P&Q University Lesson 4- Drilling & Blasting

By |  June 26, 2015

The initial production steps in a quarry operation – drilling and blasting ­– can significantly impact the productivity and costs of most downstream operations, including loading, hauling, crushing and screening, and product yield – the relative volume of high-value crushed stone products versus low-value byproducts. Properly designed and carefully executed, drilling and blasting offer the first, and perhaps greatest, opportunity to optimize quarry operations and control costs.

Drilling

Required production, terrain and material characteristics are factors to consider when selecting a drill.

Required production, terrain and material characteristics are factors
to consider when selecting a drill.

Since the dawn of the industrial age, mechanized drilling has been one of those fundamental technologies that radically transformed the cost and scale of production in a way that enabled and made viable economically the rest of the basic industries, which in turn has given us the kind of economy and lifestyle that the ancients could not conceive of.

It was the drill combined with explosives that made all this possible. And yet, like so many things, we take it for granted now. In fact, because we are able to do so much with so little now, combined with the fact that almost all of the blasting and much of the drilling is contracted, quarry operations have tended to treat the unit operations of drilling and blasting as a commodity rather than a core part of their business process in the time running up to the financial cliff of 2007 – an era that one might refer to as the “old normal.” But then we went over the cliff, and the vacuum it created in market demand for crushed stone along with the rest of the economic implosion gave us a slack condition the likes of which we have not seen or experienced in modern memory. The instantaneous reflex was to cut costs to a minimum, especially for those things that generated an invoice, including contracted services. In the moment of crisis, one has to respond, but for the long term the new market, new demographics, new economics, new constraints and requirements give rise to what we can now call, the “new normal.”

In this new era, working assumptions about what we do, why we do it and how we do it, needs to re-evaluated in the context of the entire process that defines our quarry operations. And there is no better place to begin that work than with the drill function. Old normal objectives basically revolved around rubblizing the bench and staying out of trouble in the blast. Standards and technology, while good, were not what they are today. That, combined with the robust market demand we enjoyed, defined a certain criteria for working standards of practice. In the new normal, the fundamentals that should drive our working standards have turned around almost 180 degrees.

The implication for drilling and its operational companion of blasting is that the explosive energy we release needs to be delivered at exactly the right place, in exactly the right quantity, at exactly the right time. The keyword is “exactly.” In the field, this begins with the drill and the driller, not the blaster.

The good news is that the technology has never been more capable, the opportunity greater and the potential gains in safety, productivity and total cost reduction more achievable.

The difference between then and now is the need to turn the blast into the primary crusher by controlling fragmentation response, and measure costs in light of total production cost effect in the stockpile, not just direct expense. In short, we need to master and control the process to achieve the sustainable outcomes that are now required for success in the “new normal.” When this is understood and embraced, it will transform something that we have treated as a necessary evil into the catalyst that creates greater safety, productivity and total cost reduction throughout the production process. In short, drilling and blasting becomes an investment, not a cost.

To begin the technical discussion, a blasthole is merely a cylindrical vehicle designed and strategically situated to hold and contain an explosive charge so that it can be detonated in the most efficient and optimum manner possible. No blasting system will be truly effective if the hole is poorly placed – excessive burden and spacing – sized incorrectly for desired results, insufficient in depth, etc.

The drilling phase is the most expensive in the drilling and blasting portion of production, requiring a sizable investment and upkeep. The impact of improper drilling can be felt throughout the remainder of the production cycle, such as excavating and hauling, crushing, screening and so on.
The purpose here is to concentrate on some of the factors that should be considered when evaluating and selecting drilling equipment for a mining project. The primary focus is on surface operations, rather than underground.

SELECTION OF HOLE DIAMETER

Properly sized and placed blastholes are critical for effective blasting.

Properly sized and placed blastholes
are critical for effective blasting.

The first step in the process is to determine what hole size or diameter most befits the application, bearing in mind that this could change over time as the operation grows and matures. This is probably the most important single factor since it will in large part determine the size, quantity and type drill or drills that will be needed.

Here are some (but not necessarily all) of the factors involved to determine the optimum drill capability:

■ Required production.
■ Terrain.
■ Material characteristics.
■ Type and size of excavating and hauling equipment.
■ Proximity to vibration-sensitive areas.
■ Bench or lift height.
■ Explosives type and size.

All items should be examined and considered before making a final decision.

REQUIRED PROTECTION

If intended annual production is 1 million tons, to choose a medium-sized rotary drill rig that is capable of drilling a 7 7/8-in. (200 mm) diameter hole and producing 3 to 5 million tons per year on a single-shift operation would be excessive.

Conversely, an installation that requires 5 million tons or more per year might consider using something other than a crawler-mounted drifter drill capable of drilling only up to a 4-in. (102 mm) hole, as a large number of drills would be required. However, all other things being considered, such as improved blast fragmentation, this might still be the better overall choice.

Also to be considered is the possibility of selectivity – where hole size may have to be reduced to effectively define the ore area or address a vibration issue. Another need might be for a more maneuverable drill rig for difficult terrain or a truck-mounted unit, able to move more rapidly from one selected area to another, thereby reducing non-productive drilling time.

TERRAIN

The larger the hole diameter, generally the larger the drilling platform. Big drills, even those mounted on tracks, are more limited in their ability to traverse adverse landscapes than smaller machines.

If in a pioneering phase, then definitely pursue a study of the gradability and traversability of various drill units. As the operation matures and gradates into a smoother series of benches, the constraints will be altered. Therefore, initially it might be wise to choose a smaller hole diameter and a smaller drill(s) until such time as the development proceeds to the point where larger holes and larger drill rigs can be utilized.

If the initial development portion is of insufficient duration to justify purchasing lighter, smaller drills, then consider rentals or contract drilling for the interim, purchasing larger rigs when full-blown production is achieved.

MATERIAL CHARACTERISTICS

No amount of explosives technology can overcome a poor hole diameter and drilling method.

No amount of explosives technology can overcome a
poor hole diameter and drilling method.

The previously mentioned subject of selectivity under “Required Production” has a bearing in this section, as well. However, what is referred to in this section is primarily the characteristics of the rock that lend themselves to drillability and blasting fragmentation.

Following are some of these characteristics that influence hole size and subsequently drill selection:

■ Hardness is in effect compressive strength of the rock. This subject will be approached in more detail in the following sections, but basically, percussive drilling is not as radically affected by rock hardness as is rotary drilling. This is because there are certain restraints placed on rotary bit bearing loading, particularly as the diameter decreases. The other effect of rock hardness is its resistance to blast fragmentation, particularly if it is somewhat homogeneous in nature. This can affect the distribution of the explosives in the bank, so that smaller holes, closer together, may have to be considered, depending on desired fragmentation.

■ Rock structure denotes the existence of joints, fractures, bedding planes, or faults. If the rock, even though hard, is rather friable in structure (such as a well-jointed basalt or trap rock), then larger holes farther apart may be used and still retain optimum fragmentation. On the other hand, if a radical series of fracturing is present, causing rapid attenuation of explosives detonation waves, then smaller holes closer together may be necessary.

Another structure factor might be the presence of a blocky cap rock at the upper part of the hole. This can particularly occur in the top lift or bench nearest the weathered zone of the material. This bears consideration because the larger the blasthole diameter, the more stemming is required to better confine the explosive charge. In other words, the top of the explosive charge may be below the cap rock portion, thereby contributing nothing to its fragmentation unless decking charges or satellite holes are drilled and loaded, or secondary blasting or breaking with a hydraulic hammer or drop ball is done on the pit floor. This situation should be evaluated from the standpoint of cost savings via the larger hole versus the cost of secondary breakage of the cap rock.

Yet another structural feature of the rock that can affect hole-size determination is the tendency for it to back break excessively, causing large fragments or blocks of unblasted material to be dislodged into the muckpile where secondary sizing will have to be done.

This problem can occur most severely when blasting up-dip parallels with the strike of the rock. This back break can also cause problems on drilling succeeding blasts, since it may call for angling or “looking” holes in the first row or rows in order to maintain proper toe burdens.

Solutions to alleviate this situation may be to use deck charges in the normal stemming area of larger holes, drill smaller-diameter satellite holes in between the larger holes or drill smaller-diameter holes exclusively. Where the explosives column can be brought higher and loaded lighter, better-distributed charges can be used, thereby reducing damage and minimizing the back break to a manageable level.

If toe problems still persist and angle drilling is required, remember that large drills have limited capabilities in this regard (up to 30 degrees as an optional feature). Smaller drills drilling smaller-diameter holes have better capabilities in angle drilling, but trade-offs are present in difficult setup and hole accuracy. Also, bench heights may have to decreased. This will be discussed in more detail later.

TYPE/SIZE OF EXCAVATING/HAULING EQUIPMENT

Substantial cost savings can occur when going beyond the "old normal" standards.

Substantial cost savings can occur when going beyond the “old normal” standards.

Some are of the opinion that because large excavators and haul units are employed, the blastholes should be as large as possible and the drill pattern spread, since larger sizes of blasted rock can be handled. To a certain extent this is true, but it can be argued that the primary purpose in larger excavating and hauling units is to promote greater production capabilities more economically, not to save money on the drilling and blasting phases.
Caution should be exercised not to determine blasthole size purely on the basis of this factor. On the other hand, if excavating and hauling equipment is relatively small, then carefully consider hole size relative to desired fragmentation.

