Abstract:
This thesis describes the development of a directional primer charge for use in blasting in
mining operations.
The directional primer charge is an explosive gun which takes the place of a standard
primer charge in a blasthole. It is a shaped charge which directs the explosive energy
forwards into the blasthole. Its effectiveness is enhanced by a metal liner which is located at
a specified stand-off distance from the toe of the hole.
The explosive energy of the column charge is converted into the kinetic energy of the metal
liner which transforms into an ultrasonic slug. This in turn converts into the impact energy of
the slug impacting on the rock. This rock is axially compressed to such a degree that a
radial fracture is developed. This radial fracture is termed an umbrella crack. Prior to the
development of the directional primer charge, the phenomenon of the umbrella crack had
only been observed in experimental Perspex blasting models and its formation mechanism
had never been satisfactorily explained. If the directional primer charge could cause an
umbrella crack in hard rock mining at the end of blastholes, then more rock would be broken
out per blast than is currently achieved in practice.
This thesis records the historical development of the explosive shaped charge with particular
reference to the development of the explosively-forged projectile. It describes the classical
theories and models which apply in determining the theoretical prediction of the physical
properties of the designed directional primer charge. It describes the experimental
procedures and measurements using flash X-ray radiography and electronic shorting
screens to freeze the flight of a metal slug traveling at speeds of over 2000 metres per
second. Underground tests were undertaken under full mining production conditions to
compare the rock breaking effects of various designs.
The theoretical calculation of the extent of the movement of the rock at the toe of the
blasthole indicates that umbrella cracks should be formed. The underground tests confirm
their formation.
It is concluded that the use of the directional primer charge in stoping operations can
improve the blasting efficiency in South African hard rock mines by up to 15 %.
