The Behaviour of Mine Tailings During Hydraulic Deposition by G

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The behaviour of mine tailings during hydraulic deposition by G. E. BLlGHT*. Pr.Eng.. Ph.D.. D.Sc.(Eng.). F.S.A.I.C.E. and G. M. BENTELt. M.Sc.(Eng.). Grad. S.A.I.C.E.

SYNOPSIS The environmentally acceptable disposal of fine-particled mining and industrial wastes by the formltion of hydraulic-fill slimes dams is becoming an increasingly important aspect of the total mining endeavour. Relatively little is known of the behaviour of waste slurries during deposition. This paper describes and analyses the following aspects of slurry behaviour: (i) the relationship between viscosity, shear strength, and water content, (ii) the slope assumed by a thickened slurry, (iii) particle-size sorting on a hydraulic-fill beach, (iv) gradients of hydraulic-fill beaches, and (v) internal erosion during the deposition of slurry.

SAMEVATTING Die wegdoening van mynbou- en nywerheidsafval met bale fyn partikels deur die vorming van hidroulies gevulde slykdamme, wat vir die omgewing aanvaarbaar is, word 'n al hoe belangriker aspek van die totale mynboupoging. Dur is betreklik min bekend oor die gedrag van afvalflodders tydens afsetting. Hierdie referaat beskryf en ontleed die volgende aspekte van die gedrag van flodder: (i) die verhouding tussen viskositeit, skuifsterkte en waterinhoud, (ii) die helling wat 'n verdikte flodder inneem, (iii) partikelgroottesortering op 'n hidroulies gevulde strand, (iv) gradient van hidroulies gevulde strande, en (v) inwendige erosie tydens die afsetting van flodder.

Introduction TABLE I SELECTED INFORMATION ON RATES OF PRODUCTION OF MINING The disposal of fine-grained mining and industrial WASTE wastes by the formation of hydraulic-fill tailings dams is becoming a design and construction activity of ever- Product mined Region producing Dry solid waste waste produced per year increasing scale and importance to the mining industry. Mt Technical knowledge of waste problems is sparse, and the disposal of mine waste has been identified as a high- Asbestos Canada 27 China clay United Kingdom 22 priority area for geotechnical research 1. Coal U.S.A. 200 Hydraulic filling is used world-wide because Copper U.S.A. 460 (a) wastes such as mine tailings, phosphogypsum, Gold South Africa 100 pulverized fuel ash, and coal-washery sludges are usually produced by wet processes or are removed towards fewer but larger waste deposits. In the D.S.A., from the plant by sluicing, and several tailings dams of up to 180 m have either been (b) hydraulic transportation by pipeline to the dis- planned or are under construction, and will contain posal site is currently the cheapest form of solids 500 Mt of waste or more3. In South Africa, seven gold- transportation. tailings dams currently under construction are planned to contain between 50 and 150 Mt and to be up to 80 m The quantities of waste transported and disposed of in height4. In addition to the surface disposal of mining on surface by hydraulic means are enormous, and are wastes, interest is increasing in the disposal of waste growing at a greater rate than the demand for, and underground, where it can provide both structural sup- production rate of, the products from which they derive. port and access in the-mining of ore bodies of large ver- This is because of the trend towards increasingly large- tical extentS,5. The practice of underground filling may scale exploitation of low-grade ore bodies, high-ash coals, entail the pumping of tailings slurries over relatively etc. Some idea of the enormity of today's waste-disposal large distances both vertically and horizontally. In one problems is given by Table 1. Further information on case7, tailings-cement slurry was pumped a distance of production for a more extensive range of wastes has been 2 km vertically and 3,5 km horizontally before being given by Blight2. deposited as a stope filling. Environmental pressures, decreasing availability of This paper considers the following aspects of the trans- land, and increasing costs have encouraged a trend porta tion and deposition of waste slurry: (i) relationships between viscosity, shear strength, * Professor of Construction Materials, University of the Wit- and water content for slurries, watersrand, P.O. Box 1176, Johannesburg 2000. (ii) the slope assumed by thickened slurries on t Research Student, University of the Witwatersrand, P.O. Box 1176, Johannesburg 2000. deposition,

