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A REVIEW OF CATASTROPHIC FLOW FAILURES OF DEPOSITS OF MINE WASTE AND MUNICIPAL REFUSE G.E. BLIGHT & A.B. FOURIE University of the Witwatersrand, Johannesburg, South Africa ABSTRACT: Catastrophic flow failures occur in mine tailings dams and dumps of discards and other mine waste with alarming frequency. In recent years catastrophic flow failures have also occurred in dumps of municipal refuse and even in what were considered 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 the Umraniye-Hekimbasi refuse dump in Turkey in 1993, killed 39 people, destroying their homes in the process. The paper will briefly review some of the more typical flow slides in waste materials, analysing the mechanics of failure and pointing to ways of preventing this 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 that serves to retain the body of tailings, supernatant water (and, occasionally, storm precipitation) upstream of it. If, for any reason, this outer impoundment is breached there will be the danger that the impounded tailings will escape the impoundment. Figure 1 shows statistics for 184 incidents involving tailings dams, collected by the US National Committee on Large Dams (1994). Here, the definition of "failure" is "any breach in the embankment leading to a release of the impounded tailings". The 184 incidents were not all failures, nor were the failures all flow failures. The statistics, however, show that, of the known causes of failures or breaches, the most likely to occur are slope instability, earthquakes and overtopping, in that order. Foundation and seepage failures come next, followed by structural failures. The meanings of the latter three categories are not 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 could result in breaching of an impoundment. Slope instability may result in a distortion or flattening of the slope of the retaining impoundment, without any escape of impounded tailings, or the tailings may move only a short distance beyond the impoundment wall. An example of a dyke failure 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 escaped the impoundment. However, a year later the same dyke was breached catastrophically as a result of overtopping, releasing 3x106m3 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 obviously occur before tailings can escape. At Bafokeng, overtopping probably resulted in breaching of the dyke by erosion (see Figure 3b for mechanism), but 20 years later, the disastrous flow failure that occurred from the Merriespruit tailings dam (also in South Africa), Figure 4 (13, Table 1), resulted from breaching of the dyke by overtopping, causing slope erosion and progressive shear failure, as shown in Figure 5 (Wagener et al 1997, Blight 2000). Figures 3a and 4 illustrate the characteristics of a tailings dam flow failure. These include liquefaction of a large volume of the retained tailings, which flow out of the breach as a viscous liquid and 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 106m3 was stopped when the flow entered a water retaining dam. The flow

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Figure 2: Failure (or slump) of retaining dyke of Bafokeng tailings dam (South Africa) that did not result in escape of tailings (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 containments designed to retain the consolidating solids and supernatant and storm water. However, numbers of flow failures have also occurred in mechanically placed "dry" mine waste deposits. The prime example of a flow failure in a "dry" mine waste occurred at the village of Aberfan, Wales (4, Table 1). Here, in 1966, a dump or tip of coal waste failed, liquefied (largely as a result of dumping waste over a spring) and flowed into the village of Aberfan, killing 144 people of whom 116 were school children (Anonymous 1967, Bishop, 1973). Figure 6 shows the course of the 1966 flow slide. The figure also shows that the Aberfan tip had failed and flowed twice previously, in 1944 and 1963, but these earlier flows did not reach the village and did not serve as sufficient 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 in Jupille, Belgium (Bishop 1973) (2, Table 1) is another archetype of a flow failure in "dry" material. Figure 7a shows a plan of the course of the flow which travelled down a dry valley for 0.5km. At Jupille, it appears that the ash may have been fluidized by air contained by its pores when the fly ash contracted during the failure. It was reported that fly ash that entered houses, overwhelmed in the flow, appeared to be "dry". Figure 7b shows that as the ash flowed down a natural valley it "lined" the valley with ash, the stream of fluid ash eventually flowing in a "canal" of solid ash. Of course, this also happens to some extent with flows of wet materials: the course of the flow is marked by material stranded as the main flow passes. During the 42km long flow at Bafokeng an estimated 1 x 106m3 of the 3 x 106m3 of tailings 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 municipal solid 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 at Merriespruit, 1994.

