AIR-DECK & WATER-DECK TECHNIQUES

The Theory of Air-Decking

As long ago as 1940, Russian scientist came up with the idea that by using explosive charges spaced with air gaps the efficiency of the blast could be improved. Further research carried out in Russia during the 1970's was confirmed by work in Australia and the USA.

When a charge is detonated in a blasthole the tremendous initial pressure greatly exceeds the strength of the rock. A strong shock wave begins to propagate into the rock, crushing and breaking it into extremely small particles. A large portion of the blast energy is spent unproductively in the area nearest the charge (the so-called 'crush zone') as the pressure produced by commercial explosives are far in excess of that required to simply fracture the rock. The pulverized region can often be observed in the area immediately around a half-barrel hole in the face.

By introduction an air gap (airdeck) 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. The reduced blasthole pressure caused by the air-deck is still capable of creating an extended fracture system and there is sufficient high-pressure gas to obtain the desired amount of ground movement. The lower peak blasthole pressure reduces the loss of explosive energy associated with excessive crushing of the rock adjacent to the hole. This process adds only microseconds to the event and an observer would not notice anything different about the blast.

Creating Air-Decks

The most convenient way of creating an air-deck is to use a gas bag. The earliest types were no more than a development of a football bladder. They were simply lowered into the hole and than inflated from the surface using a small compressor or gas bottle. They could only be used near the top of the hole since they were not capable of sustaining more than the weight of stemming. The second-generation models were of the chemical type. A sachet of vinegar and some bicarbonate of soda were placed inside a sealed plastic bag. The sachet was broken, causing the two ingredients to react., producing carbon dioxide. This type is still widely used in large-diameter holes (greater than 200 mm). The third generation incorporate an aerosol within the plastic bag. Pressing the cap down on top activates a time delay of about 20s before the gas emerges. This allows sufficient time to place the bag anywhere in the hole. The bag fully inflates in a further 30s. The high-strength plastic used for the third generation one allows the bag to bear the heavy load of an explosive column for long periods of time.

 Figure 1: The Fist-Generation of Gas Bag

Figure 2: The Second-Generation of Gas Bag

(Stem Lock Gas Bag 8 - made for 3 to 16 in diameter hole)

Figure 3: The-Third Generation of Gas Bag

(Infladeck Gas Bag - made for 4 to 12.25 in diameter holes)

Air-Deck Volume

Figure 4 shows the results of some research work down in Australia. The air-deck volume is expressed as a percentage (or proportion) of the explosive volume plus the air-decked volume. In effect, this is the amount of explosives that can be removed from the blasthole and substituted with air (or water).

 Figure 1: Mean Fragment Size vs. Air Deck Volume

The graph indicates that as much as 30-40% of the explosive charge can be replaced by an air-deck before a significant deterioration in fragmentation is experienced. These results were produced from laboratory experiments and have been widely reported. Experience within the UK has confirmed that air-deck volumes of 25-30% can be employed in all rock types without noticeable loss of fragmentation.

Standard Air-Deck Techniques

The earliest application (and one still widely used in large-diameter holes) was simply to reduce the quantity of explosives in the hole by placing a gas bag at the base of the stemming. Figure 2 indicates how a UK quarry drilling 200 mm diameter holes saves 150 kg of ANFO from each blasthole. The air-deck volume is 33%.

 Figure 5: Earliest Application of Gas Bag

Table 1 illustrates the quantity of explosive that can be removed from a hole with  a 20% air-deck volume using 110 mm , 125 mm and 150 mm diameter holes 16 m deep.

Table 1: 20% Air-Deck Volume

Figure 6: Reducing Stemming Length by Application of Gas Bag

Reducing Oversize from The Stemming Area

The production of oversize material is an inevitable consequence in blasting operations. A typical blast may be produce 10-15 % of oversize material. The majority of oversize is produced from the stemming area and the rule of thumb in conventional blasting is that stemming depth should be [(0.7 till 1.0) x the burden]. Reducing it further may increase the risk of stemming ejection and flyrock. By placing a gas bag in the stemming area, allowing the explosive gasses to do useful work higher up without the risk of stemming ejection. Figure 6 illustrates such an example. The quarry has created a 5 m air-deck by stopping the explosive column at 7 m (as opposed to the normal 5 m) and by reducing the stemming from 5 m to 2 m. This represents a 28% air-deck, through which a saving of about 25 kg of ANFO has been achieved. By reducing the stemming depth the quarry has dramatically reduced the quantity of oversize produced in blast from around 10% to 2%.

Water-Deck Techniques

In order to reduce costs, most quarries try to maximize the use of ANFO, which is a relatively cheap and highly effective explosive. Where water is present it is necessary to resort to waterproof, packaged explosives.

Figure 7: Alternative 1 of The Water-Decked Volume

Some quarries prefer to place their normal waterproof base charge in the hole. This may still leave water above it. Rather than continue with a waterproof charge, Figure 8 shows how a gas bag can be placed on top of the water. There may be additional initiating costs.

 Figure 8: Alternative 2 of The Water-Decked Volume

Reduction in Maximum Instantaneous Charge

If there is a need to reduce ground vibrations from blasting operations, the first option is usually to employ multi-delay techniques. Two delays per hole cope with most situations, but as the site boundary or property are approached, more drastic measures may have to be considered. Reducing the borehole diameter and/or splitting the face are both costly and create logistical problems. The next option is to go to three delays per hole. Such an arrangement greatly increases the cost and complexity of the initiation system.

Figure 9 shows how a hole was loaded in a quarry where vibration control was essential. In order to maintain an agreed peak particle velocity (PPV), two delays per hole were used to achieve a maximum instantaneous charge (MIC) of 96 kg.

Figure 9: Application of  The Air-Decked Volume with Two Delays per Hole

The site management were considering three per hole, but decided to try air-deck techniques first. The diagram shows how they reduced the MIC from 96 kg to 68 kg. Based on data, there was no deterioration in the fragmentation or throw of the blast and they achieved a satisfactory reduction in PPV levels.

From : Quarry Management, April 1997