The Selection of the Proper Borehole Size Is the First Decision That Is Made and Will Directly Impact All Other Blasting Decisions.
By Anthony Konya and Calvin J. Konya
An often-overlooked topic in drilling and blasting is the selection of the borehole diameter. Often this is decided on an economic basis where it is cheaper to reduce drilling by using larger boreholes with the understanding that an extremely large borehole can be problematic. However, the selection of the proper borehole size is the first decision that is made and will directly impact all other blasting decisions.
The outcome of this decision will also heavily influence the fragmentation, throw and environmental considerations of a blast. Assuming bulk explosives are being used, this single decision may be the most important decision a mine can make for its drilling and blasting program; however, this is typically decided by a guess and check method to see if the diameter makes sense for the operating conditions.
In many situations, quarries believe they must use a smaller diameter borehole and large metal/nonmetal mines that utilize large shovels to move material should use a larger borehole. This is not always the case and in fact the fragmentation is not directly tied to the borehole diameter.
In blasting two types of variables exist, uncontrollable variables and controllable variables. Uncontrollable variables are variables such as rock type, structural geology and government regulations. Controllable variables are those that are directly under the control of the operator, such as borehole diameter, burden and stemming.
These sets of variables then decide the outcomes of the blast, such as the fragmentation and throw. The first controllable variable that is to be decided at a site is the borehole diameter. This borehole diameter will directly decide the burden, and the remainder of the blast variables are then decided based on the burden. The borehole diameter will then influence all other parts of the blast and must be of careful consideration, especially when one desires optimal fragmentation.
Borehole Diameter
The authors have consulted for numerous operations where the current borehole that is being used is presenting problems for a mine, yet they are attempting to implement a larger diameter borehole. These mines believe that by implementing a larger borehole they will be able to alleviate the current problems.
However, this is the exact opposite of what will occur because when a larger borehole is implemented, and the blast is scaled accordingly, the problems that the mine currently faces are exponentially increased. Additionally, some operations think that the problems will remain the same if the powder factor remains the same. This is also not true as powder factor is not an appropriate design tool and as a larger borehole is used the actual mechanics of breakage will change.
The use of a larger blasthole with the same scaling parameters will result in courser, less uniform fragmentation; worse throw of material; larger ground vibration; larger air overpressure; more flyrock; and increased backbreak. This can be thought of as a scaling effect, when a larger borehole is used the burden is scaled accordingly.
Just as the burden is scaled and increased the outcomes of the blast are scaled and increased in a negative manner. Let us look at two scenarios where the fragmentation has been projected based on two different borehole diameters where all parameters of the blast, except for the bench height, were scaled for different borehole diameters.
Scenario One:
Borehole Diameter = 3.5 in.
Burden = 7.5 ft.
Spacing = 10.25 ft.
Bench Height = 30 ft.
Scenario Two:
Borehole Diameter = 7 in.
Burden = 15 ft.
Spacing = 21 ft.
Bench Height = 30 ft.
As can be seen from Table 1, by doubling the borehole size and scaling the blast dimensions, or the other controllable variables, proportionally the fragmentation size significantly increases. This could have dramatic considerations for a mine that has oversize limits such as a primary crusher.
Take for a moment a mine that has a 36-in. maximum size crusher, in Scenario One this mine would hardly ever have oversize. However, with Scenario Two the mine would have about 15 percent of material left as oversize which would require either rock picking or secondary blasting. This would dramatically increase the cost of the operation over just drilling additional boreholes at a smaller diameter.
Stiffness Ratio
One of the major problems with increasing the borehole diameter is that it will change the mechanics of the blast due to the stiffness of the blast. When a bench is long, compared to the burden, the blast has horizontal displacement and properly bends out toward the free face.
When a bench is short, compared to the burden, the blast has a significant amount of vertical displacement and is categorized as a violent blast. These blasts are not random but are decisions that are made in the blast design process, the long bench is breaking with the borehole effect and will have good throw, good fragmentation and lower ground vibration. The blast that is breaking through a vertical component is breaking through a cratering mechanism and will have worse fragmentation, bad throw, and much higher ground vibration.
The way this is quantified is from the stiffness ratio and with proper design the stiffness ratio will be optimized to give proper breakage mechanisms to ensure proper fragmentation sizing is achieved. The stiffness ratio is the bench height (L) divided by the true burden (B).
