Field of the Invention

The present invention relates to mining, and more specifically to stemming plugs for blocking off mining blast holes. The invention has particular application for blocking off underground mining blast drill holes; however the product may also be applied to surface mining drill holes.

Background to the Invention

"Stemming" describes both the inert material, and the act of placing inert material into a blast hole to contain the blast gases as much as possible on detonation. Without stemming, blast holes remain open and the explosives on detonation will seek the path of least resistance, being out through the open collar of the blast hole in which the explosives were placed. Resistance is desirable to make the explosives more efficient. The more resistance that can be put into a blast hole to contain the explosives, the more work the explosives gases generated will do in breaking the rock material around the hole to escape on detonation.

Typically in open pit mining, blast holes are stemmed with drill cuttings. These are shovelled in on top of the explosives and the weight of those drill cuttings provides resistance to the explosives on detonation. The advantage in open pit mining is of course that the holes are vertical in a downward direction, making the act of stemming them very easy.

However most of the blast holes in underground mining are vertical, or angled, in an upward direction (termed "upholes"). Therefore stemming those upholes typically either is not carried out, or is carried out by inferior products in comparison to the effectiveness of stemming open pit holes.

Prior art methods for stemming blast holes include various mechanical devices, such as: • Rubber or plastic caps that are pushed into the hole and provide very little effectiveness other than to ensure the explosives stay in the hole;

• Expansion foams, generally of a two component mix or sprayed from a can, many of which are toxic and provide little resistance in the blast hole;

• Wedge type arrangements; and,

• Inflatable sleeve (packer) configurations.

Some examples of prior art stemming devices are described in the following documents: US201 10259228 (2010)

Seismic shot holes (vertical, downward surface holes) are filled with grout material to contain the blast.

US7690307 (2008)

This patent describes a mechanical wedge apparatus that stems blast holes (again vertical, downward surface holes).

KR20090068697A (2007)

This Korean patent describes a bidirectional wedge arrangement with guide wings. While it is not specifically stated in the brief English summary, the diagram indicates the wedge shape that would need to be made from metal or wood, and could not be made from concrete or grout.

RU2329463 (2006)

This Russian patent describes a cured concrete female outer wedge arrangement placed in the collar of a blast hole. The male inner conical wedge or piston is made from plastic of hardboard, and is mounted onto a foam polystyrene or foam plastic pack which fills the void between it and the explosives charge. US20080173205 (2005)

This patent describes a mechanical packer arrangement made from plastic. It would work in any angle holes - upholes, downholes, or horizontal holes but is specifically designed for mining and tunnelling. An internal threaded tube provides the thread where, on turning, the mechanical force pushes the plastic outer against the borehole wall.

US6324980 (1999)

This patent describes a conical plug which is folded and clipped together to fit in the blast hole. A weight is then lowered down the hole which breaks the clip and causes the conical wedge to spring open and lock in the hole. It is only suitable for surface down holes.

US5936187 (1997)

This patent describes a stemming plug which is cup shaped, made out of a durable, resilient material - PVC, urethane, rubber or the like. It is directed at surface downholes.

US5979327 (1996) and updated at US6209458 (2001 )

This is an Australian patent which describes an inflatable packer which can be used to stem upholes. It also allows explosives to be pumped through it while in position. US5497829 (1993)

This patent describes a method of stemming using a two part expanding foam, each part contained in a pouch and both contained in a further pouch. Setting is initiated by a control string which breaks the internal pouches allowing the foam components to mix. This method works with both upholes and downholes.

US5247886 (1992)

This patent is for a two part mechanical wedge plug that provides stemming to a blast hole. US3179049 (1959)

This patent describes a concrete cylinder that is hoisted into a blast hole for detonation of a nuclear explosive device. The cylinder is removable should a misfire occur so that access is gained to the misfired nuclear device. Note the concrete cylinder is cured prior to installation, and it relies on collapse of the hole casing on detonation to work as stemming in the hole.

There is also a product sold under the trademark Stempac through Dyno Nobel, which is inserted with an insertion tool. The Stempac is basically a clothing sock filled with bluemetal (angular, crushed rock), which is compressed by the insertion tool so that it maintains its position in the hole.

The poor performance of most commercially available items at present leads most mines to not stem upholes at all, which of course results in higher explosive use (and therefore cost), poor blast fragmentation, less effectiveness of the explosive charge, and greater damage to surrounding areas of the mine than would be the case with a suitable stemming.

The present invention was developed with a view to providing a stemming plug and method of stemming a mining blast hole that provides greater resistance in the blast hole than the prior art devices noted above.

References to prior art in this specification are provided for illustrative purposes only and are not to be taken as an admission that such prior art is part of the common general knowledge in Australia or elsewhere.

Summary of the Invention

According to one aspect of the present invention there is provided a stemming plug for stemming a blast hole in a mine, the plug comprising: an elongate sleeve of porous material having a first inner join line and a second outer join line, the inner join line being made of water-soluble material and defining a first diameter of the sleeve which is smaller than a second diameter defined by the second join line and is adapted to be received in a blast hole;

. a volume of dry grout material in particulate form contained in the porous sleeve wherein, in use, when the plug is immersed in water it dissolves the first inner join line and mixes with the grout material to form a paste which can seep through the porous sleeve when the plug is tamped into the blast hole, and wherein the sleeve expands to the second diameter as the plug is ■ tamped to block the blast hole after which the grout material cures.

