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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