Tailings Disposal - Mining and the Environment: From Ore to Metal - page 744

A much higher incidence of tailings storage incidents, including catastrophic events,
occurs in the mountainous, wet tropics; this, despite the fact that many operations in such
geographic circumstances, recognizing the risks involved, have opted for riverine or sub-
marine disposal. Over the past two decades an increasing proportion of new mines have
been developed in such areas, which are geologically active and prone to instability. The
challenges involved in any development in such areas are much greater than in the tradi-
tional mining regions of Australia, Western USA, Canada, South Africa and Russia.
Table 18.5 identii es some of the more common causes and types of tailings storage fail-
ure and identii es typical measures adopted to reduce or remove the risks.

TABLE 18.5
Measures to Ensure Safety of Tailings Systems

Type and Cause of Failure Measure(s) Incorporated in Design to Avoid Failure

Overtopping of embankment due to fl ood infl ows, with
consequent erosion. Such an event is more likely to
occur where upstream fl ows are diverted, in which case
blockage of the diversion channel or tunnel can lead to
major infl ow.

● Installation of interception/diversion drains outside the tailings storage perimeter.
● Provision and maintenance of adequate freeboard.
● Provision of capacity (and in some cases, back-up capacity) to remove water by pumping during
the operating life of the impoundment, and by discharge through the spillway after operations end.

Erosion of embankment by prolonged wave action, with
consequent overfl ow and scour.

● Development of tailings 'beach' against the embankment.
● Provision of adequate freeboard.
● Installation of rip rap on the upstream slope.

Sliding or collapse of downstream slope, due to
inadequate strength/overly steep slope, and/or exces-
sive pore pressures, with or without earthquake loading.

● Selection of low slope angles, and earth-fi ll materials of adequate strength.
● Installation of drainage systems for the embankment and foundations.

Failure of upstream slope due to inadequate strength
and/or excessive pore pressures, with or without
earthquake loading.

● Selection of low slope angles, and earth-fi ll materials of adequate strength.
● Installation of drainage systems for the embankment and foundations.
● Avoidance of rapid drawdown during operations.

Mass downstream movement of embankment due to
sliding along the foundation interface or along a weak
foundation layer, caused by excess pore pressure or
hydrostatic pressure, with or without earthquake
loading.

● Comprehensive site investigations including engineering geological assessment to identify
potential failure modes; incorporation of specifi c foundation treatments or structural design
elements to ensure stability under all operating conditions.
● Foundation drainage system ensures dissipation of pore pressures, and avoids development of
uplift pressures.
● Removal of weak foundation material.

Collapse, as a result of liquefaction under cyclic
earthquake loading.

● Use of cohesive embankment materials and incorporation of drains to ensure a low phreatic
surface within the embankment.
● Avoidance of sites with saturated, granular foundation materials; alternatively removal or
densifi cation of such materials.

Internal erosion, due to 'piping' of embankment or
foundation materials.

● Embankment materials must be non-dispersive. i.e. not susceptible to 'piping.'
● Hydraulic gradients should be low, and toe drains should be installed, if necessary, to ensure that
the phreatic surface does not intersect the downstream face of the embankment.
● Foundation cutoff wall can be installed to substantially increase seepage path through foundations.

Cracking of embankment due to deep dessication or
differential settlement, with consequent leakage, internal
erosion, and collapse. Deep dessication can be caused
by uptake of moisture by deep-rooted plants.

● Differential settlement of foundations is minimized by avoidance or removal of highly
compressible foundation materials.
● Differential settlement of embankment materials is avoided by placement in thin layers with ade-
quate compaction, with construction control testing to ensure that design compaction
criteria are achieved.
● Cracking due to dessication is reduced by compaction and further minimized by application of a
surface mulch, and by avoiding establishment of vegetation other than shallow-rooted species.