Page 83 - Rock Mechanics For Underground Mining

COLLECTING STRUCTURAL DATA

of the rock mass. However, they cannot yet substitute for sampling the rock by
coring.
The aim of geotechnical drilling is to obtain a continuous, correctly oriented sample
of the rock mass in as nearly undisturbed a form as possible. Therefore, the standard
of the drilling must be considerably higher than that required for normal exploration
drilling. In geotechnical drilling, it is necessary to aim for 100% core recovery. Any
weak materials such as weathered rock, fault gouge, clay seams or partings in bedding
planes, should be recovered, because a knowledge of their presence and properties
is essential in predicting the likely behaviour of the rock mass during and following
excavation. In normal exploration drilling, these materials are seen to be of little
importance and no effort is made to recover them.
Diamond core drilling is expensive, and it is important that the operation be ade-
quately controlled if full value is to be gained from the expenditure. Several factors
can influence the quality of the results obtained.

Drilling machine. A hydraulic feed drilling machine is essential to ensure high core
recovery. The independent control of thrust permits the bit to adjust its penetration
rate to the properties of the rock being drilled and, in particular, to move rapidly
through weathered rock and fault zones before they are eroded away by the drilling
fluid.
There is a range of hydraulic feed machines that are suitable for geotechnical
drilling from surface and underground locations. The use of lightweight (aluminium)
drill rods and hydraulic chucks permits rapid coupling and uncoupling of rods in
a one-man operation. Figure 3.18 shows some results obtained with one of these
machines, a Craelius Diamec 250, in drilling a horizontal 56 mm diameter hole in an
underground limestone quarry. Electronic transducers were used to monitor thrust,
rotary speed, penetration, torque and delivery and return water pressures and flows as
drilling proceeded. Changes in rock strength were reflected by changes in penetration
rate. Open fractures were typified by local steps in the penetration trace and by spikes
in the rotary speed and torque traces. Clay- or gouge-filled features also produced
irregular torque and rotary speed traces. In the case shown in Figure 3.18, a 12 cm
wide clay-filled fissure encountered at a hole depth of 17.61 m, caused the bit to block
and the drill to stall.

Figure 3.18 Strip chart record of a
short instrumented diamond drilling
run (after Barr and Brown, 1983).

65