技术报告-尾矿库设计及评估 (英文)

技术报告-尾矿库设计及评估 (英文)

EPA 530-R-94-038

NTIS PB94-201845

TECHNICAL REPORT

DESIGN AND EVALUATION OF

TAILINGS DAMS

August 1994

U.S. Environmental Protection Agency

Office of Solid Waste

Special Waste Branch

401 M Street, SW

Washington, DC 20460

技术报告-尾矿库设计及评估 (英文)

Design and Evaluation of Tailings Dams

DISCLAIMER AND ACKNOWLEDGEMENTS

This document was prepared by the U.S. Environmental Protection Agency

(EPA). The mention of company or product names is not to be considered

an endorsement by the U.S. Government or the EPA.

Sections of this document rely heavily on Steven G. Vick's Planning,

Design, and Analysis of Tailings Dams (BiTech Publishers Ltd. 1990).

This is particularly true of certain concepts and organizational emphases, as

well as many of the tables and figures. In some cases, this document

presents a digest of Vick's overall approach to tailings dam planning and

design. Permission to use Planning, Design, and Analysis of Tailings

Dams as a major source was provided by Mr. Vick, who is not responsible

for any errors of omission or interpretation in the present document.

技术报告-尾矿库设计及评估 (英文)

Design and Evaluation of Tailings Dams

TABLE OF CONTENTS1. INTRODUCTION ................................................................ 1

2. OVERVIEW OF TAILINGS DISPOSAL .............................................. 3

2.1 Methods for Tailings Disposal ................................................... 4

2.2 Types of Impoundments ........................................................ 5

2.2.1 Valley Impoundments ................................................. 7

2.2.2 Ring-Dike Impoundments .............................................. 12

2.2.3In-Pit Impoundments.................................................. 14

2.2.4 Specially Dug Pit Impoundment Design ................................... 14

3. TAILINGS IMPOUNDMENT DESIGN ............................................... 15

3.1 Basic Design Concepts ......................................................... 15

3.2 Design Variables .............................................................. 17

3.2.1 Tailings-Specific Factors ............................................... 17

3.2.2 Site-Specific Factors .................................................. 18

4. EMBANKMENT CONSTRUCTION, STABILITY, AND FAILURE ....................... 22

4.1 Embankment Construction ...................................................... 22

4.2 Construction Methods .......................................................... 23

4.2.1 Construction Using Tailings Material ..................................... 23

4.2.2 Upstream Method ..................................................... 25

4.2.3 Downstream Method .................................................. 26

4.2.4 Centerline Method .................................................... 28

4.2.5 Embankments Constructed Using Alternative Materials ...................... 30

4.3 Tailings Deposition ............................................................ 30

4.3.1 Single Point Discharge ................................................. 30

4.3.2 Spigotting ........................................................... 31

4.3.3 Cycloning ........................................................... 31

4.4 Stability Analysis .............................................................. 33

4.4.1 Flow Net Analysis .................................................... 34

4.5 Failure Modes ................................................................ 36

4.5.1 Rotational Sliding ..................................................... 36

4.5.2 Foundation Failure .................................................... 37

4.5.3 Overtopping ......................................................... 37

4.5.4 Erosion ............................................................. 37

4.5.5 Piping .............................................................. 37

4.5.6 Liquefaction ......................................................... 38

4.6 Performance Monitoring ........................................................ 38

5. WATER CONTROL AND MANAGEMENT .......................................... 40

5.1 Surface Water ................................................................ 40

5.1.1 Surface Water Evaluation .............................................. 40

5.1.2 Surface Water Controls ................................................ 42

5.2 Tailings Seepage .............................................................. 43

技术报告-尾矿库设计及评估 (英文)

Design and Evaluation of Tailings Dams

5.2.1 Seepage Flow (Direction and Quantity) .................................... 44

5.2.2 Seepage Quality ...................................................... 44

5.2.3 Seepage Control ...................................................... 45

5.3 Tailings Water Treatment ....................................................... 48

6. CASE STUDY: STILLWATER MINING COMPANY TAILINGS IMPOUNDMENT ......... 49

6.1 Site Evaluation, Field Exploration and Laboratory Tests ............................... 50

6.1.1 Site Evaluation ....................................................... 50

6.1.2 Field Exploration ..................................................... 50

6.1.3 Laboratory Tests ..................................................... 51

6.2 Office Evaluations ............................................................. 52

6.2.1 Hydrology Evaluation ................................................. 52

6.3 Tailings Impoundment Design ................................................... 54

7. REFERENCES ................................................................... 56

LIST OF TABLESTable 1. Comparison of Embankment Types ............................................. 24

Table 2. Stillwater Mining Company Calculated Design Floods .............................. 53

LIST OF FIGURESFigure 1. Water-Retention Type Dam for Tailings Disposal ................................. 6

