7.2 SAMPLING EQUIPMENT
7.2.2 Drilling Techniques
The ancient Egyptians introduced a drilling principle for boring short holes during construction of the Great Pyra- mids of Giza during 2560 BC. Thefirst drill machine was manufactured in 1862e63, operated by manual rotation.
The need for drilling increasingly stimulated manufacture for effective design in machines and accessories to improve efficiency and cost. The search for deep-seated orebody, petroleum, and gas reservoirs was unthinkable without efficient drill machines that can collect samples at
depths of þ2000 m (Banerjee and Ghosh, 1997; Atlas Copco, 2014).
The drilling techniques are essentially based on three motions: percussive, rotary, and a combined effect of percussive and rotary (Bremner et al., 1996; Devereux, 1999; DMP, 2012). The various drill types are:
1. Percussion drilling.
2. Percussive cum rotary drilling.
3. Auger drilling.
4. Diamond drilling.
5. Wire-line drilling.
6. Reverse circulation drilling.
7. Air-core drilling.
8. Sonic drilling.
9. Directional drilling.
10. All hydraulic drilling.
7.2.2.1 Percussion Drilling
Thepercussionorchurn drilldigs a vertical hole using the principle of a freely falling chisel bit hanging by a cable to which percussive motion is imparted by power units. The power units work by manual lift and drop, compressed air, and electrically driven winches. A tungsten carbide bit fitted in a hammer is lifted a few meters up and allowed to drop (Fig. 7.1) to hit the bottom of the hole. The process continues in succession. The churning motion of the bit
FIGURE 7.1 Schematic conceptual diagram of the percussion drilling procedure often used in engineering geology for foundation testing and occasionally in the initial stage of mineral exploration and particularly for a low-profile budget.
crushes and scrapes the ground to dig a hole. The rock cuttings produce mud or slurry brought about by lowering water. The crushed material is removed from the hole bottom at regular intervals to form a sample. Churn drilling is suitable for soft and medium formation. The cutting bit is required to be resharpened frequently in hard formations resulting in low progress at high labor cost. The capacity of the churn drill is limited to relatively short holes of 10e50 m.
The cost of sampling is comparatively much lower than diamond drilling under similar conditions. However, the chances of contamination between samples are high mak- ing the method inefficient to demarcate correct orebody contacts and to assess the average grade of the deposit. It provides information regarding the presence of minerali- zation that can be precisely explored by diamond drilling.
Sample collection is modified by connecting a water pump (Fig. 7.2). The returning water brings rock cuttings to the surface to form a regular sample. A further modification is made by cutting a slit in the bit to hold cuttings (samples) at standard intervals. The method is preferred for tube wells and foundation testing.
7.2.2.2 Percussive Cum Rotary Drilling
The percussive cum rotary drill uses an integral or detachable tungsten carbide bit that penetrates the ground due to the resultant action of both percussive and rotary motions, e.g., jackhammer (Fig. 7.3) and wagon drills. The percussive action produces a vertical impact on the drill rod to break rock particles. The rotational motion exerts a force
on the bit head to penetrate into the rocks. The drill depth is limited to 6 m. Drills are compressed air driven. Water is injected through hollow steel drill rods to cool the bit head from excessive heating. The returning water flushes out cutting material from the hole for free movement of bit and rods. The cuttings serve as samples to understand the metal content of the advancing face. These drills are used pri- marily for the development of tunnels, advanced mining faces, and for breaking big boulders in construction areas.
They have limited use in mineral exploration. Samples are used to estimate approximate metal content of big rock exposures and mine blast quality in advance for grade control and scheduling. They provide information about the roof andfloor of the coal seam, including the thickness of coal bands within.
7.2.2.3 Auger Drilling
Theauger drill has limited use, but it plays a significant role in sampling and evaluation of soft and loose ground like soil, beach sand, mine dump, concentrate, and tailings.
