BACKGROUND OF THE INVENTION In the blasting of rock using drilling and blasting techniques, large amounts of explosives are used to economically condition rock prior to removal from an excavation. The techniques are conducted by drilling blast holes at defined distances spaced in each of three dimensions. These blast holes are drilled to a pre-determined depth/length and then filled with an appropriate explosive material. The explosive is then detonated, conditioning the rock ready for cost-effective excavation.

Currently, the most popular explosive is Ammonium Nitrate Fuel Oil (commonly known as ANFO). ANFO is very popular due to its ease of manufacture, detonation characteristics (high heave potential), and safety aspects including its low sensitivity to impact, heat and friction. Further, ANFO is relatively inexpensive, thus, making it highly desirable.

However, the major disadvantage of ANFO is its lack of water resistance and its solubility in water. ANFO's lack of water resistance causes major problems such as generation of noxious fumes, poor energy output and subsequent poor blasting results. Often blast holes contain water that originates as surface runoff or ground water. Therefore, to enable ANFO to be used in these wet blast holes, the effect of water must be neutralised.

Loading ANFO into a blast hole that contains water can cause an inefficient and/or incomplete explosion or even a total failure of detonation. Previous methods used to overcome wet blast holes include pumping water from the hole prior to loading with explosives. In most cases, however, the water re-enters the hole between de-watering and loading of the explosive.

Cartridged water resistant explosives that prevent degradation of the explosive are often used. A number of these cartridges are loaded into a blast hole depending on the depth of the blast hole. To ensure that the cartridges do not become lodged part-way down the blast hole, the cartridges must be significantly smaller in diameter than the blast hole. This causes a de-coupled explosive charge column which at detonation does not allow efficient transmission of the explosive energy to the rock and consequently does not produce a desired blast result. Also, debris often becomes lodged between cartridges. This may lead to incomplete detonation of the explosive cartridges. Further, the cartridged water resistant explosives are very expensive and also labour intensive to handle and to load into blast holes.

Alternatively, waterproof liners are used and are placed in blast holes. ANFO is loaded into these liners to prevent water contacting the ANFO. To ensure that the liners do not become lodged part-way down the blast hole, the liners must be significantly smaller in diameter than the blast hole. This causes a de-coupled explosive charge column which at detonation does not allow efficient transmission of the explosive energy to

the rock and consequently does not produce a desired blast result. Also, these liners are often split when introducing the liner into the blast hole, allowing water into the liner and to contact the ANFO. Further, these liners add to the expense of the explosives and are very labour intensive to install into the blast hole.

In more recent times, bulk water gel and emulsion products, containing a minority proportion of ANFO, have been developed to obtain higher water resistance than ANFO alone. Such explosives provide varying degrees of water resistance compared to ANFO. Unfortunately, such products must be loaded into blast holes using complex equipment and are very costly to prepare and use.

OBJECT OF THE INVENTION It is therefore an object of this invention to overcome at least one or more of the above disadvantages or to provide the consumer with a useful or commercial choice.

SUMMARY OF THE INVENTION Accordingly, the invention resides in a method of charging a blast hole, including: inserting a pre-determined amount of a highly absorbent material into the blast hole ; and inserting a pre-determined amount of an explosive into the blast hole.

The absorbent material may be any material that absorbs water. Preferably, when the absorbent material is introduced into the blast

hole, it will absorb water without substantially increasing the total height of the water within the blast hole. Accordingly, the absorbent material is usually a super-absorbent material. Preferably the super-absorbent material is a super-absorbent polymer.

Typically, the super-absorbent polymer is chosen from one of three classes, namely starch graft co-polymers, cross-linked carboxymethyl cellulose derivatives and modified hydrophilic polyacrylates. Preferably, acrylic polymers are used.

The super-absorbent polymer may absorb over 40 times its own weight when placed in water. More preferably, the super-absorbent polymer will absorb over 500 times its own weight when placed in water. The super-absorbent polymers are typically in a powdered, granular or flake form but may be in a fibrous form.