Figure 6: Flow failures of coal waste at Aberfan, Wales, 1944, 1963, 1966. numbers of landfills have not, until recently, reached a large size or because failures that did occur caused no deaths and therefore were not newsworthy and were not reported. However, in 1977 a flow failure took place in a landfill at Sarajevo (Gandolla et al 1979), and in 1993 a massive flow failure took place in the Umraniye-Hekimbasi refuse dump in Istanbul, Turkey (Kocasoy, Curi 1995) (11, Table 1). Figure 8 shows sections through the dump before and after the failure, as well as the course of the debris flow. After reaching the bottom of the valley, the momentum 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 of sewage on the upstream side of the obstruction. Since this occurrence, two and possibly three more flow failures of municipal solid waste deposits have been reported (Hendron et al 1999 (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 waste landfills 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, the volumes of material involved and the consequences. The death statistics at the foot of the table (1400 deaths in 72 years) also show that flow slides are not particularly dangerous occurrences. A single flying accident can cause 400 deaths, and we expect to have at least one or two of these per year, yet commercial air travel is not considered dangerous, nor is travel by road, even though the annual road death toll is hundreds of thousands. Not all flow failures of waste deposits make headlines, and regrettably as mentioned above, some failures may never reach the news and are never recorded in the geotechnical literature. Figure 9 (Blight 2000) (12, Table 1) shows three flow failures that occurred in a tailings impoundment at Saaiplaas, South Africa, in three days. Because the failures caused no deaths or

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Figure 8: Failure of Umraniye-Hekimbasi municipal solid waste dump, Istanbul, Turkey. injuries and the flows were confined to mine property, this incident was never reported by the news media, and never comprehensively investigated. The Saaiplaas impoundment is only a few km from the Merriespruit impoundment that failed a year later. If the Saaiplaas failure had been publicized, it may have served as a warning to the operators of other tailings dams in the area to inspect their dams carefully for safety, and the Merriespruit disaster might have been avoided. However, most of us are confident that we are immune from disasters that befall others, so it is more likely that the warning would have been ignored, as in the case of Aberfan. 2 STRAIN-SOFTENING OR LIQUEFACTION OF MINE

AND MUNICIPAL WASTES 2.1 The mechanics of strain-softening or liquefaction When a particulate material, be it a soil, tailings, dry mine waste or municipal solid waste, is subjected to shear stresses, it will tend to change volume and hence void ratio. Dense materials will tend to dilate, loose materials will tend to contract and materials of intermediate, or near critical state density will have little tendency to change volume. The consequences of this behaviour when a saturated material is sheared undrained, are illustrated by Figure 10 (after Castro 1969). The important features of Figure 10, 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) showing

locations 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 (or

strain-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 shape of the stress-strain curves of consolidated undrained tests on saturated sand. (Castro, 1969) increasing strain. The strength of the intermediate specimen remained more or less constant. .3 In the loose specimen, the pore pressure rapidly rose with

increasing strain, to reach a constant maximum. In the dense specimen, after rising to a peak, the pore pressure reduced continuously and the shear strength increased.

The loss of shear strength of the loose specimen from 180kPa to 20kPa after a strain of 3 to 5% constitutes strain-softening which in an extreme state can be called liquefaction. Because the shear stress was applied monotonically, this is termed static strain-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 of Figure 10. These tests, though, also show that if a loose material fails under stress-controlled conditions, which is usually the case in a slope failure, the failure can occur very rapidly, in fact, almost instantaneously as the shear stress is applied. Figure 11b shows the stress paths for the tests of Figure 11a, illustrating that the ultimate effective stress state reached in these tests lies on the Kf or failure line for strain-controlled tests. The behaviour of a loose saturated sand silt under dynamically applied shear stress is illustrated by Figure 12 (Blight 1990). Each application of the shear stress of 300kPa caused an increment of pore pressure that reduced the mean effective stress, until application 13 moved the stress path onto the Kf or failure line. On stress application 14 it was not possible to reach the shear stress of 300kPa. If the test had been continued past stress application 14, the ultimate condition would have been reached, with the stress path on the Kf-line, a constant mean effective stress, and a very low shear strength. 2.2 Strain-softening or liquefaction of tailings Tailings are usually deposited as slurries and settle out as the tailings 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 successive deposition 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-controlled anisotropically consolidated undrained tests on saturated loose sand ( s 31

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

Figure 12: Stress path for dynamic shear test on loose saturated natural sandy silt. weather, varying thickness of layers of deposition, demands for increased deposition rates at times of increased production to meet market demands, etc., the degree of densification inevitably varies both with time and position on the tailings dam (both in elevation and plan). For example, for a year before the three failures on the Saaiplaas No. 5A dam (Figure 9) took place, the rate of rise of the dam had been increased from its long-term average of 1.8m/y to 2.6m/y and shortly before the failures occurred, 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 of successive layers of tailings may be, and usually is highly variable. The effects of this variable density on the measured shear strength of tailings are illustrated by Figures 14 and 15 (Blight 1997, Fourie et al 2001). Figure 14a shows stress paths

Deviator Stress (kPa)

Pore Pressureu (kPa)

Deviator Stress (kPa)

Pore Pressureu (kPa)

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Figure 13: Densification of tailings slurry by drying shrinkage

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

Figure 14b: Effective stress changes in undisturbed specimens during unconsolidated undrained triaxial compression. for four consolidated undrained triaxial shear tests on 38mm diameter x 76mm high samples taken from a single Shelby tube sample from the Merriespruit tailings dam (see Figures 4 and 5). The two tests at consolidation stresses of 50 and 100kPa showed contractive behaviour, the one at 200kPa showed critical state behaviour, while that at 400kPa was weakly dilative. Figure 14b summarizes changes in effective stress from the start of shearing to the ultimate state for 16 consolidated undrained shear tests on Shelby tube specimens from Merriespruit having various void ratios. The specimens were tested under their original in situ effective overburden stress. While nine of the specimens dilated during 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 the large scale failure, carrying other denser layers with them.

