The gasbags we supply are manufactured by Mintech, an Australian Company, and are patented worldwide. Mintech conceived and developed the gasbags to help simplify shotfiring operations.

The gasbag is a self-inflating, high-strength balloon capable of locking off a borehole and bearing heavy loads for long periods of time. Initially the product's main use was to seal off nuisance water in boreholes to allow maximisation of ANFO in preference to water resistant explosives.

The ease of use of the product and its reliability has enabled shotfirers to reduce powder factors by using air-decks instead of solid decks of chippings. Stemming heights can also be safely minimised and explosives placed adjacent to hard bands leaving air gaps in the softer zones of the borehole.

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)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.

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.

What Size Air/Water Deck?

The standard answer is that it depends on the conditions prevailing at the site and will most certainly vary from site to site. Typically we recommend that on a 15m (50ft) face you can comfortably seal off as much as 3m (10ft) of water - and we have users who regularly do that!

The graph shows the results of some research done in Australia and is worth studying in the context of this question. 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). Put in simple terms it is the amount of explosives that can be removed from a blasthole and substituted with air or water.

As you can see from the graph 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. They have also been subsequently confirmed by field studies. Whilst acknowledging different rock types will display different characteristics the results of UK users' experiences have been broadly consistent with these findings.
The Stemliner is a plastic layflat extrusion designed to open in the borehole forming a continuos tubular lining which acts as a barrier protecting ANFO from water damage. The leading end of a Stemliner is strengthened to resist scuff damage and has a 'vee' shaped 'cuttings' pouch to allow the correct location of the Stemliner at the bottom of the dewatered vertical or angled blasthole. The open end of the Stemliner is secured at the top of the blasthole by a simple lightweight frame which holds the layflat open for easy loading with bulk ANFO.

The plastic is a combination of polythene resins co-extruded to produce a film impermeable to water and diesel with much greater tensile strength, tear and puncture resistance than that of standard low density polythene layflats. The liner is anti-static and anti-stick for saftey, loading explosives at high volume rates and for prevention of bridging and blocking.

Deploy the liner immediately after de-watering the hole. Put either chippings or a primer cartridge in the ballast pocket. Hold ends of rod in either hand and allow the liner to unroll at a controlled rate down the blasthole until the liner rests at the bottom of the hole. Control speed by applying pressure to the roll. Remove the rod and cardboard core from the liner. Pass open end of liner through centre of stand. Fold at least 200mm over the outside of the pipe and clamp the liner with the funnel to prevent it being forced down the blast hole during loading.

Load with ANFO. 'Settling' of the liner of around 100mm may occur - this is normal. After loading, remove the stand, place liner to one side of the blasthole and stem as usual.
Click here for information on dewatering pumps.