BACKGROUND ART Where boreholes are drilled so that an explosive charge may be delivered to an underground earth structure, it is a problem in that water located at the lower regions of the borehole may prevent or inhibit detonation of the explosive. In an attempt to overcome this problem, waterproof explosives are used, however, such waterproof explosives are generally expensive in comparison to other explosives. Where more than one explosive deposit is to be placed at spaced intervals in a borehole, it has been conventional to apply a column of concrete or the like to a portion of the borehole so that a subsequent explosive charge may be spaced at a distance from the bottom of the borehole. The application of concrete or the like to this region of the borehole is both time consuming and expensive. The placing of plugs at selected depths in a borehole is generally called decking. Presently known decking methods are costly and inefficient. One example of a decking is a wooden or concrete disk having a cross section substantially corresponding with the bore-cross section and lowered by rope to the water surface. It has also been proposed to use as a decking a polyester resin which floats on the water and solidifies after about 45 minutes. More recently, it has been suggested to replace the decking with a gel explosive slurry which floats on the water layer. Each of the foregoing methods suffers disadvantages. Timber or concrete deckings are not sufficiently reliable while gel explosive decking systems are costly and not easily prepared. Polyester resin is difficult to emplace without contamination of detonating cord, bore wall, and/or the resin and is slow to set. There thus remains a need for a decking system that is inexpensive, quick, convenient and

which provides a reliable barrier above the water.

DISCLOSURE OF INVENTION It is the object of the present invention to overcome the above problems. The invention in one broad form comprises a borehole plug comprising two or more co-reagents which expand when mixed, said co-reagents being contained in a manner such that they do not inadvertently mix but are mixable when required, said co-reagents being further contained in an outer container, said outer container being adapted to retain the co-reagents as they are mixed and being dimensioned for dropping or lowering down a borehole.

The invention also includes the method of placing an explosive charge in a borehole comprising the steps of expanding foaming reagents at a required position in a borehole to form a deck anchored to the borehole wall and loading explosives on the deck. The reagents may be mixed before or after lowering down the borehole.

The invention has many advantages over known systems - compact package; light, easy to carry; quick in operation, say 1 minute to locate in desired position; package has shelf life of at least one year; particularly, field workers unskilled in the handling of dangerous chemicals cannot come in contact with them; completely safe in use, storage and transport; has been given full approval of all relevant Regulatory Bodies; low cost; multi versatile; requires no specialised equipment; easy to locate in borehole by floating on water line or suspending from light string. BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described with respect to the accompanying drawings in which:

Figure 1 shows an example of the use of the plug in a borehole; Figure 2 shows an example of the plug of the invention.

MODES OF CARRYING OUT THE INVENTION In a preferred form of the present invention, it is proposed to use a bag of suitable material to be located at

a desired position in a borehole. The bag would contain an expandable material which is expanded to securely locate the bag at a desired position in the borehole. For example, in order to seal an explosive charge from water located in the lower portion of the borehole, the bag is to be lowered to a position above or at the water line and the expandable material allowed to expand thereby sealing the water from an explosive charge located above the bag. As a further alternative, a bag containing the expandable material could be lowered to a predetermined position on a rope or the like and the material allowed to expand so the material becomes anchored to the borehole walls so that the bag will locate an explosive charge at a position spaced from the bottom of the borehole. These two examples are shown in Figure 1.

The expanding material can be any material that expands on activation. It would usually comprise at least two co-reagents which on admixture cause a nett increase in volume and set to a solid. There would generally be a change from liquid to solid state with a generation of gas forming preferably a closed-cell foam. The solid may be rigid or flexible. In this specification such preferred materials are called plastic foams.

Examples of these plastic foams are polyurethane , polystyrene, chlorinated or chlorosulfonated polyethylene, copolymers of vinylidine fluoride and hexafluoropropylene, polyamides, polytetrafluoroethylene, styrene-acrylonitrite copolymers, polyvinylchlo ide, polyisocyanurates, polyphenols, epoxy resins, silicone resins, cellulose acetate, cellular rubber, latex foam rubber, urea-formaldehyde resins, polyimides and polyolefins such as polyethylene, polypropylene and ethylene-propylene copolymers.

The expansion mechanism preferred is polyurethane foam.

Polyurethane is the generic name for a wide range of products formed from the reaction of an isocyanate and a polyol.

Isocyanates

There are two isocyanates commercially available in Australia.

Toluene diisocyanate (T.D.I.) , (predominantly used in flexible foams) .

