Hydraulic Tailings
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kChapter 22 STEVEN G. VIcK HYDRAULIC TAILINGS 1. INTRODUCflON 2.CHARACTERIZATION AND PROPERTIES ailings, defined as sand-, silt-, or clay-sized For most metalliferous ores, tailings represent the Tsolid wastes from mineral-processing opera- end product of crushing and grinding operations tions that were originally produced, handled, and after extraction of the valuable mineral particles. deposited in slurry form, represent a special case of For other ores, such as phosphates, coal, or con- slope stability in that they are artificially con- struction aggregates, tailings are produced by sim- structed deposits of manufactured soil. Tailings are ple washing and screening operations. In either distinct from other types of coarse mining wastes case, the maximum particle size typically varies or smelter slag that are dumped or piled under dry from 0.1 to 1.0 mm, with the relative proportions conditions. Tailings deposits share many geotech- of sand- and silt-sized particles governed by the nical characteristics of such other hydraulic fills as. type of processing required for optimum mineral offshore drilling islands, hydraulic-fill water dams, extraction efficiency. Mechanical grinding pro- dredged material deposits, and certain other indus- duces particles that are highly angular; clay is con- trial wastes (e.g., fly ash and papermill sludge). tained in the tailings only if present in the parent These related materials are not discussed here, but ore, and clay content is usually significant only for pertinent information can be found in two publi- tailings produced by washing operations. Specific cations from the American Society of Civil gravity of tailings solids varies from as low as 1.5 Engineers: Geotechnical Practice for Disposal of Solid for coal to almost 4.0 for tailings derived from sul- Waste Materials (ASCE 1977) and Hydraulic Fill fide-type orebodies where pyrite is rejected during Structures (Van Zyl and Vick 1988). processing. Vick (1990) summarized gradation and Tailings are often identified by their association index properties for tailings from a variety of ore with mining and milling operations. Where types. encountered along transportation routes, tailings deposits may affect the stability or safety of these 2.1 Construction of Tailings facilities by (a) causing instability of excavated cuts, Impoundments (b) creating potential landslide hazards from adja- cent deposits, (c) increasing subgrade compressibil- During operation of a milling facility, the tailings ity and settlement, and (d) possessing adverse slurry is usually pumped to a tailings impound- environmental characteristics (high erodibility, ment at a pulp density of 15 to 50 percent solids by. heavy-metal pollution, and acid drainage). weight. Within the impoundment the solids settle 577 578 Landslides: Investigation and Mitigation from suspension, and the supematant water collects des carried farther in the slurry stream. This sedi- in a decant pond from which it is discharged or recy- mentation pattern gives rise to an above-water cled to the mill process. Central to geotechnical beach of sand tailings (defined as those with less characterization of any tailings deposit is the man- than 50 percent passing the No. 200 sieve) and a ner in which discharge and deposition of the tail- zone of slimes (more than 50 percent passing the ings slurry have been conducted, and conventional No. 200 sieve) extending from the more distant exploration and sampling techniques alone cannot regions or the above-water beach to beneath the substitute for an understanding of these operations. decant pond, as shown in an idealized manner in Usually the tailings, slurry is discharged from Figure 2 2-2. The degree of particle-size segregation the embankment perimeter, typically as shown in on the beach, and therefore the extent to which Figure 22-1. To the extent that a sand fraction may the sand tailings and slimes are differentiated, de- be present in the slurry, these particles tend to set- pends on such factors as gradation of the original tle nearest the point of discharge, with finer parti- slurry and its solids content (Abadjiev 1985). The FIGURE 22-1 Tailings slurry discharge and Spigots beach deposition Tailings discharge pipe (modified from Vick 1990). Decant pond - — — OMM FONS Embankment Dike Decant Spigotted Tailings Tailings Beach\ Dike FIGURE 22-2 _ Sequential tailings dam raising and tailings deposition showing idealized sand and slimes regions (modified from Vick Slimes 1 Sands 1990). Hydraulic Tailings 579 transition between zones of sand tailings and slimes 2.4 Typical Strength Characteristics in a tailings deposit is gradual, interlayered, and in- distinct and is governed by the various techniques Strength