Environmental Engineering Reference
In-Depth Information
or no particle sorting. Hence there will be more uniform particle size and deposited slope
than for tailings thickened to only say 35%-40% solids content.
19.4.3
Permeability
The permeability of tailings is dependent on the particle size distribution and void ratio,
so is therefore dependent on the distance from the discharge point, the method of deposi-
tion, e.g. subaqueous or subaerial, and the depth in the storage.
The following procedure is therefore recommended for the estimation of permeability
at any particular place:
(1) Predict the particle size at that place from the grading of the whole tailings and the
sorting as predicted by the approaches discussed above in Section 19.4.2;
(2) Predict the void ratio from sedimentation, and the sorting as predicted by the
approaches discussed below in Section 19.4.4;
(3) Prepare samples based on (1) and (2) above;
(4) Calculate permeability from either falling head or constant head tests, or by back
calculation from consolidation tests, conducted on these samples.
For sand sized tailings the permeability can be approximately estimated from Hazen's for-
mula, i.e.
kD/ 0
10
2
(19.15)
where k
permeability in m/sec; D
10
grain size in millimetres, for which 10% of the
particles are finer.
Using this and the relationship between particle size and distance from the discharge
point discussed in Section 19.4.2, Blight (1987) suggests that the permeability of the
beached tailings will vary as:
K e
bH
(19.16)
where “a” and “b” are dependent on the tailings; H is the distance down the beach.
19.4.4
Dry density
Prediction of the dry density of tailings in the storage is of prime importance, because the
amount of tailings which can be stored in a given storage volume depends on the density
achieved. This, in turn, is dependent on the type of tailings, method of deposition (sub-
aqueous or subaerial), drainage conditions (e.g. if underdrains are provided or if the soil
and rock underlying a tailings dam is of high permeability) degree of desiccation, distance
from the discharge point and proximity to the water pond etc.
For tailings which are deposited under water there are two phases:
(a) Sedimentation;
(b) Consolidation under self weight and the weight of tailings above.
The sedimentation phase occurs when the tailings are first placed in the dam, and the
particles settle, with a clear water interface forming on the surface of the tailings. In sub-
aerial deposition this “bleed” water will run off down the beach slope.
The consolidation phase follows and may occur concurrently with sedimentation.
Prediction of consolidation of tailings requires a different approach from that for normal
