Lesson 6.3: The Design of Blast Rounds

Lesson 6.3: The Design of Blast Rounds

We’re ready to design a blast… so, what do we need to do?

The design of a blast round generally means, at a minimum, to do the following.

• determine the size, i.e., the diameter, of the borehole, and the length of the hole;
• determine the geometric arrangement of the holes;
• determine the burden, spacing, and stemming;
• determine the delays for different holes, and the number of holes at each delay period.

Some elements of the design are common or applicable to the design of all types of blasting, while others are specific to a given type. We’ll cover the common elements in this course, and look at two types in more detail. As with many aspects of mining-engineering design, the design of blasts tends to be as much art as science. Make no mistake, however: the “art-of-practice” is informed by the science! And one goal here is to help you become proficient in applying the science.

The two types that we will focus on are bench blasting and drifting, and, together, these two account for the vast majority of all blasting done in mining. Bench blasting is used in most surface and underground mining methods, and drifting is used in the underground methods that utilize a drill and blast cycle.

Other important, but less common types that will not be examined in more detail here are:

• ring drilling & blasting: specific applications in certain underground M/NM mines, e.g., bell and draw point development in caving methods, and sublevel stoping;
• longhole drilling & blasting: specific applications in certain underground mining methods, e.g., sublevel stoping and shrinkage stoping;
• crater blasting: used in the vertical Crater Retreat (VCR) mining method;
• cast blasting: adaptation of bench blasting used in certain area surface applications, predominantly in the Powder River Basin surface coal mines.

In past lessons, we’ve defined some key terms for the design of a blast round. By way of review, these included the following:

• Free Face is an unconfined surface through which the broken material can move.
• Burden is the distance between the hole and the nearest free face.
• Spacing is the distance between holes.
• Stemming is the inert material used to confine the shot at the collar of the hole.
• Decking is unloaded space in the hole. Inert material is used as spacers for the unloaded space.
• Sub-drill is the added length of the borehole past the advance plane (horizontal) or the desired grade (vertical). In other words, if you wanted to blast a bench that was 50’ high, you might drill a that was 55’ deep. Why would you drill 5’ past the level that you wanted the bench? By doing so, i.e., sub-drilling, you will ensure a more even surface at 50’, which will make it easier to move equipment about the bench.

You will recall that we defined bulk strength, BS, as the product of the density and the weight strength of the explosive. While the weight strength tells us how much energy the explosive will release per unit weight, bulk strength is a more informative metric. The bulk strength is telling us how much energy we can place into the hole, which is of direct interest to us. The bulk strength is also a useful metric to compare explosives. In the old days, dynamites were rated as 1X, 2X, and so on, based on their bulk strength. Let’s look at an example using bulk strength to compare two different products.

where $\rho$ = density, (kg/m³) and Q = weight strength, (kcal/kg).

Consider typical values for ANFO, Q=912 kcal/kg and $\rho$ =800 kg/m3,

and let’s calculate the bulk strength of a “typical” ANFO.

Now, let’s imagine that someone has brought a different product to our attention, with the suggestion that we may want to use it. Its weight strength is 850 kcal/kg, whereas the one that we are currently using has weight strength of 912 kcal/kg. The product that we are using has larger weight strength than the new one that is being suggested to us. Do we even need to bother looking into this?

Well, humor me. Let’s look at it a bit more. Remember that what really matters is how much energy we can fit into the hole. To evaluate that, we need to calculate what?

Bulk strength!

We’ll need the density of this new product… let’s call it product “A”.

We find that ${\rho }_{\text{A}}=1200{\text{kg/m}}^3$ . We were told that ${\text{Q}}_{\text{A}}=850\text{kcal/kg}$ . Ok, now we can calculate the bulk strength of “A”.

Wow, imagine that! Product A packs a 40% bigger punch than the product that we are currently using!

A word of caution, however: these “raw” energy comparisons are useful, but not absolute indicators of blast efficacy. The amount of energy that produces seismic shock as compared to the amount that produces gases, for example, affects efficacy. A reasonable split is 15% - 85% to produce the cracks and then cause separation.

Let’s move onto the design, now that we’ve got this background behind us.