characteristics governing slope stability and locations of tailings slurry discharge adopted vary according to sand or slimes characterization. during operation of the impoundment. Never- For most types of sand tailings, the drained (effec- theless, characterization of a tailings deposit into tive-stress) friction angles typically range from 33 to regions of predominantly sand and slimes is funda- 38 degrees. These relatively high values for loose mental to establishing broad ranges of physical, materials are attributable to particle angularity. index, and engineering properties and to targeting Sand tailings display virtually no effective cohesion subsurface exploration in key areas. intercept (c') except in special cases involving chemical cementation, such as gypsum tailings or tailings affected by oxidation of sulfide minerals 2.2 Sand Tailings Characteristics (usually pyrite). Vick (1990) extensively summa- Sand tailings derived from beach deposition are rized drained strengths for various tailings types. usually loose, with relative densities ranging from Provided that the nature, origin, and deposi- 30 to 50 percent. Upon initial deposition, tional characteristics of sand tailings deposits are they typically exhibit corrected standard penetra- fully understood, estimates of drained strength tion test (SPT) blowcounts, expressed as (N1 ), of based on published values for similar tailings types 3 to 5. There is evidence to support some increase are often sufficient for effective -stress analyses. in (N1) values with time due to "aging" effects Slope stability is seldom sufficiently sensitive to (Troncoso 1990). ordinary variations in drained strength to justify extensive laboratory testing. For slimes tailings, either drained or undrained 2.3 Slimes Characteristics strength may be pertinentto slope stability, depend- Slimes, on the other hand, qualify as low-plasticity ing on such factors as the degree of desiccation- to nonplastic silts for tailings derived from induced overconsolidation that the slimes may grinding of metalliferous ores. Other ores and have experienced during deposition. For slimes processes, most notably Florida phosphatic ores, ex- derived from metalliferous ores, drained (effective- hibit less favorable plasticity, sedimentation, and stress) angles of internal friction are generally in the consolidation characteristics in their clay slimes range of 28 to 37 degrees (Vick 1990). Slimes (Bromwell and Raden 1979; Schiffman et al. 1988). exhibit little or no effective cohesion. A void ratio of 0.8 to 1.3 is typical of many slimes The undrained strength (s) of slimes is, how- deposits, with (N1) values often ranging from 1 to ever, sensitive to details of the original deposition 3. Of particular significance to slope engineering process. For slimes deposited within and beneath issues is the high specific retention of void water the decant pond, normally consolidated conditions exhibited by slimes. Once sedimentation and con- prevail after dissipation of deposition- induced solidation processes are complete, further gravity excess pore pressures, and the variation of drainage seldom results in appreciable reduction in undrained strength with depth can be character- water content or gain in strength except in the zone ized by stress-normalized ratios (s /a') ranging affected by surface desiccation, which seldom from about 0.20 to 0.27 (Vick 1990; Ladd 1991). extends below depths of 1 to 2 m. Thus, slimes In contrast, slimes deposited above water or on deposits, even in dry climates, have remained soft intermittently saturated portions of the beach or and virtually fully saturated for up to 60 years after otherwise influenced by local desiccation during deposition has ceased and surface water has been deposition may exhibit far more irregular un- removed. This feature, along with the associated drained strength patterns, which may be difficult low undrained shear strength, makes slimes the to assess (Maclver 1961). Although recovery of most troublesome tailings material from the point undisturbed samples of saturated slimes for labora- of view of slope engineering or embankment con- tory testing is possible using fixed-piston samplers, struction. Slimes deposits also exhibit high com- vibration during sample transport or sample extru- pressibility, although rates of consolidation are sion frequently causes gross disturbance and densi- usually relatively rapid. fication. Draining of sample tubes in the field 580 Landslides: Investigation and Mitigation before transport and freezing of recovered samples apply to tailings deposits, but more important than have both been used successfully to reduce these the computational procedure is the adoption of effects. Alternatively, Wahier and Associates appropriate strength parameters.