U.S. patent application number 11/622252 was filed with the patent office on 2007-09-20 for detonating cord and methods of making and using the same.
Invention is credited to Thomas L. Barkley, Dennis P. Johnson, Stanley R. Kelly, Robert A. Lee, Mark E. Woodall.
Application Number | 20070214990 11/622252 |
Document ID | / |
Family ID | 22768344 |
Filed Date | 2007-09-20 |
United States Patent
Application |
20070214990 |
Kind Code |
A1 |
Barkley; Thomas L. ; et
al. |
September 20, 2007 |
DETONATING CORD AND METHODS OF MAKING AND USING THE SAME
Abstract
The velocity of detonation of an explosive such as detonating
cord (18, 22) is controlled by the addition of a diluent to the
explosive, e.g., to the core of the detonating cord (18, 22). An
explosively inert diluent, or a diluent comprised of an explosive
of lower brisance than the principal explosive comprising the core
of the detonating cord, will serve to reduce the velocity of
detonation. Such reduced velocity of detonation has beneficial
effects in certain operations, including cleaving rock (10),
wherein it is observed to significantly reduce radial cracks (24)
and stickers (26) (long radial cracks) in the vicinity of the
boreholes (12) in which the low-velocity detonating cord (18, 22)
is functioned to cleave the rock (10). The low-velocity detonating
cord also facilitates leaving behind a smoother face in cutting
trenches and tunnels through rock. The method of manufacture of
low-velocity detonating cord includes incorporating a suitable
diluent, such as phenolic microballoons, into an explosive core of,
e.g., PETN.
Inventors: |
Barkley; Thomas L.;
(Birchwood, WI) ; Johnson; Dennis P.; (Lexington,
KY) ; Kelly; Stanley R.; (Dixon, KY) ; Lee;
Robert A.; (Simsbury, CT) ; Woodall; Mark E.;
(Greenville, KY) |
Correspondence
Address: |
CANTOR COLBURN, LLP
55 GRIFFIN ROAD SOUTH
BLOOMFIELD
CT
06002
US
|
Family ID: |
22768344 |
Appl. No.: |
11/622252 |
Filed: |
January 11, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09863795 |
May 23, 2001 |
|
|
|
11622252 |
Jan 11, 2007 |
|
|
|
60206877 |
May 24, 2000 |
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Current U.S.
Class: |
102/275.8 |
Current CPC
Class: |
F42D 1/00 20130101; C06B
23/003 20130101; C06C 5/04 20130101 |
Class at
Publication: |
102/275.8 |
International
Class: |
C06C 5/04 20060101
C06C005/04 |
Claims
1. A detonating cord comprising an elongate tubular sheath encasing
a core of an explosive material comprising a first pulverulent
explosive admixed with a diluent comprising explosively inert
microballoons present in an amount which reduces the velocity of
detonation of the detonating cord as compared to that of an
otherwise identical detonating cord in which the explosive material
contains no explosively inert microballoons.
2-3. (canceled)
4. The detonating cord of claim 1 wherein the microballoons are
selected from the class consisting of glass microballoons and resin
microballoons, the microballoons having a diameter of from about 10
to about 175 microns.
5. The detonating cord of claim 1 wherein the microballoons
comprise resin microballoons.
6. The detonating cord of claim 5 wherein the microballoons have a
diameter of from about 10 to about 175 microns.
7. The detonating cord of claim 6 wherein the microballoons
comprise phenolic resin microballoons.
8. The detonating cord of claim 1 wherein the diluent explosive
material further comprises a second explosive having a lower
velocity of detonation than the first explosive.
9. The detonating cord of claim 8 wherein the second explosive
comprises ammonium nitrate.
10. The detonating cord of claim 1 containing from about 0.5 to 15%
by weight of the diluent, based on the dry weight of the core.
11. The detonating cord of claim 10 containing from about 0.5 to 5%
by weight diluent.
12. The detonating cord of claim 1 wherein the first explosive is
selected from the class consisting of PETN, HMX, HNS, TNC, PYX and
RDX, and mixtures of two or more thereof.
13. A method of cleaving a rock formation comprising: drilling a
plurality of substantially parallel boreholes into the formation to
define between adjacent boreholes a web of rock interconnecting
adjacent boreholes with each other; placing within the boreholes at
least one length of detonating cord extending along the length of
the respective boreholes; connecting the length of detonating cord
to an explosive initiating device and initiating the length of
detonating cord to cleave the formation; wherein the detonating
cord comprises an elongate tubular sheath encasing a core of an
explosive material comprising a first pulverulent explosive admixed
with a diluent comprising explosively inert microballoon present in
an amount which reduces the velocity of detonation of the
detonating cord as compared to that of an otherwise identical
detonating cord in which the explosive material contains no
explosively inert microballoons.
14. The method of claim 13 wherein the explosive material contains
from about 0.5 to 15% by weight of the diluent, based on the dry
weight of the core.
15. The method of claim 13 wherein the explosive material contains
from about 0.5 to 5% by weight of the diluent, based on the dry
weight of the core.
16. The method of claim 13 wherein the first explosive is selected
from the group consisting of PETN, HMX, HNS, TNC, PYX and RDX.
17-18. (canceled)
19. The method of claim 13 wherein the diluent further comprises a
second explosive material having a lower velocity of detonation
than the first explosive material.
20. The method of claim 13 wherein the microballoons comprise resin
microballoons having a diameter of from about 10 to about 175
microns.
