Note: Descriptions are shown in the official language in which they were submitted.
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EXPLOSIVE PRESSURE WAVE CONCENTRATOR
BACKGROUND OF THE INVENTION
This invention is concerned generally with a customized low energy method of
breaking rock
in a controlled manner.
As used herein the word "rock" includes rock, ore, coal, concrete and any
similar hard mass,
whether above ground or underground, which is difficult to break or fracture.
It is to be
understood that "rock" is to be interpreted broadly.
A number of techniques have been developed for the breaking of rock using non-
explosive
means. These include a carbon dioxide gas pressurisation method (referred to
as the
Cardox method), the use of gas injectors (the Sunburst technique),
hydrofracturing and
various methods by which cartridges containing energetic substances pressurise
the walls or
base of a sealed drill hole to produce a penetrating cone fracture (known as
PCF).
These techniques may be an order of magnitude more efficient than conventional
blasting in
that they require approximately 1/10 of the energy to break a given amount of
rock compared
to conventional blasting using high explosives. The lower energy reduces the
resulting
quantity of fly rock and air blast and to an extent allows the rockbreaking
operation to
proceed on a continuous basis as opposed to the batch-type situation which
prevails with
conventional blasting.
Most non-explosive rockbreaking techniques rely on the generation of high gas
pressures to
initiate a tensile fracture at the bottom or sides of a relatively short drill
hole.
Efficient confinement of the gas produced in the hole is a prerequisite for
ensuring that the
available energy is effectively used to break the rock. Problems with
confining the gas in the
hole which arise with current methods of non-explosive breaking are often due
to the jointed
or fractured nature of the rock in its natural state.
A jointed rock with open joints that traverse the drill hole in the rock will
tend to terminate any
cracks that are propagated by high-pressure gas toward the open joint by
dissipating the gas
pressure in the cracks at the intersection of the open joint. The result is
that in a hole which
is relatively long, where open joints are present, there is a difficulty in
fragmenting the rock
effectively over the length of the hole.
Attempts to deck the hole with separate charges of energetic substance
separated by plugs
of stemming run into the problem that each pressurised portion of the hole
must develop a
breaking point in the rock in order to propagate cracks. Due to the relatively
low pressure
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environment which prevails in~the hole when use is made of propellants,
compared to the
high pressure environments which exist with explosives, it is not always
possible for the
pressurised sections of the hole to create new cracks in the rock with the
result that the
pressure in the hole tends to dislodge its confining stemming material to form
a "blow-out' of
the stemming through the collar of the hole.
Thus, if the hole can be pressurised in separate sections and each of the
pressurised
sections can act independently to break its respective section of the hole,
the problem of
premature termination of crack propagation and the problem of blow-outs can be
overcome
or alleviated.
Low energy rockbreaking methods, such as those using propellants, generally
use high gas
pressures to propagate fractures originating from microfractures and points of
weakness in
the rock such as joints, fissures and faults. Depending on rock conditions and
mining
requirements such as breaking rock to a particular size, it may be desirable
to induce a
region of high stress concentration at any chosen location, at the bottom or
otherwise, in the
drill hole, to initiate new fractures in the rock. An object of the present
invention is to achieve
such a result.
It is also desirable for a variety of reasons to be able to initiate fracture
of the rock at a
predetermined location which is not, necessarily, at the bottom of the drill
hole.
When the propellant is initiated a pressure wave is generated which propagates
away from
the point of initiation. It is desirable to be able to reinforce the pressure
wave or increase the
energy density which is obtainable from the ignited propellant so that
localised high pressure
regions can be generated to initiate rock fracture at predetermined points.
SUMMARY OF INVENTION
According to the invention a method of breaking rock includes the steps of:
(a) loading at least a first cartridge into a hole in a rock face;
(b) confining the cartridge in the hole;
(c) initiating a propellant in the cartridge thereby to cause the release of
pressurised
material,
(d) supporting a base of the cartridge to prevent the base from fracturing
under the effect
of the pressurised material, and
(e) directing the pressurised material at least to a periphery of the base to
initiate
breakage of rock adjacent the periphery.
In one form of the invention the cartridge is supported at a bottom of the
hole.
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In an alternative embodiment the cartridge is supported inside the hole at a
location which is
spaced from the bottom of the hole. The cartridge may be supported, for
example, on
stemming. With this form of the invention the base of the cartridge is thus
separated from
the bottom of the hole.
A plurality of cartridges may be used inside the hole. Thus a first cartridge
may be
positioned at a first location at or near a bottom of the hole and a second
cartridge may be
positioned at a second location in the hole which is spaced from the first
location. Third or
even fourth cartridges may be employed according to requirement.
Stemming may be positioned inside the hole between successive cartridges.
According to a different aspect of the invention there is provided a method of
breaking rock
which includes the steps of:
(a) supporting a plurality of cartridges at respective locations in a hole in
a rock face, the
respective locations being spaced from each other in an axial direction of the
hole,
(b) igniting propellant in the respective cartridges thereby to cause the
release of
pressurised material inside each cartridge, and
(c) at each location directing force which is generated by the respective
pressurised
material onto a respective surface of a wall of the hole at or near a base of
the
respective cartridge.
The invention may include the step of deforming the pressure wave to create at
least one
region inside the hole which has an increased stress concentration.
At the region the energy density of the wave may be greater than elsewhere in
the hole.
This creates a stress point in the rock at or adjacent the region.
