Approaches to Waste Rock Disposal - Mining and the Environment: From Ore to Metal - page 765

FIGURE 19.1
Profi le of Open Pit with Narrow,
Steeply Dipping Ore Body and High
Stripping Ratio

Pit Wall

Ore Body

A stripping ratio of 3 means that
3 tonnes of waste is excavated for
each tonne of ore.

appropriate design and management measures. Among the most important properties are
the following.

Grading or Size Range

The grading of waste rock materials depends on the nature of the rock mass prior to min-
ing and the degree of fragmentation caused during mining. Generally, mining methods
are selected to achieve sufficient fragmentation that the waste rock can be retrieved by
shovel or front end loader and loaded onto trucks without the need for additional break-
age. In smaller mines and in underground mines where low capacity equipment is used,
a higher degree of fragmentation is required than for large mines. In the case of weak
rocks the fragmentation achieved depends on the size and nature of the excavator, includ-
ing bucket dimensions, teeth size and spacing, and operator method. Excavation of weak
rock generally produces appreciable quantities of fines, which are particles of sand, silt and
clay. In stronger rocks, explosives are used to achieve the desired fragmentation. Generally,
only small quantities of fines are produced in excavation of strong rocks.

In smaller mines and in
underground mines where low
capacity equipment is used, a
higher degree of fragmentation is
required than for large mines.

Shape

The shape of particles, particularly the larger waste rock fragments, influences the way the
materials behave when dumped. For example, bulky or roughly equi-dimensional frag-
ments roll down a dump face while elongated or slabby fragments do not roll far.

Strength

Weaker waste rock particles undergo further fragmentation, and produce increasing
quantities of fines during loading, transportation and dumping, whereas strong particles
retain their size and shape.

Durability

Some rocks undergo rapid changes in physical properties once exposed to air and/or water.
In some cases, boulders can be reduced to gravel within a few weeks of exposure. Some