Auger drills can be hand operated (Fig. 7.4A) or mechan- ically powered. The advantages of auger drilling are mobility and speed at low cost. The hand-operated augers can penetrate up to 3 m with hole diameters between 10 and 15 cm. The mechanically driven augers with efficiently designed cutter heads (Fig. 7.4B) can drill 30 m or more depending on subsoil condition. The samples are useful to provide grade, moisture content, and other specifications quickly at low cost. However, samples may often fail to provide accurate information due to wall collapse and related contamination.
7.2.2.4 Diamond Drilling
Diamond drills, surface and underground, are most ver- satile tools, and extensively used in mineral exploration, at
FIGURE 7.2 Percussion drilling in operation for rock quality and quick target test for mineral occurrences at low cost and reduced time. The hammer is detailed in the inset.
FIGURE 7.3 Jackhammer drilling in rich sulfide mineralization for multipurpose use as underground mine face development and sample source for ore continuity and grade control.
dam sites, for other foundation tests, drainage of mine workings, underground mine ventilation, oil structure in- vestigations, and oil/gas well drilling (Heinz, 2009). The extreme hardness of diamond enables it to cut all types of rocks and minerals found in Earth’s crust. The diamond drill unit (Fig. 7.5) consists of an engine (motor) attached to a drilling head and hoisting units, cutting bit with crown and reamer shell, water pump, drill rods, core barrel, core lifter, casing pipes, cutting tools, and a tripod or single stand.
The engine is powered by diesel, electricity, or com- pressed air. The motor, mounted on a cemented platform or truck, transmits rotating power through the transmission and clutch to a set of gears to the drilling head. There are three to four set of feed gears within the swivel head, with capacity ranging between 100 and 1000 revolutions per inch of rock cutting. The chuck equipped with jaws is placed at the bottom of the feed screw through which drill
FIGURE 7.4 Sketch diagram of (A) auger drilling, a simplified, easy-to- operate, low-cost sampling unit, and (B) cutter head.
FIGURE 7.5 Schematic diagram of surface diamond drilling unit showing various components and functions to sink a borehole for recovery of core samples of all rock types passing through, including structural features and mineralization.
rods pass. The drill rods are attached to a core barrel and diamond bit. The total drill string is forced downward with high-speed rotation of the chuck resulting in cutting of the core and the making of a hole.
Atripodis commissioned by erecting three poles about 30 feet long around the drill unit. The function of the tripod is to raise and lower 10 or 20 feet rods during the drilling operation with the help of ahoist.Scaffoldingis nailed and chained to the tripod where the drill crew can stand safely and operate rod hoisting/lowering. The screwing and unscrewing of rods is done efficiently by automatic mechanized means. The rods are withdrawn at intervals of 10 feet or less depending on drilling conditions. The core is removed from the barrel for geological studies (logging) and storage.
The modern truck-mounted drill rig has a single hoist- ing column with screwing/unscrewing ability and the ca- pacity to sink toþ2000 m depth through fully mechanized operation with two to three crews (Fig. 7.6). The drill unit can move quickly in hilly terrain, settle in a new location, and start drilling at minimum shift loss.
The drill rods are hollow steel,flush jointed or coupled, 10 feet (3.05 m) long through which water is pumped to cool the bit andflush rock cuttings.Diamond core bit, the main cutting tool, is a cylindrical hollow tube made of special alloys with a crown at one end (Fig. 7.7). The crown is composed of superior diamond-holding qualities of powdered metal alloys on which diamonds of different sizes, quantities, and designs are set depending on the rock types to be drilled. The fragment sizes are denoted as“spc” (stones per carat), say 80/120 spc, i.e., between 80 and 120 spc (1 carat¼200 mg).
Thereaming shellis mounted between the cutting bit and core barrel. It is an annular bit with diamonds set only on the outer surface or periphery (Fig. 7.8). The reamer shell widens the borehole diameter drilled by the diamond bit by about 0.30e0.40 mm. It maintains a uniform hole
FIGURE 7.6 Truck-mounted diamond drill rig in operation for base metal exploration in Australia. The device is capable of drilling 50e100 m a day.
FIGURE 7.7 Standard diamond core bit studded with technically designed tiny diamonds at the crown composed of superior quality alloy grooved with channels to cool the bit head andflash thefine cuttings.