Preferably the plug further comprises a liner of water-soluble material, the liner being provided within the porous sleeve for containing the grout material within the porous sleeve in its dry form and wherein, in use, when the plug is immersed in water the liner dissolves.

Typically the porous sleeve is made from a water-absorbent material wherein, in use, the porous sleeve helps maintain a moist environment in the hole during curing by wicking moisture to the grout to assist with curing the grout. Preferably the porous sleeve is made from a lightweight, biodegradable mesh material. Advantageously the porous sleeve is made from hessian or jute, which is a low cost, environmentally sustainable material. Typically the liner is in the form of an inner sleeve received within the porous sleeve. Preferably the liner is in the form of an inner sleeve made from a thin film of plastics material which is separate from the porous sleeve. Alternatively the liner is formed integral to the porous sleeve. Preferably the liner is made from polyvinyl alcohol (PVA) which is a water soluble plastic that dissolves within seconds when immersed in water.

Preferably the stemming plug further comprises a breather tube that passes from one end of the sleeve to the other end wherein, in use, an accumulation of gases produced in the stemmed blast hole by an emulsion explosive prior to blasting can be vented through the tube during curing of the grout material. Advantageously the breather tube is perforated along its length, and additionally spirals around the outside of the sleeve so as to prevent blockage during tamping. Preferably the breather tube is threaded around the plug through the weave of the sleeve material at spaced intervals so as not to be dislodged during the installation process. This embodiment requires tamping (pressing hard until the contents fill the blast hole void) against the explosives column. For situations where this is not possible or desirable, there is a further embodiment where the plug contains a wedge arrangement which avoids the need to tamp against the explosives column. Advantageously the stemming plug further comprises a pair of elongate wedge-shaped members received in sliding relationship with respect to each other within the porous sleeve wherein, in use, when the plug is tamped into the blast hole the wedge-shaped members slide relative to each other so as to wedge into the blast hole. Preferably the wedge-shaped members are formed from a single cylindrical elongate member cut through at an angle to form the pair of wedge-shaped members. Preferably the cylindrical member forms a solid core of the plug, and is abutted at one end by the volume of dry grout material within the porous sleeve. Typically the solid core of the plug is formed from cured grout material. Typically the dry grout material largely comprises Portland cement. However depending on the application various additives may be required such as plasticisers, retarders or accelerators, and aggregates. Preferably the dry grout material is a fast setting, low heat of hydration cement mixture. Typically the bulk of the cement mixture comprises a low heat cement. Preferably the cement mixture is a low heat cement comprising about 70% by weight of the mixture with the balance of the mixture comprising equal parts of general purpose cement, a rapid set cement (5 minute), and an immediate set cement (1 minute).

According to another aspect of the present invention there is provided a method of stemming a blast hole in a mine, the method comprising: filling an elongate sleeve of porous material with a volume of dry grout material in particulate form to form a stemming plug, the sleeve having a first inner join line and a second outer join line, the inner join line being made of water-soluble material and defining a first diameter of the sleeve which is smaller than a second diameter defined by the second join line and is adapted to be received in a blast hole; providing one or more of the stemming plugs to a mine site ready for use in the event that a blast hole needs to be blocked; immersing one or more of the stemming plugs in water for a prescribed time until the water dissolves the first inner join line and mixes with the grout material to form a paste; inserting the one or more wetted stemming plugs in the blast hole and tamping each stemming plug into the blast hole so that the sleeve expands towards the second diameter and some of the grout material squeezes out through the porous sleeve; and, allowing the grout material to cure so that the one or more stemming plugs block the blast hole.

According to a further aspect of the present invention there is provided a stemming plug for stemming a blast hole in a mine, the plug comprising: an elongate sleeve of porous material having a first inner join line and a second outer join line, the inner join line being made of water-soluble material and defining a first diameter of the sleeve which is smaller than a second diameter defined by the second join line and is adapted to be received in a blast hole; and, a pair of elongate wedge-shaped members received in sliding relationship with respect to each other within the porous sleeve; wherein, in use, when the plug is immersed in water it dissolves the first inner join line, and when the plug is tamped into the blast hole the sleeve expands to the second diameter and the wedge-shaped members slide relative to each other so as to wedge into the blast hole and block the blast hole.

Preferably the wedge-shaped members are formed from a single cylindrical elongate member cut through at an angle to form the pair of wedge-shaped members. Preferably the cylindrical elongate member is cut through at an acute angle of between about 45° and 85°. More preferably the cylindrical elongate member is cut through at an acute angle of between about 62° and 80°. Preferably the cylindrical member forms a solid core of the plug. Typically the solid core has an outer diameter about 10% less than the nominal drill bit size to allow room for bit wear, and a further 3mm smaller to allow for a signal tube. Preferably the solid core is about 50mm to 250mm in length.

