We looked briefly at drills in Lesson 5.1, as part of an overall survey of the unit operations for winning the ore. I showed you some pictures of different types of drills, and we talked about a few drilling applications. Here in this short lesson, I want to approach the topic of drilling more systematically.
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Drilling is done for many reasons, and examples include:
This is a fairly complete list. While the drilling equipment may look differently depending on the application, the principles are similar. Here, we want to focus on the common principles.
Regardless of our reason for drilling, the goal is to penetrate the rock. We are going to focus on mechanical penetration, although there are a tiny number of applications for water jet and thermal penetration. Penetration occurs when we cause a local failure of the rock and remove the failed pieces, known as cuttings, from the hole. Simplistically, the rock fails in compression or tension. Drill bits create one or both of these forces, which causes the rock to fail. Often a given bit design will create predominately one type of force and the other to a lesser extent.
Percussive energy causes failure of the rock in compression. Imagine taking a sharp chisel, placing the edge on a rock, and striking it with a hammer. As you strike the chisel, it is pressed into the rock, and puts the rock underneath the chisel edge in compression. There is some crushing under the chisel edge, but importantly, cracks will propagate outward, and chips of rock will be released. We could strike the chisel with our hammer again, and similar effect will follow, although perhaps not giving us the same degree of fracturing. We could strike a third blow, and while we would see our indentation or groove become slightly deeper, the penetration would be again less. If we want to achieve a better penetration rate, we need to index our chisel bit; i.e., after each strike, we need to rotate the chisel edge by say 15°. This results in dramatically improved penetration. Thus, percussion drills index the bit after each strike. A percussion drill may strike more than once per second, and will index the bit after each strike.
Rotary energy causes failure of rock in tension and shear. Imagine taking our chisel, and while bearing down on the chisel, we also rotate it, dragging the chisel edge across the surface of the rock. In so doing, we will exceed the tensile and shear strength of the rock, and rock fragments will “peel” away. Of course, a mechanical drill will be able to rotate the drag bit much faster than you ever could! Rotation rates may exceed one per second. Intuitively, you probably suspect that rotary methods are only effective with softer materials, and you are correct. Rotary bits with diamond cutters, suitable for use in hard materials, are commercially available, but not normally used in mining applications.
If we wanted to make a broad statement, we could say that the lowest drilling costs are realized by using rotary drag bits in soft materials, rotary-percussion bits in medium hard to hard materials, and percussion bits in very hard materials.
The resistance of the rock to drilling is termed its drilling strength. This is an empirical term used to make comparisons, but it is not quantified in the way we can quantify engineering parameters such as compressive strength or strain, for example. Nonetheless, you will find tables giving the relative drilling strength of different materials, and these tables are informative.
One could spend an entire course on the theory of drilling and rock fragmentation, but for our purposes to utilize drills, there’s not much more that you need to know about the mechanics of rock penetration!
The piece of equipment that we refer to as a “drill” is a system of four components.
Drill types may be classified by:
When you are out and about the industry, you will hear people say they have a two-boom jumbo that’s electric over hydraulic, or we’re using an air-track drill in our quarry. These colloquial names are built on this list.
We looked at some pictures of drills in Lesson 5.1. Here I want to show you a few photos of bits.
Here is a cross section of a down-the-hole bit.
And give it a sense of scale, here is the same bit in the hands of a driller.
On the left, we have small 2” percussive bit. On the right side, we have a small rotary drag bit.
Here is the real workhorse of the drilling industry – the roller tricone bit. Downward force, i.e., thrust, is applied to this bit as well as rotation. As the bit rotates, the individual roller cones are caused to rotate, and then each bit tooth successively impacts the bottom of the hole. These are very effective bits. The size of the teeth on the roller cones will depend on the hardness of the rock. In softer materials, the teeth will be large and wedge shaped. Bits for harder materials will become smaller, and for the hardest materials, the teeth are small buttons or beads. Why do you think the size of the teeth would change?
The drilling strength of the rock, i.e., its resistance to penetration, will have a significant effect on the cost of drilling. It is not the only parameter to do so, however. Other factors that will affect drilling performance can be organized into four groups:
These affect the four components of the drilling system (drill, rod, bit, and fluid).
They are largely controllable and include factors that affect the tools (drill power, blow energy and frequency, rotary speed, thrust, and rod design) and drillhole cleaning (fluid properties and flow rate).
These include hole size, length, and inclination; they are dictated by outside requirements and thus are largely uncontrollable. Hole diameters in surface mining are typically 6 to 18 in. (150 to 450 mm); underground, they typically range from 1.5 to 7 in. (40 to 175 mm).
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These consist of properties of the rock, geological conditions surrounding the rock, and the state of stress acting on the drillhole. Often referred to as drillability factors, they determine the strength of the rock and limit drill performance. Because these factors are a result of the geologic environment, they are largely uncontrollable. Make a note of the importance of the Blast Hole log! We’ll discuss that further when we talking about explosives and blasting.
These variables include labor and supervision, power supply, job site conditions, weather, and so forth. Except for labor and supervision, they are independent factors and cannot often be affected by the drill operator.
The performance metrics for drilling are:
You will find all three of these in common use.
Never forget that we strive to optimize the system, not just the subsystems. This it is not just about the drilling cost, but also the associated blasting cost, since we are drilling specifically to conduct blasting. So, a goal is to achieve a low drilling and blasting cost.
I will mention it now for completeness, and we’ll discuss it in the next set of lessons: the typical performance metrics for blasting are:
Drillability factor and powder factor are two very important metrics for use to keep track of, in our operations.
