rock bu ttress is bli nding by fines carried down by run-of f f rom the tailings su rface above .

d ) P iping

Piping failu res of ten develop very rapidly once the f i rst nar­row pipe has been fo rmed and theref ore any s ign of piping re­quires immediate act ion . The f i rst step will theref ore be to lower the pond leve l , especially in areas near the suspect ed p ip ing , as rapidly as poss ible . I f it is poss ible to locate the inlet to a suspect ed pipe on the ins ide of the dam, the area can be compacted and s eale d . However, it will rarely be poss ible to do this as the inlet wil l most of ten be unde r wa ter and as p iping occurs from the outside and erodes its way inwards , it wi ll only show on the ins ide when well developed . Underdrained tailings and f i l tered rockfill bu ttresses will prevent pip ing fai lures at the toe of a tailings wall . If damp patches or a fine t rickle of water indicates a piping cond it ion at a higher leve l , the affected area should immediately be covered by a f i lter and ballas t ed with waste rock to prevent eros ion of material and enlargements of the pipe while still letting the water f low out . This may be very difficult on a s teep outer face and wil l probably require the cons truct ion of a rock buttress up to that leve l .

Fi l ter Layers

FIG. 4-28

Re media/ measures : Addition of filtered rock buttress.

2 1 3

FIG. 4-31

Reme ia/ measures : Incorrect construction ofrockfi buttress

e ) S lope Ins tabi lity

S lope ins tability depends on the height of the dam, the ou ter s lope angle, the s t rength characteristics of the material and the pos i t ion of the phreat i c surface . L i t t le can be done about the s t rength characteristics in an exis ting wall and remedial measures are theref ore concentrated on the relationship between height and s lope with the concept that instabi l i ty is caused by the outer slope being too s teep for the height of the dam,

Where an exi s ting wal l is cons idered safe at its present height and s lope but raising it to some further height , if that same s lope is ma intained , wil l make it unsaf e , the remedy is to re­duce the overall s lope by stepping back . Fig . 4 - 2 9

il lus trates this .

2 1 5

Where an exis ting wall is found to be inherently unsaf e , the s lope of the wall must be reduced and this can best be done by the addition of a tailings or rock but t ress as illustrated in Fig . 4 - 3 0 . The exact height to whi ch the butt ress must be rais ed to gain maximum benefit depends on the geome t ry of the wall and the f ailure surface . It should be obvious that if the but tress is raised to the full height of the wall , no , or very lit t le , benefit of f lattening the s lope has been gained. I n general it can be said that a but tress between 0 , 3 and 0 , 5 times the height of the tai lings wall will give maximum benef it .

NOTE : The angle of repose of rockf ill is invariably cons ider­ably steeper than the ou ter slope of tailings walls . If rock­fill is dumped from the top by side or end tipping without t rimming the outer face down to the slope of the tailings wal l , an inverted wedge-shaped but tress can be formed whi ch will reduce the stability of the wall ins tead of increas ing it , as i llus trated in Fig . 4 - 3 1 .

2 1 7

B IBLIOGRAPHIE

l , AMERICAN SOC IETY OF C IVIL ENGINEERS , proceedings of the confe rence on geotechnical pract ice for the disposa! of solid was te mat erials , 1 3- 1 5 June , 1 97 9 ,

2 . ANDR� , K . V . et al . , cont rol of pollut ion from tai lings dams containing a high proportion of coarse material. F rom: Commi ss ion I nternat ionale des grands barrages . 1 2 th Congress , Mexi co 1 976 .

3 . CALDWELL , Jack , baf okeng f indings : guidelines reques ted for slimes dam cons t ruction, maintenance . F rom: S .A . Mining and E ngineering Journa l , October 1 9 7 8 .

4 , CANADA , DEPARTMENI OF ENERGY , MINES AND RESOURCES , tentative des i gn guide for mine waste embankment s . F rom: Technical Bulletin T B 1 45 , 1 97 2 .