Typically the solid core of the plug is formed from cured grout material. Preferably the solid core is a grout plug made of general purpose (Portland) cement reinforced with fibres for additional strength and toughness. Typically the reinforcing fibres are either one or a combination of 47mm monofilament poly fibres and 19mm monofilament fibres, of the kind manufactured by Radmix.

Typically the porous sleeve also contains a volume of dry grout material in particulate form. Preferably the wedge-shaped members are placed at a head of the plug and the grout material is contained behind the wedge- shaped members in the porous sleeve.

Typically the porous sleeve is made from a water-absorbent material wherein, in use, the porous sleeve helps maintain a moist environment in the hole during curing by wicking moisture to the grout to assist with curing the grout. Preferably the porous sleeve is made from a lightweight, biodegradable mesh material. Advantageously the porous sleeve is made from hessian or jute, which is a low cost, environmentally sustainable material. Preferably the stemming plug further comprises a breather tube that passes around a section of the sleeve containing the grout material wherein, in use, an accumulation of gases produced in the stemmed blast hole by an emulsion explosive prior to blasting can be vented through the tube during curing of the grout material. Advantageously the breather tube spirals around the outside of the sleeve so as to prevent blockage during tamping. Additionally the breather tube may have perforations along its length to further ensure no blockages can occur within it.

According to a still further aspect of the present invention there is provided a method of stemming a blast hole in a mine, the method comprising: filling an elongate sleeve of porous material with a pair of elongate wedge- shaped members received in sliding relationship with respect to each to form a stemming plug, the sleeve having a first inner join line and a second outer join line, the inner join line being made of water-soluble material and defining a first diameter of the sleeve which is smaller than a second diameter defined by the second join line and is adapted to be received in a blast hole; providing one or more of the stemming plugs to a mine site ready for use in the event that a blast hole needs to be blocked; immersing one or more of the stemming plugs in water for a prescribed time until the water dissolves the first inner join line; inserting the one or more wetted stemming plugs in the blast hole; and, jolting each stemming plug so that the sleeve expands to the second diameter and the wedge-shaped members slide relative to each other so as to wedge into the blast hole and block the blast hole. Preferably the method further comprises the step of tamping each stemming plug to firmly locate the wedge-shaped members in the blast hole. Preferably the pair of elongate wedge-shaped members is abutted at one end by a volume of dry grout material in particulate form within the porous sleeve, and the step of tamping each stemming plug also forces the now wet grout material at the one end to fill the blast hole void beneath the wedges.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. Likewise the word "preferably" or variations such as "preferred", will be understood to imply that a stated integer or group of integers is desirable but not essential to the working of the invention.

Brief Description of the Drawings

The nature of the invention will be better understood from the following detailed description of several specific embodiments of stemming plug and method of stemming a mining blast hole, given by way of example only, with reference to the accompanying drawings, in which:

Figure 1 illustrates a first embodiment of a stemming plug in accordance with the present invention;

Figure 2 is a cut-away section view of the stemming plug of Figure 1 ;

Figure 3 illustrates a first step in the process of constructing the stemming plug of Figure 1 ;

Figure 4 illustrates a second step in the process of constructing the stemming plug of Figure 1 ;

Figure 5 illustrates a preferred method in accordance with the present invention of installing the stemming plug of Figure 1 in a blast hole; Figure 6 illustrates a solid core for a second embodiment of a stemming plug in accordance with the present invention;

Figure 7 illustrates the solid core of Figure 6 cut into two wedge-shaped members;

Figure 8 shows the wedge-shaped members of Figure 7 separated to reveal their shape;

Figure 9 illustrates another step in the process of constructing the second embodiment of a stemming plug in accordance with the present invention;

Figure 10 illustrates the second embodiment of a stemming plug in accordance with the present invention;

Figures 1 1 a, 1 1 b and 1 1c illustrate a preferred method in accordance with the present invention of installing the stemming plug of Figure 10 in a blast hole;

Figure 12 shows the effective area of the top wedge-shaped member exposed to a blast in a drill hole;

Figure 13 illustrates a solid core for a third embodiment of a stemming plug in accordance with the present invention;

Figure 14 illustrates the solid core of Figure 13 cut into two wedge- shaped members and with the wedge-shaped members separated to reveal their shape; and,

Figure .15 illustrates the third embodiment of a stemming plug in accordance with the present invention.

Detailed Description of Preferred Embodiments

A first embodiment of stemming plug 10 in accordance with the invention, as illustrated in Figures 1 to 4, comprises an elongate sleeve 12 of porous material having a first inner join line 14 and a second outer join line 16 (see Figure A). The inner join line 14 is made of water-soluble material and defining a first diameter of the sleeve 12 which is smaller than a second diameter defined by the second join line 16 and which is adapted to be received in a blast hole. The porous sleeve 12 contains a volume of dry grout material 18 in particulate form. When the plug 10 is immersed in water it dissolves the first inner join line and mixes with the grout material 18 to form a paste which can squeeze through the porous sleeve when the plug is tamped into the drill hole. The porous sleeve 12 is now free to expand towards the second diameter as the plug is tamped and in this way the stemming plug 10 can be used to block the blast hole once the grout material 18 cures.

