In our previous blog post we discussed powder factor and how it is the foundation for all blast designs. Powder factor is the core measurement of how much force is being exerted per quantity of rock.
The next element to consider with powder factor is distribution and how energy from the explosives is distributed within the rock. Powder factor gives us how much energy is in a specific amount of rock. For example, we have 1000 yards of solid rock and want to have a 1.0 powder factor. We would need to put 1000 pounds of explosives inside that 1000 yards of rock somehow. Then distribution is the next step.
Vehicles of distribution
The two vehicles of distribution are hole size and spacing of the boreholes. You have the diameter of the borehole and the spacing of the boreholes on a specific pattern on the area you wish to blast. Similar to lifting a heavy object, if you have something that weighs 500 pounds, you need 500 pounds of force to lift that object. There are different ways to lift that weight. You can have one strong person lift the object themselves, which doesn’t require much distribution. Or you can distribute the weight between five people lifting 100 pounds each. Similarly, with a rock blast that you need a 1.0 powder factor with 1000 pounds of explosives for 1000 yd.³ of rock you could have one hole with 1000 pounds of explosives or you could have 10 holes distributed throughout the rod with 100 pounds of explosives.
Determining number of holes
How we determine whether we need one hole or several holes is based on many factors including the natural fragmentation of the rock and how many natural joints are in the material. If it is a very blocky type of material we may need to distribute the energy throughout the body of rock, meaning smaller holes and a tighter pattern. Conversely if the rock has many natural fracture lines already and it doesn’t need grade distribution you could use very few holes to achieve the same results.
Much like the example of lifting the 500 pound object with one person, that one person is only going to be able to manipulate that object so much. But if you had five people doing the same work you could manipulate the object in five individual points which gives you more control over that object. This is very similar to a blast with several smaller holes which allows you to have much more control over that quantity of rock than one large hole would allow you.