2. Learning Objective
Having worked through this lecture, the student will be
able to:
Describe the various controlled blasting techniques
3. Introduction
This second technique may or may not be cosmetically
appealing, but from stability standpoint, performs its
function.
Overbreak control methods can be broken down into
three types:
Presplitting
Trim (Cushion) blasting
Line drilling
4. Introduction
Blasting techniques have been developed to control
overbreak at excavation limits.
The operator must decide the ultimate purpose of the
control technique before selecting of the technique can be
made.
Some techniques are used to produce cosmetically
appealing final walls with little or no concern for stability
within the rock mass.
Other techniques are used to provide stability by forming a
fracture plane before any production blast is conducted.
5. Presplitting
“Pre-splitting” is a controlled (special) blasting
technique that involves simultaneously firing a line of
blastholes that are drilled close together and have a very
large burden.
The blastholes are usually drilled along the design
excavation boundary or “final limit” in a construction
project, mine, or quarry.
Simultaneous firing of the charges produces a tensile
stress in the rock, creating a planar crack between
blastholes.
6. Presplitting
Presplitting along the excavation boundary is designed
to provide an open plane of weakness that will protect
surrounding rock from the effects of production
blastholes that are fired after the presplit.
7. Presplitting
Presplit blastholes are usually charged with a relatively
small amount of explosives, compared to production
blastholes in the same rock mass.
Explosives energy is used efficiently to produce stresses
that exceed the rock’s tensile strength, but are below its
compressive strength (ie. Energy is not wasted by
crushing the rock).
Firing the pre-split charges with an excessive burden
ensures that explosion gases are forced to open up a crack
instead of displacing the rock mass.
8. Presplitting
Firing explosives that are confined by an excessive burden
always produces relatively large ground vibrations.
The PPVs from pre-split blasting can be up to 5 times the
levels produced by free face blasts with the same explosives
charge weight per delay.
Results produced by pre-splitting can vary greatly,
depending on rock mass properties.
9. Presplitting
The technique is designed to produce a uniform crack
without causing any significant damage to rock on either
side of the “split”.
However, it is possible for a pre-split blast to produce
extensive radial cracks and “open up” geological
discontinuities (eg. joints and bedding planes) if
blastholes are too close together or overcharged.
11. Presplitting
Pre-splitting can produce effective results in blasthole
diameters from 38 to 311 mm, although 75 mm is the most
common size for construction projects.
Blastholes can be charged with decoupled explosives, or
“deck charged” with small amounts of bulk explosives to
produce the required energy distribution.
12. Presplitting
Decoupled charges may also be prepared by attaching
small diameter packaged explosives such as ‘POWERGEL’
“MAGNUM” to high energy detonating cord downlines.
Pre-splitting with decoupled charges appears to be most
effective with a “decoupling ratio” of 2 to 2.5 (ie. the
blasthole diameter is about 2 to 2.5 times the charge
diameter).
14. Presplitting
If airblast must be minimised, pre-split blastholes should
be effectively stemmed (eg. with drill cuttings supported
by a plastic bag/“plug”) and all detonating cord trunklines
covered by suitable material (eg. 200 mm of sand) to
muffle noise.
A pre-split line would ideally be drilled and blasted as a
separate operation before the adjacent production
blastholes are fired.
15. Presplitting
However, due to scheduling issues, it is usually more
convenient to fire pre-split charges as the first part of the
adjacent production blast (ie. as this avoids the need to
bring drilling and charging equipment into the area
twice).
When a pre-split is fired as part of a production blast, pre-
split charges should fire about 100 to 200 ms before the
nearest production blastholes (ie. to enable the pre-split
fracture to develop properly before more powerful charges
detonate nearby).
16. Trim Blasting
Trim blasting involves firing a line of blastholes that are
drilled close together along the design excavation
boundary, after the adjacent production blast has been
excavated.
These blastholes are usually drilled with a relatively
small burden and spacing, after the production blast has
been fired (ie. To prevent them from being damaged by
any ground movement or “spalling”).
17. Trim Blasting
Trimmer blastholes are charged with low energy
explosives or decoupled charges and fired
simultaneously to remove the small burden.
“Trimmer” blastholes are usually charged and fired in a
similar way to pre-splitting, to crack the rock along a
plane between blastholes and produce a smooth rock
face with minimum damage and over-break.
18. Trim Blasting
A trim blast charge usually consists of small diameter
packaged explosives (eg. ‘POWERGEL’ “MAGNUM”)
attached to a high energy detonating cord downline, or
continuous lengths of a special product such as
“Powersplit”.
The burden should generally be about 20 blasthole
diameters, with the spacing 30% less than the burden to
assist cracking along a plane between blastholes.
19. Trim Blasting
Trim blasting tends to give better results than pre-
splitting in weak or “blocky” rock, and is usually less
expensive than pre-splitting because blasthole spacings
are larger.
A rock face can be badly damaged by production
blastholes that are fired nearby before a “trim blast” is
drilled.
Thus “buffer” blastholes that are charged with less
powerful explosives may be used to protect rock faces on
final design limits:
21. Line Drilling
Line drilling is a special technique that may be used to
minimise blast damage to the surrounding rock mass in
critical situations (eg. excavations for rock crushers or
bins).
This method involves drilling a row of parallel holes
very close together along the design excavation limit,
and leaving them uncharged.
22. Line Drilling
These open holes are intended to form a “plane of
weakness” that will cause production blastholes to
break neatly along the design excavation boundary.
The hole diameter for line drilling is usually 38 to 75
mm, with a spacing between holes of about 2 to 4 times
the diameter.
23. Line Drilling
The distance between the last row of charged blastholes
and the line-drilled holes is usually about 50 to 75% of
the normal burden distance.
If any charged blastholes are too close to the design
boundary, they may cause damage to the rock mass
beyond the line of uncharged holes.
The very close hole spacing between line-drilled holes
requires exceptional drilling accuracy to avoid holes
intersecting each other.
24. Line Drilling
This factor will generally restrict the maximum length
of line drilled holes to relatively short “lifts”.
Line-drilling is only occasionally used and then in
special applications where any other technique would
cause too much overbreak (ie. because of the time and
expense of drilling many closely spaced holes).
