Page 85 - Rock Mechanics For Underground Mining

COLLECTING STRUCTURAL DATA

The more sophisticated core barrels are not required for very good quality,
strong rocks or when larger-diameter cores are taken. The usual core diameter for
geotechnical drilling is 50–55 mm. With larger core diameters, mechanical breakage
of the core and erosion at the bit are less likely to occur, and recovery is correspond-
ingly higher. Unfortunately, drilling costs vary approximately with the square of the
core diameter, and so a compromise must be reached between cost and drilling quality.
Rosengren (1970) describes successful large-diameter core drilling operations car-
ried out from underground locations at the Mount Isa Mine, Australia. Thin wall bits
and up to 6 m long single-tube core barrels were used to take 102 mm and 152 mm
diameter cores in hard silica dolomite.

Drilling techniques and contracts. Because the emphasis is on core recovery
rather than on depth drilled, the drillers must exercise greater care in geotechnical
than in other types of drilling, and must be motivated and rewarded accordingly. It
is desirable that geotechnical drilling crews be given special training and that their
contracts take account of the specialised nature of their work. The normal method
of payment for exploration drilling (fixed rate plus payment per unit length drilled)
is generally unworkable for geotechnical drilling. A preferred alternative is to pay
drillers on the basis of drilling time, with a bonus for core recovery achieved above a
specified value which will vary with the nature of the rock mass.
To obtain good core recovery and avoid excessive breakage of the core, it is essential
that the drilling machine be firmly secured to its base, that special care be taken
when drilling through weak materials (the readings of instruments monitoring drilling
parameters can be invaluable here), and that extreme care be used in transferring the
core from the core barrel to the core box and in transporting it to the core shed.

Core orientation. If the fullest possible structural data are to be obtained from the
core, it is essential that the core be oriented correctly in space. Not only must the trend
and plunge of the borehole axis be measured (the trend and plunge of a line are
analogous to the dip direction and dip of a plane), but the orientation of the core
around the full 360 of the borehole periphery must be recorded. If this is not done,
◦
then the true orientations of discontinuities intersected by the borehole cannot be
determined.
Three general approaches may be used to orient the core correctly.

(a) Use the known orientations of geological markers, such as bedding planes,
cleavage or an easily identified joint set, to determine the correct orientation
of the core and of the other structural features that it contains. Even the most
regular geological features do not always have the same attitudes at widely
spaced locations within the rock mass and so this approach can be relied upon
only in exceptional cases.
(b) Use a device in the core barrel that places orientation marks on the core. Ex-
amples of such mechanical devices are the Craelius core orienter which uses
a set of lockable prongs to orient the first piece of core in a drilling run using
the existing core stub as a guide, and the Christensen-Hugel core barrel which
scribes reference marks on the core in an orientation known from a magnetic
borehole survey instrument mounted in the core barrel. Acid etching and clay
imprint methods are also examples of this general approach.
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