How intimate are you with your rock
Published: Thursday, February 09, 2017 6:51pm
Geologies change across every axis of the Earth’s crust and core. While every mine can rightfully claim that their rock is unique, they are all still subject to the same basic principles of physics; if you hit something hard enough, it will break.
Here is our problem: We all agree that the amount of energy required to effectively blast an underground excavation is tremendous but how, in underground development, can we control this energy to only remove what we need whilst minimising the damage to the ground we wish to leave behind?
Hint: The impact your explosives has on your rock is a function of the detonating pressures imparted on the walls of the blast hole and the massive volumes of explosive gases we violently generate.
Consider Boyle’s Law of Gases where Boyle states there is an inverse relationship between an ideal gas’ pressure and volume at a steady temperature. Therefore a higher detonating pressure will equate to a lower volume of explosive gas being generated. We also know that an explosives’ detonating pressure is directly related to its detonating velocity (VOD):
So overdriving the VOD of our explosives will increase the detonating pressure and, in turn, reduce the volume of gases created.
Here is our Answer: get the fastest possible explosive into the perimeter holes.
However, the common approach to maintaining our development profile is to lower the density of the explosive in the perimeter holes. This only serves to lower the detonating pressure, so while the initial shock waves are not as intense, we have done little to decrease the gas volumes that are forced into the surrounding rock.
Having established that a high VOD is critical for our perimeter control, what else can we do to minimise the explosive gas expansion beyond our designed profile?
Use less explosives
But you need to blast your perimeter to establish a profile, right? How can we achieve this without sacrificing our all-important detonating pressure? We decouple the charge from the bore hole, i.e. USE LESS EXPLOSIVES.
Considering that the energy from a constant explosive mass is finite, a 48mm hole filled with good old reliable ANFO will have a relative 100% energy available and a VOD of approximately 3000 m/s. Now let’s overdrive the VOD to 7000 m/s and reduce the diameter of the charge to 21mm. Using the basic science above, we have just increased our detonating pressure and reduced the volume of gas produced.
A simple tabulation is laid out below:
In the table above, it’s been assumed that the velocity has remained the same for the ANFO and the low density product. Give and take a few, it is for illustrative purposes. The calculations are a little loose but serve to illustrate the dynamics of the different explosives used in a development heading. The fully coupled ANFO and reduced density product have gassing potentials of 100% and 50% respectively whereas a specialised perimeter control product only produces 3.6% gas (by comparison).
JOHNEX’s ECONOTRIM Buttbuster has been specifically designed to work in line with the science of blasting and a hole exposed to ECONOTRIM BUTTBUSTER vs a hole exposed to ANFO experiences 5 times the detonating pressure but only 20% of the gas is produced due to it being 21mm in diameter. Assuming that the Laws of Physics haven’t changed and that Boyle was correct, then the damaging gases from ECONOTRIM BUTTBUSTER are less than 5% of that from a fully coupled ANFO charged hole.