Figure 15a shows the results of a piezo-cone penetrometer test conducted on the Merriespruit impoundment after the failure illustrated by Figures 4 and 5. The cone penetration resistance fluctuated over a range of up to 2MPa as the cone penetrated successive layers of tailings. The pore pressure, in sympathy, showed low or even negative values as dense, dilative layers were penetrated, and high values as loose contractive layers were encountered. Figure 15b summarizes the results of 16 piezo-cone penetrometer tests at Merriespruit, made at various distances from the toe of the dam. Each cone penetration profile has been characterized by its maximum and minimum slopes in terms of penetration resistance per unit depth (in kPa/m). There was a considerable difference between these two slopes, and both the slopes and the difference between maxima and minima decreased with distance from the toe. However, there was no sudden change in penetration characteristics between tailings forming the outer slope of the impounding dyke and those contained in the interior of the impoundment. In other words, this was not a case of a consolidated outer embankment retaining a partly consolidated semi-fluid core. Certain layers of tailings forming the beach of the dam must have suffered static strain-softening or liquefaction 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 deposited without compaction, a phenomenon long known in concrete technology with relation to volume batching of aggregate. In general terms, if a given mass of dry cohesionless sand or gravel is deposited loosely, it will assume a certain volume and void ratio. If water is gradually added, the volume of the mass (and hence its void ratio) will increase up to an optimum water content after which the volume will decrease again. When the material is saturated, it will have approximately the same volume and void ratio as when it is dry. Bulking is well illustrated by the results shown 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 void ratios, but regardless of initial void ratio, showed much the same maximum increase in void ratio as bulking proceeded. At water contents approaching saturation, the void ratios were much the same as the initial values. Note that the sand content of the material had little effect on the bulking, but the addition of sand did affect initial void ratios for the same compactive effort. For these materials, specimens prepared at void ratios of 1.0 or above were contractive in consolidated undrained triaxial shear. Below a 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 decrease in volume on saturation. Figure 17 shows results for triaxial shear tests on bulked colliery waste (Dawson et al 1998) which was set up at a void ratio of 0.51, consolidated isotropically to 0.40 under an effective stress of 200kPa and sheared undrained (although it is not clear at what stage the specimen was saturated). The strain-softening behaviour was very similar to that shown in Figures 10 and 11. "Dry" mine wastes are usually deposited in a bulked condition without compaction. Subsequent saturation by heavy or continuous rain or some other source of water can cause a tendency for a sudden decrease in void ratio with its consequent strain-softening loss of shear strength. Fortunately, there is a current trend in South Africa for mines to compact their dry wastes. In the case of colliery wastes, this is done to reduce the air permeability of the waste and thus prevent spontaneous combustion, sustained by the entry of oxygen.

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Figure 15a: Typical cone penetration test in Merriespruit tailings impoundment.

Figure 15b: Variation of shear strength with distance from toe of outer 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 for coal mine waste (rock sandy gravel). Some gold mines in South Africa sluice their coarse wastes with waste mine water as a means of disposing of waste water. The sluicing 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 ground water pollution. 2.4 Strain-softening or liquefaction of municipal solid waste Largely because the phenomenon of flow failures in municipal waste landfills has only recently become an obvious problem, relatively little is known of the strain-softening behaviour of municipal solid waste (MSW). MSW is particularly difficult to characterise because of its heterogeneity and fibrous texture which makes it almost impossible to sample in an undisturbed condition. 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 is only recently that data have been published on volume and pore pressure changes during shear (Vilar and Carvalho 2002, Caicedo et al 2002). In particular, Caicedo et al performed consolidated undrained triaxial tests on saturated 300mm diameter by 600mm high reconstituted specimens from the Dona Juana landfill in Bogota (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 ratio material, increasing continuously with strain. But the shear strength also increased continuously, the net effect being for the MSW to behave as if dilatant. These tests were terminated at an axial strain of 13%. However, drained triaxial tests by Vilar and Carvalho (2002) on MSW from a landfill in Sao Paulo, Brazil were taken to an axi al strain of 40% without the shear strength reaching a maximum, or the volume

