A Review of Catastrophic Flow Failures of Deposits of Mine Wate

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A REVIEW OF CATASTROPHIC FLOW FAILURES OF DEPOSITS OF MINEWASTE AND MUNICIPAL REFUSE

G.E. BLIGHT & A.B. FOURIEUniversity of the Witwatersrand, Johannesburg, South Africa

ABSTRACT: Catastrophic flow failures occur in mine tailings dams and dumps of discards and other mine waste with alarmingfrequency. In recent years catastrophic flow failures have also occurred in dumps of municipal refuse and even in what wereconsidered to be carefully controlled and well engineered landfills. Apart from the environmental devastation caused by these flows,they are also dangerous to human life and society. For examples the Buffalo Creek disaster in the USA in 1972 killed 118 people,made 4000 homeless and destroyed 50 million US dollars worth of property and facilities, the flow slide that occurred in theUmraniye-Hekimbasi refuse dump in Turkey in 1993, killed 39 people, destroying their homes in the process. The paper will brieflyreview some of the more typical flow slides in waste materials, analysing the mechanics of failure and pointing to ways of preventingthis type of failure by a combination of sound design and operating procedures. In the case of existing deposits modified operating procedures can be adopted, reducing the probability of failure as well as constructing deflecting structures to protect communities and

facilities from the consequences of failure.

Keywords: Flow failure, mine waste, municipal solid waste.

1 INTRODUCTION: STATISTICS, FAILURES, BREACHES,FLOW FAILURES, EXAMPLES

1.1 Flow failures in tailings impoundments

Tailings dams, whether of the valley or ring impoundment type,usually consist of an outer impounding dyke or dam wall thatserves to retain the body of tailings, supernatant water (and,occasionally, storm precipitation) upstream of it. If, for anyreason, this outer impoundment is breached there will be thedanger that the impounded tailings will escape the impoundment.Figure 1 shows statistics for 184 incidents involving tailingsdams, collected by the US National Committee on Large Dams(1994). Here, the definition of "failure" is "any breach in theembankment leading to a release of the impounded tailings". The184 incidents were not all failures, nor were the failures all flowfailures. The statistics, however, show that, of the known causesof failures or breaches, the most likely to occur are slopeinstability, earthquakes and overtopping, in that order.Foundation and seepage failures come next, followed bystructural failures. The meanings of the latter three categories arenot very clear, but they presumably include foundation shear or piping failure, piping through the dam wall and inward collapse

of a decant tower or decant outfall, any or all of which couldresult in breaching of an impoundment.

Slope instability may result in a distortion or flattening of theslope of the retaining impoundment, without any escape ofimpounded tailings, or the tailings may move only a shortdistance beyond the impoundment wall. An example of a dykefailure in which no tailings escaped is illustrated in Figure 2,which shows a rotational shear failure in the outer dyke of a platinum tailings dam at Bafokeng, South Africa (Blight 1997)(7, Table 1). Here, the dyke stabilized at a flatter average slope,as a result of the failure, and none of the retained tailings escapedthe impoundment. However, a year later the same dyke was breached catastrophically as a result of overtopping, releasing3x106m3 of tailings (as shown by Figure 3a) and causing 13

deaths as a result (Jennings 1979, Blight 2000).

Figure 1: Analysis of causes of tailings dam failure(USCOLD, 1944).

Hence the failure of a tailings dam dyke does not inevitably

result in an escape of tailings but a dyke breach must obviouslyoccur before tailings can escape. At Bafokeng, overtopping probably resulted in breaching of the dyke by erosion (see Figure3b for mechanism), but 20 years later, the disastrous flow failurethat occurred from the Merriespruit tailings dam (also in SouthAfrica), Figure 4 (13, Table 1), resulted from breaching of thedyke by overtopping, causing slope erosion and progressive shearfailure, as shown in Figure 5 (Wagener et al 1997, Blight 2000).

Figures 3a and 4 illustrate the characteristics of a tailings damflow failure. These include liquefaction of a large volume of theretained tailings, which flow out of the breach as a viscous liquidand are capable of moving large distances before coming to rest.The 3 x 106m3 tailings escape at Bafokeng travelled 42km,covering its path with slurry, before the remaining 2 x 10 6m3 wasstopped when the flow entered a water retaining dam. The flow


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Figure 2: Failure (or slump) of retaining dyke of Bafokengtailings dam (South Africa) that did not result in escape oftailings (1973).

Figure 3a) Plan of Bafokeng tailings dam showing position of breach, course of flow failure, and extent of pools prior to failure(1974). b) Stages of failure of Bafokeng tailings dam.

at Merriespruit (a lesser quantity of 600 x 103m3) travelled 2km before being halted and contained by an ornamental lake.

1.2 Flow failures of "dry" mine and industrial waste

Tailings are hydraulically deposited as slurries, into containmentsdesigned to retain the consolidating solids and supernatant andstorm water. However, numbers of flow failures have alsooccurred in mechanically placed "dry" mine waste deposits. The prime example of a flow failure in a "dry" mine waste occurredat the village of Aberfan, Wales (4, Table 1). Here, in 1966, adump or tip of coal waste failed, liquefied (largely as a result ofdumping waste over a spring) and flowed into the village ofAberfan, killing 144 people of whom 116 were school children(Anonymous 1967, Bishop, 1973). Figure 6 shows the course ofthe 1966 flow slide. The figure also shows that the Aberfan tiphad failed and flowed twice previously, in 1944 and 1963, but

these earlier flows did not reach the village and did not serve assufficient warning to the owners of the tip, or regulatory officials,of the eventual 1966 disaster.

The flow failure of a fly ash dump that occurred in 1961 inJupille, Belgium (Bishop 1973) (2, Table 1) is another archetypeof a flow failure in "dry" material. Figure 7a shows a plan of thecourse of the flow which travelled down a dry valley for 0.5km.At Jupille, it appears that the ash may have been fluidized by aircontained by its pores when the fly ash contracted during thefailure. It was reported that fly ash that entered houses,overwhelmed in the flow, appeared to be "dry". Figure 7b showsthat as the ash flowed down a natural valley it "lined" the valleywith ash, the stream of fluid ash eventually flowing in a "canal"of solid ash. Of course, this also happens to some extent withflows of wet materials: the course of the flow is marked bymaterial stranded as the main flow passes. During the 42km longflow at Bafokeng an estimated 1 x 106m3 of the 3 x 106m3 oftailings that escaped was left marking the course of the flow.

1.3 Flow failures of municipal solid waste

Until recently, flow failures in dumps or landfills of municipalsolid waste have been unknown. This may be because significant

Figure 4: Plan of Merriespruit dam, South Africa, showing position of pool at time of failure, intended position of pool, breach in dyke, and path of tailings flood (1994).


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Figure 5: Most likely development of flow failure atMerriespruit, 1994.

