Explosives Today S4 Nr15

Blasting News I Second Quarter 201612
EXPLOSIVES
TODAY
Series 4 I No 15
BLASTHOLE DRILLING AND INITIATION PATTERNS IN
SURFACE BLASTING Part 1 By Henk Esterhuizen – Senior Mining Engineer
The layout of the drill holes,
burden and spacing and
ratio between them have an
important effect on blasting
results. In this issue a
theme will be developed
with particular reference
to blasthole drilling and
initiation patterns. In
order to understand the
significance of these
patterns however, one of the
basic mechanisms of rock
fracture by explosives must
be briefly considered.
Radial Fracture Under Gas Pressure
When blastholes are fired
independently a cylindrical ‘plug’ of
broken ground is created around each
hole before movement of the burden
takes place. The diameter of this
‘plug’ is determined by the pressure
of the explosives gases and the
time for which they act in the radial
cracks and fissures to the free face as
illustrated in steps 1 to 4 in figure 1.
If the burden is small the gas is
released very quickly and its unused
energy is spent in heaving the
broken burden forward with great
momentum. A large burden, on the
other hand, promotes the formation
of longer cracks hence a greater
diameter plug of fractured ground,
accompanied by minimal heave. The
optimum burden is that which results
in a maximum of ground being broken
and heaved into a rockpile loose
enough to be handled by the available
loading equipment.
Blasthole Drilling Patterns
Seen in plan on the surface of the
bench, the fractured areas around
the blastholes can be represented as
circles. It is logical to assume that
every point on the surface must fall
within at least one of these fracture

Blasting News I Second Quarter 2016 13
Figure 1. Gas pressure and crack extension
Figure 2. Bench coverage by blasthole fracture circles assuming constant powder factor and S/B ratio
= 1.25 - Note: In the square pattern unfractured areas and excessive overlap between circles; In the
staggered pattern, total bench coverage
Figure 3. Interaction of stresses from closely spaced blastholes fired simultaneously
circles for effective fragmentation
to occur. Figure 2 contrasts the
arrangement of square drilling
patterns with that of staggered
drilling patterns, for a Spacing/
Burden (S/B) ratio of 1.25:1.
The staggered pattern produces
a more uniform distribution of
fracture circles and thus more
even fragmentation in the rockpile
for the same powder factor.
In fact, optimum coverage is
obtained when the holes form
equilateral triangles, but as can
be seen from table 1 this pattern
varies only slightly in coverage
from the staggered patterns
based on S/B ratios of between
1.0 and 1.5.
Implications of Drilling Pattern
The greatest potential for good
breaking with the most extended
drilling pattern thus lies in using
a staggered pattern having an
S/B ratio of between 1.0 and
1.5. The square layouts all give
significantly less coverage. Note
that if the S/B ratio increases
beyond 2, the radial fractures
reach the free face before
becoming fully developed. As a
result pressure is released earlier
resulting in a smaller diameter
fractured area and more flyrock.
There can therefore be little
benefit in exceeding an S/B ratio
of 1.5.
Real as the benefits of staggered
patterns may be, they are less
evident in highly fractured
S/B Ratio Square pattern % Staggered pattern %
1.00 77 98.5
1.15 Δ 76 100
1.25 75 99.5
1.50 71 94.6
2.00 62 77.0
Table 1: Effect of drilling patterns and S/B ratios on area covered by fracture circles. Equilateral
triangular layout = 100%
Note: Δ, equilateral triangle

