AIR DECK TECHNIQUE & APPLICATION

ByDr G K Pradhan

Addl. Director, PCRA, Kolkatapradhangk@pcra.org

Introduction

The concept of using more explosive to break in any process of blasting is now irrelevant, from the point of explosive energy utilisation. Effective blasting is now key to success of any production and other areas of blasting. Several attempts have been made in the past to adopt special techniques to effectively utilize explosive energy. The first known attempts were in dimensional stone quarries using Black Powder & patented in the year 1893, followed by underground coal mine blasting. In case case of coal mining, water ampoules were used to effectively improve mine environment vis-à-vis dust generation and suppression of ignition if any.

Air-Deck Blasting Theory

In the detonation of a fully confined explosive a single high amplitude stress wave is produced which crushes the borehole wall and moves further into the surrounding rock producing a crack mechanism. During or after the stress wave propagation high temperature/pressure gases assist and extend the crack formation and produce the expansion and movement of the rock mass. The borehole pressure produced by a commercial explosive is far in excess of that required to fracture the rock.

By incorporating an air column (air-deck) (Fig.1) above or within the explosive column secondary or multiple stress waves are produced which extend the duration of their action thus increasing the extent of crack propagation.

Fig.1 : Presents Air-Deck usage in a blast hole

As make Air-Deck layer in bottom of charging borehole , increased the Action pressure 2~7times, Kinetic energy가 50~100times

[R. Frank Chiappetta (BLASTING ANALYSIS INTERNATIONAL,INC), 2004, ISEE]

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The reduced borehole pressure caused by the air-deck is still capable of creating an extended fracture system and there is sufficient high pressure gas produced to obtain the desired amount of ground movement. The lower peak borehole pressure reduces the loss of explosive energy associated with excessive crushing of the rock adjacent to the borehole. Fig.2, shows the comparison between conventional and air-deck blasting.

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Conventional VS AIR-DECK Blasting◈ Blasting mechanism compare sheet

Action pressure & Kinetic Energy Stress aspect

AIR-DECKBlasting

ConventionalBlasting

Technology

Drilling & Charging pattern

◈ Mutuality break mechanism of Air-Deck blasting holeThe concentration stress from AIR-DECK in borehole make horizontal crack below AIR-DECK layer or line crack and the shock wave from blasting hole transformation to reflected wave at line-crack surface. The reflected wave make maximization tensile fracture.

No need Sub-drilling

Fig.2, shows the comparison between conventional and air-deck blasting

What Size Air Deck?

The standard answer is that it depends on the conditions prevailing at the blast site and will most certainly vary from site to site. Typically it is recommended that on a 15m face we can comfortably insert an air gap of as much as 20% of the blast hole length i.e. 3m. This has been the practice so far. For holes in 7-15 M range the air deck length will be approx. 1 to 1.5 M.

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The air-deck volume is expressed as a percentage (or proportion) of the explosive volume plus the air-decked volume (or the total volume of the explosive charge if the hole had been fully charged). In simple terms it is the amount of explosives that can be removed from a blast hole and substituted with air or water.

How to create an Air-Deck ?

Air Deck is created by inserting a non-explosive inert shaped material which is stuck at the desired depth inside a blast hole. Chemically activated air-gap was devised for this purpose. In India chemically activated blast hole plug is manufactured by M/s Sua Explosives Ltd, where two chemicals when pressed mix to form a foaming gas, which makes the plug to stick inside the borehole at the pre-determined depth (Fig. 3 & 4).

Fig.3 : Photograph showing Air gap insertion in a blast hole

Fig. 4: Photograph showing chemically activated blast hole plug

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In view of cost of this type of plugs, efforts were made to devise other cheaper options.

Air Deck Application

(a) Flyrock control - Where due to overbreak or unfavourable geology the burden on the front row of holes has significantly reduced. In such situations AIRDECK loading begins below the reduced burden and extends upward to above the weakened zone. This results some degree of breaking and fragmentation of the burden. The fully loaded portion of the holes prevent a high bottom or excessive tow at the base of the bench where the burden is more substantial.

(b) Tunnel ove rbreak protection : For perimeter holes in tunnels one third portion of the hole loaded with explosives and leaving an air deck followed by plugging and stemming. This leave a smooth wall.

(c) Reduction of fines : Where fines generated are dumped as waste by introducing 'air deck' the crushing around the blast holes are minimised leading to less fines generation. In these holes powder factor is 15 to 20 % less than normal, 'air deck' gives the explosives reaction more time to work while contributing to increased gas volume. Fines have been reduced as much as 50 %.

(d) Blasting for Dimension Stone : By 'air deck' loading of specially prepared cartridges at the bottom of the holes and stemming with crushed stone, the dimension stones when blasted with electric or NONEL detonators result crack free surfaces.

(e) Ore-Waste Separation : Two different degree of fragmentation are obtained which helps in easily separating the ore and waste rock. Portion of the bore hole loaded with explosives and 'air deck' in zones of waste rock, coarser fragments help in segregating the ore and waste. This helps in less contamination as well as better recovery of the ore.

(f) Control of vibration : By substitution 'AIRDECK' better high wall, faster loading, improved fragmentation and controllable levels of vibration could be achieved at Block - II ( in Jharia Coalfield).

