Tailings Disposal - Mining and the Environment: From Ore to Metal - page 731

TABLE 18.4
Comparison of Surface Impoundment Embankment Types - Increasingly, regulators, fi nancial institutions and insurers are requiring that tailings
retention structures be designed as water retention structures. Essentially, this means that an embankment needs to provide the mass, strength and sealing properties as if only
water is stored

Water Retention Upstream Downstream Centreline

Mill Tailings Suitable for any type of

tailings

At least 40-60% sand in whole tailings.
Low pulp density is desirable to
promote draining-size segregation

Suitable for any type of
tailings

Sands or low-plasticity slimes

Discharge Requirements Any discharge proce-

dure suitable

Peripheral discharge and well-
controlled beach necessary

Varies according to design
details

Peripheral discharge of
at least nominal beach
necessary

Water Storage Suitability Good Not suitable for signifi cant water

storage

Good Not recommended for per-

manent storage. Temporary
fl ood storage acceptable with
proper design.

Seismic Resistance Good Poor in high seismic areas Good Acceptable

Raising Rate Restrictions Entire embankment

constructed initially

Less than 4.5-9 m/yr most desir-
able. Greater than 15 m/yr can be
hazardous

None Height restrictions for indi-

vidual raises may apply

Embankment Fill
Requirements

Natural soil borrow Natural soil, sand tailings, or mine

waste

Sand tailings or mine waste
if production rates are suf-
fi cient, or natural soul

Sand tailings or mine waste
of production rates are suf-
fi cient, or natural soul

Relative Embankment Cost High Low High Moderate

Source:
Vick (1983)

of tailings by water or wind. Upstream raising can also be carried out using waste rock or
borrow material placed on top of deposited tailings that have been allowed to desiccate.
Table 18.4 highlights the very much greater volume of embankment material required
for downstream raising compared to upstream raising, and the downstream advance of
the toe of the containment wall in downstream raising. The schematic diagrams in Figure
18.11 do not include details about internal drainage or clay cores within the containment
walls, which may be required to ensure geotechnical stability and/or to control seepage.
Downstream construction represents the safest, most conservative approach, but also
the most expensive unless waste rock suitable for the bulk of the embankment is available
from the mine. Both upstream and centreline construction require that the deposited tail-
ings, where they will be overlain by embankment i ll, have achieved sufi cient strength to
support the imposed loads without unacceptable deformation. In practice, it is common to
undertake a geotechnical investigation to evaluate in situ strength properties, as a basis for
the decision as to which approach is used for the next embankment raise.
Due mainly to its low cost, upstream construction has been used for most tailings embank-
ments worldwide, until recently. However, experience indicates that upstream construc-
tion involves the highest risks. Dam failure can occur if the decant pond encroaches on the
embankment so that the phreatic surface of groundwater within the embankment rises
above the downstream toe. While the phreatic surface location is important for all types

Downstream construction
represents the safest, most
conservative approach, but also
the most expensive.

Due mainly to its low cost,
upstream construction has
been used for most tailings
embankments worldwide, until
recently. However, experience
indicates that upstream
construction involves the highest
risks.