Figure 2. Embankment Types: (a) Upstream, (b) Centerline, (c) Downstream or Water Retention

Type ..................................................................... 7

Figure 3. Single (a) and Multiple (b) Cross-Valley Impoundments ............................ 9

Figure 4. Single (a) and Multiple (b) Side-Hill Impoundments ............................... 10

Figure 5. Single (a) and Multiple (b) Valley-Bottom Impoundments .......................... 12

Figure 6. Single (a) and Segmented (b) Ring-Dike Impoundment Configurations ................ 13

Figure 7. Phreatic Surface Through a Tailings Impoundment ................................ 16

Figure 8. Upstream Tailings Embankment Construction .................................... 25

Figure 9. Downstream Embankment Construction ........................................ 27

Figure 10. Centerline Embankment Construction .......................................... 29

Figure 11. Examples of Tailings Embankment Flow Nets .................................... 36

技术报告-尾矿库设计及评估 (英文)

Design and Evaluation of Tailings Dams

DESIGN AND EVALUATION OF TAILINGS DAMS

1.INTRODUCTION

In order to obtain the metals and other minerals needed for industrial processes, fertilizers, homes, cars, and

other consumer products, large quantities of rock are mined, crushed, pulverized, and processed to recover

metal and other mineral values. A fine grind is often necessary to release metals and minerals, so the mining

industry produces enormous quantities of fine rock particles, in sizes ranging from sand-sized down to as low

as a few microns. These fine-grained wastes are known as "tailings."

Until recent decades, the majority of mines were small underground operations with correspondingly modest

requirements for tailings disposal. Since that time, due to increasing demand, it has become economical to

mine large lower-grade deposits by utilizing advances made by mining equipment manufacturers and

developments in mining and milling technology. This has greatly increased the amount of tailings and other

wastes generated by individual mining projects and by the mining industry as a whole.

There are approximately 1,000 active metal mines in the United States (Randol, 1993) Many of these have at

least one tailings impoundment and often several impoundments grouped together in cells. EPA estimates

that there may be several thousand tailings impoundments associated with active non-coal mining, and tens of

thousands of inactive or abandoned impoundments.

By far the larger proportion of ore mined in most industry sectors ultimately becomes tailings that must be

disposed of. In the gold industry, for example, only a few hundredths of an ounce of gold may be produced

for every ton of dry tailings generated. Similarly, the copper industry and others typically mine relatively

low-grade ores that contain less than a few percent of metal values; the residue becomes tailings. Thus,

tailings disposal is a significant part of the overall mining and milling operation at most hardrock mining

projects. There are several methods used for tailings disposal. These include disposal of dry or thickened

tailings in impoundments or free-standing piles, backfilling underground mine workings and open-pits,

subaqueous disposal, and the most common method, the disposal of tailings slurry in impoundments.

Modern tailings impoundments are engineered structures for permanently disposing of the fine-grained waste

from mining and milling operations. At some projects, tailings embankments reach several hundred feet in

height and the impoundments cover several square miles.

Historically, tailings were disposed of where convenient and most cost-effective, often in flowing water or

directly into drainages. As local concerns arose about sedimentation in downstream watercourses, water use,

and other issues, mining operations began impounding tailings behind earthen dams, which were often

constructed of tailings and other waste materials. The impoundments served the dual purpose of containing

the tailings and, particularly in the arid west, allowing the re-use of scarce water.

技术报告-尾矿库设计及评估 (英文)

Design and Evaluation of Tailings Dams

More recently, concerns have been raised about the stability and environmental performance of tailings dams

and impoundments. Stability concerns are raised in part by the use of tailings material in tailings

dams/embankments; to mitigate these concerns, such embankments often rely on a certain amount of

controlled seepage to enhance stability, which in turn affects environmental performance. Ritcey (1989) has

speculated that the need for sound impoundments in the uranium industry "probably" accounts for much of

the recent attention paid to impoundment design in other types of facilities. Perhaps triggered by the initial

attention to uranium impoundments, the increasing concern for environmental performance has led to better

engineering design of tailings dams in other mining industry sectors, for both stability and environmental

performance. For instance, experience gained with leach pad liners is being transferred to linings for tailings

ponds, and the use of synthetic lining materials is growing (although use of liners is still far from being the

industry norm). In addition, the use of cyanide and other toxic reagents in mill processes has raised special

concerns for some tailings and is leading to increased treatment prior to disposal as well as increased

attention to containment. Finally, continuing concerns over acid mine drainage is resulting in a growing body

of research and emerging concepts of long-term control or mitigation.

Inactive tailings impoundments also are receiving more attention due to the long-term effects of windblown

dispersal, ground water contamination, and acid drainage. In many cases, the costs of remediation can be

considerable, exceeding the costs of original design and operation of the tailings impoundment.

