Tailings Disposal - Mining and the Environment: From Ore to Metal - page 740

embankments pose not only design and construction difi culties but also a high risk of
excessive deformation during earthquakes, leading to failure. The choice of impound-
ment type and embankment height will depend on local topography and its associated
drainage pattern, and on the materials available for embankment construction. Generally,
areas of higher relief provide the opportunity to obtain more storage volume per unit of
embankment volume. However, high relief may come with its own problems such as steep
unstable slopes, access difi culties and/or stream l ows to be diverted or otherwise accom-
modated by means of spillways or alternative strategies.

Catchment Area
Unless the tailings storage is also used as the main water reservoir for the project, a rel-
atively uncommon scenario, it is advantageous to minimize surface runoff into the
impoundment. This is done in two ways: (1) by minimizing the catchment area of the
impoundment, usually by locating the tailings storage in the uppermost part of the drain-
age; and/or (2) by diverting as much surface runoff as possible around the impoundment,
by means of surface drains or tunnels. In a high rainfall area, these issues are exacerbated if
the catchment area of the tailings storage facility is large. Therefore tailings storage facili-
ties with small catchment areas are preferred.

It is advantageous to minimize
surface runoff into the
impoundment.

Geology
Geological factors impact upon construction costs, reservoir leakage, foundation conditions
and embankment stability. Site selection needs to take account of the availability and proxim-
ity of construction materials. Clay i ll for the construction of the low permeability core may
be available from unmineralized, oxidized near-surface parts of the mine or from within the
storage impoundment itself. Waste rock from the mine is commonly used for the construction
of the outer zones of the tailings embankment, involving the bulk of the rock-i ll required.
Materials used for i lter zones and drainage zones incorporated in the embankment, are usu-
ally required to meet tight grading specii cations and are usually manufactured from crushed
and screened rock. The quantities are relatively small, however, so that these materials can be
transported economically from distant locations if local sources are not available.
Foundation conditions also need to be investigated, initially to identify any geological
defects that could render the site unsuitable for construction, and to evaluate the need for
foundation preparation, prior to embankment construction. Seepage conditions also need
to be investigated, not only in the vicinity of the embankment, but from the remainder of
the impoundment.
In practice, site selection involves a series of trade-offs. For example, minimizing
the catchment area by locating the storage high in the drainage means that the tailings
impoundment will be at a higher elevation than the process plant, resulting in higher
pumping costs.
A typical site selection process contains the following steps (see also Robertson 1988):
1. The topography within a reasonable radius - generally less than 20 km - of the site(s)
for the process plant is examined and, using rough measurements, storage areas capa-
ble of accommodating the expected quantities of tailings, are outlined.
2. For each potential storage area, a preliminary embankment layout is made and a
cumulative storage volume versus embankment height curve is established.
3. For the required storage volume, the corresponding embankment heights are deter-
mined for each site.
4. The catchment area of each site is then measured, and feasibility of diverting some or
all of the upstream drainage is determined.

In practice, site selection involves
a series of trade-offs.