Preferably the stemming plug 10 further comprises a liner 20 of water-soluble material. The liner 20 is provided within the porous sleeve 12 for containing the grout material 18 within the porous sleeve 12 in its dry form. In use, when the plug 10 is immersed in water, the liner 20 dissolves and the water mixes with the grout material 18 to form a paste. The paste can seep through the porous sleeve when the plug 10 is tamped into the drill hole to block the drill hole when the grout material cures.

Typically the porous sleeve 12 is made from a water-absorbent material wherein, in use, the porous sleeve 12 helps maintain a moist environment in the hole during curing by wicking moisture to the grout material 18 to assist with curing the grout. Preferably the porous sleeve 12 is made from a lightweight, biodegradable mesh material. Advantageously the porous sleeve 12 is made from hessian or jute, which is a low cost, environmentally sustainable material. In the described embodiment 18oz hessian is employed to make the porous sleeve 12.

Typically the liner 20 is in the form of an inner sleeve received within the porous sleeve 12. Preferably the liner 20 is made from a thin film of plastics material which is separate from the porous sleeve 12. Alternatively the liner 20 is formed integral to the porous sleeve 12. Preferably the liner 20 is made from polyvinyl alcohol (PVA) which is a water soluble polymer that dissolves within seconds when immersed in water.

Typically the grout material 18 largely comprises non-expanding Portland cement. However depending on the application various , additives may be required such as plasticisers, retarders or accelerators, and aggregates. The grout mix needs the following characteristics: • Heat of hydration to be as low as possible since there is a temperature restriction on the explosives being used, which is 70° Celsius. The ambient rock temperature in some mines may be as high as 40° Celsius, so ideally the lower the additional temperature through heat of hydration, the more suitable the stemming plug 10 will be.

• Despite this, fast curing is an advantage to provide the maximum amount of resistance to the blast. However, most fast curing additives and cements dramatically increase the heat of hydration. Therefore these competing requirements need to be carefully balanced.

• The entire stemming plug needs to be mill friendly. The stemming plug 10 will be used at a variety of mines where flotation circuits and other mineral processing techniques apply. The use of Portland cement and additives such as accelerators which are essentially shotcrete materials in the stemming plug 0 is therefore an advantage because these materials are familiar to and readily dealt with by mineral processing mills.

• The stemming plug 10 needs to be placed manually up a blast hole and stay there, therefore the mixture needs to be sticky and have minimal bleed (bleed is grout contaminated water runoff). Bleed is not desirable since it will drip on the installing operator standing beneath.

• A further reason to reduce bleed to an absolute minimum is that there is a chemical reaction between the calcium in cement and ammonium nitrate, a component of explosives. This reaction liberates ammonium gas, which in high doses in a poorly ventilated area can be hazardous. However, where the stemming plug 10 is used on upholes any bleed will run down and away from the explosive charge.

A suitable dry grout mixture is as follows, by weight:

1 part General Purpose cement;

1 part Rapid Set cement (5 minute setting);

1 part immediate set cement (1 minute setting); 7 parts Low Heat cement;

1 .5% dry powder shotcrete accelerator; and

1.5% Methyl Cellulose.

Other variations to the mixture are possible to satisfy the requirements of a fast setting, low heat of hydration grout with little or no bleed.

Preferably the stemming plug 10 needs to allow freshly placed explosives such as emulsion to degas. Emulsion is typically pumped into overhead holes at 39° Celsius and it gives off carbon dioxide as it cools and expands as part of the sensitisation process. This carbon dioxide needs to be vented away or it will build up pressure in the hole and not allow the sensitisation of the explosives, and also risk pushing the stemming plug 10 out of the hole before it can cure. For this reason the stemming plug 10 typically further comprises a perforated breather tube 21 that passes from one end of the sleeve 12 to the other end. Therefore, in use, an accumulation of gases, produced in the stemmed blast hole by an emulsion explosive prior to blasting, can be vented around the plug 10 during curing of the grout material. Advantageously the breather tube 21 spirals around the outside of the sleeve 12, and is perforated along its length so as to prevent blockage during tamping. Preferably the breather tube 21 is threaded around the plug 10 through the weave of the sleeve material 22 at spaced intervals so as not to be dislodged during the installation process.

A currently preferred process of manufacturing the stemming plugs 10 will now be described with reference to Figures 1 to 4. A rectangular, elongate sheet of hessian or jute material 22 is cut and folded in half length ways as shown in Figure 3. Then the open side and one end of the sheet 22 is joined ^' together along a first inner join line 14 and a second outer join line 16 as shown in Figure 4. Typically the join lines 14 and 16 are formed by double stitching the hessian or jute material together along the join lines. The inner join line is formed of water-soluble material, typically by stitching entirely or in part with a water soluble (PVA) thread. The resulting sleeve 12 is turned inside out to form the smaller diameter sleeve. The inner join line 14 defines a first diameter of the sleeve 12 which is smaller than a second diameter defined by the second join line 16 and is adapted to allow the plug 10 to be readily received in a blast hole. The sleeve 12 is designed with a curved leading edge 28 for installation to assist in guiding the stemming plug 10 into a hole of only marginally larger diameter. The plug 10 still needs to be capable of installation even where the hole may have surface irregularities through deviation or broken ground. At this stage the tail end 30 of the sleeve 12 is left open. The breather tube 21 is wound around the outside of the porous sleeve 12 in a spiral, and is preferably threaded through the weave of the hessian or jute material 22 every quarter turn so as not to become dislodged during the installation process.

