Percussive drilling

In this method which is the oldest one of drilling, the hole is drilled by striking a number of blows at short in intervals on the rock by a chisel-type tool and between the blows the tool is rotated slightly. The rock is chipped away with each blow and a circular hole is formed. During drilling the chisel is suspended from the surface by rods or wire rope and the weight of the chisel, rods, etc. is utilised to give the striking force.

Drilling with rods

The rods are Ni-Cr or carbon steel. Each rod has a male screw at one end and a female screw (screwed socket) at the other. Steel rods are usually in lengths of 3 m with nearly 38 mm X 38 mm square cross-section. For rotary drilling, described later, the rods are hollow, circular in cross section, have flush joints, and the length varies from 0.5 to 3 m.

The drilling tool used varies greatly in shape and cutting edge according to the type of ground to penetrate. A few common types are shown Fig. 3.1. For soft surface deposits which consist of alluvium, clay auger and worm auger may be used. These are given a rotary rather than a percussive motion. The straight chisel is commonly used for hard strata and the V chisel and T chisel, for very hard strata.

Every type of drilling requires a derrick which may consist of three or four legs and may be of wood or tubular steel. It is used chiefly for lifting the rods from the hole with the aid of a winch. In the mining localities some petty drilling contractors undertake percussive drilling of 50 mm to 75 mm holes up to a maximum of 25 m depth with the help of rods without installing any derrick.

Manual drilling

The general arrangement for manual drilling is nearly like that shown in Fig. 3.3 except that water is not supplied through hose pipes under pressure and the crank operated beam is replaced by a rocking lever connected to the drill rods. "The drilling rods are given a percussive motion with the help of a rocking lever to which they are attached through a stirrup and a brace-head. A brace-head is simply a pair of crossed handles fixed to the end of a short top rod which is screwed to the column of the rods. Two or three men press down the free end of the rocking lever, thereby lifting the rods while one man turns them slightly by means of the brace-head. The men then let go the free end so that the rods fall and the drilling tool gives a blow on the rock. Water is poured in the hole at intervals and the process is repeated. As the hole gets deeper, the rods are lowered in the stirrup by a screw, and when this can no longer provide for the increasing depth, a short rod is added to the column of rods and the screw run back to repeat the same process of drilling. Instead of the stirrup, D-links may be used. Short rods are added till the depth drilled by such small rods is slightly more than the length of a full-length rod and the short rods are then replaced by a full-length rod. A device known as retaining key is used at the time of raising or lowering the rods of square cross-section. The same purpose is served in the case of rods of circular cross section with flush joints, by a device known as "bulldog safety clamp".

During drilling the bottom of the hole soon gets filled with cuttings and has to be cleaned out frequently. This is done with the help of a sludger which consists of a long cylinder or pipe, open at the top and with a flap valve at its lower end. The flap valve opens upwards. When attached to the end of the rods and worked up and down the sludger gets filled with the sludge. It is then withdrawn to the surface and the process repeated till the hole is cleaned. The cuttings brought to the surface in the sludger give an indication of the rock being drilled. The bottom of the drill hole is always kept full with water during drilling.

Sometimes a rod or chisel breaks in the drill hole during drilling. Devices like the crow's-foot and the spiral worm (Fig 3.2) may be used to catch the broken rod under a Joint in the borehole. Broken pieces of chisel are sometimes raised with the help of powerful magnets. In diamond drilling, described later, the diamonds sometimes become loose and fall in the hole. The operation of tracing the broken and lost parts in the hole and withdrawing them to the surface is known as fishing the borehole.

TABLE I.

Uses and limitations of common methods of exploration drilling.

Type of drill	Common Sizes of drill holes(mm)	Maximum economic depth of hole (m)	Types of bits used	Rock formations where used,
1, Percussive with rods	50-75	250-300	Chisel shaped	Sand and clay and sedimentary rocks of soft and medium hardness; in fissured formations; Does not give core.
2. Churn drilling or cable drilling	75-500	300-600	Steel choppy bits of many styles.	Placer deposits; evenly firm and moderately soft formations.
Rotary drilling
3. Non-coring drilling	100-300	500	Tri-cone rock-rollers bits	Any rock formations except very hard
4. Diamond drilling	38-200	100-3000	Diamond bits of various types tungsten carbide bits; tricone bits arc mostly noncoring.	Any rock formation except fissured. Holes can be drilled from surface or underground working at any angle to the horizontal; used mainly for coring
5. Calyx or chilled shot drilling	75-1800	up to 450	Calyx; chilled steel shots;	All rocks except the hardest; unsuitable for soft and fissured formations; can drill at angles up to 35° from the vertical.

Lining a drill hole:  During drilling a steel pipe is used for lining the drill hole from surface up to the hard rock, and the drilling tool and rods pass through the pipe. Obviously, the length of the drill-hole up to which the lining pipe has to be fitted should be of a larger diameter. The lining pipe is generally withdrawn after the hole is completed though it may sometimes be necessary to leave it in its position to prevent caving of sides, e.g. drill hole for stowing, water pipes, etc.

The fining is done by hammering first a special steel pipe with a cutting edge. Pipes of 6 m lengths and having screwed joints are added to that pipe.

Power drilling with rods

Drilling with manual labour without the help of power is suitable for holes upto 150 mm in diameter and upto a depth of 30m or so. Beyond that depth, it is impossible to drill without the use of power from a diesel or petrol engine, the common source of power in isolated drilling sites. Vertical boilers have sometimes been used to avail of steam power, specially where drilling had to be done in a colliery area where coal is easily available.

Fig. 3.3 shows the general arrangement where power is available for drilling with hollow rods. A power operated winch is used to raise and lower the rods. The walking beam is operated by a crank through gearing from an engine to give the drilling tool 25 to 30 blows per minute and a stroke of nearly 225 mm. The beam is mounted on steel springs which give elasticity and cause sudden recoil of the frilling bit thereby preventing jamming. The rods are attached to the rope with a swivel attachment and brace head. The rope of power-operated beam is slackened from time to time to keep the drilling tool in contact with the rock. Water is forced down the hole of the hollow drill rods by a pump to keep the cutting tool cool. Such water returns to the surface from the outside of the rods with the sludge. With this arrangement it is possible to drill a depth equal to the length of one full rod at a time. Water flushing practically eliminates the use of a sludger.

Rope drilling or cable drilling

Where the percussive method of drilling is employed cable drilling is commonly adopted for holes deeper than 30 m. In this system the rigid rods are replaced by a steel wire rope to which the drilling tool is attached. The surface arrangement is practically the same as for drilling with rods, but the end of the walking beam is attached to a temper screw (Fig. 3.4). The rope from a winch is taken to the clamps of the temper screw across the pulley of the derrick.

Feed of up to 1.2 m is possible with the use of the temper screw. When no more feed is possible the temper screw is run back and the rope re-clamped 1.2 m higher up.

During rope drilling no device is necessary to give a twist to the drilling tool between successive blows as the lay of the stranded rope causes the tool to twist slightly. The steel rope may be 18 mm diameter for a depth of 300 metres. The ropes have always a left hand lay, so that the spin of the rope which tends to rotate the drilling tool also tends to tighten the joints between them.

Cable drilling is also known as churn drilling.