FIGURE 7.8 Standard reamer shell coupled behind the core bit embedded with diamonds at the outer surface to increase the hole diameter for easyflow of returning water andfine cuttings.
diameter, reduces wear and tear of the core bit and barrel, and improves theflow of returning water.
Acore barrelis attached between the lower end of the drill rods and the reamer shell. It holds the core inside while drilling and is brought to the surface. The core barrels are single tubesused for drilling under the best core recovery conditions or noncoring bits in blast hole drilling. The double tube core barrels are suitable for better core re- covery, where the inner and outer tubes are connected and rotate simultaneously. The triple tube core barrels are complex and expensive, and used in broken, friable, and sheared formations where samples are analyzed for esti- mation. Acore lifteror core spring (Fig. 7.9) is placed at the lower end of the barrel that stops the core from drop- ping out of the barrel.
Core, the best and most authentic sample, is a cylin- drical piece of one or multiple pieces of rock cut (Fig. 7.10) with advancement of the bit. The core represents the sub- surface geology of the section passing through in general, and serves as samples for petrology and precise chemical properties for grade assessment. The core provides physical and accurate records of formations through which the drilling continues.
Wateris an essential component of the drilling opera- tion. Often, drill sites are located at remote places far away from water sources. In such conditions, water is stored in a tank dug close to the borehole collar. The water is either
pumped from a nearby source or supplied periodically by tanker. Therefore utmost care is taken to recirculate water after settling of rock cuttings in the returning water tank.
Any water loss in the drill hole can be prevented bycasing the hole or by using localized cement grouting on the fractured areas. In the case of highly fractured ground conditions, water can be substituted by drill mud (bentonite clay, polymer). This will significantly reduce fluid loss, hole collapse, and improve drilling efficiency.
Salvaging is the recovery of the remaining diamonds from the matrix of worn-out bits and reaming shells by acid bath. The recovered stones are sorted for size and condi- tion. They are then mixed proportionately with fresh dia- mond for setting new bits. The value of the recovered diamonds is credited to the purchaser.
Underground drills are lightweight compressed air or electrically driven machines mounted on a single or double drill column (Fig. 7.11). The drill rods have a 5 or 10 feet pull due to inadequate space of mine workings. The drill units can work from horizontal to 90 degrees up and down.
However, the preferred drill angle is between 0 and 45 degrees to avoid excessive load of rods and better safety of the drill crew. Capacity isþ300 m.
Capacity of the drilling depth depends on power and condition of the machine, terrain, angle of the hole, and type of rocks to be penetrated. The efficiency will reduce with steep angle holes and longer depth. Drilling can be done beyond 2000 m depth. The size of the core is decided by the exploration agency based on the type of minerals under investigation and prevailing rock condition. The size
FIGURE 7.9 Standard spring-type core lifter that protects the core from slipping into the hole and regrinding.
FIGURE 7.10 Standard drill core showing stratiform sphalerite and galena in calc-silicate host rock with the red pencil line marked for splitting into two near-identical halves at Rajpura-Dariba Mine, India.
FIGURE 7.11 Typical compressed air-operated underground drill unit that can operate at 0e360 degrees rotations.
of the bit, reamer shell, barrel, and drill rods is selected accordingly. Generally, drilling starts with HX size and progressively reduces to NX, BX, AX, and rarely to EX (Table 7.1). The cost of drilling will be higher with larger core diameter and more reliable sample representation, and vice versa.
The collar (starting point) of the surface exploration drill hole must be closed after completion with a wooden/
cement plug to prevent pebbles and soil being dropped down the hole, thus making it difficult to move back in the hole again if required. It is also desirable tofill the entire hole with cement at a later date to prevent inundation of the underground mine by water gushing during heavy rains or from interconnected water channels. The top of the collar should be covered with a cemented platform marking the project and drill hole number, coordinates, angle, direction, depth, and start and end of drilling date for future reference.
The collars of underground boreholes must be plugged to avoid accidents at the upper or lower levels by explosive gases during mine blasts.