Figure 6: Flow failures of coal waste at Aberfan, Wales, 1944,1963, 1966.

numbers of landfills have not, until recently, reached a large sizeor because failures that did occur caused no deaths and thereforewere not newsworthy and were not reported. However, in 1977 aflow failure took place in a landfill at Sarajevo (Gandolla et al1979), and in 1993 a massive flow failure took place in theUmraniye-Hekimbasi refuse dump in Istanbul, Turkey (Kocasoy,Curi 1995) (11, Table 1). Figure 8 shows sections through thedump before and after the failure, as well as the course of thedebris flow. After reaching the bottom of the valley, themomentum of the flow carried it up the opposite slope,destroying a number of informal houses and killing 39 people.

The slide also fractured a main sewer pipeline that ran, on the

Figure 7: Flow of fly ash at Jupille, Belgium.

surface, along the valley. The sewage that poured from the sewer pipe was dammed by the slide debris and formed a lake ofsewage on the upstream side of the obstruction. Since thisoccurrence, two and possibly three more flow failures ofmunicipal solid waste deposits have been reported (Hendron et al1999 (17, Table 1), Brink et al 1999 (18, Table 1)).

1.4 Record of notable flow failures

Table 1 records 22 failures of "dry" mine waste deposits,hydraulic fill tailings impoundments and municipal solid wastelandfills that occurred over the 72 years from 1928 to 2000. The

table gives an idea of how widespread these failures can be,geographically and in terms of the materials that have flowed, thevolumes of material involved and the consequences. The deathstatistics at the foot of the table (1400 deaths in 72 years) alsoshow that flow slides are not particularly dangerous occurrences.A single flying accident can cause 400 deaths, and we expect tohave at least one or two of these per year, yet commercial airtravel is not considered dangerous, nor is travel by road, eventhough the annual road death toll is hundreds of thousands.

Not all flow failures of waste deposits make headlines, andregrettably as mentioned above, some failures may never reachthe news and are never recorded in the geotechnical literature.Figure 9 (Blight 2000) (12, Table 1) shows three flow failuresthat occurred in a tailings impoundment at Saaiplaas, SouthAfrica, in three days. Because the failures caused no deaths or


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Figure 8: Failure of Umraniye-Hekimbasi municipal solid wastedump, Istanbul, Turkey.

injuries and the flows were confined to mine property, thisincident was never reported by the news media, and nevercomprehensively investigated. The Saaiplaas impoundment isonly a few km from the Merriespruit impoundment that failed ayear later. If the Saaiplaas failure had been publicized, it mayhave served as a warning to the operators of other tailings damsin the area to inspect their dams carefully for safety, and theMerriespruit disaster might have been avoided. However, most ofus are confident that we are immune from disasters that befallothers, so it is more likely that the warning would have beenignored, as in the case of Aberfan.

2 STRAIN-SOFTENING OR LIQUEFACTION OF MINEAND MUNICIPAL WASTES

2.1 The mechanics of strain-softening or liquefaction When a particulate material, be it a soil, tailings, dry mine wasteor municipal solid waste, is subjected to shear stresses, it willtend to change volume and hence void ratio. Dense materials willtend to dilate, loose materials will tend to contract and materialsof intermediate, or near critical state density will have littletendency to change volume. The consequences of this behaviourwhen a saturated material is sheared undrained, are illustrated byFigure 10 (after Castro 1969). The important features of Figure10, which shows results for consolidated undrained strain-controlled shear, are:

.1 The initial peak shear strength achieved by all specimens was

Figure 9: Plan of Saaiplaas dam, (South Africa) showinglocations of failures A, B, & C.

of the same order, and occurred at approximately the same(small) axial strain.

.2 After the initial peak, the loose specimen lost strength (orstrain-softened) as the strain increased, whereas the dense

specimen continued to gain strength (strain-hardened) with


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Figure 10: The effect of initial relative density (Dr) on the shapeof the stress-strain curves of consolidated undrained tests on

saturated sand. (Castro, 1969)

increasing strain. The strength of the intermediate specimenremained more or less constant..3 In the loose specimen, the pore pressure rapidly rose with

increasing strain, to reach a constant maximum. In the densespecimen, after rising to a peak, the pore pressure reducedcontinuously and the shear strength increased.

The loss of shear strength of the loose specimen from 180kPato 20kPa after a strain of 3 to 5% constitutes strain-softeningwhich in an extreme state can be called liquefaction. Because theshear stress was applied monotonically, this is termed staticstrain-softening or liquefaction.

Figure 11 a shows the results of three load-controlled tests,also by Castro (1969), on specimens of loose saturated sand.These show very similar behaviour to the strain-controlled test ofFigure 10. These tests, though, also show that if a loose materialfails under stress-controlled conditions, which is usually the casein a slope failure, the failure can occur very rapidly, in fact,almost instantaneously as the shear stress is applied. Figure 11bshows the stress paths for the tests of Figure 11a, illustrating thatthe ultimate effective stress state reached in these tests lies on theK f or failure line for strain-controlled tests.

The behaviour of a loose saturated sand silt under dynamicallyapplied shear stress is illustrated by Figure 12 (Blight 1990).Each application of the shear stress of 300kPa caused anincrement of pore pressure that reduced the mean effective stress,until application 13 moved the stress path onto the K f or failure

line. On stress application 14 it was not possible to reach theshear stress of 300kPa. If the test had been continued past stressapplication 14, the ultimate condition would have been reached,with the stress path on the K f -line, a constant mean effectivestress, and a very low shear strength.

2.2 Strain-softening or liquefaction of tailings

Tailings are usually deposited as slurries and settle out as thetailings are beached in the impoundment at a high water content.They therefore settle on the beach with a loose particle structure.If the tailings beach is allowed to dry out between successivedeposition cycles, which is usually the case, the slurry layer

shrinks and densifies as it dries, as shown in Figure 13. However, because of seasonal and other variations in the

Figure 11 : a) Stress-strain curves for stress-controlledanisotropically consolidated undrained tests on saturated loose

sand ( s 31c = 400kPa) b) Corresponding stress paths (Castro, 1969).

Figure 12: Stress path for dynamic shear test on loose saturatednatural sandy silt.

weather, varying thickness of layers of deposition, demands forincreased deposition rates at times of increased production tomeet market demands, etc., the degree of densification inevitablyvaries both with time and position on the tailings dam (both inelevation and plan). For example, for a year before the threefailures on the Saaiplaas No. 5A dam (Figure 9) took place, therate of rise of the dam had been increased from its long-termaverage of 1.8m/y to 2.6m/y and shortly before the failuresoccurred, the rate of rise had been further increased to 2.8m/y.Each increase in rate of rise would have reduced the time between tailings deposition cycles. Thus the density ofsuccessive layers of tailings may be, and usually is highlyvariable. The effects of this variable density on the measuredshear strength of tailings are illustrated by Figures 14 and 15

(Blight 1997, Fourie et al 2001). Figure 14a shows stress paths

DeviatorStress

(kPa)

PorePressureu

(kPa)

DeviatorStress

(kPa)

PorePressure

u

(kPa)


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Figure 13: Densification of tailings slurry by drying shrinkage

Figure 14a: Stress paths for consolidated-undrained triaxial shearof undisturbed tailings specimens.

