Rhythmic timing practices in drilling and blasting applications

Traditionally, blast timing utilized two separate timing sequences to allow for proper breakage of rock, piling of material and minimization of ground vibration and air overpressure.

These timing sequences included the timing from one hole to another that are in the same row, and timing from one hole to another. These timing practices were typically labeled like a 17/42, meaning each hole in a row would fire 17 milliseconds after the previous and each row would start firing 42 milliseconds after the previous row.

Concepts to grasp

The first topic along these lines to understand is hole-to-hole timing basics. Firing all the holes at the same time – instantaneously – would actually result in significant boulders and very poor breakage, yet it would throw and scatter the material more. In addition, instantaneous firing would allow for larger spacing, which would reduce drill and blast costs but increase fragmentation size.

Putting a delay between the boreholes actually increases the fragmentation of the blast but does not throw the material as far. Putting an extremely long delay (hundreds to thousands of milliseconds) can lead to larger fragmentation.

The next topic to understand is the row-to-row timing of a blast. To help understand this, imagine for a second your blast has a normal design burden of 10 ft. For some reason, the blast now was shot with the same amount of explosives as the 10-ft. burden, but a 20-ft. burden was used. What would be the result?

Well, very poor breakage, no throw of the muckpile, vibration up to five times more than the normal burden, extreme flyrock and high air overpressure. Normally, a good blaster or driller would never allow this to happen on the drilled burden. But what about the timed burden?

It is critical to understand that we have numerous types of “burden” in a blast. The drilled burden is the distance from the borehole to the nearest free face. This is typically looked at in a plan view or in a two-dimensional realm.

The toe burden is then the distance from the bottom of the borehole to the free face at the bottom of the bench. This is typically the location of the largest burden throughout the powder column. If this is not, the largest burden area throughout the powder column would be considered the maximum drilled burden. This view now looks at burden in three dimensions.

However, blasting is actually four dimensional because the time the boreholes fire is taken into account. The distance from the blasthole to the free face at the time the hole fires and pressurizes is called the actual or true burden. This is the burden the borehole actually senses.

Therefore, if a pattern is drilled firing straight on rows with a 10-ft. burden but the timing row to row is too fast, the true burden could actually be 20 ft.

Now, this is obvious in cases of out-of-order firing due to cap scatter or poor hookup. But what if the proper hookup and sequencing is used but the timing is too fast?

For example, the previous pattern with a 25-millisecond timing between rows would have a true burden of 20 ft. However, by changing the timing to 42 milliseconds, the true burden would now be 10 ft. This could even be increased to a 67-millisecond delay between rows to allow for better breakage and throw of material. This is the difference between timing a blast and sequencing a blast.

Rhythmic timing practices

Rhythmic timing is a modern timing design that is utilized in accordance with signature hole techniques. Old-school timing methods, such as hole-to-hole and row-to-row, can be used to time and sequence blasts to allow for appropriate delays between boreholes. But the non-standardized timing of these techniques creates differences in time between holes firing.

For example, on a 17-millisecond hole-to-hole and 42-millisecond row-to-row timing sequence, look at the first table (see page 31), which has the time each hole fires. In this example, some holes fire 17 milliseconds apart, some fire 8 milliseconds apart, some fire 9 milliseconds apart and some fire 1 millisecond apart.

The time between holes firing is different between each hole. This causes a sporadic vibration that very often will build in the middle of a blast. This is often seen on seismograph reports with the increase of the vibration toward the middle of the vibration wave, followed by a decrease in vibration as the blast is ending.

In many cases, this is caused by clumping or bunching because a large number of holes are firing within a short period of time – even if the time between holes firing is over 8 milliseconds. For example, in the first table the time between the first and second holes firing is 17 milliseconds. Following this, the time between the second and third blast is 8 milliseconds. This still counts as a separate delay for scaled distance calculations, but will generally increase vibration compared to the 17-millisecond delay between holes.

