In terms of both equipment and sampling, RC drilling is different from diamond core drilling. The major difference being the rate of penetration, cost per foot, and the sample type. RC drilling equipment is relatively larger with a high-capacity air compressor in order to handle larger down hole equipment. Pneumatic reciprocating piston or DHH drives the drill bit, which has round protruding “TC” buttons to cut the rock, and the drill rods rotate at speeds of 3050 rpm.
Another type of bit is called “hammer bit,” which rapidly pounds and pulverizes the dense and hard rock formation normally above the water table. When the bit encounters the water table, the shattering becomes less effective. Bits consisting of “three revolving cone shaped grinders” (Tricone bit) are very effective in softer formations and in wet drilling conditions but slower in hard rock conditions.
A PVC or metal piping is installed at the surface to prevent collapse of unconsolidated material into the hole before commencing drilling. The stability of the surface formations dictates the depth. Air is pumped down the drill rods between inner and outer tubes. The returning air through inner tube brings up cuttings to the surface. The compressor forces air down the outer space of a double-wall pipe.
A cyclone chamber slows down the rock chips that travel at high velocity and then make the chips spiral downward to the cyclone bottom. As the drill advances, continuous sampling of rock chips can be done. The RC drilling is usually either 6v or 8v in diameter.
Drill cutting samples are collected at regular intervals (50). Owing to large diameter hole, huge volume of sample material constitute each sample which are usually split into a reasonable volume to handle (one-eighth of total) and sent to laboratory. A dry sample splitter (Jones splitter) is used in dry drilling conditions. On the contrary, when the drill reached the depth of water table, a rotary wet splitter is used, which spins around and splits the sample using a series of fins like the fins of a turbine engine.
Samples of small rock chips are collected in plastic boxes (chip trays). Geologist-in-Charge observes and carefully logs the samples. It is obvious that structural details, etc., of the formations drilled may not be available since the samples obtained are rock chips, but systematic microscopic examination of the samples will reveal valuable information. Besides, the other tests like fluorescence and effervescence can easily be carried out.
Compared to RAB or air-core rigs, RC rigs are larger, slower in operation, expensive, and can achieve better penetration in hard rocks, sometimes up to the depths of about 500 m. RC drilling is one of the preferred methods for preliminary mineral exploration work, since it is less expensive than diamond core drilling. Owing to their size, these rigs need good roads to mobilize them to drill sites. RC drills generally are ably supported by auxiliary vehicle(s) for diesel, water, maintenance supplies, spare compressor, booster compressor, etc.