21. A method for making a detonating cord comprises the steps of
preparing an explosive material by admixing a pulverulent explosive
with a diluent comprising explosively inert microballoons, the
diluent being present in an amount which reduces the velocity of
detonation of the detonating cord as compared to an otherwise
identical detonating cord in which the explosive material contains
no explosively inert microballoons; and enclosing the explosive
material within a tubular sheath to provide a detonating cord
having a core of the explosive material.
22. The method of claim 21 including admixing a sufficient quantity
of the explosively inert microballoons with the first explosive to
provide in the core from about 0.5 to 15% by weight of the
explosively inert microballoons, based on the dry weight of the
core.
23. The method of claim 21 or claim 22 wherein the first explosive
is selected from the class consisting of PETN, HMX, HNS, TNC, PYX
and RDX, and mixtures of two or more thereof.
24-25. (canceled)
26. The method of claim 21 or claim 22 wherein the microballoons
comprise resin microballoons.
27. The method of claim 26 wherein the microballoons comprise
phenolic resin microballoons.
28. The method of claim 21 or claim 22 wherein the microballoons
have a diameter of from about 10 to about 175 microns.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of patent application
Ser. No. 09/863,795, filed May 23, 2001, entitled "Detonating Cord
and Methods of Making and Using the Same", which claims the benefit
of U.S. provisional patent application Ser. No. 60/206,877, filed
May 24, 2000.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention is concerned with detonating cord
having a controlled energy release which is attained by
incorporating a diluent into the explosive core of the cord to
control the velocity of detonation of the core. The present
invention is also concerned with a method of malting the detonating
cord, and a method of utilizing the detonating cord to effectuate
desired cutting or rupturing of any structures, rock formations or
the like while minimizing undesired ancillary damage.
[0004] 2. Related Art
[0005] Detonating cord is, of course, well known in the art and
comprises a solid core of high explosive encased in a protective
jacket which is usually waterproofed, such as by being coated with
a suitable synthetic polymeric (plastic) material. Typically, the
solid core of high explosive is a compressed pulverulent explosive
which may or may not be plastic-bonded. Detonating cords are made
in various sizes (core diameters) conventionally measured in grains
of explosive per unit length. A typical explosive core for
detonating cord is pentaerythritol tetranitrate ("PETN") and
typical core sizes range from about 5 grains of explosive per
linear foot of cord ("gr/ft") to about 400 gr/ft. There are 15.432
grains per gram, so that, e.g., 100 gr/ft is, in the metric system,
about 21.3 grams per meter ("g/m"). Typical velocities of
detonation for detonating cord made of PETN are on the order of
about 6,500 to 7,500 meters per second ("m/sec").
[0006] In order to provide a detonating cord having explosive and
other properties which are uniform along its length, it is
necessary to compress the explosive core in order to standardize
the density of the core along its length because the velocity of
detonation, and thereby the explosive energy output, is
proportional to the density of the explosive core. Generally,
increased density of the core increases the velocity of detonation
and thereby the explosive energy output per unit length of the
cord. As is well known, the particle size of the explosive also
greatly affects the velocity of detonation and a critical diameter
exists for propagation of the explosion along the length of the
detonating cord. Generally, as the diameter of the explosive core
of the detonating cord decreases, so should the particle size. For
example, for a small detonating cord, e.g., one containing about 5
gr/ft (1.1 g/m) of explosive, a particle size of about 20 microns
diameter is suitable, whereas for a detonating cord of much larger
diameter, e.g., a detonating cord of about 400 gr/ft (85.2 g/m),
adequate propagation of the explosion along the length of the cord
may be attained with a particle diameter size of from 100 to 200
microns.
[0007] In commercial blasting operations, detonating cord is
generally used to transfer an explosive signal to various
components of a blasting setup. For example, detonating cord may be
utilized as a surface trunkline to impart a detonation signal to a
series of down-hole fuses such as shock tube or other detonating
cords. While, as noted above, PETN is the usual choice of explosive
for detonating cord, other explosives may be used. For example, for
operations such as those in the oil and gas industry in which high
temperatures are experienced by the detonating cord before it is to
be functioned (initiated), explosives such as
cyclo-1,3,5-trimethylene-2,4,6-trinitramine (Cyclonite, or "RDX")
or cyclotetramethylene tetranitramine (Homocyclonite, or "HMX") may
be utilized for the core of the detonating cord. The explosives
hexaliitrostilbene ("HNS"), tetranitrocarbazol ("TNC"), and 2-6,
bis picryoamino 3,5, di-nitro pyridine ("PYX") are among other
explosives which may be used as the first explosive material of the
core. It is also known, usually in military operations, to utilize
detonating cord to sever structural members such as the beams or
braces of a bridge, trees, etc. As to the tubular sheath which
encloses the core of explosive material, any suitable material or
combinations thereof, as is well known in the art, may be employed.
Such sheaths are pliable enough to enable the detonating cord to be
deployed in any desired pattern, wrapped around structural members,
etc. The sheath may also be a rigid sheath such as that described
in patent application Ser. No. 09/645,276, filed on Aug. 24, 2000
in the name of Mark E. Woodall et al for "Rigid Reactive Cord And
Methods Of Use And Manufacture". That patent application de-scribes
a detonating cord having a non-metal outer sheath which imparts a
sufficient flexural modulus, e.g., of about 250,000 psi
(17.236.times.10.sup.2 MPa), which enables a 6-foot length of the
cord to be sufficiently rigid to perforate and penetrate fly ash.
This rigid-type cord finds use in removing fly ash from boiler
tubes.
