The explosion of a charge in a drillhole sets the surrounding rock mass
into vibrating stress wave motion. Except in the immediate vicinity of the
drillhole, the dynamic stresses associated with this motion do damage
only to pre-existing joints, cracks, or other weak planes, not to the rock
material in between these. The joints are weak in tension, therefore the
damage occurs as a result of tensile stresses. The initial damage process
in the rock mass that ultimately breaks loose in front of the hole is similar
to that in the remaining rock behind the drillhole. Recorded or calculated
values of the vibration velocity and frequency contain a wealth of
information about the combination of stress and strain that causes the
damage. This paper outlines a new technique by which the peak strain energy
derived from measured or calculated vibration velocity records is used to
determine the local fragment size distribution. It combines two
previously known and well tested techniques, namely the
Holmberg-Persson calculation of the peak vibration velocity generated by
an extended charge and King's calculations of the fragment size
distribution as a function of the strain energy in rock crushing. Both of
these calculations are based on experimental data and have been tested
and found to agree well with actual conditions in their respective fields.
Holmberg-Persson's calculated peak vibration velocities have been used
successfully to predict and control damage to the remaining rock in
cautious blasting, while King's calculation successfully describes the
comminution of rock in mechanical crushing. Preliminary predictions of fragmentation in two types of rock blasting,
a large hole open pit mining blast and a tunnel round, indicate that the
new technique for fragmentation prediction has the potential for
predicting fragment size distributions within the rock removed by the
Blast.