Modern Underground Blasting Methods
What face round would work best and why?

By Anthony Konya, Dr. Calvin J. Konya and Dr. Paul Worsey


Figure 1—The burn cut in real life, the
relief holes are painted black and the
loaded holes are painted red.

Underground mining and construction projects face several challenges unique to the underground environment compared to the surface environment. The goals are the development of an effective blast pattern that minimizes cost, maximizes face advance, and provides the desired fragmentation and face profile. In addition to these goals, mines must consider the very real operational problems that occur, such as drill deviation, ground vibration, and deadpressing1 or sympathetic detonation2 of explosives. These operation problems lead to increased costs and decreased production in the mine, hurting the bottom line.

Combined with these problems, underground blasts can be difficult to monitor and evaluate due to tight spaces, dust during the blast preventing video, and the need to evacuate the immediate area with no clear line of sight to observe performance. This creates an environment in which most mines know that blasting can be improved in their underground operations, yet most are hesitant to change due to fears of the shot freezing, misfires and questions about how to achieve a proper design. Unlike with surface blasting, the explosives companies and shot service providers also offer limited assistance or knowledge in the underground blasting realm, leaving underground operations in the dark. It is for these reasons that most underground operations are operating decades behind the current technologies and incurring significant costs, both real and hidden, from their blasting operations.

For these reasons, the authors are working with Engineering & Mining Journal (E&MJ) to bring an article series to the readers to aid in the design and optimization of underground blasting practices. In this article, the major methods of underground blasting for face rounds will be discussed from an operational standpoint with the focus of helping mines choose which method of blasting would be most advantageous for their operation. In future articles, these patterns will be examined and methods for 21st century design will be discussed.

The Burn Cut
The burn cut is one of the older forms of underground blasting, where a part of the blast (the burn) of a shot is heavily loaded with additional empty (relief) holes with the goal of smashing (aka burn) and pushing/blowing all the material in the burn out to provide relief for subsequent blastholes. The relief holes in the burn cut provide a parallel relief for the holes that are firing to break also, creating a second plane of relief, however, these are extremely small and can cause many problems if a design is improper.

Burn cuts are perhaps the most complex methods of underground blasting and the design can be one of the difficult starting points due to the sheer number of possible designs. There are dozens of different forms of burn cuts that could all be tested at a site to determine which is more or less effective, however, this would span months to years of testing with frequent below standard performance from burn cuts that do not perform well. Because of this, most mines stick to the same burn cut that has been used for decades. However, the burn cut is the most important part of a burn cut round, as it will control the maximum face advance, susceptibility of the round to freeze3, and with improper burn cuts, a vibration increase of up to five times the original vibration levels for the rest of the round (production holes). The burn cut is commonly used in mining in places like the Nevada gold mines and Lake Erie salt mines, and other operations around the world.


Figure 2—Burn cut plan view.
Advantages of a Burn Cut
Burn cuts are one of the only options for underground blasting in small drifts as they require no angled drilling and little “elbow room” (for the drill boom) to achieve. This allows them to be used in almost all underground environments and they can be easily modified for larger room openings. Burn cuts become more economical as the room size expands due to a smaller percentage of cost from the burn compared to the entire round.

In addition, burn cut rounds can produce very fine fragmentation and a well-placed muck pile depending on the location of the burn cut. The actual burn cut can be moved vertically and horizontally to change the cost, fragmentation, and throw of the material allowing an engineer or supervisor to modify the general layout of the burn cut to achieve a higher bucket fill factor or increase crusher throughput.

Additionally, burn cut rounds, in general, will have less boulders than other rounds due to the higher explosive loads and straight holes creating very similar back burdens on the round. This characteristic, along with the increased fragmentation and the ability to place material, makes burn rounds very popular in metal mining situations.

Disadvantages of a Burn Cut
While the burn cut has many advantages such as control, placement, and minimization of boulders, several disadvantages also exist. The first major disadvantage is the complexity in design and hookup. As previously discussed, the burn cut can be designed numerous ways just with borehole placement. Combine this with extremely critical timing of both the burn and the production holes and a burn cut can be a challenge to properly design and hookup, especially with non-electric blasting caps, which are very common because of cheap cost and relative safety.


Figure 3—Burn cut section (A-A) view.
Another disadvantage with the burn cut is with the drilling accuracy and flexibility. In a burn cut, boreholes may be within 6 to 8 inches (in.) of each other and reach a depth of 10 to 15 feet (ft). These boreholes can wander or deviate many inches or feet, causing boreholes to intersect and have very small burdens, or conversely have extremely large burdens. In all of these situations the blasting will be negatively affected and total face advance will decrease. Additionally, the collaring locations are often not placed exactly where the design indicates, again changing the burden from hole to hole and generally degrading the functionality of the burn.

