WINNING
Geological characteristics and economic considerations have a considerable influence on the
layout of opencast and quarry operations.
design of the plant and the required plant
equipment. In the case of quarries, such as
dimension stone, the samples will be used
to determine the physical appearance and
dominant grain orientation, to establish the
best extraction layout direction.
The geological and assay or quality data
are then used to construct a model. In
the case of an opencast mining operation,
a geological- and block model with the
results of the assays is constructed. This
will determine the capital footprint and the
most suitable area to make the first box cut.
It will also give an indication of the amount
of material that will have to be removed
as part of the overburden or as waste. The
block model will give an indication of the
tonnages and areal extent of ore reserves.
The pit shell
The next step would be to determine the
pit shell. This could be done by means of
various geological or mining software,
such as running a Whittle Optimisation
of various scenarios, taking the modifying
factors into consideration. Such an exercise
enables the planners to generate a mine
plan and schedule. The schedules usually
cover short-term, mid-term, long-term,
and life of mine, which can be adjusted
according to changing economic or
demand conditions.
The design and layout of an open pit
or quarry need to take various factors
into consideration and will determine
the pit limits. The first is the types of ore
or material that can be mined in the case
of an opencast operation, for example
14 _ QUARRY SA | JULY/AUGUST 2018
oxidised and/or fresh ore. Quarries and
opencast operations have to consider
geological losses, including intrusions such
as dolerite dykes or fault zones. The pit shell
dimensions are largely influenced by the
cut-off grade as it will determine the depth
or distance from the plant that material can
still be economically mined.
The physical design and layout of the
open pit or quarry take the following factors
into consideration: stripping ratio, pit slopes,
bench heights, road grades, dewatering,
and property lines. The stripping ratio
refers to the amount of overburden or
waste material that needs to be removed
before accessing the ore (mineralised
material) or the saleable quarry product.
Overburden usually refers to the residual
or transported soils that cover the outcrop.
Most environmental management plans
require this top soil not to be mined, but
to be stockpiled and used during the mine
closure and rehabilitation phase.
The mine layout needs to make
provision for areas where waste rock and
low-grade stock piles can be built, without
influencing mining activities or sterilising
mineable ground. The ground control
domains determine the angle of the pit
slopes and the bench heights to meet
the minimum factors of safety. The road
grades and width of the permanent ramps,
spirals, and loading and tipping points
are influenced by the size of the trackless
equipment and the effective safe operating
slope angle.
The layout of a mine or quarry is
also affected by legal and legislative
constrains, such as the property limits
of the approved mining rights area and
servitudes associated with public roads
and infrastructure, for example power- or
waterlines. The presence of protected flora
and historical sites or graves might also
influence the layout.
Economic factors
At the end of the day, the aim of all
operations is to make a profit. Economic
factors will determine the viability of
operating a quarry or open pit. The grade
of material might be good, but if there is no
demand or if the cost of extracting it is too
high, it will not be worth developing it.
Evaluating the costs associated with
a quarry or opencast mine can be done
using techno-financial modelling. The
different costs relate to establishing
and running of the operation. Capital
The equipment will be sized to meet the requirements set out by the mining schedule and the
capacity of the plant.