Page 86 - Rock Mechanics For Underground Mining

ROCK MASS STRUCTURE AND CHARACTERISATION

(c) Examine the borehole walls with a suitably oriented downhole tool and relate
orientations of features measured on the walls to those found on the core at the
corresponding depth. Instruments used for this purpose include cameras, televi-
sion cameras, periscopes, the seisviewer (an acoustic device) and the borehole
impression packer.

A complete discussion of these various techniques is outside the scope of this
text. Fuller details are given by Rosengren (1970), Goodman (1976), Sullivan et al.
(1992) and Nickson et al. (2000). Unfortunately, all of these techniques have their own
disadvantages and generally only operate successfully under restricted conditions. As
with many aspects of mining engineering, there is no universal or simple answer to the
question of how to orient core. Quite often, when the standard techniques do not give
satisfactory results, techniques suited to local conditions can be devised. Rosengren
(1970), for example, describes a technique developed for orienting core taken from flat
dipping, large-diameter holes at the Mount Isa Mine, Australia. The device ‘consists
of a marking pen fitted in a short dummy barrel and attached to a mercury orienting
switch. The barrel is lowered into the hole with aluminium rods and when nearly on the
face, is rotated until the pen is in a known position, as indicated by the mercury switch.
Thebarrelisthenpushedontotheface,andsomarksthecorestubinaknownposition.’

Core logging. The final stage in the geotechnical drilling process is the recording of
the information obtained from the core. Here again, the value of the entire expensive
exercise can be put at risk by the use of poor techniques or insufficient care. Generally,
the structural or geotechnical logging of the core is carried out by specially trained
operators in a location removed from the drilling site. The log so obtained is additional
to the normal driller’s or geologist’s log.
It must be recognised that the data obtained from geotechnical drilling may not be
used in planning studies or detailed mine design until after some time has elapsed.
Because of the considerable cost of obtaining the core, measures should be taken to
ensure that the fullest amount of useful information is recovered from it. An essential
first step in this regard is to take colour photographs of the boxes of core as soon as
they become available and before they are disturbed by the logging process or pieces
of core are removed for testing or assaying. The use of digital technologies makes
the taking, archiving and use of core photographs much easier and more reliable than
it had been previously. Brown (2003) gives an example of the use of a photograph
gallery manager within a geotechnical data management system.
The design of the logging sheet and the logging procedures used will vary with the
nature of the rock mass and with the project concerned. However, the geotechnical or
structural log will usually include information on the size, location and orientation of
the borehole, a description of the rock types encountered, together with a strength in-
dex (generally the point load index) and/or a weathering index, and, most importantly,
data on all discontinuities intersecting the core. These data will include the depth at
which the discontinuity is intersected, its nature (joint, bedding plane, drilling break),
its orientation, its roughness (generally on a multi-point scale) and the presence and
nature of infilling materials. Values of RQD or the results of in situ testing, such as
permeability tests, may be added as required. From the data recorded on the core log
and the driller’s or geologist’s log, composite logs may be prepared for subsequent
use by the planning or rock mechanics engineer.
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