[0008] It is known in the explosives art to utilize glass
microballoons as sensitizing agents in emulsion explosives and the
like. In this regard, see U.S. Pat. No. 6,165,297 issued Dec. 26,
2000 to J.G.B. Smith et al, entitled "Process And Apparatus For The
Manufacture Of Emulsion Explosive Compositions". At column 1, lines
41-45, it is noted that sensitizing agents such as glass
microballoons may be a component of emulsion explosives. Column 2,
lines 6-7 and column 2, lines 63-67 of the same patent, describe
microballoons as a species of "closed cell void material."
Similarly, U.S. Pat. No. 6,200,398, issued Mar. 13, 2001 to J. H.
Bush and entitled "Emulsion Explosive Compositions" discloses, at
column 24, line 46 to column 25, line 35 the use of closed-cell
void containing materials such as discrete glass spheres having a
particle size within the range of about 10 to about 175 microns and
a bulk density within the range of about 0.1 to about 0.4 g/cc.
Various other microballoons are described. U.S. Pat. No. 5,714,711,
issued Feb. 3, 1998 to J. B. Schumacher et al, is entitled
"Encapsulated Propellant Grain Composition, Method Of Preparation,
Article Fabricated Therefrom And Method Of Fabrication". This
patent deals with a propellant grain composition for use in
solid-fuel rocket engines and discloses an oxidizer first reactant
encapsulated by a polymeric barrier coating and a reducer fuel
second reactant disposed on the polymeric barrier coating, with a
final polymeric coating placed over the entire propellant grain to
yield a unitary metal fuel-oxidizer propellant grain structure for
use as a solid rocket fuel. U.S. Pat. No. 5,859,264 issued Jan. 12,
1999 to K. Coupland et al is entitled "Explosive Compositions" and
discloses emulsifiers for use in emulsion explosives comprising a
continuous organic phase and a discontinuous aqueous phase. At
column 2, lines 59-67, this patent discloses the use of glass or
resin microspheres or other gas-containing particulate
materials.
[0009] In known mining, construction and quarrying operations,
explosives are used to break a web of rock extending along a line
of relatively closely spaced, small diameter parallel boreholes, in
order to cleave the rock mass along the line of boreholes. Any
cracking, spalling or fragmentation of stone not contributing to
this cleaving is undesirable. In construction and mining
operations, it is desired to leave behind relatively smooth walls
and roofs in cuts and tunnels, and in quarrying operations the
objective is to recover from the rock formation blocks of stone
which are as undamaged as possible. In some cases, the cleaving is
performed by using mechanical wedges instead of explosives. Wedges
are placed in each borehole of a line of boreholes and each wedge
is gradually mechanically loaded in order to develop an even
tensile stress field along the web of stone extending between and
connecting the row of boreholes, until the web fragments to cleave
the rock mass.
[0010] Blasting methods attempt to mimic the mechanical splitting
process. In most cases, however, the shock energy from a high
explosive will cause a discrete stress field to form around
individual boreholes before subsequent boreholes are loaded by the
explosive force, and impose high particle velocities to fractured
rock particles, both factors resulting in undesired radial fracture
damage around the boreholes. Stated otherwise, when the energy
shock wave engendered by the explosive gases generated from the
explosive charges placed in the boreholes pressurizes the holes and
fragments the web of rock between the holes, high-amplitude shock
energy exacerbates unwanted spalling and cracking along the radial
fractures emanating from the boreholes and associated damage to the
rock.
[0011] The dimension stone industry is concerned with cutting from
rock formations in quarries stone which is sized for use in
construction and for headstones, markers and the like. In the
dimension stone industry, black powder was one of the original
explosive materials used in boreholes for cleaving stone by
blasting. Black powder has a very low velocity of detonation and a
very low explosive output and shock energy. These characteristics
are advantageous in reducing collateral damage to stone cut from
rock formations. The disadvantages of black powder are safety
problems which inhere in its use, because black powder is extremely
sensitive to static, sparks and fire, making it extremely
dangerous. Black powder is also de-activated by water, precluding
its use in wet areas.
[0012] An alternative to black powder in cleaving stone by blasting
is dynamite. Dynamite is nitroglycerine soaked into an absorbent
material and packaged in a cylindrical cartridge. The velocity of
detonation of dynamite, about 4500 feet per second (about 1,372
meters per second), is slightly higher than that of black powder.
Dynamite also has slightly more radial explosive output. The
primary disadvantage of dynamite is the excruciating headaches
experienced by personnel who handle nitroglycerine-based material.
Dynamite may also require a relatively large explosive diameter to
function, rendering it unusable for smaller-diameter boreholes.
SUMMARY OF THE INVENTION
[0013] Although it is generally desired to have a high explosive
output for a given diameter detonating cord, it has been found that
for some applications it is desirable to control, e.g., to reduce,
the velocity of detonation because such reduction reduces the peak
output shock wave pressure caused by the explosion. High peak
outlet shock wave pressure causes rapid pressure loading of the
structure, e.g., rock, being ruptured by the detonating cord, which
nucleates fractures causing them to spread into portions of the
structure which it is desired to leave intact. A reduction in peak
shock wave output pressure, i.e., a reduction in the peak amplitude
of the shock energy released by detonation of the cord, has been
found to be highly beneficial in some applications where
high-amplitude shock energy may cause or exacerbate unwanted
collateral damage. Such applications include certain construction
and tunneling activities, quarrying operations and cutting
structures such as dimension stone, as described more fully
below.
[0014] Generally, in accordance with the present invention there is
provided a detonating cord having a controlled velocity of
detonation and comprising a solid core of an explosive containing
therein a first explosive and one or more diluents which reduce the
velocity of detonation of the core. The detonating cord of the
present invention finds use in any application in which reduced
peak shock energy is required or desired.
