Trench Blasting

The purpose for blasting is to break the rock sufficiently in order that it can be excavated and removed.  To accomplish this, the blaster drills a pattern of holes distributed evenly throughout the rock to be shattered.  These holes are then filled with a pre-determined amount of explosives.  When these explosives are detonated, they release huge amounts of energy in the form of shock waves and high gas pressure.  This energy confined in the rock shatters the surrounding rock and moves it from its original location.  A small percentage of the gas pressure escapes into the atmosphere and produces noise and air concussion.  The shock wave, and the force exerted on the rock produces vibrations in the rock and earth surrounding the blast site.

A competent blaster will design his blast so that the maximum amount of energy released by the explosive goes into breaking and displacing the rock.  The energy that escapes as noise and vibrations is wasted since it serves no useful purpose.  Such wasted energy represents higher production costs.  There is no way to design or detonate a blast that will use 100% of its energy in useful work.  There will always be a small amount that will cause the undesirable effects of noise and vibration.

Trench Drilling

The diameter of the blast holes is normally smaller which provides better distribution of the explosive in the rock and avoids excessive over break outside the theoretical contour.
As a rule of thumb, the hole diameter should be 1/60 of the width of the trench, so if the width is 2.0 m the diameter of the hole should be 0.033 m or 33 mm.