Note: Descriptions are shown in the official language in which they were submitted.
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A METHOD AND APPARATUS FOR PRESSURE WAVE SUPPRESSION
IN SMALL-CHARGE BLASTING
FIELD OF THE INVENTION
The present invention is directed generally to methods and devices for small
charge
blasting of rock and other materials and specifically to methods and devices
for controlling
pressure wave emissions and/or flyrock generated by the small-charge blasting
process.
BACKGROUND OF THE INVENTION
In civil excavation projects in urban environments, many restrictions are
imposed
on operators that substantially increase the operator's capital and operating
costs. The
operator must generally comply with strict requirements regarding not only the
transportation, storage and use of explosives but also airblast, noise, and
airborne flyrock
particles. "Airblast" refers to pressure waves in air emanating from a rapid
release of
energy (e.g., a blast). Airblast noise is the audible part of the airblast
energy spectrum,
having frequencies in the range from 20 to 20, 000 Hz. Airblast concussion is
the inaudible
part of the airblast energy spectrum, having a frequency content below 20 Hz.
"Noise"
refers to pressure waves in air generated by equipment other than the small
charge blasting
equipment, such as the drill during formation of one or more holes for small
charge
blasting and/or the impact breaker during removal of fractured material.
"Flyrock" refers
to rock particles thrown into the air by the rapid release of energy (e.g.,
blast). Flyrock
may be in the form of a shower of small pieces at relatively high velocities
(20 to 50 mls
typical), which typically originate from the collar region of the drill hole.
Flyrock may
also be in the form of larger pieces of rock at relatively low velocity (1 to
10 m/s typical),
which typically originate in the mass of rock excavated from the crater formed
by the
blasting event.
Existing drill and blast methods may be inapplicable in many applications as a
result of these restrictions, even though the blasting methods are the most
cost effective
method for the specific application. For example, small charge blasting which
commonly
has a lower seismic and airblast signature, cause less flyrock, and have lower
operating
costs compared to conventional drill and blast techniques, can nonetheless
generate
airblast, equipment noise, and/or flyrock levels that exceed the maximum
permissible
levels in many applications. "Small charge blasting" refers to any excavation
method
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where relatively small amounts of an energetic substance (typically a few
kilograms or
less) are consumed for each hole in a rock fracturing sequence.
SUNINIARY OF THE INVENTION
Objectives of the present invention include providing a drilling and blasting
methodology for excavating rock, particularly hard rock, in airblast, noise,
and/or flyrock
restricted areas, such as urban settings, and providing a methodology and
apparatuses)
for use with small-charge blasting techniques for controlling and/or
suppressing airblast
in airblast restricted areas, controlling and/or suppressing equipment noise
in noise
restricted areas, and/or for controlling and/or suppressing flyrock in flyrock
restricted
areas.
These and other objectives are addressed by the methodology and apparatuses of
the present invention. In a first embodiment, a method is provided for
selecting one or
more pressure wave suppression devices for use with small-charge blasting of a
material
that is near a pressure wave (i.e., an airblast and/or noise) restricted area.
The method
broadly includes the steps of
(a) determining one or more pressure wave level requirements) at
corresponding selected distances) from the material to be broken;
(b) determining a corresponding unsuppressed pressure wave levels) at each
selected distance produced by the excavation equipment in the absence of a
pressure wave
suppression device;
(c) comparing the pressure wave level requirements) with the corresponding
unsuppressed pressure wave levels) to determine a desired amount of pressure
wave
suppression;
(d) comparing the desired amounts) of pressure wave suppression at one or
more distances from the material to be broken with an amount of pressure wave
suppression for each of a plurality of pressure wave suppression devices; and
(e) thereafter selecting one or more pressure wave suppression devices based
on the comparing step (d) to produce the desired amounts) of pressure wave
suppression.
As used herein, a "pressure wave level requirement" refers to the maximum
permissible
pressure wave emission levels and is often expressed as a maximum allowable
pressure
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wave emission level at a specified distance from the blasting site (e.g., the
location of the
energetic substance used in the small charge blasting process). The method
provides an
operator with versatility in meeting the unique requirements of each job and a
relatively
low cost and simple excavation technique that complies with the often
demanding
requirements in pressure wave restricted areas.
