AU5423601A - Drill and blast method for excavating or fracturing hard material - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant(s): RockMin Pty Ltd Invention Title: DRILL AND BLAST METHOD FOR EXCAVATING OR FRACTURING HARD MATERIAL The following statement is a full description of this invention, including the best method of performing it known to me/us: DRILL AND BLAST METHOD FOR EXCAVATING OR FRACTURING HARD MATERIAL The present invention relates to a drill and blast method for excavating or fracturing hard material such as rock, concrete and the like.

Traditional drill and blast methods used for rock excavation in mining involves drilling one or more holes in a rock face, filling the holes with an energetic material such as an explosive, placing an initiator in each hole in operational contact with the energetic material and then setting off the initiator to cause detonation of the energetic material. In order to blast or excavate rock to a particular depth it is necessary to sub drill (also known in the art as "over drill") beyond that depth typically by around 3% to 15% of the hole depth. For example, if one were attempting to effect a 20 metre excavation of rock using the drill and blast method typically one would drill to a depth of 22 metres. This arises because of the dynamics of the detonation wave which tends to plastically deform rock at the bottom of a hole rather than induce fractures.

Therefore, the traditional drill and blast method is inherently inefficient because one must overdrill a hole to achieve a desired excavation depth. This in itself increases drilling cost and time. Also the increased hole depth usually results in use of a greater amount of energetic material.

Conventional practice also indicates that it is preferable at times to vary the charge concentration in the hole so that the bottom charge has a higher concentration that the column charge. Thus for example in common bench blasting it is known practice to use an explosive or other energetic material with less energy per unit volume in the column charge than in the bottom charge. By having the same type of explosive and the same linear charge concentration for the bottom charge and the column charge, it is commonly 3 0 believed that the hole will not break properly, thus the frequent use of a booster charge at the bottom of the hole to ensure better breakage.

Ht\jnelaon\keep\peci\DRILL AND BLAST METHOD FOR EXCAVATING OR FRACTURING HARD MATERIAL COMPLET.doc -3- It is an object of the present invention to provide a drill and blast method of improved efficiency.

According to one aspect of the present invention there is provided a drill and blast method for excavating or fracturing a hard material comprising at least the steps of: drilling a hole in the hard material to a required depth D metres of excavation or fracture; charging the hole with an energetic material to a depth of x y metres where x y is less than or equal to D, x is measured up from the bottom of the hole and y is measured contiguously up from x, with the charge concentration of the energetic material for the depth x being less than the charge concentration of energetic material for the depth y; and, detonating the energetic material in the hole to create a pressure wave for inducing fractures in the hard material at the bottom of the hole.

Preferably the method includes arranging depth x to be in a range of 2% to 20% of the depth D of the hole.

Preferably the step of charging the hole to the depth of x metres includes placing a spacer means at the bottom of the hole for creating a compressible space up to the depth x from the bottom of the hole whereby the charge concentration of energetic material for the depth x is less than the charge concentration of energetic material for the depth y.

According to another form of the present invention there is provided a drill and blast method for excavating or fracturing a hard material including the steps of: drilling a hole in the hard material to the required depth of excavation or fracture; placing spacer means at the bottom of hole for creating a compressible space at the bottom of the hole; charging the hole with an energetic material so that the energetic material is supported by the spacer; and, H,\jnelson\keep\speci\DRILL AND BLAST METHOD FOR EXCAVATING OR FRACTURING HARD MATERIAL COMPLTB. doc -4detonating the energetic material in the hole to create a pressure wave that can travel through the compressible space thereby inducing fractures at the bottom of the hole.

Preferably the method includes forming the spacer means in a manner so as to provide a compressible space of a length between 2% to 20% of the depth of the hole.