The pressure wave may be deformed in any appropriate way, using any suitable
technique.
Without being limiting the pressure wave may be deformed by at least one of
the following:
by shaping the cartridge at one or more regions to induce pressure wave
deformation; by
inserting or forming one or more wave deforming members on an inner or outer
side of the
cartridge; by locating one or more wave deforming members inside the
cartridge.
In one form of the invention the pressure wave is deformed by suitably shaping
a base or a
side wall of the cartridge.
In one embodiment the method includes the step detonating a first high-
explosive inside the
hole to generate a localised explosive shock wave in the rock.
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Step (c) may be carried out, according to requirement, substantially
simultaneously with or
slightly before or slight after, the last mentioned step.
In the last mentioned step more than one high-explosive may be detonated.
These high-
explosives may be detonated substantially simultaneously or a second high-
explosive may
be detonated a predetermined time period after detonation of the first high
explosive.
The method may include the step of deforming the pressure wave which is
produced by the
propellant. The pressure wave may be deformed in a region which is close to or
substantially coincident with the region in which the high explosive is
detonated in step (c).
In a variation the method includes the step of generating a high pressure jet
of a second
material which has a density which is greater than the density of the
pressurised material.
Stemming material of any appropriate kind may be placed in the hole over the
cartridge in a
manner which is known in the art.
The cartridge may be used to confine the pressurised materials in the
cartridge whereby the
cartridge is expanded into sealing engagement with a wall of the hole
surrounding the
cartridge so that, initially, the cartridge reinforces the wall of the hole.
To allow the aforementioned sealing engagement of the cartridge with the wall
of the hole
the cartridge may be made from a malleable material. "Malleable" in this sense
includes a
material which is capable of plastic deformation, without fracture, at least
to the point at
which the cartridge is in close contact with the surrounding wall of the hole.
The cartridge, when it fractures, allows the high pressure materials to
initiate rock breakage.
The high pressure jet of the second material may be generated at one or more
predetermined positions in the cartridge.
The high pressure jet of the second material may be generated by the action of
the
pressurised material, released in step (c), on at least one member which
includes the second
material.
Alternatively or additionally to the aforegoing the high pressure jet of the
second material
may be generated by the action of an explosive on at least one member which
includes the
second material.
The explosive may be detonated by the action of the pressurised material or it
may be
directly detonated by suitable control means substantially at the same time as
the propellant
is initiated or slightly before or slightly after the time at which the
propellant is initiated.
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The propellant may be initiated at a first predetermined time, at least at a
first zone, and the
method may include the step at a second predetermined time of carrying out at
least one of:
(i) detonating an explosive in the hole, and
(ii) initiating the propellant at least at a second zone in the hole.
5 The first and second predetermined times can, in essence, be coincident.
However a
predetermined interval may exist between the first and second predetermined
times.
According to requirement the first predetermined time may be before the second
predetermined time, or vice versa.
The explosive in the hole may be separate from the cartridge and may be
physically
displaced from the cartridge. Alternatively the explosive may be inside the
cartridge or on an
outer side of the cartridge.
Where a time interval exists between the first and second predetermined times
the duration
of the time interval may be controlled by using electronic techniques.
The propellant and the explosive may be initiated and detonated, respectively,
by means of
respective control signals which are transmitted from a control unit or units
via control lines
or by using wireless techniques.
The method may include the steps of creating at least two pressure waves,
resulting from
initiation of the propellant or propellants, and allowing the pressure waves
to interfere with
each other at a predetermined region.
In one form of the invention each pressure wave is generated by initiating a
respective
propellant.
In a different form of the invention the pressure waves are generated by
initiating the
propellant at two respective points which are spaced from each other.
The pressure waves may be generated inside a single enclosure. In a different
form of the
invention each pressure wave is generated inside a respective enclosure.
According to a different aspect of the invention there is provided a method of
breaking rock
which includes the steps of:
(a) loading a cartridge into a hole in a rock face;
(b) initiating a propellant in the cartridge, at least at first and second
points which are
spaced from each other in the cartridge, thereby to generate at least two wave
fronts
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which are caused to interact with each other, each wave front causing the
release of
pressurised material; and
(c) confining the pressurised material in the cartridge.
The cartridge may be elongate and the first and second points may be located
respectively
at opposed ends of the cartridge.
The initiation of the propellant at the first and second points may occur
substantially
simultaneously or initiation at one point may take place at a predetermined
time interval after
initiation at the other point.
According to another form of the invention there is provided a method of
breaking rock which
includes the steps of loading first and second cartridges into a hole in a
rock face and
initiating respective propellants in the cartridges at respective first and
second points thereby
to cause the generation of pressure waves which are allowed to interact with
each other at a
location which is between the first and second points.
The invention also provides apparatus for breaking rock which includes a first
cartridge with
a base and a side wall which form an enclosure, and a propellant inside the
enclosure, and
wherein a discontinuous relatively weaker region of the container is formed at
a junction
between the wall and the base.
The cartridge may be generally cylindrical in shape and the base may be
substantially at
right angles to a longitudinal axis of the cartridge.
The base may be substantially more robust than the wall of the container and
to achieve this
the base may be made from a stronger or thicker material than the wall.
In one embodiment of the invention the base is shaped to direct a wave of
pressurised
material, produced by the ignited propellant, towards a periphery of the base.
This may be
achieved in any appropriate way and, for example, the base, on an internal
surface, may be
, substantially conical in shape.