PROXIMITY TO VIBRATION-SENSITIVE AREAS

Most operators who employ drilling and blasting techniques are well aware of the vibration restrictions imposed on them in today’s lawsuit-conscious world. Obviously, the larger the hole, the more weight of explosives per hole. Unless you can employ more than one delay per hole or advanced vibration control techniques, greater explosive weight charges can yield higher vibrations.

Many factors should be considered before selecting a drill. Of course, you won’t want to drill and load 12-1/4 in. (311 mm) holes adjacent to a new subdivision, but with proper delay utilization and monitoring, vibration concerns could be addressed with a 6-in. (152 mm) hole at a considerable cost savings compared with even smaller-diameter holes.

To ensure proper drill selection, some vibration-control research and testing may be necessary, particularly in a new operation or construction project without previous background history.

BENCH LIFT OR HEIGHT

If there is an existing bench height, then selection of blasthole size has to be made with this in mind. In some areas, legislation dictates maximum heights, which eliminates some of the selection process. Wall control and stability may also be a significant factor in deciding bench heights.

On the other hand, if bench height is variable, there is more latitude. (In many cases, bench or bank height is predetermined by production requirements and hauling and digging equipment.)

There are other considerations that need to be evaluated in this process:

■ Blow energy to the drill bit from an out-of-the-hole drifter drill dissipates incrementally as depth increases. This subject will be discussed in a subsequent section, but holes drilled by this type of drill engine are in the range of 5.5 in. (140 mm) and smaller. Traditionally, top hammer equipment allows effective drilling to a depth of 30 to 40 ft. (9 to 12 meters). However, drill manufacturers’ ongoing work and development has improved control and drill string tooling to increase effective hole depth to the 60 to 80 ft. under reasonable site geologic conditions. Technologies vary by manufacturer, and energy dynamics of any design must be tuned and balanced to achieve the efficiencies requisite for economy and productivity.

■ Percent of available hole volume for loading with explosives increases with bench height. Simply stated, this is the relationship between the volume of the explosives column to the “pay volume” or drilled volume of the hole above the floor (without sub drilling) stated as a percent.

At the outset, it was stated that a blasthole is a cylinder whose prime purpose is to efficiently accommodate an explosives charge. A blasthole cannot be loaded right up to the top of the ground with explosives since there is no confinement of the energy to break the rock and the hole will “rifle” or “gun-barrel,” throwing fly rock and creating a safety concern, high air-over pressures and poor fragmentation.

Therefore, it is necessary to maintain a specific depth from the top of the hole to the top of the explosives column, and fill this portion of the hole with an inert material such as small angular crushed stone (stemming). Larger-diameter blastholes will require larger crushed stone to ensure the blast will be confined for optimum performance and safety.

Since detonation of an explosives column tends to direct its energy toward the path of least resistance, which most often is up and forwards, it is generally necessary to sub drill, or drill below the floor or final grade, in order to place explosives to ensure the blast breaks to grade. When detonated, the explosives in the sub drill will help to create a shear plane and displace the blasted material at or below the desired floor level.

(In some instances such as surface coal or horizontally bedded sedimentary rocks, this may not be necessary or desirable.)

Since larger-diameter holes are generally placed farther apart, sub drilling is somewhat greater in order to break between the holes and not cause “high-bottom.” Therefore, the length of the explosives column is the bench height plus the sub drilling less the stemming. However, the pay portion of the hole is only the length corresponding to the bench height. A relationship can then be set up showing the ratio or percent of the volume of the explosives column to the pay volume of the hole. If the explosive used is fully coupled, or fills the complete cross section of the hole, as with poured or pumped explosives, then this relationship becomes merely the ratio of the length of explosives column to pay length of the hole.

It should be noted that as hole size increases, the pattern spreads to keep the same powder factor. Consequently, to maintain effective breakage at the required toe elevation, sub drilling generally increases to bridge the increased gap between holes at the required toe elevation. A derivative of this geometry is that the broken/fractured zone on the resulting bench top is more broken and variable in height cross section. This is a negative as it tends to slow down the drill cycle, makes establishing the collar more difficult, and decreases blasting safety by introducing more structure variability and complexity in the stemming zone of the bench. Fly rock is more likely and the fragmentation result tends to be larger and more blocky.

Just as with physics in the rest of the real world, one thing affects another. Almost nothing exists in isolation from each other. Consequently, success in the drill/blast phase requires a quantitative understanding of how these variables affect each other if one is to optimize their production formula.

Here a worker drills a horizontal blasthole at an underground operation.

Here a worker drills a horizontal blasthole at an underground operation.

EXPLOSIVE TYPES AND SIZE

Finally, another factor that should be taken into consideration is explosives type, size and method of loading. Explosives generally take on more favorable properties of detonation in larger holes and are usually easier to load, especially with the advent of bulk loading systems. Some cost advantages may prevail that are worth attention.

DRILLING METHODS

The variables discussed cover what should be considered in selecting a blasthole diameter and an approximate bench height. It has also stressed how critical this is in determining the quantity, type and size of drilling equipment that is required. An intermediate step is required before final determination – an evaluation of what general drilling method (or methods) will be best suited for a particular application.

Drilling methods, herein referring to the mechanics of the process, can be broken down into two broad categories, and then further divided into subsections as follows:

■ Percussion: Top hammer or out-of-the-hole drills (OHDs).
– Compressed air-actuated.
– Hydraulic fluid-actuated.

■ Percussion: Down-the-hole-drills (DHDs).
– Compressed air-actuated.

■ Rotary
– Blade (or drag) bit rotary drilling.
– Roller-cone bit rotary drilling.

PERCUSSION DRILLING

This method of drilling is, in simple terms, nothing more than mechanization of the venerable hand steel and hammer method used in mining 100 years ago and longer, wherein the end of an integrated piece of steel and bit were struck by a hand-held hammer and then rotated by hand between each blow to re-index the bit so that it didn’t “rifle.”

The components in present-day percussive drilling technology are still the same:

■ A bit to penetrate the rock and cause it to fail.
■ A method of imparting a sharp blow that can be transmitted to the bit.
■ A rotational device so that the bit can be incrementally indexed in the hole.

In addition to the above, a means of removing or flushing the drill cuttings and dust from the hole must be provided. The drill bit must, by necessity, be highly resistant and hardened in order to absorb the blow energy and penetrate the rock. Today’s bits are generally detachable and replaceable and have cutting edges of a very hard alloy, such as tungsten carbide in the form of inserts or buttons.

The blow is delivered by a piston reciprocating within an enclosed cylinder. Rotation can be achieved by either a mechanism contained within the drill engine or an external device, such as the rotation or power head on a rotary drill (see subsequent discussion). Hole flushing is generally performed using compressed air when drilling on the surface. Many times water is injected into the air stream to create a water-vapor mist that helps dampen fine dust generation as well as assist in stabilizing the collar zone. A beneficial byproduct is that it helps cool the drill string operating temperature. With underground applications, straight water flushing is commonly used.

DRIFTER DRILLS (OHDS OR TOP HAMMER)

In this method of drilling, the drifter, or drill engine, moves along a guide or track on the drill carrier. It contains both the piston and the rotation components. Piston actuation can be achieved by either compressed air or hydraulic action (the merits of each will be briefly discussed later). The piston imparts its energy to a shank piece or striking bar, inserted in a chuck, located at the bottom of the drifter. This shank passes the blow along to the bit via a series of drill steels or rods connected by detachable couplings.

Within the drifter, there is a blow tube that directs compressed flushing air down through a drilled hole in the piston and drill steel to drilled passages in the bit that force the drill cuttings out the annular space between the hole diameter and the drill steel to the surface. In addition to compressed air alone, water, foam or some type of detergent can be combined with the flushing air to cause cuttings and dust to adhere, thereby controlling airborne emissions.

The drifter and drill string are fed up and down the drill guide by (generally) a motor and chain device, in order to keep down-pressure on the bit and also to allow the addition and removal of drill steel. Air-operated drifter drills generally use air at pressures of about 100 to 125 psig (690 kPa to 862 kPa). To use higher pressures causes excessive wear and breakage of the drilling accessories.

Hydraulic-operated drifters employ similar principles of operation, except that hydraulic fluid is relatively non-compressible.

The piston stroke is therefore much shorter than air drifters, but the work performed is compensated by greatly increased blows per minute (BPM). Hydraulic drifters still need a certain amount of compressed air to flush the cuttings from the hole.

Drifter drills, whether air or hydraulic, are limited to hole sizes smaller than 6 in. (153 mm). From a power production standpoint relative to drifter design, there is no limit to the hammer size as it is not restricted by space or weight outside the hole. Rather, the limiting factor is the stress/strain strength capacity of the drill string system relative to the percussive energy required to drive a given bit diameter.

DOWN-THE-HOLE DRILLS (DHD)

In drifter drilling, one of the big drawbacks is that the blow energy to the bit from the out-of-the-hole piston is attenuated incrementally through the absorption by the drill string. The deeper the hole, the less the efficiency. A solution to this would be to put the piston actually in the hole where its energy could be applied directly to the bit. This is the principle of the DHD, wherein the cylinder and piston are encased in a sleeve that goes into the drill hole with the bit. The rotation mechanism is still retained externally. At the present, all DHDs are compressed air operated.