[0015] Reference herein to "reduced-velocity detonating cord",
"low-velocity detonating cord", or the like, means a detonating
cord whose explosive core contains a diluent which reduces the
velocity of detonation of the detonating cord as compared to an
otherwise identical detonating cord which does not contain the
diluent.
[0016] The diluent may be either an explosively inert material,
such as closed-cell void materials (referred to herein as
microballoons, e.g., glass or resin microballoons or very fine
plastic or glass beads, etc., or it may be an explosive material,
for example, ammonium nitrate. As used herein, reference to a
"solid" core of explosive material means that the tubular sheath of
the detonating cord is completely filled with the explosive
material. The presence of microballoons dispersed in the explosive
material provides enclosed voids therein, but as the explosive
material substantially completely fills the enclosing tubular
sheath, the core is nonetheless described as a solid core.
[0017] Specifically in accordance with the present invention there
is provided a detonating cord comprising an elongate tubular sheath
encasing a solid core of an explosive material, the explosive
material being comprised of a first explosive and a diluent. The
diluent is present in an amount which reduces the velocity of
detonation of the detonating cord as compared to that of an
otherwise identical detonating cord in which the explosive material
contains no diluent. In one aspect of the present invention, the
diluent comprises particles of an explosively inert material, e.g.,
explosively inert microballoons. The microballoons may be selected
from the class consisting of glass microballoons and resin
microballoons, preferably the latter, the microballoons having a
diameter of from about 10 to about 175 microns. Thus, the
microballoons may comprise resin microballoons, e.g., phenolic
resin microballoons, having a diameter of from about 10 to about
175 microns.
[0018] In another aspect of the present invention, diluent
comprises a second explosive material, e.g., ammonium nitrate,
having a lower velocity of detonation than the first explosive
material.
[0019] Another aspect of the present invention provides that the
detonating cord contains from about 0.5 to 15%, e.g., from about
0.5 to 5%, by weight of the diluent, based on the dry weight of the
core.
[0020] The first explosive may be any suitable explosive, such as
one or more of PETN, HMX, HNS, TNC, PYX and RDX.
[0021] A method aspect of the present invention provides an
improvement in a method of cleaving a rock formation. The method
comprises drilling a plurality of substantially parallel boreholes
into the formation to define between adjacent boreholes a web of
rock interconnecting adjacent boreholes with each other, placing
within the boreholes at least one length of detonating cord
extending along the length of the respective boreholes, connecting
the length of detonating cord to an explosive initiating device and
initiating the length of detonating cord to cleave the formation.
The improvement comprises that the detonating cord is one as
described above.
[0022] Another method aspect of the present invention provides a
method for making a detonating cord as described above. The method
comprises the steps of preparing an explosive material by admixing
a first explosive with a diluent selected from the group consisting
of (a) explosively inert diluents; (b) a second explosive having a
velocity of detonation less than that of the first explosive; and
(c) mixtures of (a) and (b), the diluent being present in an amount
which reduces the velocity of detonation of the detonating cord as
compared to an otherwise identical detonating cord in which the
explosive material contains no diluent. The explosive material is
enclosed within a tubular sheath to provide a detonating cord
having a core of the explosive material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a cross-sectional view in elevation of a segment
of a slab of granite having a plurality of boreholes drilled
therethrough;
[0024] FIG. 2 is a plan view of the granite slab of FIG. 1 showing
an end view of the boreholes;
[0025] FIG. 3 is a schematic view corresponding to that of FIG. 1
showing a length of detonating cord disposed within each of the
boreholes and connected to a trunkline for initiation of the
detonating cords;
[0026] FIG. 3A is a cross-sectional view, enlarged relative to FIG.
3, of a segment of the length of detonating cord illustrated in
FIG. 3;
[0027] FIG. 4 is a schematic view corresponding to that of FIG. 1,
but showing a stitching arrangement of a continuous length of
detonating cord disposed as a loop within the boreholes;
[0028] FIG. 5 is a plan view of the granite slab of FIG. 1 after it
has been split in two by initiation of detonating cord emplaced
with the boreholes;
[0029] FIG. 6 is a perspective, schematic view of a two-borehole
setup for conducting tests of detonating cord; and
[0030] FIGS. 7-12 are graphs plotting the pressure output in
thousands of pounds per square inch ("IPS") against time in seconds
of various samples of detonating cord tested in the setup
illustrated in FIG. 6.
DETAILED DESCRIPTION OF THE INVENTION AND SPECIFIC EMBODIMENTS
THEREOF
[0031] As noted above, the velocity of detonation of detonating
cord can be controlled by mixing a diluent with the pulverulent
explosive from which the explosive core of detonating cord is
formed. By utilizing as the diluent inert particulate material or a
second explosive less brisant than the first explosive, i.e., one
having a lower velocity of detonation than the first explosive, the
velocity of detonation of the detonating cord may be reduced. It
has been discovered that by reducing the velocity of detonation, a
reduction in the peak output pressures caused by detonation of the
cord is attained without significantly affecting the total energy
output of the cord. Such reduction of peak output pressure has been
found to be highly desirable in certain circumstances, as it
reduces undesired damage to areas of the rock immediately
surrounding the cord. The benefits of the present invention are
clearly shown, for example, in cutting what is referred to as
"dimension stone" from in-ground formations of stone such as
granite and the like, or in cutting slabs of dimension stone from
larger blocks (sometimes referred to as "production loaves") of the
stone.