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY APRIL 1983 73 the deposition of slurries on hydraulic-fill beaches, (iv) profiles of hydraulic-fill beaches and cyclone T=TO+:~'7]. (2) underflow cones, and (v) internal erosion resulting from the flow of slurries In (2), TO is a yield shear strength that applies when through drying cracks in previously deposited dyjdt = O. tailings. The coaxial-cylinder viscometer relies on the torque generated at a given rate of rotatio~ to i.ndicate sh~ar strength and viscosity. If the conventIOnallllt~rpretatIOn Relationships between Viscosity, Shear Strength, of the viscometer readings is followed, what IS actually and Water Content measured is an apparent viscosity: A knowledge of the relationship between the viscosity 7]a = 7] + ...... (3) or shear strength and the water content of tailings slur- drldt ries is important in the assessment of If the true viscosity is required, the value of TOhas first (1) head losses in slurry pumping lines8, to be established from the relationship between T and (2) the slope assumed by thickened slurries when d yjdt. The true viscosity can then be determined from deposited by the thickened-discharge method 9, and T - TO 7] ...... (4) (3) the flow pattern of a slurry after it has escaped = dyfdt through a breach in the outer wall of a tailings As most flow relationships have been derived for dam1O. viscous fluids and not Bingham plastics, the apparent A wide range of water contents is of interest. For viscosity, 7]a, is in fact a useful engineering property for deposition on a conventional tailings dam, the water approximating the behaviour of a slurry to that of a content of the tailings during transportation from the Newtonian fluid. mill to the dam is typically in the range 150 to 100 per Fig. 1, which shows the properties of a tailings slurry cent by mass of dry solids. (a relative density for the from a diamond (kim berlite) operation at a water content slurry of 1,3 to 1,5). For deposition by the thickened- of 125 per cent, illustrates the relationship between shear discharge method, the water content is typically about strength and rate of shear strain, and indicates a yield 50 per cent (relative density 1,7 to 1,8). For the slurry shear strength, of 0,35 Fa. The fact that the shear filling of underground mining excavations, the water TO' strength does not increase linearly with the rate of shear content ranges from 50 to 30 per cent (relative density strain shows that, although the behaviour of the slurry 1,7 to 1,9). Actual water contents depend very much on approximates that of a Bingham plastic, it is actually the particle-size distribution of the slurry and on any more complex than equation (2) would indicate.. clay minerals present. The relative densities are, of . Fig. 2 shows relationships between apparent Vl~COSlty, course, influenced by that of the particles. 7]a,and rate of shear strain (calculated from equatIOn (1)) A number of methods are in common use for the and between 'true' viscosity, 7], and rate of shear stralll measurement of viscosity, which can be measured in (from equation (2)). It is again clear that equation (2) is absolute or relative terms 11. The variable-speed coaxial- only an approximation to the real behaviour of the cylinder type of viscometer has been found to be suitable slurry. for the measurement of the absolute viscosity of tailings slurries, and all the measurements reported here were I,it made with an instrument of this type (the Ferranti portable viscometer, model VL). It was found necessary for the range of cylinders supplied with the standard ~o instrument to be extended by means of a number of specially made cylinders. The instrument was calibrated in absolute terms by the application of torque through 0,8 a system of weights, strings, and pulleys pivoted on ball ll. :I: races. I- Cl Z 0,6 The viscosity of Newtonian fluids is defined by the w relationship a: V)I- TJa . ab lob dy a: 0,4 T=---a-t7], ...... (1) « w :I: V) in which T is the shear strength of the fluid, 7]is its viscosity, and O;? 'to ~is the rate of shear strain. dt 0 b However, tailings slurries are non-Newtonian and behave 0 10 20 30 more as Bingham plastics, for which viscosity is related RATE OF SHEAR STRAIN d Y/dt ,- I to shear strength by Fig. I-Relationship between shear strength and rate of shear .All water contents are given on the basis of dry mass. strain for diamond tailings at a water content of 115 per cent

74 APRIL 11183 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY (108 n-l T)a = nK( (5) :~ ) ...... 0,06 Integration of this equation with respect to (d yjdt) gives .. Cl the result for shear strength: Q. n >- dy I- 0,04 (5a) en T = TO + K ( ...... 0 Tt) u The applicability of equation (5) to the behaviour of ~> 0,02 tailings slurries is illustrated in Fig. 5, in which the data 1') for Figs. 1 to 4 were replotted on a logarithmic basis. Data similar to but less detailed than those shown in 0 Figs. 1 to 5 have been published by Whitney et al.13 for 0 10 20 30 phosphate tailings. A useful summary of other available data on the viscosity of mineral slurries is given by RATE OF SHEAR STRAIN dY/dl.-1 JeyapalanI4 and Lucia et al. 15.

Fig.2-Relationship between viscosity and rate of shear 0/4 strain for diamond tailings at a water content of 125 per cent w% apparent viscosity T) 2!! T)a = = 'true' viscosity 0,/2.

Figs. 3 and 4 show the results of a series of shear 0,10 strength and viscosity measurements on a slurry of 0. platinum tailings. The measurements were made as .. ,. 0,8 part of an investigation into the flow of tailings through t- a breach in a tailings damIO,I2, and the water contents ~0,6 0 correspond to settled and partly consolidated tailings 35 "'- that have become liquefied by shear disturbance. > 0,4 Fig. 3 shows relationships between shear strength 39 and rate of shear strain for a range of water contents. 0,2 25 Although the shear strengths are an order of magnitude -30 39U5 larger than those shown in Fig. 1, similar behaviour is 0 apparent. 0 2 !J 4 !J 6 7 8 9 10

/jO RATE OF SHEAR STRAN d YIM .-1

W% Fig.4--Measurements of viscosity on platinum tailings, showing the influence of varying water content ~Il. w = water content of the slurry T)a= apparent viscosity 25 T)= 'true' viscosity :z: t- ~o Z Flow of Slurry in Pipelines " Head losses in the flow of both Newtonian and non- ~'" '" Newtonian fluids through pipes can be estimated by the i 2p application ofthe D'Arcy-Weisbach equation 8: 35 1 V2 39 i (6) ~o = 6hJL =j'r['2g'

0 where i = 6hJL is the hydraulic gradient along the pipe, i.e. the ratio of head lost, h, to the 0 2 !J 4 !J 6 7 8 9 10

