12.11 Upstream Method
Upstream construction is the oldest and most economical method, and begins with a starter dam constructed at the downstream toe (Figure 9). It should be capable of passing seepage water and the downstream portion should be resistant to piping. The tailings are discharged peripherally from the crest of the starter dam using spigots or cyclones. This deposition develops a dike and wide beach area composed of coarse material. The beach becomes the foundation for the next dike. In some applications, dikes are mechanically placed and the discharge is used to build the beach only. These dikes can be built with borrow fill or beach sand tailings can be excavated from the beach and placed by either dragline or bulldozer. Either way, some type of mechanical compaction of the dike is typically conducted before the next stage of dam is constructed.
The single most important criteria for application of the upstream construction method is that the tailings beach must form a competent foundation for support of the next dike. Vick (1990) states that, as a general rule, the discharge should not contain less than 40- 60% sand. This can preclude the use of upstream method for those mill tailings that contain very low percentages of sand. In addition to tailings gradation, several other factors can limit the applicability of this method. These factors include prelatic surface control, water storage capacity, seismic liquefaction susceptibility and the rate of dam raising. Upstream embankment construction offers few structural measures for control of the prelatic surface within the embankment. Vick (1990) identified four important factors influencing the phreatic surface location: permeability of the foundation relative to the tailings, the degree of grain sizes segregation and lateral permeability variation within the deposit, and the location of pounded water relative to the embankment crest.
Tailings embankments constructed using the upstream method generally has a low relative density with high water saturation. This combination can result in liquefaction of the tailings embankment in the event of seismic activity. In addition, vibration of sufficient intensity and magnitude caused by blasting, trains, heavy trucks, etc., may cause liquefaction. The shear strength can be reduced to near zero such that the fluidized slimes easily burst through the remaining thin, unsaturated sand-dike shell and the dam collapses and flows.
Upstream construction is not appropriate in areas with a high potential for seismic activity. The rate of embankment raises is limited by the built-up of excess pore pressure within the deposit. This built-up of pore pressures can lead to a shear failure, which may result in breaching of the dam and the release of contained tailings (Brawner 1973). The height, at which potential failures are triggered, depends on the strength of the tailings within the zone of shearing, the downstream slope of the dam, and the location of the phreatic line. Horizontal drainage zones may be installed during starter dike construction to help maintaining low pore pressure within the embankment.
The upstream method is used with most tailings dams worldwide Because of its low cost, but it must be built and operated with great care and attention as it has the highest risk of failure among all the methods.
Figure 9: Upstream construction of retaining dam
Upstream dams are highly susceptible to liquefaction under severe seismic groundmovement. This may result from earthquakes, from mine blasting, or even from themovement of heavy equipment.