In this article we will discuss about the types of drilling methods used in mining. The types are: 1. Percussive Drilling 2. Rotary Drilling.
Method # 1. Percussive Drilling:
In this method which is the oldest one of drilling, the hole is drilled by striking a number 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.
I. 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.
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The drilling tool used varies greatly in shape and cutting edge according to the type of ground to penetrate. 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 upto a maximum of 25 mm depth with the help of rods without installing any derrick.
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 cranck 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 corking level to which they are attached through a stirrup and a brace-head.
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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 level, 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 runs back to repeat the same process of drilling.
Instead of the stirrup, Dlinks may be used. Short rods are added till the depth drilled by such small rods is slightly more than the length of 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.
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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 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 diamonds drilling, 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.
Lining a Drill Hole:
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During drilling a steel pipe is used for lining the drill hole from surface upto the hard rock, and the drilling tool and rods pass through the pipe. Obviously, the length of the drill-hole upto 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 lining 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.
Drilling with manual labour without the help of power is suitable for holes upto 150 mm in diameter and upto a depth of 30 m 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.
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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 and 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.
III. 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 temperature screw. The rope from a winch is taken to the clamps of the temper screw across the pulley of the derrick.
Feed of upto 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 reclamped 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.
Method # 2. Rotary Drilling:
For rotary drilling, hollow drill rods of steel or aluminium are used. These are thread- connected and transmit torque and feed pressure to the drilling bit or drilling tool which is attached at the end of a column of the drill rods. Rotation of the drill rods is through gearing driven by a prime mover at the surface.
As the rods rotate, the drilling tool abrades the rock and the cuttings are cleared by pumping water under pressure or compressed air down the hole through the hollow drill rods. The water or air, along with the cuttings, comes to the surface through the space between the drill rods and the sides of the drill hole.
In some drillings, specially those for oil exploration, mud which is not very viscous, is circulated instead of water. The mud which keeps back any water, gas or oil pressure encountered during drilling is known by various trade names such as bentonite, aqua gel, etc. and there muds serve different in the mining areas it may be necessary to resort to mud flushing when passing through a fractured or friable zone.
Aluminium rods weight only half as much as steel rods, but owing to their bigger gauge they possess 90% of the mechanical strength of the latter. The couplings, which are the parts most exposed to wear, are made of chromium-nickel steel.
Aluminium rods offer numerous advantages, such as increased machine capacity, easier handling, more rapid and simple recovery of the drill string and faster rotation, all of which contribution to simplifying drilling and reducing costs.
The various methods of rotary drilling are known by the type of drilling tool used but the diamond drilling method is quite common.
This method is commonly adopted where cores of rocks passed through are desired for accurate records of the strata or for testing the rocks for their strength, composition, porosity, etc., the common type of drill bit which consists of a cylindrical cast steel shell having in its lower face a number of small sockets in which pieces of black diamonds are set.
These diamonds are not useful as jewelry but are used in the drill bits for their hardness and the bit is suitable for the hardest rocks. The hole sizes in diamond drilling are designated as NX, BX, AX, and EX. The drill rods and the drill bits are specified under two main groups, X series and W series, as per the standards laid down by DCDMA (Diamond Core Drill Manufacturer’s Association), an international Association.
The drill hole diameters and core sizes (in mm) available are given below:
NW series rods are of W and conform to international standards for conventional drilling. NQ series drill rods are manufactured by Long year for wire line drilling technique. There are Q series standards for wire line drilling rods but some manufacturers have their own sizes.
The core sizes where wire line drilling technique is adopted are- NX holes – 44 mm; BX holes – 25 mm.
The surface arrangements for diamond drilling include:
I. A derrick,
II. An engine for supplying power,
III. An winch,
IV. A pump for supplying water under pressure for flushing,
V. A setting tank,
VI. A platform for keeping the drilling rods lifted for removal of core or changing the bit,
VII. Core boxes for keeping the cores, and
VIII. A driving and feed mechanism for the drill rods.
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.
On smaller machines, driven either by hand-power or mechanically, the feed is by a screw feed arrangement comprising a series of differential gears.
In this arrangement the drill rods pass through a hollow screw shaft, threaded on the outside, and provide with a long keyway. A bevel pinion, rotated by the bevel gear of the main driving shaft, has feathers engaging the keyway on the screw shaft, to which it imparts rotation. It also drives gear wheel a, engaging with gear wheel B on a rotation.
It also drives gear wheel A, engaging with gear wheel B on a countershaft. Usually three different combinations of gear are provided here, any of which can be utilised to vary the rate of advance to suit the type of rock.
Gear B, through the counter-shaft, drives C, which engages a fourth wheel D, threaded internally to fit the threads of the screw shaft. If, for example, the number of teeth on gears A, B, C and D are as shown in the diagram, viz. 38, 36, 24 and 25, one revolution of A = 38/36 revolution of B and C, = 3/836 x 24/25, revolutions of D.
