WO2008144846A1

WO2008144846A1 - An inflatable void former for use in explosives boreholes

Info

Publication number: WO2008144846A1
Application number: PCT/AU2008/000790
Authority: WO
Inventors: Michael John Martin
Original assignee: Trouperdale Pty Ltd
Priority date: 2007-05-31
Filing date: 2008-06-02
Publication date: 2008-12-04

Abstract

An inflatable void forming device for forming a void in a column of explosives composition in a borehole. The device comprises an elongate inflatable bladder fabricated from a flexible polymeric film and a flexible sheath of stretch resistant material. The flexible sheath, in use, is capable of resisting an undesired increase in volume of the bladder after the bladder has been pressurized to a predetermined degree.

Description

An inflatable void former for use in explosives boreholes

FIELD OF THE INVENTION

This invention is concerned with a method and apparatus for forming voids in an explosives composition. The invention is concerned particularly, although not exclusively, with an inflatable device for forming a void in a column of explosive composition in a borehole.

BACKGROUND OF THE INVENTION

It is known to use inflatable plastics bladders to form a plug at a desired depth in a borehole formed in an earth formation. When the plug expands within the borehole, it engages against the borehole wall under pressure thereby permitting one or more columns of explosives composition to be supported within the borehole at a desired depth.

Generally speaking, these inflatable bladders fall into two groups of which one group comprises sealed bladders with a selectively actuable expandable fluid generating device sealed within the bladder and the other group comprises an externally inflatable bladder having a valve to which an air hose or the like is detachably connected.

Australian Patents 579395 and 595887 describe selectively mixable co-reagents which react to form, respectively, an expandable polyurethane foam and a gas within the bladder.

Australian Patent 656051 describes a bladder inflation device with a time delay mechanism which relies on exhaustion of a liquid through a narrow nozzle before an expandable gas can escape.

Other internally inflatable bladders are described in Australian Patents 763474 and 779463 which utilize a mixture of a low boiling point organic solvent, water and ethanol.

Although not exhaustive, United States Patents 4,913,233, 5,000,261 ,

5,035,286 and 5,273,100 each describe externally inflatable bladders having a self-sealing nozzle detachably connectable to an air hose. The inflatable bladders are lowered into boreholes by the air hose and, at a desired depth, a source of pressurized air is introduced to inflate the bladders firmly against the wall of the borehole.

To overcome the problem of accidental puncturing of the inflatable bladders by sharp rocks or the like protruding from the borehole wall, it was proposed in United States Patent 5,273,110 and Australian Patent 779463, to encase the bladders within a woven polyester fabric outer bag. By ensuring that the outer bag had height and diametric dimensions greater than the inner bladder when in an expanded state, maximum frictional contact with the borehole wall could be ensured without interference from the outer bag. Generally speaking, the greater the contact area between the inflatable bladder and the borehole wall, the less pressure was required to inflate the bladder to support a given mass of explosives composition supported thereon. With externally inflatable bladders in particular, the lower inflation pressure reduced the risk of leakage from the self-sealing valve arrangement. A rock blasting method described in WO 01/20248 utilizes an externally inflatable cylindrical air tube which forms an air decking to support layers of explosives composition and stemming material as a means of reducing the number of detonators required in a given blast pattern.

United States Patent 6,631 ,684 describes the use of inflated cylindrical tubes having a diameter less than that of the borehole so that when surrounded by a flowable explosives composition such as ANFO, the inflated tube formed a void within the column of explosive which reduced the quantity of explosive required. To avoid the prospect of an incomplete detonation of explosive, a centering device having radially projecting arms was proposed, but it was considered that these centering devices would prevent precise location of the void forming tubes and actually add to the risk of incomplete detonation.

International Publication WO 2006/095938 proposed an inflatable cylindrical void former with diametrically extending flexible wing-like formations extending over the length of the cylindrical bladder. These wings were intended to assist in centering the cylindrical bladder in the borehole without interfering with the introduction of a flowable explosives composition to surround the void forming device in the borehole.

A difficulty with the aforementioned inflatable void formers is that they are required to be inflated before insertion into a borehole and thus, to overcome frictional forces between the wing-like formations and the bore hole wall, a mass such as stemmings, sand or earth must be introduced into a specially fabricated pocket to enable the void former to proceed down the borehole without jamming.