7.2.2.5 Wire-Line Drilling
Wire-line drilling works by withdrawing the core and inner tube assembly from the hole without pulling out the hollow drill rods by a separate hoisting unit fixed on a different pulley. The inner tube assembly is lowered down inside the barrel after taking out the core and drilling continues. Therefore lowering and hoisting of drill string, barrel, and drilling head is not required after every run drilled. It also saves considerable time and energy.
Continuous core drilling works on the principle of reverse circulation. Reverse circulation refers to circulating
water down to a bit head outside the drill rods and returning it up through the inside the bit, core barrel, and drill rods.
The effect of reverse circulating water is to continuously float the core back to the surface. The drill rods and barrel are hollow enough to move the core upward as drilling continues. The cores are collected at the surface and placed in a core box with proper orientation and depth. The continuous core-drilling process operates for a long dura- tion until the bit is ineffective. This saves time by not lowering and hoisting the drill string every 3 m, and avoids overgrinding of the core. Efficiency of drilling increases many times resulting in substantial cost reduction and improved core recovery.
7.2.2.6 Reverse Circulation Drilling
Reverse circulation drilling prefers compressed air pro- duced by a hydraulic top drive motor under dry drilling conditions. The compressed air is introduced into the drill through a dual concentric air pipe (between the outer and inner pipe) andflows to adown-the-holetungsten carbide hammer bit. The compressed air and hammering initiate breaking of chips at drill head depth, and are set in continuous motion upward by high-pressure return air through the inner sample tube. The return air is supplied by installing a high-pressure vacuum pump. The entire rock cuttings move to the surface, and are collected as an inverted sequence in a cyclone above ground level (Fig. 7.12). The samples are separated at regular intervals representing particular depths.
Reverse circulation drills(Fig. 7.13) are used for open pit excavation of iron ore, bauxite, limestone, rock phos- phate, and coal seam. They drill 10e15 cm large diameter vertical blast holes at high speed and low cost. The modi- fied reverse circulation drills can rotate the drill string from vertical to any steep angle and are popular for mineral prospecting. The reverse circulation drill sample enables quick testing of drill targets during reconnaissance and prospecting. It creates a base for further diamond drilling programs to acquire precise sample locations and high- quality core samples.
The best advantage of reverse circulation drilling is its capability of drilling in the hardest of formations with easy penetration at high speed and recovery of uncontaminated samples even in broken and fractured formations. The fast rate of drilling helps the planning of future core drilling programs and mining operations. It is cost effective and provides mobility of the rig from one location to another for both prospecting and mining.
7.2.2.7 Air-Core Drilling
Air-coreorrotary air blast drillingis preferred to cut into unconsolidated weathered regolith ground mass. The drill bit has three hardened steel or tungsten carbide blades TABLE 7.1 Standard Drilling Type, Hole, and
Core Diameter Drilling Type
Hole Diameter (mm)
Core Diameter (mm)
American Sizes
HX 99.20 76.20
NX 75.70 54.70
BX 59.90 42.00
AX 48.00 30.00
EX 37.70 21.40
Wire-Line
HQ 96.10 63.50
NQ 75.80 47.60
BQ 60.00 36.50
AQ 48.00 27.00
arranged around a bit head. The drill rod comprises a hollow inner tube placed inside a hollow outer rod barrel. A small compressor provides rotation of the cutting bit and blows air to remove rock cuttings to the surface through an inner tube, which passes through a sample separating system to form a sample. It is low cost with a faster speed up to a depth of w125 m, and is often used in first pass exploration drill programs. Air-core drilling is efficient in the delineation of
lateritic aluminum, nickel, cobalt, and platinum-group ele- ments within saprolite and other horizons (Fig. 5.4).
7.2.2.8 Sonic Drilling
Sonicor rotasonic drilling is a high-speed, low-cost soil penetration technique for collecting soil/rock samples. It is suitable to identify detailed stratigraphic information of
FIGURE 7.12 Conceptual framework of noncore reverse circulation drilling and sample collection, widely adopted by exploration companies all over the world due to fast sampling at low cost with reasonable reliability.