These deviations in the hole and at the collar are compounded if the same drill is being used to drill the relief holes. In general, relief holes need to be larger than the blasthole to provide adequate relief for material to move. This either requires multiple drills/drill bits or drilling multiple small holes together to simulate a large hole. For example, if a 4-in. hole was desired for the relief hole, either one 4-in. hole could be drilled, or four 2-in. holes could be drilled within inches of each other. Any deviation on these holes could significantly impact the performance of the burn cut.

Burn cuts also suffer from the close spacing causing flow of the rock and deformation of boreholes near each other. This deformation can cause a densification and deadpressing of the explosive, leading to misfires, freezing of the face, and poor face advance.


Figure 4—Burn cut nonel hookup.
The last disadvantage to be discussed with the burn cut is the increased cost of production compared to other blasting methods. A general assumption can be made that the burn cut will be the most expensive option due to the large number of holes and extremely large powder factors in the actual burn cut. This is eliminated with other production methods and the loss of rock due to fines are reasons most aggregate operations and other cost competitive mines stay away from the burn cut. In the end, an economic analysis must be done considering not only the blasting cost, but the pull obtained and the mucking, crushing and secondary blasting (or lost production from boulders) in a full mine-to-mill optimization.

The V-cut
A V-cut is one of the most commonly used blasting patterns in underground aggregates due to its comparatively low drilling and blasting costs. The typical V-cut has the drift divided in half with the first hole put on a large angle to break the middle of the drift out. After these first rows of holes, each row is angled slightly less to allow for the V-cut to eventually have holes that slightly look out along the ribs. Because of the need for extra room for the angled drill boom for the middle V, larger rooms are generally required for V-cuts. While V-cuts can be simple to design, they can be difficult to accurately drill due to the changing angles and accuracy of drillers to adjust and measure these angles. The V-cut is used in numerous underground operations in the U.S. for aggregates mining. It is also used internationally, such as in underground coal mining in Brazil without the use of an undercut.

Advantages of a V-cut
The V-cut has numerous advantages over other cuts, if the space is available for them to function. The first of these advantages is a reduction in cost due to less overall drilling and lower explosive load. A majority of holes have full burden and spacing, with only the V holes having an increased powder factor. One important note, reducing total drilling does not equal reduced total drilling time.

Another advantage of the V-cut is that the larger the width of the room, the larger the total face advance can be achieved, with the proper drilling equipment. This is an advantage over the burn cut that will reach a maximum depth due to the ability of the burn to eject. With the V-cut, as the width gets larger, the pattern can be designed to reach deeper and maintain the required angle for breakage and material ejection. In most cases, the depth of the V-cut is limited by the width of the room and ability of drilling equipment.


Figure 5—Multiple relief holes to simulate a
larger relief hole.
The V-cut also greatly simplifies the operation drilling, charging, and hook-up process for the drilling and blasting crew. The drilling of a V-cut does require angled drilling (which will be discussed in disadvantages). However, the holes are generally drilled in rows from ceiling to floor and the complexity of drilling the burn cut is no longer present. In addition, while significant drill deviation does affect the V-cut, it is not nearly as sensitive as the burn cut. The blaster’s role is also greatly simplified with the V-cut because, in general, all boreholes are loaded in the same manner. The timing hookup is also greatly simplified, with rows either firing instantaneously or with small delays (depending on vibration constraints). The V-cut also has the ability to use new vibration control methods to greatly reduce vibration.

Disadvantages of a V-cut
While the V-cut has many advantages, it also has numerous disadvantages. The first that will be discussed is the limited face advance. As previously stated, the wider the drift the farther the maximum possible face advance. However, this is also a limit that is imposed on the V-cut along with the maximum reach of the drilling equipment. Because the V-cut requires angled drilling, the drill reach is not fully utilized and is generally around 80% of the total reach of the drill steel thus a reduced maximum pull depth. In addition, many drifts are not wide enough that the V-cut will be able to reach farther than a burn cut, even with proper drilling equipment. For example, a burn cut with a 15-ft face advance can generally be achieved without a great deal of complexity. However, a V-cut with a 15-ft face advance would require a room over 35 ft wide and a drill steel that is 20 ft long, which can be difficult to find underground. In addition, this could create extremely large boulders at the site.

The second major disadvantage of the V-cut is that large boulders can be produced from the center of the shot. These boulders are then either put aside and never processed (creating a hidden cost of lost material) or secondary blasting methods are used to break them (increasing cost). However, these can be eliminated by the use of baby Vs or buster holes. This will add additional cost and create additional difficulty in the drilling process. In addition to these large boulders, the throw of a V-cut will leave a scattered muckpile throughout the drift.