The super-absorbent material may be introduced into the blast hole separate from the explosive. Alternatively, the super-absorbent polymer may be blended with the explosive.

When the super-absorbent material is introduced into the blast hole separate from the explosive, the super-absorbent material may be poured freely into the blast hole.

Alternately, the absorbent material may be introduced into the blast hole by first locating the super-absorbent material into a water permeable container. The container may be a flexible bag. The flexible bag may be designed to float in water. The flexible bag may be designed to release the super-absorbent material when it contacts the water. This may

be achieved by constructing the bag out of a material that dissolves in water.

The bag may be overfilled with super-absorbent polymer such that when the super-absorbent polymer absorbs a predetermined amount of water, the super-absorbent polymer swells to such that the bag is caused to break releasing the super-absorbent polymer into the water.

Still, alternately, the flexible bag may be designed to float and be made of material that stretchable. The absorbent material, located with the flexible bag, may expand when the bag contacts the water. The expansion of the super-absorbent polymer may cause the flexible bag to form a deck within the blast hole onto which the explosive is located.

Still alternately, the container may be removed from the blast hole when the water has been absorbed.

The explosive material may be of any suitable explosive.

Usually the explosive is a particulate explosive. Normally, the explosive is non-water resistant. Preferably, the explosive is Ammonium Nitrate Fuel Oil.

Alternately, the explosive is derived from Ammonium Nitrate and Fuel Oil.

In another form, the invention resides in a charge for use in a blast hole, the charge comprising: a predetermined amount of explosive ; and a predetermined amount of absorbent material.

The explosive may be mixed prior to locating the charge within the blast hole.

The explosive and the absorbent material may be the same as described previously.

In another aspect, the invention resides in a device for preparing a blast hole to be loaded with explosive, the device comprising: a container pervious to water; and an absorbent material located within the container.

The container and the absorbent material may be the same as described previously.

DETAILED DESCRIPTION The invention will be further described by reference to the following examples.

Example 1 A blast hole of approximately 38 mm diameter was drilled approximately 1.2 m deep. 0.23 kg of water was introduced into the blast hole. The depth of the water measured and it was discovered that there was 0.2 m of water located at the bottom of the blast hole.

An amount of super-absorbent polymer (Dow chemicals DRYTECH 2035M also known as Partial Sodium salt of cross-linked poly- propenoic acid) was then measured that would sufficiently absorb the water located within the blast hole. It was calculated that between 2% and 3% of the total weight of the water would be a sufficient amount of super-absorbent polymer to be used. Therefore, approximately 0.006 kg of super-absorbent polymer was measured for placement into the blast hole.

The super-absorbent polymer was placed in a bag manufactured from paper and that was introduced into the blast hole. Water permeated through the bag and was absorbed by the super-absorbent

polymer until the bag split. The interior space of the bag was sized so that after the super-absorbent polymer had reached approximately 25% of its absorption potential, the expansion of the polymer caused the bag to break open. The super-absorbent polymer was released into the column of water absorbing the remaining water. A solid platform was formed from the absorbed water and super-absorbent polymer. The height of the water was again measured.

A common explosive known in the art as ANFO was then inserted into the blast hole. The charge was then left overnight and detonated the following day.

The charge operated in the same manner as if the charge was introduced into a dry blast hole. The depth of the water within the hole did not substantially differ when the absorbent material was introduced.

Example 2 A blast hole of approximately 89 mm diameter was drilled approximately 6 m deep. 5 kg of water was introduced into the blast hole.

The depth of the water measured and it was discovered that there was 0.8 m of water located at the bottom of the blast hole.