A further difficulty arises from the limited degree to which the flexible plastics cylinders can be inflated. Although at reduced inflation pressures the risk of deflation in the borehole is reduced, the mass of explosive composition and stemmings above the cylinder can be sufficient to cause the inflated cylinder to kink and partially collapse within the borehole thereby constricting the volume of explosive in the region of the partial collapse with a consequent risk of forming a discontinuity in the explosive column.

A major problem with inflatable void formers arises from the capacity of the inflatable plastics bladder to continue to expand over time. Due to a phenomenon known as "cold creep", a plastics film under a tensile load will continue to stretch thereby increasing in volume. Apart from risking a "partial choking" of the borehole, if the bladder continues to expand such that is in contact with the borehole walls, with consequent incomplete blast propagation within the borehole, Boyle's Law predicts a lowering of bladder pressure with an increase in bladder volume according to the relationship PiVi = P2 V₂. As internal pressure diminishes due to volume increase, the increasing risk of void former collapse under load increases the risk of borehole blockage or formation of incomplete columns of explosive.

All the aforesaid pressure control problems with inflatable void formers are greatly exacerbated where ambient or downhole temperatures exceed about 25°C. In some mining regions, ambient temperatures can exceed 6O⁰C while downhole temperatures can exceed 120⁰C in thermally reactive substrates such as coal or pyritic minerals. United States Patent Application 10/388738 describes the use of hollow void formers or cores made from plastics tubes capped at both ends, a hollow cylindrical cardboard core, or similar hollow cylindrical cores from timber, plastics, rubber or the like. No method of centering of the cores is described although this is indicated as necessary.

Accordingly, it is an aim of the present invention to overcome or alleviate at least some of the shortcomings of prior art void formers and/or otherwise to provide consumers with a more convenient choice.

SUMMARY OF THE INVENTION According to one aspect of the invention there is provided an inflatable void forming device for use in explosives boreholes comprising: an elongate inflatable bladder fabricated from a flexible polymeric film; and a flexible sheath of stretch resistant material, the flexible sheath, in use, resisting an increase in volume of the bladder when the bladder is pressurized to a predetermined degree.

Preferably, the flexible sheath is comprised of a stretch resistant polymeric film.

If required, the stretch resistant polymeric film may comprise a laminate or multi-layer construction including a stretch resistant substrate. Suitably, the stretch resistant polymeric film comprises an aluminium substrate.

Preferably, the inflatable bladder is fabricated from a polyolefin, polyester, polyamide, polyurethane, vinyl or copolymers, co-extrusions or laminates thereof.

Suitably, the inflatable bladder is an elongate inflatable tubular bladder.

If required, the inflatable bladder may comprise a gas-tight sealed chamber containing a selectively actuable container of expansible fluid. Alternatively, said inflatable tubular bladder may comprise a gas-tight sealed chamber comprising an inflation valve to permit inflation with an external source of pressurized fluid.

The flexible sheath may be comprised of a high tensile strength plastics film.

Preferably, the flexible sheath is comprised of a woven or non-woven fabric selected from the group consisting of polyolefins, polyesters, polyamides, polyurethanes, glass, carbon or Kevlar™ fibres. Suitably, said flexible sheath has antistatic properties. If required, said void forming device may include a layer of heat reflective material intermediate said inflatable bladder and said flexible sheath.

According to another aspect of the invention there is provided a method of forming voids in a column of explosives composition in a borehole comprising the steps of:- inflating, to a predetermined pressure, a void forming device comprising an elongate inflatable bladder fabricated from a flexible polymeric film, the bladder being located within a flexible stretch resistant sheath whereby, in use, the stretch resistant sheath resists an increase in volume of the bladder when pressurized to a predetermined degree; inserting the inflated void forming device into a borehole having a diameter greater than said void forming device; and introducing into said borehole, a quantity of flowable explosives composition to surround said void forming device, thereby forming a void in a column of explosives composition within said borehole.

Throughout this specification and claims which follow, unless the context requires otherwise, the word "comprise", and variations such as

"comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers or steps but not the exclusion of any other integer or group of integers.

BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be fully understood and put into practical effect, reference will now be made to preferred embodiments illustrated in the accompanying drawings, in which:

FIG. 1 shows an inflatable tubular bladder;

FIG. 2 shows a void forming device according to the invention;

FIG. 3 shows an enlarged view of an inflation valve associated with said tubular bladder; and

FIG. 4 shows a cross-sectional view of a borehole containing a column of explosives composition surrounding a void forming device.

