Blasting Mechanics Revisited: The mechanics of rock breakage

Blasting directly controls the profitability of quarry operations because it is at the face that the production cycle begins.

Poor blasting will invariably result in economic difficulties, from poor fragmentation to throw (heave). Additionally, the ever-changing nature of the blast pattern, with general operations causing small but noticeable pattern changes, results in the compounding of these problems and operators who are unsatisfied with blast results.

A 1963 statement from Richard Ash, a longtime professor of mining engineering at the School of Mines & Metallurgy at the University of Missouri-Rolla, still holds true today: “The usual trial-and-error approach as such is expensive and often hazardous, and it rarely leads to complete success because of a lack in flexibility of application. Also, information that is generally available on blasting is not usually applicable from a practical point of view.”

In 1963, some basic standards were outlined in Ash’s “Mechanics of Rock Breakage,” appearing within Pit & Quarry and giving readers an understanding that could immediately be applied to sites for improvements to blast programs.

In part one of P&Q’s new four-part “Mechanics of Blasting” series, Ash’s “Mechanics of Rock Breakage” is revisited with an emphasis on the practical aspects of breakage and how they are relevant to quarry sites today. Today, blasters have the same goals that they did 50 years ago: fragmentation and displacement. These goals were defined by Ash, and they continue to hold true for fragmentation, the uniformity of particle-size distribution and the limits of actual sizing. The minimum and maximum sizes of blasted material is critical.

Similarly, rock movement is critical for operators. Too little or too much displacement is unwanted for economic and safety considerations.

In addition, with new regulations since the 1960s – specifically related to the environmental effects of blasting – most operations now deal with staying below ground vibration and air overpressure limits. Operations also mitigate flyrock and toxic gases.

The mechanics of rock breakage

The role of the explosive has two functions in blasting: first, to break the rock, and second, to move it.

As far back as 1963, it was known that borehole pressures reached in excess of 2 million psi due to gas pressure. At the time, however, new high-speed cameras were developed and the shock wave produced by the explosive could now be observed. Researchers began studying this new phenomenon.

This led to new hypotheses about how blasting phenomena worked, which did not hold true for normal bench blasting. It did not hold true, though, because it took at least a powder factor of 8.4 pounds of explosive per cu. yd. of rock to cause the first shock spalling to begin at the face.

Quarries work with powder factors of about 0.7 to about 1.3 pounds of explosive per cu. yd. of rock. With the last 50 years of experimentation, we understand what roles these components play and how they affect the blast.

When an explosive is detonated, a large impact is felt in the rock through a fast-moving shockwave. But due to the rapid movement of the shockwave and the quick degradation of the impulse as it moves through the rock, the shockwave has minimal effect in the breakage of rock at any distance from the blasthole.

Instead, it is now understood that the gas pressure from the explosive does a majority of the work – first through fracturing of the rock and then through bending and movement of the bench.

It was originally believed that due to the shock wave moving equally in all directions that breakage would be equal in all directions. However, with modern understanding of explosive pressures, it is known that this is not the case.

The breakage from a blasthole will begin immediately around the blasthole and crack outward toward the free face. This fracturing process is called “radial cracking,” and with a completely confined blasthole will be equidistant in all directions.

Still, when the blasthole is placed a specified distance from a bench (free) face, these fractures will concentrate toward the free face. It is known that with a properly designed blast, fractures behind the blasthole will be minimal and overbreak should not occur.

Following this fracturing, the breaking action of rock will begin at the free face and move backward toward the borehole in a process known as flexural failure. This is when the bench begins to bend and move outward, causing a large degree of fragmentation and also movement, or throw, of the material.

Fragmentation effects

In 1963, researchers observed blasts that had poor breakage and saw one trend that persisted: the burden was often very large.

At this time, it was concluded that “if the amount of initial explosive energy is inadequate for the total travel distance, so that the tensile strengths are not exceeded both outward and on return, one can expect to find the unbroken rock or very course fragmentation.”

Researchers at the time struggled to explain why phenomena such as stemming would then influence fragmentation. If the shock pressure was doing the work, then stemming would have no impact because the shockwave is not changed by stemming a borehole.

Today, the industry understands that this breakage is due to the gas pressure, and inadequate break is due to confinement – which in some cases is too great. Other scenarios that lead to over-confinement of a blast are poor stemming, row-to-row timing that is too fast, large toe burdens and improper loading procedures, especially with emulsions. Typically, poor fragmentation from this is seen throughout the muckpile – especially in the middle of a muckpile.

Poor fragmentation in the front of the muckpile is typically a sign that overbreak occurred from the previous pattern. This causes large boulders to sit on the face of the next shot. Without an explosive charge put in these boulders, they are pushed forward to the front of the blast.

Large boulders on the top of a muckpile typically are formed from the stemming zone and poor flexural failure or bending of the bench. If massive rock is found in the stemming zone, then stem charges typically will be used to cause breakage with the remainder of the blast.

Conversely, when the shot is under-confined, meaning the explosive generates too much gas pressure for the burden, the fragmentation will be very fine and the throw will typically be excessive.

This was defined by Ash: “Where excess energy is used, the broken rock will be thrown farther out from the face, and there may be some overbreak in back of holes and on edges.”

Confinement is the major principle when looking at any effects from blasting. This is why confinement is the major principle in blasting and is determined by how much energy is available to break and move the burden. Rock type, structure and explosive type will all contribute to what proper confinement would be, and the blast design variables are the decisions that are made that determine how the confinement is, or is not, achieved.

Stiffness ratio

Two mechanisms of breakage exist when blasting. The first is cratering, in which movement of a blast is upward and equidistant around the borehole. This can be thought of like this: When a charge is drilled into the ground and no burden exists, the charge will crater.

The second mechanism is through the borehole effect, in which a very long borehole is drilled with a free face in front of it. This free face is then broken, horizontally from the borehole. This borehole effect is typically maximized when the bench height is four times the burden on the shot.

The cratering effect occurs when the bench height is near equal to the burden. Between this range, a combination of the two mechanisms typically occurs. The shorter the bench, the larger the cratering effect. The longer the bench, the greater the borehole effect.

This borehole effect has been realized for centuries. Even in the 1800s, books published on blasting discuss that long boreholes will perform better than short boreholes. This concept was also present in Ash’s 1963 paper.

“As hole depths increase, the difference in blast effects will become greater,” Ash wrote.

Shortly after this paper was written, Ash and Calvin Konya researched this phenomenon. Later, Konya fully developed the term “stiffness ratio” to describe the borehole length effects on blast results by using a ratio of the bench height compared to the burden.

Stiffness ratio is the second major principle for understanding the mechanics of rock breakage. As the borehole becomes longer, compared to the burden, the blast performs better.

This is similar to a slenderness ratio in civil engineering. As a beam becomes longer, compared to its thickness, it is easier to break.

In order for blasts to perform well, the design decisions made need to accommodate this stiffness ratio so the bench height is three times the burden, or greater. This can be a choice made based on the borehole diameter, the explosive type or the bench height.

Furthermore, the stiffness ratio ties into the confinement aspect. The shorter the bench, the more explosive energy it takes to break.

Conclusion

The 1963 papers written by Richard Ash were the pioneers in the understanding of the field of explosive engineering.

Ash’s papers began the expansion of a field on the mechanics of rock breakage and optimization of blasts based on sound engineering principles. Ash recognized that blasting was truly an engineering field in which the different variables worked together to produce the final results. He also realized that the use of powder factor alone was not the answer to good blasting results.

Many of the topics covered in Ash’s papers are still used today. With the advancement of the blasting field over the last 50 years, new explanations exist to back up Ash’s findings. The two principles Ash discusses in his first paper – confinement and bench height – still hold true today and are the major principles that decide the function of a blast.

These principles come up over and over, from design decisions and explosive choice to blast effects. The principles must be fully understood before the first design considerations are made.

Understanding these principles allows blasters to begin to move away from a trial-and-error approach and into a design approach based on real fundamentals.

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About this four-part series

In 1963, Pit & Quarry published a series of articles on the mechanics of blasting authored by Richard Ash, a longtime professor of mining engineering at the School of Mines & Metallurgy at the University of Missouri-Rolla. The content within each article was ahead of its time, putting forth cutting-edge concepts about the mechanics of rock breakage, standards for blasting design, the characteristics of explosives and material properties, powder factor and the cost of blasting.

The Academy of Blasting & Explosive Technology, including Anthony Konya and Dr. Calvin J. Konya, pay tribute to the time-honored principles Ash put forth more than 50 years ago with a special 2019 series for Pit & Quarry readers. In their “Blasting Mechanics Revisited” series, the Konyas look back on Ash’s work and explore how modern-day technology and methods currently fit into each of the areas Ash covered a half-century ago.

Look for these articles from the Konyas within Pit & Quarry now and in the months to come:

Part 1. Mechanics of Rock Breakage (January)
Part 2. Standards for Blasting Design (February)
Part 3. Characteristics of Explosives (March)
Part 4. Material Properties, Powder Factor, Blasting Cost (April)

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Anthony Konya is the senior explosive engineer for Precision Blasting Services who consults around the world in rock blasting and vibration from blasting and an instructor for the Academy of Blasting and Explosive Technology. Calvin J. Konya is the president of Precision Blasting Services and director for the Academy of Blasting & Explosive Technology, consulting and training worldwide in rock blasting, vibration and emulsion manufacturing.