While this report discusses general features of tailings dams and impoundments, actual designs for tailings

disposal are highly site-specific. Design depends on the quantity and the individual characteristics of the

tailings produced by the mining and milling operation, as well as the climatic, topographic, geologic,

hydrogeologic and geotechnical characteristics of the disposal site, and on regulatory requirements related to

dam safety and to environmental performance. What may work for one type of tailings may not work for

another type, and may not work for the same tailings at different sites. Hence each situation requires its own

design process. The estimated quantity of tailings to be disposed of is particularly important given the

evolving nature of most mining projects. Tailings quantity estimates are based on estimated reserves that

change continuously as mine development progresses. Accordingly, the final size and design of tailings

impoundments can differ substantially from initial projections. This presents major challenges to Federal

land managers and State permit writers, who are faced with reviewing and overseeing tailings impoundment

planning, design, and performance, and to the general public, who may ultimately pay for miscalculations

resulting in environmental damages.

The purpose of this report is to provide an introduction for Federal land managers, permit writers, and the

general public to the subject of tailings dams and impoundments, particularly with regard to their engineering

features and their ability to mitigate or minimize adverse effects to the environment. The report is based on

the current literature on tailings impoundment engineering. While broad in scope, the report is necessarily

limited in depth: a comprehensive guide to the design and evaluation of tailings impoundments would

incorporate most of the materials in a number of examinations of tailings dam engineering and environmental

performance, including those in texts by Vick (1990), Ritcey (1989), and CANMET (1977), among others.

技术报告-尾矿库设计及评估 (英文)

Design and Evaluation of Tailings Dams

It should also be noted that tailings dam engineering is continually evolving. The relatively recent emphasis

on environmental performance is leading to many changes in the field, many of which are as yet not fully

tested. Vick (1990) may be the most recent and most comprehensive examination of the topics covered by

this report. Consequently, certain sections of this report rely heavily on Vick's approach.

The next section of this report provides an overview of the various methods used to dispose of mine tailings

and the types of impoundments that are used. Section 3 describes the basic concepts used in the design of

impoundments, including a number of site-specific variables of concern. Section 4 discusses tailings

embankment and stability, while Section 5 briefly discusses water management in tailings impoundments. A

case study on a lined tailings impoundment is presented in Section 6. Finally, Section 7 lists all references

cited in the text.

2.OVERVIEW OF TAILINGS DISPOSAL

The ultimate purpose of a tailings impoundment is to contain fine-grained tailings, often with a secondary or

co-purpose of conserving water for use in the mine and mill. This has to be accomplished in a cost-effective

manner that provides for long-term stability of the embankment structure and the impounded tailings and the

long-term protection of the environment. In the process of designing any tailings embankment and

impoundment, these three interests, cost, stability, and environmental performance, must be balanced, with

situation-specific conditions establishing the balance at each stage of the process. It is worth noting that the

long-term costs of tailings disposal depend in part on mechanical stability and environmental integrity, such

that stable and environmentally acceptable structures promote cost effectiveness.

Impoundment of slurry tailings is the most common method of disposal and are the main focus of this report.

Impoundments are favored because, among other things, they are "economically attractive and relatively easy

to operate" (Environment Canada 1987). Tailings impoundments can be and are designed to perform a

number of functions, including treatment functions. These include (Environment Canada 1987):

Removal of suspended solids by sedimentation

Precipitation of heavy metals as hydroxides

Permanent containment of settled tailings

Equalization of wastewater quality

Stabilization of some oxidizable constituents (e.g., thiosalts, cyanides, flotation reagents)

Storage and stabilization of process recycle water

Incidental flow balancing of storm water flows.

技术报告-尾矿库设计及评估 (英文)

Design and Evaluation of Tailings Dams

There are, however, a number of disadvantages to tailings impoundments requiring attention in design,

including (Environment Canada 1987):

Difficulty in achieving good flow distribution

Difficulty in segregating drainage from uncontaminated areas

Difficulty in reclamation, particularly with acid-generating tailings, because of the large surface

area and materials characteristics

Inconsistent treatment performance due to seasonal variations in bio-oxidation efficiency

Costly and difficult collection and treatment of seepage through impoundment structures

Potentially serious wind dispersion of fine materials unless the surface is stabilized by

revegetation, chemical binders, or rock cover.

2.1Methods for Tailings Disposal

Because mine tailings produced by the mill are usually in slurry form, disposal of slurry tailings in

impoundments made of local materials is the most common and economical method of disposal. There are

four main types of slurry impoundment layouts; valley impoundments, ring dikes, in-pit impoundments, and

specially-dug pits (Ritcey 1989). These impoundment configurations are explained in more detail below,

with major emphasis on valley impoundments, as they are the most common. Before describing

impoundments, several other methods of tailings disposed are briefly described below.