RATE OF SHEAR STRAIN dT/dl.-1 length, L, over which it is lost, d is the diameter of the pipe, Fig. 3-Measurementsof shear strength on platinum tailings, showing the influence of varying water content v is the flowNelocity, g is the acceleration due to gravity, and w = water content of the slurry .f is a dimensionless friction factor. Fig. 4 shows relationships between apparent viscosity, For laminaI' flow of Newtonian fluids, equation (6) T)a,and rate of shear strain and between 'true' viscosity, is identical to the Hagen- Poiseuille equation: T), and rate of shear strain. Once again the pattern is 32 vT) .f ...,...... (7) similar to that shown by Figs. 1 and 2. = pgd2' It appears from Figs. 3 and 4 that, while the water (in which p is the mass per unit volume of the fluid) content has a major influence on the yield shear strength, provided that TO' and the apparent viscosity, T)a, it has much less influence on the 'true' viscosity, T). 64 .f = As an alternative, the apparent viscosity of tailings NR dvp. slurries can be characterized by an empirical equation of and the Reynolds number, N R, = (8b) the form T)

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY APRIL 1983 75 2,0 .-- r- to rest at a flat slope to form a roughly conical surface having a small slope away from the point of deposition.

I The system has the advantage of considerably increasing /,0 the storage capacity of a given disposal area. To illustrate this statement, Fig. 7 shows a section through a small tailings impoundment that is to be constructed by the 0,5 thickened-discharge method. The difference in stored volume between a conventional system in which material Q.0 ;; PLATINUM TAILINGS is beached upstream from the impounding wall and the 0,2 thickened discharge method is immediately obvious. ~ (. ) The method has the disadvantage that storm-water ;;~ run-off and any supernatant water collects immediately 0,/ adjacent to the impounding wall, which must be de- I- ~ signed either to retain or to pass this water. ~ Fig. 8 shows the relationship between the storage ~ 0,05 volume and the elevation of the tailings surface corres- (b) ponding to Fig. 7. When the elevation of the tailings at the dam is 65 m, this particular impoundment can hold 67 per cent more tailings when operated by the thickened- 0,02 DIAMDND TAILINGS

W=12S% discharge method than by conventional filling. In the design of such a system, the relationship between the water content of the tailings and the slope assumed by 0,0/ / 2 5 /0 20 50 /00 the deposited material has to be predicted. Robinsky9 has presented empirical curves relating the slope of the RATE OF SHEAR STRAIN dY Id!,.1 slurry surface to the water content of the tailings slurry, Fig. 5-Relationship between apparent viscosity and rate of but there is a need for a more rational method of predict- shear strain for diamond and platinum tailings ing the slope. If the relationship between shear strength This equation can be applied to mine tailings slurries if and water content is known, the slope can be predicted the apparent viscosity, 1Ja,at the appropriate water con- rationally on the basis of the theory of infinite tent and rate of shear strain is used to approximate the slope stability. The slurry is assumed to behave as a flow behaviour of the slurry to that of a N ewtonian fluid. purely cohesive (cp= 0) material for which T = TO'the Alternatively, a more rigorous expression such as that yield shear strength contained by equations (2) and (5a). for a Bingham plastic16 can be applied. If the depth of deposition of each slurry layer is S, then It appears8 that the transition from laminar to turb- the consideration of the sliding equilibrium of an infinite ulent flow can be expected at Reynolds numbers exceed- slope of slurry results in the conclusion that it will come ing about 2000. For turbulent flow, it has also been sug- to rest in limiting equilibrium at an angle of inclination, gested 17that flow behaviour can be approximated by use i, given by of the apparent viscosity, 1Ja.Fig. 6 is the familiar Moody i=sin-1h/pgS). (9) diagram18 relating the friction factor to the Reynolds Fig. 9 shows a comparison between slope angles predic- number for both viscous and turbulent flow with the ted from equation (9) and angles measured in laboratory apparent viscosity replacing the true viscosity. tests. The material was a froth-flotation discard from a The limited data available to the authors on the process for the upgrading of calcium carbonate. The pumping of slurries are illustrated in Fig. 6. The data all slurry was spread to a thickness of 5 mm on a horizontal relate to slurries of high viscosity (in the range 0,3 to sheet of plate-glass, and the sheet was tilted slowly until 1 Pa) and lie in the laminar-flow region. The results the slurry started to slide. The tilt was then slightly show that, in this region of the Moody diagram, friction decreased and, if the slurry retained this slope, the angle factors can be predicted reasonably accurately from was noted as a point in Fig. 9. Because the slurry measured slurry densities and viscosities. This region of was sliding on a glass surface rather than on a surface of flow applies particularly to the transport of thickened slurry, the experimentally observed values of i probably slurries such as those used for underground filling and represent a lower limit to the inclinations corresponding for the thickened-discharge method of tailings disposal. to equation (9). The agreement between the predictions of It is recommended that equation (7) be used for the pre- equation (9) and the observed angles is nevertheless liminary design of pipelines for the transportation of considered sufficiently satisfactory for practical design thickened slurries. purposes. The thickened-discharge method of deposition has much to commend it, especially now that a rational Slope of a Thickened Slurry Surface method of design has been demonstrated. In the thickened-discharge method of tailings disposal, a thickened slurry is deposited from a point upstream of Flow of Slurry after Breaching of a Dam the outer retaining wall (Fig. 7) and is allowed to flow Failure of the outside wall of a hydraulic-fill tailings towards the wall. Because of the reduced water content impoundment may cause the contents of the impound- of the thickened slurry, little or no supernatant or bleed- ment to liquefy and flow out through the breach. Blight ing water arises during deposition. The material comes et al.10 and Jeyapalan14 have shown that the progress of