Figure 18: Results of consolidated undrained triaxial shear test on reconstituted specimens of MSW measuring 300mm dia. by 600mm high (Caicedo, etal, 2002). contraction ceasing. Similar results were obtained in drained triaxial 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 in MSW 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 IN

TAILINGS IMPOUNDMENTS, "DRY" MINE WASTE DUMPS 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 seismic action are (Troncoso, in Blight et al 2000): .1 the presence of a large pond in the impoundment that has

encroached on the outer impoundment dyke; .2 an outer dyke formed of loose, poorly compacted or

uncompacted tailings sand that is contractive when subjected to shear stress;

.3 poor separation of the sand used to build the impoundment dyke from the silts stored within the impoundment, with weak 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 failure develops as follows: .1 the shear strains and the corresponding shear stresses imposed

by the earthquake cause the weaker, fine, possibly partly consolidated tailings in the basin of the impoundment to strain-soften. If the shear strength falls to a low enough value,

.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 forming a breach, while water and liquid tailings may flow into and through cracks in the dyke, eroding and enlarging them;

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

.6 as the breach in the dyke rapidly enlarges, the contents of the impoundment flow out of the breach starting the tailings flood, which is sustained by retrogressive liquefaction of the tailings 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 stable surface profile is developed by the breached dyke and the tailings flood that has escaped from the impoundment. This profile must be sustainable by the reduced shear strength of the strain-softened tailings.

The El Cobre (Antiguo) failure (3(1), Table 1) is a good example of a failure caused by an earthquake (Dobry, Alvares 1967). Figure 19 shows cross-sections through the side-hill impoundment before and after failure. The impoundment was commissioned in 1930, but after the Nuevo (new dam) (3(2), Table 1) was constructed in 1963, the Antiguo (old) dam was used only periodically as a standby. The dyke had been built by upstream hydraulic filling, and the downstream slope of the dyke was 35m high at the time of the failure. The epicentre of the 7.5 Richter magnitude La Ligua earthquake that resulted in the failure was 70km from the dam with a focal point at a depth of 61km.