In some cases, tailings are dewatered (thickened to 60 percent pulp density or more) or dried (to a moisture

content of 25 percent or below) prior to disposal. The efficiency and applicability of using thickened or dry

tailings depends on the ore grind and concentrations of gypsum and clay as well as the availability of

alternative methods. Except under special circumstances, these methods may be prohibitively expensive due

to additional equipment and energy costs. However, the advantages include minimizing seepage volumes and

land needed for an impoundment, and simultaneous tailings deposition and reclamation. (Vick 1990)

Slurry tailings are sometimes disposed in underground mines as backfill to provide ground or wall support.

This decreases the above-ground surface disturbance and can stabilize mined-out areas. For stability reasons,

underground backfilling requires tailings that have a high permeability, low compressibility, and the ability to

rapidly dewater (i.e., a large sand fraction). As a result, only the sand fraction of whole tailings is generally

used as backfill. Whole tailings may be cycloned to separate out the coarse sand fraction for backfilling,

leaving only the slimes to be disposed in an impoundment. To increase structural competence, cement may

be added to the sand fraction before backfilling (Environment Canada 1987).

Open-pit backfilling is also practiced, where tailings are deposited into abandoned pits or portions of active

pits. The Pinto Valley tailings reprocessing operation, located in Arizona, uses this method to dispose of

技术报告-尾矿库设计及评估 (英文)

Design and Evaluation of Tailings Dams

copper tailings. In active pits, embankments may be necessary to keep the tailings from the active area.

However, since seepage from the tailings can adversely affect the stability of the pit walls or embankments, it

is unusual to see disposal in active pits. Williams (1979), for example, discusses a failure due to pore water

pressure in the floor of a pit in Australia. Ritcey (1989) notes that the hydrogeological parameters affecting

the migration of seepage and contaminants are poorly understood, so tailings with toxic contaminants or

reactive tailings may be poor candidates for this type of impoundment. The U.S. Bureau of Mines points out

that other limitations for using active open pits for tailings disposal are loss of the pit areas for future

resources, and subsequent mine operating and design restrictions to which mine operators would be

subjected.

Subaqueous disposal in a deep lake or ocean is also a possible disposal method. Underwater disposal may

prevent the oxidation of sulfide minerals in tailings, thus inhibiting acid generation. Subaqueous disposal has

recently been practiced by eight mines in Canada, with three still active as of 1990 (Environment Canada

1992). Subaqueous disposal is used in areas with high precipitation, steep terrain, or high seismicity or, in

Canada, where its use predated current regulations. This method is also limited to coarse tailings that can

settle quickly. CANMET (Canadian Centre for Mineral and Energy Technology) completed a bench-scale

16-year simulation of deep-lake disposal using Ottawa River water (Ritcey and Silver 1987). They found

that the tailings had little effect on pH when using ores with a low sulfide content. Ripley, et al. (1978),

found that the tailings can cover large areas on the ocean or lake floor and cause turbidity problems if the

disposal practice is not designed correctly. There is little data on the long-term effect of subaqueous disposal

(Environment Canada 1987), although it is being studied in Canada and peer reviewed by CANMET

(CANMET 1993).

A variation on subaqueous disposal in the ocean or lakes would be permanent immersion of tailings in a pit or

impoundment. This could present many of the same advantages of underwater disposal (i.e., reduced

oxidation of sulfide minerals) but also would require long-term attention to ensure constant water levels and

possibly monitoring for potential ground water impacts.

2.2Types of Impoundments

There are two basic types of structures used to retain tailings in impoundments, the raised embankment and

the retention dam. Because raised embankments are much more common than retention dams, they are

emphasized in this report. Either type of structure, raised embankments or retention dams, can be used to

form different types or configurations of tailings impoundments. The four main types of impoundments

include the Ring-Dike, In-Pit, Specially Dug Pit, and variations of the Valley design. The design choice is

primarily dependent upon natural topography, site conditions, and economic factors. Most tailings dams in

operation today are a form of the Valley design. Because costs are often directly related to the amount of fill

material used in the dam or embankment (i.e., its size), major savings can be realized by minimizing the size

of the dam and by maximizing the use of local materials, particularly the tailings themselves.

技术报告-尾矿库设计及评估 (英文)

Design and Evaluation of Tailings Dams

Retention dams are constructed at full height at the beginning of the disposal whereas raised embankments

are constructed in phases as the need for additional disposal capacity arises. Raised embankments begin with

a starter dike with more height added to the embankment as the volume of tailings increases in the

impoundment.

Tailings retention dams (Figure 1) are similar to water retention dams in regard to soil properties, surface

water and ground water controls, and stability considerations. They are suitable for any type of tailings and

deposition method.

