2. INTRODUCTION
All seams in underground coal mines are considered
gassy, the degree of gassiness may vary from I to III.
Methane gas that is released from during excavation
when mixed in the proportions between 5-14% forms an
explosive mixture.
In underground coal mines, there is also an additional
risk as coal dust which undergoes a dust explosion when
suitably ignited with air.
3. Methane ignition is known to take place due to following
reasons:
Direct action of shock wave from the explosive.
Indirect action of the shock wave after it has been
reflected from solid surfaces in the vicinity of explosives.
Hot gaseous products of detonation mixing with the
methane-air mixture.
Hot reacting solid particles of explosives getting
ejected in to the methane-air mixture.
4. Hence , specially designed explosives and detonators are
required for use in underground coal mines.
PERMITTED EXPLOSIVES
Only those explosives, which are included in the official list
of authorised explosives issued by the Chief Controller of
Explosives in India, may be used for blasting in mines.
Only on passing the tests in an explosive approved as a
“permitted” explosive.
In India there are 3 types of permitted explosives, each
group being designed for a particular type of operation to
give maximum safety.
5. P-1Type Permitted Explosives:These explosives can be
used for drifting or ripping and simultaneously firing in an
undercut/middlecut/overcut and in depillaring faces, in coal
seams of first degree of gassiness.
P-3Type Permitted Explosives:These are Equivalent-to-
Sheathed Explosives used for drifting or ripping and
simultaneously firing an undercut/middlecut/overcut and in
depillaring faces, in coal seams of first degree of gassiness.
P-5Type Permitted Explosives: These are special type of
explosives with a higher degree of intrinsic safety against all
types of likely hazards in delay blasting in coal and specially
designed for Solid Blasting in conjunction with non-
incendive copper short delay detonators.
6. NON PERMITTED EXPLOSIVES
For certain operations such as shaft sinking and stone
drifting, where there is no gas or coal dust present,
explosives other than permitted explosives can be used,
subject to such conditions as may be laid down by the
DGMS in specific instances.
7. DETONATORS
In conjunction with Permitted Explosives, only approved
detonators may be used for coal blasting.
Only electric detonators with copper tube are permitted
for use in coal blasting.
There are 2 types of detonators:
=> Instantaneous Copper Electric Detonators(CED)
=> Copper Non-Incendive Short Delay
Detonators(CDD)
8. PRINCIPLES OF BLASTING IN COAL
Underground coal mining operations could be divided
into two categories , namely, Development and
Depillaring.
Both for bord -and-pillar as well as for longwall mining,
driving galleries through coal seams are done. there are
two techniques employed for blasting in development
galleries.
=>Blasting on pre-cut face
=>Solid blasting of coal
9. BLASTING ON A PRE-CUT FACE
In many mines coal-cutting machines are used to provide
an additional free face for blasting.While this may be
under , over, middle or side cut, the most common practise
in Indian mines is to have undercut face. For blasting on
cut faces the following conditions must be compiled with:
The length of the shot hole should be at least 15cm (6
inches) less than the depth of the cut.
The detonators in a round should be fired
simultaneously , preferably in series.
The maximum charge in hole should not exceed 800gms
for PI explosives and 1000gms for P3 and P5 explosives.
10. SOLID BLASTING OF COAL(BOS)
Both in Longwall and development faces coal can
beblasted without giving an undercut by a coal cutting
machine.
The technique of blasting of the solid is simpler, more
economical and less hazardous than conventional method
of breaking coal with machine cut and blasting.
Only P5 Permitted Explosives (Solarcoal-5) and ‘Non-
incendive” copper short delay detonators shall be used.
11. CODE OF GOOD PRACTISES IN
UNDERGROUND BLASTING
Velocities of Air Current (DGMS stipulations):
The velocity of air current measured in metres per
minutes at the place shown in column (2) shall not be less
than that shown in column(3) for the different seams
shown in column(1) of the table
12. Degree of
Gassiness
Place where the velocity of air is to be
measured
Velocity
of air
First, Second or Third
Degree
Intermediate out by ventilation connection from the
face 30
First, second degree
(i)4.5 mts from any face whether working or
discontinued 30
on the take side of the brattice or partition
(ii)7.5 mts out by the discharge end of an air pipe 15
(iii) At the maximum span of a Longwall face 60
Third Degree
(i)4.5 mts from any face whether working or
discontinued 45
on the intake side of the brattice or partition
(ii)7.5 mts out by the discharge end of an air pipe 25
(iii)At the maximum span of a Longwall face 75