It should be noted that after a failure, the flow of liquefied tailings from the impoundment will continue until a surface profile compatible with the reduced strength of the tailings has developed. Once this stable surface has formed, loss of tailings from the impoundment will cease. In the case of El Cobre (Antiguo) the average stable slope was about 3.5°, under static conditions because the quaking had stopped. Any aftershocks could have resulted in further flattening of the profile, and further loss of tailings. 3.1.2 Flow failure resulting from static liquefaction For a flow failure to occur as a result of a static liquefaction, the outer 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 by overtopping followed by erosion and shear failure (e.g. Figure 5). The formation of a breach in the outer dyke acts as a trigger for strain-softening or liquefaction of the impounded tailings by imposing sudden shear strains in the tailings adjacent to the breach by the removal of lateral support. If certain layers sandwiched 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 a substantial part of the total tailings mass moves towards and out of the breach. This process continues until the stable surface profile, compatible with the reduced strength of the tailings that was mentioned above, has developed. Note also, that the basin that forms the source of the flow must not only be stable on the line of the breach (the exit direction of the escaping tailings), but also transversely, i.e. the basin sides must everywhere develop a stable slope before the tailings flow can cease. The Merriespruit failure (13, Table 1 and Figure 4) is a good example of a flow failure that resulted from static liquefaction. On 22 February 1994 a rainstorm deposited 25mm of water on to the Merriespruit gold tailings ring-dyke impoundment in the Free State province of South Africa. A large quantity of water had been stored in the impoundment, reducing the free-board to an unknown, but small value. Shortly thereafter, as runoff from rainfall on the impoundment surface concentrated in the pool, the dyke 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 surface of the only periodically used impoundment. The flow failure continued for 20 minutes after the quake had ended, as 1.9 x 106m3 of a total storage of 4.25 x 106m3 of tailings flowed down a dry valley for a distance of 12km. A town in the path of the flow was annihilated with 300 deaths occurring. As shown by Figure 19, the dam was constructed on sloping ground with a slope angle of 3° and the average slope of the post-failure profile through the breach was only 3.5°. The flow was reported to have covered its 12km course in a few minutes. This is 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 destruction through the village of Merriespruit downhill of the tailings dam. Seventeen people were killed and scores of houses were demolished and swept away by the flood. Eventually, the flow stopped about 2km from the breach when the tailings entered an ornamental lake, constructed in a natural wetland. After the afternoon rainstorm, clear water (presumably from the dam) ran through the streets of the village from about 7 p.m. to 9 p.m. when failure occurred. The failure was accompanied by a 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 (see Figure 5). Figure 20 shows sections of the post-failure equilibrium surface for the failure basin of the Merriespruit tailings impoundment. Section E'E' is the pre-failure section normal to the wall and EE is a section through the breach. Section FF runsat right angles to EE, and GG runs at 45° to EE. The intersections of FF and GG with EE are marked in Figure 20. The slope of the tongue of escaped tailings was 2°, which is very similar to the slope of large portions of FF and GG. In other words, the post-failure surface had flattened to a general slope of 2°-3°, with some portions around the perimeter of the failure scar being as steep as 10°-20°. Presumably, these areas had formed late in the failure process, had been subjected to lesser shear strains because they were not so high, and were therefore stable at steeper surface slopes. Because of the disturbance caused by the failure, it is very difficult to know from what depth in the impoundment the material that composes the post-failure surface originated. The surface is also too soft to be accessible after a failure until a drying crust has formed. Hence it is not possible to sample a post-failure surface straight after the failure to help identify its depth of origin. It seems likely, however, that the tailings that move out of the breach will consist of the upper, more recently deposited layers, and that the post-failure surface will consist of deeper 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 strength measured in an operating gold tailings impoundment. In the event of the outer dyke being breached, it is obvious from their relatively low strength that the top 10m of tailings would tend to flow off more readily than the deeper layers. Figure 21 also demonstrates the loss of in situ strength of the tailings when disturbed, 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 mine waste 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 operating gold 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 and the slopes of the tip were at the angle of repose of the waste of about 37°. Under the toe of tip 7 was a spring, fed by water in the underlying sandstone under artesian pressure between the uppermost coal seam and the surface layer of alluvial boulder clay, which acted as aquicludes. The height of the tip when the failure occurred was about 67m from toe to crest. The failure was probably initiated by a series of shallow slips triggered 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 of the failure, the tipping gang found that the crest of tip 7 had moved downwards by 3m over a distance of 10 to 12m from the edge. By 08.30 this displacement had increased to 6m. At 09.10 the toe of the tip started moving down the 12½° hillside and within a few minutes the rapid flow down the hillside had commenced. The flow travelled 1600m before reaching the school which it destroyed, and came to a halt 350m further downhill. 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 Aberfan waste was 9m. The speed of the flow was estimated to have been 15 to 30kmh-1. 3.3 Flow failures in municipal solid waste The flow failure at Istanbul (11, Table 1) will be taken as the archetypical example of this type of flow failure (Kocasoy and Curi 1995). It is remarkable not only for the destruction it wrought, but also for the lack of common sense of the authorities that established and operated the landfill. Figure 8 shows that the landfill must have been sited where it was, purely for reasons of expediency. Given some flexibility in siting, no engineer in his right mind would have sited a waste deposit on a 27° slope. The waste was dumped near the edge of the slope, sorted through by informal reclaimers (i.e. scavengers), and then pushed over the edge 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 to cover it either. As a result, the waste absorbed all the rain that fell on it, as well as the runoff from the dumping platform. The waste

was burning in several places and streams of noisome leachate issued from the toe of the dump and ran down the slope into the valley bottom. In 1992 the "technical advisor" to the Mayor of Istanbul decided that the waste should be covered, and later that year the site operator complied by covering the sub-horizontal top platform with 3 to 5m of demolition wastes and soil. This additional 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 methane explosion, 1.2 x 106m3 of waste rapidly moved down the valley and was carried a short way up the opposite slope, where the houses were situated that the slide demolished. Whereas the failure at Istanbul took place as a result of a complete lack of engineering or technical input or understanding, the failure of the Dona Juana landfill in Bogota, Columbia (17 Table 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 with a 1mm PVC geomembrane resting on either compacted clay or in situ soils. A sand drainage layer and a protective soil layer were above the liner. A horizontal soil cover layer was provided on top of each 2.5m lift of compacted waste, while the lifts of waste between cover layers were interconnected with rock-filled drains to allow leachate to percolate downwards to the drain above the liner. There was also a passive gas venting system consisting of vertical perforated pipes on a 50m grid. A leachate recirculation system 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 from the base of the landfill, back into the waste, so as to operate the landfill 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 been triggered by high liquid pore pressures caused by the re-injection of leachate. The zone that failed was the only zone where leachate recirculation had been applied. The design stability analysis had assumed that no pore pressures would occur in the waste. The inset on Figure 23 shows how the calculated factor of safety for the failed section must have declined as the waste thickness increased during the initial 22 months prior to the start of leachate injection (Caicedo et al 2002). The additional pore pressures caused by re-injection caused the already low factor of safety to fall to 1.0 and the failure followed. The failure investigation reached the obvious conclusion that when 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-GROUND INTERFACIAL SHEAR STRENGTH