Figure 1. Water-Retention Type Dam for Tailings Disposal

(Source: Vick 1990)

Raised embankments can be constructed using upstream, downstream, or centerline methods, which are

explained in more detail in a later section (see Figure 2). Each of the structures in Figure 2, for instance, is

constructed in four successive lifts, with constructing material and fill capacity increasing incrementally with

each successive lift. They have a lower initial capital cost than retention dams because fill material and

placement costs are phased over the life of the impoundment. The choices available for construction material

are increased because of the smaller quantities needed at any one time. For example, retention dams generally

use natural soil whereas raised embankments can use natural soil, tailings, and waste rock in any

combination. (Vick 1990)

技术报告-尾矿库设计及评估 (英文)

Design and Evaluation of Tailings Dams

Figure 2. Embankment Types: (a) Upstream, (b) Centerline, (c) Downstream or Water Retention

Type

(Source: Vick 1990)

Finally, the phased nature of raised embankments makes it possible to attempt to address problems that may

arise during the life of a tailings impoundment. For example, at the Rain facility in Nevada, unplanned

seepage under and through the base of the tailings embankment made design changes necessary. The fact

that this was a raised embankment made it possible to attempt engineered solutions to the problem as the dam

was enlarged and raised during later phases of construction, and this could be accomplished without taking

the impoundment out of service and without moving enormous quantities of fill material or impounded

tailings.

2.2.1Valley Impoundments

Other things being equal, it is economically advantageous to use natural depressions to contain tailings.

Among other advantages are reduced dam size, since the sides of the valley or other depression serve to

contain tailings. In addition, tailings in valleys or other natural depressions present less relief for air

dispersion of tailings material. As a result, valley impoundments (and variations) are the most commonly

used. Valley-type impoundments can be constructed singly, in which the tailings are contained behind a

技术报告-尾矿库设计及评估 (英文)

Design and Evaluation of Tailings Dams

single dam or embankment; or in multiple form, in which case a series of embankments contain the tailings in

connected "stair-step" impoundments.

There are several variations of valley-type impoundments. The Cross-Valley design is frequently used

because it can be applied to almost any topographical depression in either single or multiple form. Laid out

similarly to a conventional water-storage dam, the dam is constructed connecting two valley walls, confining

the tailings in the natural valley topography. This configuration requires the least fill material and

consequently is favored for economic reasons. The impoundment is best located near the head of the drainage

basin to minimize flood inflows. Side hill diversion ditches may be used to reduce normal runon if

topography allows, but large flood runoff may be handled by dam storage capacity, spillways, or separate

water-control dams located upstream of the impoundment. Figure 3 shows single and multiple cross-valley

impoundment configurations.

技术报告-尾矿库设计及评估 (英文)

Design and Evaluation of Tailings Dams

Figure 3. Single (a) and Multiple (b) Cross-Valley Impoundments

(Source: Vick 1990)

技术报告-尾矿库设计及评估 (英文)

Design and Evaluation of Tailings Dams

Figure 4. Single (a) and Multiple (b) Side-Hill Impoundments

(Source: Vick 1990)

Other types of valley impoundments may be employed when there is an excessively large drainage catchment

area and/or there is a lack of necessary valley topography. Two variations are the side-hill impoundment and

the valley-bottom impoundment. The side-hill layout consists of a three-sided dam constructed against a

hillside (Figure 4). This design is optimal for slopes of less than 10% grade. Construction on steeper slopes

requires much more fill volume to achieve sufficient storage volume (especially when using the downstream

method of construction).

If the drainage catchment area is too large for a cross-valley dam and the slope of the terrain is too steep for a

side-hill layout, then a combination of these two designs, the valley-bottom impoundment, may be considered

(Figure 5). Valley-bottom impoundments are often laid out in multiple form as the valley floor rises, in order

to achieve greater storage volume. Because the upstream catchment area is relatively large, it is often, or

usually, necessary to convey upstream flows around (and/or under) valley-bottom impoundments.

The valley dam configurations are often the optimum choice for economic reasons. This is because the valley

walls form one or more sides, so that the dam length is reduced, minimizing construction costs. However,

decreased construction costs and low average depth of tailings in the embankment may be offset by increased

environmental mitigation and increased costs of shut-down and reclamation.

技术报告-尾矿库设计及评估 (英文)

Design and Evaluation of Tailings Dams

The valley dam design is particularly sensitive to overtopping by flood waters, erosion near the intersection of

the dam and the valley hillside, and liquefaction due to higher volumes of surface water inflow from drainages

within the natural catchment basin and from high precipitation runon/runoff. As is described in more detail

later, the stability of a valley dam depends largely on the level of hydrostatic pressure within fill material and

the embankment. An unusual, one-time rise in the hydrostatic pressure above design levels may be sufficient

to trigger failure. The control of inflows across, around, or under the impoundment is important to retaining

structural stability and to controlling environmental impacts. Providing adequate internal drainage can help

guard against liquefaction, and improve the permeability and consolidation of the tailings, thereby improving

the stability of the structure.

