Top Initiation: A Different Approach to Blasting

Most blasting theory courses teach that bottom initiating blast holes is the best and sometimes only way to detonate blast holes.  This is true in cases with open free faces when we don’t mind a bit of throw.  But what about all of the other times?  What about in a typical non-ideal mining environment where there is no free face, there is mud in the holes, and there are vibration/fume issues?
 

Top initiating (aka delaying the bottom primer rather the top primer) can be a generic risk mitigation tool when blasting conditions are less than ideal.  Picture a typical single 20m hole with 4m of stemming and a booster at the top and bottom of the hole.  Consider the following,

Where is the least contaminated explosives product in the hole?  Top or Bottom?

Which booster is more likely to fire?  Top or Bottom?
With no free face, where is the best relief point for the hole?
Which initiation location will cause the least vibration?

TOP!  If there is mud, water, or left over drill cuttings in the bottom of the hole, it is likely that the bottom meter or two or explosives is contaminated in some way.  It is also difficult to ensure that the booster is placed in an ideal location at the bottom of the hole.  Detonating into bad product can slow run up times for the explosive.  This can waste energy and cause fume.  At the top of the hole, there is little to no contamination and it is very easy to place a booster exactly where you want in the top of the hole.  A clean top initiation means ideal run up time for the explosives column.  This also means that any contaminated product at the bottom of the hole has a higher chance of detonating properly since the detonation front is at full speed when it hits muddy or wet product.

The Bottom booster also has more weight on it which increases the risk, although still low, for line failure.

The top of the hole is the best free face for confined blasts.  Top priming takes advantage of this free face in the same way as firing face holes first in a throw blast.  Top initiating encourages material to move upward and can lead to better heave. 

Better heave, and more relief leads to less vibration.

Top initiating can also help with cap rock.  Blast energy always moves towards the initiation point.  If the hard rock is at the top, shouldn’t we encourage the energy to go to the top?  With that said, top initiating will do very little for a hard toe so please use wisely.

As was once told to me… Top Initiating:  “If you can find an excuse to use it, use it”