Ingeniero

UNDERGROUND BLASTING
TUNNELING

There are two reasons to go underground and excavate:
i.

to use the excavated space, e.g. for storage, transport etc.

ii. to use the excavated material, e.g. mining and quarrying
operations.
In both cases tunneling forms an integral part of the entire
operation.
The main difference between tunnel blasting and bench
blasting is that tunnel blasting isdone towards one free surface
while bench blasting is done towards two or more free surface.

Various drilling patterns have been developed for blasting solid
rock faces, such as:
i.

wedge cut or V cut

ii. pyramid or diamond cut
iii. drag cut
iv. fan cut
v. burn cut
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Wedge cut
Blasthole are drilled at an angle to the face in a uniform wedge
formation so that the axisof symmetry is at the centre line of
the face.
The cut displaces a wedge of rock out of the face in the initial
blast and this wedge is widened to the full width of the drift in
subsequent blasts, each blast being fired with detonators of
suitable delay time.
The apex angle is as near as possible to 600 (Figure 1)

This type of cut is particularly suited to large size drifts, which
havewell laminated or fissured rocks. Hole placement should
be carefully preplanned and the alignment of each hole should
be accurately drilled.

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Figure 1: Wedge cut (after ICI)

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Pyramid or diamond cut
The pyramid or diamond cut is a variation of the wedge cut
where the blastholes for the initial cavity may have a line of
symmetry along horizontal axis as well asthe vertical axis
(Figure 2).

Figure 2: Pyramid or diamond cut (after ICI)

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Drag cut
The drag cut is particularly suitable in small sectional drifts
where a pull of up to 1 m is very useful (Figure 3).

Figure 3: Drag cut

Fan cut
The fan cut is one-half of a wedge cut and is applicable mainly
where only one machine is employed in a narrow drive.
Generally the depth ofpull obtainable is limited to 1.5 m
(Figure 4).

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Figure 4: Fan cut

Burn cut
A series of parallel holes are drilled closely spaced at right
angles to the face. One hole or more at the centre of the face
are uncharged. This is called the burn cut (Figure 5).
The uncharged holes are often of larger diameter than the
charged holes and form zones of weakness that assist theadjacent charged holes in breaking out the ground.
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Since all holes are at right angles to the face, hole placement
and alignment are easier than in other types of cuts. The burn
cut is particularly suitable for use in massive rock such as
granite, basalt etc.

Figure 5: Burn cut (after ICI)

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Sequence of detonation
For both fragmentation and throw, blastingefficiency depends
on the delay sequence of blasthole detonation.

Delayed

detonation improves loadability of the entire cut, contributes to
a

better

strata

control

and

reduction

of

blast-induced

vibrations.

Figure 6: Bunch blasting with Nonel tubes and detonating cord.

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Figure 7: Firing of tunnel round with Nonel (after Olofsson)

Figure 8: Firingsequence for tunnel numerical order.

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SHAFT SINKING
In mining, shafts form a system of vertically or inclined
passageways, which are used for transportation of ore, refill,
personnel, equipment, air, electricity, ventilation etc.

In

quarrying, glory holes for transportation of materials such as at
Perak Hanjoong.
An important requirement in shaft sinking is to provideoptimum fragmentation of the rock so that it can be cleared
quickly from the congested shaft-face area.

Blasting operation is carried out against gravity, and the scatter
of the broken rock is confined in the shaft. It is common to
use generous distribution of explosives throughout the rock
using a large number of small diameter (35 – 42 mm)
shotholes.
The number of holes N required for...