Figure 14b: Effective stress changes in undisturbed specimensduring unconsolidated undrained triaxial compression.

for four consolidated undrained triaxial shear tests on 38mmdiameter x 76mm high samples taken from a single Shelby tubesample from the Merriespruit tailings dam (see Figures 4 and 5).The two tests at consolidation stresses of 50 and 100kPa showedcontractive behaviour, the one at 200kPa showed critical state behaviour, while that at 400kPa was weakly dilative. Figure 14bsummarizes changes in effective stress from the start of shearingto the ultimate state for 16 consolidated undrained shear tests onShelby tube specimens from Merriespruit having various voidratios. The specimens were tested under their original in situeffective overburden stress. While nine of the specimens dilatedduring shear, seven showed contractant or almost neutral behaviour. In other words certain of the layers in the tailings

could have strain-softened or liquefied and flowed during thelarge scale failure, carrying other denser layers with them.

Figure 15a shows the results of a piezo-cone penetrometer testconducted on the Merriespruit impoundment after the failureillustrated by Figures 4 and 5. The cone penetration resistancefluctuated over a range of up to 2MPa as the cone penetratedsuccessive layers of tailings. The pore pressure, in sympathy,showed low or even negative values as dense, dilative layerswere penetrated, and high values as loose contractive layers wereencountered. Figure 15b summarizes the results of 16 piezo-cone penetrometer tests at Merriespruit, made at various distancesfrom the toe of the dam. Each cone penetration profile has beencharacterized by its maximum and minimum slopes in terms of penetration resistance per unit depth (in kPa/m). There was aconsiderable difference between these two slopes, and both theslopes and the difference between maxima and minima decreasedwith distance from the toe. However, there was no suddenchange in penetration characteristics between tailings forming theouter slope of the impounding dyke and those contained in theinterior of the impoundment. In other words, this was not a caseof a consolidated outer embankment retaining a partlyconsolidated semi-fluid core. Certain layers of tailings formingthe beach of the dam must have suffered static strain-softening orliquefaction for the flow failure to have occurred.

2.3 Strain-softening or liquefaction of "dry" mine wastes

Bishop (1973) drew attention to the phenomenon of the"bulking" of unsaturated sands and gravels when depositedwithout compaction, a phenomenon long known in concretetechnology with relation to volume batching of aggregate. Ingeneral terms, if a given mass of dry cohesionless sand or gravelis deposited loosely, it will assume a certain volume and voidratio. If water is gradually added, the volume of the mass (andhence its void ratio) will increase up to an optimum water contentafter which the volume will decrease again. When the material issaturated, it will have approximately the same volume and voidratio as when it is dry. Bulking is well illustrated by the resultsshown in Figure 16a for mixtures of the coarse gravel and sand-

fractions of diamond mining waste. The two sets of curves were prepared with different compactive efforts, and hence initial voidratios, but regardless of initial void ratio, showed much the samemaximum increase in void ratio as bulking proceeded. At watercontents approaching saturation, the void ratios were much thesame as the initial values. Note that the sand content of thematerial had little effect on the bulking, but the addition of sanddid affect initial void ratios for the same compactive effort. Forthese materials, specimens prepared at void ratios of 1.0 or abovewere contractive in consolidated undrained triaxial shear. Belowa void ratio of 1.0, the materials were neutral to dilative.

Figure 16b shows bulking results presented by Bishop (1973)for waste from the Aberfan tip, which show the percent decreasein volume on saturation. Figure 17 shows results for triaxialshear tests on bulked colliery waste (Dawson et al 1998) which

was set up at a void ratio of 0.51, consolidated isotropically to0.40 under an effective stress of 200kPa and sheared undrained(although it is not clear at what stage the specimen wassaturated). The strain-softening behaviour was very similar tothat shown in Figures 10 and 11.

"Dry" mine wastes are usually deposited in a bulked conditionwithout compaction. Subsequent saturation by heavy orcontinuous rain or some other source of water can cause atendency for a sudden decrease in void ratio with its consequentstrain-softening loss of shear strength.

Fortunately, there is a current trend in South Africa for minesto compact their dry wastes. In the case of colliery wastes, this isdone to reduce the air permeability of the waste and thus preventspontaneous combustion, sustained by the entry of oxygen.


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Figure 15a: Typical cone penetration test in Merriespruit tailingsimpoundment.

Figure 15b: Variation of shear strength with distance from toe ofouter wall for Merriespruit tailings impoundment. Figure 16b: Bulking effects in coal waste from Aberfan.

Figure16a: Bulking curves for diamond tailings


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Figure 17: Typical isotropically consolidated undrained test forcoal mine waste (rock sandy gravel).

Some gold mines in South Africa sluice their coarse wastes withwaste mine water as a means of disposing of waste water. Thesluicing causes the rock to compact, reducing its tendency to

contract, but may unfortunately increase acid seepage from the base of the dumps, leading to undesirable surface and groundwater pollution.

2.4 Strain-softening or liquefaction of municipal solid waste

Largely because the phenomenon of flow failures in municipalwaste landfills has only recently become an obvious problem,relatively little is known of the strain-softening behaviour ofmunicipal solid waste (MSW). MSW is particularly difficult tocharacterise because of its heterogeneity and fibrous texturewhich makes it almost impossible to sample in an undisturbedcondition. Also, the properties of MSW change with age and the progress of decomposition. Although there were some published

data on strength parameters (e.g. Singh and Murphy 1990) it isonly recently that data have been published on volume and pore pressure changes during shear (Vilar and Carvalho 2002, Caicedoet al 2002). In particular, Caicedo et al performed consolidatedundrained triaxial tests on saturated 300mm diameter by 600mmhigh reconstituted specimens from the Dona Juana landfill inBogota (17, Table 1), obtaining the results shown in Figure 18.(The density of the specimens is not given.) The pore pressure behaviour is what would be expected of a high void ratiomaterial, increasing continuously with strain. But the shearstrength also increased continuously, the net effect being for theMSW to behave as if dilatant. These tests were terminated at anaxial strain of 13%. However, drained triaxial tests by Vilar andCarvalho (2002) on MSW from a landfill in Sao Paulo, Brazil

were taken to an axial strain of 40% without the shear strengthreaching a maximum, or the volume

Figure 18: Results of consolidated undrained triaxial shear test onreconstituted specimens of MSW measuring 300mm dia. by600mm high (Caicedo, etal, 2002).

contraction ceasing. Similar results were obtained in drainedtriaxial tests on reconstituted MSW specimens from the Bulbul

landfill in Durban, South Africa (18, Table 1).Hence at present there appears to be no clear evidence from

laboratory tests that MSW can be strain-softening. However,there is no doubt from the three (possibly four) flow failures inMSW landfills recorded (11, 17, 18 and possibly 22, Table 1)that MSW can strain-soften, resulting in flow failure.

