WO 2010/091455 PCT/AU2010/000130 1 METHOD OF BLASTING TECHNICAL FIELD The present invention relates to a method of blasting. 5 BACKGROUND TO THE INVENTION In underground mining, ground material normally needs to be removed to provide access to the material to be mined. To achieve such removal successfully, it is usually necessary to create a void, called a "rise" or "winze" in production io blasting and a "burn-cut" in development blasting. This is to provide a space into which material removed by subsequent blasting can be accommodated. In the absence of such a void, the material loosened by the blasting may not be able to be properly freed. 15 The present invention relates to the creation of such voids, whether they extend vertically, horizontally or at any other angle. 20 One manner of forming a rise is first to form a large number of smaller, initial voids called "reamers", which are generally between 150 and 200 mm in diameter. In addition, the method includes forming a large number of even smaller blast holes that are closely spaced in 25 relation to the reamers. The reamers form voids for accommodating material loosened by blasting carried out in the blast holes. The same principal applies in a development burn-cut, but the discussion below is in relation to rises. 30 The level of success creating the rises depends on a number of factors including accuracy of drilling of the blast holes, the nature of the geology in the blasting location, depths of the risers and whether they are blind rises 35 (having only one opening) or break throughs (rises with openings at both ends). Poor drilling of the blast holes can cause them to extend (advance) in undesirable WO 2010/091455 PCT/AU2010/000130 2 directions relative to one another which can result in effectively "desensitizing" neighbouring blast holes where the holes are too close together, or "sympathetic detonation" where the holes are even closer spaced. 5 This level of success is very important in the mining industry. If the rises are not properly formed, this may result in inadequate removal of material, including mined material such as ore, and this can result in production io losses of millions of dollars per year. Indeed, a 50% to 75% success rate is not uncommon in certain mines. For example, if a rise is to be formed and is proposed to be a 20m rise but only advances 15m (or 75 % of the proposed depth) this is likely to result in the production blasts on 15 the same level, which are intended to blast into the rise, also only advancing approximately 75%. This is especially problematic in up-hole blind rises. In such cases, there may be only one opportunity to correctly carry out the blasting, with little or no opportunity or scope to rectify 20 the inadequate advance. In an attempt to compensate for losses due to inadequate removal of material, slashing holes (i.e. holes that lead to the regions in which material removal was incomplete) 25 may be formed to provide access to the material to be removed. However, this also involves significant man-hours and cost so that, in some cases, it is decided simply to accept the losses. 30 Another manner of attempting to overcome such failures to remove material, in break through rises, is to remove the material of the rise in successive portions of approximately 5 to 8 m each. However, although this may achieve desirable results with regard to the adequacy of 35 material removal, it is very time consuming and costly. Another method that has a close to 100% success rate WO 2010/091455 PCT/AU2010/000130 3 involves the use of raise boring, which is a method of drilling out a very large void using boring machinery which is guided by a pre-drilled pilot hole. However, particularly because of the equipment used, such a method 5 is extremely costly and time consuming. SUMMARY OF THE INVENTION According to an aspect of the invention there is provided a io method of blasting a segment in a ground mass, the method comprising: forming an array of holes around a central portion of the ground mass segment; placing explosives in the array of holes; and 15 detonating the explosives. Forming the array of holes may comprise forming six holes. However, it is to be understood that the present invention may not be limited to any particular number of holes. 20 The holes may extend substantially parallel to one another. In another arrangement, the holes may converge on each other. 25 Each hole may be at an angle of one degree to the direction in which the hole would have extended had the holes extended parallel to one another. 30 In a preferred embodiment, the method is a method of blasting for removal of ground material from the ground mass. BRIEF DESCRIPTION OF THE DRAWINGS 35 Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: WO 2010/091455 PCT/AU2010/000130 4 Figure 1 is a schematic front view of a surface wall of a ground mass with holes opening out through the face; and Figure 2 is a schematic side view of the ground mass of Figure 1. 5 DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring to the Figures, there is shown a ground mass 10 which, according to an embodiment of the invention, is located in an underground mine. 10 According to the embodiment described, the ground mass 10 has a surface wall 12 that constitutes the ceiling of a drive 14 that has been formed for production mining in the mine. 15 The blast holes 16 are formed in the ground mass 10 preferably by suitable drilling of the holes 16, for example with hydraulic drilling machines. The blast holes 16 are drilled so that each opens out through the surface 20 wall 12 at a respective blast hole aperture 18, with each blast hole extending away from the surface wall into the ground mass 10. The holes 16 are formed in an array around a central 25 portion of a segment of the ground mass 10 which is to be blasted. The apertures 18 are formed in an arrangement 20 so that imaginary straight lines 22 (shown in phantom lines) 30 joining each pair of apertures together form a polygon - in particular, a hexagon - as shown in Figure 1. The method also involves inserting explosive charges 24 in each of the blast holes 16. According to one embodiment, 35 the explosive charges 24 fully fill the blast holes and are in the form of emulsions. However, it is to be understood that any other suitable explosives may be used such as WO 2010/091455 PCT/AU2010/000130 5 cartridge explosives and ANFO for example. A detonator 26 is also suitably placed in each blast hole 16. The detonator 26 may be any suitable detonator such as 5 an electronic detonator, a nonel detonator or detonating cord for example. Preferably, the detonator selected is one with little or no delay so as to minimise timing scatter. In other embodiments, the position and number of detonators 10 26 may depend on the particular blasting application. Thus, according to the example shown in Figure 2 the detonators 26 are each placed within boosters. Each detonator and booster combination may be referred to as a "primer". The booster is an explosive charge which is used to detonate 15 the mass explosive charge in the blast holes 16 upon detonation of the detonator 26. That is, the detonator 26 provides the required timing delay and initial blast energy to initiate the booster which itself has the required energy to initiate detonation of the explosives 24. The 20 primer (detonator 26 and booster combination) are preferably located towards the aperture 18 (commonly referred to as the "collar") of each blast hole 26. According to another example (not shown), a plurality of 25 detonators may be inserted in each blast hole 16, being a primary detonator and one or more back-up detonators. The back-up detonators can be employed in the event that the primary detonator fails. 