field of usefulness in foundation grouting. However, they are more costly than
coring bits for drilling in extremely hard foundations and in badly fractured
rock because of greater diamond cost. Since they produce only cuttings, more
diamonds are required to make a given footage of hole than if a large part of
the rock encountered is removed as core. The loss of one or two diamonds
from the center of a noncoring bit (a not infrequent occurrence in drilling
shattered rock) renders the bit useless for further cutting. The plug bit is
less expensive than the core bit in deep holes due to the time saved by not
having to pull out of the hole to empty the core barrel or to clean a blocked bit.
( c ) Size. The sizes of diamond bits are standard and are generally shown
by the code letters EX, AX, BX, and NX. The dimensions of each size are
tabulated below. Most diamond-drilled grout holes are EX or AX in size.
There is insufficient advantage in larger bits to justify their use. The possi-
ble advantage that the larger diameter bit may have in encountering more
fractures than the smaller is more than offset by the fact that the greater
economy of the small bit permits a closer spacing of holes for the same
o v e r a l l cost.
(2) Hard metal bits. Drill bits of hardened steel notched to resemble
the teeth of a saw can b e placed on the core barrel to substitute for a more
costly diamond bit. In some soft rocks this type of bit will make a hole much
faster, is not as easily blocked, and is much cheaper than a diamond bit. Of-
ten the teeth of such bits are faced with one of the alloys of tungsten carbide,
or replaceable inserts of a hard alloy are welded into holes cutinto the bit
blank. A noncoring bit can also be made with the hard alloys by studding the
cap for a piece of drill pipe with bits of the steel rod containing the powdered
alloy and adding waterways.
(3) Rock bits. Rock bits, like diamond bits, are attached to the bottom
of a column of h O ll OW drill pipe. The bit is made of toothed rollers or cones,
each of which turns or rolls on the rock as the bit rotates with the drill pipe.
Cutting is accomplished by crushing and chipping. The shape of the teeth,
their attitude and number, and the number of rollers vary. Most bits have
three or four cones or rollers; some have two. The teeth and other parts of
the bits subjected to intense abrasion are made of hard alloys. Cuttings and
sludge are washed out of the hole by circulating water or drilling mud through
the drill pipe and back to the surface between the drill pipe and the walls of
the hole. The roller rock bit is not extensively used for grout-hole drilling
because the smallest available size is approximately the same as that of an
NX diamond bit.