3 DESCRIPTIONS OF TYPICAL FLOW FAILURES INTAILINGS IMPOUNDMENTS, "DRY" MINE WASTEDUMPS AND MUNICIPAL SOLID WASTE LANDFILLS

3.1 Tailings impoundments

3.1.1 Failure caused by seismic action

Common features of failures in tailings dams caused by seismicaction are (Troncoso, in Blight et al 2000):

.1 the presence of a large pond in the impoundment that hasencroached on the outer impoundment dyke;

.2 an outer dyke formed of loose, poorly compacted oruncompacted tailings sand that is contractive when subjectedto shear stress;

.3 poor separation of the sand used to build the impoundmentdyke from the silts stored within the impoundment, withweak lenses of silt included in the dyke; and

.4 dykes usually built (at least partially) by upstream deposition.


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When an earthquake of sufficient magnitude occurs, a failuredevelops as follows:

.1 the shear strains and the corresponding shear stresses imposed by the earthquake cause the weaker, fine, possibly partlyconsolidated tailings in the basin of the impoundment tostrain-soften. If the shear strength falls to a low enoughvalue,

.2 liquefied tailings and ponded water will move in waves,alternately drawing down and overtopping the upstream

slope and crest of the confining dyke; .3 the upstream slope of the dyke may slide into the

impoundment, and the dyke may crack; .4 when the wave of water and liquid tailings returns, it may

overtop the failed section of the dyke, eroding it and forminga breach, while water and liquid tailings may flow into andthrough cracks in the dyke, eroding and enlarging them;

.5 the downstream slope of the dyke may fail in shear, as a resultof strain-softening accompanied by erosion;

.6 as the breach in the dyke rapidly enlarges, the contents of theimpoundment flow out of the breach starting the tailingsflood, which is sustained by retrogressive liquefaction of thetailings within the impoundment (as illustrated by Figure 5);

.7 the failure process and flow of tailings cease once the shear

strains imposed by the earthquake diminish and a stablesurface profile is developed by the breached dyke and thetailings flood that has escaped from the impoundment. This profile must be sustainable by the reduced shear strength ofthe strain-softened tailings.

The El Cobre (Antiguo) failure (3(1), Table 1) is a goodexample of a failure caused by an earthquake (Dobry, Alvares1967). Figure 19 shows cross-sections through the side-hillimpoundment before and after failure. The impoundment wascommissioned in 1930, but after the Nuevo (new dam) (3(2),Table 1) was constructed in 1963, the Antiguo (old) dam wasused only periodically as a standby. The dyke had been built byupstream hydraulic filling, and the downstream slope of the dykewas 35m high at the time of the failure. The epicentre of the 7.5Richter magnitude La Ligua earthquake that resulted in thefailure was 70km from the dam with a focal point at a depth of61km.

It should be noted that after a failure, the flow of liquefiedtailings from the impoundment will continue until a surface profile compatible with the reduced strength of the tailings hasdeveloped. Once this stable surface has formed, loss of tailingsfrom the impoundment will cease. In the case of El Cobre(Antiguo) the average stable slope was about 3.5°, under staticconditions because the quaking had stopped. Any aftershockscould have resulted in further flattening of the profile, and furtherloss of tailings.

3.1.2 Flow failure resulting from static liquefaction

For a flow failure to occur as a result of a static liquefaction, theouter dyke of the tailings impoundment must be breached either by shear (e.g. Figure 2) possibly followed by overtopping, or by piping erosion followed by overtopping (e.g. Figure 3b), or byovertopping followed by erosion and shear failure (e.g. Figure 5).The formation of a breach in the outer dyke acts as a trigger forstrain-softening or liquefaction of the impounded tailings byimposing sudden shear strains in the tailings adjacent to the breach by the removal of lateral support. If certain layerssandwiched in the mass of tailings are susceptible to liquefaction,they lose strength and cause the adjacent, possibly dilative layers

(see Figure 15a) to disintegrate as well, with the result that asubstantial part of the total tailings mass moves towards and outof the breach. This process continues until the stable surface profile, compatible with the reduced strength of the tailings thatwas mentioned above, has developed. Note also, that the basinthat forms the source of the flow must not only be stable on theline of the breach (the exit direction of the escaping tailings), butalso transversely, i.e. the basin sides must everywhere develop astable slope before the tailings flow can cease.

The Merriespruit failure (13, Table 1 and Figure 4) is a goodexample of a flow failure that resulted from static liquefaction.On 22 February 1994 a rainstorm deposited 25mm of water on tothe Merriespruit gold tailings ring-dyke impoundment in the FreeState province of South Africa. A large quantity of water had been stored in the impoundment, reducing the free-board to anunknown, but small value. Shortly thereafter, as runoff fromrainfall on the impoundment surface concentrated in the pool, thedyke was overtopped and breached. A flow failure ensued that

Figure 19: Pre-and post- failure profiles of EI Cobre old dam

During the quake a cloud of dust arose from the dried surfaceof the only periodically used impoundment. The flow failurecontinued for 20 minutes after the quake had ended, as 1.9 x106m3 of a total storage of 4.25 x 106m3 of tailings flowed downa dry valley for a distance of 12km. A town in the path of theflow was annihilated with 300 deaths occurring.

As shown by Figure 19, the dam was constructed on slopingground with a slope angle of 3° and the average slope of the post-failure profile through the breach was only 3.5°. The flow wasreported to have covered its 12km course in a few minutes. Thisis too imprecise to allow the speed of the flow to be estimated, but it must have been about 20kmh-1 (see Section 4).

involved 600 000m3 of tailings and cut a swathe of destructionthrough the village of Merriespruit downhill of the tailings dam.Seventeen people were killed and scores of houses weredemolished and swept away by the flood. Eventually, the flowstopped about 2km from the breach when the tailings entered anornamental lake, constructed in a natural wetland.

After the afternoon rainstorm, clear water (presumably fromthe dam) ran through the streets of the village from about 7 p.m.to 9 p.m. when failure occurred. The failure was accompanied bya series of bangs. It was dark, but there was light from the moon.Unfortunately, eye-witness accounts as to how the failure took place do not give a consistent picture. The wall appears to have

disintegrated into a series of large slabs that crashed down,


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Figure 20: Sections through failure at Merriespruit showing post-failure equilibrium surface.

causing the noise and being followed by a wave of mud (seeFigure 5).