Note that while provision still has to be made to provide a passage of a certain portion of the air for hole cleaning, the DHD uses exhaust air after it has been applied to the piston, rather than a diverted portion of the main air supply as in drifter drilling. Also, a water check valve is supplied to prevent groundwater from entering should the air be shut off. The only moving part is the piston, which acts as its own valve by opening and closing the air intake and exhaust ports. It should be obvious, that in order to accommodate all these components in a confined space, the piston becomes relatively small and indeed must be considerably smaller in diameter than the bit.

The reduced piston size reflects two important aspects of DHD drilling:

■ In order to maintain ample blow energy to the bit with a piston of less area of applied pressure and smaller mass, higher-pressure compressed air must be used. Generally, this is 250 psig (1723 kPa) nominal pressure at the compressor receiver tank and greater (to about 350 psig or 2412 kPa at present).

■ The minimum hole size is necessarily limited due to the space requirements and the metallurgy of the components, particularly the piston, to withstand the forces involved. For blasthole purposes, and to preserve the economics of production, this minimum hole diameter would be about 4-1/2 to 5 in. (114 to 127 mm).

DHDs have one other consideration in their favor: Since the blow is produced in the hole, airborne noise is reduced considerably compared with drifter drills. Percussion drilling, whether by OHD drifter or DHD, is possible in all types of material regardless of hardness, strength, or abrasion.

One thing should be noted: From a thermodynamic mechanical perspective, creating and transfer power and energy using compressed air is significantly less efficient than using compressed fluids (hydraulic oil). This physics reality manifests itself notably in the form of substantial fuel savings when using hydraulic top hammer rigs as compared with DHDs so long as the basis for comparison is referencing the same hole size and depth for the respective rig types. Given present technology, that savings translates into a 30 percent fuel consumption rate benefit or about 20 to 25 cents per drilled foot in the 5-in. hole size range. The proviso here is that the top hammer rig can handle ground conditions and accuracy requirements at site. There are some locations where the geology and blasting over break combine to make hole flushing capacity the primary issue in drilling productivity as so much air flushing volume is lost into the broken rock around the hole. This directly affects the up-hole bailing velocity of the air stream and the ability of the drill to effectively bail the hole of cuttings. In such extreme conditions, it is all about air volume delivered to the hole, and for that, DHDs and rotary drills generally remain the preferred choice.

ROTARY DRILLING

Drill bits and other equipment are an important consideration in the drilling and blasting process.

Drill bits and other equipment are an important consideration in the drilling
and blasting process.

The concept of rotary drilling as a method differs entirely from percussive drilling. In the latter, the rock is caused to fail by imparting a sharp blow onto a bit and using a rotation device to merely turn the bit a sufficient amount to keep it indexed to cut a circular path. This rotation in percussive drilling is best kept to a minimal value, and feed or down-pressure should just be sufficient to keep the bit fed to the bottom of the hole.

In rotary drilling, no blow is struck, and the rock is made to fail by a combination of down pressure and rotation speed. In blasthole work, the cuttings are flushed from the hole by compressed air, but consideration here is more toward providing enough volume to maintain a suitable bailing velocity rather than pressure. Pressures in rotary drilling range from 50 to 100 psig (345 to 689 kpa).

This is an important factor that will come into play when selecting equipment. Rotary drilling, then, is more of a brute-strength operation and requires massive pull-down systems and raw rotation power. However, with a suitable high-pressure air supply, a rotary drilling machine can be used with DHDs should conditions dictate. Since the principles are different, it stands to reason that bit design for rotary drilling differs considerably from that of percussion drilling.

For blasthole rotary drilling, bits usually take two configurations:
■ Drag or blade bits
■ Roller-cone bits

DRAG OR BLADE BITS

The drag or blade bit has fixed wings that penetrate the rock and gouge it out. Some of these bits are supplied with replaceable blades. For harder materials, the blades are tipped with tungsten carbide for longer wear life.

These bits are generally confined to quite soft formations (usually less than 20,000 psi compressive strength) and in smaller diameters in the range of 3-1/2 in. to 6 in. (89 to 152 mm). Their rigid design prohibits the metallurgy from withstanding the higher pull-down and rotation stresses necessary in harder formations and larger diameters.

ROLLER-CONE ROTARY BITS

Roller-cone (or sometimes referred to as tri-cone or rolling cone) bits are the most utilized for blasthole drilling. The bit consists of three conical-shaped members that rotate on a combination of roller and ball bearings. Mounted on these cones is some kind of cutting tooth design that is dependent on the hardness and compressive strength of the material being drilled.

On some of these bits, the teeth are cut on the cone body itself for softer formations, whereas for harder rock, the cutters are hardened steel or tungsten carbide inserts. The tooth patterns on adjacent cones complement each other so that they will contact different portions of the rock. On some bit designs, the axes of rotation of each cone is offset from the others to impart a scraping action to the material to be drilled.

The most critical point of any roller-cone rotary bit is the bearings. In addition to failure due to excessive pull-down or bit loading (this will be discussed later), these bearings need to be kept cool and clean in an otherwise hostile environment. Previously mentioned was the need for sufficient compressed air to be delivered through the drill pipe to the bit to properly flush the cuttings from the hole with a suitable bailing velocity. In addition, a portion of this air needs to be diverted through passages in the bit body to clean and cool the bearings.

While air pressure is of less importance in rotary drilling, it is vital that sufficient pressure be delivered to the bit to adequately perform the cooling-cleaning function.

Curiously, when rating rotary drilling rigs, the emphasis is placed on the pull-down power in terms of pounds or kilograms of force exerted by the feed system. In reality, most of the power consumption in a rotary drill rig is apportioned off in driving the air compressor (about 60 percent) and in rotation power or torque to maintain rpm under heavy resistance (about 25 percent), whereas pull-down consumes less than 5 percent.

ROTARY BIT DESIGN WEIGHT

This operation is using a rotary DTH drill rig.

This operation is using a rotary DTH drill rig.

Another vital consideration in selecting a rotary blasthole rig is rotary bit design weight. This is a function of the amount of bit loading that the roller-cone bit bearings can take before failure, assuming that adequate cooling and cleaning air is present. The larger the bit diameter, the larger the bearings, and all else being equal, the more weight they can sustain.

To illustrate this, assume selection of a hole diameter between 184 and 200 mm (7 3/8 and 7 7/8 in.) and that rotary drilling with roller-cone bits is determined the way to proceed. It should be obvious that choosing a rotary drill capable of exerting bit weights of 60,000 lbs. (27,216 kg) would be too much machine for the application, and pull-down would have to be held back to keep from tearing up the bearings in the bit.

On the other hand, selecting a rotary rig capable of only 30,000 lbs. (13,600 kg) would not allow maximum efficiency and productivity out of the hole size. This factor of bit loading also bears another important consideration in drill selection. Since as bit size decreases the amount of allowable loading also decreases, smaller pull-down forces are necessary.

Penetration rate has pull-down as one of its variables. Therefore, rotary drilling is not as effective as the diameter decreases and the rock hardness increases. In this region, it may be possible that DHDs are more productive. Remember, however, that DHDs require higher inlet air pressures, so rig selection will have to take this into account.

DRILL RIGS AND MOUNTINGS

Up to this point, we have looked into the optimum hole diameter for an operation and also have related the various drilling methods toward that hole size.
The next step is to investigate and select the type of drill and drill mounting that would be best suited for maximum overall production at minimum cost. The previous steps have, in effect, greatly narrowed down the choices.

In addition to just penetration rate, view the drill rig and mounting from the following aspects:

1. Mobility – Can it effectively move to and traverse the drilling area or areas with a minimum amount of difficulty and time consumed?

2. Simplicity – Is it a machine that can be operated effectively without undergoing extensive training and is its functioning such that operator fatigue will not be a problem?

3. Reliability and mechanical availability – Is it a rig that will not pose severe maintenance and repair problems? Are repair parts readily available and is technical assistance obtainable if necessary?

4. Productivity – Can it meet the necessary production schedule? This is overall productivity, considering not just the mechanical availability but the time consumed in the non-drilling functions such as moving from hole to hole, changing drill steel or pipe, pulling out of the hole, etc.

5. Safety – Is the rig safe to operate? Based on selection of hole diameter and drilling method, following are some areas for consideration:

PERCUSSION DRILL MOUNTINGS

Both air-actuated and hydraulically-actuated drifter drills generally are only available in a crawler mounting so that terrain mobility generally is not a problem. However, since grousers and pads, sprockets and rollers, etc. are susceptible to wear and tear when tramming over long distances, and are slow, it is recommended that these rigs be transported on some form of truck or trailer in such instances. Hydraulic drills only need a small air compressor for blow air, and this is usually self-contained. Air drills, on the other hand, need a much larger compressor (600 cfm or 17 m3/min or larger), which is a separate unit and requires hose and/or piping to connect to the drill. Hydraulic drills, while usually heavier, are generally easier to move as a package than air crawlers. However, if the air crawler compressor supply can be centralized, then the drill itself is more mobile than the hydraulic version.

