1. TECHNIQUES OF CONTROLLED BLASTING TO
IMPROVE OVERALL ENVIRONMENTAL &
SAFETY STANDARD
***
MEANS TO MITIGATE ADVERSE IMPACT OF
BLASTING IN OPEN PITS, QUARRIES, TUNNELS,
UG METAL MINES AND CONSTRUCTION
WORKINGS
Partha Das Sharma (sharmapd1@gmail.com), website: http://miningandblasting.wordpress.com/
2. In both the mining and construction
industries, blasting is the predominant method
for fragmentation of consolidated mineral
deposits and rocks.
The public relation problems of users of
explosives have increased greatly in the past
few years as explosives are being consumed in
increasing quantities.
Increased population and spread of
urbanization near to the construction & mining
sites have affected more people by blasting.
2
3. Controlled blasting methods are used to control
adverse impacts such as:
Over-break
Reduce ground vibrations
Reduce fractures within remaining rock walls
Reduce noise
Reduce dilution / waste of ore etc.
3
4. Following are the techniques of controlled
blasting:
Line drilling,
Trim (Cushion) blasting,
Smooth (contour or perimeter) blasting,
Pre-splitting,
Selecting and employing various parameters
of blast design, using modern technology,
Precise and accurate timing delays,
Muffle blasting at critical and congested areas.
4
5. Line drilling:
This system involves a single row of closely spaced uncharged holes
along the neat excavation line.
This provides a plane of weakness to which the primary blast can
break.
It also causes some of the shock waves generated by the blast to be
reflected, which reduces shattering and stressing in the finished wall of
the host rock.
Line drill holes are generally percussive hammer holes having spaced
two to four times the hole diameter, drilled along the excavation line.
The blast holes directly adjacent to the line drill holes (buffer holes)
are generally loaded lighter (about 50% of primary holes) and are closely
spaced (about 50 to 75 %) than primary holes.
This technique gives maximum protection to the host rock to
preserve its original strength.
The disadvantage of this system is high drilling cost due to closed
spacing and results are often unsatisfactory because of poor hole
alignment. 5
6. Trim (Cushion) blasting :
Like line drilling trim or cushion blasting involves a single
row of holes along the specified final excavation line.
This technique generally uses 2 to 4 inch diameter holes.
Holes are loaded with light charge, well-distributed,
completely stemmed and fired after the main excavation is
removed.
By firing the trim holes with minimum or no delay between
holes, the detonation tend to shear the rock web between
holes and give a smooth wall with minimum over-break.
It is better to put trim holes just before removing the final
berm.
As compare to line drilling technique trim or cushion
blasting is simpler and economical as increased hole spacing
are used. 6
7. Smooth (contour or perimeter) blasting:
A technique used mostly in underground blasting, closely spaced
drill holes are loaded with decoupled charges and fired
simultaneously to produce an excavation contour without fracturing
or damaging the rock behind or adjacent to the blasted face.
For promoting safety and economy in underground workings,
performance of blasting in headings, drivages, tunnels and stopes
becomes very important factor.
An ideal blast results in a minimum of damage to the host rock
with minimum of over-break.
Perimeter or contour holes are drilled along specified final
excavation limits and are lightly loaded than that of buffer holes and
production holes.
The spacing is kept closer than buffer holes and production holes.
As a thumb rule 10 to 12 times hole diameter in medium to tough
rock and 5 to 6 times hole diameter in poor, fragmented rock are kept
as spacing. 7
8. Example: Blast damage and overbreak in Tunneling
Results of Controlled Blast: Smooth walls, Minimal fractures, Minimal overbreak,
8
Less support required, Better economy & safety.
10. Pre-splitting:
Pre-splitting is the smooth blasting method in which cracks for the
final contour are created by blasting prior to the drilling of the rest of
the holes for the blast pattern.
This is an effective way of restricting back-break and ground
vibration in large open pit, quarry blasting.
Pre-splitting helps in isolating blasting area from remaining rock
mass by creating an artificial discontinuity along the final designed
excavation line / plane against which subsequent main blast breaks.
A row of holes are drilled at the periphery (three sides) of the main
blasting block at a closer spacing, charged with lesser quantity of
explosives than the production blast and blasted prior to the main
blast in an effort to create a fractured line and a reflective plane at
the excavation limit or plane.
Some of the shock waves from subsequent main blast are
reflected at the pre-split plane which results in arresting a
considerable portion of blast induced ground vibration generated in
the main blast to propagate. 10
13. Muffle blasting:
Fly-rock is another important adverse impact of
blasting operations, specially, when conducted in the
vicinity of dense human habitation / congested areas.