Therefore 75 revolutions of A will cause D to rotate 75 x 38/36 x 24/25 = 76 times. Consequently for every 75 revolutions of A, D revolves 76 times, and the screw shaft moves forward by a distance equal to the pitch of its thread. If this is 6 mm, then the rods advance 24 mm for every 300 revolutions. The movement of the screw shaft is imparted to the rods by the chuck.
A pressure gauge attached to the roller friction collar records the varying pressure on the bit. The setting of the feed decided upon is- obtained by sliding a lever to the desired position for locking the appropriate, loose gear, giving the speed required for the ground being bored.
The feed ratio may be changed while the drill is operating; but if any necessary change is neglected, so causing the drill rods to advance too rapidly (e.g., when a sofer stratum is suddenly encountered) the rate of advance is automatically checked by the slipping of a spring- loaded friction cone-clutch on the bottom of the countershaft.
Fig. 3.8 shows the general principle of a typical hydraulic feed mechanism (excluding the engine and frame) for a deep boring, the main features, starting from the bottom, being the chuck, the bevel driving gears, the hydraulic cylinder and the ball-bearing suspension box. At the top is seen a lifting bail and water-swivel whose functions are self-evident.
There are three hollow rods, one within the other, namely, the inner bore rod, the middle drive rod and the outer piston rod. The hollow bore-rod is clamped to the drive-rod by the chuck. The drive rod is secured to and supported by the collar-plate in the suspension box, the latter remaining supported by being fixed to the hollow piston rod which carries the piston in the hydraulic cylinder. This cylinder is firmly fixed to the engine frame work.
The rate of feed of the bore-rods is governed by the rate at which the piston descends, for this governs the descent of the suspension box and therefore, of the collar plate and drive-rod to which the uppermost bore- rod is clamped. It will be seen that the drive rod is free to rotate within the suspensions box and within the hollows piston rod.
The drive-rod had on its outer surface ribs or splines which pass through slots or grooves in the box of the horizontal bevel drive-wheel. It can thus be rotated by the gear wheels (whose position is fixed) but it is free to descend through the horizontal wheel, carrying with it the bore-rods and boring tools at their lower end.
The piston in the hydraulic cylinder may be moved either up or down by admitting water under pressure to the appropriate side of the piston through the inlet pipe and one of the controlling valves, V1 or V2 and by simultaneously releasing an equal amount of water from the other side of the piston through one of the valves V3 or V4 and the outlet.
A single lever operates the four valves simultaneously to produce any desired pressure, either downward or upward, on the piston. Thus the weight of the rods may be partly taken off the boring tools by upward pressure; or the whole weight of the rods can rest on the boring tools and additional downward pressure can be applied. In this way complete control may be obtained over the pressure on the boring tool and over the rate of forward feed of the rods.
The cuttings are cleared from the drill hole by circulating water under pressure which is forced down the drill rods by a pump through a flexible hose pipe and water swivel connection. The return water from the hole goes to a settling tank and it is used over and over again.
To collect the core of the rock drilled, a device known as the core barrel is used. It is length varies from 0.5 to 3 m.
There are two types of this:
1. The single tube core barrel, and
1. Single Tube Core Barrel:
A single tube core barrel is suitable for homogeneous formations where the core is not eroded by flushing water and a solid core can be taken without risk of blockage in the barrel.
The connection of the diamond crown, the single-tube core barrel and the mud bucket (also called calyx) are shown in fig. 3.10. The core lifter is placed within the bevel shell which has its inside conically shaped to receive the former. The core lifter is corrugated on the inner face and is a split ring. It occupies the wider portion of the bevel shell when drilling takes place so that it has little or no tendency to grip the core.
After certain progress in drilling when the rods are lifted to take out the core, the split ring descends inside the bevel shell and grips the core. The latter may now be broken off by a twist and raised to the surface. The core is replaced after about 250 m of drilling.
The larger particles of drill cuttings which the circulating water fails to carry upto the surface settle down in the mud bucket. Where supply of flushing water is plentiful, calyx is not necessary. The water under circulation is nearly 900 litres per minute.
In soft and friable rocks, the core is partially washed away due to the circulating water flowing past it. The rotation of the barrel greatly assists in grinding the core so that its recovery in a single tube core barrel is poor. To avoid these difficulties a double tube core barrel is used, specially where good core of soft rock is desired.
2. Double Tube Core Barrel:
In a double tube core barrel in inner tube with holds the core does not rotate during drilling as it is suspended on ball bearings mounted in the block at the top of the barrel permitting the inner core barrel to remain stationary. Moreover, water does not flow past the core but in the annual space between the inner tube and the outer barrel and through channels near the bottom of the hole.
A double tube core barrel improves drill bit economy and overall drilling performance. Core recovery is good in hard uniform rocks, but poor in loose, soft, friable or weathered rocks. Vibrations of drill rods result in poor recovery. In hard rocks, to achieve good core recovery the drill should be run at low speed and heavy pressure; in soft rocks, reverse procedure should be adopted. The combination of pressure on drill bit and its rotational speed should be such as to give vibration-free drill string during drilling.
Normally core barrels of 0.5 m to 3 m length are employed. For removal of core during the conventional method of drilling, all the drill rods have to be withdrawn to the surface, after drilling a length equal to length of core barrel. The withdrawal of the rods and their re-introduction into the borehole with the additional drill rod, after removal of core, takes a considerable time, nearly 75 to 90% of the total time spent on drilling.