5 . CHAMBER OF MINES OF SOUTH AFRICA , code of pract ice for cons t ruct ion of s limes dams and the condit ion in whi ch they should be lef t at the time of mine closure .

6 . COUNCIL FOR SCIENIIFIC AND I NDUSTRIAL RESEARCH - Donalds on , G .W. , the des ign and cons t ruct ion of special type hydraulic fill dams .

7 . DONALDSON , G , W , , the design and const ruction of special type hydraulic fill dams . F rom: The T ransactions of the S .A. Inst i tut ion of Civil Engineers . Vol. 5 , Nos . 9 and 1 2 , 1 9 5 5 ,

8 , DONALDSON , G .W . , s limes dams f or gold mine t ai lings and o ther res idues in S outh Africa. F rom: Commiss ion Internat ionale des grandes barrages , 1 2 th Congress , Mexico 1 9 7 6 .

9 , DONALDSON, G .W . , a summary of s limes dams p ract ice in the United S tates of America and C anada .

1 0 . Klohn , Earle J , , des ign and const ruction of tailings dams . From: The Canadian Mining and Metallurgical Bulle tin for April , 1 97 2 .

2 1 8

1 1 . MROST , M . , s limes disposal at S ou th African gold mine s . From: Journal o f the S . A . Ins t itute o f Mining & Metallurgy , F ebruary , 1 9 74 .

1 2 . PIESOLD , D . D . A . - s ome aspects i n the design of the new dam cons t ructed for Mufuli ra Copper Mines L t d . F rom: Northern Rhodesian Society of Engineers ' J ou rnal , May 1 963 .

1 3 . TAILINGS DISPOSAL TO-DAY , Vols . 1 & 2 (Proceedings of International Conferences ) 1 972 + 1 97 8 .

2 1 9

5 . CLOSURE AND ABANDONMENT

5 . 1 General

The closure or abandonment of a tailings lagoon may occur because of a variety of circumstances . Thes e include reaching an economic or regulation height , partial f ai lure o f the lagoon and closure o f the mine o r works that i t served . Continued drainage and consoli­dation can improve the stability o f the discarded tai lings , but provision mus t be made to avoid future danger to mankind. Hazards that mus t be guarded agains t include :

1 . St ormwater damage. Th is can adversely affect s t ability and cause slope erosion wit h consequent spread o f tailings on t o adjo ining land .

2 . Water pollut ion by dissolved materials o r tailings thems elves carried by drained wate r ,

3 . Mining subsidence or earthquake shocks that could cause insta­bility.

4 . Ai r pollution due to wind e rosion .

Tailings lagoons a r e o f ten buil t like impounding reservoirs in a valley . Several may be const ructed in one valley , presenting a cascade hazard exemplif ied by the Buf falo Creek disaster of 1 97 2 . Af ter abandonment , arrangements mus t b e made to safely conduct the maximuo poss ible f lood that can occur in the valley pas t t he o ld lagoons .

The seriousnes s o f water pollution may depend on the t yp e of tail­ings in t he old lagoons . If they contain poison i t will be neces­sary to isolate d rainage : if not , i t may only be necessary t o arrange for rainfall run-off to occur without caus ing erosion.

Shear s t rains o r shock loading induced by subsidence o r ear th­quakes can produce very high pore pressures in t ailings and could lead to failures of the type experienced in Chile in 1 96 5 . The risk of this mus t be assessed , together with the consequences o f failure and poss ible mud flows over great d i s tances .

221

I f veget ation can be induced to grow over the sur face of abandoned dams and lagoons wind eros ion may be eliminated. If vegetation will not g row , wind eros ion can be reduced by placing a coating of coarse material over the f ine d iscard.

Vegetation aspects and o ther types of protect ive cover for the abandonment of tai lings dams and impoundments and land rehabilita­tion were the subj ect o f some papers at the First and Second International Tail ing Sumposia in Tucson, Ari zona in 1 97 2 and in Denver , Colorad o , i n 1 9 7 8 .