Because a shorter embankment is required in this configuration, it is more feasible to consider impervious

cores and internal drains as a means of controlling the phreatic surface and promoting stability of the

embankment. Surface water controls may also be necessary. Diversion channels may not always be an

option due to the difficulty of construction along steep valley sides. However, closed conduits may be an

alternative diversion method. Another alternative surface water control in the valley layout is to construct a

smaller water-retaining dam upstream of the tailings dam to collect the water to divert it around the tailings or

use it in the mill. A water-related factor that also must be considered, particularly in valley impoundments, is

the presence of shallow alluvial ground water. Ground water can infiltrate the tailings, thus raising the level

of saturation within the tailings; this can be seasonal, in response to seasonal high surface water flows that

interconnect with the alluvium upgradient of the impoundment (or under the impoundment itself).

It should be noted that any design that calls for diverting or otherwise controlling water flows during the

active life of the impoundment has to consider later periods as well. The water balance may be more

favorable after tailings slurry water is no longer being added to the impoundment/and the dam stability may

be less of a concern. However, if there are toxic contaminants in the tailings, or if the tailings are reactive, the

design must account for environmental performance following surface stabilization and reclamation.

The stability of the tailings impoundment is also dependent on (or at least related to) foundation

characteristics, such as shear strength, compressibility, and permeability. Depending on soil characteristics,

the valley layout can be adapted to account for high permeability materials in the design through the use of

liners and/or adequate internal drainage. Soil characteristics often can be improved through soil compaction.

In addition, the method of tailings deposition and construction have an increased impact on the valley

impoundment layout. The deposition of tailings affects consolidation, permeability, strength and,

subsequently, the stability of the embankment material. All these factors are discussed in later sections.

In some cases, liners or zones of low permeability may be appropriate means of controlling seepage to

enhance stability or environmental performance. The upstream face of tailings dams/embankments (i.e., the

side that contacts the tailings), for example, is frequently designed to provide a layer of low permeability or to

be impermeable. The effect is to lower the phreatic surface through the embankment. This is usually

accomplished with the slimes fraction of tailings and/or with synthetic materials.

技术报告-尾矿库设计及评估 (英文)

Design and Evaluation of Tailings Dams

Lining the entire impoundment area is more problematic, both because of the expense and because

irregularities in valley side walls and floors make it difficult to ensure consistent liner integrity. Liners or

layers of low permeability may be necessary, however, to impede flows to and from underlying ground water.

More common than impermeable synthetic or clay liners is the practice of compacting native soil, including

any available local clays, to reduce permeability to an acceptable level; dewatered or dried-in-place slimes

may also be used in some cases. Should a liner or low-permeability layer be necessary, it must be designed to

account for impoundment loadings, differential settlement, toxic or corrosive seepage, and weathering effects.

If impoundments will desaturate after reclamation, for example, clay or slimes can crack and provide a

pathway for ground water to enter the tailings or for contaminated seepage to enter ground water. Similarly,

layers of clay or slimes that are prepared in anticipation of late impoundment expansion can develop cracks if

they are allowed to dry before being covered with tailings.

2.2.2Ring-Dike Impoundments

Where natural topographic depressions are not available, the Ring-Dike configuration may be appropriate

(Figure 6). Instead of one large embankment (as in the valley design), embankments (or dikes) are required

on all sides to contain the tailings. Construction can be similar to valley dams, with tailings, waste rock,

and/or other native materials typically used in later phases of construction. Because of the length of the

dike/dam, more materials are necessary for this configuration, and material for the initial surrounding dikes is

typically excavated from the impoundment area.

Figure 5. Single (a) and Multiple (b) Valley-Bottom Impoundments

(Source: Vick 1990)

技术报告-尾矿库设计及评估 (英文)

Design and Evaluation of Tailings Dams

Figure 6. Single (a) and Segmented (b) Ring-Dike Impoundment Configurations

(Source: Vick 1990)

According to Ritcey (1989), most recent dike dams have been built using downstream or centerline methods

rather than the upstream method (see below for descriptions of the various types of construction); Ritcey cites

Green (1980) as reporting that long-term stability of upstream dikes is not certain.

Embankments are required on all sides, so this method utilizes a large amount of embankment fill in relation

to the storage volume. This layout can be arranged in single or segmented form. The regular geometry

typically used with this configuration makes it amenable to the installation of various kinds of liners. (Vick

1990)

If the terrain is flat and thus suitable for ring-dikes, this configuration allows maximum flexibility in actually

selecting a site. Since the dikes are relatively low in height, the design is often simpler than a high valley dam

design. Containment can be achieved by using an impervious core in the dikes and/or the use of a liner below

the impoundment.