Figure 20 shows sections of the post-failure equilibriumsurface for the failure basin of the Merriespruit tailingsimpoundment. Section E'E' is the pre-failure section normal tothe wall and EE is a section through the breach. Section FFrunsat right angles to EE, and GG runs at 45° to EE. Theintersections of FF and GG with EE are marked in Figure 20. Theslope of the tongue of escaped tailings was 2°, which is verysimilar to the slope of large portions of FF and GG. In otherwords, the post-failure surface had flattened to a general slope of2°-3°, with some portions around the perimeter of the failure scar

being as steep as 10°-20°. Presumably, these areas had formedlate in the failure process, had been subjected to lesser shearstrains because they were not so high, and were therefore stableat steeper surface slopes.

Because of the disturbance caused by the failure, it is verydifficult to know from what depth in the impoundment thematerial that composes the post-failure surface originated. Thesurface is also too soft to be accessible after a failure until adrying crust has formed. Hence it is not possible to sample a post-failure surface straight after the failure to help identify itsdepth of origin. It seems likely, however, that the tailings thatmove out of the breach will consist of the upper, more recentlydeposited layers, and that the post-failure surface will consist ofdeeper layers exposed as the slope of the failure basin is flattened

by the outward flow of the tailings.For example, Figure 21 shows profiles of vane shear strengthmeasured in an operating gold tailings impoundment. In theevent of the outer dyke being breached, it is obvious from theirrelatively low strength that the top 10m of tailings would tend toflow off more readily than the deeper layers. Figure 21 alsodemonstrates the loss of in situ strength of the tailings whendisturbed, with a sensitivity ratio or strength reduction factor(undisturbed/remoulded strength) of about 2.7.

3.2 Flow failures of "dry" mine waste dumps

Perhaps the best example of a flow slide involving dry minewaste was the final of the series of three failures that occurred at

Aberfan (4, Table 1). Figure 22 shows a section through tips 5

Figure 21: Vane shear strength profiles measured in an operatinggold tailings impoundment

and 7 at Aberfan, 7 being the tip that failed and flowed in 1966.The colliery waste was tipped loosely by a mechanical tipper andthe slopes of the tip were at the angle of repose of the waste ofabout 37°. Under the toe of tip 7 was a spring, fed by water in the

underlying sandstone under artesian pressure between theuppermost coal seam and the surface layer of alluvial boulderclay, which acted as aquicludes. The height of the tip when thefailure occurred was about 67m from toe to crest.

The failure was probably initiated by a series of shallow slipstriggered by the artesian pressure of the spring and exacerbated by contraction of the loose, bulked waste as it became saturated by upward seepage from the spring. At 07.30 on the morning ofthe failure, the tipping gang found that the crest of tip 7 hadmoved downwards by 3m over a distance of 10 to 12m from theedge. By 08.30 this displacement had increased to 6m. At 09.10the toe of the tip started moving down the 12½° hillside andwithin a few minutes the rapid flow down the hillside hadcommenced. The flow travelled 1600m before reaching the

school which it destroyed, and came to a halt 350m furtherdownhill. Referring to Figure 6, at Aberfan road, the depth of the

1 2 3


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Figure 22: Section through tips 5 and 7 at Aberfanwaste was 9m. The speed of the flow was estimated to have been15 to 30kmh-1.

3.3 Flow failures in municipal solid waste

The flow failure at Istanbul (11, Table 1) will be taken as thearchetypical example of this type of flow failure (Kocasoy andCuri 1995). It is remarkable not only for the destruction itwrought, but also for the lack of common sense of the authoritiesthat established and operated the landfill. Figure 8 shows that thelandfill must have been sited where it was, purely for reasons ofexpediency. Given some flexibility in siting, no engineer in hisright mind would have sited a waste deposit on a 27° slope. The

waste was dumped near the edge of the slope, sorted through byinformal reclaimers (i.e. scavengers), and then pushed over theedge by dozer where it came to rest at an angle of repose of 45°.There was no attempt to compact the waste and no attempt tocover it either. As a result, the waste absorbed all the rain that fellon it, as well as the runoff from the dumping platform. The waste

was burning in several places and streams of noisome leachateissued from the toe of the dump and ran down the slope into thevalley bottom. In 1992 the "technical advisor" to the Mayor ofIstanbul decided that the waste should be covered, and later thatyear the site operator complied by covering the sub-horizontaltop platform with 3 to 5m of demolition wastes and soil. Thisadditional disturbing force was the straw that broke the camel's back.The failure took place in April 1993. Heralded by a loud bang,which was later ascribed (probably wrongly) to a methaneexplosion, 1.2 x 106m3 of waste rapidly moved down the valleyand was carried a short way up the opposite slope, where thehouses were situated that the slide demolished.

Whereas the failure at Istanbul took place as a result of acomplete lack of engineering or technical input or understanding,

the failure of the Dona Juana landfill in Bogota, Columbia (17Table 1) appears to have occurred as a result of a combination of poor design understanding and poor appreciation of operating principles (Hendron et al, 1999, Caicedo et al, 2002.)

.

Figure 23: Progression of failure of Dona Juana landfill (Hendron, etal, 1999).


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The zone of the landfill that failed (see Figure 23) was lined witha 1mm PVC geomembrane resting on either compacted clay or insitu soils. A sand drainage layer and a protective soil layer wereabove the liner. A horizontal soil cover layer was provided on topof each 2.5m lift of compacted waste, while the lifts of waste between cover layers were interconnected with rock-filled drainsto allow leachate to percolate downwards to the drain above theliner. There was also a passive gas venting system consisting ofvertical perforated pipes on a 50m grid. A leachate recirculationsystem was installed consisting of horizontal perforated pipes placed on top of each waste lift before placing the cover layer.The object of this piping was to inject leachate, collected fromthe base of the landfill, back into the waste, so as to operate thelandfill as a biological waste reactor, thus purifying the leachate before releasing it into the nearby river.

The investigation of the failure concluded that it had beentriggered by high liquid pore pressures caused by the re-injectionof leachate. The zone that failed was the only zone whereleachate recirculation had been applied. The design stabilityanalysis had assumed that no pore pressures would occur in thewaste. The inset on Figure 23 shows how the calculated factor ofsafety for the failed section must have declined as the wastethickness increased during the initial 22 months prior to the startof leachate injection (Caicedo et al 2002). The additional pore

pressures caused by re-injection caused the already low factor ofsafety to fall to 1.0 and the failure followed.

The failure investigation reached the obvious conclusion thatwhen designing a landfill where leachate is to be re-circulated, pore pressures must be properly evaluated and their effect must be considered in the stability analysis

Figure 24: Analysis of equilibrium of flowing waste

4 RELATIONSHIP BETWEEN GROUND AND POST-FAILURE SURFACE SLOPES AND TAILINGS-GROUNDINTERFACIAL SHEAR STRENGTH

Figure 24a shows the basis for a simple sliding block analysis tocalculate the relationship between the post-failure slope, ß, of atongue of escaped tailings, dry mine waste or municipal solidwaste, the slope, i, of the ground surface and the interfacial shearstrength, t, between the ground surface and the fugitive waste.Alternatively, the analysis can be used to calculate the shearstrength of the surface of the failure basin within a breached orfailed impoundment, dump or landfill, or the acceleration of aflow of material once it exits the boundary of the waste deposit(Blight et al 1981, Blight 1997).

