Tailings Disposal - Mining and the Environment: From Ore to Metal - page 732

of embankment, it is more complex for upstream embankment types. Upstream dams are
particularly susceptible to liquefaction under severe seismic ground motion. This motion
may result from earthquakes, from nearby mine blasting, or even from nearby motion of
heavy equipment. Stability also decreases with increased rates of raising the embankment.
Vick (1983) considers raising rates greater than 15 m/yr as hazardous. High raising rates can
produce excess pore pressure within the deposit, decreasing stability.
In the past, many tailings retention embankments were constructed largely or entirely
of tailings. These designs usually depended for their long-term stability, at least in part, on
strength provided by the settled tailings. In some cases the sealing properties of the tailings
were required to minimize seepage from the storage. In the Western Australian Goldi elds,
many such structures remain intact decades after being completed and abandoned. Others
have failed or have been damaged by erosion. In some cases, coarse sand used as i ll to raise
the embankment, has been obtained from the tailings, usually by cycloning to separate the
sand from the i nes that could impede drainage. Some very large tailings storage facilities
have been constructed in this way. One at the Padcal Copper Mine in the Baguio district of
the Philippines actually survived the Baguio earthquake of July, 1990, except that blockage of
the portal of an upstream diversion tunnel caused water to l ow across the storage and down
the embankment face, resulting in scouring, but leaving most of the storage intact.
Increasingly, regulators, i 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; i.e. the additional strength and sealing properties of the deposited tailings cannot be
included in stability and seepage calculations. This has increased the stability and safety of
tailings retention structures and has also increased the costs. However, embankment design
based on water retention may not provide sufi cient stability in the event of an earthquake,
as the dynamic forces imposed by liquei ed tailings exceed those imposed by water.
A particular type of tailings storage was developed by a Canadian - Eli Robinsky
(1968). This system known as the Central Discharge or Thickened Tailings Discharge
(TTD) uses tailings thickened to the maximum extent consistent with being pumped,
which varies considerably between different types of tailings but is typically 60% solids.
The thickened tailings are discharged from a riser located in the middle of the tailings
storage. Excess water including runoff from precipitation, drains to a low dyke as depicted
in Figure 18.12 . Deposition results in high beach angles, typically 1.1º to 3.4º, which assists
rapid desiccation. The resulting tailings deposit is in the form of a shallow cone, which
provides an attractive landform, readily accessible and amenable to rehabilitation. Other,
natural-looking landforms, including fans and coalescing fans, can also be formed on gen-
tly sloping land surfaces. Tailings deposited in this way do not segregate, so that proper-
ties will be relatively consistent throughout the storage. A major benei t of this and other
thickened tailings systems is the recovery of most water and process chemicals.
Perhaps the most advanced tailings storage designs in common use are those that follow
the requirements of the US State of Nevada. These incorporate all or most of the follow-
ing features: (1) underdrains at the base of the tailings deposit; (2) double liners with leak
detection between; (3) liquid recovery systems; (4) drains beneath the embankment; and
(5) secondary containment. A typical cross-section of a tailings retention system of this type
is shown on Figure 18.13 .
Clearly, tailings retention structures designed to meet these stringent requirements are
expensive to build. For many tailings storage facilities which will not receive toxic or haz-
ardous constituents, simpler and much less costly designs may be entirely appropriate.
Most surface tailings disposal systems involve settling of the tailings solids within an
impoundment and recycling of the supernatant water. The exceptions are some operations