5 . 2 Regulat ions

5 . 2 . 1

S ome countries have enacted laws and regu lat ions to govern the s i te selection, des i gn , const ruction and abandonment of waste em­bankments . Regulat ions relating to closure of wast e embankments vary f rom country to count ry . The following examp le is typical o f such regulat ions :

Regulations in G reat Britain

In G reat Britain was te t ips or embankments are cont rolled by :

a ) t he Mines and Quarries Act , 1 954 b ) the Mines and Quarries (Tips ) Act , 1 96 9 c ) the Mines and Quarries (Tip s ) Regulations , 1 9 7 1 d ) the Mines /Quarri es ( Notification o f Dangerous Occurrences )

O rders , 1 95 9 e ) t he T own and Country Planning Act s 1 96 2 t o 1 96 8 f ) t he Rivers ( P revention o f Pollution) Act s , 1 95 1 t o 1 96 1 g ) the Clean Air Acts , 1 95 6 t o 1 96 8 .

In these regulat ions , " t ip " means an accumulation or deposit of refuse from a mine or quarry (whether in a solid state or in solu­t i on or suspension ) other t han an accumulation or deposi t s i tuated underground , and whe re any wall or other s t ructure retains or con­f ines a t ip then , whether or not that wall or s tructure is itself composed o f refuse , it shall be deemed to form part of the t ip for the purposes of the regulations .

The regulations apply to any tailings lagoon that is more than 4 m above adj oining land or has a volume greater than 1 0 , 000 m3 .

A demarcation is made be tween a tip that bas become full o r for s ome other reason is no longer used , but is associated with a working mine and a tip attached to a closed mine . The first is called a "closed" t ip and is the respons ibility of the mine owner . The second is a "disused " t ip and it is put unde r the jurisdiction of the local authority ( i . e . county or borough council ) .

223

C losed T i p :

When a t i p i s closed , the mine or quarry owner mus t give not ice of this to the mines inspector for the d is t ri c t , within two months . He must then have a closed tailings lagoon ins pected by a compe­tent persan at int e rvals not e xceeding s ix months . The compe tent persan mus t make and s ign a full and accurate rep ort of the in­spection. These inspection repo rts must be kept at the o f f ice o f the mine or quarry for at least three years and in addi t ion , the competent persan must record every defect revealed by his inspec­t ion in a separate record book . The persan res pons ible for the t ip must record the action taken to remedy each defect . In the cas e of a closed tailings lagoon , these records must be kep t for at least five years .

Re ports on the security o f a closed lagoon mu st be obtained by the mine owner f rom a competent persan at least every f ive years or as s oon as practicable af ter any dangerous occurrence. Such reports mus t give an opinion whether the t i p i s s ecure , detai l s of any subsidence or sur face movements , account s o f surveys , bo reho les , and tests , and the nature of any ins pect ion, supervision and safety measures necessary to ensure the s ecur ity o f the t i p . These should b e kept , together wi th records and d rawings , a t the mine o f f ice for use by the inspectors or other author ised pers ons .

Di sused T i p :

T o ensure the s tab i l ity o f a d isused t i p and prevent i t f rom becoming a danger t o members o f the publi c , the local autho ri ty have been g iven powers to :

a) require the owner of the tip to produce document s relating tothe t i p and the land it is on ;

b ) obtain entry to the t ip and adj oining land for purposes o f inspect ion and explorat i on ;

c ) require the owne r to carry out remedial wo rk;

d ) carry out es sential remedial work themselve s ;

e ) recover the cost o f the remedial work f rorn :

i ) the owner o r owne rs o f the t i p during the past 1 2 years ;

i i ) any ot her pe rsan who has used the t i p during that period ;

i i i ) any other person who has caused or cont r:ibuted to the instability of the t i p , during the pas t 1 2 year s ;

f ) seek a grant t o pay for some o r all o f the remedial work .

