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Presplitting
This is the most successful and widely adopted controlled blasting method
and creates a plane of shear on the desired line of break, exposing the half
barrel of the blasthole after excavation &figure !'.
$igure !% ( presplit highwall.
The holes are usually ) mm to #)) mm in diameter in civil engineering
applications whereas larger diameter holes &sometimes more than *)) mm
in diameter' are proving to be successful in surface mining operations. The
spacing between holes varies between ten to twenty times the hole
diameter. The presplit shots transmit compressive shock waves, which, at
their point of meeting between the boles, create a +one of tension, which
fractures and shears the rock. This method has limited application in
underground work.
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$igure *% tress conditions in presplitting.
$igure "% -ecoupled charges for presplitting.
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Perimeter blasting
This type of blasting is generally used for underground blasting. owever
in surface blasting the same technique is known as /smooth blasting0. In
underground operations the perimeter holes of the backs &roof' of headings
and tunnels are drilled along the design profile parallel to the direction of
the excacation. 1enerally the spacing between the final lines of holes is
less than #. times the burden
$igure % 2rack +one from blasting with
conventional explosives.Figure 6: Crack zone from perimeter
blsting with Gurit 17X 500 mm small
!iameter light e"plosi#es with low $%&'
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$igure 7% 3erimeter blasting arrangement
In surface mining the decoupled perimeter holes are drilled on a closer
pattern than the production blastholes. These holes are detonated last, in
order to maximise the relief of burden. This reduces overbreak. 4nlike
normal blasting underground, the spacing between holes in the same row
for surface excavation is less than the burden. The usual relation is 5
).6.
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$igure 6% 3rinciple of smooth blasting in surface excavation.
Cushion blasting
2ushion blasting is applicable in surface mining where the ob8ect is to trim
the excess material from the final high wall to improve stability. ( single
row of holes is drilled along the perimeter of the excavation. The si+e ofthe drillholes varies between ) mm to #9" mm. 2ushion blastholes are
charged with small, well disributed charges in completely stemmed holes,
which are fired after the main blast is excavated. The charges are fired
with no delay, or minimum delay between boles.
$igure :% 2ushion blasting
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