For the potentially sliding block illustrated in Figure 24a,Downstream forces - upstream forces = mass of block xacceleration

i.e. (P1 - P2)cosi + Wsini - tL/cosi = W.a/g (1)

The symbols are defined in Figure 24a anda = acceleration of the block, g = gravimetric acceleration.From equation (1)

a = [(P1 - P2)cosi + Wsini - tL/cosi]g/W (1a)

If the block of material comes to rest, a = 0 and

t = [(P1 - P2)cosi + Wsini]cosi/L (1b)

If the block is accelerating, its increase in velocity after time ?twill be

?v = a?t (2)

In equation (1)

W = [2h - L(tanß - tani)? L/2 (3)and

h =H1 + L(tanß - tani)(4)

where ? ( is the bulk unit weight of the material in the block.)

If the surface of the flow (i.e. of the block) is parallel to theground surface, ß = i and P1 = P2 , h =H

1.If the pore water pressure in the block is taken as hydrostatic

with free water at the surface of the slide,

(P1 - P2) = (K?1 + ?w)[h

2 - (H1)2]/2 (5)

where ?1 is the effective unit weight, ?w is the unit weight ofwater and K is the active lateral pressure coefficient, K A.

In Table 2, equation (1b) has been applied to the surfaces ofsome failure basins of tailings impoundments (Blight 1997). Allof these failures have been listed in Table 1, except the Arcturusfailure that occurred in a gold tailings dam in Zimbabwe in 1978(Shakesby and Whitlow, 1991).

It is important to note that in all of these cases, the interfacialshear strength required for stability was relatively small whencompared (for example) with the values shown in Figure 21. Thissupports the view that very thin layers of low strength maygovern the overall strength of a sliding mass.

It should also be noted that if a liquefied waste flow debouchesonto wet ground, e.g. when failure follows a heavy rainfall, theinterfacial shear strength will be reduced by the water already atthe waste-to-ground surface interface, and the flow will be moremobile than if the ground surface had been dry. For example, inTable 2, the calculated interfacial shear strength at Saaiplaas for


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Table 1: 22 flow failures of mine waste tips (or dumps), tailings dams and municipal solid waste landfills that have resulted in deaths,major environmental damage, or major damage to structures and infrastructure(Note: Entries have been selected, list is not comprehensive)

Year & Number Location Waste Cause of Failure

Volume ofFlow Consequences

1928(1)

Barahona, Chile copper tailings 8.2 Richterearthquake

3 x 106m3 fine tailings environmentaldevastation

1961(2)

Jupille,Belgium

fly ash removal of toesupport of dump

100-150 x 103m3 flyash

11 deaths, housesdestroyed

1965(3)

El Cobre (2impoundments)

copper tailings 7.5 Richterearthquake

1) 1.9 x 106m3

2) 0.5 x 106m3fine tailings

300 deaths, village buried in tailings

1966(4)

Aberfan, UK coal waste dumping of wasteover spring

108 x103m3 waste 144 deaths, 116children, extensivedamage to property

1970

(5)

Mufulira

Zambia

copper tailings collapse of tailingsdam into workings

89 miners killedunderground

1972

(6)

Buffalo Creek,

USA

coal waste overtopping of waste

impoundment

500 x 103m3 water +

waste

118 deaths, 4 000

homeless, US$50million damage

1974(7)

Bafokeng,South Africa

platinum tailings overtopping oftailings dam

3 x 106m3 fine tailings 13 deaths, extensivedamage to mineinstallation andenvironment

1978(8)

Mochikoshi,Japan

gold tailings 7.0 Mercalliearthquake

80 x 103m3 finetailings

environmentaldevastation

1985

(9)

Stava, Italy fluorite tailings shear failure ofretaining dyke


268 deaths, extensivedamage to propertyand environment

1985(10)

QuintetteMaËmot, BC,Canada

coal waste pore pressureresulting fromcollapse settlement

2.5 x 106m3 environmentaldamage - river valleyfilled with waste for2.5km

1993(11)

Istanbul,Turkey(Umraniye-Hekimbasi)

municipal solidwaste

shear instability ofuncompacted waste

1.2 x 106m3 39 deaths, 11 housesdestroyed, mainsewer fractured,sewer flow dammed by slide debris

1993(12)

Saaiplaas,South Africa (3failures in 3days)

gold tailings high phreatic surfacein ring dyke

140 x 103m3 (slides 1& 2)140 x 103m3 (slide 3)

minimalenvironmentaldamage. Notreported by newsmedia

1994(13)

Merriespruit,South Africa

gold tailings overtopping oftailings dam


17 deaths, extensivedamage to housingand environment

1995(14)

Omai, Guyana gold tailings piping erosion ofretaining dyke

4.2 x 106m3 slurry 80km of riverdevastated

1995(15)

Surigao del Norte,Philippines

gold dyke failure 50 x 103m3 12 deaths, coastal pollution

1996(16)

Sgurigrad,Bulgaria

lead, zinc, copper overtopping ofretaining dyke

220 x103m3 107 deaths,environmentaldevastation


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Year & Number

Location Waste Cause of FailureVolume ofFlow

Consequences

1997(17)

Bogota,Colombia


pore pressure caused by recirculation ofleachate

800 x103m3 river dammed bydebris

1997(18)

Durban, SouthAfrica


pore pressure caused by co-disposal ofliquid wastes

160 x 103m3 slide containedwithin boundary ofsite

1998(19)

Los Frailes,Spain

lead, zinc, copper foundation failure oftailings dam

4 x 106m3 slurry environmentaldevastation

1999

(20)

Surigao del Norte,Phillippines

gold Tailings slurryescaping from burst pipe

700 x 103m3 17 houses destroyed,agricultural landdevastated

2000(21)

Inez, Kentucky,USA

coal wastes tailings dam failurefrom collapse ofundergroundworkings

950 x103m3 120km of riversdevastated by slurry

2000

(22)

Manila,

Philippines

municipal solid

waste

shear failure

following heavytyphoon rains

not known minimum of 218

deaths

At least 1 400 deaths in 72 years (a maximum of perhaps 20 per year) compared with thousands of millions killed by war, disease,famine, traffic accidents, etc. in the same period.

Table 1 was drawn from a number of sources, most of which appear in the reference list. For post 1991 failures, the list given byFahey et al (2002) has been useful.