ground where the fracture
planes seriously hamper the
development of radial fractures.
Also, with small diameter
blastholes in high benches,
drilling inaccuracy and hole
deviation can result in the pattern
at the toe being unrelated to the
laid out pattern on the bench. In
these conditions, therefore, the
merits of staggered patterns may
be outweighed by the convenience
of drilling square patterns.
Blasthole Initiation Patterns
The overall performance
of production blasts can be
controlled by altering delay
timing to vary the degree of
interaction between adjacent
blastholes. Whilst absolute
values of inter-row and intra-row
delays are important, the ratio
of these times is also significant.
This can be explained by the
following concepts (which are
over-simplifications of a complex
subject).
• The intra-row delay controls
blastholes and determines
whether blastholes act
independently or together.
• The inter-row delay controls
interaction between
dependent blastholes, as
it affects the progressive
creation of new effective free
faces during the blast.
• The ratio of inter-row delay to
intra-row delay controls the
Figure 4. Interaction of stresses from closely spaced blastholes fired with excess time delay
Figure 5. Interaction of stresses from closely spaced blastholes fired with optimal time delay
Figure 6. Squared and staggered V-cut chevron patterns
geometry and orientation of new
free faces created as the blast
progresses. For a later-firing
blasthole, the location, shape
and extent of any effective free
face will depend on this ratio
of delay times. This influences
the direction and extent of
displacement of the burden of
each blasthole and thus the final
muckpile shape and position.
This is sometimes referred to
as the apparent direction of
movement of a blasthole or the
overall blast.

Single row fired with excess time
delay
If a single row of similar blastholes is
detonated in sequence with relatively
long time delay (e.g. several seconds)
between successive detonations,
the result will be different again as
illustrated in figure 4.
In general, this type of blast would
produce:
• Better fragmentation than
the instantaneous single row
blast, as cracks between
blastholes would not tend to
link up preferentially. However,
fragmentation may be poorer
than the single hole blast
because there is no positive
blastholes, and earlier-firing
charges may disrupt adjacent
explosives charges or the rock
mass surrounding them.
• Less forward movement than
the single hole or simultaneous
single row blasts, as the rock
displaced by the first holes
to fire will come to rest and
become a buffer which restrains
subsequent burden movement.
The opening of cracks from
earlier-firing charges may
also permit premature venting
of gases from subsequent
detonations.
• Less overbreak than the
instantaneous row of blastholes,
but more than the single
hole blast because forward
displacement is restrained by the
broken rock buffer.
• Lower ground vibrations and
airblast than the instantaneous
single hole blast, because energy
release and ground movement
are spread over a longer period
of time. Ground vibrations may be
higher than the single hole blast,
because of the restraining effects
of the broken rock buffer.
Single row firing with optimal time
delay
Alternatively, a single row of similar
blastholes could be fired in sequence
with a relatively small time (e.g.
several milliseconds) between
adjacent detonations. In general, a
delay interval of a few milliseconds
per metre of spacing between
adjacent blastholes will produce quite
different results compared to the
previous examples.
Optimal time delay is known as intra-
row delay, the essential difference
is that each blast hole charge is
detonated whilst the surrounding
rock mass is pre-stressed but not
completely disrupted by the effects of
earlier-firing charges.
Adjacent blastholes thus interact
positively, producing superior results
because the explosives energy is
released in a controlled manner
and applied to the rock mass more
effectively. In fact, for any pair of
blastholes there is a unique delay
time which will produce the best
possible result as can be seen in
figure 5.
“Initiation” Burden and Spacing
Spacing delay
Identifying the right intra-row delay is
one of the key factors to predictable
and efficient blasting. For a brittle,
elastic, homogeneous rock type,
a short intra-row delay is usually
appropriate. In contrast, a porous,
plastic, highly jointed rock mass
would require more time between
detonation of adjacent blastholes.
Short delays promote a united effort
between adjacent blastholes, tending
to maximise forward displacement
at the expense of fragmentation and
vibration levels. Long delays tend
to make each blasthole work more
independently, reducing positive
interaction.
Results from a wide range of
conditions indicate that the
appropriate intra-row delay for
conventional blasting is usually less
than 5 milliseconds per metre of
burden (as measured between rows
of holes). The ideal delay for each
situation is clearly influenced by rock
properties, blast geometry and the
desired result, but 3 to 6 milliseconds
per metre of burden is recommended.
Burden delay
Fragmentation will be enhanced,
particularly in the toe region of