An amount of super-absorbent polymer (Dow chemicals DRYTECH 2035M also known as Partial Sodium salt of cross-linked poly- propenoic acid) was then measured that would sufficiently absorb the water located within the blast hole. It was calculated that between 2% and 3% of the total weight of the water would be a sufficient amount of super-absorbent polymer to be used. Therefore, approximately 0.125 kg of super-absorbent

polymer was measured for placement into the blast hole.

The super-absorbent polymer was placed in a bag manufactured from paper and that was introduced into the blast hole. Water permeated through the bag and was absorbed by the super-absorbent polymer until the bag split. The interior space of the bag was sized so that after the super-absorbent polymer had reached approximately 25% of its absorption potential, the expansion of the polymer caused the bag to break open. The super-absorbent polymer was released into the column of water absorbing the remaining water. A solid platform was formed from the absorbed water and super-absorbent polymer. The height of the water was again measured.

A common explosive known in the art as ANFO was then inserted into the blast hole. The charge was then left overnight and detonated the following day.

The charge operated in the same manner as if the charge was introduced into a dry blast hole. The depth of the water within the hole did not substantially differ when the absorbent material was introduced.

Example 3 A blast hole of approximately 154 mm diameter was drilled approximately 15 m deep. 36 kg of water was introduced into the blast hole.

The depth of the water measured and it was discovered that there was 2 m of water located at the bottom of the blast hole.

An amount of super-absorbent polymer (Dow chemicals DRYTECH 2035M also known as Partial Sodium salt of cross-linked poly-

propenoic acid) was then measured that would sufficiently absorb the water located within the blast hole. It was calculated that between 2% and 3% of the total weight of the water would be a sufficient amount of super-absorbent polymer to be used. Therefore, approximately 1 kg of super-absorbent polymer was measured for placement into the blast hole.

The super-absorbent polymer was placed in a bag manufactured from paper and that was introduced into the blast hole. Water permeated through the bag and was absorbed by the super-absorbent polymer until the bag split. The interior space of the bag was sized so that after the super-absorbent polymer had reached approximately 25% of its absorption potential, the expansion of the polymer caused the bag to break open. The super-absorbent polymer was released into the column of water absorbing the remaining water. A solid platform was formed from the absorbed water and super-absorbent polymer. The height of the water was again measured.

A common explosive known in the art as ANFO was then inserted into the blast hole. The charge was then left overnight and detonated the following day.

The charge operated in the same manner as if the charge was introduced into a dry blast hole. The depth of the water within the hole did not substantially differ when the absorbent material was introduced.

Example 4 A blast hole of approximately 381 mm diameter was drilled approximately 35 m deep. 342 kg of water was introduced into the blast

hole. The depth of the water measured and it was discovered that there was 3 m of water located at the bottom of the blast hole.

An amount of super-absorbent polymer (Dow chemicals DRYTECH 2035M also known as Partial Sodium salt of cross-linked poly- propenoic acid) was then measured that would sufficiently absorb the water located within the blast hole. It was calculated that between 2% and 3% of the total weight of the water would be a sufficient amount of super-absorbent polymer to be used. Therefore, approximately 8.5 kg of super-absorbent polymer was measured for placement into the blast hole.

The super-absorbent polymer was placed in a bag manufactured from paper and that was introduced into the blast hole. Water permeated through the bag and was absorbed by the super-absorbent polymer until the bag split. The interior space of the bag was sized so that after the super-absorbent polymer had reached approximately 25% of its absorption potential, the expansion of the polymer caused the bag to break open. The super-absorbent polymer was released into the column of water absorbing the remaining water. A solid platform was formed from the absorbed water and super-absorbent polymer. The height of the water was again measured.

A common explosive known in the art as ANFO was then inserted into the blast hole. The charge was then left overnight and detonated the following day.

The charge operated in the same manner as if the charge was introduced into a dry blast hole. The depth of the water within the hole did

not substantially differ when the absorbent material was introduced.

It should be remembered that various other changes and modifications may be made to the examples described without departing from the spirit or scope of the invention.