controlled blasting

1. mtmy 1
CONTROLLED BLASTING
In tunneling, road and railroad cuts, it is of the utmost
importance that he remaining rock is of high quality in order to
avoid rockfall, rockslides and excessive maintenance work.
Line drilling
Line drilling is the earliest controlled blasting method used.
The purpose of line drilling is to create a plane of weakness by
drilling closely spaced, small diameter holes along the
perimeter of the excavation to which the blast can break.
Line drill holes are usually not over 75 mm in diameter and the
spacing is 2 to 4 times the diameter of the hole.
The hole depth should not be more than 12 m, since deviation
in longer holes may produce adverse results.
These holes are not charged.

2. mtmy 2
Figure 1: Line drilling.
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 2).

3. mtmy 3
Figure 2: A presplit highwall.
The holes are usually 50 mm to 100 mm in diameter in civil
engineering applications whereas larger diameter holes
(sometimes more than 300 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 zone of
tension, which fractures and shears the rock. This method has
limited application in underground work.

4. mtmy 4
Figure 3: Stress conditions in presplitting.
Figure 4: Decoupled charges for presplitting.

5. mtmy 5
Perimeter blasting
This type of blasting is generally used for underground
blasting. However in surface blasting the same technique is
known as ‘smooth blasting’.
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.
Generally the spacing between the final lines of holes is less
than 1.5 times the burden
Figure 5: Crack zone from blasting with
conventional explosives.
Figure 6: Crack zone from perimeter
blsting with Gurit 17X 500 mm (small
diameter light explosives with low VOD)

6. mtmy 6
Figure 7: Perimeter 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.
Unlike normal blasting underground, the spacing S between
holes in the same row for surface excavation is less than the
burden. The usual relation is S = 0.8B.

7. mtmy 7
Figure 8: Principle of smooth blasting in surface excavation.
Cushion blasting
Cushion blasting is applicable in surface mining where the
object is to trim the excess material from the final high wall to
improve stability.
A single row of holes is drilled along the perimeter of the
excavation. The size of the drillholes varies between 50 mm to
164 mm.

8. mtmy 8
Cushion 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.
Figure 9: Cushion blasting