The third disadvantage of the V-cut is in the difficulty of drilling the round. Due to numerous angles, which are often poorly measured and rarely recorded, the driller must spend a significant amount of time attempting to get close to the correct angle. Additionally, the drill must set up in numerous places and in different positions to properly drill the pattern. This is also one reason the V-cut has difficulty being used in small rooms — the drill cannot set up far enough to the side to drill a large enough angle for the center V.


Figure 6—V-cut plan view.
The final disadvantage of the V-cut to be discussed is, with improper design, modern explosive products can easily deadpress and fail to detonate, or they deflagrate instead. This is a more recent problem, as the historically used explosive of choice (dynamite) would sympathetically detonate in these situations. This has led to a redesign of the V-cut to be used with modern explosives, which will be discussed in a future article.

The Fan Cut
The fan cut is a less commonly used form of angled drilling that can be used in drifts that are smaller than a V-cut. However, the fan cut still needs larger drifts compared to a burn cut. A fan cut involves angled drilling on one side of the face with the goal of removing half of the face and blasting the remainder into that opened area. This is an effective method for blasting in small- to medium-sized drifts where V-cuts are not applicable; however, a price lower than a burn cut is desired. This technique is primarily used in aggregates and is used for the first round of a cross cut in room-and-pillar mining in metals or hard rock mining environments, such as in the Missouri Lead Belt, and is known in some operations as “slashing.”

Advantages of a Fan Cut
The fan cut provides some advantages in underground aggregates blasting when the drift width is smaller than what is possible with a V-cut. This can reduce the cost of drilling and blasting when compared to a burn cut by reducing the total number of holes, however, this could reduce the total face advance. In the end, an economic analysis will need to be completed in order to determine whether a shorter face advance with less holes is more or less economic than a longer face advance with more holes. Production considerations must be made with this type of cut.


Figure 7—V-cut section (B-B) view.
Additionally, the fan cut can allow rooms that are high but narrow a unique advantage to quickly transition from an opening cut into a normal production blast, reducing the overall cost. The fan cut also allows for a simpler drill setup than the V-cut, which can lead to a reduction in overall drill time. The final advantage to be discussed with a fan cut is that the angled boreholes are drilled in the middle of the face. This allows for easier mobility of the drill and a reduction in boulders compared to the V-cut.

Disadvantages of a Fan Cut
The major disadvantages of the fan cut are that it can result in a higher cost than a V-cut in larger rooms and the minimal face advance compared to the burn cut in smaller rooms. For these reasons, fan cuts are selectively used in specific mining situations where their use has proven more economical than either the V-cut or the burn cut. In the right conditions, the fan cut can provide a cost-effective way to use the advantages of a V-cut in a smaller drift.

Another disadvantage of a fan cut is that a straight face profile is rarely developed, with an arching face more commonly present due to the geometry of the blast. This causes blasting crews to have to “square up” or create a straight face after every few blasts. This can be a hidden cost that would result from the use of a fan cut.


Figure 8—Fan cut plan view.
Conclusion
The three most commonly used face rounds in underground mining are the burn cut, the V-cut and the fan cut, each having unique advantages and disadvantages. The first round discussed was the burn cut, which is a parallel hole cut most traditionally used in smaller drifts where space does not allow for angled drilling. The burn cut is also more commonly used in stronger rock types and metal mining. The burn cut typically provides greater fragmentation and face advance compared to the width of the room and keeps the muck pile closer to the face. Major problems associated with the burn cut include drill deviation, difficulty in design, difficult loading and hookup, and highest likelihood for deadpressing modern explosives.


Figure 9—Fan cut section (C-C) view.
The next type of cuts discussed were angled hole cuts, the V-cut and fan cut. These cuts operate in similar manners, by angling drillholes to create a burden to the open face and creating a larger opening as the blast progresses. While the V-cut and fan cut do differ in many areas, the V-cut requires a much larger opening than the fan cut and can be more cumbersome to drill. However, the V-cut can generally allow for larger face advance and the ability to employ advanced vibration control measures. Unless a room is extremely large and the proper drilling equipment is used, both of these forms of angled cuts will produce a face advance less than a similar burn cut.

The decision for choosing and optimizing any underground round requires an understanding of the processes and principles of operation with each of the underground rounds as well as a fullscale view of the mine-to-mill processes. The goals of the mine in relation to the desired fragmentation, desired condition of the muck pile, handling of boulders, drilling equipment, and other operational problems need to be identified to choose an optimal pattern to be used at any site.

After all these issues have been identified and costs associated with them, economics will then determine the optimal pattern. However, testing of any pattern being considered needs to be completed to ensure that rounds function as expected. In future articles of this series, the design for these rounds will be discussed.

Calvin Konya is the founder of Precision Blasting Services (PBS). Anthony Konya serves as project engineer for PBS and Dr. Paul Worsey is a mining professor and well-known explosives expert and researcher at the Missouri University of Science and Technology. www.idc-pbs.com

As featured in Womp 2017 Vol 06 - www.womp-int.com