Diamond Drilling Equipment

Good results cannot be obtained without suitable equipment. By far the greatest amount of diamond drilling carried out in Australia is for mineral exploration, but the equipment which may be suitable for this purpose is not necessarily suitable for engineering investigations. Also diamond drilling equipment has evolved considerably during the past fifteen years but there is still much technically obsolete equipment in use.

The ideal requirement for engineering investigations is the complete recovery of undisturbed, oriented cores.

In practice some core losses appear to be unavoidable, and it is a major disadvantage inherent in diamond drilling that these losses are most likely to occur in the weakest parts of the rock mass. In other words there is a tendency for the most important material not to be recovered as core. Excavation of ground, in which core losses occurred, has shown that the losses are usually, but not invariably, due to some important weakness such as crushed rock, clay along a fault, a zone of extreme weathering or hydrothermal alteration.

However, it is possible to come close to complete recovery of undisturbed cores in many types of ground, by using the following equipment.

Drilling machine equipped with hydraulic feed mechanism.— The hydraulic rams tend to maintain a constant pressure on the rock face and the speed of advance of the bit is automatically adjusted to the resistance of the rock.

In particular the bit tends to move forward rapidly through soft seams and this action greatly increases the chances of their recovery. Screw-feed, the other common type of feed mechanism, is quite unsuitable since the speed of advance of the bit is controlled by the motor speed through sets of gears, and has to be changed by the driller who has only very indirect indications of the changes occurring at the face of the bit.

Drilling machine as large as practicable.—In general the larger and heavier drills give better results because there is less vibration in the drilling string than with smaller and lighter drills.

The largest standard size core barrels and bits.—The greater the diameter of the core the greater the resistance to damage by vibration and torsion. The Authority has standardised on coring equipment of N-size, this being the largest standard size readily available. This produces cores from 2 1/8 to 1 7/8 inches in diameter, depending on the type of core barrel and bit.

N-type core barrels with bottom discharge bits.—This combination practically eliminates the two chief causes of core loss, namely grinding which occurs when a loose piece of core rotates on top of stationary core and erosion of weak rock by the circulating drilling water.

Triple-tube core barreI.—This barrel permits the core to be extracted from the barrel without the disturbance which inevitably occurs when it is shaken out of the end of the conventional NM barrel. The third tube fitting inside the stationary inner tube, is either a steel tube which can be opened lengthwise, or is made of transparent plastic.