30 The detonators 26 are configured to detonate the explosive charges 24 in all of the blast holes 16 substantially simultaneously which may include a delay of a few milliseconds between detonation of some of the explosive charges 24. 35 Detonating the explosive charges 24 causes shock (energy) waves 28 to be propagated from the blast holes 16. In WO 2010/091455 PCT/AU2010/000130 6 addition, the simultaneous detonation assists in causing the formation, in the ground mass 10, of cracks or cuts each extending from one blast hole to the adjacent blast holes on either side (also as illustrated by the lines 22). 5 This assists in loosening the central portion or core of the segment of the ground mass 10 which is bounded by the cracks or cuts 22. This in turn, facilitates the removal of the central portion from the ground mass 10. In particular, 10 the loosening of the central portion can assist it to fall, under the effect of gravity, from the ground mass into the space of the drive 14 below. It will be appreciated that parts of the shock waves 28 15 propagated from each of the blast holes 16 travel towards the other blast holes. This results in a concentration of the shock energy in the centre of the arrangement 20. The shock waves 28 propagated within the arrangement 20, 20 and particularly the concentration of shock waves near the centre of the arrangement, may facilitate the breaking up of the ground material between the blast holes 16 which may, in turn, facilitate the removal of the central portion of the segment of the ground mass 10. The effectiveness of 25 this may, however, be dependant on the strength of the blast, intensity of the shock waves 28, the nature of the material forming the ground mass 10, the dimensions of the arrangement 20, and so on. 30 According to the preferred embodiment, the blast holes 16 are tightly spaced in relation to one another, with a high quantity of explosive charge 14. This, in turn, results in a very high powder factor (ratio of explosives to kilogram or cubic metre of ground material within the arrangement). 35 Although the blast holes 16 are indicated as being substantially parallel to one another in Figure 2, in other WO 2010/091455 PCT/AU2010/000130 7 embodiments they may be angled with respect to each other. For instance, the blast holes may be angled to converge on one another in a direction away from the surface wall 12, at an angle of approximately one degree from the parallel 5 configuration. Such an angle may be referred to as a "dump" angle as it may facilitate the dumping of the ground material from the ground mass 10, under the effect of gravity, once the blasting has taken place. 10 In further embodiments (not shown), there are one to three additional blast holes at the approximate centre of the arrangement 20 and these are provided with explosive charge that is detonated a few milliseconds after the charge in the first-mentioned blast holes 16 is detonated. According 15 to this embodiment, the shock waves propagated from these additional blast holes meet the shock waves 28 propagated from the first-mentioned blast holes 16 at a position within the bounds of the arrangement 20. This may further assist in dislodging the ground material. 20 A further embodiment (not shown) provides a reamer at the approximate centre of the arrangement 20, the reamer not being charged with explosives. This may also assist in dislodging the ground material as it provides a void into 25 which part of that material can be accommodated after detonation. A further embodiment (not shown) involves carrying out the detonation in the blast holes in sequential stages, say in 30 a direction away from the apertures 18 - i.e. in a "decked" fashion. This may assist in loosening the ground material a layer at a time within the arrangement 20, which may also facilitate the removal of the ground material. Such an embodiment may be particularly beneficial in a mine or 35 tunnelling project where a high level of vibration can cause problems in the surrounding areas, for example where there is sensitive equipment, residential or office WO 2010/091455 PCT/AU2010/000130 8 accommodation, and so on. This is because a lesser charge is required for loosening an individual layer than that required for loosening the entire core of ground matter within the arrangement 20. Thus, the vibration caused by 5 each simultaneous detonation as each layer is removed is lower than those caused if the blast holes are fully charged and the full charge is detonated. According to a further embodiment (not shown), collar 10 priming is carried out. This involves removing some ground material from the surface wall 12 within the arrangement 20 to facilitate release of the ground material after blasting. 15 The invention is not limited to the particular embodiments described above. In particular, the invention may be utilised for underground mining or tunnelling, especially for the creation of vertical or near-vertical voids (rises or winzes) in production blasting, or shafts in tunnelling. 20 The invention could also be used in development mining or tunnelling to create horizontal or near-horizontal voids for providing space in which material loosened by subsequent blasts can be accommodated. It will thus be appreciated that the invention is not limited to upwardly 25 extending blast holes as illustrated in the drawing. The configuration of the blast holes 16 is simple and therefore not susceptible to significant deviation from the intended hole advance direction during drilling of the 30 holes. In addition, the simultaneous blasting in all of the blast holes 16 will minimise the chance that blasting in one blast hole of a particular arrangement will negatively interfere with a neighbouring blast hole in that arrangement as a result of poor drilling practices or 35 undesirable geology. Although the invention is described above in relation to WO 2010/091455 PCT/AU2010/000130 9 preferred embodiments, it will be appreciated by those skilled in the art that it is not limited to those embodiments, but may be embodied in many other forms. 5 In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify 10 the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.