Percussive Drilling
It is the oldest method of drilling. In this method a great variety of bits suspended from cables or rods have been used to bore through different kinds of rock. Rope boring was widely used for oil well drilling in which they raised a heavy blunt chisel type of bit jumping on the end of a cable. This enabled a great speed in fairly hard rocks. This blunt type chisel bit is not suitable for clays and soft marls which not only cushion the blows but ball up the bit until it attains a sucking action that renders the bit almost useless. For soft mudstones and shales a long heavy and narrow tool proved successful.
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In some cases, a bit of fish-tail pattern was used in soft but less sticky formations. But for harder rocks the chisel type bit or blunt faced heavy chisel bit was the one most used today in percussive drilling. Following figure shows different types of bits used in percussion drilling.
Different types of drill bits used in percussive drilling
Percussive method of boring is particularly suitable for drilling in heterogeneous glacial deposits and other stony overburden, or where hard quartzite pebbles are so loosely set in a matrix of soft sand that they become like a broken ball rock and are often very destructive of any kind of rotary bit, diamond or otherwise.
This method is at its best in rocks which fragment well under blows. It requires less quantity of water, just sufficient to make the debris into a slurry which can be removed by a sludger. But with this method of boring samples are often contaminated by a portion of higher strata dislodged during the drilling process.
Diamond Drilling
The surface arrangements for diamond drilling include, as shown in given figure.
a derrick,
an engine for supplying power,
an winch,
a pump for supplying water under pressure for flushing,
a setting tank,
a platform for keeping the drilling rods lifted for removal of core or changing the bit,
core boxes for keeping the cores, and
a driving and feed mechanism for the drill rods
Surface arrangement for rotary drilling
The diamond drill bit is rotated at a speed of nearly 300 r.p.m. and the pressure on the diamonds is between 1.5 and 2 kgf/cm2. The pressure acting upon the diamonds of the drill bit and the rate of advance of the drill bit into the rock are controlled by an arrangement known as "feed mechanism". The feed mechanism is hydraulic for deep holes, but may be replaced by screw feed for shallow holes. Beyond a depth of nearly 60 m, the weight of the rods keeps the bit pressed against the rocks and the feed mechanism may not be necessary. At greater depth the feed mechanism is operated in such a way that the weight on the drill bit is not excessive.
Drilling Patterns
V, V1, V2 Patterns
These Pattern are far superior, to row delays. These result in superior fragmentation due to reduce hole burdens and increased spacing at the time of hole initiation and also due to in-flight collision of broken rock during its movement. The delayed action of holes in the back row reduces over break ensuring increased wall stability.
The best available pattern is one where the holes are drilled (staggered) on a equilateral triangle pattern. This in a drilled spacing to burden ratio of approximately 1.16. It has been observed that an effective spacing (Se) to Burden (Be), ratio of about 3.5 is achieved with holes drilled on an equilateral triangle grid and fired using a V1 initiation sequence.
Drilling (staggered) equilateral triangular pattern require more operator skill and supervision as compared to in-line patterns. Clear marking of the hole positions in advance by a responsible person would help the drillers immensely. Fig. gives various delay patterns discussed above.
Down the Hole (DTH) Drill
"Down the Hole" or "DTH Hammer" drill is meant for drilling borehole in hard compact rock formation. It combines both percussion and rotary action and powered by compressed air. If required, DTH units are provided with mud pump for negotiating the loose overburden. The hole diameter varies between 115 and 165 mm for a depth of 125 m. The drilling tool is a cross bit or a button bit. In DTH hammer drill the assembly of the drill bit and its short-length pipe is called the DTH hammer. The lowering and hoisting of the drill rods are operated by a chain.
The drill is levelled with three hydraulic jacks. The drilling is usually vertical with a possible swing of 20 on either side from vertical in some model. DTH hammer rock drills have been leading the way in performance, reliability and longevity since their introduction 15 years ago.
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