76 APRIL 1983 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY 0,5

0,2

0,1

0,05

OfJ2

.....

a: 0 " I- u.~ 0,01 LAM INAR z Q FLOW I- U " ~ 0,

0,(XJ2 "

0,001 100 200 500 1000 2000 5000 10 OCXJ 20000 50000

REYNOLDS NUMBER: HR: ~vy c.a 9

Fig. 6-Moody diagram showing some experimental results for the pumping of thick slurries the slUITY after it has left the breach can be predicted such a slUITY will spread can be calculated by consider- rationally. Both these papers consider the situation in ations of force equilibrium as demonstrated by Lucia which the escaping slurry has a high water content and a et al.15. very low shear strength, as was the case, for example, The basis of such an analysis is shown in Fig. 10. It is in the Buffalo Creek failure described by Wahler et al.19. assumed that the slurry will flow and its surface slope SIUITies having much higher shear strengths are not as will flatten until an equilibrium is established between mobile as those at Buffalo Creek or Bafokeng12, but they the fluid pressure exerted by the slurry retained in the ooze and spread from a breach to take up an equilibrium breach (of depth H) and the shear strength along the base surface slope in much the same way as the thickened- of the wedge of slurry. At that stage, movement will tailings slurries dealt with earlier. The distance to which cease. The height of the 'tongue' of the slide will be the

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY APRIL 1983 77 120

l()!j SLOPE 20 (3°) UPWARDS I'

90 -E TAILINGS RETAINING WALL OR DAM z 75 0 >= LEVEL SURFACE 60 ;;j~ 50 YEAR FlOOD 45 LEVEL SLOPE 50 (J O) DOWNWARDS 30 I' I 0 100 200 300 400 500

DISTANCE FROM RIVER BANK (m)

Fig. 7-Longitudinal section through a tailings impoundment showing the difference between stored volumes by conventional beaching and thickened-discharge deposition

LEVEL SURFACE would travel for 60 m. Equation (10) can be used in the 6' estimation of the consequences of a dam failure, especially where escaping slurry may damage or inundate surface installations. Particle-size Gradation on Hydraulic-fill Beaches ! 60 0 UPWARDS Although tailings slurries may be transported as g reasonably homogeneous materials, this state ceases to .... exist once the slurry has been deposited in the tailings .. impoundment. ii:'" In conventional hydraulic-fill construction, tailings '",,! " ~ slurry is deposited along the length of the impounding ... wall and runs down the beach to the pool from which the 0 plant return water is decanted. z 0 The tailings slurry is usually deposited at a number of ~ '0 > separate points, and the flow spreads after deposition, '"..J resulting in some decrease of velocity as the slurry moves down the beach. Fig. 11, for example, shows a plan and surface contours of a typical multi-delivery point ring- ", dyke dam used for the disposal of gold tailings (9 deposi- 0 '0 100 /50 200 tion points are shown).

STORAGE VOLUME ( 1000 m3) I() ° Fig.8-Relationship between storage volume and elevation of tailings surface for conventional and thickened-discharge ~ methods of deposition ANGLE P EDICTED FOR 8° 5mm THI K LAY height to which an unsupported bank of slurry will stand 2!J unsupported. The relationship between the length of ~c flow (L) and the depth of slurry at the breach (H) for iii 6° 20 given values of shear strength (r), density (pg), and ~ ~ '";:'" topographical gradient (i) can be written v: FOR '"u 15 'i:~ ~ L/H = ..' . . . (10) 4° ~ r /pg.H 2(1 - ~'")SIn ~ - sm ~ ~ ... ~ .... This relationship can be shown graphically by plotting of 10 ~ ..~ ... the dimensionIess ratios L/H and r/ pgH for various values To '" 2" c of i as shown in Fig. 10. ;;j 5 For the thickened-tailings slurry referred to in Fig. 9, >= with i = 0 and pg = 13 kNm-3, the height of the tongue of the slide would be only 1,5 mm and, for H = 5 m, 0 0 r/pgH = 3 X 10-4. 1,1 1,2 1,3 1,4 1,5 1,6 In this case L/H is very large, and the slurry would flow DENSITY OF SLURRY 11000 kg m - 3) OR SG OF for a great distance (1700 m). For a value of L/H = 12, S ~URRY a strength of 2,6 kPa is required, in which case the Fig.9-Relationship between relative density of tailings tongue of the slide would be 0,8 m high and the slurry slurry, angle of rest of slurry, and shear strength

78 APRIL 1983 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY surface can be predicted2O~ and hence the shear strength 12 of the wall can be established. Consider a particle of diameter D moving down a beach

10 of gradient i at a horizontal velocity Vh while it simul------taneously settles at velocity Vv. Following Graf8 and ignoring the interference of adjacent particles in the slurry, the settling velocity will be given by 4(ps v~= ~w~~gD = KD (say), ...... (H)

L /H in which Psis the density of the particle, pw is the density of the slurrying liquid, and C is a coefficient. If the depth of the sheet of flowing slurry is 8, the maximum time taken for a particle of diameter D to settle onto the surface of the beach will be 8 ts = -...... (12) 2 Vv In this time, the particle will have travelled down the I 1,0 0 0,2 0,4 beach a maximum horizontal distance of / "gH 8 H , = Vh . -Vy BREACH IN DAM ASSUMED LINEAR SURFACE Dr SLIDE and Vh will be governed by an equation similar to that of fA ",. open-channel flow of the form i C(8i) ~..'. .1 4 rip] Vh = .