The apparatus may include at least one pressure wave deforming member which is
exposed
to a pressure wave generated by initiating the propellant.
The cartridge may include a device for initiating the propellant.
The pressure wave deforming member may be selected from the following: at
least one
formation on or near the base; at least one formation on an inner or outer
surface of the side
wall; at least one suitably shaped member inside the cartridge, or outside the
cartridge; at
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least one suitably shaped member inside the propellant positioned at a desired
distance
relatively to the base.
The apparatus may include at least one high-explosive charge on or inside the
cartridge.
The apparatus may include at least one pressure wave deforming member which is
positioned inside or outside the cartridge and at a region which is adjacent
the location at
which the high-explosive charge is located.
The cartridge is preferably made from a plastically deformable material. Thus
the cartridge
may be made from a material which is capable of plastic deformation, without
rupturing, by at
least a predetermined extent eg. by at least 10%.
In one embodiment the apparatus includes at least one member, which is made
from a
material which has a density greater than the density of the propellant, on or
inside the
cartridge.
The density of the member should, within reason, be as high as possible. For
example iron
has a density of the order of 7,8 and other metals or substances have
densities in excess of
this. Lead for example has a density of approximately 11,3. Uranium has a
density of the
order of 19. These substances are given merely by way of example as being
suitable for use
in the apparatus of the invention. In the last mentioned case it is preferable
to make use of
uranium in its depleted form to minimise radioactivity consequences.
The member may be positioned at a predetermined point inside the cartridge.
The member may be turned into a high pressure jet by the action of the
propellant when it is
ignited. Alternatively or additionally an explosive which acts directly on the
member may be
used to generate a high pressure jet of the material.
The means which is used for igniting the blasting agent may be used for
detonating the
explosive. Alternatively a separate initiator is used for detonating the
explosive
independently of the means which is used for igniting the blasting agent.
The apparatus may include an explosive, and a control unit which initiates the
propellant at a
first predetermined time and which detonates the explosive at a second
predetermined time.
The first and second predetermined times may be coincident or a predetermined
time
interval may exist between the first and second predetermined times.
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The control unit may be an integralmechanism or may include a first mechanism
which is
used for initiating the propellant and a second mechanism which is used for
detonating the
explosive.
The explosive may be separate, ie. physically displaced, from the cartridge,
positioned on an
outer surface of the cartridge, or located inside the cartridge.
The control unit may be used for generating control signals which are
transmitted to the
propellant and to the explosive respectively, for initiation and detonation
thereof. The control
signals may be transmitted using communication links of any appropriate kind
eg. physical
conductors or optic links, or by making use of wireless techniques or the
like.
The apparatus may include at least first and second initiators for igniting
the propellant at
respective first and second points which are spaced from each other inside the
cartridge.
In another form of the invention there is provided apparatus for breaking rock
which includes
first and second cartridges, each cartridge forming a respective enclosure for
a respective
propellant, each cartridge including a respective initiator for igniting the
propellant in the
respective enclosure, and wherein the cartridges are positioned in an assembly
with the
initiators at opposed remote points in the assembly.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is further described by way of examples with reference to the
accompanying
drawings in which:
Figure 1 illustrates from the side and in cross section apparatus according to
one form of the
invention;
Figures 2 to 8 respectively illustrate from the side and in cross section
different forms of
apparatus for breaking rock according to the invention;
Figures 9, 10 and 11 respectively illustrate different forms of apparatus for
breaking rock
according to the invention;
Figure 12 is a side view in cross section of apparatus for breaking rock
according to another
form of the invention;
Figure 13 is a side view in cross section of apparatus for breaking rock
according to a further
form of the invention;
Figure 14 is a side view in cross section of apparatus for breaking rock
according to a
variation of the invention;
Figure 15 schematically illustrates a control circuit which is used in the
method of the
invention;
Figure 16 illustrates an alternative control circuit for use in the method of
the invention; and
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Figures 17 to 19 respectively illustrate apparatus for breaking rock in
accordance with
different embodiments of the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
Figure 1 of the accompanying drawings illustrates a hole 10 which is formed in
rock 12 by
drilling from a rock face 14 using conventional drilling machines and
techniques which are
not further described herein.
The hole 10 has a length 16 and a diameter 18. The hole has a bottom 20 which,
ideally, is
substantially at right-angles to side walls 22 of the hole. It is to be noted
however that this
ideal is rarely reached in practice for due to wear of the bit which is used
for drilling the hole
10 or poor operator technique the "corners" 26 between the bottom 20 and the
wall 22 are
often concave in shape with the result that the bottom 20 is normally at least
slightly
rounded.
A cartridge 30 is loaded into the hole so that its base 32 is in contact with
the bottom 20 of
the hole. The cartridge is made from any appropriate material, such as, for
example, a high
density plastics material. The cartridge includes a side wall 34 which extends
upwardly from
the base 32 and which is generally of circular cylindrical shape. At an upper
end 36 the
cartridge 30 is domed in shape.
The base 32 has a thickness 38 which is significantly greater than the
thickness 40 of the
wall 34. The base 38 is therefore substantially more robust than the wall 40.
The cartridge
is filled with a propellant 42 which can be ignited by means of an initiator
44, of known
construction, which is located within the cartridge . Control wires 46 lead
from the initiator 44
to a control unit which is used for controlling the breaking operation. The
control unit is of a
type which is known in the art and consequently is not further described
herein.
