application of air-deck technique in surface blasting

Application of Air-deck in surface blasting

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Author: Partha Das Sharma, B.Tech(Hons.) in Mining Engineering;

E.mail: [email protected] ; Blog/Website: http://miningandblasting.wordpress.com/

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Application of Air-Deck Technique in Surface Blasting


E.mail: [email protected];

Blog/Website: http://miningandblasting.wordpress.com/

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1. Introduction – One of the basic mine operations for quarries is blasting. Primarily, the

blasting helps breaking the earth materials into fragments before fed into aggregate

plants. The explosives used in rock blasting can be commonly, classified, into two types;

primer and blasting agent. Primer, a high explosive, is placed at the bottom of blast holes

along with its ignition system. A calculated amount of the blasting agent is then filled

into the hole. The commonly used blasting agent in the mining industry is ammonium-

nitrate pill and fuel oil mixture (ANFO). Each blast hole is loaded with these explosives.

Stemming such as rock cutting, the small rock chips from drilling, is filled into the holes.

Normally in rock blasting, a number of blast holes are drilled and placed systematically.

An example of blast hole pattern is shown.


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The distance between the first row of the blast holes and free face or between the rows is

called burden (B). The distance between two adjacent blast holes in the same row is

called spacing (S). The burden and spacing greatly affect the rock fragmentation and

fragment sizes. If the burden and spacing are large, the rock fragment size trends to be

large. If stemming is too short, a large number of fly rocks are likely to occur, and hence

decrease the blasting efficiency. For a proper stemming, the blasting efficiency can be

improved and diminish the air blast that impacts the environments.

To reduce the environmental impacts and maintain the desirable fragment sizes, the air

deck blasting has been trialed and applied during the several last decades. Air decking,

basically, is an empty space in a blast hole. It could be located at the bottom, middle or

top of the charge column. Figure shows the location of air deck.

Originally air-deck was used in surface or OC blasting as a means to distribute the

explosives energy more evenly throughout the rock mass being blasted. This system was

critical for reduction of shattering in specially in dimensional stone blasting, yet was also


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recognized as a means to "stretch the explosive energy" in conventional OC blasting.

Initially, this system was restricted primarily to small diameter holes using low

explosives such as black blasting powder etc., in dimensional stone work; but today,

application of this system has gone much beyond dimensional stone quarries.

Now air decking system is being used successfully to allow reductions from 10% to 30%

in total explosives required for production blasting. With the increased pressure on

operators in large volume operations to reduce blasting costs, potential savings of this

magnitude cannot be ignored. Other common applications of air-decking are for wall

control, rip-rap production, to reduce vibration/overpressure and to create a barrier to

groundwater using pre-splitting techniques. In all these cases, air decking can result in

substantial savings and improved efficiencies over conventional methods.

2. Modality in using Air-deck system in OC blasts: In the blast holes, air decks create

empty spaces which provide additional free faces. The additional free faces are necessary

for shock wave during the explosion. The free faces in the blast holes induce additional

reflecting shock waves that help fracturing the rock. The benefits from applying air deck


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blasting obviously are the blast efficiency improvement and the charge length reduction.

The shorten charge length saves the explosives, and hence reduces environmental

impacts, such as ground vibrations and air blast. These are discussed in next paragraphs

below:

Theory of Air Deck Blasting

The Russian Mel'nikov proposed the original air deck or air column blasting theory.

Detonation of a typical, full column confined charge produces a single high amplitude stress

wave which crushes the borehole wall and moves out into the surrounding rock producing a crack

mechanism. In conjunction with the stress wave, high temperature gases assist in extending the

crack formation and moving the rock mass.

By incorporation an air gap (air deck) above or within the explosive column, shock wave

reflections within the hole produce a secondary stress wave. This wave extends the crack

formation before gas pressurization. In fact, repeated oscillation of shock waves within the air gap

increases the time over which it acts on the surrounding rock mass by a factor at between 2 and 5.

The ultimate effect lies in increasing the crack network in the surrounding rock and reducing the

burden movement.