, . _,H . - -:-' - - 1 "~ , 1- C'83/2ii Hence, D= (14) KH Fig. IO-Stability chart for slumping failures In other words, the predominant particle size to be found Because the coarser particles contained by the slurry at a distance H from the point of deposition will be settle more rapidly than the finer particles, a gradation roughly inversely proportional to H. This is the form of of particle size down the beach results, with coarser relationship shown in Fig. 12. Alternatively, if the sett- material adjacent to the points of deposition and pro- ling velocity is governed by Stokes law, gressively finer material being deposited with increasing (Ps - pw)gD2 Vv = KID2 (say), . (Ha) distance from these points. Fig. 12 illustrates the particle- 187J = size sorting that has taken place on a 280 m long beach of diamond tailings. Particle-size sorting also occurs vertically within each layer of deposition. Vertical sorting and and D=[~~~2ir, ..., (14a) the anisotropy that results from it has been examined by i.e. the predominant particle size to be found at distance Blight and Steffen4. H from the point of deposition will be inversely propor- The inset in Fig. 12 shows the results of a separate set tional to the square root of H. of measurements taken on the same beach some 12 months later. The distance down the beach, H, has been The above analysis is necessarily approximate because normalized by dividing by the distance from the point of of the idealizations on which it is based. Apart from the deposition to the edge ofthe pool, x. For the construction particle interference that must occur, the gradient, i, of both diagrams, samples were taken that penetrated of the beach surface varies with H, and, as the flow three layers of deposition. spreads from the deposition point, 8 decreases. It is also The particle-size gradation on a beach can have a probable that some transport of the material down the significant effect on the stability of the tailings dam. If beach occurs after it has settled out of the slurry. This the walls and outer zone of the beaches consist of coarser, may result in further particle sorting if there is any rolling more pervious material, the phreatic surface will be lower motion of settled particles along the beach surface. It is and the dam more stable. Advantage can be taken of the also possible that material deposited during one deposi- particle-size gradation that results from hydraulic filling tion cycle could be eroded, picked up, and re-deposited only if it can be predicted at the design stage from an during a subsequent deposition cycle. analysis of the total tailings product, or if a similar dam Then there is the question of the beach gradient, i, constructed of a similar product is available to sample. which both equations (14) and (14a) contain. It is prob- In what follows, an attempt is made to derive a method able that i and D are variables that are interlinked in of predicting the predominant particle size, and hence some complex way. The question of beach gradient will indirectly the permeability, at any point along a hydraulic be investigated later, but for the time being the varia- beach. If this can be done, the position of the phreatic bility of the beach gradient will be ignored.

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY APRIL 1983 79 150m

Fig. I I-Plan of a typical ring-dyke tailings dam (reproduced by kind permission of Messrs Steffen, Robertson and Kirsten Inc.)

80 APRIL 1983 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY 1,0 is a photograph showing rill flow occurring down a beach of platinum tailings. The unstable nature of the flow is 1,0 evident from the hydraulic jump in the photograph. This 0,8 8 \ instability is probably responsible for much of the scatter ! D.m.. \ evident in Fig. 12. The overflowing of rills and the result- \j 0.6 0 .'\: ant deposition of fine material high up the beach can i 0,6 result in a layered fill in which the vertical permeability ... N ;;; 0.2 " """'" is considerably less than the horizontal permeability. ~ r-- - This, in turn, can lead to a raised phreatic surface with 0,4 . . its attendant slope-stability problems. A ratio of horizon- i 0.2 0,4 0,6 0,8 1,0 ... H /x tal to vertical permeability of 4000 has been measured > § on a beach of diamond tailings. In that dam a conven- ::!0.2 tional phreatic surface was not present at all, but was 0 replaced by a series of horizontal perched water table s. It is important to note that, because of the presence of 0 0 0 process chemicals in the water, many tailings slurries 0 !lO 100 150 roo 2fiO n> are strongly flocculated. A particle-size analysis carried D1STANCE FROM PONT OF DEPOSITION I..) out by hydrometer in the conventional way using distilled Fig. 12- The particle-size sorting that occurs on a hydraulic water and deflocculants will therefore not be represen- beach tative of the effective particle-size distribution on a Because the particles are effectively settling through hydraulic beach. For this reason, all the particle-size a viscous slurry and not through water, Vv in the above analyses used in this section were conducted on plant- analysis is very small. For example, if we consider a return water, and no attempt was made to deflocculate 20 mm thick sheet of slurry flowing down a 280 m beach the slurries. at 1 m/s, the settling velocity of particles precipitating If equation (14) is valid, the product, DH, should be out at the end of the beach will be a maximum of only constant at all points along a beach for which H exceeds 70 X 10-6 m/so The idealized laminar-sheet flow assumed for the zero, while, if equation (14a) is valid, the product, D2H, analysis does not always occur. Instead, an unstable, should be constant. The two possibilities are explored in meandering, and turbulent rill flow is often observed. Figs. 14 and 15. In these figures, the ratio H/X is the When such a rill or channel overflows, the settling regime ratio of the distance H from the point of deposition to is upset and fine material is deposited higher up the the length X of the beach, measured to the edge of the beach than equation (14) or (14a) would predict. Fig. 13 pool.

