Site Investigation - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 359

Figure 13.5 Diamond and tungsten carbide bits (Atlas Copco Group)
A drilling fluid flushes the borehole in order to cool the bit and to lubricate the drill rod
to reduce friction and wear. It may also serve to stabilize the borehole wall. Depending
on the rock mass conditions, water, clay or bentonite-based drilling fluids (slurry) and
other liquids or gases such as air, for dry drilling, may be used. Additives help to obtain
a better core recovery, a better penetration rate and a higher lifetime of the equipment.
Core barrels may be classified into single tube, double tube and wire line core barrels.
Core recovery up to depths of more than a thousand meters is nowadays carried out
almost exclusively by wire line core barrels, while for shallower depths single and double
core barrels are used.
Single tube core barrels (Fig. 13.6, left) are predominantly used in hard and massive
rock. Since rotation and flush attack the core, loose rock may be eroded or washed away
when drilling in soft or weak rock containing fines. However, if air flushing is used,
single tube core barrels may also be applied in these rock types. When using single tube
core barrels in brittle rock there is a risk of disking or crushing of the core.