FIGURE 7.13 Reverse circulation drilling in operation and sample collection for iron ore deposits.
thick glacial sediments. It recovers uncontaminated continuous core and works on a variable frequency drill head to transmit vibration energy through a drill pipe and core barrel. It can penetrate overburden, fine sand, boul- ders, hard rock, and collect samples up to 254 mm in diameter fromw200e250 m deep vertical/inclined holes.
Sonic drilling recovers 100% core in glacial till, clay, sands, gravels, and hard rock. Sonic holes are useful for the installation of groundwater monitoring devices, geotech- nical instruments, and geothermal sensors (Gandhi and Sarkar, 2016).
7.2.2.9 Directional Drilling
Directional drillingunder controlled deviation will result in multiple intersections from a single drill collar, follow a common path in waste formation without any physical drilling, and change drilling by using a wedge to deviate the drilling course in desired angles and directions (refer to Figs. 15.3 and 15.8). The system achieves multiple in- tersections at much reduced time and cost.
7.2.2.10 All Hydraulic Drilling
All hydraulic drilling is performed by single-handed, computer-controlled total automation that includes rotation, penetration, hoisting and lowering, screwing and unscrewing of rods, and lifting of accessories from rack to feed frame and back. The unit follows safety features in operations, bit efficiency, productivity, quality output, and clean work environments.
7.2.2.11 Borehole Survey
Boreholes have a tendency to deviate both in inclination (angle) and direction from original settings due to drilling through rock types of different hardness and structures, and using overspeed drilling, defective drill rods, barrel, and bits. Drill hole deviation at certain depth intervals (30e50 m), depending on the rock strength and formation structure, is suggested and measured by various ways (Devico, 2010). There are three types of borehole survey method available: the old style etch testing, low-cost Tro- pari, and advanced reflex multishot borehole camera with individual merits.
Etch testingis the simplest way to survey the angles of a borehole. It consists of a hollow container fitted at the lowering end of the drill rod in place of a barrel and bit. A special type of glass test tubew13 cm long, partiallyfilled with hydrofluoric acid and corked with a rubber stopper is placed in the container. The container is lowered to the desired depth of survey and kept stationary forw45 min.
The hydrofluoric acid reacts with the glass tube and forms a horizontal etch in the inner surface during the stationary period. The container is withdrawn from the drill hole, the
tube is washed, and an etching line is marked with ink. The angle of the hole at that point is measured as shown in Fig. 7.14. The method could provide deviation of angle only, has no direction, and could be used in the case of low- budget exploration for limestone, etc.
Tropari is a single-shot, micromechanical borehole survey instrument operated by a timing device. The unit provides both inclination and direction at high precision that defines the borehole attitude at a desired survey depth.
The instrument is essentially a pivoted ring-mounted compass (Fig. 7.15). It is attached with a time-clock mechanism to lock the compass needle and dip indicator within a preset delay between 5 and 90 min from lowering the unit. It is hoisted to the surface after automatic locking of the system to record inclination and azimuth direction.
The device can record one set of readings at a time. It has to be lowered/hoisted repetitively to all desired depths resulting in loss of drill shifts. The time loss will be considerable in the case of a drill hole with extended depth.
Regular surveys at intervals as the borehole progresses will allow a realistic plot of the borehole.
The reflex multishot borehole camera resolved the foregoing issues by using a digital multishot digital device ideal for nonmagnetic borehole paths. Features include capabilities like robust, reliable, and fully integrated per- formance on a precise electronic measuring principle, measurements in all directions, fast speed, and user friendliness. The unit comprises a multishot camera, a tiny magnetic compass, aflashlight, and an autolocking clock (Fig. 7.16). The shot and flash are synchronized with the clock. The angle and directional data are recorded on a 16 mmfilm/chip/data card. The string slowly moves in the
FIGURE 7.14 Borehole survey by special type of glass test tube-acid etching method at lowest cost for measuring the deviation of angle but not direction.