Environmental Engineering Reference
In-Depth Information
Table 19.4.
Typical in place densities and void ratio for non-desiccated tailings (adapted from Vick, 1983).
Dry density (t/m 3 )
Tailings type
Specific gravity
Void ratio
Fine coal refuse
Eastern US
1.5-1.8
0.8-1.1
0.7-0.9
Western US
1.4-1.6
0.6-1.0
0.7-1.1
Great Britain
1.6-2.1
0.5-1.0
0.9-1.35
Oil sands
Sands
-
0.9
1.4
Slimes
-
6.0-10.0
Lead-zinc slimes (1)
2.9-3.0
0.6-1.0
1.5-1.8
Gold-silver slimes
-
1.1-1.2
Molybdenum sands
2.7-2.8
0.7-0.9
1.45-1.5
Copper sands
2.6-2.8
0.9-1.4
1.1-1.45
Taconite sands
3.0
0.7
1.75
Taconite slimes
3.1
1.1
1.5
Phosphate slimes
2.5-2.8
11.0
0.25
Gypsum treated tailings
2.4
0.7-1.5
0.3 (2)
Bauxite slimes
2.8-3.3
8.0
Notes: (1) For hard rock tailings; (2) low by Australian standards 0.5-0.9 t/m 3 more likely.
The relative density of the silt and sand sized tailings has an important influence on the
potential for liquefaction. The relative density of spigotted tailings sands above water, i.e.
in the beach zone, can be expected to be in the range 30% to 50% (Vick, 1983).
Morgenstern (1988) indicates that relative densities of 50% to 65% can be expected for
sands placed above water and 35% to 40% when placed underwater. De Groot et al.
(1988) show relative densities from 10% to 50% (mainly 25% to 40%) for sands placed
below water. Thus silt/sand tailings placed below water are likely to be susceptible to liq-
uefaction under earthquake and static shear.
19.2.3.5 Permeability
The permeability of mine tailings depends on the type of tailings, particle size and miner-
alogy, deposition method, degree of consolidation and/or desiccation (dry density/void
ratio), cracking or other structure and whether the tailings are saturated or partially saturated.
When tailings are deposited unthickened, there is segregation of the coarser particles relative
to the fines with distance from the discharge point so the tailings permeability will vary in the
deposited tailings, typically higher near the spigot point, if the sands and coarse silts separate
out, and lower distant from the discharge point where the fine tailings or “slimes” deposit. It
is also lower, deeper in the deposit than at the surface, due to consolidation.
In addition, within each layer of tailings, the larger particles settle at a different rate from
the finer ones, so there is vertical segregation in the layers. For most tailings, the larger
particles settle to the bottom of the layer and may form a thin sandy “parting”. For coal
washing tailings, the coarser particles are often coal, with a low specific gravity, and these
rise to the top of the layer.
Table 19.5 gives some typical tailings permeabilities, which are probably representative
of vertical permeabilities.
The horizontal permeability will change with the degree of segregation and variability of
deposition. Near the discharge area (say the first 50-100 metres), it is likely the vertical seg-
regation will give an equivalent horizontal permeability at least 10 times and probably 100
times the vertical. In the slimes the effect is likely to be small, with horizontal and vertical
permeabilities similar.
 
 
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