[0032] This technique of cleaving rock by fracturing the web of
intact rock extending between adjacent parallel boreholes is used
in a variety of situations. In dimension stone quarries, sections
of granite or other desirable stone are cleaved from the geologic
formation. Radial fractures emanating from the boreholes reduce the
yield of useable stone. Further, a clean, relatively smooth surface
of the rock face from which a segment of rock is cleaved is often
desired in applications other than dimension stone quarrying. For
example, in surface mining and construction blasting, a smooth,
flat wall of rock in the remaining formation is often desired,
e.g., to provide a structurally sound and reasonably smooth wall to
minimize rock-fall danger. This smooth-wall technique is also
employed in underground mining and tunneling applications to blast
a secure, reasonably smooth roof or "back" in the mine, the
smoothness of which reduces the requirements for mechanical
supports such as roof bolts and the like.
[0033] One aspect of the present invention contemplates the use of
an explosive, e.g., the use of detonating cord, of reduced velocity
of detonation to cleave rock or stone along a line defined by a
series of boreholes. Detonating cord is waterproof and safer to
handle than black powder. It typically is a PETN-based explosive
and produces none of the handling problems related to
nitroglycerine-based explosives. Detonating cord can function in
very small cord diameters, e.g., as small as less than 0.25 inch,
i.e., 0.635 centimeter ("cm"), in diameter. The disadvantage of
conventional, high-velocity detonating cord in applications
designed to form in the cleaved stone, and/or leave behind a
smooth, cleaved stone face, is its typically high detonation
velocity of up to more than 7000 meters/second, and high radial
shock wave output. These characteristics, which are conventionally
considered to be desirable attributes necessary to enable
conventional detonating cord to initiate explosive charges of low
sensitivity, contribute to excessive fracturing and cracking around
the boreholes when conventional detonating cord is used to cleave
rock. Only the detonating cord of the present invention provides an
explosive of such small diameter, typically from about 0.125 to
0.250 inch (0.318 to 0.635 cm) diameter, having such a low velocity
of detonation.
[0034] Another aspect of the present invention utilizes a
low-velocity detonating cord, preferably one having a velocity of
detonation less than about 5000 meters per second ("m/sec"). The
invention enables taking advantage of the desirable features of
detonating cord, such as the ability to be cut at any point along
its length, and its relatively small cross-sectional diameter as
compared to other explosives, with the feature of optimizing
blasting performance by modifying the velocity of detonation of the
cord. The low-velocity detonating cord of this aspect of the
present invention results in decreased shock loading and increased
gas pressurization within the boreholes in which the detonating
cord is functioned, with no significant reduction in total energy
output. This characteristic results in reduced radial fracturing
around the periphery of the borehole, more gradual development of
the stress field, and more efficient fracturing of the web of rock
between adjacent, parallel boreholes. In short, the low-velocity
detonating cord aspect of the present invention better mimics the
action of mechanical wedges in cleaving stone than do conventional
explosives, including conventional, high-velocity detonating cord,
as is demonstrated by the data provided below. All of these
advantages apply to any rock-cleaving application. The present
invention permits the radial output energy of detonating cord to be
tailored to a specific blasting application by changing, e.g.,
reducing, the velocity of detonation of the cord.
[0035] Referring now to FIG. 1, there is shown a somewhat schematic
cross-sectional view taken along line I-I of FIG. 2 of a granite
block 10 having a plurality of boreholes 12 formed therein and
extending substantially parallel to each other from and through top
surface 14a to and through bottom surface 14b of granite block 10.
As best seen in FIG. 2, parallel boreholes 12 are aligned along a
straight line to define a web 16 of stone extending between and
connecting boreholes 12 to each other. Web 16 is indicated in FIG.
2 by dash lines.
[0036] Generally, it is desirable to minimize the number of
boreholes needed to effectuate a particular task in order to reduce
costs of drilling. Reducing the number of boreholes, and thereby
increasing the spacing between boreholes for a given length of rock
formation to be cleaved, increases the amount of explosive required
per borehole. The increased amount of explosive per borehole is
necessary in order to ensure breakage of the increased length of
the web of rock between adjacent boreholes, but also increases
radial fractures and collateral damage of the cleaved rock.
Conversely, reducing the explosive load of each borehole requires
decreasing the spacing between adjacent boreholes, and therefore
increasing the number of boreholes and consequently increasing the
drilling costs. The present invention, by providing an explosive
which generates reduced peak shock wave pressures without a
substantial reduction in total energy output, enhances the ability
of the explosive of the present invention to break the rock web
while reducing collateral radial damage to the rock. This enables
increasing the spacing between adjacent boreholes without a
corresponding increase in collateral damage.
[0037] FIG. 3 shows schematically one arrangement for providing
throughout the length of each of boreholes 12, lengths 18 of
reduced-velocity detonating cord in accordance with an embodiment
of the present invention. In the embodiment schematically
illustrated in FIG. 3, individual lengths 18 of reduced-velocity
detonating cord are connected to a trunkline 20 which may itself
comprise detonating cord and which may, but need not, be
reduced-velocity detonating cord in accordance with an embodiment
of the present invention. Trunkline 20 is initiated at one end
thereof by any suitable known means (not illustrated) and a
detonation signal travels along trunkline 20 to initiate each of
the lengths 18 of detonating cord to fracture the web 16 of
stone.
[0038] FIG. 3A shows that detonating cord 18 comprises a tubular
sheath 18a encasing a solid core 18b of explosive material
throughout which is dispersed a particulate diluent, provided in
the illustrated embodiment by microballoons 18c. Each of
microballoons 18c is a hollow particulate body enclosing a void
containing a gas, e.g., air. Tubular sheath 18a is made of any
suitable material to provide adequate mechanical strength and to be
resistant to penetration of water or other liquids into core
18b.