Figure 24a shows the basis for a simple sliding block analysis to calculate the relationship between the post-failure slope, ß, of a tongue of escaped tailings, dry mine waste or municipal solid waste, the slope, i, of the ground surface and the interfacial shear strength, t, between the ground surface and the fugitive waste. Alternatively, the analysis can be used to calculate the shear strength of the surface of the failure basin within a breached or failed impoundment, dump or landfill, or the acceleration of a flow 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 x acceleration i.e. (P1 - P2)cosi + Wsini - tL/cosi = W.a/g (1) The symbols are defined in Figure 24a and a = 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 ?t will 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 the ground surface, ß = i and P1 = P2 , h =H1. 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)[h2 - (H1)2]/2 (5) where ?1 is the effective unit weight, ?w is the unit weight of water and K is the active lateral pressure coefficient, KA. In Table 2, equation (1b) has been applied to the surfaces of some failure basins of tailings impoundments (Blight 1997). All of these failures have been listed in Table 1, except the Arcturus failure 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 interfacial shear strength required for stability was relatively small when compared (for example) with the values shown in Figure 21. This supports the view that very thin layers of low strength may govern the overall strength of a sliding mass. It should also be noted that if a liquefied waste flow debouches onto wet ground, e.g. when failure follows a heavy rainfall, the interfacial shear strength will be reduced by the water already at the waste-to-ground surface interface, and the flow will be more mobile than if the ground surface had been dry. For example, in Table 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 of

Flow Consequences

1928 (1)

Barahona, Chile copper tailings 8.2 Richter earthquake

3 x 106m3 fine tailings environmental devastation

1961 (2)

Jupille, Belgium

fly ash removal of toe support of dump

100-150 x 103m3 fly ash

11 deaths, houses destroyed

1965 (3)

El Cobre (2 impoundments)

copper tailings 7.5 Richter earthquake

1) 1.9 x 106m3

2) 0.5 x 106m3 fine tailings

300 deaths, village buried in tailings

1966 (4)

Aberfan, UK coal waste dumping of waste over spring

108 x103m3 waste 144 deaths, 116 children, extensive damage to property

1970

(5)

Mufulira

Zambia

copper tailings collapse of tailings dam into workings

89 miners killed underground

1972 (6)

Buffalo Creek, USA

coal waste overtopping of waste impoundment

500 x 103m3 water + waste

118 deaths, 4 000 homeless, US$50 million damage

1974 (7)

Bafokeng, South Africa

platinum tailings overtopping of tailings dam

3 x 106m3 fine tailings 13 deaths, extensive damage to mine installation and environment

1978 (8)

Mochikoshi, Japan

gold tailings 7.0 Mercalli earthquake

80 x 103m3 fine tailings

environmental devastation

1985

(9)

Stava, Italy fluorite tailings shear failure of retaining dyke

190 x 103m3 fine tailings

268 deaths, extensive damage to property and environment

1985 (10)

Quintette MaËmot, BC, Canada

coal waste pore pressure resulting from collapse settlement

2.5 x 106m3 environmental damage - river valley filled with waste for 2.5km

1993 (11)

Istanbul, Turkey (Umraniye-Hekimbasi)

municipal solid waste

shear instability of uncompacted waste

1.2 x 106m3 39 deaths, 11 houses destroyed, main sewer fractured, sewer flow dammed by slide debris

1993 (12)

Saaiplaas, South Africa (3 failures in 3 days)

gold tailings high phreatic surface in ring dyke

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

minimal environmental damage. Not reported by news media

1994 (13)

Merriespruit, South Africa

gold tailings overtopping of tailings dam

600 x 103m3 fine tailings

17 deaths, extensive damage to housing and environment

1995 (14)

Omai, Guyana gold tailings piping erosion of retaining dyke

4.2 x 106m3 slurry 80km of river devastated

1995 (15)

Surigao del Norte, Philippines

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

1996 (16)

Sgurigrad, Bulgaria

lead, zinc, copper overtopping of retaining dyke

220 x103m3 107 deaths, environmental devastation

14

Year & Number Location Waste Cause of Failure Volume of

Flow Consequences

1997 (17)

Bogota, Colombia

municipal solid waste

pore pressure caused by recirculation of leachate

800 x103m3 river dammed by debris

1997 (18)

Durban, South Africa

municipal solid waste

pore pressure caused by co-disposal of liquid wastes

160 x 103m3 slide contained within boundary of site

1998 (19)

Los Frailes, Spain

lead, zinc, copper foundation failure of tailings dam

4 x 106m3 slurry environmental devastation

1999

(20)

Surigao del Norte, Phillippines

gold Tailings slurry escaping from burst pipe

700 x 103m3 17 houses destroyed, agricultural land devastated

2000 (21)