The reduced borehole pressure caused by the air column reduces excessive crushing of the rock

adjacent to the borehole wall but still is capable of extending the crack formation and moving the

rock out away from the face.

Tests run by Moxon, et al, have shown that substantial air deck volumes can be used before there

is any reduction in fragmentation. The chart on this page is from his paper showing the air deck

volume versus the change in fragmentation.

As you can see, you can save as much as 35% of your explosives without much change in

fragmentation.


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(a) In Production blast-holes - In designing the burden for production blast-holes,

blasting engineer is restricted to volume or weight of the ground the hole can pull at the

toe by the explosives energy used. This is the most heavily constricted area of the hole

and proper fragmentation and movement in this zone is the most critical. At the top (or

"collar") of the hole, there is much less vertical constriction, so it follows that

considerably less explosive energy is required to fragment and move the material at the

top of the hole than at the bottom. Common practice to compensate for this is to use a

lower energy/density explosive in the upper region of the hole.

Air decking offers an economical and energy efficient alternative to this practice. The

physical steps in creating an air-deck are:

* Lower the design top of the explosive column

* Set a barricade to the stemming material at some point above the original top of column

location

* Stem from this barricade to the top of the hole, as usual

The explosive gases will be able to expand into the air deck created above the column

and exert a reduced but prolonged stress in the collar zone of the hole. This can allow

significant reductions in the total explosives loads in production holes without significant

loss in either fragmentation or movement of the collar zone.

The most common design criteria used to estimate an air deck volume is a percentage of

the total explosive column length. When working in a fairly tight range of diameters and

hole lengths, this approach works quite well. This method has the added advantage of

automatically giving the operator an accurate estimate of how much will be saved in

explosives costs. The new stemming length is usually started at 75% of the original - but

this is dependant on the degree of risk for stemming ejection or fly-rock. Gradual

reductions in stemming length will determine the minimum allowable, and this can often

be as little as half of the original stemming length provided good stemming material is

used. Moreover, controlled field tests are required to stabilize the system for exceptional

conditions such as, rock or explosives types, holes diameters or depths. It is

recommended that these tests begin with the minimum amount of stemming and

explosives removed, with gradual increases in each until the maximums that can be

removed are determined.

(b) For Pre-split or Pre-shearing holes – Pre-shearing (or ”Pre-splitting”) is generally

an expensive process, both from a drilling and from a blasting perspective. Drill factors

can be as low as 15% of normal production which in turn means that the drilling cost per

ton is increased over six-fold. In addition, there is a low yield of rock per man-hour

loading when compared to normal production holes. In the loading process, high cost,

specialty explosives are often used, or labour intensive preparations of string charges are

involved.

Air decking with bulk explosives offers two methods of reducing the impact of lower

drill factors and high explosives costs.


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* Air deck pre-shearing is conducive to large diameter holes - normal production

diameters can be used, usually at much wider spacings than small diameter holes. This, in

effect, reduces the impact of lower drill factors on drilling and loading costs.

* Bulk explosives are considerably less expensive to purchase and load than specialty or

hand-prepared string charges.

(c) Controlling blast induced Ground Vibration - When vibration problems are the

result of charge per delay, the simple expedient of introducing air decks in the production

holes can help. The resultant reductions in vibration levels are primarily due to the simple

reduction of charge in each hole. There is some evidence to support the claim that the air

deck also acts as an "accumulator", trapping previously wasted energy in the collar zone

and converting it into useful work. This may further reduce vibration levels in the near

field.

(d) Over-pressure reduction - Over-pressure due to stemming ejection can also be

reduced through the use of air decks. In addition to effectively lowering the top of the

explosive column, air decks can act as stemming enhancers.

A second major cause of overpressure is the transmission of the explosive shock wave

from the rock surface into the atmosphere. Efficient pattern design and implementation,

coupled with proper delay sequencing, can go a long way to reducing the magnitude of

the shock wave introduced into the atmosphere, but can't eliminate it. Air decking, by the

simple expedient of reducing the total explosive amount, can reduce it even further.

