March 21, 2017
A common problem for engineers and consultants for aggregates mines is the critical challenge of achieving proper blasting. Many of the problems faced came from operational inefficiencies in the drilling and blasting program. Most mines believe that better blast design will help save costs, improve fragmentation, and decrease vibration, and, while this is all true, none of this can be achieved if the design cannot be achieved with reasonable accuracy. By achieving this accuracy, before implementing better blast designs and processes, most mines save upwards of 15 to 30 percent on their blasting costs. There are many ways to achieve accuracy, such as designing for inaccuracies, automating jobs, and blasting management.
Designing for inaccuracies
How does one design for inaccuracies in blasting? This is a common question when inaccuracies are first encountered, because engineering out these inaccuracies is one of the easiest options with the least capital cost. However, this approach is extremely costly in the hidden costs of drilling and blasting.
One of the first people to study drill deviation on a large scale was Langefors. Langefors documented his extensive results from dozens of mines in his book The Modern Technique of Rock Blasting and discussed ways to ‘engineer out’ drilling inefficiencies. He did this by analyzing the average errors in drilling from:
When Langefors studied a large multitude of blasts in Sweden, he concluded that, on average, improper collaring locations accounted for 4 inches of deviation, and alignment inaccuracies were approximately 0.04 inches per foot in bench blasting. To most, this may seem insignificant, but this is almost ±6 inches on a 40-foot bench. This means that between two holes the spacing may be reduced by 1 foot and the burden increased by 0.5 feet.
Forsyth et. al studied this same problem and reported up to 0.12 inches per foot on longer blast holes with a 7 ¾ inch drill bit. As the drill bit is reduced, the drilling errors are larger based on the stiffness of the drill steel. In this case, both studies were with larger drill bits.
To determine the significance of this, we can look at a fragmentation prediction method called the Kuz-Ram model. The Kuz-Ram model is a way to predict fragmentation before a blast and has a variable to define the drillhole deviation. Let’s analyze the drill deviation with an example blast in limestone using the following specifications:
Figure 1 – Drill deviation on fragmentation
From Figure 1, one can observe the actual impact of drill deviation. In this graph, the horizontal axis is the fragmentation (screen size) in inches from the blast, and the vertical axis is the percent passing that screen size. Table 1 has a summary of these results, with the term Px meaning the screen size that ‘x%’ of the material would pass through. This table shows that, with increasing deviation, the fragmentation oversize significantly increases, which will increase crushing costs as well as secondary breakage costs.
Table 1 – Fragmentation comparison
To overcome this drill deviation, Langefors suggested to reduce the burden to account for this deviation. He suggested that the actual design burden should be the optimal burden subtracted by the deviation per foot. This is what is meant by ‘engineering out’ deviation from drilling. While this can be a good method, it has some drastic ramifications.
By engineering out deviation, not only will the burden change, but all blasting parameters have to be modified. To analyze this effect, the Kuz-Ram model will again be used for the design in each of these situations:
Table 2 – Engineered out solution
A few problems are brought to light instantly when analyzing these patterns, even before looking at the fragmentation.
When analyzing the fragmentation in Figure 1 and Table 3, one can see the extreme results this can have on the fragmentation. One can observe that, in terms of the fragmentation, Langefors method works well with his drill deviation, however, with larger deviation such as Forsyth, this becomes impractical and costly with a large increase in fines and boulders, with relatively little good material.
Figure 2 – Engineered-out fragmentation
Table 3 – Engineered fragmentation
How much does this ‘engineering out’ method cost a mine? The figures below show the cost for blasting a bench that is 60 feet long by 22 feet wide, based on a drilling cost of $5 per foot for drilling and $0.30 per pound of ANFO. One can clearly see that by engineering out the solution, the cost is increased drastically. If better fragmentation is achieved, some of these costs can be offset in reduced crushing and better product. But what if large deviation, like in Forsyth, are displayed at the mine?