Preferably the PVA liner 20 is formed in a similar way, either by stitching or heat welding along the open side and one end to form an inner sleeve. The PVA liner 20 is not turned inside out. Assembly is simple and involves inserting the PVA liner 20 into the hessian or jute sleeve 12 and filling it with the dry grout powder 18. The stemming plugs preferably have a bulk density after filling with grout of 0 - 2.0g/cc, more preferably about 1.5g/cc. This is driven by the need for the stemming plug to retain its shape for installation, and to maintain a low water: cement ratio (WC ratio) on immersion. Both sleeves 12 and 20 are then closed with a cable tie 29 at the open tail end 30, resulting in the completed stemming plug 10 as shown in Figures 1 and 2.

The assembled stemming plug 10 is cylindrical or sausage shaped and typically is manufactured in lengths of 250mm, with an outer diameter about 10% less and a further 3mm smaller than, the nominal hole size to allow for drill bit wear. For example, a plug 10 is made to pass 89mm to go into a 102mm hole. The minimum (worn) bit size encountered will be 92mm, the detonator signal tube is 3mm, and the maximum the plug 10 will expand to with the outer stitching is slightly larger than 102mm. In this way the plug is guaranteed to fit any blast holes drilled with nominal 102mm drill bits, regardless of how much bit wear affects an individual bit. On tamping, the grout material extrudes through the hessian sleeve 12 to engage more fully with the hole and make up the hole diameter. The outer diameter of the porous sleeve 12 may be selected to suit the nominal drill hole size. Colour- coded hessian may be used to clearly identify different diameter plugs for ease of use for the various standard diameter blast holes. The PVA liner may not be in the form of a separate inner sleeve 20 as in the illustrated embodiment. The liner 20 could be formed integral to the porous sleeve 12, as layer on the inside or outside of the sleeve 12 which blocks the pores in the porous hessian material. The liner helps to prevent the dry grout powder from leaking through the pores of the porous sleeve 12. However the liner 20 could be dispensed with altogether if a degree of leakage of the dry grout material during transport and storage can be tolerated. Alternatively the grout material may be selected to be of a particle size that cannot easily escape through the pores of the porous sleeve 12 in its dry form.

A preferred method of plugging a mining drill hole using the stemming plugs 10 according to the invention will now be described with reference to Figure 5. The method preferably comprises filling an elongate sleeve 12 of porous material, adapted to be received in a blast hole, with a volume of dry grout material 18 in particulate form to form a stemming plug 10 as described above. One or more of the stemming plugs 10 is then provided to a mine site ready for use in the event that a blast hole needs to be blocked.

When a blast hole is required to be blocked, one or more of the stemming plugs 10 is carried down to the blast hole location. Each stemming plug 10 is immersed in water for a prescribed time until the water mixes with the grout material 18 to form a paste. Soaking the stemming plug 0 until the cessation of production of bubbles, generally around 5 minutes, dissolves the PVA liner 20 and automatically wets the grout to a WC ratio of 0.35, which develops a strong, low shrink grout. This avoids the requirement for mixing grout. The plug 10 needs to be drained for 1 minute to remove excess water. The stemming plug 10 can then be installed into a blast hole. It does not have to be installed immediately, the installation time frame may be up to 30 minutes or so after immersion. The water also dissolves the water soluble thread, forming the first inner stitch line 14. However, note that during and after immersion, even though the soluble stitching will dissolve away, the plug retains its shape due to the low water cement ratio, and the memory of the liner material. It can then be installed into the blast hole with the diameter remaining at the smaller size, but is free to expand on tamping.

PVA is commonly used in the concreting industry and has a documented effect of increasing the strength and decreasing the porosity of the cured product. However for the purpose of this application, the diluted PVA content of the water in which the stemming plugs soaks is unlikely to provide any advantage. More importantly it will not be detrimental to the integrity of the cured grout.

