Mine and industrial dams - Geotechnical Engineering of Dams - page 811

There are some practical aspects which should be considered in the provision of clay liners:

- The permeability of the clay depends on the soil available. For many naturally occurring
soils the compacted permeability across the liner will be of the order of 10 8 m/sec to
10 9 m/sec. In the authors' experience, few soils have permeability as low as 10 10 m/sec.
- The permeability is affected by the compaction water content and density. To achieve a
low permeability, the soil should be compacted to a density ratio of 98% of standard
maximum dry density, at a water content of between

2% of opti-
mum. The permeability can be increased by an order of magnitude by compacting dry
of optimum - see Lambe and Whitman (1981).
- The thickness of the clay liner may not be particularly critical, depending on the par-
ticular circumstances. It is better to have a relatively thin (say 0.6 m) high quality layer
(i.e. good selection, good compaction control) than a thicker, less controlled layer.
- The clay liner is susceptible to cracking on exposure to the sun which can increase its per-
meability by orders of magnitude. This is particularly critical on sloping sites, where the
liner may not be covered by tailings for months after construction. On flat sites, covering
the liner with, say, 150 mm of clean sand or silty sand can act to prevent drying of the clay.
On sloping sites the sand cover may be eroded by rainfall and may need to be held in place
with a geotextile. As shown by Kleppe and Olson (1985), once cracked, the liner perme-
ability will remain high, even if the cracks are apparently closed by swelling on re-wetting.
- The permeability of clay liners was questioned in the early 1980s, when some researchers
found that the permeability was increased by some orders of magnitude when particu-
lar organic leachates were passed through the clay. Later research has shown this is not
a problem for water containing inorganic chemicals, even if at high concentrations.
Since most tailings liquor would not contain organic leachate, the permeability meas-
ured in the laboratory should be a reasonable guide to its long term behaviour.
- If the foundation has openwork gravel or wide open joints, clay liners can be subject to
“sinkhole” development, i.e. erosion of the liner into the underlying foundation.
- The clay liner is quite expensive. A 0.6 m thick liner could be expected to cost of the
order of $10-15/square metre, assuming there was a ready source of clay fill available.
Protection against drying and erosion, plus surface preparation to give a smooth con-
tour, could be expected to add to this cost giving an overall cost of say $20 to $25 per
square metre. Because large areas are often involved, the costs can be very large, e.g. a
1km

1% of optimum to

1 km area would cost about $20 to $25 million to line with clay.
- The clay liner should cover the whole of the tailings storage. Use of a liner as an
“upstream blanket” is unlikely to result in significant reduction in seepage quantities.
- There will be significant seepage through a clay liner. For example, a 0.6 m thick clay liner
with permeability of 10 9 m/sec would discharge 90 m 3 /day over an area of 1 km square
under unit gradient.
- The naturally occurring clays in a storage area are unlikely to have a low permeability
unless they are excavated and recompacted. In situ they are likely to have a permeability
of the order of 10 5 m/sec to 10 6 m/sec due to the presence of root holes, fissures etc.

Unfortunately some regulatory authorities have unrealistic views in regards to clay lin-
ers for tailings dams and may request provision of a liner, jeopardizing the viability of a
project, when the geochemistry is such that seepage of an unlined storage is not an envi-
ronmental hazard.

19.6.4.5 Underdrains
Drains may be provided under the tailings as shown in Figures 19.28 , 19.29 and 19.42 ,
with or without a clay liner. The drains act to attract the seepage water and discharge it
to a collector system, often for recycling to the process plant.