The table below (Table 2) discusses the effects of the stiffness ratio:
While no additional benefits to cost, fragmentation, throw or environmental factors exist for increasing the stiffness ratio above four, it can be done and the bench suffers no problems.Once a benches stiffness ratio has increased to four or greater, another additional benefit is that the borehole spacing can be expanded further. This is due to the relationships between borehole interaction and bench height. In fact, as the stiffness ratio increases from 1 to 4, the spacing is increased as well. This not only leads to consistent powder factor as bench height increases, but it is also much more economical to have tall benches.
Next, three further scenarios will be examined this time keeping the borehole diameter the same and changing only the bench height to show the difference in stiffness ratios:
Scenario One:
Borehole Diameter = 3.5 in.
Burden = 7.5 ft.
Spacing = 10.25 ft.
Bench Height = 15 ft.
Scenario One:
Borehole Diameter = 3.5 in.
Burden = 7.5 ft.
Spacing = 10.25 ft.
Bench Height = 22.5 ft.
Scenario One:
Borehole Diameter = 3.5 in.
Burden = 7.5 ft.
Spacing = 10.25 ft.
Bench Height = 30 ft.
Table 3 shows the common fragmentation effects of stiffness ratio. As the bench is lower the fragmentation of the blast contains more fines and more oversize – the opposite of what is desired in any situation. This is because the goals of fragmentation are most often two parts, the first part is what is the size of the rock. Typically, this is defined as a P50 or P80; however, the second and one of the most important goals is how evenly distributed can the fragmentation be.
Imagine the scenario of an aggregates producer who desires that blasted material, on average, be around 6 in. They bring two powder companies in to conduct side-by-side blasts to see which can achieve this result. Powder Company A conducts their blast and produces an average size of 6.5 in. material, with 95 percent of material between 4 and 8 in.
Powder Company B conducts its blast and produces an average size of 6 in. material, but minimal material is between 4 and 8 in. Instead there is a large amount of material that is under 2 in. and a large amount of material that is over 12 in. Who will get the contract?
Powder Company A, because distribution is in many cases more important than having the exact size. Larger stiffness ratios produce better distributed product than lower stiffness ratio, if boulders and fines are costing an operation achieving proper stiffness ratio is one method to eliminate these problems.
Selection of Borehole Sizes
As many mines have a certain bench size requirement, the main way a mine can engineer an improved stiffness ratio is through the selection of a proper borehole diameter. This is because the burden of a blast is directly proportional to the diameter of the borehole. Therefore, selecting a borehole diameter that produced a higher stiffness ratio is typically better. What does this mean though?
Typically this goes against what many think is a proper method to reduce costs, instead of going to a larger diameter borehole the use of a smaller diameter borehole is preferred. While this increases drilling and initiator costs, the ability to spread the spacing out and obtain uniform fragmentation offset this cost. Additionally, through the use of scientifically based, not sales based, priming recommendations and the ability to spread the patterns out further on higher benches many mines actually see a large decrease in overall mine costs when conducting mine-to-mill optimization techniques when going to an appropriately sized, smaller borehole.
What is the proper borehole size for an operation? Well, it varies based on the operation but typically an operation should design there borehole so that they achieve a stiffness ratio above a three, after the burden has been calculated. For an upper limit, it is typical that at slightly above four will give the best fragmentation while keeping costs minimal. While sites can blast at stiffness ratios well above four, they may be able to reduce costs while keeping fragmentation similar if they move closer to a stiffness ratio of four.
Conclusion
Mining is an extremely competitive environment, especially for aggregate producers that are constantly trying to achieve minimal costs through an operation. The typical bottleneck for many operations though is not in the blasting but in the crushing and processing; this leads aggregate producers to greatly gain by introducing mine-to-mill optimization programs and considering the total cost of an operation, not just of individual units. This path will often introduce the goal of obtaining proper sizing and distribution of blasted materials to ensure that the crusher(s) are running at maximum capacity and minimal re-handling/secondary blasting costs are introduced.
In order to achieve these goals, it is often more economical to select a borehole that will produce a stiffness ratio in the range of 3 to 4.5, giving the mine a good fragmentation distribution and sizing while also keeping costs manageable.
In many mining situations, the way to achieve this will be to go to a smaller diameter borehole that is typically the opposite of what producers first imagine when looking at lowering costs. This is because at first this can appear costlier, but with proper spacing arrangements oftentimes powder factors remain the same as with a larger borehole. Increases in cost then arise from additional drilling and initiators.
However, this will lead to properly sized product where the reduction in crushing and secondary breakage costs, as well as reduction of waste fines, which will ultimately put the producer into a better economic condition with a lower overall cost.
Dr. Calvin Konya is the president of Precision Blasting Services, and Anthony Konya is a project engineer for the company. They can be reached at 440-823-2263, or [email protected].