One or more wetted stemming plugs 10 is then inserted in the blast hole and each stemming plug is tamped into the blast hole so that some of the grout material extrudes out through the weave of the porous hessian sleeve 12. Tamping also causes the porous sleeve 12 to expand outwards towards the second diameter defined be the second outer join line 16. The tamping force on the end of the plug 10 (the left hand end in Figure 5) pushes the grout outwards to meet the wall of the hole. The liner allows this movement since the first line of stitching has dissolved away during immersion and no longer confines the diameter of the plug 10. The plug is only confined by the second outer seam 16 of stitching; however this is sized to allow it to expand to at least the diameter of the hole so that this is not a constraint during tamping. Tamping is typically done using a rigid wooden pole or plastic tube (not shown), and is similar to the process used for tamping explosives into a blast hole. The self-wetting aspect of the stemming plug removes the human error involved in achieving a certain WC ratio. The grout material is then allowed to cure so that the one or more of the stemming plugs block the drill hole. Any suitable number of stemming plugs 10 may be tamped into the collar of the blast hole to block the hole. The blast hole is effectively sealed off with the stemming plug(s), with a suitable number of such plugs tamped into place one behind the other, for the purpose of providing resistance to the expanding gases following detonation of the explosive. Mines would be free to use more or less stemming plugs as the circumstances dictate. The plug(s) 10 need to cure for 6 to 24 hours to provide the resistance required to perform as adequate stemming. This is something that larger mines are able to work with since they can often take several days to load explosives into the blast holes for their blasts. The hessian or jute sleeve 12 is economical, environmentally sustainable, and allows some storage of moisture and the wicking of moisture to the grout material 18 to assist with curing the grout. The curing of grout is essentially the same as curing concrete. It is assisted by continued hydration, resulting in a stronger end result than if it cures in a dry environment. The stemming plugs 10 are intended to block blast drill holes to resist the expanding gases upon detonation of the explosive in the blast hole, and therefore provide more effective stemming. The stemming plugs 10 are designed to quickly, efficiently and economically block a mine blast hole.

A second embodiment of a stemming plug 40 in accordance with the invention is illustrated in Figures 6 to 12. The stemming plug 40 is similar in many respects to the first embodiment of the stemming plug 10, and therefore the similar parts will be identified with the same reference numerals and will not be described again in detail. The stemming plug 40 comprises an elongate sleeve 12 of porous material having a first inner join line 44 and a second outer join line 16 (see Figure 9).

The inner join line 44 is made of water-soluble material and defines a first diameter of the sleeve 12 which is smaller than a second diameter defined by the second join line 16 and which is adapted to be received in a blast hole. Advantageously the first inner join line 44 in this embodiment is also taken around the head of the sleeve to define a first length of the sleeve 12 which is smaller than a second length defined by the second join line 16. The stemming plug 40 further comprises a pair of elongate wedge-shaped members 42 received in sliding relationship with respect to each other so as to form a solid core within the porous sleeve. In use, when the plug 40 is immersed in water it dissolves the first inner join line 44, and when the plug is tamped into the blast hole the sleeve 12 expands to the second diameter and the wedge-shaped members 42 are free to slide relative to each other so as to wedge into the blast hole and block the blast hole. A preferred method in accordance with the invention of installing the stemming plug 40 in a blast hole will be described in more detail below. Preferably the wedge-shaped members 42 are formed from a single cylindrical elongate member or solid core 48 as shown in Figure 6 cut through at an angle to form the pair of wedge-shaped members 42. Advantageously the cylindrical elongate member 48 is constructed from a grout plug which has been soaked in water and cured. The plug is dimensioned with the correct diameter to allow for drill bit wear, to allow room for the signal tube to the explosives which fits in the hole alongside, and to allow for the thickness of the outer liner.

The solid core 48 is a grout plug made from cement grout (General Purpose cement) with the addition of fibres for tensile strength and toughness. In this embodiment readily available 47mm Radmix monofilament poly fibres are used, in combination with 19mm Radmix monofilament microfibers. However, fibres may not be necessary for the effectiveness of the plug. Additionally, steel fibres which are also commonly available could be used.

The solid core 48 is cut into sections of approximately 50mm. Typically the solid core has an outer diameter about 15mm less than the nominal drill bit size. In the case of an 89mm (nominal) drill bit size, allowing 10% for bit wear and allowing 3mm for the signal tube, and 2mm for the outer liner of the plug, the preferred diameter of the solid core 48 is 75mm. Similar logic would apply for 102mm, 76mm, and 64mm drill bit sizes which are all commonly used for underground production holes. Preferably the solid core 48 is cut through at an acute angle of between about 45° and 85°. More preferably the solid core 48 is cut through at an angle of about 62° to form the two wedge-shaped members 42 as shown in Figures 7 and 8. Advantageously the presence of the fibres in the core allows the wedge-shaped members 42 to maintain the acute angle. Without the fibres, the leading edge of each wedge-shaped member 42 would crack away, making a blunt wedge which would be less effective.

The porous sleeve 12 is preferably made from a sheet of hessian 22 as in the previous embodiment, except in this instance the dissolvable line of stitching 44 is taken further around the head of the plug to allow greater movement of the wedge-shaped members 42. Figure 9 shows the cotton or non-soluble stitching at 16, and the soluble stitching at 44 in the construction of the porous sleeve 12.