For the sake of simplicity, like reference numerals are used for like features in the drawings. DETAILED DESCRIPTION OF THE DRAWINGS

In FIG. 1 , the inflatable bladder 1 may be formed as an elongate hollow body 2 by thermally fusing two sheets of a thermoplastic film about the perimetral edge 3 of body 2. Preferably, however, the bladder 1 is formed from lay flat tubing with transverse thermal welds at opposite ends. An inflation valve 4 is sealed into body 2 by a transverse thermal weld 5 at the top of body 2 and a similar weld 5a forms a closure at the bottom of the body. When inflated, body 2 assumes a tubular configuration with generally rounded ends 6, 7 to withstand inflation pressures of up to 100 psi without placing undue stress on welds 5, 5a. Other shapes are, of course, possible but the tubular configuration shown in FIG. 1 is preferred to lessen the risk of the borehole becoming blocked.

The thermoplastic film from which bladder 1 is fabricated may be selected from polyolefins, polyamides, polyurethanes, polyesters, vinyls or any other suitable plastics films. The polymeric compounds may be monomeric or polymeric and the films thereof may be co-extruded or laminated. Polyethylene terephthalate (PET) is a particularly preferred film for this purpose.

FIG. 2 shows a void forming device 1a according to the invention in an inflated state. A bladder of the type shown in FIG. 1 is encased in a sheath 8 of a tough stretch and puncture resistant material such as a woven polypropylene fabric. Other stretch resistant materials may include axially stretched polymeric films, knitted, woven or non-woven fabrics of polymeric materials, glass fibre, carbon fibre, KEVLAR™ or like high tensile fibres, natural fibres such as cotton, jute, hemp and the like or mixtures thereof. An opening 9 is provided in the top of sheath 8 to accommodate inflation valve 4.

The sheath 8 will be designed to be of a size such that, in operation, it allows the bladder 1 to be inflated, and hence to expand, to the desired level but physically restricts the volume to which the bladder 1 is able to expand beyond this point. This is achieved by the sheath 8 being formed from a stretch resistant material.

This prevents further, unwanted, expansion of the bladder 1 after inflation. If allowed to continue unchecked this would result in a decrease in pressure within bladder 1 which could result in the void former 1a collapsing due to the surrounding load of explosive composition. An unrestricted volume increase could also result in the void former 1a contacting the walls of the borehole and effectively sealing the borehole. The stretch resistant material is sized so as restrict bladder 1 from expanding to fill the entire borehole or increasing its volume such that its internal pressure becomes undesirably low. In this manner, when inflated to the appropriate level, the external surface of inflatable bladder 1 will be in substantial contact with the inner surface of stretch resistant sheath 8 and in this way further volume expansion of bladder 1 is restricted without the application of a force external to the sheath such as pressure from the borehole walls. The diameter of the inflatable bladder remains substantially the same due to the restrictive action of the sheath 8. This represents an improvement over the prior art wherein the inflatable bladders are always capable of further volume expansion in at least one direction after the inflation fluid has been fully dispensed into the bladder. This can lead to many problems as described previously. To inflate the bladder within sheath 8, a tapered nozzle of an air hose

(not shown) coupled to a source of compressed air is inserted into valve 4 and the bladder is inflated to a predetermined pressure of between say, 60 psi and 100 psi, depending upon such factors as the depth of the borehole. When the bladder has been inflated to a desired pressure, the hose nozzle is withdrawn and the valve self seals as will be described later. These types of valves are known and do not form part of the invention.

FIG. 3 shows an enlarged view of the valve 4. Valve 4 is a flat elongate tubular member formed either as a lay flat tube by extrusion or otherwise formed by two strips of plastics flim, thermally fused along longitudinal edges 4a with upper and lower ends 10, 11 both being open. In fabrication of the bladder 1 , the valve tube 4 is placed between the two sheets of film forming the body 2 before the upper end of body 2 is sealed by thermal welding along weld seam 5. The valve tubing 4 is selected from a polymeric material compatible with the body film to ensure air tight integrity. Located within the valve tubing 4 on opposite faces thereof are strips or layers 12 of a thermally incompatible polymeric material which prevent fusion between the opposing layers of valve tube 4 thereby creating a small opening 13 at the intersection of weld seam 5 and the strip 12 of thermally incompatible material.