Inez, Kentucky, USA

coal wastes tailings dam failure from collapse of underground workings

950 x103m3 120km of rivers devastated by slurry

2000 (22)

Manila, Philippines

municipal solid waste

shear failure following heavy typhoon 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 by Fahey et al (2002) has been useful. Table 2: Summary of observed post-failure surface slopes and corresponding ground/tailings interfacial shear strengths for flow failures in tailings impoundments

Tailings dam Post -failure surface slope ß

Ground slope i

At rest interfacial shear strength, t (kPa)

Bafokeng (Figure 2) Bafokeng (Figure 3) Arcturus Saaiplaas (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

5.2 1.6 2.6 2.3 3.4 3.6 1.0 1.8

the failure that occurred after rain was 65% of that corresponding to flows over a dry ground surface. At Merriespruit, the fugitive tailings flow over wet ground had an interfacial shear strength of 55% of that of the final surface of the failure basin. Figures 24b and c show some data on the shear strength required for stability (zero acceleration) on various ground slopes (b) and also the acceleration that will occur if these shear strengths are not met (c). The data correspond to a simple case in which the surface of the flowing waste is parallel to the ground surface, but via equation (2) give some idea of the speed with which a flow slide can move. For example, if the acceleration from rest is only 0.1ms-2 and this is maintained for 1 minute, the flow will accelerate to 6ms -1 or 20kmh-1 in this period. The consequences of higher rates of acceleration are frightening. In the flow failure at Bafokeng, the flow velocity a short distance after leaving the breach in the impoundment was estimated from stagnation flow heights on damaged buildings (by equating the potential energy

of the stagnation height against the building to the kinetic energy of adjacent unimpeded flow) (Blight, Robinson, Diering 1981) to have been 10ms -1 or 36kmh-1, even though the ground surface was almost level. Hence the lower accelerations shown on Figure 24 appear to be realistic. A similar approach to estimating flow velocity can be applied in cases where a downhill flow crosses a valley and stagnates at a given elevation on the opposite slope, as in the Istanbul MSW flow. Here, the flow reached stagnation at an elevation of 15m above the bottom of the valley. Assuming the bulk density ( of the liquefied waste to have been 1 000kgm-2, an approximate energy 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 ?h is the stagnation height above the bottom of the valley. For the Istanbul case, ?h = 15m and the (minimum) v = 17ms -1, or 60kmh-1. This ignores energy consumed in overcoming shear at the interface of the hillside and the flowing waste. Applying the same reasoning to the flow at Aberfan, if the stagnation height is taken as 9m, the minimum speed of the flow would have been 13ms-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 used to 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. For example, in the Aberfan slide, of the 118 x 103m3 that participated in the slide, only 42 x 103m3 crossed the rail embankment between the village and the waste tip. If the rail embankment had been designed as a safety barrier and been constructed 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 geotechnical structures 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 under

construction, 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 will extend 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 disposal is therefore low on the list of priorities, both in terms of capital and running expenditure, and in terms of the quality of 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 and monitored for periods often thought of in terms of decades or 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 companies that ceased to exist before the end of the 19th century, are still causing pollution and nuisance at the start of the 21st.

Many considerations are obvious from the above points, others not so obvious, as will be seen below. However, the prime causes of disasters involving waste deposits are the financial greed of the owners, the mental and physical sloth of the operators, and in the case of landfills, vote-seeking by local politicians (which in most forms of democracy translates into personal financial greed). In reviewing the failures at Bafokeng, Saaiplaas and Merriespruit, the first author (Blight 2000) pointed out that these failures were not the result of unknown geotechnical factors, or design faults (although it must be noted that in all three cases site investigation and design studies had been perfunctory). All three were the result of poor operation, lack of proper management and cost saving pressures applied by the mines involved to the contractor operating the tailings impoundments. (The fact that the same contractor was involved in all three failures, points up Winston Churchill's observation that all we learn from history is that 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 that invite the occurrence of disasters. Examples are the Jupille, Aberfan and Quintette Marmot waste dumps (2, 4 and 10, Table 1), the El Cobre, Mochikoshi, Stava and Merriespruit tailings impoundments (3, 8, 9 and 13, Table 1), all of which were sited on hillsides or hill crests above villages, the Bafokeng (7, Table 1) tailings impoundment, sited 200m from an unprotected mine shaft and the Istanbul MSW dump (11, Table 1) sited on the crest of a very steep slope. These are obviously unacceptable sites for structures of this type. In all likelihood, most of these sites were chosen 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 waste disposal, 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 ridge may be easily accessible, and dumping can proceed by building out a horizontal platform using edge-tipping with gravity to transport the waste down the hill, over the "wasteland". This was the reason for the choice of the Istanbul site and several others like 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 directed towards the waste body in addition to providing a steep base for the landfill to rest on. Siting of waste deposits in swampy areas has been the root cause of many failures (e.g. Blight 1997). In 1970 a tailings dam collapsed into underground workings in Zambia, trapping and killing 89 miners in the workings, and this was also the cause of the failure at Inez, Kentucky (21, Table 1) in 2000. The Bafokeng tailings dam was sited with one of its outer dykes on the bank of a dry valley, and it was the presence of this valley, carrying water after rain, that caused the 42km long flow of the escaped tailings. 5.2 Design Because of the long-term nature of waste deposition operations, and because the characteristics of the waste will inevitably change during the deposit's operating life, pre-construction designs are really site preparation designs, based on available knowledge of the waste characteristics. Design for stability must be reviewed and, if necessary, revised once the installation is operating, waste has been deposited and its in situ properties have been measured. It is also quite likely that the envisaged method of deposition will prove unsuccessful or unsuitable and will have to be changed. For example spigot deposition of unthickened tailings from a ring delivery main may be replaced by paddock deposition or thickened tailings, or placing of dry waste 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, the design should provide for: ? holding an absolute minimum of water on the deposit, and the