In fact, in design, two surfaces must be considered - the vertical surface at the front of the

shot (normally called the free face) and the horizontal surface on the top. Shock waves

from the vertical face are least affected by air-decking, since the air deck itself comprises

a relatively small portion of the vertical section of the blast. The shock wave off the

horizontal face is most affected by air decking, where the deck (i) reduces the total

explosive amount, and (ii) increases the distance the shock wave must travel through the

rock before it reaches the surface.

A third mechanism in over-pressure reduction could be the "accumulator" effect, as

discussed previously.

3. Conclusions - As in any other technique, there will naturally be situations where air

decking will not be feasible - either due to economics, convenience or geology. In

summary, the potential benefits of air decking include:

* Improved fragmentation in the collar zone of the blast

* A faster loading cycle

* Reduced crushed rock requirements for stemming material

* Reduced vibration and overpressure levels

* Improved costs and production in wall control drilling and blasting

* A savings in overall explosives costs


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References:

* Chiappetta, R. Frank, Mammele, M. E., Analystical high speed photography to evaluate air

decks, stemming retention and gross motion studies, First High Tech Seminar State of the Art

Blasting Technology Instrumentation and Explosives Applications, Orlando, Florida, 1989

* Davids, T. and Botha, B.J. J., The application of mid-column airdecks in full scale production

blasts, Fifth High Tech Seminar State of the Art Blasting Technology Instrumentation and

Explosives Applications, New Orleans, Louisiana, 1994

* Moxon, N. T., Mead, D., Danell, R. E., Richardson, S. B., The use of airdecks in production

blasting, Proceedings of the Nineteenth Annual Conference on Explosives and Blasting

Technique, International Society of Explosives Engineers, San Diego, California, 1993

* Theory of Air Deck Blasting: http://www.stemlock.com/airdeck.html

* Gamma, C.D. & Jimeno, C.L., 1993. Rock fragmentation control for blasting cost

minimization and environment impact abatement. Rock Fragmentation by Blasting.

Rossmanith (Ed.). Balkema. pp. 273.

* Liu, L. & Katsabanis, P.D., 1996. Numerical modeling of the effects of air

decking/decoupling in production and controlled blasting. Rock Fragmentation by

Blasting. Mohanty (Ed.). Balkema. pp. 319-320.

* Mead, D.J., Moxon, N.T., Danell, R.E. & Richardson, S.B., 1993. The use of air deck in

production blasting. In Proc. 4th Int. Conf. on Rock Fragmentation by Blasting.

Vienna, Austria. pp. 437.

* Partha Das Sharma; Techniques of Controlled Blasting:

http://www.docstoc.com/docs/10641987/Techniques-of-Controlled-Blasting

----------------------------------------------------------------------------------------------------- Author’s Bio-data:

Partha Das Sharma is Graduate (B.Tech – Hons.) in Mining Engineering from IIT, Kharagpur,

India (1979) and was associated with number of mining and explosives organizations, namely

MOIL, BALCO, Century Cement, Anil Chemicals, VBC Industries, Mah. Explosives etc., before

joining the present organization, Solar Group of Explosives Industries at Nagpur (India), few

years ago.

Author has presented number of technical papers in many of the seminars and journals on varied

topics like Overburden side casting by blasting, Blast induced Ground Vibration and its control,

Tunnel blasting, Drilling & blasting in metalliferous underground mines, Controlled blasting

techniques, Development of Non-primary explosive detonators (NPED), Hot hole blasting,

Signature hole blast analysis with Electronic detonator etc.

Author’s Published Book: "Acid mine drainage (AMD) and It's control", Lambert

Academic Publishing, Germany (ISBN 978-3-8383-5522-1).

Currently, author has following useful blogs on Web:

• http://miningandblasting.wordpress.com/ • http://saferenvironment.wordpress.com

• http://www.environmentengineering.blogspot.com

• www.coalandfuel.blogspot.com

Author can be contacted at E-mail: [email protected], [email protected],

------------------------------------------------------------------------------------------------------------------- Disclaimer: Views expressed in the article are solely of the author’s own and do not necessarily

belong to any of the Company.

application of air-deck technique in surface blasting

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