Breaking Boulders in Top Burden
Stemming is widely used to ensure that a loaded borehole achieves optimum blast results, but the practice can leave large boulders unbroken in the upper layer of burden. The authors explore several proven methods for boulder-busting using explosives.
An explosive detonation releases a large amount of gas pressure in a borehole that, with proper design, is contained and forced to do maximum work horizontally away from the borehole. The main method to ensure the blast does not lift upward, causing minimal fragmentation and up to a 6 decibels (dB) increase in air overpressure, is through the use of proper stemming material and length. Stemming is a double-edged sword leading to better fragmentation of the face, but also with increased boulders from the top burden (see figure below) in the stemming zone of the blast. This can cause problems if the boulders are too large to be handled with the digging equipment or cannot be effectively cast with the overburden.
There are many ways to break these boulders with explosives; however, it is always better to break the boulders in the blast or prevent these boulders from forming during the blasting process rather than having to break them in the muck pile after the blast.