As used herein "propellant" is to be interpreted broadly to include a
propellant, blasting
agent, explosive, gas-evolving substance, or similar means which, once
initiated, generates
high pressure material typically at least partly in gaseous form. Propellants
of this nature are
known in the art. Propellant and blasting agent are used interchangeably.
Stemming 50 is positioned inside the hole 10 over the cartridge 30 to a
desired extent.
Thereafter a cartridge 52 is loaded into the hole, resting on the underlying
stemming. The
amount of stemming 50 placed in the hole is such that the cartridge 52 is
consequently
supported inside the hole at a desired spacing from the lower cartridge 30.
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It is apparent that further cartridges can be supported inside the hole,
according to the length
of the hole and blasting requirements. The present invention is however
described with
reference to the use of two cartridges inside the hole but this is a non-
limiting example.
The cartridge 52 is in many respects similar to the cartridge 30 and
components which are
5 the same as components in the cartridge 30 bear identical reference numerals
and are not
further described.
It is to be noted however that the base of the cartridge 52, designated 32A,
is substantially
conical in cross section with an apex 54 of the cone extending into the
interior of the
cartridge at a central location thereof.
10 Stemming 56 is placed over the cartridge 52 and tamped in position.
The propellants 42 in the various cartridges are ignited substantially
simultaneously by
means of control signals applied through the wires 46 to the respective
initiators 44. Ignition
of the propellants causes the release of high pressure jet material,
substantially in gaseous
form, in each of the cartridges.
In respect of the cartridge 30 the base 32 is forced downwardly by the high
pressure jet
material expanding inside the cartridge interior and is driven into close
contact with the
bottom 20 of the hole. Due to the robust nature of the base gas inside the
cartridge is
prevented from venting directly onto the bottom 20. The gas is instead
directed towards the
right angled junction 60 between the walls 34 and the base 32 which, at least
due to its
shape, is discontinuous and therefore constitutes a line of weakness. The
junction 60 could,
if desired, be deliberately weakened by reducing the quantity of material
which is used at the
junction.
The base 32 thus provides a solid surface which, at least initially, is gas
impermeable and
the high pressure gas thus fractures the junction 60 and is thereby directed
into that portion
of the wall 26 ("the corners") which surrounds the junction. Fracture of the
wall is thereby
induced or initiated at this region.
In respect of the cartridge 52 the base 32A is, as before, significantly
robust and is also
forced by the high pressure jet material expanding inside the confinement
structure
constituted by the cartridge onto the underlying stemming 50. The stemming 50
in
conjunction with the base 32A effectively defines a "false" bottom of the
hole, insofar as the
cartridge 52 is concerned. The high pressure material inside this cartridge is
then directed
by the conical upper surface of the base 32A towards the peripheral region 26A
of the base
which, as before, is discontinuous or weakened so that pressure release takes
place, at least
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initially, at this region. The gas which is released at the side wall, in the
region of the
periphery of the base, fractures the rock at this region.
The invention thus provides a technique whereby the wall of a relatively
elongate hole, in the
rock face, can be fractured at two or more points which are spaced from one
another by
inducing localised stresses at these points. At locations other than the
bottom of the hole the
localised stresses result from creating a "false" hole bottom by using a
robust base of the
cartridge which is supported by means of the underlying stemming and which is
shaped to
direct the pressurised material, released by the ignited propellant, towards
the surrounding
wall of the hole to cause its fracture.
A principal benefit of the method of the invention is that a relatively large
amount of rock over
an extended hole distance can be released in a manner which makes efficient
use of
propellant and which allows cycle times for blasting and clearing to be
contained.
In the following description reference numerals which are the same as those
used in
connection with Figure 1 are used, where applicable, to indicate like
components.
Figure 2 illustrates a hole 10 with a length L which is of the order of at
least four times the
diameter D of the hole.
A cartridge 30A is loaded into the hole. The cartridge has a base 32 and a
cylindrical side
wall 34 which extends upwardly from the base and which, at an end which is
remote from the
base, has a rounded shape.
The base and the wall 34 form an enclosure for a propellant 42 of any
appropriate
composition. The propellant is compressed into the cartridge under factory
conditions using
techniques which are known in the art. An initiator 44 is loaded into the
cartridge. The
initiator is located at the rounded upper end but this is by no means limiting
and the initiator
can be loaded into the cartridge at any appropriate point.
Control wires 46 lead from the initiator to a unit, not shown, which is used
in a known manner
for initiating the blasting process.
Stemming 50 is placed into the hole and is tamped or otherwise secured in
position.
The base 32A is formed with a central socket-like region 60 which directly
opposes a bottom
20 of the hole. The region 60 is flanked by a sloping or conical-like
formation 62 which
extends downwardly towards a lower extremity of the wall 34.
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Ignition of the propellant 42 ~by the initiator 44 causes the release of high
pressure jet
material which is substantially in gaseous form. The cartridge 30A is designed
to contain the
expanding high pressure jet material and is allowed to deform outwardly,
without rupturing,
so that the wall of the cartridge is forced into sealing contact with an
opposing surface of the
wall 22 of the hole. The cartridge does not fracture during this process for
it is fabricated
from a plastically deformable material.
At an upper end the cartridge is contained by the stemming 50.
The high pressure jet material released by ignition of the propellant gives
rise to a pressure
wave which propagates in the cartridge downwardly as the propellant ignites.