Unlike valley impoundments, which are located in a natural catchment area, the ring-dike design enables

better maintenance of water control. The quantity of pond water is limited to that transported with the

tailings and any precipitation falling directly onto the impoundment. There is no runoff other than from outer

slopes. Since surface runoff and flood impacts are reduced, a smaller pond area and/or less elaborate water

control measures are required. A trade-off can be made with a high tailings depth that reduces surface area

技术报告-尾矿库设计及评估 (英文)

Design and Evaluation of Tailings Dams

and results in less seepage. There are also drawbacks to this design, including the relatively large volumes of

material necessary for construction, and its effect on cost. The increased length of the embankment walls also

may increase the possibility of failure (Robertson 1984, cited in Ritcey 1989). Other disadvantages of the

ring dike system are that the impoundment rises above the surrounding terrain, creating an aesthetic problem

in some locations, and there can be considerable wind erosion of the tailings. In many areas, also, there is no

flat terrain suitable for ring-dike designs.

Although each situation needs to be evaluated on its own merits, the ring dike system has the potential for

better control of seepage than that found in most valley dam locations. If warranted by the characteristics of a

particular tailings, almost total containment and collection of effluent can be achieved using a suitable

combination of low permeability cores, liners, and drainage system. Since seepage control is often a pressing

environmental concern with tailings impoundments, the ring dike system can have an important advantage

over most other layouts.

2.2.3In-Pit Impoundments

This method is much less common than the valley and ring-dike impoundments. It consists of disposing

tailings material into a previously mined pit. The design initially eliminates the need for dike construction.

Since the tailings are protected by pit walls, wind dispersion is minimized. Good drainage can be

incorporated into the design. Many of the failure modes common to tailings embankments (e.g., piping,

liquefaction) do not apply to this design. The lack of dam walls reduces the possibility of slope failure, but

the stability of the pit slopes do have to be checked.

Unless the purpose is to isolate sulfide tailings underneath water, the water table should be below the tailings

disposed in the pit. This may require backfilling with mine rock or overburden. If backfilling underneath the

tailings is necessary, and/or if the surrounding rock is not sufficiently impermeable, a liner may be required.

Ritcey (1989) notes that the hydrogeological parameters affecting the migration of seepage and contaminants

are poorly understood, so tailings with toxic contaminants or reactive tailings may be poor candidates for this

type of impoundment.

When mining in an active pit is proceeding laterally, the mined-out portion of the pit may be suitable for

tailings disposal. In such cases, dikes would be constructed to impound the tailings in the mined-out area.

This embankment could then be raised in a phased approach (Ritcey 1989).

2.2.4Specially Dug Pit Impoundment Design

This design is fairly unusual and involves the excavation of a pit specifically for the purpose of tailings

disposal. The impoundment consists of four or more cells with impermeable liners and surrounded by an

abovegrade dam. Material removed from the pit is used in construction of the dam. This dug pit/dam design

has some of the same advantages as the ring-dike design, including site independence and uniform shape.

技术报告-尾矿库设计及评估 (英文)

Design and Evaluation of Tailings Dams

Site independence benefits the design, since less effort and cost are needed to counteract topographic

obstacles, soil conditions, climatic conditions, and construction obstacles. The uniform layout, shape, and

flat terrain prevents surface runoff from entering the impoundment and decreases the requirements for flood

control measures.

3.TAILINGS IMPOUNDMENT DESIGN

The actual design of a tailings dam and impoundment occurs only after the site has been selected. However,

the site selection and design are best considered to be a dynamic process. A number of design principles

should affect the site selection process as well as the determination of the embankment type and the

impoundment configuration. This section first describes some of these fundamental design principles as well

as major design variables and site-specific factors that influence ultimate design. As noted previously, the

major considerations in the design of a tailings dam and impoundment are stability, cost, and environmental

performance.

3.1Basic Design Concepts

In general, tailings impoundments (and the embankments that confine them) are designed using information

on tailings characteristics, available construction materials, site specific factors (such as topography, geology,

hydrology and seismicity) and costs, with dynamic interplay between these factors influencing the location (or

siting) and actual design of the impoundment. Because water is a major component in any tailings

impoundment system, principles of hydrology (applied to flow of water through and around the tailings

embankment) dictate many of the rules of tailings impoundment design. Indeed, because impoundment and

dam stability are in large part a function of the water level, these principles are of fundamental concern in the

design of any tailings impoundment.

One of the basic principles used in the design of impoundments and their embankments is the maintenance of

the phreatic surface within the embankment. The phreatic surface is the level of saturation in the

impoundment and embankment (the surface along which pressure in the fluid equals atmospheric pressure

(CANMET 1977)); in natural systems it is often called the water table. The phreatic surface exerts a large

degree of control over the stability of the embankment, under both static and seismic loading conditions (Vick

1990). The major design precept is that the phreatic surface should not emerge from the embankment and

should be as low as possible near the embankment face (Vick 1990). This basically maintains a pore

pressure at the face of the embankment lower than atmospheric pressure plus the weight of the embankment

particles and maintains the face of the dam. Thus any factors that might affect the phreatic surface in the

embankment may also affect stability of the embankment. The primary method of maintaining a low phreatic

surface near the embankment face is to increase the relative permeability (or hydraulic conductivity, since

water is the fluid) of the embankment in the direction of flow. (See Figure 7.)

