Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02410465 2005-10-14
DETONATING CORD AND
METHODS OF MAKING AND USING THE SAME
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] 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
making the detonating cord; and a method of utilizing the detonating cord to
effectuate desired
cuffing or rupturing of any structures, rock formations or the like while
minimizing undesired
ancillary damage.
Related Art
[0003] Detonating cord is, of course, well ltnown 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 puiverulent 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 tet-
ranitrate ("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 ~/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").
[0004] 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 stan-
dardize 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, in-
creased density of the core increases the velocity of detonation and thereby
the explosive en-
ergy 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
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2
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.
[0005] 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-triniframine (Cyclonite, or
"RDX") or
cyclotetramethylene tetranitramine (Homocyclonite, or "HMX") may be utilized
for the core of the
detonating cord. 'The explosives hexanitrostilbene ("HNS"), tetranitrocarbazol
("TNC"), and 2-6, bis
picryoamino 3, 5, dinitro 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, frees, 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, maybe 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 U.S. Patent Application No. 6,694,886. 'That
patent describes a
detonating cord having a non-metal outer sheath which imparts a sufficient
flexural modulus, e.g., of
about 250,000 psi (17.236 x 102 MPa), which enables a 6-foot length of the
cord to be sufficiently rigid
to perforate and penetrate fly ash. This ridge-type cord fins use in removing
fly ash from boiler tubes.
[0006] 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. Patent
6,165,297 issued
December 26, 2000 to J.G.B. Smith et al, entitled "Process And Apparatus For
The Manufacture Of
Emulsion Explosive Compositions". At column l, lines 41-45, it is noted that
sensitizing agents such
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
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void material." Similarly, U.S. Patent 6,200,398, issued March 13, 2001 to
J:H. Bush and enti-
tled "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 aad a bulk density
within the range of
about O.i to about 0.4 g/cc. Various othermicroballoons are described. U.S.
Patent 5,714,711,
issued February 3,1998 to J.B. Schumacher et al, is entitled "Encapsulated
Propellant Grasp
Composition, Method Of Preparation, Article Fabricated Therefrom And Method Of
Fabrica
lion". 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 re-
ducer fuel second reactant disposed on the polymeric barrier coaxing, 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. Patent 5,859,264 issued
January 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. '
[0007] In known mining, construction and quarrying operations, explosives are
used to
break a web of rock extending along a line of relatively closely spa:,ed,
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 us-
ing 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, un-
til the web fragments to cleave the rock mass.
[0008] Blasting methods attempt to minnic 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 bore-
holes pressurizes the holes and fi~agments the web of rock between the holes,
high amplitude
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shock energy exacerbates unwanted spelling and cxacking along the radial
fractmres emanating
from the boreholes and associated damage to the rock.
[0009] The dimension stone industry is concerned with cutting from rock
fom~ions 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 origina.I
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 sensi-
tive to static, sparks and fire, making it extremely dangerous. Black powder
is also de-activated
by water, precluding its use in wet areas.
[0010] 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
Iarge explosive diameter to function, rendering it unusable for smaller-
diameter boreholes.
SIJI~vIARY OF TIC INVENTION
[0011] 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 op-
erations and cutting structines such as dimension stone, as described more
fully below.
[OOI2] 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 deto-
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nation of the core. The dctonating cord of the present invention finds use is
any application in
which reduced peak shock energy is required or desired.
[0013] Reference herein to "reduced-velocity detonating cord", "Iow velocity
detonating
cord", or the like, mesas a detonating cord whose explosive core contains a
diluent which re
daces the velocity of detonation of the detonating cord as compared to an
otherwise identical
detonating cord which does not contain the diluent.
[0014] 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 micro-
balloons dispersed in the explosive material provides enclosed voids therein,
but as the explo-
sive material substantially completely fills the enclosing tubular sheath, the
core is nonetheless
described as a solid core.
[0015] Specifically in accordance with the present inventioxl 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 ccrd as
compare. 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 mi-
croballoons having a diameter of from about 10 to about I75 microns. Thus, the
noicroballoons
may comprise resin microballoons, e.g., phenolic resin microballoons, having a
diameter of
from about 10 to about 175 microns. .
[0016] 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.
[001' , 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.
[0018] The first explosive may be any suitable explosive, such as one or more
of PETN,
HIVIX, HNS, TrTC, PYX and RDX.
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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.
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.
According to an aspect of the present invention, there is provided a
detonating
cord comprising an elongate tubular sheath encasing a solid core of an
explosive
material obtained by mixing with a first pulverulent explosive 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 that
of an
otherwise identical detonating cord in which the explosive material contains
no
diluent, wherein the diluent comprises explosively inert microballoons.