The pressure
wave strikes the base 32A and the conical formation 62 directs the pressure
wave, which
impinges on the formation, radially outwardly towards the periphery of the
base. The
pressure wave is thus deformed and a high energy density region of the
pressure wave is
produced at lower peripheral extremities of the base more or less at the
junction of the
bottom of the hole with the side wall 22. This causes fracturing of adjacent
regions of the
rock.
It is possible to deform the pressure wave generated by the ignited propellant
in a variety of
ways. The invention is not restricted in this regard. In Figure 2 a base of
the cartridge is
shaped to produce the desired way of deformation. As a consequence rock
fracture is
initiated at the bottom of the hole.
Figure 3 illustrates an alternative technique wherein a cartridge 30B is
placed in a hole 10
and covered with stemming 50. There are strong similarities between the
arrangement
shown in Figure 3 and that shown in Figure 2 and for this reason components
which are the
same in the two embodiments bear like reference numerals. It is to be noted
however that in
the Figure 3 embodiment of the invention the base 32 is planar and, to a
substantial extent,
rests on the bottom 20 of the hole. Thus the base is not used, in itself, to
deform the
pressure wave inside the cartridge.
Two rings 64 and 66 are positioned inside the cartridge and are secured to the
inner surface
of the wall 34 by means of a suitable adhesive. This step is taken before the
propellant 42 is
placed inside the cartridge.
When the propellant is ignited a pressure wave is transmitted through the
combusting
charge. Discontinuities are created by the rings 64 and 66 which present
localised barriers
to propagation of the wave. Although the resulting effect on the pressure wave
is complex
high energy regions of the pressure wave are generated in the vicinity of each
ring. It is
believed that the deformation of the pressure wave gives rise to interference
between two or
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more pressure wave fronts and this in turn gives rise to an increase in the
energy density.
Another factor is that the pressure wave passes through a region of a first
set of properties,
ie. those of the combusting propellant, into a region with a second set of
properties, ie. those
arising from the material of each ring. This causes diffraction effects and,
again, the
pressure wave is deformed. The applicant has established through
experimentation that by
choosing the size and position of the rings correctly the rock can be caused
to fracture at
regions other than the bottom of the hole.
In the arrangement shown in Figure 4 a discontinuity is created inside the
interior of a
cartridge 30C by forming the side wall 34 with an internally extending
circumferential channel
or recess 68. The channel 68 has an effect similar to that of the ring 64 in
that a localised
high stress region is produced by deforming the propagating pressure wave
which is
generated by combustion of the propellant 42.
Figure 5 illustrates a cartridge 30D which has a ring 70 at a desired location
on an outer
surface of the side wall 34. When the propellant 42 is ignited the wall is
forced radially
outwardly into close sealing contact with the wall 22 of the hole. The ring 70
is however not
compressible to any significant extent and consequently forms an inwardly
extending
peripheral rib or ridge which acts in a manner which is similar to that of the
ring 64 in Figure
3. Once again the pressure wave is deformed and a localised high energy region
is
produced which gives rise to fracture of the rock at a region which is close
to the ring 70.
Figure 6 shows a cartridge 30E with external ribs 72 and 74 respectively which
are integrally
formed with the side wall at selected locations. The ribs 72 and 74 function
in the same way
as the ring 70 shown in Figure 5 in that they deform the pressure wave and
produce high
energy stress regions which promote localised cracking or fracture of the
opposing rock
surface.
Figures 7 and 8 illustrate that it is possible to deform the pressure wave
inside the cartridge
by using members which are not at a periphery of the cartridge. In the Figure
7 arrangement
a cartridge 30F with a regular side wall 34 and a planar base 32 is positioned
in a hole 10. A
solid insert 76 is positioned inside the propellant 42. The insert is
supported on a stalk 78
which extends upwardly from the base 32. The size of the insert 76 may vary
according to
the degree of pressure wave deformation which is required. Although the
resulting situation
is complex and at least to some extent the size and position of the insert may
be required to
be determined empirically it is possible to produce localised high stress
regions in order to
promote rock fracture at one or more predetermined locations.
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14
The cartridge 30G shown in Figure 8~is similar to that shown in Figure 7 in
that an insert 80 is
effectively imbedded in and surrounded by the propellant 42. In this instance
however the
insert is supported on small arms 82 which extend from an inner surface of the
side wall 34.
It is to be noted that the cartridge confines the expanding high pressure jet
material which is
released by the ignited propellant in such a way that the cartridge is
expanded and is thereby
forced into contact with the surrounding wall of the rock. As the pressure
wave propagates
through the cartridge interior the pressure wave deforming member or members,
which can
take on a variety of forms, produce localised high energy regions which
promote rock
fracture at predetermined points in the rock mass.
In the arrangement shown in Figure 9 two rings 84 and 86 of high-explosive
material are
positioned inside a cartridge 30H at desired locations and are secured in
position using any
appropriate technique. In the illustrated example the ring 86 rests on the
base 32 of the
cartridge while the ring 84 is adhesively secured to the inner surface of the
wall 34. The
nature of the high-explosive material may vary according to requirement and
for example
may comprise CORTEX or P10 ( these are registered trade marks) or the like.
The high-
explosive material may also comprise or include aluminium powder or any other
high energy
content material.