Case Study-1

At the bottom of the hole a specific quantity of explosive is loaded and a large part of it remains empty (except for the stemming). When fired the rebounding shock waves produce a cleanly split rock. This technique of pre-splitting is now quite popular. In this method a single explosive charge (cap-sensitive - primed) and a bore-hole plug (either a power plug, air bag; bore plug - chemically activated or a jute bag filled with sand_ hung from the surface at the desired depth. In India, for the first time this technique of pre splitting was utilised for blasting in a dragline bench at Black - II open cast coal mine of Jharia Coalfield In the recent years many mines have reported the application of this technique is Dongri Buzurg mine of MOIL, some limestone mines in Rajasthan. They have reported 13 % savings in explosive consumption, 14.30& improvement in loading efficiency, 40% reduction in ground vibration, 50% in over break and 45-50% in throw. While wooden blocks or other plugs have been used, the introduction of 'blast hole plugs' by M/s Sua Explosives, of inflatable device will make of the operation more simpler

Hole depth 28m, hole dia. 250mm, some holes had 1.5 to 2m. water column also. Stemming height 6m. Qty, of explosive/hole 75kgs (1:2 prime : column slurry). Burden distance from last row of the production blast B/2. spacing between holes S/2.

Plug : 50 kg jute bag filled with 35 kgs. of sand lowered to the desired depth by a nylon cord.

While conducting blasting the following precautions were taken ;

(i) All holes should be of equal depth.(ii) Burden and spacing has to be accurate.(iii) Stemming column : After placing the jute bag plug, initially

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stemming material is carefully filled. After half a meter of depth is filled, usual means of stemming done.(iv) All holes not confirming to the designed depth should be re-drilled.(v) All holes were blasted instantaneously taking 10 to 15 holes at a time.

The results were excellent, fracturing along the holes except for few holes drilled 10 to 12m., causing heavy cratering. Some holes were also charged with SMS Indogel - 614 primed with 200 gm. cast booster.

Advantages (Fig.5)

1. Improved highwall stability leading to better safety and percentage recovery.

2. Reduction of tow on highwalls.

3. 40-50% cheaper than conventional method of presplitting.

4. Best suited for 'cast blasting' in high dragline benches.

5. Reduction of ground vibration levels.

Fig 5 : Shows advantages with the use of Air Deck.

Case Study 2

Using air-gap with SME Explosive

Unlike SMS explosives, at site density of bulk SME cannot be varied. SME use can only be

viable if charging of SME is done in a manner that the overall column density is not

disturbed. With normal SME it is not possible to reduce the column density (kg/m), and

Save explosives(10~25%)■Save explosive charging space instead of Air Deck■Reduced Vibration, noise,비산(10~30%)■Save cost explosives

■Increased blast space of borehole■Produced smaller sized boulders■Reduced 2nd breaking work(20%)

Improved fragmentation(40 -50%)

■2-7 times increased Action pressure of borehole■50-100 times increased Kinetic energy

Power & Strong Blasting

■Save drilling costNo need sub-drilling ■Protected over-blasting as make AIR- DECK layer in bottom of borehole at tunnel site ■Reduced amount of explosives each blasting time

Precision Blasting

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decking has also limited scope. Based on literature study and also experience of air-deck

concept, it was suggested to incorporate an ‘air-deck’ suitably in each hole.

The ‘stem induced fracturing technique’ (Fourney et al 2006) has provided ready to apply

solution. It was suggested to provide an air gap of 0.5 to 1.0m above the explosive column.

The air gap so created has not only increased the pressure magnitude and spread out of the

duration of pressure pulse, but also increased the explosive occupancy and column height.

By introduction of an air gap, the stemming depth is uniformly maintained. In order to

maintain uniform stemming height, this came as a possible solution to experiment with

SME. SME from M/s Sua Explosive was brought in insulated tankers and charged into the

holes by a calibrated pump truck Fig. 6. Fig. 7, shows the charging of SME into blast hole.

The ease in transportation and use in the site associated with SME, is gaining popularity over

SMS explosives.

Fig. 6 : Photograph showing chemically activated blast hole plug after reaction

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Fig. 7 : SME pump truck at the experimental site

Details of Experimental Blasts with SME

Table 1, details of the experimental blast with SME is presented. In this case 1.0 m length of air deck was placed, by gas bags for the first blast, thereby maintaining an explosive column density at 0.73, although density of SME was 1.14. g/cc. Table 9.10, presents the blast details of 13 blasts which were conducted, with this method of charging and density management with SME.

Fig. 8 - Full-view of Blast No.1 (Table 1)7

Table 1 - Details of blast results bulk SME explosives with air-deck

BlastNos.

Benchno.

No. of holes

Hole Depth B x S x H Qty of

SME EFAir Gap

length

Stemming Ht.

Expl.Col.

lengthQty/m.

E.F.with air-

gap

(m) (m3) (kg) (Kcal/m3) (m) (m) (m) (t) (Kcal/m3)1* IV 60 9 5X10X9 6152 205 1.00 2.5 5.50 18.64 2052 IV 21 9.5 5X10X9.5 3234.2 292 1.00 2.75 5.75 26.78 2923 IV 48 10 5X10X10 6709.6 252 1.00 2.75 6.25 22.37 2524 IV 24 10 5X10X10 3304.8 248 1.50 2.75 5.75 23.95 2485 IV 32 10 5X10X10 4785.8 269 1.00 3 6.00 24.93 2696 I 27 8.5 5X7X8.5 3865.4 433 1.00 2.5 5.00 28.63 4337 I 62 8.5 5X7X8.5 6782.2 331 1.00 2.75 4.75 23.03 3318 I 41 9 5X7X9 5088.2 355 1.00 2.75 5.25 23.64 3559 I 42 9 5X7X9 4928.4 335 1.00 3.00 5.00 23.47 33510 I 46 8.5 5X7X8.5 5809.3 382 1.00 2.5 5.00 25.26 38211 II 57 8.75 5X10X8.75 6731.4 243 1.00 2.75 5.00 23.62 24312 II 36 8.5 5X10X8.5 4207.2 247 1.00 2.5 5.00 23.37 24713 VI(A) 48 6 5X7X6 3284.8 293 0.50 2.5 3.00 22.81 293

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