According to another aspect of the present invention, there is provided in 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
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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 comprising that the detonating cord comprises an elongate
tubular sheath encasing a solid core of an explosive material obtained by
mixing with
a first pulverulent explosive a diluent comprising explosively inert
microballoons 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.
According to a further aspect of the present invention, there is provided a
method for making a detonating cord comprising the steps of preparing an
explosive
material by admixing a first pulverulent explosive with a diluent comprising
explosively inert microballoons in an amount which reduces the velocity of
deto-
nation of the detonating cord as compared to an otherwise identical detonating
cord in
which the explosive material contains no diluent; and
enclosing the explosive material within a tubular sheath to provide a
detonating cord having a core of the explosive material.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a cross-sectional view in elevation of a segment of a slab of
granite
having a plurality of boreholes drilled therethrough;
Figure 2 is a plan view of the granite slab of Figure 1 showing an end view of
the boreholes;
Figure 3 is a schematic view corresponding to that of Figure 1 showing a
length of detonating cord disposed within each of the boreholes and connected
to a
trunkline for initiation of the detonating cords;
Figure 3A is a cross-sectional view, enlarged relative to Figure 3, of a
segment
of the length of detonating cord illustrated in Figure 3;
Figure 4 is a schematic view corresponding to that of Figure 1, but showing a
stitching arrangement of a continuous length of detonating cord disposed as a
loop
within the boreholes;
Figure 5 is a plan view of the granite slab of Figure 1 after it has been
split in
two by initiation of detonating cord emplaced with the boreholes;
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[0027) Figure 6 is a perspective, schematic view of a two-borehole setup for
conducting
tests of detonating cord; and
[0028] Figures 7-IZ are graphs plotting the pressure output in thousands of
pounds per
square inch ("KIPS") against time in seconds of various samples of detonating
cord tested in the
setup illustrated in Figure 6.
DETAILED DESCRIPTION OF THE >ZWEN'JCION
[0029] As noted above, the velocity of detonation of detonating cord can be
controlled by
mixing a diluent with the pulveruleat 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 ex-
plosive, the velocity of detonation of the detonating cord may be reduced. It
has been discov-
ered 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 cer-
tain circumstances, as it reduces undesired damage to areas of the rock
immediately sureouad-
ing the cord The benefits of the present invention are clearly shown, for
example, in cutting
what is refereed to as "dimension stone" from in-ground foons of stone such as
granite and
the like, or in cutting slabs of dimension stone from larger blocks (sometimes
refen~ed to as
"production loaves") of the stone.
[0030] This technique of cleaving rock by fracturing the web of intact rock
extending be-
tween 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
fiactures 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 de-
sir~ed in applications other than dimension stone quarr3ring. 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 mindmize
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 re-
daces the requirements for mechanical supports such as roof bolts and the
like.
[0031] One aspect of the present invartion 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
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_$-
powder. It typically is a PETN based explosive and pmdnces none of the
handling problems
related to nitroglycerine-based explosives. Detonating cord can function in
very small cord di-
ameters, e.g., as small as less than 0.25 inch, i.e., 0.635 centimeter ("cm"),
in diameter. The
disadvantage of conventional, higlrvelocity detonating cord in applications
designed to form in
S the cleaved stone, and/or leave behind a smooth, cleaved stone face, is its
typically high deto-
nation velocity of up to more than 7000 meters/second, and high radial shock
wave output.
These characteristics, which ate conventionally considered to be desirable
attributes necessary
to enable conventional detonating cord to initiate explosive charges of low
sensitivity, contrib-
ute 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 ex-
plosive 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.
[0032) Another aspect of the present invention utiilizGS a low-velocity
detonating cord, pref
erably one having a velocity of detonation less than about 5000 meters per
second ("mlsec").
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 modify-
ing 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 e~cicnt
fi~acturing 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.
[0033] Referring now to Figure 1, there is shown a somewhat schematic cross-
sectional
view taken along line I I of Figure 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
Figure 2, parallel
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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 Figure 2 by
dash lines.
[0034] 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 ex-
plosive 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 Ioad of each borehole
requires decreasing
the spacing between adj scent 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 en
ergy 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 be-
tween adjacent boreholes without a corresponding increase in collateral
damage.
[0035] Figure 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 Figure 3,
individual lengths 18 of reduced-velocity detonating cord are connected to a
tr~mkline 20 which
may itself comprise detonating cord and which may, but need not, be reduced-
velocity deto-
nating cord in accordance with an embodiment of the present invention.