[0039] FIG. 4 shows an alternate arrangement, sometimes referred to
as "stitching", in which a single length 22 of reduced-velocity
detonating cord in accordance with an embodiment of the present
invention is threaded in a serpentine, stitching-like arrangement
through each of boreholes 12 by inserting a return-loop of
detonating cord through substantially the entire length of each
borehole. Length 22 of detonating cord is initiated by any suitable
means (not illustrated) and detonates along the length thereof to
fracture the web 16 of stone. In this arrangement the total length
of reduced-velocity detonating cord in a borehole is effectively
doubled as compared to the arrangement of FIG. 3. Therefore, other
factors being equal, the diameter of the reduced-velocity
detonating cord used in the arrangement of FIG. 4 would be about
one-half that of the reduced-velocity detonating cord used in the
arrangement of FIG. 3.
[0040] The result of fracturing web 16 is schematically illustrated
in FIG. 5, which is a plan view corresponding to that of FIG. 2,
but showing the granite block after it is cleaved in two by either
the lengths 18 of detonating cord as illustrated in FIG. 3, or the
length 22 of detonating cord as illustrated in FIG. 4. Granite
block 10 (FIG. 2) has now been split into two granite blocks, 10a
and 10b (FIG. 5). Adjacent the arcs 12a, 12b (FIG. 5) of the former
boreholes 12 (FIG. 2) are a series of small radial cracks 24 and a
few considerably longer radial cracks, referred to as "stickers" in
the dimension stone industry, shown at 26. In order to reduce the
amount of wasted stone, it is, of course, desired that the radial
cracks 24 and stickers 26 be reduced in number and/or shortened as
much as possible.
[0041] A series of blocks was cleaved from larger blocks
(production loaves) of granite using either 18 gr/ft (3.8 g/m),
low-velocity detonating cord in accordance with an embodiment of
the present invention, or conventional 18 gr/ft (3.8 g/m)
detonating cord. All holes in the test blocks were loaded with
Viking B-gel, an inert gel product with microballoons, available
from Viking Explosives & Supply, Inc. of Rosemont, Minn. The
B-gel is a product which is well known for use as a coupling agent
for the purpose of buffering the initial shock pressure generated
by functioning of the detonating cord. This or an equivalent gel
product is used in an effort to reduce unwanted radial fracturing.
A total of four blocks were cleaved using reduced-velocity
detonating cord in accordance with an aspect of the present
invention, and three blocks were cleaved using conventional
high-velocity detonating cord. All blocks were sawed into 6 to 8
inch (about 15.2 to 20.3 cm) thick slabs and then polished. The
yield from each slab, along with the degree of radial fracturing,
was determined through observation and digital imaging analysis in
order to compare the performance of each type of detonating cord.
The observed results of these tests are tabulated in the following
Table A. The average length of stickers was twelve inches.
TABLE-US-00001 TABLE A Average Number of Holes Affected Average
Minimum Average Maximum Average Number by Cracking, Crack Crack of
Stickers per per Slab Length (Inches) Length (Inches) Slab A.
Reduced- 17.0 2.4 4.1 0.5 Velocity Detonating Cord B. Comparative
18.3 3.6 5.7 2.9 High-Velocity Detonating Cord Difference 1.3 1.2
1.6 2.4 % Improvement 7% 32% 28% 82% Provided By A Over B
[0042] It is seen from Table A that the reduced-velocity detonating
cord in accordance with an embodiment of the invention, Sample A,
having a velocity of detonation of 4,800 m/sec, significantly
reduced cracking as compared to comparative high-velocity
detonating cord, Sample B, having a velocity of detonation of 7,100
m/sec. The detonating cord of Sample A had a loading of 18 gr/ft
(3.83 g/m) of 90% by weight PETN, 5% by weight ammonium nitrate,
and 5% by weight of phenolic microballoons having a particle size
distribution shown in Table B, below. (The percents by weight are
on the basis of percent by weight of the combined weight of PETN,
ammonium nitrate and phenolic microballoons.) The comparative
detonating cord of Sample B had a loading of 18 gr/ft (3.83 g/m) of
100% PETN, i.e., it contained no diluent.
[0043] Blast-pressure profiles were measured for a series of
cord/coupling-agent combinations. The test arrangement is
schematically illustrated in FIG. 6 in which two test boreholes, 28
and 30, terminating in respective borehole bottoms 28a and 30a,
were bored in the tone parallel to each other with their
peripheries spaced 6 inches (15.24 cm) from each other, this
distance being illustrated as D in FIG. 6. A test detonating cord
32 was inserted through-out the length of test borehole 28 and, in
those cases in which a coupling agent other than air was utilized,
borehole 28 was filled with a coupling agent, e.g., gel or water
(not shown). A loop (not shown) of test detonating cord 32 was
inserted into borehole 28 in order to test a "stitching"
arrangement of detonating cord, as schematically illustrated in
FIG. 4. Test borehole 30 was filled with water as a coupling agent,
and an underwater blast pressure sensor 34 was placed within the
water-filled test borehole 30. Underwater blast pressure sensor 34
was connected by a cable 36 to computerized recording equipment
(not illustrated) to record "pressure profiles", i.e., graphs of
the pressure of the shock wave or pressure pulse generated by
detonation of test detonating cord 32 as a function of time. The
pressure profiles generated from test detonating cord 32 are shown
in FIGS. 7-16, wherein the shock wave pressures are plotted on the
left-hand vertical axes in kilopounds per square inch ("ICMS") and
on the right-hand vertical axes in kilograms per square centimeter
("kg/cm.sup.2"). The time after functioning (detonation) is plotted
on the horizontal axes in seconds. Among the tested detonating
cords 32 were 18 gr/ft (3.83 g/m) low-velocity cord in accordance
with an embodiment of the present invention, and 7.5 gr/ft (1.60
g/m) and 18 gr/ft (3.83 g/m) comparative high-velocity cords. In
some tests, in lieu of water as the coupling agent media in test
borehole 28, B-gel or air were used as the coupling agent.
