Tailings Disposal - Mining and the Environment: From Ore to Metal - page 742

Other relevant data are compiled from regional or national authorities, which pro-
vide important inputs to the subsequent engineering analyses. These include climatol-
ogy/hydrology data to determine the design rainfall event from which the design inl ow
event is computed. Similarly, seismic records are reviewed to determine seismic risk, and a
design earthquake event is selected.
Once the results of site investigations are available, engineering analyses are carried out
to assess the stability of various embankment layouts and designs under the entire range of
operating conditions and scenarios, including 'upset events' in the process plant, extreme
rainfall events and earthquake loadings. Other analyses evaluate seepage rates from the
impoundment, and through, around or beneath the embankment. These are used to assess
the need for sealing treatments or for cutoff walls beneath the embankment or drainage
zones within the embankment. From all these studies, an embankment design is selected
which satisi es stability and seepage criteria.
Stability is usually expressed in terms of factor of safety (FS). For a tailings embank-
ment, an FS of at least 1.2 is required for steady-state operating conditions, with an FS of
at least 1.1 for short-term events such as earthquakes. Acceptable rates of seepage depend
mainly on the nature of the tailings solution, whether or not it contains dissolved sub-
stances that could cause environmental contamination, and the contaminant pathways
which are determined by the hydrogeological situation.
While site investigations are carried out primarily for engineering design purposes, they
also provide the opportunity to obtain useful environmental data with little, if any additional
cost or effort. Therefore, the relevant environmental professional(s) should have the opportu-
nity to provide input to the scoping of the site investigations and should be supplied with the
relevant data as it becomes available. Environmental data generated from site investigation
studies usually include: (1) water table proi les, (2) seasonal information on water table l uctu-
ations, (3) aquifer characteristics such as storativity and transmissivity, (4) groundwater l ow
directions, (5) groundwater quality, (6) soil proi le, (7) rock mass weathering proi le informa-
tion, (8) surface ini ltration characteristics, (9) statistical analyses of rainfall events of differ-
ent intensity and duration for different return periods, (10) rainfall/runoff relationships, and
(11) seismicity. Apart from generating data for planning purposes, piezometers installed dur-
ing site investigations may be incorporated in the ongoing monitoring programme.

Risks of Onshore Tailings Storage

In any discussion of tailings disposal, it is important that the risk of failure is recognized. The
causes of failure are many and varied. The world-wide experience of tailings storage failures
includes many incidents involving: (1) overtopping of tailings impoundments caused by unex-
pectedly large rainfall events; (2) collapse of tailings dams due to foundation failure; (3) inter-
nal erosion or piping, leading to the collapse of tailings embankments; (4) earthquake damage
leading to overtopping or collapse; and (5) human error in terms of inadequate design, poor
construction or inappropriate operating practices. In some cases, catastrophic failure involved
loss of life and widespread environmental damage (ICOLD 2001, UNEP 2001).
TSF failures may be categorized as follows (Aldous 2002):
1. Structural failures generally involve the collapse, subsidence or slippage of a part of a
containment structure. Structural failures often result in the discharge of large quanti-
ties of tailings with severe environmental damage, including loss of life due to inunda-
tion or damage inl icted on structures.

2. Operational failures occur where the root cause of the incident is the failure to operate

or control the operation of a facility adequately. For example, failure to monitor the