David Summers is correct in saying that you can get 'short' barrels of the
holes at the end of a longer round.
The giveaway that this was a relatively modern hole is the shattering that
you will see around the bottom, or toe, of the hole. This only occurs with
high explosives. Any hole charged with black powder will not exhibit this
as balck powder, which does not strictly speaking detonate, breaks
predominately by gas pressure. The mechanism of rock breaking with high
explosive is fundamentally different. HE on detonation sets up a fracture
pattern around the hole - these fractures are then 'levered' open by the
following high pressure gas in the hole.
Most holes are charged leaving an uncharged collar of something a little
less than the burden on that hole. Thus say, for example, there is a burden
of one foot then there will be an uncharged collar of something similar.
With hand drilling it is going to be a balance between a longer hole and
therefore preportionally more hole charged against the problems of drilling
longer holes and the fact that long holes do not break so well.
Tony Brooks
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