With a tapered air hose nozzle frictionally engaged in the opening 13, air is introduced until a desired bladder pressure is achieved whereupon the valve is withdrawn. Internal pressure acting on the relatively long internal tail 4b of valve 4 urges the two layers of plastics film into close contact with each other thereby forming an effective self-sealing valve 4.

FIG. 4 shows the mode of use of the void forming devices according to the invention. In FIG.4, a borehole 15 is filled to a predetermined depth 16 with a flowable explosives composition such as ANFO. A void forming device 1a of the type described above is then inflated to a predetermined pressure, say, 80 psi and is dropped down the borehole where typically it will be supported at its base on the surface 16 of the initial ANFO charge 17 and inclined against the borehole wall 18. The inflatable bladder 1 within the void former will be in intimate contact over its external surface (apart from the valve area) with the internal surface of stretch resistant sheath 8 to prevent unwanted volume expansion of bladder 1 and thereby maintain the internal pressure within a set range.

A second charge 19 of ANFO is then introduced into the borehole 15 whereby it completely surrounds the void forming device except perhaps for the region 21 where the upper end of the void forming device 1a is in contact with the borehole wall 18. It will be noted however, that around the void former, the mass of explosives composition does not, at any point along its length, form a region less than the critical diameter of the composition to ensure full propagation of explosive forces within the borehole. The process may be repeated to have a plurality of void formers spaced throughout the borehole and otherwise encased in explosives composition. Alternatively, while a continuous feed of ANFO is introduced into the borehole, inflated void formers may be introduced along with the ANFO at predetermined intervals representing a desired spacing between void formers in the borehole.

It readily will be apparent to persons skilled in the art that many modifications or variations may be made to the invention without departing from the spirit and scope thereof. For example, the sheath 8 may be adapted to possess antistatic properties to avoid the build up of triboelectrically generated static charges which could cause a spark sufficient to activate the NONEL detonating tube 20 within the borehole. The sheath fabric may be coated with a proprietary antistatic fabric spray, or the sheath material may be fabricated from or include electrically conductive fibres.

Where extreme downhole temperatures of up to 15O⁰C may be encountered in boreholes in thermally reactive materials such as coal or pyritic minerals, the void forming device may include an intermediate layer of aluminized MYLAR™ film between the inner bladder and the outer sheath to form a thermal barrier.

In yet another adaptation of the invention, the bladder 1 may be formed as a gas-tight sealed chamber within which is located an aerosol can of expansible fluid selectively releasable upon manual activation of a valve actuator by pressure applied from outside the sealed chamber. A suitable container of expansible fluid is described in Australian Patent 779463, the contents of which are incorporated herein by reference. After actuating the valve of the aerosol container, the inner bladder is inflated until it fully occupies the stretch resistant sheath 8. A predetermined bladder pressure may be achieved by selecting an appropriate volume of expansible material for the volume of the bladder or otherwise by adapting the composition of the expansible fluid such that the vapour pressure of the expansible fluid reaches equilibrium at a predetermined bladder pressure.

The bladder 1 and sheath 8 may be manufactured from a translucent or, preferably, transparent material, particularly when an aerosol can is used as the inflation device. This allows the actuator mechanism of said aerosol to be more conveniently located and actuated by the user.

In one preferred embodiment the materials from which the inflatable bladder 1 and sheath 8 are constructed from, respectively, may be manufactured in a form such that they are laminated together. In this manner it may be envisaged that a large sheet of the laminate is rolled out which may have the material to form the bladder 1 in a lay flat tubing-like form as previously described with the sheath material pre-formed to surround it. The shape of the void former 1a is cut out in a single press using a welding plate or the like. The welding plate would be such that, as the void former 1 a is cut out of the laminate the open edges are thermally sealed apart from a small opening for the valve which is formed substantially as described in relation to FIG. 3. If an externally actuable aerosol can of expansible fluid is employed as described above then the laminate cut out would be sealed as before but leaving an opening for insertion of the aerosol can after which time this opening would also be sealed. It should be understood that the laminate is such that the inflatable bladder 1 can still be inflated by the desired degree before its walls come into restrictive contact with the sheath 8 and further expansion of the bladder 1 is thereby prevented. A laminate of this kind can provide advantages in ease and speed of manufacture by allowing a number of void formers 1a to be "stamped out" from one large piece of laminate. It should now be understood that the present invention provides for an inflatable device for forming a void in a column of explosives composition in a borehole. The device is comprised of an inflatable bladder made from a plastics material which can be inflated to a desired pressure and volume. The bladder is encased in a sheath of a stretch and puncture resistant material which prevents the bladder from continuing to expand over time after active inflation has ceased due to the pressure placed upon the plastics material by the inflation fluid. The sheath forms a physical barrier to the continuing expansion of the bladder beyond the desired volume as once the bladder stretches to contact and press against the internal surface of the sheath its stretch resistant nature will restrict any further expansion.