Drifter-type drill rigs, while fairly easy to operate, have been historically quite labor intensive, requiring manual steel handling and a lot of “bull work” in general. Also, the operator has always been exposed to the noise of the drifter and the drill dust created. Lately, especially in hydraulic models, more sophisticated rod-handling methods have been developed, and in many cases the operator can function from a sound-attenuated, climatized cab rather than be exposed to dust, noise, and the elements.

Since the greatest enemy of hydraulic systems is dirt and dust, and drilling is the worst environment to provide these elements, hydraulic drifter drills up to now have been more susceptible to downtime. Also, repair of a hydraulic drifter should be done in a clean environment; whereas, air drifters can be disassembled on the site, and reassembled with little more than wiping off the grime and re-oiling the parts. Better sealing and improved technology have increased the uptime of hydraulic drills, but still a rigid schedule of regular inspection and overhauls is highly recommended. Vibration is a problem with drifter-type drill rigs. They therefore have a reduced useful life and require more maintenance than DHD or rotary drills.

If the downside of hydraulic drifters is the maintenance, the upside is that generally they drill faster (generally twice as fast) than their air cousins. Evaluating one factor versus the other is a major element on deciding on air or hydraulic. Both types of drifters usually are equipped to drill angle holes. One other factor to consider if the material is generally seamy and broken: Air drifters can effectively hammer themselves out of a hole if stuck; hydraulic drifters can do the same thing depending on their specific design. With some manufacturers, this back-hammering feature works just like the pneumatic drifter. However, other designs that do not allow for the free circulation of oil in the head of the drifter require an additional reverse-percussion attachment to back-hammer out of the hole. This is because when the drifter is in free ossification, the oil must be able to circulate freely to dissipate the heat that builds up in the piston strike zone, or else the metal expands too much, metal on metal contact occurs and the drifter fails, requiring expensive repair.

From the beginning of mechanical percussion drilling up until the late 1970s or early 1980s, the dominant technology was based on compressed air (pneumatics). With the advent of using compressed hydraulic oil instead of compressed air to power the drifter, the increased production rate, improved drill string life and fuel efficiency benefits that accrue to the hydraulic drill systems has caused the industry to move substantially away from pneumatics for most mainstream production. Pneumatics still work the way they always have, but for the most part see limited use save for small (< 500,000 tons/year) annual production requirements in mines and quarries and special construction/pioneering applications.

Drilling machines are inherently dangerous just from their nature. Legs can be run over, fingers and hands lost in changing rods, clothing and flesh caught in moving chains, etc. Training and awareness are the best sources for prevention of accidents along with choosing a rig from a reliable manufacturer wherein experience has dictated incorporation of as much safety as possible.

COMPRESSED AIR

DHD and rotary drill technology grew out of rig designs and systems that were the basis for large carriers (large rigid-frame track or truck carriers) used for rotary blasthole drill production.

Today, the need to be able to operate in smaller hole size ranges, tight and rough benches and heavily broken rock strata has given rise to a merging of carrier/configuration arrangements that look and function like a modern large-scale top hammer track drill, but utilize an on-board compressed air system to drive a down-the-hole hammer using drill pipe and a tophead drive.

This marriage of carrier/boom/feed arrangement with down-the-hole drill technology gives rise to a new drill class ideally suited to many quarry applications.

Where smooth level benches with large working areas exist along with larger annual production requirements, DHD drilling in blasthole applications is still performed using larger rotary drill rigs, modified to provide the necessary high-pressure air. Therefore, the discussion of characteristics of drill rigs and mountings for this method of drilling is best covered under the “Rotary drill” section.

ROTARY DRILL MOUNTINGS

Mobility is an important factor when deciding between a truck- or track-mounted drill rig.

Mobility is an important factor when deciding between
a truck- or track-mounted drill rig.

Blade (or drag) bit rotary drilling is limited to about 3 1/2 in. to 6 in. (89 to 152 mm) in soft to medium formations only. The ideal drill rig for this application is a lightweight rotary drill with less than 25,000 lbs. (11,300 kg) of pull-down, preferably with single-pass capabilities (no addition of drill pipe). By using a table drive rather than a tophead drive rotation mechanism, a lighter weight tower can be used, thereby lowering the center of gravity and making moving from setup to setup faster.

A drill mounting has to be fairly substantial for roller cone bit (and DHD) drill mounting in which the hole size is fairly large (other than the instances mentioned where DHDs are mounted on drifter-type crawler mountings).

DHD drilling doesn’t require the pull-down and rotation forces of roller-cone drilling, but generally drill pipe is longer and heavier and a substantial tower or derrick is necessary to facilitate handling. With the inclusion of high-pressure air, both DHD and rotary drilling can be accomplished with the same drill in most instances.

Drill mountings fall into two categories:
■ Truck-mounted.
■ Track-mounted.

TRUCK-MOUNTED

This type of mounting is really an adaptation of a water well rig. Its obvious biggest advantage is speed of mobility between sites and drill areas. Its disadvantages are:

■ It can’t traverse severely adverse terrain.
■ It does not provide as substantial and as heavy a platform for straight rotary drilling as track machines.
■ It usually has two engines to maintain.
■ Tire maintenance can be a problem.
■ It takes longer to set up on a hole and usually requires an additional person to spot holes. (Note: remote propel systems are available, but there is a definite hazard encountered here when control over a truck is performed from the rear of the machine.)

TRACK-MOUNTED

This configuration overcomes most of the disadvantages of the truck mounting except it is less adaptable for rapid deployment between drilling areas or job sites.

Both rotary drill-mounting types are limited in their ability to drill angle blastholes. This capability is purchased as an option and is usually limited to 20- to 30-degree holes in 5-degree increments and in one plane only. This causes increased setup time because the rig has to approach the hole location in a plane perpendicular to the face rather than along a parallel plane as with vertical holes. This factor has to be offset against the advantages of angle drilling, particularly in cast blasting (or blast casting) applications in surface coal mines.

CHOICE OF ROTARY DRILL MOUNTINGS

First and foremost, it is assumed that the drill will be of sufficient size and power to handle the designated hole diameter. The air plant has to be of sufficient pressure and capacity to meet all requirements, including bailing velocities and DHD needs.

Secondly, when choosing between a truck- or track-mounted rotary drill rig, the evaluation has to be made as to the importance of the mobility factor versus the possibility of less maintenance and probably more efficient drilling in the sense that while on the shot, the track rig should consume less time in moving from hole to hole, thereby reducing the non-drilling time of the cycle.

OVERALL PRODUCTIVITY AS A FUNCTION OF COST

This chapter has referred to overall productivity. What this really means is the time spent actually drilling (bottom-hole time) as related to the time spent in mechanical downtime and time spent in the non-drilling functions, which include moving, setting up, adding drill pipe or steel, pulling rod or steel from the hole, time stuck in the hole, etc.

Choosing a drill rig only on the basis of its ability to punch down holes is therefore not enough without consideration of these other parameters.

CAUTION

There is a pitfall that the layman can easily fall prey to in running drill cost calculations. When one takes all the fixed capital, maintenance, and variable consumable costs for a given drill application scenario and applies them to a given hole size at a fixed annual tonnage requirement, on paper, the lower drill (and blast) cost per ton result will always show a lower cost result as you increase the hole size and pattern (burden and spacing) dimensions.

While it is tempting to move in the direction of the lowest cost/largest hole size outcome, such a calculation demonstrates nothing about the effect on fragmentation, shot control and all the side and downstream effects that are influenced or controlled by the blast result.

Unless you are starting from a condition of overcharging the blast, or are introducing a new technology feature that is absent from your current practice, the result of chasing the expanded hole pattern scenario will almost always be a liability when you measure and evaluate the downstream effects and total production cost result measured in the product stockpile.

Remember that drilling and blasting is not an end in and of itself. Rather, it is a means to an end. If you treat the subject in isolation, you are apt to make a sub-optimal or wrong choice as far as the enterprise is concerned.

CONCLUSION

Evaluating and selecting a blasthole drill can be an involved process if done correctly in light of the factors in play in the “new normal” working environment. Often, it does not receive the consideration that it deserves. When hole size and rig selection is evaluated for its potential impact on total cost of product measured in the finished stockpile at the end of a typical quarry process, considerable cost savings are not uncommon relative to “old normal” standards and practice. In the current situation and looking forward, process-oriented application and practice will be critical to the sustainable success of an entire project or quarry operation. It essentially hinges on the ability of a drill to contribute to the quantity and proper sizing of the raw material when combined with the proper blast design and execution of that design in the field. No amount of explosives expertise and technology can overcome a hastily and ill-informed selection of hole diameter, drilling method, and drill rig. At best, it can only hope to compensate for sub-par drilling results, which can only go against process improvement and the economic bottom line. When such results derive from favoring invoice costing over process impact, the result is false economy – a luxury no one should allow themselves today’s market environment.

Blasting

Blasting is heavily regulated and watched by federal, state and local agencies. In terms of processing, blasting is the critical first step in the rock-fragmentation process. Maximum profitability depends largely on an optimized blast. Consider that drilling and blasting are the first operations performed in any hard rock quarry operation. Therefore, the results of these operations will affect more down-line activities, such as loading, hauling and crushing, than any other processing operation.