技术报告-尾矿库设计及评估 (英文)

Design and Evaluation of Tailings Dams

Figure 7. Phreatic Surface Through a Tailings Impoundment

(Source: CANMET 1977)

Creating a relative increase in permeability downstream can be accomplished in one of two ways, or a

combination of the two: by incorporating lower permeability zones in the upstream areas of the embankment

(typically by constructing embankments with low permeability cores) and by using higher permeability zones

downstream (typically using internal drainage zones). The selection of which technique to use is often based

on the availability of materials, such as clays for cores and/or clean sands for drains. The use of cores and

drainage zones to maintain embankment stability are further discussed in a later section. It should be kept in

mind, however, that major changes in phreatic surface require permeability differences in adjacent zones to be

two or more orders of magnitude (Vick 1990).

The low permeability layer generally controls the overall flow rate through the impoundment. This allows

higher permeability layers located downstream of the low permeability layer to drain and avoid increased pore

pressure. The rule on increasing permeability in the direction of flow only applies in areas near the

embankment face; if a low permeability core in the center of the embankment is used and permeability

increases downstream toward the face, permeability of the material on the upstream side of the embankment

may have little effect on the phreatic surface downstream of the low permeability core (Vick 1990).

In most embankments, materials in the various zones are also arranged to meet filter requirements, which are

designed to prevent migration of tailings and finer materials into coarser zones. Otherwise voids will be

produced that can form a pathway through the dam along which water can escape. As seepage rates

accelerate along the pathway, erosion of the dam material occurs leading to failure of the dam. Such failures

are referred to as piping failures, because of the natural "pipe" that is formed through the embankment.

Piping failures can be avoided by the proper application of various filter rules that have been established in

the design of water-retention dams. (Vick 1990)

Factors that affect the phreatic surface in the embankment affect its stability. These factors include the

depositional characteristics of the tailings (permeability, compressibility, grading, pulp density, etc.) and

site-specific features such as foundation characteristics and the hydrology and hydrogeology of the

impoundment area and its upstream catchment area. Changes in the phreatic surface in a waste embankment

will change the pore water pressures and consequently the resistance of the dam materials to sliding. Changes

技术报告-尾矿库设计及评估 (英文)

Design and Evaluation of Tailings Dams

to the phreatic surface can be caused by: malfunction of drainage systems, freezing of surface layers on the

downstream slope of the dam, changes in construction method (including the characteristics of the placed

material), and changes in the elevation of the pond. The level of the water table also may be altered by

changes in the permeability of the underlying foundation material; sometimes these are caused by strains

induced by mining subsidence (Vick 1990).

In addition to maintaining the phreatic surface for stability purposes, dam design now includes factors related

to environmental impacts associated with tailings seepage. By the use of liners, drains, and pumpback

systems, tailings seepage may be controlled. These techniques are discussed in more detail in a later section

of this report. The design should also address the future reclamation of the site.

3.2

3.2.1Design VariablesTailings-Specific Factors

Tailings composition, pulp density, grading, and other characteristics are used in the design of tailings

impoundments in three basic ways: tailings analysis to assess the potential use of tailings sands in

constructing the embankment, analysis of tailings to be placed in the impoundment to determine their

potential impact on structural stability and seepage characteristics, and mineralogical analysis to determine

the potential chemical aspects of seepage or other discharges from the impoundment. In addition to the

physical characteristics, the method of deposition of tailings into the impoundment plays a role in the

"engineering characteristics." (Vick 1990)

Tailings sands are often used as an inexpensive source of material for embankment construction; by removing

the sands for embankment construction the volume of tailings to be disposed of is reduced. Depending on the

gradation (grain size distribution) of the tailings, a cyclone may be used to separate sufficient amounts of

coarse sand from the whole tailings to construct the embankment, leaving a higher percentage of slimes to be

deposited behind the embankment. Cycloned sands can have both high effective strength and high

permeability, the two major characteristics necessary for downstream embankment material. In addition,

cycloning results in the deposit of the less permeable slimes behind the embankment, possibly reducing

impoundment seepage.

With regard to their general physical properties, tailings are considered to be soils, subject to traditional soil

mechanics patterns of behavior. Index properties (gradation, specific gravity, and plasticity) are determined

by relatively simple tests that can be performed on tailings produced in bench testing of the mill process.

These index tests are a guide to the engineering properties of the tailings. Caution is required, however, since

tailings differ in subtle ways from soils having similar index properties (Vick 1990).

Tailings properties that impact design, stability and drainage of the impoundment include in-place and

relative density, permeability, plasticity, compressibility, consolidation, shear strengths, and stress parameters

(Vick 1990). In-place density is an important factor in determining the size of impoundment required for a

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