225 15

5 . 2 . 2

For the purposes o f the Act , a disused t i p shall be t reated a s un­s table i f , and only if , t here i s , o r there is reas onabl e ground for believing that there is l ike ly to be , such a movement of the refuse which makes up the t ip as to cause a signif icant increase in the area o f l and covered by the t i p .

Britain contains very few disused tailings lagoons . Many o f those are below g ene ral g round leve l , in old quarries or clay wo rki ngs and are of chemically ine r t material s . The only hazards they pre­sent are to works that mi ght be carried out over them such as pipes or cables that might be laid across them.

In the United Kingdom, the Nat ional Coal Board has p roduced Codes and Rules for Tips ( 1 9 7 1 ) relat ing to act ive and closed t i p s . lt normal ly ins pects i t s own disused tips in the same way as i t s c losed t i p s . The Cent ral Elect ricity Gene rat ing Board cons ider t he i r fly-ash lagoons as reservoirs under the 1910 Re servoirs ( Safety Provis ions ) Ac t and have them des igned and inspected by Panel Engineer s . Thi s involves detailed inspection at leas t every ten year s .

The new Re servoirs Ac t that has n o t y e t been imp lemented requi res cons tant survei llance of reservoirs and may lead to more frequent inspect ion of the fly-ash lagoons .

Re gulat ions in Other Coun t r ies

Abandoned tips in o ther countries

The Canad ian Cent re for Mi ne ral and Ene rgy Te chnology has publ ish­ed a Pit Slope Manual ( 1 9 7 7 ) . Chapter 9 of this manual deals with was te embankment s , includ ing tai lings l agoons . It re commends t hat embankments should be put into such a cond i t ion before being aban­doned that only occas ional maintenance will be required to p revent deteriorat ion to the point where instability can develop . Thi s will usually req ui re care t o :

a) provide rel iable drainage s chemes , capable o f funct ioning adequately for many years wi th little maintenance , and

b ) p ro t e c t embankment slopes agains t surface ero s ion andweathe ring .

In Sout h Af rica regulat ions made in 1 9 7 6 reinforce exis t i ng regu­lations on wat er pollution and require mine managers to measure flow rates and analyse e f f luent four t imes a year. Under the Water Ac t 1 9 5 6 , t he manager of a mine is req u i red to make adequate p rovis ion t o prevent rain run-off f rom erod ing s l imes dams and tail ings l agoon s . Stormwater mu s t b e controlled so tha t i t flows clear of s limes dams , minera l , tail ings and was t e-rock d ump s and

227

o the r s ources of pollut ion. Ba rrier dams and evaporation dams shall be provided where necessary to retain run-of f from tai lings dams that may include e roded mat erial. The s torage capacities of these dams shall be sufficient to ensure a f reeboard o f at leas t 0 , 5 m above the expected maximum wat er leve l , based on averagemonthly rainfal l , less gro s s mean evapo ration, plus the maximum precipitation expected ove r a 24 hour period once in a hundred years .

The mine manager shall ensure that as far as practicable , tailings or s l imes dams , barrier or evaporation dams shall be located and des igned to minimi se the possibility of damage by subs idenc e , set­t l eraent , shock and cracking , due to any mining operation s . Di s­used tailings lagoons and was t e <l umps shall be kept adequately fenced and sha l l not be used for any purpose without approval. Any person removing material shall be responsible for reinstate­ment to e f f ectively avoid wat er pollut ion.

229

B IBLIOGRAPHIE

1 . BACH , Dan A . , the u se of drip irrigation f or vege tat ing mi ne was te areas . From: Tai ling Disp osa! To-day : Proceedings of the F i rs t Inte rnat ional Tai ling Symposium, Tu cson , Ari zona . October 3 1 , November 1 , 2 and 3 , 1 9 7 2 . Chap ter 2 3 : pp . 563-5 7 0 .