When the propellant 42 is ignited a pressure wave propagates through the
cartridge interior
as combustion of the propellant takes place. High pressure material is
released by the
combusted propellant. The high-explosive rings 84 and 86 are also detonated at
precisely
controlled times. When the high-explosive ring 84 is detonated an explosive
shock wave is
generated which reinforces the propellant pressure wave. Another effect which
comes into
play is that the explosive shock wave can increase the efficiency of
combustion of the
propellant and in this way enhance the propellant pressure wave. It is
believed that the
explosive shock wave can act as a booster to the propellant pressure wave and
thereby
increase the intensity of the shockwave on the rock. In essence therefore
respective or
separate shock waves and pressure waves are generated by each high-explosive
ring and
the propellant respectively. Particularly in the regions of the high-explosive
rings 84 and 86
high energy stress regions are created in the adjacent rock masses.
The cartridge 30H initially expands plastically confining the high pressure
material which is
released by the combustion and explosive processes. Substantial force is
thereby generated
inside the cartridge. As the cartridge fractures the energy released by the
explosive rings 84
and 86 combined with the energy contained in the pressure wave from the
propellant 42
results in localised fracture of the rock at least initially in the region of
the ring 84 and at the
bottom of the hole.
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In the example of the inventiori shown in Figure 10 components which are the
same as those
described in connection with Figure 9 bear like reference numerals and are not
further
described herein.
In the Figure 10 embodiment an explosive ring 88 is positioned on an external
surface of the
5 cartridge 30J. The explosive ring is detonated at a carefully chosen time
relatively to the
instant at which the propellant 42 is ignited. Once again the pressure wave
produce by the
propellant is enhanced or boosted by the shock wave generated by the
detonation of the
explosive 88.
Figure 10 illustrates a further variation in that alternatively or in addition
to the ring 88 an
10 explosive charge 90 may be positioned inside the propellant 42. In the
first instance the
charge 90 acts to deform the pressure wave which is produced by the propellant
42 while a
second effect arises, in a manner similar to what has been described, once the
explosive 90
is detonated in that a booster shock wave is generated which enhances the
effect of the
propellant pressure wave.
15 Figure 11 illustrates an embodiment of the invention in which deformation
of the propellant
pressure wave is achieved by means of a ring or any other appropriate
deforming member
92 which, in this example, is positioned inside the cartridge 30K. An inwardly
extending
peripheral groove 94 is formed in the side wall 34 of the cartridge and the
ring 92 rests on
the groove. The groove is externally filled with explosives 96 which is sealed
in position by
means of a surrounding cover strip 98. When the propellant 42 is ignited the
ring 92 acts to
deform the resulting pressure wave and this gives rise to a high energy region
of the
pressure wave in the vicinity of the ring 92. The explosive 96, once
detonated, produces an
explosive shock wave which enhances the high energy region and this promotes
fracture of
the rock body in the locality of the ring and the groove 94.
An important aspect of the invention resides in the ability of the cartridge
to contain the
propellant pressure wave so that premature release of the energy generated by
the
propellant combustion does not take place. The shock wave which is caused by
detonation
of the explosive enhances the propellant pressure wave and once the cartridge
fractures the
rock mass is caused to fracture by the release of high pressure jet material
directed at the
rock at a controlled region which is determined beforehand.
The explosive charges can be detonated by means of control signals transmitted
over wires
46A which are connected directly to the wires 46.
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16
It is possible though to connect the wires 46A to a separate control unit so
that, in each case,
the respective initiator 44 is initiated at a first time while the respective
explosive is detonated
at a second time which may be a predetermined period before or after the first
time.
In Figure 12 two ring-shaped inserts 100 and 102 are positioned inside a
cartridge 30L. The
insert 102 rests on the base 32 while the insert 100 is positioned at a
predetermined
intermediate location. The insert is kept in position by means of a suitable
adhesive or
alternatively is frictionally engaged with an inner surface of the wall 34.
Any other
appropriate technique can be used to secure the insert at a desired position
inside the
cartridge.
The inserts 100 and 102 are made from a material which has a density greater
than the
density of the propellant 42. Ideally the density of each insert should be as
high as possible
under the circumstances. The inserts can be made from any appropriate
substance which
does not have undesirable side effects. For example the inserts can be made
from lead or a
composition which contains any other heavy metal which is not harmful. It is
also possible to
make use of depleted uranium, a dense substance which has a reduced
radioactivity level.
Ignition of the propellant 42 by the initiator 44 causes the release of high
pressure jet
material which is substantially in gaseous form and which is directly
generated by the
combustion of the propellant. The cartridge 30L is designed to contain the
expanding high
pressure material initially in that it is allowed to deform outwardly without
rupturing, so that
the wall of the cartridge is forced into close sealing contact with an
opposing surface 22 of
the wall of the hole. The cartridge does not initially fracture for, as noted,
it is fabricated from
a plastically deformable material.
The cartridge confines the high pressure gas released by the propellant 42.
During this
process the inserts 100 and 102, at least to a substantial extent, remain
integral.
The pressure wave which is generated by ignition of the propellant 42 advances
through the
cartridge. The insert 100 deforms the pressure wave and this results in a high
energy region
being established at the locality of the insert. This, in itself, helps to
cause the rock to crack
once the cartridge fractures as the pressure inside the cartridge builds up to
a predetermined
point.
At the base of the cartridge the insert 102 also causes wave deformation and,
in a manner
similar to what has been described, this gives rise to a high pressure region
more or less at
the junction of the side wall with the base 32. A further factor comes into
play in that the
junction of the wall 34 with the base 32 is discontinuous and this further
promotes the
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17
generation of localised high pressures which subsequently cause fracturing of
the rock in the
region of the interface between the bottom 20 and the side wall 22 the hole.