Photo by Tom Hingst

Photo by Tom Hingst

Blasting should always be viewed in the global sense. One should examine not only the effect of changes on the drilling and blasting program, but also how changes will affect the productivity and economics of other down-line cost centers. It is also important to note that breaking rock through chemical means (explosives) is more cost effective than by mechanical methods. Blasting should also be viewed in the local sense. No other quarry operation has more capacity to cause community dissent than blasting.

Photo by Tom Hingst

Photo by Tom Hingst

The blasts must be shot in a safe manner, with the area properly barricaded and all persons removed a safe distance away. Environmental effects such as ground vibration, airblast and fume production must also be controlled.

All quarry operations should have in place a proper public relations program designed to communicate to the community that proper safety precautions and procedures are in place with regard to its blasting program.

FRAGMENTATION

Photo by Tom Hingst

Photo by Tom Hingst

In the 1990s, increasing emphasis was placed on the role of fragmentation at the operation. In particular, the effect of fragmentation on crushing, load and haul and run-of-mine leach pad efficiency has received considerable attention. Better predictive techniques have been developed, and computer-aided methods for determining the fragmentation distributions in actual blasts are now available. Fragmentation studies can lead to improved profits at many operations. For example, studies at one operation showed that the same production could be obtained with one less excavator in good digging, when compared with poor digging conditions. This is a result with both capital and operating-cost implications.

This sequence of photos shows a typical blast at a quarry operation. Photo by Tom Hingst

This sequence of photos shows a typical blast at a quarry operation. Photo by Tom Hingst

For maximum success, it is essential that the mine or quarry carefully design its blasts to achieve the desired results. These designs must be accurately implemented in the field. Advanced three-dimensional photogrammetric profiling technology and high-accuracy GPS systems provide explosives engineers with the required data to design patterns to obtain more uniform fragmentation.

EXPLOSIVES

The last 15 years or so have seen new explosive formulations reach the marketplace, and reductions in the use of some products that have been in use for longer periods. The principal newcomers have been gassed and sensitized emulsions and emulsion-ANFO blends, usually called Heavy ANFO, which denotes its greater density than ANFO dry mixes.

EMULSIONS

The formulation of an emulsion is similar to that of blasting slurries (water gels). However, the cross-linking agent used to stiffen the slurry is replaced by an emulsifying agent. This emulsifying agent suspends minute droplets of the ammonium nitrate (AN), or a combination of AN with either calcium nitrate or sodium nitrate, oxidizer within the fuel. This yields an intimate oxidizer and fuel mix that leads to high detonation velocities.

Emulsions may be bulk loaded or used in packaged form. Packaged products are usually employed in small-hole diameters. They are mechanically sensitized using microballoons to introduce sufficient air into the mix and control the density. Bulk emulsions are used in larger-diameter holes and may be mechanically or chemically sensitized (gassing), with chemical sensitization being less costly. Bulk-loaded product fills the cross sectional area of the hole and delivers maximum energy to the surrounding rock. This is a characteristic of all bulk-loaded products unless they are intentionally decoupled as is often the case in presplitting. Packaged emulsion will usually result in some decoupling with a reduction in borehole pressures. This generally is not a great problem in small-diameter blastholes.

AMMONIUM NITRATE FUEL OIL (ANFO)

ANFO remains one of the most commonly used products in blasting. It is a combination of ammonium nitrate (oxidizer) and No. 2 fuel oil (fuel). No. 1 fuel oil may be used in cold-weather applications.

Blasting-grade AN prills are made by spraying molten AN into a prilling tower. Droplets fall under carefully controlled cooling conditions. The AN solidifies while falling, taking on an approximately spherical shape of relatively uniform size. Prilling tower conditions must be such that will produce a porous prill that will absorb the proper amount of fuel oil (6 percent by weight). For those with overseas operations especially, it is important to confirm that a porous prill is being produced. High-density prills will not properly absorb the fuel oil and blasting performance will suffer, unless these have been crushed to about -20 mesh.

Blasting-grade AN prills are typically +6, -14 mesh in size. This uniformity in the size of the prills results in a poor packing density, with considerable interstitial voids present, hence a product that typically bulk loads in a density range of 0.80 to 0.85 gm/cc. Some packaged ANFO products use a blend of sizes, where a portion of the prill is crushed, leading to densities of about 1.05 gm/cc. This product can be loaded in wet holes provided it is contained in a suitably waterproof bag.

ANFO has virtually no water resistance. Many people are of the impression that it takes several hours before water attack seriously affects ANFO. The reality, however, is that degradation of the product is immediate. Even if holes will be detonated two or three hours after loading, performance will have been seriously affected.

Therefore, whenever ANFO is to be loaded into wet holes, the blastholes should first be dewatered and a plastic liner placed in the hole. The ANFO is loaded inside the liner. Care should be taken to obtain a liner that has a high integrity. Even a few pinholes are enough to allow water to attack ANFO. For hole diameters smaller than 5 in., using plastic liners is generally difficult. Therefore, small-diameter waterproof products such as cartridged or pumped emulsions or slurries are generally used for small-diameter, wet holes.

One way to increase the energy output in ANFO is to add aluminum. The reaction of ammonium nitrate with aluminum releases more energy per unit of weight. The aluminum must be of a suitable size to be reactive, but not so fine as to constitute an explosion hazard. This generally means a size range of -20, +150 mesh.

The upper limit on aluminum addition is usually about 15 percent. As more aluminum is added to the mix, increasing percentages of the energy are trapped in a solid product of detonation. Beyond 15 percent aluminum by weight, there is little additional energy output for the aluminum added.

HEAVY ANFO

A top hammer drill rig at work.

A top hammer drill rig at work.

Another way to increase the energy output of ANFO is to add emulsion to it. The emulsion fills the voids between the prills, the density increases, and there is more energy output per unit of blasthole volume. This type of explosives is known as Heavy ANFO. It provides a cost-effective way to increase the energy output of ANFO.

Heavy ANFO may be produced solely for the purpose of increasing the energy output. However, at higher emulsion percentages by weight these products become water resistant. Such formulations can be bulk loaded into dewatered holes without a hole liner. However, in cases where dynamic water exists in the blasthole, alternative products and custom hole loading may be required.

Experience has shown that the performance of Heavy ANFO becomes sluggish as more emulsion is added unless the emulsion has been sensitized by gassing or microballoons. It appears that in hard rock, performance will suffer when there is more than 30 percent of unsensitized emulsion in the mix. In softer formations, greater percentages of unsensitized product can usually be employed because suitable fragmentation of the rock depends to a greater degree on heave energy. The degree of non-ideal detonation introduced by the lack of sensitization means that a greater degree of the total energy is released as heave energy.

A water-resistant product is typically produced at 50 percent emulsion addition. However, to obtain a product that can be pumped reliably it is common to use a water-resistant Heavy ANFO containing 60 to 70 percent emulsion. Such products should always be made with a sensitized emulsion or performance will suffer.

When water-resistant heavy ANFO blend is loaded into wet holes, it should always be loaded from the bottom up. This is achieved using a bulk truck with a hose that can extend to the bottom of the blasthole. The product is pumped through the hose. The hose is retracted as loading proceeds, but is always kept in the explosive. The water rises on top of the advancing column of more dense explosives. Contamination of the explosives column does not occur if loading is performed carefully.

When Heavy ANFO is augured into wet holes, it spatters on impact with the water and prill goes into the solution. Water is mixed into the explosive column degrading the performance of the Heavy ANFO. Bridging may occur with portions of the explosive column separated by a water gap. Since the gap sensitivity of these products is not large, this may lead to the failure of a portion of the explosive column to detonate unless it happens to be primed on both sides of the water gap.

Heavy ANFO also is produced as a packaged product. In this case, it is sensitized using microballons, which improves the shelf life. Package products can be used where there are insufficient wet holes to warrant bulk loading, or in small tonnage operations. It also is used as a toe load in holes that have only a few feet of water in the bottom of the hole, and can be used in small-diameter packaged formulations.

DYNAMITE

There is still a considerable amount of dynamite sold annually in the U.S. However, pits and quarries have almost completely moved from the use of dynamites to small-diameter, cap-sensitive emulsions and slurries for appropriate applications. Dynamites are explosive substances that depend on nitroglycerin or nitrostarch for sensitiveness. These products are usually cap sensitive with a detonation velocity dependent on the diameter and density.
Dynamites are used in small-diameter construction blasting and as decoupled charges in presplitting. They are also used sometimes to prime ANFO in small diameters. For this latter application, a product with high detonation velocity should be chosen because it will have the higher detonation pressures (a function of the square of the detonation velocity) that are important for efficient priming of ANFO.

EXPLOSIVES AND BLAST-INITIATION ACCESSORIES

For this section, it is important to know that a booster is any unit of explosives or blasting agent used for the purposes of perpetuating or intensifying an initial detonation; and a primer is a unit, package or cartridge of explosives used to initiate other explosives or blasting agents, and which contains a detonator.