According to another aspect of this invention there is provided spacer means for use in a drill and blast method of excavating or fracturing rock and other hard materials, the spacer means shaped and configured to enable passage through the hole and support energetic material placed in the hole above the bottom of the hole, the spacer means defining a compressible space through which a pressure wave can travel upon initiation of the energetic material. In one embodiment, the spacer means is in the form of a tube, shell or elongate element made of a compressible material. For example, the spacer means can be in the form of a thin walled polyethylene cylindrical tube that is closed its upper end when located in the hole. In another form, the space means is made from a plastic or cardboard elongate element having a cross shaped cross section with a plate extending transversely across its upper most end when deposited in the hole. In an alternate form, the spacer means can be in the form of a polystyrene or sponge rubber blank or cylinder. In yet a further embodiment it is envisaged that the space means is made up from a plurality of compressible elements or beads for example a plurality of polystyrene beads or hollow plastic beads or alternately substantially non compressible particulate materials such as coarse aggregates or gravel that take up a volume greater than the sum of the volume of each particle of aggregate or gravel thereby creating a compressible space. Indeed, the spacer means can take any convenient form that facilitates its intended function, that being to reduce the effect of the detonation pressure wave to that of a rapidly applied hydrostatic pressure increase at the bottom of the hole, thus initiating a tensile PLF fracture from the bottom corner of the hole.

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: HI\jnelaon\keep\speci\DRILL AND BLAST MRTI(OD FOR EXCAVATINO OR FRACTRING HARD MATERIAL COPLtrB.doc Figure 1 is a schematic representation of the prior art drill and blast method for excavating or fracturing a hard material; Figure 2 is a schematic representation of an embodiment of a drill and blast method in accordance with the present invention; Figure 3 is a perspective view from the side of a spacer means useable in the method reflected in Figure 2; Figure 4 is a perspective view from the side of a second embodiment of the spacer means useable in the method in accordance with the present invention; Figure 5 is a schematic representation of a third embodiment of a spacer means useable in a method in accordance with the present invention; and, Figure 6 is a schematic representation of a fourth embodiment of a spacer means useable in a method in accordance with the present invention.

Referring to Figure 1, if it is desired to excavate a rock bench 10 to a depth Dl then in accordance with the prior art one or more holes 12 are drilled into the rock 10 to a depth of D1 D2 where D2 is typically about 10% of DI. That is, to excavate to a depth of Dl one must subdrill (also known as "overdrill") by an additional distance D2. Once the holes 12 have been drilled, detonators 14 are placed in the holes 12 and then the holes packed with energetic materials such as an energetic material 16 and an upper level of stemming 17 such as sand or drill cuttings. The detonators 14 are then set off in a conventional manner to thereby detonate the energetic material 16. This causes a shock wave to travel through the rock 10. It has been found that near the bottom of the holes 12 for the length approximately equal to D2, the shock wave tends to plastically deform the rock rather than cause fracturing. It is for this reason that in the prior art it is necessary to sub drill to a depth greater than the desired depth of excavation. Clearly this is inefficient as drilling is charged on a per/metre basis and therefore instead of paying for, for example a driller to drill 20 metres to achieve a 20 metre excavation one must bear the additional Hl\jnelson\keep\spec\DRILL AND BLAST MBTHOD FOR EXCAVATING OR PRACTURING HARD ATEBRIAL COMPLETE.doc cost of drilling a further 2 metres to achieve a desired excavation depth of 20 metres.

Also, because the holes 12 are overdrilled there is a tendency to use more energetic material.

Embodiments of the present invention attempt to alleviate the above deficiencies in the prior art. In the drill and blast methods according to embodiments of this invention each hole 12 is drilled to the actual required depth Dl of excavation. No overdrilling or subdrilling is required. The hole is then charged in a way so that the charge concentration is less for a length of the hole adjacent the bottom of the hole. This can be done through the use of a spacer as shown in Figures 2-5 or by the use of energetic materials of different charge concentration with a bottom charge having the lowest charge concentration as shown in figure 6. In both cases the intended effect is to reduce the speed of gas pressure increase near the bottom of the hole to assist in the generation of penetrating cone fracture (PCF), rather than plastically deform or pulverise the hard material.