facility for rapid drainage of rainfall and run-on water during and after the design storm;

? compacting or densifying the waste to above the critical density, so that it is not contractive under the application of shear stresses;

? outer slopes that are flatter than those calculated for an acceptable factor of safety against shear failure (it must be remembered that the outer slopes will need to be rehabilitated, and that for vegetation to be stable, and surface erosion minimal, the maximum outer slope should not exceed 15°);

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

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5.3 Operation It must be recognized that waste deposits are complex structures that need careful and intelligent operation. Every waste deposit should have its own operating manual that is regularly updated as conditions change and operating experience is gained. The operating manual should include both "do's" and don't's" and must have sections covering recognition of the development or existence of dangerous and emergency situations, emergency procedures, public warning systems, etc. However, even the best operating manual is completely useless if it stays unread on the bookshelf of the waste disposal manager. Because staff change continually, and because people forget, regular refresher courses on 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 or consulted. 5.4 Review Reviewing and measurements of at least the following should be made six-monthly: ? Properties of wastes disposed (grading, shear strength,

consolidation parameters of the waste for mine wastes and composition for municipal solid waste).

? Properties of wastes as placed (slurry density, beach slopes and gradings down the beach, in situ shear strength and dry density for tailings, in situ densities and water contents for dry 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 from landfills, seepage from all waste deposits, erosion from slopes).

? 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 or engineering panel in the light of the current design for the waste deposit, including reviews of: ? the water balance for the deposit; ? the stability of the slopes in terms of geometry, height, rate of

rise, 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 its operation must then be corrected immediately, and the corrections reported at the next review. If and where necessary, amendments must be made to the design and to the operating manual, for immediate implementation. 6. CONCLUDING REMARKS Tailings impoundments, dry mine waste dumps and landfills are different from natural slopes in that they all are, or should be

engineered structures that have been suitably sited on prepared sites, designed for stability and constructed under careful and continuing supervision and design review. Whereas a decade or so ago, regulations relating to these structures were minimal and those regulations that existed were often laxly applied, attitudes now appear to have improved. Mining companies appear to be adopting more responsible attitudes to both public safety and environmental issues, and in most parts of the world, regulations are more comprehensive and better enforced. Accidents will, however, still happen if the mining and geotechnical engineering professions do not continually remain vigilant, 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 falls within the area of responsibility and the field of competence of the professional civil and mining engineer and is therefore subject to the moral standards and ethics accepted by members of the engineering profession. Professional engineers have a moral obligation not only to their employers and clients, but also 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 technology and modern knowledge of geotechnology, plant biology, surface and groundwater hydrology, soil chemistry and other applied sciences, the worst aspects of waste disposal can be mitigated and some adverse effects can be entirely eliminated. However, if the ideal situation is to be approached, our attitudes must change. Mining and industrial corporations, the professions and government agencies must unite and collaborate to bring the disposal of waste within an acceptable framework of control. It will be noted that government agencies have been mentioned last in the above sentence. It is firmly believed that the initiative in formulating clear and practical guidelines for waste disposal should be taken by industry, who must pay for the cost of environmental protection measures, and the professions, who must plan, design, institute and control those measures. ... It is well to concede at this point that any mining or industrial activity will inevitably cause some environmental damage. The overall benefit to the country must be offset against this damage. It must also be recognized that whatever control measures are instituted, due regard must be paid to local conditions and current circumstances. The costs of the waste disposal operation in relation to the revenue-producing operation that must pay for it, the practicability of the environmental protection measures proposed, and the short and long-term consequences of these measures, both for the safety of the public and for their quality of life, must all receive careful and due consideration."

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