Preferably the porous sleeve 12 of the stemming plug 40 also contains a volume of dry grout material 8 in particulate form of similar characteristics to that previously described. When the plug 40 is immersed in water it dissolves the first inner join line and mixes with the grout material 18 to form a paste which can squeeze through the porous sleeve when the plug is tamped into the drill hole. The plug 40 is preferably constructed with the wedge-shaped members 42 placed at the head of the plug, as shown in Figure 10. (Part of the porous sleeve 12 has been omitted in Figures 10 to 12 for clarity.) Behind and abutting the wedge-shaped members 42 in the stemming plug 40 is a quick-setting grout 18 as previously described, although it may also be bentonite. Preferably the grout material is a low bleed general purpose cement grout. Necessarily the grout has a curing temperature well below 70° Celsius. The grout mixture described below may be used, or a grout with similar qualities or bentonite which will not cure. Bentonite is an option in the plug 40 since curing is not an essential part of the grout function. Bentonite avoids the chemical reaction between the calcium carbonates in cement and ammonium nitrate in explosives, which liberates small amounts of ammonia gas, the avoidance of which may be seen as advantageous in some mines.

A preferred dry particulate grout mixture for the stemming plug 40 comprises:

7 parts Low Heat cement;

1 part General Purpose cement;

1 part Rapid Set cement such as Baroid Rapid Set 5 - 5 minute setting; 1 part immediate set cement such as Sika Plug - 1 minute setting (it actually sets during the "immersion" phase producing a putty like consistency in the plug); and,

0.7% dry powder shotcrete accelerator such as Riversands.

Alternately, due to the smaller volume of grout material required in the wedge plug arrangement, the lack of curing time available, and the lesser requirement on grout strength in this arrangement, 5 minute rapid setting cement alone may be sufficient.

The grout section of the plug 40 may require a perforated breather tube 21 so that, in use, the grout does not completely seal the blast hole enabling emulsion explosives to degas. The breather tube 21 can be easily removed by the installation crew if the user is using ANFO explosives where the breather tube is not required.

A preferred method in accordance with the invention of installing the stemming plug 40 in a blast hole will now be described with reference to Figures 1 1a, 1 1b and 1 1c.

The double stitching of the sleeve 12 keeps the plug 40 in shape during transport, and the hessian material of the sleeve 12 develops a certain amount of memory, a bit like a fold in a piece of paper. The diameter of the plug 40 is critical for installation. On immersion in water, the memory of the hessian sleeve 12 retains the plug's size, and in particular the diameter by keeping the wedges 42 in place, to allow installation at the reduced diameter, as shown in Figure 1a. As the plug is pushed up into the blast hole with either a tamping stick or loading hose, it can be easily located anywhere along the hole.

Once it reaches the desired location, the operator just provides a sharp jab or jolting motion to the plug 40. This causes the fold in the hessian sleeve 12 to open and the wedge-shaped members 42 to separate and slide across each other so as to lock into the walls of the blast hole, as shown in Figure 1 1 b. As the inner soluble line of stitching 44 dissolves, the sleeve 12 can accommodate the increase in diameter of the plug 40 caused by the sliding movement of the wedge-shaped members 42 relative to each other. The wedge-shaped members 42 now engage with and grip the walls of the blast hole. Note it need not be tamped against the explosives column but can be located anywhere in the blast hole.

Finally, the operator tamps the plug 40 harder to firmly locate the wedges 42 and also tamp the grout inside the tail of the plug so that it contacts the walls of the hole, as shown in Figure 11c. The double stitching of the porous sleeve 12 also facilitates freedom of movement inside the sleeve lengthwise. As the lower wedge-shaped member 42b is pushed up, its angled leading edge will protrude past the top of the upper wedge-shaped member 42a, hitting the constraining hessian of sleeve 12 if the inner dissolvable line of stitching 44 which extends further around the head of the plug 40 did not dissolve away to allow room for this movement.

The design of the plug 40 with the wedge-shaped members 42 allows the very simple installation described by jolting the plug at the correct location. The wedges 42 then provide the resistance to tamp the grout in the base of the plug. It is this plug design - stitching, outer sleeve, and to a lesser extent grout content which combine to make the wedges 42 work effectively in providing the stemming resistance on initiation of the explosives. This will then allow immediate use, rather than having to wait 6-24 hours to allow the grout to cure prior to firing a blast. The grout material 18 now plays a secondary role to that of the wedges 42. The incorporation of the wedge-shaped members 42 in the stemming plug 40 provides a number of advantages as follows: Firstly, the plugs 40 can be located at any position in the blast hole without having to be tamped up against the explosive column. At the desired location they are simply jolted to enable the wedges to hold in the hole. Secondly, the grout supports the plug 40 in the hole when the explosive shot is initiated so that the wedges are not shaken loose by the other holes exploding around them. Thirdly, the plug 40 can be used immediately with no need to wait for grout to cure.

Finally, the largest area the shock wave sees on initiation of the blast is the flat circular face 50 of the uppermost wedge 42a (the hashed area 50 shown in Figure 12). Since Force = Pressure x Area, the pressure being the thrust of the Shockwave on initiation of the explosives, the greatest force is applied to the largest area being the flat top of the uppermost wedge 42a. This drives the upper wedge-shaped member 42a down into the lower wedge-shaped member 42b and locks it into the hole harder as the Shockwave hits it. The lower wedge 42b is supported by the grout at its base, in addition to the interference with the side of the blast hole, providing cohesive and frictional resistance to the upper wedge 42a. The inclusion of the reinforcing fibres in the grout material of the wedge-shaped members 42 means the wedges will resist the explosive force better than if the fibres were not present.