The process is particularly applicable to small charge blasting techniques
using
controlled fracturing to break the material. Generally, controlled fracturing
is performed
by drilling a hole in the material to be broken, inserting a sealing member,
which can be
a stemming bar, a gas injector barrel, or other pressurizing device, into the
drill hole, and
releasing a pressurized working fluid rapidly into a portion of the drill
hole, usually the
bottom portion. "Sealing" refers to partial or total blockage of the hole to
inhibit the
escape of the fluid from the drill hole. "Sealing member" refers to any
downhole device
capable of sealing a pressurized working fluid in the bottom of a hole,
including without
limitation loosely consolidated or unconsolidated particles such as sand,
gravel, rock
fragments, and the like, and a solid material such as grout, a stemming bar, a
gas injector
barrel, and the like. The pressurized fluid is typically generated by
combustion of a
propellant or explosive source, by an electrical discharge into a conductive
fluid, or by
compression of the working fluid. The fractured material is thereafter removed
from the
face by an impact breaker and mucking equipment. Because of the relatively low
weight
of the energetic substance used to generate the working fluid and the
relatively low
pressure wave and flyrock emissions, equipment and personnel commonly remain
in the
area of the hole during the small charge blast.
The plurality of pressure wave suppression devices and their performance
capabilities can be listed in the form of a menu from which any number of
appropriate
devices can be selected. The plurality of pressure wave suppression devices
preferably
includes at least one of, more preferably at least two of, and even more
preferably at least
three of
(i) a downhole pressure wave suppression device located in a hole in the
material for directing flow of the working fluid through one or more nonlinear
pathways
and/or for contacting at least a portion of the working fluid with a thermal
energy
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absorbing material having a plurality of heat transfer surfaces, and/or for
attenuating the
noise generated by downhole equipment (e.g., a drill),
(ii) a collar pressure wave suppression device located at or near the
opening of the drill hole in communication with the drill hole for directing
flow of the
working fluid through one or more nonlinear pathways, for contacting at least
a portion
of the working fluid with a thermal energy absorbing material having a
plurality of heat
transfer surfaces, and/or for attenuating the noise generated by the downhole
equipment,
(iii) a mat positioned on the surface of the material to be broken around
the hole opening for directing flow of the working fluid through one or more
nonlinear
pathways in the mat and/or for absorbing thermal energy from the working fluid
by
contacting the working fluid with a plurality of heat transfer surfaces,
(iv) an enclosure substantially surrounding the hole opening for containing
the equipment noise and/or working fluid (i.e., the pressure waves) and
impeding the
discharge of the noise and/or working fluid into the ambient atmosphere,
(v) a barrier located between the hole opening and the pressure wave
restricted area for absorbing and/or deflecting at least a portion of the
pressure wave
energy, and
(vi) a plurality of atomized liquid droplets (preferably having a droplet size
ranging from about 0.01 to about 0.1 mm) suspended in the air adjacent to the
surface of
the material during the blast to absorb thermal energy from the working fluid.
The
plurality of devices includes not only devices capable of resisting the high
gas pressures
in close proximity to the small-charge blast but also devices capable of
resisting only
relatively low gas pressures further away from the blast. Many of the devices
can not only
suppress pressure wave energy but also reduce or eliminate flyrock.
The determining step (a) can include a number of substeps to determine the
pressure wave level requirement. By way of example, the substeps can include:
determining operator pressure wave requirements) at one or more selected
distances from the material to be broken (i.e., the "operator pressure wave
requirement"
refers to pressure wave restrictions that are generally applied by the
operator and are
independent ofthe specific job, such as mandated by the operator to protect
personnel and
equipment);
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determining job pressure wave requirements) at one or more selected distances)
from the material to be broken (i.e., the "job pressure wave requirement"
refers to
pressure wave restrictions that are unique to the specific job, such as
imposed by
regulatory authorities or by the surroundings of the job, e.g., nearby
structures or
5 thoroughfares); and
comparing the operator pressure wave requirement with the job pressure wave
requirement with the more restrictive of the two (i. e., the requirement
mandating a greater
degree of pressure wave attention) being the pressure wave level requirement.
The operator pressure wave requirements) in small charge blasting typically
includes a machine pressure wave requirements) and/or a personnel pressure
wave
requirement(s). In small charge blasting, the equipment and personnel commonly
remain
at or near the hole during the blast. Because of the small charge used in the
blast,
complete excavation of the blasting site during blasting, as normally is the
case in
conventional large charge blasting, is often unnecessary and even undesirable
for reasons
of productivity. For these reasons, the small charge blasting operator
commonly
formulates requirements unique to small charge blasting and independent of the
specific
job to protect equipment from pressure wave emissions ofthe blast ("the
machine pressure
wave requirement") and from flyrock ("the machine flyrock distance
requirement") and
personnel from pressure wave emissions of the blast ("the personnel pressure
wave
requirement") and from flyrock ("the personnel flyrock distance requirement").
The unsuppressed pressure wave levels at the selected distance from the
material
to be broken are commonly determined by numerous field measurements.