When the cartridge fractures each insert 100 and 102 disintegrates, thereby
producing a high
pressure jet of material which is directed into the adjacent rock surface.
This high pressure
jet is of a material with a density which is significantly greater than the
density of the
propellant 42 or, for that matter, than the density of the material from which
the cartridge is
made. This latter material is usually a plastics material. Each insert thus
gives rise to a high
pressure jet of massive material which has considerable rock breaking
capability.
It follows that by correctly positioning and shaping the inserts it is
possible to initiate cracking
of the rock at a chosen position.
The cartridge 30M shown in Figure 13 bears substantial similarities to what is
shown in
Figure 12 and where applicable like components are designated by means of like
reference
numerals. Again use is made of two inserts designated 100A and 1008
respectively which
are made from a suitable high density material. Each insert has a small
explosive charge
104A and 1048 respectively which is packed in close contact with the insert.
When the propellant 42 is ignited the explosive charges are detonated as the
pressure wave
advances through the propellant. In each case the explosive charge helps to
disintegrate the
heavy metal insert thereby producing a localised high pressure jet of material
which is
effective at initiating fracture of the adjacent rock. It is to be borne in
mind that the effect of
the explosive is enhanced by the ability of the insert and the explosive,
prior to detonation, to
deform the pressure wave which is generated by the combusting propellant.
It is possible to ignite the explosive charges by sending an appropriate
control signal directly
to the charges. Optionally, if necessary, use could be made of a local
detonator at the
explosive charges. The control signal can be transmitted via a control wire
46A which is
directly electrically connected to the control wires 46. Alternatively
separate control signals
can be sent on the control wires 46 and 46A in order to detonate the explosive
charges at a
predetermined time relatively to the time at which the propellant 42 is
ignited.
In Figure 14 a cartridge 30~ contains a ring of explosive material, designated
106, which is
positioned on an inner surface of the wall 34. A control lead 46D extends from
the explosive
ring to a control unit 108.
Figure 15 illustrates somewhat schematically the use of the control unit 108
in conjunction
with a circuit 110 which is associated with the initiator 44 and a circuit 112
which is
associated with the explosive 106.
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18
The control unit 108 is powered by an external source 114, for example a
battery, and
includes a wave generator 116 which produces coded pulses each of an
appropriate shape
and with a desired energy content, using techniques which are known in the art
and which
are impressed on the line 46.
The circuit 110 is preferably mounted inside a housing of the initiator 44 as
is somewhat
schematically illustrated in Figure 14. Similarly the circuit 112 is
physically located adjacent
the explosive ring 106, again as is indicated in Figure 14.
The circuit 110 includes a timer 118 and a capacitor 120. The mechanism 112
includes a
separate timer 122 and a capacitor 124. An active component of the initiator
44 which, when
energised, produces a hot spot which results in initiation of the propellant
42, is designated
126 in Figure 15. A similar hot spot initiator, which is used to detonate the
explosive 106, is
designated 128 and is connected to the timer 122. Hot spot initiators of this
type are known
in the art and consequently are not further described herein.
When the rock breaking process is commenced a control signal is sent from the
unit 116 on
the line 46 to the initiator 44. The control signal is also applied by the
line 46C to the
capacitor 124. The capacitors 120 and 124 are charged by the control signal to
respective
voltages which permit operation of the timers 118 and 122. These devices may
communicate with each other via the line 46C and their operation can therefore
be
coordinated or synchronised so that each timer commences, at the same instant,
timing a
respective predetermined time interval. Through the use of suitable electronic
circuitry highly
accurate and precisely controlled timing intervals can be achieved.
When the timer 118 reaches the end of its timing interval energy from the
charged capacitor
120 is discharged, by closing an internal switch in the timer, into the hot
spot initiator 126.
Similarly, at the end of the timing interval of the timer 122, closure of an
internal switch in the
timer causes the discharge of energy from the cap~itor 124 into the hot spot
initiator 128.
When the hot spot initiator 126 is energised it causes combustion of the
propellant 42.
Similarly energisation of the hot spot initiator 128 causes detonation of the
explosive 106.
Normally the difference between the time at which the propellant is initiated
and the time at
which the explosive is detonated is small, of the order of micro-seconds, and
although the
combustion of the propellant takes place rapidly the resulting pressure wave
does not cause
the cartridge to disintegrate before the timer 122 causes the explosive 106 to
be detonated.
In other words although the propellant and the explosive are initiated in
rapid succession the
time interval between these two events can be precisely controlled in order to
optimise the
effect which the explosive has on the pressure wave which is released by the
combustion of
the propellant.
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19
The control wires 46 and 46C may be conductive, for conveying electrical
signals, or may be
formed by fibre optic cables for conducting optical signals. In the latter
case it is not feasible
however to transmit meaningful quantities of energy from the control unit to
the circuits 110
and 112. In this instance the capacitors 120 and 124 are dispensed with and
are replaced
by small onboard batteries which provide the required energy for operating the
timers and for
energising the respective hot spot initiators.
Figure 16 illustrates a wireless technique which is used in place of a
physical connection
between the control unit 108 and the cartridge. The control unit 108 includes
a timer 130
which is powered from an electrical source 132. A transmitting antenna 134 is
used to
radiate a signal 136 to a receiving antenna 138 which is positioned at the
initiator 44. The
received signal is rectified by a diode 140 and the rectified output is used
to charge a
capacitor 142. An onboard timer 144 is powered by the capacitor 142 and, at an
appropriate
time which is measured by the timer, energy from the capacitor 142 is
discharged into a hot
spot initiator 146.