Tnmkline 20 is initi-
ated at one end thereof by any suitable known means (not illustrated) and a
detonation signal
travels along tnmkline 20 to initiate each of the lengths 18 of detonating
cord to fracture the
web 16 of stone.
[0036] Figure 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, pro-
vided 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
sheaxh 18a is made
of any suitable material to provide adequate mechanical strength and to be
resistant to penetra-
tion of water or other liquids into core 18b.
[0037] Figure 4 shows an alternate aaangement, sometimes referred to as
"stitching", in
which a single length 22 of reduced-velocity detonating cord in accordance
with an embodi-
ment of the present invention is threaded in a serpentine, stitching-li7ce
arrangement through
each of boreholes I2 by inserting a return loop of detonating cord through
substantially the en-
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tire length of each borehole. Length 22 of detonating cord is initiated by any
suitable means
(not illns~tcd) aad 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 Figure 3. Therefore, other factors
being equal, the
diameter of the reduced-velocity detonating cord used in the arrangement of
Figure 4 would be
about one-half that of the reduced-velocity detonating cord used in the
axraagement of Figure 3.
[0038] The result of fracturing web 16 is schematically illustrated in Figure
5, which is a
plan view concsponding to that of Figure 2, but showing the granite block
after it is cleaved in
two by either the lengths 18 of detonating cord as illustrated in Figure 3, or
the length 22 of
detonating cord as illustrated in Figure 4. Granite block 10 (Figure 2) has
now been split into
two granite blocks, l0a and IOb (Figure ~. Adjacent the arcs I2a, I2b (Figure
5} of the farmer
boreholes 12 (Figure 2) are a series of small radial cracks 24 and a few
considerably longer ra
dial cracks, refeaed 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.
[0039] A series of blocks was cleaved from larger blocks (production loaves)
of granite
using either 18 grlft (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 werc loaded with Viking B-gel, an inert gel product with microballoons,
available from
Viking Explosives & Supply, inc. of Rosemont, Minnesota. 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 r~luce 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 detonaxing cord.
The observed
results of these tests are tabulated in the following Table A. The average
length of stickers was
twelve inches.
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Table A
Average Number
of Holes Af
fected by Average Mini- Average Maxi-Average Number
~
Cracking, mum Crack mum Crack of Stickers
per per
Slab Length (Inches)Length (Inches)Slab
A. Reduced-
Velocity Deto-
nating Cord 17.0 2.4 4.1 0.5
B. Comparative
High Velocity
Detonating 18.3 3.6 ~ S.7 2.9
Cord
Difference 1.3 I.2 I.6 2.4
Improvement
Provided By
A Over B 7% 32% 28% 82%
[0040] 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, Sam-
ple 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, S% by weight ammonium
nitrate, and
S% 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/$ {3.83 g/m) of 100% PETN, i.e., it contained
no diluent.
[0041] Blast pressure profiles were measured for a series of cord/coupling-
agent combina-
tions. The test arrangement is schematically illustrated in Figure 6 in which
two test boreholes,
28 and 30, terminating in respective borehole bottoms 28a and 30a, were bored
in the stone par-
1 S allel to each othex with their peripheries spaced 6 inches (15.24 cm) from
each other, this dis-
tance being illustrated as D in Figure 6. A test detonating cord 32 was
inserted t'lu'oughout the
length of test borehole 28 and, in those cases in which a coupling agent other
thaw air was util-
ized, borehole 28 was filled with a coupling agent, e.g., gel or water (not
shown). A Ioop (not
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shown) of test detonating cord 32 was inserted into borehole 28 in order to
test a "stitching" ar-
rangemeat of detonating cord; as schcmatically illustrated in Figure 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 co~ected by
a cable 36 to comps 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 Figures 7-16, wherein the shock wave pmsures are plotted
on the left-
hand vertical axes in kilopoimds per square inch ("KIPS") and on the right-
hand vertical axes in
kilograms per square centimeter ("kglcm2"). 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.
[0042) The tests resulted in fracturing of the stone adjacent the boreholes in
the manner de-
scn'bed above in connection with Figure 5. Thus, radial fractures (such as
those shown at 24 in
Figure 5), which tended to be of relatively constant Iengfn, affected many of
the boreholes on
the perimeter of the slab. Stickers (such as those shown at 26 in Figure 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.
[0043] The results of the pressure testing shown in Figures 7-12 support the
visual observe-
dons of the cleaved stone. In the pressure measurements illustrat~.in Figures
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 proporkional to the length and
quantity of the resultant
radial fractures and stickers. The key to reducing radial frachuing 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
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the area beneath the curve in the Figures plotting the pressure profiles.