This results in a void former which can be inflated to a desired pressure to form the required void volume within an explosives composition which will not further increase its volume significantly and therefore will not completely fill the borehole diameter, will be less likely to collapse under the load due to its inflation pressure being maintained and will be protected from rocks and other sharp objects in the borehole by the tough sheath.

Claims

CLAIMS 1. An inflatable void forming device for use in explosives boreholes comprising: an elongate inflatable bladder fabricated from a flexible polymeric film; and a flexible sheath of stretch resistant material, the flexible sheath, in use, resisting an increase in volume of the bladder when the bladder is pressurized to a predetermined degree.

2. The inflatable void forming device of claim 1 wherein the flexible sheath is comprised of a stretch resistant polymeric film.

3. The inflatable void forming device of claim 2 wherein the stretch resistant polymeric film comprises a laminate or multi-layer construction including a stretch resistant substrate.

4. The inflatable void forming device of any one of the preceding claims wherein the inflatable bladder is fabricated from a polyolefin, polyester, polyamide, polyurethane, vinyl or copolymers, co- extrusions or laminates thereof.

5. The inflatable void forming device of claim 4 wherein the inflatable bladder is fabricated from polyethylene terephthalate.

6. The inflatable void forming device of any one of the preceding claims wherein the inflatable bladder is an elongate inflatable tubular bladder.

7. The inflatable void forming device of any one of the preceding claims wherein the inflatable bladder comprises a gas-tight sealed chamber containing a selectively actuable container of expansible fluid.

8. The inflatable void forming device of any one of claim 1 to claim 6 wherein the gas-tight sealed chamber comprises an inflation valve to permit inflation with an external source of pressurized fluid.

9. The inflatable void forming device of claim 1 wherein the flexible sheath is comprised of a high tensile strength plastics film.

10. The inflatable void forming device of claim 9 wherein the flexible sheath is comprised of a woven or non-woven fabric selected from the group consisting of polyolefins, polyesters, polyamides, polyurethanes, glass, carbon or Kevlar™ fibres.

11. The inflatable void forming device of claim 1 wherein the flexible sheath has antistatic properties.

12. The inflatable void forming device of claim 1 further comprising a layer of heat reflective material located between the inflatable bladder and flexible sheath.

13. The inflatable void forming device of claim 1 wherein the inflatable bladder and/or the flexible sheath are fabricated from a transparent or translucent material.

14. The inflatable void forming device of any one of the preceding claims wherein the inflatable bladder and the flexible sheath are fabricated as a laminate.

15. A method of forming a void in a column of explosives composition in a borehole comprising the steps of:- inflating, to a predetermined pressure, a void forming device comprising an elongate inflatable bladder fabricated from a flexible polymeric film, the bladder being located within a flexible stretch resistant sheath whereby, in use, the stretch resistant sheath resists an increase in volume of the bladder when pressurized to a predetermined degree; inserting the inflated void forming device into a borehole having a diameter greater than said void forming device; and introducing into said borehole, a quantity of flowable explosive composition to surround said void forming device, thereby forming a void in a column of explosives composition within said borehole.

16. The method of claim 15 wherein the flexible sheath is comprised of a stretch resistant polymeric film.

17. The method of claim 16 wherein the stretch resistant polymeric film comprises a laminate or multi-layer construction including a stretch resistant substrate.

18. The method of claim 15 wherein the bladder is an elongate inflatable tubular bladder.

19. The method of claim 15 wherein the flexible sheath is comprised of a high tensile strength plastics film.

20. The method of claim 19 wherein the flexible sheath is comprised of a woven or non-woven fabric selected from the group consisting of polyolefins, polyesters, polyamides, polyurethanes, glass, carbon or Kevlar™ fibres.

21. The method of any one of the preceding claims wherein the flexible sheath has antistatic properties.

22. The method of claim 15 wherein the void forming device further comprises a layer of heat reflective material located between the inflatable bladder and flexible sheath.