Before leaving the performance metrics specifically, and drilling in general, I need to add that it is not just about optimizing drilling and blasting, but what else? Well, loading in particular, and then materials handling, and of course mineral processing. Actions or inactions within any of these can adversely affect the profitability for the mining operation, and our decisions at each step must be informed by a knowledge of the impact of those actions on subsequent operations. This will become clearer as you acquire more and more knowledge throughout this course.
Drills are used to create a hole of a certain diameter and depth. Occasionally, the goal of drilling is to create an empty hole, but more often, the purpose of the hole is to accept explosives. The major components of a drill include the bit, which fragments the rock; a power source that transfers energy to the bit; and lengths of drill steel, sometimes called the drill string, that connect the bit to the drill rig proper. Drills vary by: the method of rock penetration, e.g., rotary or percussion; the location of the power source, which can be at the top of the drill string, e.g., top hammer, or at the bit, e.g., down-the-hole; primary method of powering the drill, e.g., diesel engine, electric motor, or compressed-air; and the method of mounting the drill rig, e.g., track-mounted or tire mounted.
Here, we have a track-mounted down-the-hole drill with an articulating boom to facilitate drilling holes at precise angles.
While the previous drill employs a down-the-hole hammer to apply energy to the bit, this one is a top-hammer drill, i.e., the energy source for the bit, at the top of the drill string. That means the “pounding and rotational” action has to be transmitted through the drill string to the bit. The biggest disadvantage of this approach is the loss in accuracy. The drill string tends to travel in a large helical track with the top hammer, and this causes the drill bit to “wander” off the desired location of the hole.
If these drills are going to be used underground where the headroom is limited, the mast is not as high, but otherwise the drill is similar. While the accuracy of hole location is most always important, in some underground applications, it is crucial. In those, a down-the-hole hammer will be used.
Vertical or inclined holes are commonly required in surface mining, and sometimes in underground mining. It is very likely that horizontal or nearly horizontal holes will be required in underground mining, as well as overhead vertical or overhead angled holes.
The dual boom jumbo drill is designed to drill horizontal or inclined holes at an angle off of the horizontal. The depth of these holes is typically limited by the application and is on the order of 15’.
Ring or fan drilling and longhole drilling are characteristic of a few underground metal mining methods. The holes may be 150’ or long, and must be drilled to precise depths at the exact design angles. Drills to accommodate these requirements employ computer control to achieve the required accuracy, as do more and more jumbo and other drills being used in production operation. A typical drill is shown here. Note the remote operating station for the drill operator.
This diagram illustrates the drilling pattern required in a sublevel stoping mine. Notice the layout of the holes. What do you think would happen if a hole was started, but the angle was off by a few degrees? Or what if a particular hole were drilled to 128’ instead of the designed length of 135’? Intuitively, I am sure you can imagine that it will affect the performance of the blasting, and you are correct. We’ll talk more about this when we cover blasting, but for now, please recognize that productivity, cost, and safety are adversely affected by less-than-optimal drilling practices.
Finally, we should close our overview of drills with the handheld “jackleg” drill. In years past, these were used instead of jumbo drills in hardrock mining and for ground control applications in coal and metal/nonmetal mines. The size of hole depended on the cylinder bore of the drill, as they were powered by compressed air, as well as the size of the drill bit. Hole sizes ranged from approximately 1” to 4”. Depending on the size, and consequently what the drill was used for, they were given names such as drifters or stopers. They are only used in modern mines for very specialized functions, requiring a few holes on occasion, here or there. Unfortunately, you will find them in widespread use in the underground mines of lesser-developed countries. I say unfortunately because they are brutal to use. Although much of the weight, around 75 lbs., is supported on the jackleg, the miner has to apply the thrust, i.e., pushing the drill bit into the hole. That alone is hard work. They are very loud, there is bone-jarring vibration, and they can produce dust-laden air and that along with the oil mist from the compressed air creates a respiratory hazard. Teams of miners would stand all shift-long operating these drills. Often they were paid based on the production that they achieved, and 15 years ago in the western U.S., one of these miners could easily earn $60-80 thousand dollars per year… but they earned every penny of that!
The practice of loading sticks of dynamite has all but disappeared from modern mining. Safer and more economic practices utilize explosives that are pumped, gravity-fed, or pneumatically blown into the hole. That’s not to say that we never use packaged material, because we do, and we’ll look at that when we discuss explosives and blasting practice. The vast majority of blasting, however, is accomplished with powders, gels, or emulsions that are bulk loaded.
The truck below is used in surface mining. It carries the materials to mix the explosive at the hole, and the equipment to place the explosive into the hole. Such an arrangement can be found in some underground mines as well. In this example, the explosive is being pumped into the hole.
In a mine in which the height of the active mining face is on the order of 15’ to 60’ feet, you will see equipment like this. The boom allows the blaster, standing in the basket, to reach each hole, insert a hose into the hole, and then load the explosive. In this picture, you can see the yellow tank that contains the blasting agent. There is a second tank on the other side, which is obscured from view in this photo. These tanks are known colloquially as “powder monkeys.”
Rippers are dozers that have been equipped with one or more large drag bits. These are pulled through the ore as the dozer moves forward. Typically, they break up the top 6” – 18”, depending on the mechanical properties of the ore. Rippers are not very common because they are suited to few deposits. Occasionally, I’ve seen them used in soft high calcium limestone and in coal, but there are other examples. In the picture here, this ripper (seen at the back of the image) has three large drag bits.
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