1. Omar Helal Mohamed
G r a d u a t e M i n i n g E n g i n e e r
Elements of
Blast Design
in Surface Mining
2. TABLE
OF
CONTENTS
Surface Blast Design
Free Face
Hole D and B VS Face Height
Burden and Spacing
Subdrilling
Stemming
Decking
Powder Factor
Choice of Pattern
Delays
Actual Blasting Cost
Terminology
References
01
02
03
04
05
06
07
08
09
10
11
12
13
3. Surface Blast
Design
The nomenclature used for bench blast design is shown
in Figure 1. The following discussion presents some
guidelines for blast design. The guidelines provide initial
estimates from which to design a blasting pattern.
However, the properties of the rock mass and of the
explosive should also be considered. There are guidelines
that have been established over the years for blast
designs. In the following subsections, guidelines are
presented that the authors believe represent common
practice.
Figure 1 Nomenclature for bench blast design
4. Free Face
In blasting, a free face provides a
direction for movement and, thus,
a degree of freedom. As the
number and extent of free faces
located near the blasthole
increases, the amount of rock
that can be fragmented from that
blasthole also increases
5. Hole
Diameter
and
Burden
Versus
Face
Height
The face height-to-burden ratio should be at
least 3:1 to 4:1 for good fragmentation. Short
faces are stiff and resist breakage. Also, the
amount of stemming required to seal the hole
increases as the face shortens, leaving less
room for explosive in the hole, increasing the
proportion of the bench interval remote from
adjacent explosive columns and the hole
utilization and powder factors drop.
As the face height decreases, because the
stemming height is proportional to the hole
diameter and burden, the powder column
height decreases disproportionally, resulting in
reduced borehole utilization (explosive-filled
portion). To maintain powder factor, the pattern
has to be shrunk, and this requires the drilling
of more holes, increasing drilling costs.
6. Burden and Spacing
For bench blasting, most equations
produce similar results for rock in the
specific gravity range of 2.2 to 2.8. There
are basically two specific gravity divisions
for modern bulk explosives. These are 0.8
to 0.87 for ANFO and 0.65 to 1.3 for
emulsions and blends.
In bench blasting it is accepted practice
that a burden-to-spacing ratio of about 1.15
to 1.4 for bench heights greater than four
times the burden should be used on a
staggered pattern. The reason for this is
that repeatable faces are developed with
equal burden on both faces of relief.
7. Subdrilling
Subdrilling is necessary when blasting
massive rock where there is no suitable
horizontal bedding plane to maintain floor
grade. The accepted amount of subdrilling
is one-third of the burden. Some
operations may range from 0.2 to 0.5 times
the burden, but one-third is a good
guideline. Subdrilling can also be
calculated as 5 to 8 times the diameter of
the hole.
8. Stemming
Stemming is necessary to confine the
explosives column to prevent explosive
energy from escaping and reducing the
effectiveness of the blast. If the stemming
is ejected prematurely, not only is costly
energy lost, but also a sonic boom is often
generated along with flyrock.
The guidelines for stemming are
T = 2 ⁄ 3 B for clean stemming material 1 ⁄ 8
of the hole diameter
T = 4 ⁄ 3 B for drill cuttings
9. Decking
Decking is the separation of the explosives column in a
blasthole into two or more parts with stemming between
them. The thickness of the deck material should be 6 times
the hole diameter for dry holes and 12 times the hole diameter
for wet holes.
10. Powder Factor
The powder factor is usually determined by taking
the amount of rock to be fragmented to grade and
dividing it by the weight of explosives used. It can
be calculated for each hole or for a complete
shot/round. In some mining, the powder factor is
often quoted as weight of explosive per unit
weight of rock (kilograms per metric ton).
For normal surface-mining operations a powder
factor of 0.6 kg/m3 is a good initial estimate. The
weight of explosive, W, per hole is calculated as
follows:
W = PC * Cp
where PC = powder column height
Cp = column density
11. Choice of
Patterns
A number of different patterns can be chosen,
including square, staggered, en echelon, and
diamond. It is normal to use a staggered pattern
in bench blasting, firstly, because the hole in the
row behind is blasting into more solid rock rather
than a weakened pocket and, secondly, because
holes tend to break at 45° to the free face,
making a square end to a bench almost
impossible (see the next figure)
13. Choice of
Patterns
A square pattern is more appropriate for a
confined shot, such as sumps used for
dewatering the next bench level. A square pattern
may also be appropriate in sinking cuts when
developing a new level. In construction blasting, a
square or rectangular pattern helps maintain
straight sidewalls, especially in road cuts
14. Delays
There are two main types of delay in a blast
pattern. These are the hole-to-hole (also known
as “inter-hole”) delay and the row-to-row (or
“inter-row”) delay. The optimum hole-to-hole
delay for fragmentation has been determined by
a number of different researchers.
The general consensus is that the delay is 3
ms/m of burden. However, maximum
fragmentation is not always desired.
15. Delays
When using delays, the accuracy of the delay
time is also important. delay timing with
conventional detonators is subject to scatter, and
accurate detonation delay times are most likely
to be achieved when using electronic detonators.
Because of the potential for inaccuracy in
detonator timing, it is recommended to use a
delay interval within a row larger than the 3 ms/m
optimum. Large delay intervals between holes
result in reduced fragmentation.
16. Delays
Insufficient delay between holes is also not
recommended and generally results in excessive
throw , increased back break/wall damage, and
inadequate fragmentation. The firing of
neighboring holes together results in splitting of
the rock between them and the premature
propulsion of the rock mass forward, resulting in
poor fragmentation.
17. Delays
In blasting, it is normal to use a spacing-to-
burden (S/B) ratio of 1.15:1 to 1.4:1, which means
that delays are a necessity for fragmentation but
a penalty in blast casting where burden and
spacing are often reversed to achieve less
fragmentation but more throw.
20. References
"Surface Mining" by B. A. Kennedy and E. D. McKee
"Blasting Principles for Open Pit Mining" by William A. Hustrulid and Michael K.
McCarter
"Rock Fragmentation by Blasting" by Pradeep K. Singh
SME Mining Engineering Handbook
"Explosives and Blasting Procedures Manual" by Richard A. Dick and Charles D.
Fisk