Clearly suitable safeguards must be built into the control system, which is
used for firing the
propellant, to ensure that stray signals from extraneous sources, including
noise, do not
inadvertently cause initiation of the propellant.
In Figure 17 a first initiator 44E is engaged with the cartridge 30R. The
initiator is located at
a rounded upper end of the cartridge. A second initiator 44F, which may be
identical to the
initiator 44E, is engaged with the base 32 of the cartridge.
Control wires 46C and 46D extend from the two initiators to a control unit,
not shown, which
is of conventional construction.
The control wires 46C and 46D may be electrically connected to each other or
alternatively
may extend separately to the control unit. In the former case one control
signal may be
impressed on the wires to energise the initiators 44E and 44F substantially
simultaneously.
In the second instance however separate control signals are impressed on the
wires 46C
and 46D respectively to energise the initiators 44E and 44F. With this form of
the invention it
is possible to fire the initiators at intervals which are slightly spaced, by
a predetermined time
interval, from each other.
When the initiators are fired the propellant material 42 in the region of each
initiator is ignited
and a rapid combustion process takes place which gives rise to the generation
of two high
pressure waves which advance towards each other from respective ends of the
cartridge ie.
from the initiators 44E and 44F. The pressure waves are accompanied by the
release of
high pressure jet materials which are substantially in gaseous form. The
pressure waves
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advance towards each other and depending on physical conditions inside the
cartridge and
the times at which the initiators are fired interfere or meet with each other
approximately at a
central region, designated 150, of the cartridge. Interference of the pressure
waves gives
rise to a high pressure, or high stress region, more or less at the middle of
the cartridge.
5 Initially the cartridge contains the expanding high pressure jet material
and deforms
outwardly, without rupturing, so that the wall 34 of the cartridge is forced
into intimate sealing
contact with an opposing surface of the wall 22 of the hole. The cartridge
does not fracture
during this process for, as noted, it is preferably fabricated from a
plastically deformable
material.
10 The cartridge thus effectively confines the high pressure gas and the wall
of the cartridge,
since it is in close contact with the wall of the hole, effectively reinforces
the wall.
The central region of the cartridge is, as noted, the region at which high
stress
concentrations occurs due to the interference of the two pressure waves with
each other.
Consequently when the cartridge ruptures the rock in the vicinity of the
region 150 is initially
15 fractured by the high pressure jet material.
It follows that by confining the high pressure jet material inside the
cartridge and by allowing
the two pressure waves to interfere with each other the cartridge can be
caused to fracture at
a desired point which means that the force which is released by the combusting
propellant
can then be directed onto a chosen surface of the wall of the hole adjacent
the point or
20 region at which the shock waves interfere.
Figure 17 also illustrates a variation of the invention. A dotted line 152
indicates that the
volume which is occupied by the cartridge 30E could be occupied by two
relatively smaller
cartridges designated 3081 and 3082 respectively. Each cartridge carries a
respective
initiator 44E or 44F, substantially as shown in the drawing.
The cartridges are however orientated so that their respective bases,
designated by the
dotted line 152, abut each other with the cartridge assembly, which is
elongate, being such
that the initiators 44E and 44F are at opposed respectively points of the
elongate assembly.
The initiators are fired substantially simultaneously and pressure waves in
each respective
cartridge are then propagated towards the respective bases at which point the
pressure
waves interfere with each other, substantially in the manner which has been
described and
give rise, again, to a high stress region. In this case however the bases of
the cartridges act
to deflect the pressure waves outwardly and, through suitable design, this
feature can be
used to enhance the high stress region yet further. For example it is possible
to form each
base with a conical shape, as is indicated by means of dotted lines 154A and
1548
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21
respectively, so that the pressure waves are initially deflected radially
outwardly before
interacting directly with each other.
Figure 18 illustrates a cartridge 30T which is similar in many respects to
what has been
shown in Figure 17 and where applicable like reference numerals are used to
designate like
components.
The initiators which are used in the arrangement of Figure 18 are, in this
instance, made
from an inert material such as carbon wire and are designated respectively 44G
and 44H
and are positioned adjacent a surface of the wall 34 of the cartridge. The
initiators are
directly exposed to the propellant 42 and are fixed under factory conditions
to the cartridge
30T. The control wires 46C and 46D which lead to the initiators are embedded
in the wall of
the cartridge.
In the arrangement shown in Figure 19 each initiator is constituted by a
substantially circular
loop of filament wire 44X and 44Y respectively. As is the case with the Figure
18
embodiment each filament wire is made from an inert material such as carbon
wire. "Inert" in
this sense means a material which, in the absence of an electric current
passing through the
material, is not capable of emitting a spark or showing any other phenomenon
which can
cause ignition of a propellant. With the arrangement of Figure 19 the blasting
agent is
initiated, at each of two spaced locations, over a relatively substantial
distance or area, or at
a plurality of points. In the arrangement shown in Figure 18 on the other hand
initiation takes
place at relatively small regions which are spaced from each other. Different
types of
pressure waves are produced depending on the manner of initiation. Nonetheless
the
principle remains the same which is that the pressure waves are allowed to
interfere with
each other at a location which is between the points at which they originate
in order to cause
a localised high energy region which causes rock fracture in the vicinity of
the region.