2 . BEWNG SON , Stuart A . , irrigat ion te chnique for tai ling re-vege tation in the arid Sou thwes t . From: Tai ling Di sposa! To-day , Vo l . 2 : Proceedings of the Se cond Inte rnat i ona l Tai ling Symp osium, Denve r , Co lorado , May i 9 7 8 . Section 4 - Rec lamation, Vege t ation and Abandonment Chap ter 2 4 ; pp . 487-50 4 .

3 . CHENIK , D . the p romo t i on of a vege tat ive cover on mine s limes dams and sands <lump s . From : Jou rna l of the South Af rican Ins t i tu t e of Mining and Meta llu rgy , 1 9 6 0 .

4 . CLAUSEN , H . T . , ecological aspects of s limes dam cons truction. From: The South Af r i can Ins t i tu te of Mining and Me ta llurgy . Gene ral Mee t ing and Colloguium on "The Cons truction of Slimes Dams " , 1 9 7 3 .

5 . DAY , A. D . and K. L . LUDEKE , d i s tu rbed pond reclamation in an arid envi ronment . From : Tai ling Disposa! To-day , V o l 2 : Proceedings of the Se cond Inte rnationa l Tai ling Symp os ium, Denver , Co lorad o , May 1 9 78 . Section 4 , Rec lamation, Vegetat ion and Abandonment Chap ter 2 1 pp . 4 3 7 -460 .

6 . HILL , J . R . C . , the mine dump problem in Rho di s i a . From: Rhodisia Agri cu ltu ral Jou rna l , Vo l . 6 9 ( 4 ) .

7 . HILL, J . R . C . and W . F . NOTHARD , the Rhodes ian app roach to vegetating s limes dams . From : The South Af rican Ins t i tu te of Mining and Metallu rgy , General Mee ting and Co lloquium on "The Cons t ru ct ion of S limes Dams " , 1 9 7 3 •

230

8 . HODDER , Ri chard L . , innovations i n plan t i ng techni ques to d i ve r se shrub cover on m i ne t a i l i ngs . From : Ta i l ing Di s posal To-day , Vo l . 2 : Proceedings of the Second Interna t i onal Ta i l ing Sympo s i um , Denver , Colo rado , May , 1 9 7 8 , Cha p t e r 2 2 : pp . 4 6 1-4 7 0 .

9 . KENNEDY, Richard H . , long- t e rra s ta b i l i z a t ion o f urani um mill ta i l ings at inac t i ve si te s . From : Ta i l ing Di s po sal To-day , Vo l 2 : P roceed i ngs of t he Second Interna t i onal Ta i l i ng Sympo s i um , Denve r , Co l o rado , May 1 9 7 8 : Cha pte r 2 3 : p p . 4 7 1-48 6 .

1 0 . LUDEKE , Kenneth L . , vege t a t i ve stab i l i za t i o n o f copper mi ne t a i l i ng d i s po sa! t e rms o f Pima Min i ng Compan y . From : Ta i l ing Di s po sa ! To-day . Proc eedings of the Fi r s t In t e rna t ional Ta i l ing S�mpo s i um , Tuc son , Ari zona , Oc tobe r 3 1 , November 1 , 2 and 3 , 1 9 7 2 . Cha p t e r 1 7 : pp . 3 7 7 - 4 1 0 .

1 1 . REPORT on vege ta t i on cover of m i ne t a i l i ng dams and <lumps .

Anonymous .

23 1

GLOSSARY

Angle of shearing resistance : The maximum angle of obliquity between the normal and the resultant stress acting on a surface within a soil or rock .

Atterberg limits :

a) Liquid Limit wl b) Plastic Limit wp c) Plasticitiy Index Ip Ip = wl - wp

Bund : A stage in embankment construction .

Cohesion, C : The portion o f the shear strength (S) indicated b y the term C in Coulomb's equation : S = C + P tan </>, where </> is the angle of shearing resistance . lt has the nature of an intergranular binding force .

