- proper stripping of foundation to below depth of weathering;
- controlled compaction in construction.
For discussion of the important question of flood bank alignment and pro-
tection reference should be made to Chapter 8.
2.11 Tailings dams and storage lagoonsTailings dams and storage lagoons are constructed for the retention of
waste fines, slurries and liquids or semi-liquids produced by industry.
Examples are the tailings dams storing waste fines from mining and similar
extractive industries, the extensive fly-ash storage lagoons associated with
many large coal-fired power stations, and the liquid storage lagoons
required by process and manufacturing industries. Storage lagoons are
invariably impounded behind earthfill retaining embankments; tailings
dams are commonly constructed with industrial waste materials, including
fines. Tailings dams are frequently designed to accelerate de-watering and
‘solidification’ of the stored fines by decanting surplus surface water and
controlled seepage through the body of the retaining embankment.
Recent decades have seen the size and storage capacity of the largest
tailings dams and lagoons increase very significantly, and at the upper end
of the scale eight have a height in excess of 150 m. In terms of intended
storage capacity, Syncrude Tailings Dam in Canada is the world’s largest
facility of this type, with an ultimate capacity of 540 106 m^3. On comple-
tion the 18 km long retaining embankments will range up to 90 m in height.
A further very large facility is Mission Tailings No 2 in the USA, com-
pleted in 1973 and holding 40 106 m^3 of tailings. As with conventional
dams, however, the majority are much more modest in scale and do not
exceed 15–20 m in height.
The embankments constructed for tailings disposal or for storage
lagoons differ from conventional embankment dams in a number of
important regards:
- tailings embankments, particularly the smaller examples, are almost
invariably planned as ‘minimum cost’ structures. Design and con-
struction standards, particularly on older projects, may consequently
fall well short of modern embankment dam practice and utilize
poorly selected and compacted low quality fills; - the retained fines or slurry can be of very high density, exceptionally
three or more times that of water; - a common construction technique is to raise the crest of the retaining
embankment and retention level of the stored waste incrementally
and in parallel, i.e. the embankment functions as a dam as it is itself