In the beginning, blasting was relatively simple. The explosive was placed in the holes and the holes were fired. The speed at which the fuse or an igniter cap would burn was the delay timing per explosive round along with different lengths equalling different times (Alfred Nobel, 1865). Electric pyrotechnic delay detonators (circa 1900) and cord initiating systems followed. Second interval delay detonators entered the market in the 1920s. Each progression of detonator type had its pros and cons, with many mines around the globe still choosing detonating cord on surface for their large-scale paddock blasts, as it is viewed as a cost-effective form of initiation.
With the advent of the the MS short delay detonators, introduced around the 1940s, and the shock tube first patented by Per-Anders Persson in 1967 (Holmberg et al, 2001), a cheap, effective form of initiation was born. These detonators heralded a new era of safety and accuracy. Since pyrotechnic delay detonators – electric and non-electric – became available, people have been playing with timing to improve many aspects of blasting. From the 1940s to the 1970s, many studies were conducted on delay sequences and how they improved fragmentation and environmental impacts. Research conducted by the United States Bureau of Mines and the Swedish Detonics Research Laboratory assisted in the evolution of the timing ?rules of thumb?. Today, initiation systems are improving again, in regards to precision and accuracy, with the use of electronic detonators. However, non-electric systems such as Maxam?s Rionel SCE and Rionel MS, still provide the best cost/results ratio for the majority of blasting situations.
RULES OF THUMB
The ?rules of thumb?, with regard to timing, have provided excellent starting points for the implementation of initiation systems in open cut operations. The majority of explosive suppliers produce shock tube MS delay detonators at set intervals. Of course, underground operations have often required the use of far longer delay periods, termed the LP (long period) range, and open cut/surface operations utilise the MS range (millisecond). These intervals are intertwined with the ?rules of thumb? concerning the interaction of blast holes, in-field application, and the simplicity of shock tube delays.
The most notable ?rules of thumb? relates to the moment at which two blast holes no longer act as one. This is the eight millisecond window of time for vibration.
In many situations, if a blast hole is initiated within eight milliseconds of another blast hole, the charges act as one, often increasing vibration. It is important to note that this is not always true and depends on distances and rock types; however, it is a great starting point. For overpressure, the time window is often considered much greater than eight milliseconds. These rules can be applied consistently in many operations worldwide.
Other ?rules of thumb? relate to the spacing and burden velocity, and their combination into overall blast relief. Timing relief is measured in milliseconds per metre (ms/m) of effective burden and is the rate of the initiation of a timing sequence. In-field rock movement velocities are measured at slower rates. In John Floyd?s graph (Figure 1), ?rules of thumb? are displayed in a usable form.
Spacing speed is considered to be 0.33 to 0.5 times the chosen ms/m of burden. Another industry accepted range for spacing speed is two to 15 ms/m of spacing. Burden relief amounts are considered best, if between five to 40 ms/m of burden. These ranges are quite large and therefore Figure 1 is a more useful starting place.
CONSISTENCY AND RELIEF
With all aspects of blasting, consistency is paramount to any understanding of blast results. Timing is no different, best results are achieved by keeping the blast relief consistent. In the majority of situations the volume of the rock within the blast boundary is similar in geological attributes. With this in mind, it makes sense that the initiation sequence if consistent will produce consistent results.
The concept of relief is to ensure the rock has adequate time to move into voids created by previously detonated blast holes, along with allowing sufficient time for interaction with other rocks in motion, creating looseness of muckpile and good fragmentation.
Consistency comes from considering blast timing in terms of time per distance (ms/m). Many blasting practitioners view timing in terms of the delay periods used. This produces some misconception about what is actually occurring. For example, in Figures 2a and 2b, the relief is very close to being the same, however the delays are different. The similarity between rates at which the blasts are initiated is due to Figure 2b?s pattern expansion and the obvious consideration of time per distance.
When thinking in time per distance, some very simple concepts can be realised, providing us with the ability to produce timing with a consistent methodology.
According to the Merriam-Webster Online Dictionary, the key concept and definition of a pattern is ?a discernible coherent system based on the intended interrelationship of component parts?. This pretty much says that a pattern is a repetitive form, therefore timing and other design traits of a blast design are generally repetitive. The time between holes (of the same distance separation) in a row is best kept the same throughout, as with the time between rows. Once the repetition is realised the patterns within designs can be seen, eg ?timing triangles/trapezoids? for staggered patterns and ?timing triangles/squares/rectangles? for square or rectangular patterns.