Compression index, Cc : An index indicating the compressibility of the soil , which repre­sents the slope of the cuive of void ratio versus logarithm of effective pressu re .

Consolidation : That portion of the decrease in volume of a soil mass resulting from the expulsion of only the pore fluid when subjected to load.

Deformation : The change in linear dimension of a body or the ab solute movement of a point on a body.

Degree of saturation, Sr : The ratio of the volume o f water in the soil voids to the total volume of voids.

Dump : An embankment consisting of mine o r industrial waste material (tailings) laid and stockpiled in the dry .

Factor of safety : The ratio of available shear strength to shear stress on the critical failure surface .

Filters : Well-graded material which is more permeable than the adjacent finer soil , so that it will act as a drain and freely conduct water away from the interface between the protected zone and the filter. lt must have a gradation such that its voids are sufficient small to prevent the passage of the fine soil particles from the protected soil.

Finger drains : A drainage system which consists of parallel strips of narrow width of pervious drainage materials. (instead of a continuous drainage blanket).

Freeboard : The height measured from water level to the crest of embankment . The minimum freeboard should be measured from the maximum projected flood level to the crest of the embankment.

Groundwater level (phreatic surface or water table) : The level below which the pores and fissures of the rock and sub soil , down to indefinite depth are full of water.

234

Hydrocycloning : A procedure used for dam and embankment construction in which hydrocyclones or cyclones can be used to separate sands from the slime . A series of cyclones can be placed along the crest of the embankment or a group of cyclones can be mounted in parallel as a mobile unit which travels along the longitudinal axis of the dam .

lmpoundment : An embankment consisting o f mine or industrial waste material laid and stockpiled with water in a reservoir created by the coarse friction of the waste material (tailings) or borrow material .

Liquefaction : The sudden , large decrease o f shearing resistance of a cohesionless soil caused by collapse of the soil structure , produced by shock or small shear strains, associated with sud den , b ut temporary increase of pore water pressure .

Liquid lirnit , wl : The water content at which the soil exhibits a small shearing strength is taken to be the boundary between liquid and plastic behaviour and this water content is called liquid lirnit .

Permeability coefficient , k : The rate of discharge of water under laminar flow condition through a unit cross-sectional area of a porous medium under a unit hydraulic gradient and standard temperature ( 20° C) .

Photogrammetric mapping : The mapping of surface exposures or geological structures done by photogrammetric techniques.

Pipe drains : A drainage system which consists of pipes in the embankment cross section .

Piping : The movement of soil particles as the result of unbalanced seepage forces produced by percolating water, leading to the development of boils or erosion channels.

Plastic limit : The lowest water content, as determined by the standard plastic lirnit test , at which a soil remains plastic.

Plasticity index : The difference between the liquid lirnit and the plastic limit .

Pore pressure, u : Stress transmitted through the pore water.

Porosity : The ratio usually expressed as a percentage of, ( 1 ) the volume of voids a given soil mass to (2) the total volume of the soil mass.

Pulp : Pulverized ore mixed in water.

Relief wells : Wells drilled for control of pore water pressure beneath embankment or soil slope .

Shear failure : Failure resulting from shear stresses.

Spigotting : A procedure generaly used in the upstream method of construction for dams or tailings embankments for which spigots are employed and the tailing slurry is discharged for a series of spigots along the crest of dam or emb ankment . The slurry meanders in a series of loose streams that result in discontinuous horizontal stratifi­cation .

235

Tailings : The waste product from a milling operation in which the valuable materials have been recovered.

Thixotropy : When some materials are kneeded without alternating the water content , the cohesion decreases considerably. This effect is known as thixotropy.

Void ration, e : The ratio of void volume to solid in a soi! mass.

Wall : A dam consisting of one or a series of successive embankments designed to impound water or slimes .

Waste embankments : Refers to all mine or industrial waste materials placed on surface, but excludes those materials placed underground as backfill.

