Modern Design of Fan Cuts
Understanding the principles allows more effective use of the fan cut for slashing and crosscuts

By Anthony Konya, Calvin Konya and Paul Worsey


Figure 1—Fan Cut Diagram (Olofsson, 1991).
When considering blast designs, another option is the fan cut, which is an angled cut that is similar to half a V-Cut. It is called a fan cut because it looks like an open hand fan from the last century and before, as illustrated in Figure 1. These holes operate on the principle that breaking rock with the face parallel to the blast hole (in a fashion similar to bench blasting) is one of the most optimal and easiest methods of breakage.

The fan cut allows for relief to quickly and easily be made as the hole angles are quickly transitioned into straight in holes. Because of the geometry, smaller drifts can be mined with a fan cut because a drill can be set up in the middle or far side of the round and the fan portion of the cut can be placed on the opposite edge of the face.

In general, fan cuts are more common in smaller drifts than V-Cuts that do not require as fine fragmentation as obtained with burn cuts. The face advance of a fan cut will be between 40% to 60% of the width of the tunnel (Langfors & Kihlstrom, 1967).


Figure 2—Example of a tradition fan cut
round with three-row high cut from
Langefors and Kilhlstrom.
Typically, the design of the fan cut was made using mostly 3-D geometry with similar burden and spacing dimensions to that of the V-Cut (Konya, Konya, & Worsey, Modern V-Cut Design, E&MJ September 2017) with the major difference being that the angled hole opening will be on one side of the drift (shown in figure 1). The side of the drift that it is placed on should be alternated from left to right with each blast to minimize the risk of drilling into a bootleg.

In the traditional design, the fan cut is laid out to blast a horizontal slot across the face, three holes high. The rest of the round above and below is drilled straight in. This was due to the use of older equipment, such as jackleg drills and the difficulty of drilling angled holes precisely using the older equipment especially at extended heights.

Langefors and Kihlstrom (1967) give multiple examples of double fan cuts one of which is illustrated in Figure 2, which when initially viewed appear similar to a V-Cut with the central portion between V holes fanned from one side, the V being to the opposite side and its axis skewed at an angle toward the center.

It is interesting to note that the cut as in traditional V design is used to develop a horizontal slot to provide relief for the rest of the round, which uses “straight” holes.

Traditionally, the fan round was used to mine smaller drifts. However, in modern mining with the use of large jumbo drills, the technique is primarily used for starting crosscuts off a main heading.

With modern explosives, a traditional fan cut as illustrated in Figures 1 and 2, can lead to pre-compression or dead pressing of the explosives as discussed in previous articles (Konya, Konya, & Worsey, Modern Underground Blasting Methods, E&MJ June 2017). This can result in very poor performance to complete freezing of the fan cut if some of the early fan holes fails to fire.

Modern Fan Cut Design
In a more modern design, the fan cut is extended from floor to roof, which is made possible using high-reach drilling equipment, thus extending it to a fan round rather than just cut. The fan round comprises a single-sided fan and larger diameter boreholes are typically used, which increases the distance between boreholes reducing the risk of dead pressing and pre-compression.


Figure 3—Modern jumbo drill with long drill sash illustrating the start of a fan/
slashing shot to start a crosscut. Note that there is insufficient room to turn
the drill completely sideways. Photo courtesy of Clay McNail.
These days the main use of the fan cut is as a slashing round where material is “slashed” or slabbed from the rib of a heading to start a crosscut. Over the years, the length of drill jumbo booms has increased with the push for greater productivity, extraction and advance per blast. Due to the length of modern jumbo drill booms and the cantilevered weight in front of the drill unit, underground drilling equipment has tended to become larger and longer in room and pillar mining — definitely too long to maneuver and fit sideways in a heading as illustrated in Figure 3.

Whereas in the old days, this was very easy for a jackleg drill to accomplish. Because of this, an operation that drills headings using burn cuts has to slash the rib to start a crosscut (or work around a pillar) as illustrated in Figure 4, until the opening is sufficiently large enough to allow the rig to drill, at which point the burn cut can be used again.

An artist’s rendition of the drilling process is given in Figure 4. The pillars and crosscuts are shown with the advancement, with burn cuts going forward and slashing rounds starting crosscuts.