Timing shapes are useful, hands-on tools for manual contour and burden relief calculations. A pattern of blast holes is exactly that – a ?pattern?. The designs have a few attributes and when an attribute changes, the dynamics of the blast change. Timing shapes utilise this exact principle that a tie-up in most cases is repetitive. Timing triangles present themselves everywhere in a blast pattern. To find these shapes, simply draw lines through every echelon, reverse echelon and row, as displayed in Figure 3.
Timing shape benefits can:
? Be used anywhere in the field or sitting at the computer.
? Allow you to calculate the exact times required for in-fill holes.
? Allow you to calculate burden relief.
? Allow you to calculate contours.
? Show you flaws in your reasoning.
? Allow you to provide/change consistent burden relief.
Once the information is known, the time for any in-fill hole in a blast can be worked out. Alternatively, if a hole was not drilled in the precise location, its timing can be kept consistent with the desired relief.
In Figure 4a, there is an in-fill hole. Giving it the most appropriate time for the required relief is easy because the pattern can be seen. The bolded hole with a ??? in Figure 4a is half way between 0 and 17. When using timing shapes on the shot we break the initiation sequence down to its simplest form. For this example, hole #1 is zero milliseconds (ms), hole #2 is 17ms and hole #3 is 59ms. To work out the correct time for the in-fill hole, halve the time between hole #1 and hole #2 (17 milliseconds x 0.5); as the in-fill hole is located halfway between hole #1 and hole #2, this is equal to 8.5ms and as 8.5ms non-electric surface connectors do not exist, so a 9ms delay is used. This is the simplest application of a timing shape.
How is the hole timed in the middle of the three holes as in Figure 4b? Separating the hole?s location into its component distances is how the most suitable time is discovered, eg:
? Half-way between hole #1 and hole #2.
? Two-fifths of the way between hole #3 and the row in front.
In Figure 4b?s example, finding the hole #1 to hole #2 half-way time, which is 8.5ms and then subtracting 8.5ms from hole #3?s initiation time (59ms), equals 50.5ms. Dividing 50.5 by five and multiplying it by two (two-fifths of the distance) equals 20.2ms. The addition of the 8.5ms back on to the 20.2ms provides us with the optimal time at which the in-fill hole should be initiated, 28.7ms. Using a 25ms from hole #1 or a 9ms from hole #2, as displayed in Figure 4b is the simplest solution.
These timing shapes can be used on any shot to help you time the blast consistently and with a method.
BLAST CONTOURS AND MOVEMENT
The contours of a blast are lines indicating the same time within a blast. These are similar to any contour on any map. The general direction of rock movement is perpendicular to these contours (at right angles) and usually indicated by an arrow on tie-up plans.
Calculating the initiation times of the blast holes and using timing shapes, drawing blast contours and direction of movement arrows manually is easy. This is displayed in Figures 2a and 2b by the green line with arrows.
When a contour with an unusual kink in the line is noticed, special care should be taken to discover the reason; in many cases the blast may be timed out of sequence. This can produce unwanted outcomes (back break, poor fragmentation or wall damage) or environmental concerns (fly-rock, high vibration or high overpressures). Figure 5 shows an example of an out of sequence blast. In this example, the green contour lines and movements of the rock produce a kink in the contour. This is where reverting to the rule of consistency of blast relief is helpful. Using different delays or an alternate tie-up method would quickly fix the issue. The understanding of what a blast-timing contour is indicating can be a powerful tool to all blasting operators.
THE FINAL DESIGN STEP
Timing is one of the final design steps that can greatly improve blast results. When design, drilling and loading, as well as any other implementation of blast attributes, are near optimal, timing is a an aspect that makes a good blast great. Simple timing concepts applied in any situation make the most out of your blast.
Brent Buffham is the technical manager of Maxam Australia Pty Ltd.
Holmberg R, Lee J, Persson PA. Rock blasting and explosives engineering (6th edn). CRC Press, New York, 2001.
Dessureault S. Rock excavation: Mining and geological engineering: course notes. University of Arizona, Arizona, 2006.
Floyd J. Blast Dynamics: course notes. 2005.