Some of the explosives described in the previous section are cap sensitive. This means the product can be detonated efficiently by a blasting cap or delay detonator of adequate strength, or by compatible detonating cord. Small-diameter emulsions and slurries are typically cap sensitive. Consult the manufacturer as to the minimum priming requirements. The sensitivity of some products may vary with temperature. Greater priming strength may be required when the product is to be detonated at low temperature.

Bulk-loaded explosives used in hole diameters greater than 5 in. almost always require heavier priming than a detonator alone can provide. It is well established that initiation of bulk explosive is temperature and pressure dependent. Primers yielding high detonation pressure initiate explosives more efficiently. Thus, formulations with high velocity of detonation (VOD) generally give the best results. For this reason, cast pentolite boosters were developed. These generate 2.2 to 2.8 million psi detonation pressure, depending on the formulation.

Cast boosters are produced in various weights. However, primers of greater weight may be useful in difficult applications or with an explosive having a higher minimum primer weight. The weight per primer used in the blastholes should be four to six times the minimum primer weight. Cast boosters typically have a length-to-diameter ratio of 3:1 to 4:1. The primer should have a sufficient diameter to act on an adequate cross sectional area of the explosive charge, thereby ensuring efficient initiation. It must be long enough to allow the VOD in the primer to build up, providing maximum pressure off the end of the primer. Therefore, there is a tradeoff between length and diameter to provide effective initiation with a primer of reasonable dimensions and cost.

Cast boosters are made with a single tunnel through which detonating cord can pass or with a tunnel through the primer and a cap well. The cap well accepts a down-the-hole delay detonator for in-hole delay applications.

Slider boosters are used for multiple priming on a single detonating cord downline. This is often used when deck loading is employed. These boosters are made with a tunnel affixed to the outside of the cast booster. Detonating cord passes through the tunnel. The pigtail on the end of the delay detonator is also passed through the tunnel. Upon initiation, the delay is initiated from the contact between the detonating cord and the delay pigtail. Only certain types of downlines (usually of low grain count) can be used; obtain this information from the manufacturer.

In some cases, a stick of dynamite can be used as a primer. This approach is most common when priming ANFO in small-diameters holes. Most dynamites do not generate the kind of pressures a cast booster provides. However, a few gelatin products detonate at very high VOD and do give high detonation pressure. When priming ANFO with dynamite, this type should be used.

DETONATORS AND INITIATION SYSTEMS

Detonators are used to initiate the blast. These may be electronic, electric or non-electric. For modern-day blasting, delay detonators are virtually always used. Delay detonators are available for use in the hole, and also for connecting into the surface tie-in.

A pyrotechnic delay detonator is similar to an instantaneous cap except that a delay element is included between the initiation charge that is activated by the incoming energy, primary, and then the base charge. The delay element’s compound burns at a specific rate and provides the desired delay time. The delay element’s accuracy can be affected by scatter. Scatter can be caused by variations in the pyrotechnic composition, age, temperature and environmental factors.

Down-the-hole delays can be used alone to provide the proper firing sequence or in combination with surface delays. In the former case, different delay times are used in the appropriate blastholes to provide the desired sequence of detonating holes. When used together with surface delays, a constant down-the-hole delay time is often used. Where possible, the in-hole delay should be of sufficient duration to allow all rows of surface connections and downlines to be activated in advance of blasthole detonations. This approach avoids cutoffs and misfires that reduce blast performance and introduce subsequent safety concerns. When down-the-hole delays are used, it is often possible to use longer surface delays without fear of cutoffs.

In ore bodies where hot holes are possible (such as reactive sulfides), down-the-hole detonators must be used very carefully because these are the most sensitive element to heat in the blasthole. Holes above a certain temperature are often not loaded. Top priming just before shooting is often indicated. Avoiding the use of these detonators is also an approach taken by mines where this is a severe problem.

Shock tube down-the-hole delays are available in various lengths. These may be long lead where the shock tube extends all the way to the collar, or short lead where the shock tube is an 18- to 24-in. pigtail. These latter units are used with detonating cord downlines. The detonating cord must be compatible with the delay system used.

Surface delays provide good flexibility in blast tie-in to provide for the desired sequence of detonating holes. Delay units, ms connectors, are made that can be spliced into detonating cord trunklines and used to connect the blastholes together. Systems are also available with long shock-tube leads, eliminating the need for the more noisy detonating cord. This is especially useful for quarries because these pits are often located in close proximity to residential and commercial areas. However, the latter systems cannot be made redundant in the same manner as those that employ detonating cord, so shock-tube systems must be connected together with particular care.

DETONATING CORD

Detonating cord contains a core load of high explosive (usually PETN). It detonates at about 22,000 ft. per second (fps). Detonating cord is made with various weights of PETN per foot of cord. This is usually expressed as the grains per foot. There are 7,000 grains in one pound.

Detonating cord is used as downlines in the blasthole to transfer initiation energy to primers and down-the-hole delays. It is also used for surface trunklines to connect blastholes together. It is easy to connect up, but has the disadvantage of generating substantial airblast. Therefore, it is usually used on surface when operating in remote locations. Shock tube, electric and electronic blasting systems are more commonly used when operating in proximity to built-up areas.

SHOCK-TUBE SYSTEMS

The shock-tube system is a plastic tube with a thin explosive coating on the inside of the tube. Upon detonation, this material continuously detonates at a low velocity of approximately 6,500 fps. Thus, the plastic tubes are not consumed and the noise level is low. It is, therefore, good to use as lead-in line to initiate a non-electric blast in populated areas. It is also used to connect holes together when used as part of a long lead-surface delay system. It is used in the blasthole as a long lead down-the-hole delay system to replace detonating cord downlines, or as a pigtail on down-the-hole delays used in conjunction with detonating cord. Shock tube systems, unlike some detonating cords, will not set off a primer and must always be used with a down-the-hole initiator and compatible primer.

ELECTRIC DETONATORS

Fewer blasts in surface mines and quarries are initiated with electric systems today than once was the case. However, this practice is certainly still followed by many, especially in quarrying.

Construction of electrical caps and delays is similar to non-electric components, except that the energy to ignite the ignition compound is provided electrically. This has the advantage of minimizing noise on surface, but has the disadvantage of being more susceptible to stray radio frequency and currents, lightening, etc.

The instantaneous electric blasting cap is sometimes called an E.B. cap. Like the non-electric blasting cap, it is a thin metal shell containing various sensitive ignition powders and primary initiating high explosives sealed in a waterproof assembly. The electric cap is completely sealed with water-resistant plugs with only two insulated leg wires emerging. Inside the cap, the leg wires are joined by a short piece of fine resistance wire called a bridge wire. The bridge wire may be embedded either directly into an ignition mixture or in an electric match. In either case, when an ample electric current passes through this bridge wire, it heats it to incandescence. This ignites the ignition mixture and initiates the primer and base charges in the cap. Thus, the electric blasting cap converts a relatively small amount of electrical energy into a primary-initiating explosion capable of detonating cap-sensitive high explosives with which it is in intimate contact.

Delay electric caps are similar to instantaneous caps in construction and action, except that between the ignition charge and the primer charge there is a column of powder called a delay charge, which serves as a time fuse. Delay E.B. caps are of two general types: millisecond (ms) and long-period delay. A wide choice of delay intervals are available from 0 (instantaneous) milliseconds (a millisecond is one-thousandth of a second) to about 12 seconds. Most quarries use millisecond delays because of the improved breakage and reduced vibration they provide. Many underground operations use the long-periods, although many have switched over to milliseconds.

Scores of different hook-ups may be made. Determination of electrical resistances and other details pertinent to firing electrically are discussed in manufacturers’ literature available to guide mine and quarry operators. Success requires that the operator precisely follow directions of the manufacturer who produced the electrical devices they utilize. Such directions give the exact procedure required to properly:

1. Select and lay out the blasting circuit.
2. Connect wires and protect splices.
3. Test the circuit.
4. Apply the required electrical energy.
5. Protect the circuit from extraneous electricity.

ELECTRONIC BLASTING SYSTEMS

Both the shock-tube system and electric detonators rely on a pyrotechnic delay element to attain their delay timing. These pyrotechnic delays are subject to timing inaccuracies called scatter. Scatter can be caused by variations in the pyrotechnic composition, age, temperature and environmental factors. Deviation from the detonators nominal firing time can cause out-of-sequence firing. This will result in high vibrations, airblast and poor blast performance. Recognizing the accuracy issue and the safety concerns with both the electric system (stray current) and shock tube (cannot be tested), the industry has moved toward a more advanced initiation technology called electronic-blasting systems.

Electronic blasting systems have eliminated the pyrotechnic delay element and replaced it with a high-accuracy timing chip. These systems now deliver 0.1 ms timing accuracy with delays up to 20,000 ms. The systems are available in both programmable and fixed times. Programmable systems allow the blast engineer to design blasts specific to the site conditions.

Electronic systems also bring with them many safety advantages such as being fully testable with self-diagnostics, able to operate in areas of extraneous current and greater blast control through accurate timing.

Field tests have proven that the use of electronic blasting systems with proper blast designs have reduced vibration levels, airblasts and significantly improved blast performance.