In the embodiment shown in Figure 2 after drilling of the hole 12, a spacer means 18 is placed at the bottom of the hole 12 for creating a compressible space 20 at the bottom of the hole 12. Thereafter, the method is similar to the prior art namely that a initiator 14 is lowered into the hole 12, the hole 12 then packed with energetic material 16 and stemmed with stemming 17. When a initiator 14 is activated to initiate the energetic material 16, a high pressure shock wave travels through the compressible space 20 to cause fracturing, and most preferably PCF, at the bottom of the hole 12 at the depth D1.

As is readily apparent, in this method, the holes 12 are drilled to the actual desired depth of excavation with no subdrilling required. Also, as the spacer 18 supports the energetic material 16 above the bottom of the hole, some what less energetic material 16 is required to achieve an excavation depth Dl. Therefore efficiencies are gained in the reduction of drilling time/depth and use of less energetic material 16.

The spacer 18 has a height or length so as to correspond with a depth x at the bottom of the hole 12. Energetic material 16 is poured down the hole 12 so that the hole 12 is charged to a depth of x y metres. x y will equal the excavation depth Dl when there is Hl\jnelson\keep\8peci\DRILL AND BLAST METHOD FOR EXCAVATING OR FRACTURING HARD MATERIAL COMPLBTB.doc -7no stemming. However, typically the hole will be stemmed for a depth or length z so that x y z is substantially equal to the depth Dl. Thus, it can be seen that the hole 12 is charged to a depth of x y metres. The presence of the spacer 18 results in the bottom x metres of the hole having an effective charge density substantially less than the charge density for the depth y. Depending on the form of the spacer the charge density in the depth x can be in the order of 0 (for example where the spacer 18 is in a form that effectively blocks of the bottom depth x of the hole, as shown in Figures 3 and 4).

Alternately, if the spacer 18 is in the form of a plurality of particles that compact together form voids therebetween as shown in Figure 5, some of energetic material 16 can pass into the voids to create in effect a low charge density energetic material in the bottom x metres of the hole 12. The depth or length x is most advantageously between 5% to 20% the depth Dl of the hole 12.

The spacer 12 may take many different forms. An important characteristic of the spacer 18 is that it has the effect of reducing charge density near the bottom of the hole so as to reduce the speed of gas pressure increase and thereby assist in causing PCF at the bottom of the hole.

In the embodiment shown in Figure 3, the spacer 18 is in the form of a thin walled polythene tube which is closed at at least its upper end 22 to form a support surface 24 for holding the energetic material 16. The spacer 18 need not be made from a plastics material and can in fact be made from other suitable materials such as cardboard or drawn aluminium of the type typically used in beverage cans. Indeed, in a very basic and primitive form, the spaces 18 can simply be one or more plastic beverage containers.

Figure 4 illustrates an alternate form of the spacer 18 comprising four elongate plates 26 (only two shown) joined along a common longitudinal axis so that the spacer 18 has a cross shaped cross section, with at least the upper end 22 of the spacer 18 having a circular plate top to form a support surface 24 for the explosive material 16.

In yet another embodiment shown in Figure 5, the spacer 18 can be in the form of a plurality of compressible beads 28. The beads can be made from a solid compressible Hs\jnelson\keep\peci\DRILL AND BLAST METHOD POR EXCAVATING OR FRACTURING HAMD MATERIAL COMPLETE.doc -8material such as polystyrene or, can be made hollow and from a material that will allow the bead 28 to easily compress. In this embodiment, a predetermined volume of beads 28 is poured down each hole 12 with the explosive material 16 then supported at the top of a short column formed by the beads 28.

Referring to Figure 6, a similar effect to that produced by use of the spacer 18 can be achieved by arranging a bottom charge 16a for the depth x of the hole 12 with a lower charge concentration that a column charge 16b for the depth y of the hole 12. This can be accomplished in many different ways including the use of different types of energetic material for the bottom and column charges 16a, 16b or mixing some of the energetic material used for the common charge 16b with a particulate diluent so as to effectively reduce the charge concentration and using this mixture as the bottom charge 16a. One way of doing this is to mix the energetic material with solid compressible, or alternately hollow, beads such as the spacer 18 depicted in Figure 5. The depth or length x is most 15 advantageously between 2% to 20% the depth D1 of the hole 12.