Many large mines are now going to autonomous drilling, with claims of highly improved accuracy. However, these drills are only as accurate as the GPS and instrumentation and still have errors associated with them. With drilling, a significant portion of the drilling deviation occurs when the steel is in the borehole (shown in Figure 3). This deviation is not normally controlled by these autonomous machines, and, while they will reduce deviation, most mines do not possess the capital to incorporate these for the improved drilling set-up location.
Figure 3 – In-hole deviation (photo by T. Sinkala)
Six Sigma blasting
In order to improve a mine’s blasting, management techniques must be employed. Even with engineering-out a blast, deviations must be minimized to achieve proper breakage. One famous management technique is Six Sigma, and it is being used to improve drilling, blasting, and fragmentation. The goals of this program include:
Without proper employee buy-in, projects generally are unsuccessful and results are not as expected. This is the same with improving a drilling and blasting program, so how can management, engineering, and driller/blaster all work together and buy-in to the project; especially when it has increased work-loads for all levels?
According to Maslow’s hierarchy of needs (Figure 4), a person is motivated at a higher level by belonging, self-worth, accomplishment, and personal growth. In most cases, mine employees at all levels have safety, job, insurance, food, and water. In general, people are longing for an increased belonging and accomplishment. This is where the Six Sigma effect has large benefits.
Figure 4 – Maslow’s hierarchy of needs
Six Sigma functions as a team role, where all levels of management, engineering, and the drillers and blasters work together to accomplish these goals. This team is composed of:
Figure 5 – Six Sigma team
By having all levels involved in the team, all members will feel a belonging and want to improve their work, and that of others, to ensure team success. Little wins for the team will give all members a sense of accomplishment and continue to motivate them toward the goal of improving.
In cases where contract drilling and blasting is done, the management should require that the contractor attend these meetings and achieve certain parameters. If these parameters are not met, the contractor should have a financial responsibility for poor performance. This will ensure the contractor will listen, however, many times contractors have a knowledge that can help the mine. Good communication between parties is critical and will result in better performance.
This team generally functions under the DMAIC approach
(Figure 6). This means that the group will:
Figure 6 – DMAIC approach
In the define phase, the team will need to come up with the goals of the program, these could be topics such as:
Clear goals are needed to achieve, and multiple goals can be decided on, with an order of importance. For example, one mine had the following goals:
Once the goals have been established, the next step is to define the key performance indicators (KPI) for the drilling and blasting program. These are for both the inputs and the outputs of the program. Examples include the following:
Borehole Depth to Intended Depth
Borehole Location to Intended Location
Borehole Path to Intended Path
Actual Burden to Intended Burden
Actual Spacing to Intended Spacing
Fragmentation Size (P80, P50, P10)
Cost to Blast
Time to Drill/Load
Muckpile Height and Throw
Crusher Power Output
The measure phase of the program is when the driller, blaster, engineer, and supervisor will take field measurements of the KPIs. There are many ways to now effectively and quickly measure these KPIs. A few of these are discussed below:
1. Borehole depth
The simplest way to ensure that the proper borehole depth is reached is to get a tape measure that has the length of the borehole +10 percent and tie a 5- to 10-gram lead weight on the end (fishing sinker). This can be let down the hole until the bottom is felt and the depth can be recorded. The depth is important in many places, such as:
If the depth of the bench is known, this method can be used to analyze the depth of the subdrill (borehole length – bench height). The subdrill is often critical in leaving a toe and fragmentation throughout the shot.
2. Flashlight test
The flashlight test is a relatively primitive method of determining borehole deviation, but can be useful and provide valuable information for mines with limited capital. This involves tying a flashlight onto the end of a rope with ½- to 1-foot increments marked. The flashlight is then put into the borehole with the light shining out so one can see it. This is lowered until the light no longer shows (caused by drill deviation) and the depth is recorded.
3. Burden and spacing (tape measure)
The burden (toe burden) and spacing are critical parameters in blasting and need to be field verified to ensure proper placement. This can be done by simply taking a tape measure and measuring the distance from the center of one hole to the other.
4. Burden and spacing (GPS)
For a slightly more accurate, more expensive, and quicker way to measure the burden and spacing, a GPS surveyor can be purchased and used to get exact location of each borehole. These can then be imported into a CAD program and easily measured.