SUMMARY

Following are advantages of using delay detonators in production blasting:

■ Improved fragmentation due to the greater freedom for the material to relieve.
■ Greater flexibility in firing sequences and burden-to-spacing relationships due to the ability to orient the blast through the tie-in.
■ Greater ability to control blast vibration and airblast.
■ More predictable blast movement and flyrock control.
■ Reduced backbreak behind the last row of holes.
■ Minimized cut-offs.

For extensive information about explosive and initiation products provided by many domestic and overseas manufacturers, reference the Explosives Product Guide included in the Membership Directory and Desk Reference published each year by the International Society of Explosives Engineers.

BLAST-DESIGN FACTORS

There are a number of factors to consider when designing a blast, including:

■ Material type to be blasted and the blast pattern and hole loading to use in the given rock.
■ Degree of fragmentation desired.
■ Geological structure and the attitude of the angle of initiation relative to the structure.
■ Type and performance of the explosive charge.
■ Type of initiation system and the duration of millisecond delays and accuracy needed.
■ For a given pattern, the ratio of burden to spacing as defined by the tie-in or the layout.
■ Subdrill needed to fully break to the pit floor.
■ Crest and toe locations (or average backbreak from the last row if the face is not dug out).
■ Bench height to determine hole depths.
■ Blast size required to maintain quarry or mine production.
■ Blasting ground vibration and airblast limits, and the design requirements to maintain acceptable levels.

BLASTHOLE LAYOUT

Drilling/blasting success requires a quantitative understanding of how many variables affect one another.

Drilling/blasting success requires a quantitative understanding of how many
variables affect one another.

Once a suitable pattern and loading have been determined, it is important that the holes be accurately laid out in the field and drilled in the proper location. Irregular blasthole locations lead to less acceptable blasting results, unless the improperly drilled holes are redrilled. Burden and spacing dimensions vary and the tie-in is more difficult on irregular patterns. Some portions of the blast will be overshot, while other areas will experience hard toe and coarser fragmentation.

It is especially important that the front row holes be properly located. If there is too much burden (especially at the toe), fragmentation will suffer and the remainder of the blast will have poor relief. Hard toes, loss of grade control and increased ground vibration may result. When there is too little burden, the high-pressure explosion gases cannot be contained. Rapid gas venting through the face will occur and greater flyrock throw and airblast can be expected. There may also be hard toes and blocky fragmentation.

When laying out the front row of holes, it is critical to determine the face burden of each hole to ensure proper loading and decrease the potential for flyrock, airblast and venting. In the past, burden poles were used to measure the face burdens.

While burden poles are still used on some locations, operations increasingly are transitioning to contact-free profiling equipment to increase safety and burden measurement accuracy.

With advancement in technology it is possible to determine the burden of the front row of holes using both two-dimensional and three-dimensional face profiling. Prior to drilling, a 2D face profile allows an individual to determine face profile (perpendicular cross-section of the blast face at that location) and adjust the hole location closer to or further from the face to obtain the optimal front row placement. Once the shot has been drilled, the face can be profiled to get the actual 2D cross section and burden for each hole in the front row. This gives the blaster or loading personnel the information required to load the front row more accurately and greatly decrease the chance for flyrock. This form of profiling is usually adequate for simple faces.

For more difficult blast faces, it may become necessary to use a 3D profiling system to obtain a minimum burden view of each blasthole. Two types of systems that can be used are 3D laser profilers and 3D photogrammetry systems. The 3D laser takes many points on the face and creates a 3D mesh of the blast face allowing a person to design the front row to a more accurate face. It also gives the blaster or loading crew the same options as far as the 2D cross sectional view for hole loading accuracy. A 3D photogrammetry system uses digital pictures of the blast face to create a high-accuracy 3D image of the blast face. It also gives the loading crew the ability to see the minimum burden, or burden on a 360-degree view for each hole. It can be used to design simple or difficult blasts and get the most accurate front row hole placement. A profile taken of an already drilled pattern allows the loading crew the most accurate burden view for loading purposes.

3D profiling systems also can be geo-referenced to allow an operation to import the information into its mine planning software, drill navigations systems and blast design software.

Using the face profiles generated from these systems, an operation can now place the holes in the front rows at the optimum burden, load them with greater accuracy and increase the overall safety, blast performance and cost efficiencies.

BLASTHOLE LOADING

It is important that holes be loaded correctly in accordance with the design. Improperly loaded holes can lead to poor fragmentation and/or excessive flyrock and airblast. The hole depths must be correct. Operators must decide how short a hole can be before redrilling is required. In very hard rock, a blasthole that is 1 or 2 ft. short can result in hard toe and loss of floor control. In softer rock, more variance is acceptable, but is seldom more than 4 or 5 ft.

Modern-day bulk trucks have more sophisticated measuring and control systems. The operator can set the weight to be loaded in the blasthole and the truck shuts off automatically. However, this does not eliminate the need to measure column rise in the hole during loading. The truck-control systems cannot tell about voids or cavities in the hole, nor about control-system malfunctions. Thus to avoid over or under loading, and to obtain the correct column rise, it is still important to measure the hole during loading.

Accurate loading is especially important regarding the column rise and corresponding stemming height. The explosive column must rise high enough in the given rock type to fully break to the surface of the upper bench. Good breakage is related to the depth of burial of the top of the charge. Too great a depth of burial and the top of the blast will be poorly fragmented.

On the other hand, if the explosive column rises too high in the hole, the depth of burial is low, gases vent rapidly to the surface, and there is more flyrock and airblast. Also, the radius of the crater of fully broken material formed around the hole decreases and there may be hard areas between holes.

FRONT ROW STEMMING HEIGHT

Stemming heights on the front row may need to be increased. If the bench-face angle is less than 90 degrees, the burden on the front row holes is continuously decreasing between the toe and collar of the hole. Depending on where the front row blasthole must be drilled to maintain a suitable toe distance, the burden may become too small to contain the explosion gases at the normal column rise.

To avoid gas venting to the face causing flyrock, airblast and loss of performance, stemming on front row holes may need to be increased.

For simple blast faces, a burden pole may be adequate for stemming measurements, but when using a face profiling system the loading crew will have the cross-sectional profile view or the minimum burden view and can use those to more accurately determine the proper loading and stemming height for the front row of holes.

This stemming height is required to maintain minimum burden on the charge. Failure to make appropriate adjustments to front row stemming may well lead to hazardous flyrock.

BLAST TIE-INS AND BURDEN-TO-SPACING RELATIONSHIPS

Drilled and loaded blast patterns may be tied in to create different burden to spacing relationships. Commonly used designs are:

1. Square pattern tied en-echelon or across two free faces   – Known as a V-1 tie-in, this is a non-staggered pattern. Tie-in is on the diagonal of the square and is oriented at 45 degrees to the free face. The effective burden is 0.707 times the drilled burden. The ratio of the effective spacing on the tie-in to the effective burden is 2:1.

2. Staggered square pattern tied on the diagonal of the parallelogram – This is known as the V-2 tie-in. The orientation is 34 degrees to the face. The effective burden is 0.56 times the drilled burden. The ratio of effective spacing to effective burden is 3:1.

3. Staggered equilateral pattern tied-in on the V-2 configuration – The angle to the free face is 30 degrees. The effective burden across the tie-in is 0.50 times the drilled spacing. The ratio of effective spacing to effective burden is 3-5:1.

4. Row-on-row tie-in – In this case, successive rows detonate in progression. There is no burden reduction and the effective burden and spacing are the same as the drilled dimensions. The rows detonate parallel to the face rather than at an angle. Generally in open pits and quarries the tie-ins described above are preferred.

MILLISECOND DELAY TIMING

The duration of millisecond delay times must also be considered. Field experiments have shown that 1 to 1-1/2 ms per foot of effective burden is the minimum that can be considered if any relief is to be obtained for holes firing on successive delay periods.

For good relief, it is typically found that 2 to 2-1/2 ms per foot of effective burden are required. In some cases where maximum relief is desired 5 to 6 ms per foot may be appropriate.

When delay times are long, care must be taken to avoid cutoffs and misfires, depending on the type of initiation system being used. A down-the-hole delay of sufficient duration to allow much of the surface tie-in lines and blasthole downlines to be consumed before blastholes begin detonating is the usual procedure taken to avoid these problems.


Choosing the right DTH tools

The wide range of applications and drilling conditions suitable to down-the-hole (DTH) pneumatic percussion technique requires consideration of more than one type of tool to drill efficiently and profitably. A variety of factors, from hammer size to determining whether or not an economy hammer represents the best value, all figure into the hammer’s overall cost of operation on a given job.

CHOOSING THE CORRECT HAMMER

The selection of hammer size is largely determined by the hole diameter and type of rock formation. The optimum blasthole range for DTH drilling is 3 ½ to 10 in. Smaller holes are typically drilled with a top hammer rather than DTH hammer, and larger holes generally use rotary drilling technique, mainly with the focus on hole straightness.

As a general rule, the smallest hole diameter a DTH hammer can drill is its nominal size. A 4-in. hammer is optimally designed to drill a 4-in. hole. The closer the hole diameter is to the hammer’s diameter, the more restricted the hole’s evacuating airflow is. Drilling holes at the nominal size leaves adequate space for cuttings to evacuate the hole up the annulus between the hammer and drill pipe diameters and the internal diameter of the hole’s wall.
Maximum bit size for production drilling is the nominal hammer diameter plus 1 in. For instance, a 4-in. hammer’s maximum bit size is regarded to be 5 in. in diameter.