All modifications and variations as would be apparent to a person of ordinary skill in the art are deemed to be within the scope of the present invention the nature of which is to be determined from the foregoing description.

HC\jnelson\keep\speci\DRILL AND BLAST METHOD FOR EXCAVATING OR IRACURING HARD MATERIAL COMPLETE.doc

Claims (13)

1. A drill and blast method for excavating or fracturing a hard material comprising at least the steps of: drilling a hole in the hard material to a required depth D metres of excavation or fracture; charging the hole with an energetic material to a depth of x y metres where x y is less than or equal to D, x is measured up from the bottom of the hole and y is measured contiguously up from x, with the charge concentration of the energetic material for the depth x being less than the charge concentration of energetic material for the depth y; and, detonating the energetic material in the hole to create a pressure wave for inducing fractures in the hard material at the bottom of the hole.

2. The method according to claim 1 wherein the depth x is in a range of 2% to of the depth D of the hole.

3. The method according to claim 2 wherein the step of charging the hole to the depth of x metres includes placing a spacer means at the bottom of the hole for creating a compressible space up to the depth x from the bottom of the hole whereby the charge concentration of energetic material for the depth x is less than the charge concentration of energetic material for the depth y.

4. A drill and blast method for excavating or fracturing a hard material including the steps of: drilling a hole in the hard material to the required depth of excavation or fracture; placing spacer means at the bottom of hole for creating a compressible space at the bottom of the hole; charging the hole with an energetic material so that the energetic material is supported by the spacer; and, Hs\jnelson\keep\peci\DRILL AND BLAST METHOD FOR EXCAVATING OR FRIACTURING HARD MATERIAL COMPLT.doc detonating the energetic material in the hole to create a pressure wave that can travel through the compressible space thereby inducing fractures at the bottom of the hole.

5. The method according to claim 4 further including the step of forming the spacer means in a manner so as to provide a compressible space of a length between 2% to 20% of the depth of the hole.

6. Spacer means for use in a drill and blast method of excavating or fracturing rock and other hard materials, the spacer means shaped and configured to enable passage through the hole and support energetic material placed in the hole above the bottom of the hole, the spacer means defining a compressible space through which a pressure wave can travel upon initiation of the energetic material.

7. The spacer means according to claim 6 wherein the spacer means is in the form of a tube, shell or elongate element made of a compressible material.

8. The spacer means according to claim 6 wherein the space means is made from a plastic or cardboard elongate element having a cross shaped cross section with a plate extending transversely across its upper most end when deposited in the hole.

9. The spacer means according to claim 6 wherein the spacer means can be in the form of a polystyrene or sponge rubber blank or cylinder.

10. The spacer means according to claim 6 wherein the space means is made up from a plurality of compressible elements or beads.

11. The spacer means according to claim 6 wherein the spacer means is in the form of a quantity of substantially non compressible particulate material that take up a volume greater than the sum of the volume of each particle of the material thereby creating said compressible space. Hs\Jnelson\keep\epeci\DRILL AND BLAST METHOD FOR EXCAVATING OR FRACTURING HARD MATERIAL COMPLBTB.doc -11 I-

12. A drill and blast method for excavating or fracturing a hard material substantially as herein described with reference to and as illustrated in the accompanying drawings.

13. A spacer means substantially as herein described with reference to and as illustrated in the accompanying drawings. DATED this 2 8 'h day of JUNE 2001 ROCKMIN PTY LTD By Its Patent Attorneys GRIFFITH HACK Fellows Institute of Patent and Trade Mark Attorneys of Australia. K: \jnelson\keep\speci\DRILL AN~D BLAST METHiOD FOR EXCAVATING OR FRACTU3RING HARD MATZRIAL COMpLETE.doc