5. Burden and spacing (drone)
Another method to very quickly and accurately get the coordinates of the borehole is to use a drone to capture the locations. This is normally more expensive than other methods, but has a pay-back in the time to analyze. This can also be contracted out to another company, and videos of blasts can also be monitored to see what is happening.
6. Borehole tracker (borehole length and deviation)
A more accurate method to determine the length and deviation of a borehole is a borehole tracker. These devices are inserted into the borehole and take coordinates as they are lowered. When loaded into a CAD program or other software, one can see the exact path of the borehole, including angles, curves, and length.
7. Face profiling system
Many laser and photogrammetric face profiling systems exist, which allow mines to create a point cloud of the face of their blast. This can be combined with options such as the borehole tracker and drone to create a 3-D image and calculate exact parameters of all variables.
8. Boulder counting
One quick and easy way to measure the fragmentation performance of a blast is to use a boulder count. This is counting the number of boulders over a certain size on the muckpile. To refine this method, the loader operator can count boulders throughout the pile. This method can be deceiving and doesn’t accurately display fragmentation, but will accurately assess the secondary breakage reduction of blasting.
9. Fragmentation analysis (WipFrag, WipWare)
If fragmentation is a key output to the mine (which it normally is), the Wipware systems are a great way to accurately measure the fragmentation. I have used this at many mines to help identify fragmentation. This involves taking a picture of the muckpile, in several locations, and using the software to determine the fragmentation of the muckpile. At least nine images at varying locations on the muckpile should be used per muckpile.
10. Fragmentation analysis (Reflex System)
To simplify this process and achieve more accurate results, WipWare has introduced a Reflex System which uses stationary cameras to take pictures of almost the entire muckpile. The camera can be placed on a dump-point (crusher), a point all trucks dump, or the front of a loader/shovel.
Figure 7 – Analysis of muckpile
The next step is to analyze all inputs and determine which key inputs are relied upon to achieve the desired outputs. This can take many blasts to determine, and, often, the bad blasts are more important than the good blasts for this. By monitoring both, one can determine the changes in the different parameters. This, along with pre-blast simulations of performance, can be used to carefully manipulate variables to determine the key inputs.
Important topics to look at in the statistical determination of blasting parameters include:
After the analysis phase is complete and the key inputs are mapped and correlated to the outputs, the improvement phase begins. Improvement is the most difficult step and can be handled in many ways by the mine. In certain situations, it is training an employee on proper techniques for drilling or loading. It might be providing employee motivation to achieve measureable goals. Or, it might be purchasing new equipment or technology to help achieve the goals.
The improvement phase is the area in which the black belt will need to take the lead, using help from the green belts, to determine the exact way the mine will improve a system. This can come from engineering implementations as well. Like Langefors suggestions, if minimal deviations exist, other engineering methods include:
Finally, the control phase is implemented to ensure that the parameters are met. This is normally in the form of quality control charts with maximum acceptable and minimum acceptable limits. Periodic monitoring should be done (much less than measure phase) to ensure everything is consistent. In many cases, the outputs can be monitored creating an easy, successful control plan.
Without the control plan and excellent documentation in place, mines may implement this process and spend a lot of time on it only to have new people come in and change it. One of the benefits of the Six Sigma team is that an entire team will rarely leave the site at the same time. This ensures that knowledge can be passed down, and the mine’s drilling and blasting can be maintained.
Improper drilling and blasting procedures can result in major blasting costs, which go relatively unnoticed. Design changes and new products will not be beneficial unless proper drilling and blasting management is in place. While some corrections can be made to the design to ‘engineer-out’ inaccuracies, a steady management system is one of the only ways to consistently ensure desired performance. The Six Sigma drilling and blasting approach has proven itself in the trenches. Based on teamwork at all levels, clear goals and measurements of systems, and a control system, it can help any mine achieve excellence in their drilling and blasting program.
Anthony Konya is a project engineer with Precision Blasting Services. He can be reached via email at Anthony@idc-pbs.com.