MATCHING THE HAMMER TO THE CORRECT PIPE DIAMETER

Pipe diameter should be close to the hammer diameter to provide optimum flushing, reducing the chances of getting stuck in the hole.

Key features to look for in high-quality DTH pipes are durability, accuracy and manageability. Pipe (tubes) made from cold-drawn tubing provide a superior surface finish and tolerance compared with tubes made from hot-rolled tubing. A better finish reduces the risk from metal chips from the tubes, called scaling. Scaling that flows through the hammer is a major cause of premature hammer failure.

Construction quality is at least as important as design. Friction-welded joints add strength. Heat treating the threads of end-pieces ensures optimum durability and strength of the thread profile, which results in longer thread life. Preserving the thread profile keeps coupling and uncoupling smooth, without adding time to the average rate of penetration – in other words, the cost per foot to complete holes.

In most applications, standard API threads are the best choice, although adapter subs and crossover subs are available to support any setup.

SELECTING THE CORRECT BIT

DTH bits are manufactured to match the shank and diameter of the hammers. The bodies of quality bits are precision machined from alloy steel, heat treated to a specified hardness, providing surface compression for fatigue resistance, and then fitted with precision carbide buttons.

A variety of bit designs and configurations are available for all rock types, focusing on rock psi hardnesses and application conditions. Bit life and the rate of penetration are the most important criteria in selecting the right bit for a particular application.

Convex-faced, ballistic-button designs are preferable for fastest cuttings removal. This bit design cuts clean with minimal re-crushing, making it ideal for soft to medium, non-abrasive formations.

In hard and abrasive formations, flat-front designs offer best bit life, especially those that feature strong gauge rows with large spherical buttons that are easy to regrind and maintain. An alternative in these conditions is a concave design with spherical buttons.

Concave bit faces work well in medium to hard, fractured formations. In this type of application, a concave bit minimizes hole deviation.

PREMIUM OR ECONOMY

Probably the most fundamental question is whether one needs a reliable, low-cost hammer or a premium hammer. Buying more than one less-expensive hammer to achieve the same drill feet of a single premium hammer is false economy. However, premium hammers that offer maximum rig productivity are not necessarily the best value at every drilling job.

Premium hammers are generally a better value in production work. A manufacturer’s premium hammer features state-of-the-art technology to deliver maximum productivity and profit through superior longevity and reliability. They are designed to be easily serviced and rebuilt. Premium hammers will provide the greatest external wear resistance. Its internal components will likely feature wear and corrosion protection. Its useful life can be extended through multiple rebuilds, replacing only typical wear points rather than the whole hammer.

The word “economy” should not be confused with “inferior” in the term “economy hammer.” This class of hammers represents a different strategy for lowering cost of operation in certain applications. Economy hammers are based on simplicity and serviceability, resulting in lower operating costs. For instance, such hammers might be used for deep hole operations, providing higher power outputs, but also feature selector valves to maximize air compressor productivity. Quality non-premium hammers can be rebuilt at least once and feature modular components, snap-in cylinders, a reversible casing and backhead saver sleeves.

APPLICATION CONSIDERATIONS

Manufacturers design DTH hammers to address the challenges presented by the full array of rock types and applications. For deeper hole applications, hammers are designed to work with different air requirements as bailing velocity requirements change. This allows greater volumes of flushing air for hole cleaning than what is required for optimum hammer performance. Quarry and mining hammer designs may feature heavy-duty chucks, wear sleeves and backheads fitted with tungsten carbide buttons for wear protection in harsh and abrasive conditions.

The following applications present factors to be accounted for in hammer selection:

QUARRY

Quarrying projects are long-term jobs that will easily outlast the hammer. Frequently replacing hammers would be cost-prohibitive. Drillers who are seeking the highest productivity or who are drilling in abrasive formations should consider choosing the most reliable and productive premium hammers on the market.

Dimensional stone quarrying demands consistently straight holes. DTH hammers offer more precision to hole straightness than top hammers. Dimensional stone operations typically drill smaller size holes of 3 ½ to 4 in. in limestone, granite, and marble. Though premium hammers might still offer better value in this non-abrasive application than non-premium models, in some instances non-premium hammers will be the better value.

MINERAL EXPLORATION

Mineral exploration generally requires robust hammers capable of running high pressures, often in dirty, remote environments. Operators tend to prefer simple, reliable designs in this application.

Reverse circulation (RC) with specialized DTH hammers is less expensive than diamond coring technique. The RC hammers use the same engineered technology and components as standard DTH hammers, but use RC tubes with inner and outer walls for direct airflow to allow the cuttings to pass up through the pipe to a cyclone for collection bags, rather than the typical hole cleaning up the hole’s annulus. RC hammers are used for both deep hole exploration drilling and in-pit grade control applications.

GEOTECHNICAL

Drilling of foundation, anchoring, monitoring or drainage holes demands simple, reliable, workhorses rather than premium hammers.

OPEN PIT MINING

Like quarries, mining operations typically have high equipment utilization, drilling up to 80 percent of the working day with DTH technique. The typical blasthole drilled with DTH tools in open pit mines is 5 to 8 in. in diameter. Buffer holes regularly run 5 ½ to 6 ¾ in. Pre-split drilling usually requires hole diameters between 4 ½ and 5 ½ in.

Some premium hammers for these applications can now be rebuilt twice before replacement. Drillers who choose premium hammers in these conditions also keep rebuild kits on hand to achieve maximum productivity with the longest tool life possible.

Properly training drillers to recognize and maintain optimum drilling parameters prevents premature failure of hammers, whether premium or economy hammers are used in these applications.


Prediction and evaluation of blast fragmentation

In recent years, empirical methods for predicting fragmentation that will be obtained from a given structural geology, rock type, explosive and blast pattern have become better and more useful. In addition, computer-assisted photographic techniques for measuring the size distribution of blasts have been developed.

There is currently no theory of fragmentation developed from first principles that can be used to accurately predict the fragmentation that will be obtained from a given blast. We seem to be at least a decade away from such a theory, and given all of the explosive, blast implementation and rock factors that influence fragmentation, one may well question whether such a theory is possible. For the time being at least, the empirical approach is what we have to work with.

Empirical prediction of expected fragmentation is most often done using the Kuz-Ram approach. This is an application of the Rosin-Rammler theory of comminution to blasting and was first proposed by V.M. Kuznetsov of Russia. Subsequent work by others, especially Claude Cunningham, served to make this approach much more effective.

Using this approach, one calculates a rock factor that describes the nature and geology of the rock. A uniformity index is also obtained that characterizes the explosive loading and blast pattern type and dimensions. This allows a characteristic size and size distribution to be calculated according to the Rosin-Rammler procedure. A considerable amount of fragmentation study is being performed in the industry using this procedure.

Fragmentation measurements are often done today using the edge technique to analyze photographs or video of a blast to determine the size distribution. While these techniques may not always be totally accurate for the fines fraction, they do provide a good overall assessment of the blast fragmentation. When photographs or video are taken at different times during the digging of the blasted rock, a good idea of the degree of fragmentation throughout the blast can be obtained. Digital fragmentation systems are now available to the operator that will allow them to monitor the fragmentation at the muck pile or at the processing plant.

A powerful approach appears to be combining predictive techniques with actual measurements of the same blast. If the two curves do not agree, adjustments can be made to the rock factor in the Kuz-Ram calculation to bring them closer. Thus, actual measurement can be used to fine tune the predictions. Then, when explosive, pattern size or other changes are contemplated, better prediction of the resulting fragmentation will be achieved.

Fragmentation prediction and analysis have become increasingly important as it has been realized that primary breakage is less costly than secondary breakage, and often may be more economical than crushing. In addition, crusher throughput is directly related to the size distribution of the feed in relation to the crusher opening size.

The fragmentation distribution is important to efficient run-of-mine heap leaching. When rip-rap is desired, one may wish to look for combinations of geology, explosive and blast pattern that lead to blocky, coarse fragmentation. In quarry operations, excessive fines detract from the ability to produce products of more value and may create an environmental problem with regard to dust generation.


 

Sources:

Contributors to this chapter include the following, in alphabetical order:

William Hissem
Senior Mining Engineer
Sandvik Construction

Robert McClure
Blasting Consultant
R.A. McClure Inc.
r.mcclure@ramets.com

George D. Raitt
Formerly Ingersoll-Rand Co.

Lyall Workman
Vice President, Senior Mining Consultant
Barr Engineering


Lesson 4 Quiz

1. What is the term for a cylindrical vehicle designed and strategically situated to hold and contain an explosive charge so that it can be detonated in the most efficient manner?

2. What is the most expensive phase in the drilling and blasting portion of production?

3. What is the first step in the drilling process?

4. What are the two broad categories of drilling methods?

5. What are the five aspects that need to be considered in regard to drill rig and mounting?

6. What does DTH stand for in regard to drilling and blasting?

7. What is one of the most commonly used products in blasting that combines oxidizer and fuel?

8. In modern-day blasting, what type of detonator is “virtually always” used?

 

Click here for the quiz answers.

 

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