Polymeric methylene diisocyanate (M.D.I.) , (predominantly used in rigid insulating foams) . The preferred isocyanate in the system is M.D.I, as it is a less hazardous material. However, it must be noted that M.D.I, is classified as a 6.1 (B) Hazardous Material, which must be taken into account in the design concept of the invention. Polyols

There are two basic polyol groups: polyetherols and polyesterols. Both of these groups are commercially available in Australia in an extensive subrange of products,

The properties of the finished polyurethane product are varied by choice of polyol.

The preferred polyol in the system is a polyetherol, one part of which reacts with 2 parts of M.D.I, to form the rigid polyurethane end product. Foaming

There are two basic mechanisms used to obtain the typical honeycomb structure associated with polyurethane. - Fluorinated hydrocarbon gas dissolved in the polyol is released during the reaction and trapped within the product. Very low density foams can be produced with this method and also foams of excellent insulating properties. - Carbon dioxide gas evolution. Free water in the polyol will react with the isocyanate to form carbon dioxide gas which is entrained, forming the typical cellular structure. The preferred mechanism is that of the "Water Blown System" which gives the invention a superior shelf life even at elevated temperatures. The Inner Containers (3 and 4 of Figure 2)

It is important that the co-reagents be kept separate and sealed from moisture ingress during storage.

The material composition of the containers should be compatable with the chemical components.

The construction and sealing of the containers should be such that when filled they satisfy the requirements of the relevant Regulatory Bodies regarding- transport, storage and use. Although any suitable container can be used the preferred container is a blow moulded low density polyethylene jar with a screw on lid. This container meets all of the above requirements and is also important to the operation of the invention. - the co-reagents can be released from the containers by application of pressure. For example, from the heel of one's foot. This means that the co-reagents can be released easily, even through several layers of encapsulating outer bags. The construction and materials used for the containers is such that even on rupturing no sharp edges or points result which may puncture the encapsulating bags.

Other containing arrangements are envisaged whereby the reagents are mixed in situ in the borehole. For instance, a long cord could be pulled from the top of the borehole to open the inner containers to enable mixing of the reagents. The Outer Container (Inner Bag) (2 in Figure 2)

The bag provides a dual role, initially that of safety in containing co-reagents leaking from their containers and secondly as the reaction vessel for the chemicals.

The bag is preferably: transparent in order that it can be seen that all of the co-reagents are emptied from their individual containers and that sufficient mixing of the co-reagents has been achieved. flexible enough to enable quick easy mixing of the co-reagents by hand, yet robust enough to withstand puncture during rupturing of the containers and the subsequent mixing. ? be a material compatible with the co-reagents, be strong enough to contain the reagents during reaction until at least the bag is completely filled with

foam. tolerate the heat produced in the reaction without rupturing. must be made of a material capable of being competantly sealed. the walls and seals of the bag must be free from leaks. the dimensions of the bag are such as to encourage the foam to expand laterally to the "Borehole" wall prior to vertical expansion up the "Borehole". the relative volume of the bag to the chemical foam is such as to impede free expansion and thus produce a strong, rigid, high density polyurethane plug in firm contact with the "Borehole" wall. The preferred inner bag is made from low density polyethylene clear "Lay Flat" tubing, approximate wall thickness 0.004 inch. The dimensions of the tubing and the length of the bags is designed to suit the "Borehole" in which it will be used. The bags are heat sealed around the chemical containers.

There may be provided a further container (outer bag) (1 in Figure 2) . The bag also has a dual role; one of safety providing additional insurance against leakage of co-reagents and secondly as a support to the inner bag during the reaction.

The bag is preferably: transparent in order that previously stated operation of the inner bag can be seen. flexible enough to enable easy mixing of the reagents, yet sufficiently robust to withstand rough handling e.g. application of pressure to containers with the heel of the foot. be strong enough to support the total weight and able to tolerate the heat generated from the inner bag without leakage. be capable of being sealed. walls and seals of the bag must be free from leaks. the dimensions of the outer bag are such as to

allow full expansion of the inner bag.

The preferred outer bag is made from low density polyethylene clear "Lay Flat" tubing approximate wall thickness 0.006 inch. The dimensions of the bag again relate to the diameter of the "Borehole" in which it is to be used.

The outer bag is folded and wrapped neatly around the form of the two containers further increasing the ability of the total unit to withstand rough handling in transport and storage.

In use the device of the invention is taken to a borehole and the inner containers broken and the co-reagents mixed. It is then dropped down the borehole to rest on top of any water therein or it is lowered down to a required level where it expands and anchors to the borehole wall to form a deck on which explosives are loaded.