An example of a modern slashing round is illustrated in Figures 5 and 6. In Figure 5, the round in the heading looking forward is a standard Doe Run burn cut with four relief holes. A fan/slashing round is drilled to the left to start a crosscut. In many cases, up to three rounds are drilled and charged in the same heading and if there is a cutoff of one of the rounds the entire hook up on the face of that round will is obliterated by the neighboring rounds causing difficult recovery and a lot of lost time. At Doe Run, the holes are primed with long-period (LP) shock tube delays clipped onto detonating cord “ring mains.”

To avoid the potential for a cutoff, separate detonating cord lead-in lines to each round have been implemented and the “ring mains” are also tied together with detonating cord jumpers for redundancy. This is a very important best practice.

The convention for underground for many operations is to use LP’s to allow time for movement. However, millisecond delays can be used just as easily, and in many cases will provide superior fragmentation because of the small burdens compared to bench blasting in quarries. In general, the rounds are fired at the same time with the same delays. The perimeter (pillar) holes of the burn cut round being on high delay numbers are fired well after the first holes in the slashing/fan rounds, which are fired on low delay numbers and this protects the pillar corners.


Figure 4—Diagram of a heading
going forward with the
crosscuts being slashed in.

Figure 5—Swing (heading) round with accompanying slashing (fan) round on left
at Doe Run. Photo courtesy of Clay McNail.

Figure 6—Fan/slash round close up. Only the bottom three horizontal rows of
holes are painted due to height of opening. Roof holes are unpainted. The
picture is taken roughly at the pivot point of the drill sash. The vertical lines
denote vertical rows where individual holes share the same hole orientation
and the swing around from shallow to near perpendicular (right to left) can be
seen. Photo courtesy of Clay McNail.

Figure 7—Diagram of a panel in a room-and-pillar
mine being extended in one direction and
crosscuts being worked sideways to minimize the
need for slashing rounds..

Figure 8—Diagram of a Hammer Cut with lifter holes for openings that are narrow
but taller. Lifter holes can be replaced with an undercut, but a traditional undercut is rarely used.
Losing pillar corners is inevitable in production as with most blasting, but a little more pronounced when this technique is used by itself. This can be combatted by the use of pre-splitting or cautious blasting (blasting in parts incorporating smooth wall blasting), but this is more applicable for civil excavation rather than production mining due to cost.

The metal deposit at Doe Run is lead-zinc-copper and is not regular in shape, therefore the use of the slashing round is quite common. However, in massive regular room-and-pillar mining, which is used in underground limestone mines, the slashing round only has to be used to start crosscuts from the most advanced heading. Once crosscuts are established the regularly used blasting techniques of burn or V-Cut can be used to extend each cross cut throughout the mine as illustrated in Figure 7.

Hammer Cuts
The fan cut can also be turned 90° to where the angled boreholes are going toward the floor and then used with either lifter holes or an undercut. This is traditionally called a hammer cut and follows similar design principles as the fan cut.

The hammer cut is now a rarely used method for underground heading design as the angles are difficult to achieve especially with modern large drilling equipment. However, it can be implemented in drifts that are tall and too narrow to allow for the full fan cut. Drill deviation and drill depth then become critical for ensuring toes are not left on the floor.

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

References
• Konya, A., Konya, C., & Worsey, P. (2017, June). “Modern Blasting Method Selection.” Engineering and Mining Journal, 44-51.
• Konya, A., Konya, C., & Worsey, P. (2017, September). “Modern V-Cut Design.” Engineering and Mining Journal, 40-42.
• Langfors, U., & Kihlstrom, B. (1967). “Rock Blasting.” Sweden.
• Worsey, P. (2017). V-Cut and Fan Cut Design, Section 6.3. Underground Heading Design, Missouri S&T Course Explosive Engineering 5622 “Blasting Design and Technology.” Rolla, Missouri.
• Olofsson, S., 1991, “Applied Explosives Technology for Construction and Mining,” APPLEX, ISBN 91-7970-634-7.
• Konya C., 1995, “Blast Design,” IDC, ISBN 0-9649560-0-4.

Note: Dr. Worsey has provided blaster training at Doe Run for several years.

As featured in Womp 2018 Vol 01 - www.womp-int.com