Mining_Methods_UnderGround_Mining - Mining and Blasting

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exploration drilling will not be carried
out by the same people, so reliability of
information is critical. There are many
reasons why geologists should choose
their drilling method carefully.
If there is no need for continuous information
about the geological formation
on the way down, there is no need
for samples. It is just a matter of minimizing
the drilling time. The geometry
of the orebody is already known, and
just a reconfirmation of the boundaries
is necessary. In this case, RC drilling is
an efficient method to use.
A first scanning of virgin territory
is being done where the goal is just to
obtain a preliminary indication of possible
content. In this case, the geologist
is not relying on any mineralized struc-
ture or geometry. With an evaluation
giving positive results, a programme of
core drilling is the logical way to con-
tinue in order to bring the project to a
resource/reserve status.If the minera-
lized structure is identified but the
geometry and rate of content varies,
RC drilling is used as an indicator for
ensuring continued grade control.
The geologist wants dry and repre-
sentative samples in order to make opti-
mal evaluations. RC drilling below the
groundwater table was previously believed
to undermine sample quality.
Core drilling therefore remained the
only viable method for these depths
Today, the availability of high pressure
compressors and hammer tools makes
it possible for RC drilling to reduce
costs even for these depths.
These days, professional contractors
deliver dry sampling down to depths
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of 500 m. By sealing off the bit from
the rest of the hole it can be kept dry.
A correct selection of shroud vs bit tolerance
maintains a pressurized zone
around the bit. Boosted air pressure is
needed to meet the higher water pressure
on its way down the hole. In addition,
a dry bit drills faster.
It must be remembered that infor-
mation from a core is crucial in esti-
mating the period of mineralized struc-
tures. The core helps the geologist to
calculate the cost of extracting the
mineral from the ore. Large volumes of
rock have to be excavated to obtain just
a few grammes of a valuable mineral.
Cores also yield geotechnical data.
Data about slope stability can be of the
highest importance. Ground conditions
are naturally also of great importance
and may produce questionable samples
if some of the information from
fissured zones is left behind in the
hole and not collected. In such circumstances,
core drilling could be the only
alternative.
increased usage of
rC drilling
RC drilling is on the increase, and may
well account for 55% of all metres
drilled in 2008. The diagram above
shows some estimated ratios between
core and RC drilling in different parts
of the world in 2002. In terms of metres
drilled, RC accounts for 50% and core
drilling for 50%. Tradition and environ-
mental impact play large roles. RC rigs
are heavy, and are mounted on trucks
or track carriers. This fact tends to
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Ratios between core and RC drilling. The figures reflect total exploration expenditures from national statistics for surface and underground.
favour core drilling rigs, which are much
lighter and more adaptable in order
to be flown into remote and sensitive
environments.
In areas with extremely cold climates
and where permafrost is present, RC
drilling may have its limitations. Antifreeze
rock drill oil can help to keep
the hammer and bottom of the hole free
from ice. Other purely practical issues
determine the choice of one or the other
drilling method.
An intelligent, balanced choice be-
tween the two methods is the key to
optimal results. The geologist plays an
extremely important role in finding this
balance, as do the manufacturers such
as Atlas Copco Geotechnical Drilling
and Exploration, who continue to provide
the right tools for the job.
Explorac 220RC.
Jan Jönsson
rC drilling
Core drilling
underground mining methods 19