Table 2: Summary of observed post-failure surface slopes andcorresponding ground/tailings interfacial shear strengths forflow failures in tailings impoundments

Tailings dam Post -failure

surfaceslope ß

Groundslope

i

At restinterfacial

shearstrength, t(kPa)

Bafokeng (Figure 2)Bafokeng (Figure 3)ArcturusSaaiplaas (Figure 9)

(After rain)(No rain)(No rain)

Merriespruit (Figure 4)(Flow slide)(Failure basin)

4°2°3°

3°2.3°3°

2°2°

1.5°1.3°1.5°

1°-0.5°-0.5°

1.5°0

5.21.62.6

2.33.43.6

1.01.8

the failure that occurred after rain was 65% of that corresponding

to flows over a dry ground surface. At Merriespruit, the fugitivetailings flow over wet ground had an interfacial shear strength of55% of that of the final surface of the failure basin. Figures 24band c show some data on the shear strength required for stability(zero acceleration) on various ground slopes (b) and also theacceleration that will occur if these shear strengths are not met(c). The data correspond to a simple case in which the surface ofthe flowing waste is parallel to the ground surface, but viaequation (2) give some idea of the speed with which a flow slidecan move. For example, if the acceleration from rest is only0.1ms-2 and this is maintained for 1 minute, the flow willaccelerate to 6ms -1 or 20kmh-1 in this period. The consequencesof higher rates of acceleration are frightening. In the flow failureat Bafokeng, the flow velocity a short distance after leaving the

breach in the impoundment was estimated from stagnation flowheights on damaged buildings (by equating the potential energy

of the stagnation height against the building to the kinetic energyof adjacent unimpeded flow) (Blight, Robinson, Diering 1981) tohave been 10ms -1 or 36kmh-1, even though the ground surface was almost level. Hence the lower accelerations shown on Figure24 appear to be realistic.

A similar approach to estimating flow velocity can be appliedin cases where a downhill flow crosses a valley and stagnates at a

given elevation on the opposite slope, as in the Istanbul MSWflow. Here, the flow reached stagnation at an elevation of 15mabove the bottom of the valley. Assuming the bulk density ( ofthe liquefied waste to have been 1 000kgm-2, an approximateenergy balance per m3 of waste would be:

½ ? v2 = ?g?h or v = (2g?h)½ (6)

where v is the velocity of flow at the bottom of the valley and ?his the stagnation height above the bottom of the valley. For theIstanbul case, ?h = 15m and the (minimum) v = 17ms -1, or60kmh-1. This ignores energy consumed in overcoming shear atthe interface of the hillside and the flowing waste. Applying thesame reasoning to the flow at Aberfan, if the stagnation height is

taken as 9m, the minimum speed of the flow would have been13ms-1 or 48kmh-1, whereas the speed was estimated to have been 15 to 30kmh-1.

The basis of the sliding block analysis (above) can also be usedto design protection measures such as deflection dykes and safety platforms to protect installations from the effects of waste flows(e.g. Blight, Robinson, Diering 1981, Miao et al 2001).Obstructions such as these can give very effective protection. Forexample, in the Aberfan slide, of the 118 x 103m3 that participated in the slide, only 42 x 103m3 crossed the railembankment between the village and the waste tip. If the railembankment had been designed as a safety barrier and beenconstructed higher, it could have stopped or deflected the flow,saving the village from devastation.


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5 PREVENTING FLOW SLIDES IN THE FUTURE - SITING,DESIGN, OPERATION, REVIEW AND AMENDMENT

Waste deposits are among the most difficult of geotechnicalstructures to design, manage and operate:

? Most tailings impoundments, mine waste dumps or landfills,have an operational life of 30 years or more.

?

During their operational life, they are continually underconstruction, and will experience several complete turnovers

of design, supervisory and operating staff.? Most of them have to be designed and commissioned before

the material they are intended to store has been produced.? In most cases the characteristics of the waste will change with

time, as the geology of the ore body varies and metallurgical processes are changed.

? Many of them will eventually be constructed to heights, or willextend laterally to extents not envisaged when they were planned.

?

In mining operations, waste disposal is at the tail end of the process, and is a source of cost, not revenue. Waste disposalis therefore low on the list of priorities, both in terms ofcapital and running expenditure, and in terms of the qualityof operating staff assigned to waste disposal.

?

At the end of the operating life, the waste deposit is still there,and has to be closed, rehabilitated, maintained andmonitored for periods often thought of in terms of decadesor centuries, but in reality, in perpetuity. There is no walk-away solution to closure. For example, in Johannesburg,tailings dams and mine waste dumps operated by companiesthat ceased to exist before the end of the 19th century, arestill causing pollution and nuisance at the start of the 21st.

Many considerations are obvious from the above points, othersnot so obvious, as will be seen below. However, the prime causesof disasters involving waste deposits are the financial greed ofthe owners, the mental and physical sloth of the operators, and inthe case of landfills, vote-seeking by local politicians (which in

most forms of democracy translates into personal financialgreed).In reviewing the failures at Bafokeng, Saaiplaas and

Merriespruit, the first author (Blight 2000) pointed out that thesefailures were not the result of unknown geotechnical factors, ordesign faults (although it must be noted that in all three cases siteinvestigation and design studies had been perfunctory). All threewere the result of poor operation, lack of proper management andcost saving pressures applied by the mines involved to thecontractor operating the tailings impoundments. (The fact that thesame contractor was involved in all three failures, points upWinston Churchill's observation that all we learn from history isthat we do not learn from history.)

5.1

Siting

Many waste deposits whether of hydraulic fill tailings, "dry"mine waste or municipal solid waste are sited in positions thatinvite the occurrence of disasters. Examples are the Jupille,Aberfan and Quintette Marmot waste dumps (2, 4 and 10, Table1), the El Cobre, Mochikoshi, Stava and Merriespruit tailingsimpoundments (3, 8, 9 and 13, Table 1), all of which were sitedon hillsides or hill crests above villages, the Bafokeng (7, Table1) tailings impoundment, sited 200m from an unprotected mineshaft and the Istanbul MSW dump (11, Table 1) sited on the crestof a very steep slope. These are obviously unacceptable sites forstructures of this type. In all likelihood, most of these sites werechosen for reasons of cost saving, or to use land that was

regarded as waste land, unsuitable for any other use.

Examples of "waste land" that is still often used for wastedisposal, but should never be so used are:

? steep hillsides or the crests of hills above steep hillsides,?

water-logged swampy areas, or areas crossed by streams,?

areas below the 500 year flood level,? undermined areas, and? areas crossed by usually dry valleys that could convey raging

torrents in exceptionally wet weather.

Side-hill dumps are often opted for because the top of a ridgemay be easily accessible, and dumping can proceed by buildingout a horizontal platform using edge-tipping with gravity totransport the waste down the hill, over the "wasteland". This wasthe reason for the choice of the Istanbul site and several otherslike it, as well as the Quintette Marmot site.

The Durban Bulbul landfill (18, Table 1) was sited in a steep-sided valley. This caused seepage from the hillside to be directedtowards the waste body in addition to providing a steep base forthe landfill to rest on.

