Civil Engineering Reference
In-Depth Information
(a)
stemming length will give poor fragmentation of
the rock above the column load.
The common stemming length is about
0.7 times the burden, which is adequate to keep
material from ejecting prematurely from the hole.
If unacceptably large blocks are obtained from the
top of the bench, even when the minimum stem-
ming column consistent with flyrock and airblast
problems is used, fragmentation can be improved
by locating a small “pocket” charge centrally
within the stemming (Hagan, 1975).
S
B
(b)
S
B
11.3.8 Hole spacing
When cracks are opened parallel to the free face
as a result of the reflected tensile strain wave, gas
pressure entering these cracks exerts an outward
force that fragments the rock and heaves it on
to the muck pile. Obviously, the lateral extent
to which the gas can penetrate is limited by the
size of the crack and the volume of gas avail-
able, and a stage will be reached when the force
generated is no longer large enough to fragment
and move the rock. If the effect of a single blast
hole is reinforced by holes on either side, the total
force acting on the strip of burden material will
be evened out and uniform fragmentation of this
rock will result.
Figure 11.6 illustrates drill hole patterns with a
variety of burden/spacing ratios. While the square
pattern is the easiest to layout, in some conditions
better fragmentation is obtained when the spacing
is greater than the burden. For a series of delayed
holes, the spacing
S
can be calculated from the
following two equations:
For a stiffness ratio
H/B
between 1 and 4,
(c)
EE
E
Figure 11.6
Typical blast hole patterns used in
production blasting: (a) square pattern with
burden/spacing ratio 1:1; (b) staggered pattern with
burden/spacing ratio 1:1.15; (c) easer holes (E) to
assist movement of front row burden.
which in total contain several thousand kilo-
grams of explosive. Simultaneous detonation of
this quantity of explosive would not only pro-
duce very poorly fragmented rock, but would also
damage the rock in the walls of the excavation
and create large vibrations in nearby structures.
In order to overcome this situation, the blast is
broken down into a number of sequential deton-
ations by delays. When the front row is detonated
and moves away from the rock mass to cre-
ate a new free face, it is important that time
be allowed for this new face to be established
before the next row is detonated. Figure 11.7
show examples of detonation sequences. Row-
to-row blasts are parallel to the free face, with
the row closest to the face being detonated first in
the sequence (Figure 11.7(a)). The V cut shown in
Figure 11.7(b), where the rows are inclined to the
face, is used to open a new free face, and when
(H
+
7
B)
S
=
(11.5)
8
and for a stiffness ratio
H/B
greater than 4,
S
=
1.4
×
B
(11.6)
11.3.9 Hole detonation sequence
A typical blast for a construction project or open
pit mine may contain as many as 100 blast holes,
