Civil Engineering Reference
In-Depth Information
(a)
S
excessive as the bench height increases.
Where small diameter blast holes are drilled
in high benches, blast holes may need to be
angled, at least in the front row.
3
B
2
(c)
Drilling accuracy becomes more critical in
higher benches. Where precise alignment of
holes on the final face is required, the max-
imum bench height is normally limited to
8or9m.
1
Face
Direction of
rock movement
Surface delay
(b)
11.3.3 Burden
The influence of the effective burden (the distance
between rows of holes and the nearest free face)
on fragmentation is related to the mechanism of
rock fracture described in Section 11.2. The blast
is most efficient when the shock wave is reflec-
ted in tension from a free face so that the rock is
broken and displaced to form a well-fragmented
muck pile. This efficiency depends to a large
extent on having the correct burden. Too small
a burden will allow the radial cracks to extend
to the free face resulting in venting of the explo-
sion gases with consequent loss of efficiency and
the generation of flyrock and air blast problems.
Too large a burden, where the shock wave is not
reflected from the free face, will choke the blast
resulting in poor fragmentation and a general loss
of efficiency.
The relationship between the bench height
H
and the burden
B
can be expressed in terms of the
“stiffness ratio,”
H/B
. If this ratio is low such
that the burden is about equal to the bench height,
then the blast will be highly confined resulting in
severe backbreak, airblast, flyrock and vibration.
In contrast, if the
H/B >
S
B
6
5
4
3
2
1
Face
Direction of
rock movement
(c)
Maximum front
row burden
B
Figure 11.4
Definition of blast hole spacing (S) and
burden (B): (a) burden and spacing for a square
blasting pattern; (b) burden and spacing for an
en echelon
blasting pattern; (c) effect of face angle on
front row burden.
4, there is little con-
finement and the explosive gases will be vented at
the free face also resulting in airblast and flyrock.
It is found that a stiffness ratio of 3-4 produces
good results, or
∼
pattern which is fired row by row from the face
gives an effective burden equal to the spacing
between successive rows parallel to the face. On
the other hand, an identical pattern of blast holes
fired
en echelon
results in different burdens and
spacings, and a spacing to burden ratio greater
than 1 (Figure 11.4(b)).
B
=
0.33
×
H
to 0.25
×
H
(11.1)
The burden distance calculated from equa-
tion (11.1) depends not only upon the blast hole
pattern, but also upon the sequence of firing. As
illustrated in Figure 11.4(a), a square blast hole