Siting of waste deposits in swampy areas has been the rootcause of many failures (e.g. Blight 1997). In 1970 a tailings damcollapsed into underground workings in Zambia, trapping andkilling 89 miners in the workings, and this was also the cause of

the failure at Inez, Kentucky (21, Table 1) in 2000. TheBafokeng tailings dam was sited with one of its outer dykes onthe bank of a dry valley, and it was the presence of this valley,carrying water after rain, that caused the 42km long flow of theescaped tailings.

5.2 Design

Because of the long-term nature of waste deposition operations,and because the characteristics of the waste will inevitablychange during the deposit's operating life, pre-constructiondesigns are really site preparation designs, based on availableknowledge of the waste characteristics. Design for stability must be reviewed and, if necessary, revised once the installation isoperating, waste has been deposited and its in situ propertieshave been measured. It is also quite likely that the envisagedmethod of deposition will prove unsuccessful or unsuitable andwill have to be changed. For example spigot deposition ofunthickened tailings from a ring delivery main may be replaced by paddock deposition or thickened tailings, or placing of drywaste by mechanical stacker may be replaced by spreading from bottom-dump trucks.

However, to avoid failure of a (suitably sited) waste deposit,and in particular, failure resulting in a destructive flow, thedesign should provide for:

?

holding an absolute minimum of water on the deposit, and thefacility for rapid drainage of rainfall and run-on water

during and after the design storm;?

compacting or densifying the waste to above the criticaldensity, so that it is not contractive under the application ofshear stresses;

? outer slopes that are flatter than those calculated for anacceptable factor of safety against shear failure (it must beremembered that the outer slopes will need to berehabilitated, and that for vegetation to be stable, andsurface erosion minimal, the maximum outer slope shouldnot exceed 15°);

?

the installation of an instrument system (piezometers,inclinometers, etc.) that will enable pore pressure conditionsas well as movements in the waste to be monitoredcontinuously during operation and after closure.


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5.3

Operation

It must be recognized that waste deposits are complex structuresthat need careful and intelligent operation. Every waste depositshould have its own operating manual that is regularly updated asconditions change and operating experience is gained. Theoperating manual should include both "do's" and don't's" andmust have sections covering recognition of the development orexistence of dangerous and emergency situations, emergency procedures, public warning systems, etc. However, even the bestoperating manual is completely useless if it stays unread on the bookshelf of the waste disposal manager. Because staff changecontinually, and because people forget, regular refresher courseson operating procedures should be given to the operating staff,and summaries of the emergency procedures must be posted prominently at the workplace where they can be read orconsulted.

5.4 Review

Reviewing and measurements of at least the following should bemade six-monthly:

?

Properties of wastes disposed (grading, shear strength,consolidation parameters of the waste for mine wastes andcomposition for municipal solid waste).

?

Properties of wastes as placed (slurry density, beach slopes andgradings down the beach, in situ shear strength and drydensity for tailings, in situ densities and water contents fordry mine wastes, in situ densities for MSW).

?

Dimensions of deposit (slope angles, heights and rates of rise).? Effluents from deposits (quantities and rates of flow for return

water from tailings dams, rates of flow of leachate fromlandfills, seepage from all waste deposits, erosion fromslopes).

?

Weekly maximum pool levels and minimum freeboards.? Weekly return water reservoir or leachate pond levels.?

Measurements from instruments (pore pressure, settlement,movement of slopes).? Meteorological data, rainfall, evaporation, wind speed and

direction.?

Seismic data (whether natural or seismically induced.)? A detailed site inspection by an independent engineer or panel

of engineers.

The design should then be reviewed by the engineer orengineering panel in the light of the current design for the wastedeposit, including reviews of:

?

the water balance for the deposit;?

the stability of the slopes in terms of geometry, height, rate ofrise, in situ shear strength and results of instrument

measurements;? minimum free boards and maximum return water reservoir

levels.

Any deficiencies in the performance of the deposit or itsoperation must then be corrected immediately, and thecorrections reported at the next review. If and where necessary,amendments must be made to the design and to the operatingmanual, for immediate implementation.

6. CONCLUDING REMARKS

Tailings impoundments, dry mine waste dumps and landfills aredifferent from natural slopes in that they all are, or should be

engineered structures that have been suitably sited on preparedsites, designed for stability and constructed under careful andcontinuing supervision and design review. Whereas a decade orso ago, regulations relating to these structures were minimal andthose regulations that existed were often laxly applied, attitudesnow appear to have improved. Mining companies appear to beadopting more responsible attitudes to both public safety andenvironmental issues, and in most parts of the world, regulationsare more comprehensive and better enforced.

Accidents will, however, still happen if the mining andgeotechnical engineering professions do not continually remainvigilant, and alive to the development of dangerous situations or practices.

Finally, we quote a statement made by the first author in 1979,which is as applicable 24 years later as when it was written(Blight 1979):

"The design, construction and control of deposits of waste fallswithin the area of responsibility and the field of competence ofthe professional civil and mining engineer and is thereforesubject to the moral standards and ethics accepted by membersof the engineering profession. Professional engineers have amoral obligation not only to their employers and clients, butalso to the country, the public at large and to the future

generations who will inherit their works. ...Dirt, muck, mess, pollution and desolation are not

inseparable from mining activities. With modern technologyand modern knowledge of geotechnology, plant biology,surface and groundwater hydrology, soil chemistry and otherapplied sciences, the worst aspects of waste disposal can bemitigated and some adverse effects can be entirely eliminated.

However, if the ideal situation is to be approached, ourattitudes must change. Mining and industrial corporations, the professions and government agencies must unite andcollaborate to bring the disposal of waste within an acceptableframework of control.

It will be noted that government agencies have beenmentioned last in the above sentence. It is firmly believed that

the initiative in formulating clear and practical guidelines forwaste disposal should be taken by industry, who must pay forthe cost of environmental protection measures, and the professions, who must plan, design, institute and control thosemeasures. ...

It is well to concede at this point that any mining orindustrial activity will inevitably cause some environmentaldamage. The overall benefit to the country must be offsetagainst this damage. It must also be recognized that whatevercontrol measures are instituted, due regard must be paid tolocal conditions and current circumstances. The costs of thewaste disposal operation in relation to the revenue-producingoperation that must pay for it, the practicability of theenvironmental protection measures proposed, and the short andlong-term consequences of these measures, both for the safety

of the public and for their quality of life, must all receivecareful and due consideration."

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Blight, G.E. 1979. Editorial: The disposal of mining andindustrial waste. The Civil Engineer in South Africa, June: 133.

Blight, G.E. 1990. The effect of dynamic loading on underground

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Singh, S., Murphy, B.J. 1990. Evaluation of the stability ofsanitary landfills. In: Geotechnics of Waste Fills - Theory and Practice, ASTM STP 1070, Edited: Landva, A., Knowles,G.D., American Society for Testing and Materials,Philadelphia, USA: 240-258.

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Documents

A Review of Catastrophic Flow Failures of Deposits of Mine Wate