[0044] The tests resulted in fracturing of the stone adjacent the
boreholes in the manner described above in connection with FIG. 5.
Thus, radial fractures (such as those shown at 24 in FIG. 5), which
tended to be of relatively constant length, affected many of the
boreholes on the perimeter of the slab. Stickers (such as those
shown at 26 in FIG. 5) tended to be fewer in number, but up to 2 to
10 times the length of, the radial cracks 24. The slabs shot with
reduced-velocity detonating cord were observed to have 30% shorter
radial fractures and 80% fewer stickers than slabs shot with
comparative, conventional high-velocity detonating cord. Both of
these cord types were used with B-gel as the coupling media.
[0045] The results of the pressure testing shown in FIGS. 7-12
support the visual observations of the cleaved stone. In the
pressure measurements illustrated in FIGS. 7-12, the shock pulse
from the detonating cord is seen as a very sharp and high peak
close to the front (earliest time after detonation) of the pressure
profile. This pressure pulse represented by this peak is believed
to be the primary cause of radial fracturing around the boreholes,
and the magnitude of this early pressure pulse is believed to be
proportional to the length and quantity of the resultant radial
fractures and stickers. The key to reducing radial fracturing while
breaking the web of rock connecting the boreholes is to reduce the
high-amplitude, sharp peaks while maintaining much of the pressure
that occurs generally throughout the pressure profile. Maintaining
the overall level of the pressure profile avoids reducing the total
energy output or work while eliminating or reducing shock wave
pressure peaks. The energy output or work is indicated by the area
beneath the curve in the Figures plotting the pressure profiles.
These areas represent the product of pressure multiplied by
time.
[0046] A pressure profile for 18 gr/ft (3.83 g/m) comparative
high-velocity detonating cord in water is shown in FIG. 7. (In
FIGS. 7-12, repeated tests are shown on the same set of axes and it
is seen that the results are reproducible with only very minor
variations.)
[0047] The pressure profile of FIG. 8 shows the pressure profile
from comparative high-velocity detonating cord in B-gel coupling
agent, and that of FIG. 9 shows the pressure profile from
low-velocity detonating cord in accordance with an embodiment of
the present invention in B-gel coupling agent. A comparison of
FIGS. 8 and 9 makes clear that the low-velocity detonating cord did
not produce the sharp, high-amplitude pressure transients that were
typical of the comparative high-velocity detonating cord, while
otherwise maintaining a comparable level of pressure.
[0048] The pressure profile of the low-velocity detonating cord in
water shown in FIG. 10 also shows that the reduction of the sharp
peaks was not accompanied by a general reduction of pressure
throughout the trace of the pressure profile. The performance of
the low-velocity detonating cord in water shown in FIG. 10 is of
particular interest. It will be noted that these pressure profiles
are very similar to those measured with low-velocity detonating
cord in B-gel (shown in FIG. 9) and represent a dramatic
improvement over comparative high-velocity detonating cord in
B-gel, shown in FIG. 8. These results indicate that low-velocity
cord in water performs better than the comparative high-velocity
detonating cord in B-gel, and about as well as low-velocity
detonating cord in B-gel. Given the high cost of B-gel coupling
agent and the difficulty of mixing and using the product in
day-to-day operations, the low-velocity detonating cord in
accordance with an embodiment of the present invention provides
greatly enhanced performance and may enable elimination of the need
to use B-gel.
[0049] FIG. 11A shows the pressure profile for a comparative
high-velocity 18 gr/ft (3.83 g/m) detonating cord in air and a
low-velocity 18 gr/ft (3.83 g/m) detonating cord in accordance with
an embodiment of the present invention in air, the cords having
been placed in the test borehole (28 in FIG. 6) using the stitching
arrangement schematically illustrated in FIG. 4. Using the
stitching arrangement of FIG. 4 provided a doubled length of the
test detonating cord within the test borehole. In addition, a
thimble- or cup-shaped spacer (not shown in the drawings) was
emplaced at the bottom of the test borehole atop the return-bend
portion of the looped detonating cord in order to ensure that the
detonating cord extended the full length of the borehole. The
respective legs of the doubled detonating cord emerged from under
the spacer at diametrically opposite sides of the periphery
thereof, resulting in each leg of the detonating cord being held
against, or at least in close proximity to, the wall of the
borehole, thereby enhancing coupling of the detonating cord to the
walls of the borehole. As seen by comparing FIGS. 11A and 11B to
each other, the contrast between the two types of cords is
striking. The high peak loads of pressure engendered by the
comparative high-velocity detonating cord shown in FIG. 11A are
absent from the pressure profile of the low-velocity detonating
cord in accordance with an embodiment of the present invention
shown in FIG. 11B. A comparison of the pressure profiles of FIGS.
11A and 11B also shows that the low-velocity detonating cord (FIG.
11A) nevertheless had about the same general level of pressure
output as the comparative high-velocity detonating cord (FIG. 11B).
The significant difference is the very desirable elimination of the
high-pressure peaks shown in FIG. 11B.
[0050] FIG. 12 shows the pressure profile of a comparative 7.5
gr/ft (1.60 g/m) high-velocity detonating cord in water as a
coupling agent. The initial pressure peak, while reduced as
compared to higher core load detonating cords, is nonetheless
prominent, about three times higher than the remaining non-peak
pressure level.