These areas represent
the product of pressure multiplied by time.
[0044] A pressure profile for 18 gdft (3.83 glm) comparative high-velocity
detonating cord
in water is shown in Figure 7. (In Figures 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.)
[0045] The pressure profile of Figure 8 shows the pressure profile from
comparative high- '
velocity detonating cord in B-gel coupling agent, and that of Figure 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 Figures 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
Ievel of pressure.
[0046] The pressure profile of the low-velocity detonating cord in water shown
in Figure 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 Figure 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 Figure 9) and represent a dramatic improvement over comparative high-
velocity
detonating cord in B-gel, shown in Figure 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 di~culty of mixing and using the product in day-to-dsy operations, the low-
velocity deto-
nating cord in accordance with an embodiment of the present invention provides
greatly en-
hanced performance and may enable elimination of the need to use B-gel.
[0047] Figure 11A shows the pressure profile for a comparative high-velocity
18 grlft (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 Figure 6) using the stitching arrangement schematically
illustrated. in Figure 4.
Using the stitching arrangement of Figure 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
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cord being held against or at least in close proximity to, the wall of the
borehole, thereby en-
hancing coupling of the detonating cord to the walls of the borehole. As seen
by comparing
Figures I IA and 11B to each other, the contrast between the two types of
cords is stinking. The
high peak loads of pressure engendered by the comparative high-velocity
detonating cord
shown in Figure 11A are absent from the pressure profile of the low-velocity
detonating cord in
accordance with an embodiment of the present invention shown in Figure 11B. A
comparison
of the pressure profiles of Figures 11A and 11B also shows that the low-
velocity detonating
cord (Figure 11A) nevertheless had about the same general level of pressure
output as the com-
parative high-velocity detonating cord (Figure l IB). The significant
difference is the very de-
sirable elimination of the high-pressure peaks shown in Figure 11B.
[0048] Figure 12 shows the pressure profile of a comparative 7.5 gr/ft (I.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 detonaxing cords, is nonetheless prominent,
about three times
higher than the remaining non-peak pressure level.
[0049] 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 de-
sirable than phenolic microballoons for utilization in a detonating cord.
Phenolic resin is a brit-
tle material and the addition of phenolic nucroballoons to the explosive core
does not appear to
sensitize dry PETN. Phenolic resin has a specific gravity of approximately
1.35 and the pheno-
lic 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 den-
sity",)
[0050] As indicated above, the diluent may comprise an explosive diluent, such
as ammo-
nium nitrate, which is of significantly lessen 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 dilueat,
e.g., mieroballoons, may be used as the sole diluent. The use of a less
brisant explosive such as
ammonium nitrate as the sole diluent in a detonating cord has the disadvantage
that the amount
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of less brisant explosive required to attain a su~cient reduction in velocity
of detonation will
significantly increase the diameter of the detonating cord.
[005I] The phenolic microballoons used to prepare the tested low-velocity
detonating cord
had an average particle size distn'bution as follows, wherein ~, stands for
microns.
eB
Maximum
of ParticlesDiameter
75% 71 ~,~.
50% 51 ~s
25% 35 [.~
[0052] 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 ruptur-
ing 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 lower VOD is shown for compressed cord containing the
same amount
of microballoons as the uncompressed cord. In Table 1, "SFPETN' means a
superfine grade of
PETN, having a particle size of about 20 microns diameter.
Table 1
Core % By Weight Inspection-
Load
gr/ftg/m Microballoon CompressedPETN 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
[0053] It was detcrmined 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
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substantially reduced the VOD as shown in Table 2. Reference to "MFl'ETN' in
Table 2
means a medium fine grade of PETN, having a particle. size of about 150
microns diameter.
Table 2
Core %
Load By
Weight
gr/ftg/m AN* MicroballoonPETN 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
[0054] 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 dilu
eats) 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.
[0055] 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.
[0056) When the same weight percent of phenolic microballoons was added to the
exam-
ined (post-manufacture compressed) and unexamined (not post-m~ufacture
compressed) deto-
nating cords, a loss of some of the VOD reduction effect was noted is the
examined (com-
pressed) samples. This loss of desired reduction in VOD is attributed to
crushing of the micro-
balloons 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 quan-
tified, that a substantial percentage of the microballoons were nrphu~ed 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 substan-
tial VOD reduction effect is believed to be achieved by reduction of density
of the explosive
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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 -reducing effect.
[0057] While the invention has been described in detail in connection with
specific em-
bodiments thereof, it will be appreciated that neither the invention nor the
appended claims are
limited to the specific illustrative embodiments.
