Rock blasting 243In this section, we introduced the three basic concepts of(a) attaining the post-peak portion of the complete stress-strain curve;
(b) changing the rock fragment size distribution; and
(c) the only practical methods of inputting the required energy.
These concepts were presented in order to provide an overall conceptual
background to the discussions that follow on rock blasting, specific
methods of blasting and mechanized excavation. The book's objective is to
provide an understanding that allows for maximal creative engineering
thinking, and rock excavation is one area ripe for advancement through
the application of innovative developments.
15.2 Rock blasting
In Chapter 13 rock dynamics and stress waves were discussed. Here, we
consider the specific application of stress waves (and the associated gas
pressure that is generated during blasting) to the explosive breakage of
rock. The technique of rock breakage using explosives involves drilling
blastholes by percussive or rotary-percussive means, loading the blastholes
with explosive and then detonating the explosive in each hole in sequence
and in accordance with a programme which depends on the type of
blasting round being used.
The explosion generates a stress wave and significant gas pressure,
resulting in complex reflections and refractions of stress waves at discon-
tinuities within the rock mass. However, even with this complexity, it is
possible to isolate key features of the process which allow the blasting
rounds to be designed successfully.
75.2.7 The stress wave and gas pressure effects
In Fig. 15.4, an idealized development of borehole radial stress with time
after detonation is shown. The stress rapidly builds up to a peak-within
a few microseconds-and then reduces as the gas pressure is dissipated
through discontinuities. As indicated in the figure, it is convenient to
Iml;;a;d IStress effect wave Explosive within ch#
blasthole nTimeFigure 15.4 Stress wave and gas pressure effects during an explosion.