SUSTAINABILITY Construction methods
Surface disposal of tailings uses dams and embankments of various types to form impoundments that retain both the tailings and plant effluent.
The embankment type under consideration may be of the water retention variety constructed in a single lift or one of the raised embankments, including upstream, downstream, or centerline types.
Determination of embankment type, governed by the disposal method and water handling requirements, incorporates consideration of the following key issues:
The type of embankment selected must be compatible with constraints imposed by these factors. Integrally tied to the selection of embankment type are siting and layout considerations.
Upstream wall raising starts with a starter embankment constructed at the downstream toe (Figures 1a and 1b). The tailings are discharged peripherally from the crest of the starter embankment using spigots or hydro-cyclones. This deposition approach develops a wide beach area comprising coarse material. Beach refers to the surface slope of the tailings after being deposited by water from a discharge point. The beach becomes the foundation of the next embankment raise. In some applications, the embankments are mechanically placed, and the discharge is used to build the beach only.
These embankment raises can be built with borrow fill, or beach sand tailings can be excavated from the beach and placed by either excavator or front-end loader. Either way, some mechanical compaction of the embankment raise is typically conducted before the dam's next stage is constructed.
The most important criteria for applying the upstream raised construction method is that the tailings beach must form a competent foundation for the support of the next embankment raise. In addition to tailings gradation, several other factors can limit the applicability of this method. These factors include phreatic surface control, water storage capacity, seismic liquefaction susceptibility, and the rate of dam raising.
The design requirements for the downstream method of construction are similar to conventional water storage dams. As in upstream construction, downstream construction also begins with a starter embankment constructed of compacted borrow materials. However, this starter embankment may be constructed of pervious sands and gravels or with predominately silts and clays to minimize seepage through the dam. If low permeability materials are used in the starter embankment, internal drains will need to be incorporated into the design.
The downstream method is so named because subsequent embankment construction stages are supported on top of the previous sections’ downstream slope, shifting the centreline of the embankment crest downstream as the dam stages are progressively raised (Figures 2a and 2b). A variety of tailings depositional techniques can be used in conjunction with the downstream construction method, but peripheral spigotting of tailings is very common. If coarse rock (run-of-mine waste) is used, a filter or impervious upstream membrane is required on the upstream slope to prevent piping of the tailings through the rock.
The downstream construction method allows for the incorporation of drains and impervious cores to control the phreatic surface. The downstream construction method provides a degree of stability not found in upstream construction due to the ability and ease of compaction, the incorporation of phreatic surface control measures, and the fact that the dam raises are not structurally dependent upon the tailings deposits for foundation strength. Downstream raising methods require careful planning. Because the toe of the dam progresses outward as its height increases, sufficient space must be left during the starter embankment layout to prevent encroachment of the dam on property lines and topographical constraints. Such restrictions at the toe often determine the ultimate height of the embankment.
A significant disadvantage of this method is the large volume of fill material required to raise the dam. Consequently, planning is necessary to ensure that fill material production rates will be sufficient during the embankment’s life.
Embankments constructed with downstream raises also cover a relatively large area, which can be a significant disadvantage if available space is limited.
The centreline raising method is a compromise between the upstream and downstream methods; it shares both methods' advantages to a degree while mitigating their disadvantages. Similarly to the upstream method, a starter embankment is constructed, and tailings are discharged peripherally from the embankment to form the beach. Subsequent raises are constructed by placing fill onto the beach and onto the previous raise's downstream slope. The raises' centreline is coincident as the embankment progresses upward (Figures 3a and 3b).
The centreline method generally has good seismic resistance.
Open-pit mining creates voids, and it would seem that the most environmentally responsible place to store tailings would be in the voids that were the source of the wastes, although tailings disposal in completed open pits may sterilise resources, and a single open pit might not be available for tailings disposal during active mining operations. In-pit disposal typically has minimal (depending on topography) or no containment walls (Figure 4).
Tailings can be placed in completed open pits as a slurry, thickened or filtered, or in combination with waste rock (Figure 5).