236

SYMBOLS (See Tech n ical Dictio n a ry on Dams

pages 193 to 195)

w - water content

'Ys - unit weight solid particules

'Y - unit weight of soi! (wet)

'Y sat - unit weight of saturated soi!

'Yw - unit weight of pore liquid

n - porosity

'Y d - unit weight of day soi!

Dr - relative density (formely) ; now Id = density index

e - void ratio

Cu - uniformity factor

237

OTHER TECHNICAL BULLETINS A V AILABLE (*)

Bulletin 1 5 1 960- 1 98 1

Bulletin 20 1 96 8

Bulletin 2 1 1 969

Bulletin 2 2 1 972

Bulletin 2 3 1 972

Bulletin 2 4 1 97 3

Bulletin 2 5 1 976- 1 98 1

Bulletin 2 6 1 976- 1 98 1

Bulletin 27 1 975 - 1 98 1

Bulletin 2 9 1 977

Bulletin 30 1 978- 1 98 2

Bul letin 3 1 a 1 977- 1 9 8 2

Bulletin 32a 1 9 77- 1 98 2

Bulletin 3 3 1 979

Bul letin 34 1 97 9

Bulletin 3 5 1 980

Bulletin 36a 1 980- 1 98 2

Bulletin 3 7 1 98 1

Bulletin 3 8 1 9 8 1

Bul letin 3 9 1 98 1

Bulletin 40 1 98 2

Bulletin 4 1 1 98 2

B ul letin 4 2 1 98 2

Bulletin 4 3 1 98 2

Bulletin 4 4 1 98 2

Bulletin 4 5 1 98 3

Bulletin 4 6 1 98 3

"Frost resistance of concrete"

"Guide and recommendations for tests on surface active admixture for concrete for large dams" General considerations applicable to instrumentation for earth and rockfil l dams" "Guide and recommendations on pozzolanas and slags for use in concrete for large dams" " l ) General considerations on instrumentation for concrete dams.

2) Note on the application of geodetic methods to the determination of movements of dams" "Guide and recommendations for accelerating and retarding admixtures for use in concrete for large dams" "Extensibility of concrete for large dams"

"Methods of determining effects of shrinkage, creep and temperature on concrete for large dams" "A review of earthquake resistant design of dams"

"Report from the Committee on Risks to Third Parties from Large Dams"

"Finite element methods in analysis and design of dams"

"A glossary of words and phrases related to dams"

"Bituminous concrete facings for earth and rockfill dams"

"Compendium for dam symbols"

"JCOLD Guide for the I nternational System of U nits (JS)"

"Dams and the Environment"

"Cements for concrete for large dams"

"Dam projects and environmental success"

"Use of thin membranes on fil! dams"

"Upstream facing interface with foundations and abutments" ( 1 st Supplement to Bulletin 3 2a) "Fiber reinforced concrete"

"Automated Observation for the Safety Control of Dams"

"Bituminous cores for earth and rockfill dams" (2nd Supplement to Bulletin 3 2a) "Synthetic resins for facings of dams"

"Bibl iography - Mine and industrial tailings dams and dumps"

"Manual on tailings dams and dumps"

"Seismicity and dam design"

(*) 2 dates : successive edîtions. Numbers marked '"a" : where there is an updating.

NB , Bulletins before Bulletin No. 1 5 were not Technical Bul letins.

I M P R I M E R I E L O U I S - J E A N Publications scientifiques et littéraires 05002 G A P - T é l . · 1 9 2 1 5 1 . 3 5 . 2 3 D é p ô t l é g a l · 2 0 4 - M a r s 1 9 8 3

ISSN 0534 - 8293

Copyright © ICOLD - CIGB

Archives informatisées en ligne Computerized Archives on line

The General Secretary / Le Secrétaire Général : André Bergeret - 2004

_______________________________________________________________

International Commission on Large Dams Commission Internationale des Grands Barrages 151 Bd Haussmann -PARIS –75008

http://www. icold-cigb.net ; http://www. icold-cigb.org