Fig. 13-Unstable rill flow occurring on a hydraulic beach, showing a hydraulic jump in the centre of the photograph

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY APRIL 1983 81 (i (ii) M is approximately constant for all tailings of a "" given type and for all lengths of beaches greater than 40 m, but has to be established for each type

4 of tailings. (!) (!) It is known that, if a beach is too short, a relationship (!) such as (16) does not hold. Sampling of a beach of dia- 0 AH/X ~2 mond tailings 9 m long, for example, showed that virtual- (!) (V (!) MEAN ( ly no particle separation was occurring over that distance. @~ One possible reason for the lack of rapid separation is

(!) @ (!) the high viscosity of the clay 'flocs' in suspension that 0 constitute much of a diamond-tailings slurry. 0 0,2 0,4 0,6 0,8 40 So far, the investigation of particle-size distribution

H/X has stopped at the edge of the pool, i.e. only deposition in air has been investigated. It is known that an abrupt change in the depositional regime occurs as soon as the tailings slurry reaches and runs into water. Limited measurements have shown that A (equation (15)) decreases 60 suddenly beyond the edge of the pool (i.e. for X > L), 0 and the beach gradient steepens suddenly beyond the (!) water's edge. The under-water regime usually applies only at a considerable distance from the outer perimeter 40 of a tailings dam, and for this reason is less important A2 H/X 0 in considerations ofthe stability of tailings dams. ~0 20 Gradients of Hydraulic-fill Beaches ~MEAN As mentioned earlier, the gradient of a hydraulic-fill E) (!) (!) @ g beach varies along its length and is probably related to (;) ( 0 G'I the particle-size separation that occurs on the beach. The coarser material, which settles out first, drains rapid- 0 0,2 0,4 0,6 0,8 1,0 H/X (S) X "90.. A x . 1111III Fig. I4-Particle-size distribution along a beach of diamond . X . 40.. & x . 711.. tailings 280 m long 1,5

D5o (at H down the beach) A = . . . . . (15) D5o (of the total product) 1,0 ... D5o is the particle size at which 50 per cent by mass of the solids at any point is finer than D5Q" D5o was chosen MEAN OR~LL X arbitrarily as a representative particle size at any & distance H down the beach. "H/X (\5 ~'I EAN FOR 90.. Fig. 14 represents the results of particle-size deter- G> . minations on specimens taken along the same 280 m long tJ 4 beach of diamond tailings referred to in Fig. 12. Fig. 15 0 presents similar data for four beaches of platinum tailings 0. 0,2 0,4 0,6 0.8 1,0 that vary in length from 40 to 90 m. Whether equation (14) or (14a) is followed, there is a lot of scatter about the H/X mean value of either AHjX or A2HjX. Not surprisingly, the mean values for these ratios vary with the material under consideration. In the case of the platinum tailings, the mean value of AHjX for the 90 m beach is fairly 2, close to that for all four beaches. The data appear to fit better than in the case of A2HjX. Similar results to those . . shown in Figs. 14 and 15 have been obtained for gold IllEAN FOR ALL If A2 tailings. H/X 1,.0 .. Based on the available data, the following tentative conclusions have been drawn. (pIllEAN FOR X. 90 Ill""" i 4 (i) The particle size at a given distance along a 0 ,4 0,6 0.8 I',0 hydraulic beach from the point of discharge can ." be roughly predicted from the relationship H/X AHjX = M (for H greater than zero), .. (16) Fig. IS-Particle-size distribution along four beaches of in which M is a characteristic of the tailings. platinum tailings

82 APRIL 1983 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY ly and develops shear strength faster than the finer strength of the settled slurry is an important factor in material, which settles out further down the beach and determining beach gradients. However, as the flow drains more slowly. Hence, equation (9) would predict a spreads, delta-like, from the point of deposition and in progressive flattening of the beach gradient. Fig. 16 so doing decreases in velocity (Fig. 11), the requirements illustrates typical variations in surface shear strength of continuity and conservation of energy probably also along the beaches of an operational platinum-tailings have an influence on the gradients that develop. dam. Strengths were not measured during deposition, Figure 17 illustrates beach profiles measured on six but a few hours afterwards, when sufficient capillary platinum-tailings dams. If the profiles are plotted on a strength had developed to make the area accessible. The dimensionless basis as in Fig. 18, they reduce to a single 0 to actual decline in surface strength from H/X = 'master' profile that can be approximated by an expres- H /X = 1 during deposition is almost certainly larger sion of the form than that shown in Fig. 16. X!H h/Y = Y/X (1 - H/X)n...... (17) 0 0' 8'0 9'0 The variables h, H, X, and Y are defined in Fig. 17, and n is an exponent that is characteristic of the tailings material. It should be noted that Y/X represents the i!b 'average' gradient along the beach. Fig. 19 compares dimensionless beach profiles for four different types of tailings. The profiles for platinum, "0 nw, diamond, and gold tailings were measured on hydraulic- 1 0 M'O ~ fill beaches, but that for copper tailings represents the "0 average profile of a series of cyclone underflow cones. It 8 MW 11--1( is interesting that such repeatable results with very little '",0 ---- scatter can be obtained for beach and cyclone-cone 8'0 profiles, whereas particle-size profiles, which are of more direct use to the designer, cannot be predicted nearly so certainly. Fig. 16-Variation of shear strength along three hydraulic As shown earlier when the flow of thickened slurries beaches was considered, a small change in the angle of a beach, The application of equation (9) to the data of Fig. 16 and particularly of a cyclone underflow cone, can mean predicts that the slope angle of the beach at the pool an important variation in the volume of stored tailings. would be 30 to 50 per cent of the slope at the point of It is therefore useful to have an expression such as deposition. Measurement indicates that the slope de- equation (17) when tailings-disposal systems are being creases from 4,7° to 0,8°, a reduction to 17 per cent of the planned. This is of particular importance when the design initial value. This appears to support the view that the of a tailings dam depends on the deposition of a given

