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Drilling 137

FIGUre 8.2 Auger drilling system.

Rotary drilling techniques shatter the rock by applying pressure on the rock using a rotating bit
that is bearing an imposed load. Some bits crush the rock using rolling cutters that have hardened
teeth projecting from rollers while other bits grind away at the rock using tungsten-carbide or dia-
mond studded surfaces (Figure 8.3). The roller bits are usually used for soft rocks but they can also
be used on harder rocks, if the grain size of the rock and ability of the hardened teeth to shatter the
grains is adequate. For hard rock on which the roller bits are inadequate, diamond bits are usually
used.

drillinG fluid and circulaTion
Once crushed, the rock chips and dust are removed from the hole by either a high-pressure air
stream or, more commonly, by circulating a drilling fluid through the hole. The fluid is usually
injected down the drill string and returns to the surface via the annulus around the pipe. Reverse
circulation, in which fluid moves down the annulus and back up the interior of the drill string is used
in cases where the material being drilled is particularly granular.
Integral to the drilling process is drilling fluid. Although not particularly glamorous, drilling
fluid (usually called drilling mud because it is composed of water and clay) performs functions that
are necessary to economically obtain a stable, environmentally benign hole. One of those func-
tions is the removal of cuttings from around the bit. If the cuttings from the drilling process are
not removed, energy is unnecessarily wasted on grinding the cuttings to a fine powder that will
ultimately clog the hole. To remove the cuttings, the drilling mud is pumped down the hole (either