A third embodiment of a stemming plug 60 in accordance with the invention is illustrated in Figures 13 to 15. The stemming plug 60 is substantially identical in many respects to the second embodiment of the stemming plug 40, and therefore the similar parts will be identified with the same reference numerals and will not be described again in detail. The stemming plug 60 comprises an elongate sleeve 12 of porous material having a first inner join line 44 and a second outer join line 16 (see Figure 9).

The inner join line 44 is made of water-soluble material and defines a first diameter of the sleeve 12 which is smaller than a second diameter defined by the second join- line 16 and which is adapted to be received in a blast hole. Advantageously the first inner join line 44 in this embodiment is also taken around the head of the sleeve to define a first length of the sleeve 12 which is smaller than a second length defined by the second join line 16. The porous sleeve 12 is preferably made from a sheet of hessian 22 as in the previous embodiments. The stemming plug 60 further comprises a pair of elongate wedge-shaped members 62 received in sliding relationship with respect to each other so as to form a solid core within the porous sleeve. In use, when the plug 60 is immersed in water it dissolves the first inner join line 44, and when the plug is tamped into the blast hole the sleeve 12 expands to the second diameter and , the wedge-shaped members 62 are free to slide relative to each other so as to wedge into the blast hole and block the blast hole.

A preferred method in accordance with the invention of installing the stemming plug 60 in a blast hole is substantially identical to the method of installing the stemming plug 40 described above, with reference to Figures 1 1 and 12, and will not be described again.

Preferably the wedge-shaped members 62 are formed from a single cylindrical elongate member or solid core 68 as shown in Figure 13 cut through at an angle to form the pair of wedge-shaped members 62. Advantageously the cylindrical elongate member 68 is constructed from a grout plug which has been soaked in water and cured. The solid core 68 is a grout plug made from cement grout (General Purpose cement) with the addition of fibres for tensile strength and toughness, similar to the solid core 48 described above.

The solid core 68 is cut into sections of approximately 200mm to 250 mm in length. Typically the solid core has an outer diameter about 15mm less than the nominal drill bit size, similar to the solid core 48 described above. Preferably the solid core 68 is cut through at an acute angle of between about 45° and 85°. More preferably the solid core 68 is cut through at an angle of about 80° to form the two wedge-shaped members 62 as shown in Figure 14. Advantageously the presence of the fibres in the core allows the wedge-shaped members 62 to maintain the acute angle. The principal difference between this embodiment of the stemming plug 60 and the previous embodiment of the stemming plug 40 is that the porous sleeve 12 of the stemming plug 60 does not contain a volume of dry grout material as in the previous embodiments. This also means that the breather tube is not required since there is no grout sealing off the blast hole to make it airtight (the wedges alone do not create an airtight seal). In other respects the stemming plug 60 operates in the same way as the second embodiment of the stemming plug 40 to wedge into a blast hole and block the blast hole. Figure 15 illustrates the third embodiment of the stemming plug 60, with the porous sleeve 12 partially cut-away to reveal the wedge-shaped members 62 contained inside.

The third embodiment of the stemming plug 60 described above is particularly well-suited to flattish downholes, i.e. holes dipping at 30° to 40°. The best way to do this is to elongate the wedge members to about 200mm or 220mm to accommodate the steeper angled cut of around 80° between them, and not to include any dry grout material at all in the plug. This configuration also has the potential to work just as well in upholes.

Now that preferred embodiments of the stemming plug and a method of stemming a mine blast hole have been described in detail, it will be apparent that the described embodiments provide a number of advantages over the prior art, including the following:

(i) The plugs are more effective than other uphole stemming products at containing the gases produced upon detonation of the explosive.

(ii) It is cost effective being made of low cost materials. (iii) The hessian or jute sleeve is environmentally sustainable and its porosity assists with retaining and wicking moisture during curing.

(iv) It is very quick and easy to install.

(v) No special installation tools are required. (vi) The double stitching allows the plug to be easily installed in the blast hole, sized to allow for drill bit wear, and then to expand to the full inside diameter of the blast hole during tamping.

(vii) The wedges, particularly the lower wedge, combine with the grout to locate the plug in the hole and forms resistance to both vibration and the blast itself.

(viii) The plug configuration also ensures the upper wedge-shaped member transmits the larger proportion of the Shockwave force into resistance against the blast by the wedging action. (ix) Additionally, the outer hessian sleeve assists in retaining the wedge-shaped members in the streamlined position while being pushed into the hole.

(x) A further, significant advantage of this configuration is that the wedge-shaped members can be made very economically and simply from a previously cured grout plug.

It will be readily apparent to persons skilled in the relevant arts that various modifications and improvements may be made to the foregoing embodiments, in addition to those already described, without departing from the basic inventive concepts of the present invention. For example, whilst hessian or jute has been described as the preferred material for making the porous sleeve, the sleeve may be made from any suitable porous material. Therefore, it will be appreciated that the scope of the invention is not limited to the specific embodiments described and is to be determined from the appended claims.