I&. 0 ..J I&J 8 8 11- y I&J E z h 0 ~ 3 0 ~ x W --I W w > ~ 2 -

o. 20 40 60 80 100 /60

H = DISTANCE ALONG BEACH (m) Fig. 17-Measured beach profiles on six platinum-tailings dams

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY APRIL 1983 83 shrinks, with the result that a network of open sub- 1/,0 . DAM AI surface cracks may form in the tailings deposit. Fig. 20 . DAM A2 is a sketch of a system of open shrinkage cracks that were X DAM BI observed in the side of a test pit in the surface of an 0,8 ~DAM B2 operational tailings dam. The system of open cracks was . DAM C observed to extend to a depth of 500 mm below the 0 DAM D surface. As overburden builds up, the cracks appear to

q6 FITTED CURVE close but remain as incipient surfaces of weakness. h /y The photograph in Fig. 21 shows the surface of a fresh vertical cut in an abandoned tailings dam. The network of old shrinkage cracks is shown up by the oxidation of 0,4 pyrite that has occurred preferentially along these surfaces. The height ofthe cut is more than 2 m. I I SURFACE OF DAM q2

0 q2 0,4 0,6 (0 H/X

Fig. 18-Dimensionless beach profiles for platinum tailings

proportion of the total tailings to form an embankment OPEN VERTICAL CRACKS of a predetermined profile. DOWN TO 500 mm lInternal Erosion during the Deposition of Slurry The deposition of tailings slurry on a dam is an inter- mittent process. Material deposited on one section of a dam is usually left to drain and dry out for a few days before the next layer is deposited. For example, the deposition cycle on the dam shown in Fig. 11 would be either 9 or 4! days. The drying process has the beneficial effect of considerably reducing the void ratio of the tail- ings4, but also results in the formation of a network of 5mm shrinkage cracks in the surface of the layer of dried slurry. When the next layer of slurry is deposited, these COARSE - 200 mm . . . FILLING CRAC~S~N dr ymg crac ks are filled WIth sIurry. Durmg su bsequent FINEASH drying cycles, the slurry in the shrinkage cracks also

/,0

Fig.20-Systems of shrinkage cracks near the surface of hydraulically deposited power-station fly ash

The network of sub-surface open cracks formed by drying shrinkage constitutes potential channels for piping 0,6 erosion either when slurry is deposited on the dam or when, in exceptionally wet weather, water accumulates h /y on the surface of the dam. This internal erosion often D) results in troublesome maintenance work when slurry 0,4 PL TNUM (n = erodes a pipe back to the outer slope ofthe dam and issues from the slope at a distance of 1 to 3 m below the crest. In isolated instances, piping along shrinkage cracks is 0,2 suspected of being responsible for large-scale failures. The failure at Bafokeng12, for instance, in which several lives were lost and extensive damage was done to property, appears to have been initiated by piping erosion. Fig. 22 0 0,2 0,4 0,6 op /,0 shows sketches of two erosion pipes found below the sur- HIx face of a dam, while Fig. 23 shows the mouth of an erosion Fig. 19-Dimensionless beach profiles for various types of pipe near the crest of a small dam. In this case, the exit tailings of the pipe was 5 m below the crest.

84 APRIL 1983 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY (b) the design of surface slopes for the disposal of tailings by the thickened-discharge method; (c) the investigation of the distance travelled by a slurry after escaping from a breach in a dam wall. 2. Methods of predicting both the particle-size gradation along a hydraulic-fill beach and the profile assumed by the beach were developed. Particle-size gradation can be used in the prediction of the position of the phreatic surface, and hence can help in the analysis of the stability of a proposed dam. A knowledge of the beach profile assists in an analysis of the volume-height-area relationships for dams. 3. A troublesome phenomenon, that of internal erosion during the deposition of slurry, was described, and preventive measures were tentatively suggested.

SURFACE OF DAM

l50mm

130mm

VOID 270 mm

VOID TRACED FOR 27m

F DAM Fig. 21-Pattern of shrinkage cracks shown up in the side of a cut in hydraulically placed tailings

The remedy for this phenomenon is not clear. It has been suggested that the deposition of thickened slurries VOID 200 mm may reduce the severity of shrinkage as well as reducing VOID 29m LONG LEADlNG the erosive capabilities of the slurry as it is being deposit- TO WALL OF DAM ed. As the erosion cannot occur without the presence of 1I00mm j shrinkage cracking, another possible solution is to shorten Fig. 22-Erosion pipes located beneath the surface of hydraul- the cycle of deposition so that the surface of the dam ically deposited fly ash does not dry out to the extent that shrinkage cracking occurs. However, the effect of an accelerated deposition cycle on the stability of the dam would have to be con- List of Symbols in Order of Appearance sidered. Symbol Unit Explanation (with or Summary (without In the course of describing the behaviour of mine 8ubcript) tailings during hydraulic deposition, this paper dealt 'T kilopascal (kPa) shear strength with the following. 'YJ Pascal second (Pas) viscosity 1. A method of establishing relationships between y radian shear strain water content, viscosity, and shear strength was t second (s) time given, and the application of these relationships to w dimensionIess water content the following aspects of tailings disposal was des- K empirical constant cribed: n empirical exponent (a) the design of pipelines for the transportation of i metre/metre (m/m) flow gradient, slope thickened slurries; of surface

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY APRIL 1983 85 G. M. Bentel was sponsored by the South African Council for Scientific and Industrial Research, the University of the Witwatersrand, Messrs Steffen, Robertson & Kirsten, Inc., and Fraser F. Alexander & Co. (Pty) Ltd. Thanks are due to these bodies for their generous support.