[0051] While any suitable diluent may be utilized to reduce the
velocity of the detonating cord, one which is found to be
particularly useful is phenolic microballoons, e.g., of the type
conventionally used as a filler in fiberglass resin applications to
lower weight and density of finished fiberglass items. Glass
microballoons are commonly used in blasting agents, but glass
functions as a sensitizing agent in dry explosives and therefore is
dangerous and much less desirable than phenolic microballoons for
utilization in a detonating cord. Phenolic resin is a brittle
material and the addition of phenolic microballoons to the
explosive core does not appear to sensitize dry PETN. Phenolic
resin has a specific gravity of approximately 1.35 and the phenolic
microballoons used had a tapped bulk density of 0.13 grams per
cubic centimeter. (The container of phenolic microballoons is
tapped on a solid surface to settle the material prior to measuring
its density. The density of the settled material is referred to as
its "tapped bulk density".)
[0052] As indicated above, the diluent may comprise an explosive
diluent, such as ammonium nitrate, which is of significantly lesser
brisance than the major explosive ingredient, e.g., PETN, of the
detonating cord or other explosive used in the practices of the
present invention. The explosive diluent may be used in combination
with another diluent such as phenolic or other microballoons, or
the explosive diluent or other type or types of non-explosive
diluent, e.g., microballoons, may be used as the sole diluent. The
use of a less blisant explosive such as ammonium nitrate as the
sole diluent in a detonating cord has the disadvantage that the
amount of less brisant explosive required to attain a sufficient
reduction in velocity of detonation will significantly increase the
diameter of the detonating cord.
[0053] The phenolic microballoons used to prepare the tested
low-velocity detonating cord had an average particle size
distribution as follows, wherein .mu. stands for microns.
TABLE-US-00002 TABLE B Maximum % of Particles Diameter 75% 71.mu.
50% 51.mu. 25% 35.mu.
[0054] It was determined by experimentation that the step of
compressing detonating cords in the course of post-manufacture
inspection for variations in cord diameter resulted in rupturing
some of the phenolic microballoons used as a diluent in the
low-velocity detonating cord of the present invention.
Consequently, these compressed cords did not exhibit the reduction
in velocity of detonation ("VOD") expected from the addition of the
quantity of phenolic micro balloons. This is shown by a comparison
of the data for the second and third entries in Table 1 wherein a
significantly smaller reduction in VOD is shown for compressed cord
containing the same amount of microballoons than for uncompressed
cord. In Table 1, "SFPETN" means a superfine grade of PETN, having
a particle size of about 20 microns diameter. TABLE-US-00003 TABLE
1 Core Load % By Weight Inspection- PETN gr/ft g/m Microballoon
Compressed Grade VOD (average) 18 3.83 0% Yes SFPETN 6513 m/s 18
3.83 5% Yes SFPETN 5515 m/s 18 3.83 5% No SFPETN 4795 m/s
[0055] It was determined by experimentation that simply diluting
the PETN in 200 gr/ft (42.6 g/m) detonating cord with ammonium
nitrate reduced the VOD slightly. However, the addition of
microballoons without compression of the detonating cord for
examination purposes substantially reduced the VOD as shown in
Table 2. Reference to "MFPETN" in Table 2 means a medium fine grade
of PETN, having a particle size of about 150 microns diameter.
TABLE-US-00004 TABLE 2 Core Load % By Weight gr/ft g/m AN*
Microballoon PETN Grade VOD Average 200 42.6 0% 0% MFPETN 6758 m/s
200 42.6 15% 0% MFPETN 6247 m/s 200 42.6 0% 10% MFPETN 4034 m/s *AN
= ammonium nitrate
[0056] Generally, any suitable quantity of diluent may be used to
attain a desired change, i.e., a reduction, in velocity of
detonation. For example, phenolic microballoons (or other diluents)
may be added in amounts of from about 0.5 to 5%, 0.5 to 10%, 0.5 to
15%, 1 to 5%, 1 to 7%, 1 to 10% or 1 to 15% by weight of the
combined weight of explosive and diluent.
[0057] In the experimental results shown in Tables 1 and 2, adding
to the PETN powder an ammonium nitrate diluent (Table 2) reduced
the VOD only slightly. On the other hand, adding phenolic
microballoons as the diluent (Tables 1 and 2) substantially reduced
the core density of the detonating cord and the VOD. This is
attributed to the fact that the microballoons reduced the density
of the PETN much more than did the ammonium nitrate which had
little or no effect on density.
[0058] When the same weight percent of phenolic microballoons was
added to the examined (post-manufacture compressed) and unexamined
(not post-manufacture compressed) detonating cords, a loss of some
of the VOD reduction effect was noted in the examined (compressed)
samples. This loss of desired reduction in VOD is attributed to
crushing of the microballoons by the post-manufacture compression
for inspection purposes. Since compressing the cord increased the
VOD without, of course, changing its content by weight of phenolic
material (crushed and non-crushed phenolic microballoons), the
shape of the microballoons, i.e., their effective density, must be
involved in the loss of VOD reduction. It was verified, but not
quantified, that a substantial percentage of the microballoons were
ruptured and compacted by the post-manufacture compression
examination, thus increasing the density of the phenolic material
in the compressed samples as compared to that in the uncompressed
samples. A more substantial VOD reduction effect is believed to be
achieved by reduction of density of the explosive core of the
detonating cord, not merely dilution of the explosive powder with
an inert material or a less brisant explosive which has no or very
little density-reducing effect.
[0059] While the invention has been described in detail in
connection with specific embodiments thereof, it will be
appreciated that neither the invention nor the appended claims are
limited to the specific illustrative embodiments.
* * * * *