blastholes towards the perimeter of
the blast. The correct inter-row delay
ensures that each blast hole has an
effective free face to break towards,
because preceding blastholes have
broken and detached their burdens before
the next dependent blast hole fires. This
progressive relief of burden during the
blast will affect the volume of oversize
rock produced, although fragmentation
is often influenced more by the intra-row
delay than by the inter-row delay.
Results from a wide range of conditions
indicate that the appropriate inter-row
delay for conventional blasting is usually
less than 18milliseconds (measured
between rows of holes) but 12 – 18
milliseconds per metre of burden is
recommended for initial trials.
Ground and air vibrations are minimised
and can often be maintained at levels
similar to a single row blast. This is
a direct result of progressive relief of
burden during the blast, which promotes
lateral movement and minimises uplift,
cratering and stemming ejection.
Subsequent blasts are also likely to
have less potential for airblast because
reduced overbreak means that front row
burden rock contains minimum cracking
from the previous blast.
Types and Features of Chevron Patterns
Open or Closed
Chevron patterns are classed as ‘Open’
or ‘Closed’ depending on whether it
is desired to take V-cut in the bench
or blast to two free faces. The closed
Figure 8. Squared and staggered V2-cut chevron patterns
Figure 7. Chevron initiation patterns
chevron pattern causes the rock pile to
be concentrated in a central position, and
may provide a small bonus in terms of
fragmentation, owing to impacts between
rocks projected from opposing echelons.
Open chevron patterns yield flatter, evenly
spread rock piles, that are well suited to
front-end loaders. They also avoid the
possibility of a toe problem posed by the
tighter breaking conditions of closed
patterns, and are rather easier to connect

up in terms of visualising the correct
initiation routes. Figure 6 illustrates
the difference between a squared and
staggered V-cut chevron patterns.
If we consider a square pattern of holes
as depicted in figure 6, it is evident that
several different chevrons can be drawn
through the pattern.
Considering a single front row hole, the
chevron which intersects the nearest hole
in the next row, i.e. the hole immediately
behind, defines the “VO” chevron. If the
angle of the chevron is flattened so that
it extends through the next nearest hole,
this defines the “V1” chevron, and so on,
figure 7.
Chevrons through staggered hole
patterns are similarly defined but
look rather flatter, as in each case the
sideways distance is increased by half
the spacing. Figure 8 illustrates the
difference between a squared “V2”
chevron and a more flatter staggered
“V2”chevron pattern.
Factors which influence the timing of a
blast
Several factors which have a direct
influence on timing which should be
considered are:
• Rock properties
– Strength, Young’s modulus,
density, porosity and rock
structure
• Blast geometry
– Burden, spacing, bench height
and available free faces
• Explosives
– Characteristics, degree of
coupling and decking
• Initiation system
– Surface or in-hole delays and
type of downline
• Environmental constraints
– Air, ground vibration levels and
frequency
• The desired result
– Fragmentation, muckpile
displacement and final profile
We will develop this concept in future
editions of Explosives Today. AEL Mining
Services Explosives Engineers based at
the regional offices are available to help
and advise on this subject and the use
of electronic detonators to achieve all
controlled blasting scenarios.
References:
This document is a new addition to the
Explosives Today series. This document
replaces all previous Explosives Today
on this subject including Series 2. No 12:
June 1978
Disclaimer: Any advice and/or
recommendations given by AEL
Mining Services Limited (“AEL”) in this
publication, is given by AEL in good
faith in order to provide assistance to
the reader. AEL does not however: 1.1
warrant the correctness of its advice
and/or recommendations; 1.2 warrant
that particular results or effects will
be achieved if AEL’s advice and/or
recommendations are implemented;
1.3 accept liability for any losses or
damages that may be suffered, as a
result of a party acting, or failing to act,
on the advice and/or recommendations
given by AEL;1.4 accept liability for any
acts or omissions of its employees.
Representatives and/or agents, whether
negligent or otherwise.
Copyright: All copyright that subsists in this
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annexures contained herein, which shall include all
and/or any ideas, plans, models and/or intellectual
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contrary.

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