References 1. ANON. Geotechnical research needs. Civil Engineering, ASCE, Jul. 1981. pp. 68 - 71. 2. BLIGHT, G. E. Assessment for environmentally acceptable disposal of mine wastes. The Civil Engineer in South Africa, vol. 23, no. 10. Oct. 1981. pp. 489 - 499. 3. CALDWELL,J. A. Personal communication. 4. BLIGHT, G. E., and STEFFEN, O. K. H. Geotechnics of gold mining waste disposal. Current Geotechnical Practice in Mine Waste Disposal, ASCE, 1979. pp. I - 52. 5. BORGESSON,L. The Nasliden Project - mechanical properties of hydraulic back fill. Proceedings, Conference on the Application of Rock Mechanics to Cut-and-Fill Mining. Lulea. (Sweden), 1980. vol. 2, pp. 173 - 184. 6. BLIGHT, G. E. Properties of pumped tailings fill. Journal of the South African Institute of Mining and Metallurgy, Oct. 1979. pp. 446 - 453. 7. BLIGHT, G. E., MoRE O'FERRAL, R. C., and AVALLE, D. L. Cemented tailings fill for mining excavations. Proceedings, 9th International Conference on Soil Mechanics and Found- ation Engineering. Tokyo, 1977. vol. I, pp. 47 - 54. 8. GRAF, W. H. Hydraulics of sediment transport. New York, McGraw-Hill, 1971. pp. 455 - 461. 9. ROBINSKY, E. 1. Tailing disposal by the thickened discharge method for improved economy and environmental control. Proceedings, Second International Conference on Tailings Disposal. Denver (U.S.A.), 1978. pp. 75 - 92. 10. BLIGHT, G. E., ROBINSON, M. J., and DIERING, J. A. C. The flow of slurry from a breached tailings dam. Journal of the South African Institute of Mining and' Metallurgy, Jan. 1981. pp. 1 - 8. 11. VAN WAZER, J. R., LYONS, J. W., KIM, K. Y., and COL- WELL, R. E. Viscosity and flow measurement. New York, Interscience, 1963. Fig. 23-Mouth of erosion pipe initiated by flow along shrin- 12. JENNINGS, J. E. The failure of a slimes dam at Bafokeng. kage cracks in hydraulically placed tailings Mechanisms of failure and associated design considerations. The Civil Engineer in South Africa, vol. 21, no. 6. 1979. pp. 135 - 141. t::o.h metre (m) change of head 13. WHITNEY, E. D., MOUDGIL, B. M., and ONODA, G. Y. De- L metre (m) flow distance watering of phosphate clay wastes. Department of Material f dimensionless friction factor Science and Engineering, University of Florida, 1977. d metre (m) pipe diameter 14. JEYAPALAN,K. Analyses of flow failurcs of mine tailings v metre/second (m/s) flow velocity impoundments. Doctor of Philosophy thesis, University of California, Berkeley, 1980. g metre/(second)2(m/s2) gravitational 15. LUCIA, P. C., DuNcAN, J. M., and SEED, H. B. Summary of acceleration research on case histories of flow failures of mine tailings NR dimensionless Reynolds number impoundments. Mine Waste Disposal Technology, U.S. kilogram/(metre )3 mass per unit Bureau of Mines Information Circular, IC8857/1981. pp. P 46 53. (kg/m3) volume - 16. BEHN, V. C. Derivation of flow equations for sewage sludg- S metre (m) depth of deposition es. Journal of the Sanitary Engineering Division, ASCE, h metre (m) depth or height vol. 86, no. SA6. 1966. pp. 59 - 81. C empirical constant 17. METZNER, A. B. Flow of non-Newtonian fluids. Handbook D metre (m) particle size of fluid dynamics. New York, McGraw-Hill, 1961. H metre (m) distance 18. DoDGE, D. W., and METZNER, A. B. Turbulent flow of non-Newtonian systems. Journal, American Institute of A dimension less ratio of particle Chemical Engineers, vol. 5, no. 2. 1959. sizes 19. WAHLER, W. A., and Associates. Reports on analysis of X metre (m) distance coal refuse dam failure at Buffalo Creek, West Virginia. U.S. M empirical constant Bureau of Mines, 1973. Y metre (m) depth or height 20. ABADJIEV, C. B. Seepage through mill tailings dams. Proceedings, 12th International Congrcss on Large Dams. Acknowledgements Mexico, 1976, vol. I, Q44, R42, pp. 381 - 393. 21. BENTEL, G. M. Some aspects of the behaviour of hydraulically The work on beach profiles and some of the the work deposited tailings. M.Sc. (Eng.) dissertation, University of on particle-size sorting on beaches21 was conducted while the Witwatersrand, Johannesburg, 1981.

86 APRIL 1983 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY