A Proper Presplit Creates a Safer Working Environment While Reducing Costs.
By Anthony J. Konya and Calvin J. Konya
Geology is often blamed for poor blast results, especially when presplitting is used and significant overbreak occurs. It makes sense, sometimes geologic conditions are poor, and it makes blasting more difficult – difficult but not impossible.
Today’s blasters and technical representatives are quick to blame any problem on geology. Poor performance of the blast? The geology is unique in this area. Nitrous oxides after a blast? It is certainly not the explosives, but the geology could be to blame. Presplits not breaking properly? Geology.
In nearly every situation though, it is not the geology.
Geology can influence certain parameters in a blast and lead to worse results than expected. The geologic problems though are often exacerbated by a poor understanding of fundamental blasting concepts which lead blast designers into making poor changes to a blast design.
This is often the case with presplitting and leads to either the presplit not forming or massive overbreak, especially at the crest of the bench. This leads to slopes, which are not in the proper location, not on the proper angles, and not safe for workers to be near.
Conversely, a proper presplit leads to a smooth, stable slope with minimal backbreak creating a safer working environment. A proper presplit leads to slopes, which are at the exact designed position and angle.
A proper presplit does not weaken the rock mass, which will remain in place. This can allow for slope steepening, which reduces the amount of waste mined and allows for deeper, safer mining operations. A poorly designed presplit costs money and produces little in the way of results. A proper presplit creates a safer working environment while reducing costs. The difference between these two is normally one of design, not geology.
The Fundamentals are Wrong
Presplitting began officially being used in the late 1950s and quickly took over the industry. Proper presplitting reduces the necessary scaling of benches to 10% of previous presplitting numbers. It increased productivity in construction blasting by more than ten-fold, allowing smaller crews to advance further on projects such as the Niagara Power Project (Paine, Holmes, & Clark, 1961). It increased mine safety and led to fewer rockfalls in surface operations and advanced in trim blasting led to safer underground workings.
Shortly after the advent of presplitting, engineers and scientists began to theorize how it worked. Normal blasting led to fracturing and fragmentation, the highly disputed theory at the time was that of shockwaves causing tension spalling. Without the free-face tension spalling could not occurring but the concept of shockwaves was still thrown around.
Eventually, the idea of shockwave collisions between boreholes was introduced. This theory was quickly adopted and is still lingering in the blasting industry today (International Society of Explosive Engineers, 2016). The theory of shockwaves in rock blasting has been disproven for over 40 years now, yet many still cling to the idea of shockwave collisions causing a presplit to form. This has many incorrect concepts that make it impossible with a full analysis of the shockwave theory having recently been published (Konya, 2019).
First Conflict
The first major conflict with this theory of shockwaves is that is relies on perfect timing between boreholes. The shockwave collision theory requires that two boreholes are detonated at exactly the same time in order for the shockwaves to collide between holes and form the presplit. A shockwave moves faster than the acoustic velocity of the material. Depending on the rock type, the shockwave would travel 5 ft. or more per millisecond.
Typical presplitting utilizes holes that are 3 ft. apart, this means that if the boreholes fire more than 0.5 milliseconds apart no shockwave collision occurs and no presplit would form. However, the caps which were used for decades in presplitting, electric and nonelectric, have large cap scatter that on downhole delays can be 30 milliseconds or more.
This theory would then say that the industry could not presplit with electric or nonelectric caps. Furthermore, today’s electronic caps have a claimed cap scatter of 0.5 milliseconds, or 1 millisecond around the programmed time. This would then say that electronic caps could not be used to effectively presplit. It is obvious that this is not true, and all caps work to presplit.
Shockwave Collisions
The theory of shockwave collisions would also say that boreholes could not be fired at a separate time for a presplit to function. Presplit holes are often delayed by many milliseconds and in some cases up to 25 milliseconds apart from one another due to vibration or air overpressure concerns. If the delays between holes are short, for purposes of this discussion 25 milliseconds or less, the presplit suffers no ill effects from being delayed. Yet another reason that shockwave theory does not hold in the real world.
The explosive used in a presplit blasthole is also decoupled, meaning it is not in direct contact with the rock as the diameter of the explosive is smaller than the diameter of the borehole. Using basic impedance mismatch theories, a well-established concept in shockwave mechanics, the magnitude of the shockwave at the borehole wall is less than one psi, well below the tensile strength of the rock.
After accounting for attenuation as the shockwave moves through the rock, the magnitude of the shockwave halfway between the boreholes is nearly zero. With a decoupled charge no evidence exists that a shockwave forms in the rock mass, if it does not exist it cannot be causing the presplit to form.
Finally, the resounding practical proof that a shockwave does not cause a presplit is that in full scale blasts, propellants proved to be just as effective as dynamite in forming the presplit (Konya, Barret, & Smith, 1986). In the 1980s, studies were conducted at quarries throughout Georgia comparing presplitting that was completed with dynamite and Pyrodex. Pyrodex is a propellant which deflagrates, and no shockwave is formed. In every case in these full-scale blasts conducted in granite, the Pyrodex performed identically to the dynamite. Boom, there goes the shockwave collision theory.
However, even with all this evidence that shockwaves are inapplicable in presplit blasting its proponents carry it forward and attempt to advance the concepts of shockwave collisions and methods of designed based on shockwave collisions.
This is not because the concepts are correct, but because admitting shockwave collisions do not occur in presplitting stated that all their “research” is incorrect. The mining industry is not concerned with the research of academicians, the mining industry is concerned with producing safer mines at lower costs and this is were the modern theory and modern design approaches of presplitting becomes extremely important.
Modern Presplitting
In recent groundbreaking research, the entire mechanism of presplitting was analyzed, and a mathematical proof has been developed which perfectly matches with field data confirming the actual mechanics behind presplit blasting (Konya A. , 2019). The actual mathematical concepts are beyond the purview of this paper and instead the general characteristics of the presplit will be discussed.
When a presplit blast is detonated, the explosive rapidly converts to a high-density and high-temperature gas, for a detonating cord this temperature of the gas is 8,400 F. This builds up and puts a large pressure on the borehole wall. This pressure causes a stress field similar to that of Hoop Stresses, which causes a fracture to form between boreholes.
This stress is present for a relatively large amount of time and is the reason boreholes can have delays. The mechanism of this stress field also explains how overbreak is determined from the blast. The stress field from this borehole relies on numerous variables such as, borehole diameter, explosive type, borehole pressure, spacing between boreholes, borehole length, and stemming. The rock properties are also critical including Young’s Modulus, Tensile Strength, Compressive Strength and Joint Frequency.
Since the 1980s, a method of presplitting termed Precision Presplitting has been utilized around the world to properly design presplit blasts. A traditional presplit blast typically overloaded a shot, in any rock type, and caused some degree of overbreak. In weak or highly jointed rock the overbreak would be severe and it was stated that the rock could not be presplit.
Precision Presplitting is a unique design approach which changes the explosive load to match the rock type, causing a presplit to form without overbreak. The typical load for a Precision Presplit is 1/10 to 1/3 the load of a traditional presplit. This method used to be based solely on experience, but recently has been developed into the design method known as Simplified Precision Presplit Design (Konya & Konya, 2016).
This method has been used to design Precision Presplit blasts for mines and construction projects around the world and is now adopted as the top presplitting method by groups such as the U.S. Army Corp of Engineers (U.S. Army Corp of Engineers, 2018).
This design approach has now been expanded to Advanced Precision Presplit Design (Konya A. , 2019). The advanced design approach can accurately determine the stress field generated from the explosive gasses. This approach then considered factors such as the structural geology and depositional environment as well as the rock strength to determine suitable design parameters.
This process can be combined with economic modeling to then determine what is the most economical method for the mine to Precision Presplit. This methodology typically produces 90% or greater half casts on the wall and has been done in all rock types including typical rocks such as granite, limestone and sandstone. This method also works in weak rock types such as siltstone, shale and mudstone. This methodology can then be used to steepen slopes, reduce presplit costs and create safer mines through proper presplitting methods.
Conclusion
The theories of shockwave collisions between boreholes causing a presplit to form have been disproven through the last 50 years. Today these incorrect theories lead blast designers to make incorrect choices when designing blasts, the common excuse then used as to why the blast did not perform is geology. Modern presplit mechanics have been thoroughly studied and shown to be a result of the gas pressurization of the borehole.
This has led to revolutionary design approaches which center on Precision Presplitting. These new design approaches have allowed mines and construction projects to record half casts across the entire face, putting slopes on position and at the correct angle without causing overbreak.
This has allowed mines to perform better and decrease drill and blast costs while increasing site safety. The old theories of shockwave collisions are now part of history; the new theory of gas pressure generating Hoop Stresses between boreholes is now upon us and leading to true progress in the blasting industry.
Dr. Calvin Konya is the president and Dr. Anthony Konya is the vice president of Precision Blasting Services. They can be reached at 440-823-2263, or [email protected].