Muffling or covering of blast holes properly before
blasting, is the common solution to prevent fly-rock from
damaging human habitants and structures.
Generally, mat or mesh (40 mm x 40 mm size) made of
preferably of locally available steel ropes (5 to 6 mm) are
used for muffling purpose. Sand bags weighing 40 to 50
kg are kept over the mesh at an interval of 3 m.
Efficiency of arresting of fly-rock depends mainly on
the quality of muffling system implemented.
13
14. Using millisecond delay sequence and use of in-hole delays in
decks:
Delay blasting (with millisecond delays) permits to divide the
shot into smaller charges, which are detonated in a
predetermined millisecond sequence at specific time intervals.
Millisecond delay initiation of the explosive charge is a
technique used in most open pit, quarry, tunnel and underground
rock blasting operations.
It serves to enhance fragmentation and direct rock movement
for increasing productivity.
The major advantages of delay blasting are: (i) Improved
fragmentation, (ii) Reduction of ground vibrations and air blast,
(iii) Reduction of over-break and fly-rock, (iv) Improved
productivity and lower cost.
Charge weight per delay is the most important parameter for
controlling blast induced ground vibration and air-blast.
14
18. ‘Signature-Hole’ Blast Analysis for Vibration Control by using
Accurate Delay Timing Electronic Detonator System:
Structural response to blast-induced ground vibration is a
phenomenon that has been analyzed for many years.
Residential structure’s level of response to blast induced ground
vibration is dependent on both the peak particle velocity and the
frequency of the waveform.
Researchers have shown that, above ground structures resonate
whenever they are excited by a vibration containing adequate energy
matching the fundamental frequency of the structure.
The value of this frequency is mainly dependent upon the mass,
height and stiffness of the structure.
The maximum response of a house to blast induced ground
vibration occurs whenever the frequency of the ground vibration
matches the natural resonant frequency of the house.
When little or no energy at the resonant frequency of the
structure, the structural response to the vibration will be negligible.
18
19. A method of controlling blast vibrations other than by modifying
the scaled distance came into use some time ago.
The crucial point of the methodology is the use of a pilot-blast
signal which takes account of the seismic properties of all complex
geology between the blast and the target locations.
Therefore, it does not require any geological model or
assumption.
The delay interval between blast-holes can be chosen to control
and minimize the vibration energy within the structural response
band of most houses.
Studies had indicated that blast vibration could be simulated by
detonating a “Signature Hole” with the vibration monitored at critical
locations, and then using a computer to superpose the waveforms
with varying delays.
By choosing delay times (∆t) that create ‘destructive interference’
at frequencies that are favored by the local geology, the “ringing”
vibration that excites structural elements in structures, houses and
annoys neighbors could be reduced. 19
21. In this method, accurate delay times are crucial to
effective vibration control, scatter in the firing times
limited the method severely.
Electronic detonators have scatter less than a
millisecond.
In light of all these, researchers have started finding
both limitations and new potential of this new technique
of controlling blast vibration.
21
22. In other words, “Signature Hole Analysis” is a modeling
technique, to help control adverse effects of blast induced
vibrations.
The process involves controlling the frequency content by
adjusting delay times within a blast containing several
explosive charges.
The risk to adjacent structures is thereby mitigated.
Thus, with the growing adoption rate of electronic initiation
systems as a tool to control nuisance of vibrations, the
modeling techniques are becoming more popular.
M/s Instantel has recently developed and launched a
‘Signature Hole Analysis’ software tool, which allows users to
simulate a large number of charge delay times very quickly.
The software, in fact, is a modeling technique used to help
predict and control blast induced vibrations. 22
23. Advantages of the technique, ‘Signature Hole Blast Analysis’, for
Vibration Control:
This technique provides optimum electronic timing while
maintaining high level of production with efficiency by raising
quantity of explosives per delay (kg/delay) and provide overall
structural safety of blast surroundings.
Blast with shorted duration results in mitigating effects of blast
induced vibration.
Therefore, as post-blast vibrations are reduced by raising
frequencies, much larger blasts can be undertaken with better
operational performance, without compromising stringent safety
standards of environment.
Thus, Signature Hole Analysis software tool available can be
used to help optimize and improve overall operational efficiency.
It has also been observed that this vibration control method is
feasible for underground mining ring blasts as well.
23
24. By adopting these precautions
The ground vibration is restricted to ease
the public relation problem,
The mines’ / construction’s techno-
economics are improved,
Preservation of host rock strength and
safety standard are improved to a
considerable level
24
25. As far as possible modern
techniques and equipments are also
to be used in order to mitigate the
adverse blast effects
Regular monitoring is needed
Training to the blasting
supervisors on controlled blasting
techniques is very much essential.
25
