Environmental Engineering Reference
In-Depth Information
shows that tailings with a permeability of 10
8
m/sec are as effective as a 2 foot (0.6 m)
thick clay liner with a permeability of 10
9
m/sec.
It should be remembered that rock in the upper 10 m to 30 m of most foundations has
a permeability between 1 Lugeon and 20 Lugeons, or 10
7
m/sec to 2
10
6
m/sec. Most
naturally occurring soils will have a similar or higher permeability. Since tailings slimes
(even from non oxidised ore) are likely to have a vertical permeability of less than
10
7
m/sec, the tailings will often be less permeable than the underlying soil and rock. If
the tailings are from oxidised ore or from washeries (such as coal, bauxite, iron ore) they
are likely to have a permeability of the order of 10
7
m/sec to 10
9
m/sec or less. Clearly,
in these cases, covering the storage with the tailings will be an inexpensive and effective
way of limiting seepage.
The effectiveness of the tailings as a “liner” is dependent on placement methods. If tail-
ings are placed subaerially and allowed to desiccate, lower permeabilities will result from
the drying, provided cracking does not occur. If placed subaqueously lower densities and
higher permeabilities are likely to result.
A potential difficulty with using tailings as a liner, is that the coarser fraction of the tail-
ings tend to settle out more quickly than the fine (or slimes) fraction. Hence a “beach” of
sandy tailings often occurs near the discharge point and if water is allowed to cover this
area subsequently it can allow local high seepage rates. This can be overcome by using
thickened discharge which inhibits segregation, by shifting the tailings discharge points
from one end of the storage to the other, placing slimes under the beach area and/or by
using a liner or seepage collector system under the sandy area.
Seepage can also occur along the contact between tailings and embankment if rock rip-
rap is used (
Figure 19.40
)
. Another problem is that it can be difficult to avoid water pond-
ing against the storage foundation, particularly early in the storage operating life.
19.6.4.2
Foundation grouting
As discussed in Chapter 18, grouting is not particularly effective in reducing seepage,
except in high permeability rock.
In a project on which the authors were involved, the grouting of a 5 km long dam foun-
dation to a depth of about 25 m on average, would have reduced the estimated seepage by
only 1%; nearly all of this in a relatively small portion of the foundation affected by fault-
ing and with an ungrouted permeability of the order of 100 Lugeons.
It will be seen from the above discussion, that it is unlikely that grouting of tailings dam
foundations can be justified on the grounds of reducing seepage. It may be justified on
other grounds, such as reducing potential erosion in weathered rock, or where the high
permeability zones can be identified from geological information and only these zones are
grouted. It is usually not possible to do this.
19.6.4.3
Foundation cutoffs
For tailings storages constructed on soil foundations, particularly sand or sand and
gravel, a significant reduction in seepage may be achieved by construction of an earthfill
cutoff or a slurry trench cutoff wall as discussed in Chapter 10.
These cutoffs are of high cost and applicable only in critical situations and where
ground conditions allow, i.e. generally soil. They may be applied to extremely weathered
rock, e.g. lateritised highly permeable weathered rock.
19.6.4.4
Clay liners
Clay liners can be an effective way of reducing the seepage from a tailings storage. For
example, if a tailings storage is located in a highly permeable area over sand, and the tail-
ings are moderately high permeability, a clay liner may well be appropriate.