A wire line drilling technique is an improvement to reduce this time. The rods are not taken out to remove the core which is collected in the core barrel tube during drilling. The tube is pulled out the surface through the drill rods with the help of a catcher which is lowered through the rods by a 5 mm dia. wire rope. The catcher grips the tube containing the core.
Core size less than BX is not possible with wire line coring equipment which is therefore used for drilling holes of NX and BX size only. The speed of drilling with this equipment is nearly 18 m per shift (8 hours) in the types of rocks met with in the coalfields. All the drill rods need to be withdrawn to the surface only when the bit has to be changed.
Wire line drilling is possible upto a depth of 1000 m. The rods used for wire line drilling have specifications as laid down in “Q” series decided by DCDMA. Ordinary drilling equipment can be adopted to wire-line drilling and hoisting equipment with suitable modifications.
Some of the recent drills in the market are equipped with hydrostatic drive. In such drive and electric motor or a diesel engine drive a water pump. Its pressure is used for rotation of drill rods through a hydraulic motor and also for hydraulic feed. Its main advantage is that speed can be varied from zero to a certain limit without any fixed ratios that are possible in a geared drive.
When drilling in fractured zone or strata with cavities the circulating water is lost in such zone and fails to appear at the surface. This is known as water loss. To seal up the cavities or fractures around the hole, saw dust, husk, etc., are mixed in circulating water.
If this is ineffective, it may be necessary to ream the bore hole and fix a casing pipe to cover the fractured zone. The casing pipe can be removed after completion of drilling. At depth, instead of resorting to a casing pipe a special type of mud like bentonite or kaolinite may be used to overcome the water loss.
Drilling mud is a suspension mixture of certain types of colloidal clay in water and/ or oil and used as drilling mud. The mud most commonly used in diamond drilling is slurry of clay and water, properly controlled drilling mud slurries can prevent caving or collapse of borehole sides by building thin, impermeable protective coatings of clay particles to the walls of the hole. The mud is generally used as a final resort and the water circulation pump has to be replaced by a suitable mud circulation pump for this purpose.
Diamond drilling method is suitable for drilling at any angle to obtain cores of friable strata as well as the hardest rock. It has been adopted for drilling upto a depth of 3000 m and hole diameter upto 200 mm. Drilling upto such large depths is not required in coal mining areas where the maximum drilling depth is upto 1000 m, as coal mines are rarely deeper than that.
Other methods of rotary drilling differ from the diamond drilling method essentially in the type of drilling bit used.
The drill bits used are as follows:
I. The Saw-Toothed Crown:
The drilling tool is a saw-toothed steel crown or cutter. The teeth are set alternately inward and outward to give the necessary clearance. The speed of rotation is only 5 to 10 r.p.m. and the drill bits are suitable only for drilling through rocks of medium hardness; only holes of diameter not less than 150 mm are possible.
II. Rock Roller Bits:
These are suitable for hole diameter between 75 mm and 300 mm. In mining areas these are commonly used for drilling large diameter holes in mechanised quarries. Flushing of the hole with compressed air instead of by water under pressure is the common practice with this type of bit. Rock roller bits can be used for deep hole drilling with speed and are suitable for mostly vertical downward holes.
III. Chilled Steel Shots:
These shots are prepared by heating very finely divided steel particles to a very high temperature and then suddenly cooling them in ice cold water. Chilled shots are used in conjunction with a plain steel shell or cylinder with a diagonal slot near the bottom.
They are fed through the hollow drill rods and pass to the bottom of the hole where they get caught between the bottom end of the cutting shell and the rock. As the shell and the drill rods rotate, the chilled steel shots cut the rock by a milling action.
The method is suitable for vertical and large diameter holes of 100 mm to 750 mm. It is also called calyx drilling, but is not much favoured these days as diamond drill bits have gained wide popularity and are available in large diameters upto 250 mm.
Calyx drilling proved to be a significant step during the development of drilling techniques. Except the rock roller bits, all the other drilling tools used for rotary drilling provide cores of the strata passed through.
To drill a hole from underground workings for purposes of prospecting, stowing, tapping water or gas or any other object, the drill equipment has necessarily to be of smaller dimensions. This restricts the size and weight of the machine and therefore puts limitations on the size of the bore hole and its length (or depth).
The power available underground may be electricity at 440/550 volts, or compressed air (in most of the metalliferous mines). Flame proof diesel engines are rarely employed as power sources. At some underground working places the water supply for drilling may be on a very limited scale and not as plentiful as on the surface.
Underground drills for exploration have often to be shifted to blind ends of roadways with narrow dimensions. They are, therefore, usually with skid plates. Components of aluminium alloy are nowadays increasingly used for such drills to reduce weight.
Water Development Society, Hyderabad, manufactures drills of the following sizes. The drills are available with hydraulic, semi-hydraulic and pneumatic operating systems with an option of diesel or electric prime-mover. Underground drills can be provided with DTH & Drifter attachment.