Rhythmic timing practices are then the standardization of the timing between holes. This ensures each hole fires a set distance apart in time. This, combined with a proper signature hole technique, leads to a good prediction of the entire vibration waveform – about 95 percent accuracy throughout the entire waveform – with either nonelectric or electronic initiation systems. This may mean firing every blasthole 25 milliseconds apart, but just incorporating this can have devastating effects on a site if not properly implemented.

To reiterate, rhythmic timing practices must be implemented with the understanding of proper blast design. Combining this with signature hole techniques, regardless of the initiation system used, can lead to predictions of the entire vibration waveform in the range of 90 to 95 percent accuracy, with almost perfect prediction of the peak.

In addition to the errors listed in this article, there is a large amount of material being published at explosive conferences by technicians that do not understand these basics of blasting. This causes a great deal of confusion for mines everywhere.

For example, using a signature hole study with completely confined boreholes and attempting to develop an envelope (or maximum) peak particle velocity is against both practical and theoretical principals and should not be used. An example of this may be using four or more boreholes with varied powder loads to vary the “scaled distance” and then predict that a certain area may receive up to 8 in. per second. Then, when the shot is fired, an actual vibration record of 3 in. per second is picked up. This is a completely inappropriate use of signature hole analysis and rhythmic timing techniques.

In addition to this, it is common that when attempting to mitigate vibrational concerns and applying rhythmic timing practices, a site will often sacrifice fragmentation and throw. This significantly impacts the mine. The goal then becomes not only to mitigate these vibrational concerns but to minimize the negative effects in the other aspects.

Another principle to consider

In addition, the most basic principal of blasting, confinement, needs to be understood and not violated.

For example, let’s assume that a signature hole analysis was run and the minimum peak particle velocity would be with a rhythmic timing sequence of 3 milliseconds. Let’s assume the sequencing of the blast is kept the same as the previous blast example. This would give a row-to-row timing of 6 milliseconds providing no relief to the second row. Instead of reducing ground vibration, it could increase ground vibration by as much as 500 percent due to the large increase in confinement.

How can this be approached? In the first case, let’s assume the 3 milliseconds in rhythmic timing was okay with hole-to-hole and the mine would accept the decreased fragmentation and increased scattering. Previously, a 42-millisecond delay was an acceptable amount of relief for the row-to-row blasting. This same (or greater) delay would be needed between rows. This could be achieved by firing each row with 14 boreholes, where each borehole was fired in the first row before the second row began to fire such as the second table (see page 33). However, this may still be much too fast hole-to-hole or even in the corners of the blast. Still, at least this timing sequence would keep the proper row-to-row delay. It is important to note that the total time for the blast was cut into almost one-third, which can lead to other consequences.

The other method could be to look for alternative timing windows if enough holes cannot be fired to allow for proper release of confinement from previous boreholes firing. Maybe a 3-millisecond delay rhythmic timing reduces the ground vibration by 50 percent of the previous blast. A 12-millisecond delay may reduce this by 40 percent compared to the previous blast, but only four holes would need to be fired for proper relief of the second row.

Advanced blast patterns can use a combination of these methods while providing additional relief or reducing the maximum ground vibration. Under no circumstances should a 6-millisecond delay from row to row be used, violating the main principle of confinement.

Takeaways

Rhythmic timing practices are a modern tool for timing a blast to significantly reduce ground vibration from a blast. These are normally used in accordance with signature hole techniques, but proper understanding of how to use hole-to-hole and row-to-row delays need to be considered.

Without consideration of fragmentation, throw and confinement, rhythmic timing could produce much worse results than previous blasting practices.

Shooting faster is not always beneficial and has not proven to reliably and repeatedly raise frequency without dramatically raising peak particle velocity. Instead, proper timing to reduce confinement and reduce the peak particle velocity of a blast should be implemented with an appropriate rhythm of holes firing to ensure no clumping of blastholes cause higher-than-necessary vibration.

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