Once the unsuppressed pressure wave level and the pertinent pressure wave
level
requirement are known, a sufficient number of pressure wave suppression
devices are
selected to realize the desired amount of pressure wave suppression. Each of
the pressure
wave suppression devices in the menu will yield a specific amount of pressure
wave
suppression, usually determined and quantified by field measurements. The
appropriate
pressure wave suppression devices for a given application are commonly
selected based
not only on the desired pressure wave level reduction but also on the
requirements of the
job, cost considerations, and the like.
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The process can include various substeps. For example, step (a) can include
the
substep of determining at least two of a job pressure wave requirement, a
machine
pressure wave requirement, and a personnel pressure wave requirement. In that
event,
step (b) includes the substep of determining a corresponding unsuppressed
pressure wave
level for each of the at least two of a job pressure wave requirement, a
machine pressure
wave requirement, and a personnel pressure wave requirement at a corresponding
selected
distance from at least a portion of the material to be broken and step (c) the
substep of
comparing the at least two of a job pressure wave requirement, a machine
pressure wave
requirement, and a personnel pressure wave requirement with the corresponding
unsuppressed pressure wave level to determine a corresponding degree of
pressure wave
suppression. Steps (d) and (e) are thereafter repeated for each of the
corresponding
degrees of pressure wave suppression to verify that the selected pressure wave
suppression devices are sufficient to comply with the at least two ofthe job
pressure wave
requirement, the machine pressure wave requirement, and the personnel pressure
wave
requirement.
The above-described steps are commonly repeated for each step of the process
(and each piece of equipment) that generates pressure waves. For example, the
above-
noted steps are typically repeated for the step of drilling the hole impact
breaking of the
fractured rock, and/or mucking of the fractured rock.
Commonly, a number of the pressure wave suppression devices selected in the
process (e.g., mats and barriers) remain in place during the repeated drilling
and small
charge blasting sequences. This is so because only one or at most a few holes
are shot at
any one time because the small charge blasting machine can only seal one hole
at a time.
In contrast in conventional large charge blasting a large number of holes are
simultaneously shot and the pressure wave suppression devices are thereafter
removed to
remove fractured material and drill new holes and repositioned before the next
blasting
cycle.
To control flyrock, the method can include additional steps. Specifically, the
steps
include:
determining at least two of (a) a job flyrock distance requirement (i.e., the
"job
flyrock distance requirement" refers to flyrock distance restrictions that are
unique to the
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specific job, such as imposed by regulatory authorities or by the surroundings
ofthe job,
e.g., nearby structures or thoroughfares), (b) the personnel flyrock distance
requirement,
and (c) the machine flyrock distance requirement;
comparing the at least two of the job flyrock distance requirement, the
personnel
flyrock distance requirement, and the machine flyrock distance requirement to
determine
a flyrock distance requirement; and
comparing the flyrock distance requirement with an uncontrolled (or
unsuppressed) flyrock distances) to determine whether protection against
flyrock is
required. "Unsuppressed flyrock distance" refers to the probable flight
distance of flyrock
from the hole. Ifthe flyrock distance requirement is more restrictive than the
unsuppressed
flyrock distance(s), the operator should consider the need for flyrock
protection in
selecting a devices) from among the various devices in the menu.
In a second embodiment of the present invention, a pressure wave suppression
device is provided that has at least one nonlinear pathway extending
therethrough for
channeling the flow of the gas released by the energetic material. The
nonlinear pathway
can remove energy from the airblast and thereby facilitate pressure wave
suppression.
Typically, the device will have a plurality of nonlinear (or tortuous)
pathways of different
lengths to achieve suppression. The device can include, for example, a
plurality of baffles
and dead end chambers that may be separated by an attenuation material (e.g.,
a fibrous
porous filament material such as metal (e.g., copper, steel, etc.) wool,
packed metal (e.g.,
copper, steel, etc.) balls or ball bearings, a series of perforated mats
stacked on top of the
other and having different sized and/or misaligned perforations. The device
can be located
at and/or around the hole opening and/or in the hole itself and generally
substantially
surrounds the outer perimeter ofthe sealing member. The device is particularly
effective
in airblast suppression.
In another embodiment of the present invention, a pressure wave and flyrock
suppression device is provided that is an enclosure substantially surrounding
and enclosing
the opening of the hole. For pressure wave suppression, the enclosure interior
can have
a pressure wave layer for absorbing, dissipating or reflecting the pressure
waves and an
impact resistant layer located on the interior surface of the enclosure
(interiorly of the
impact resistant layer) for deflecting flyrock. The enclosure is su~ciently
large to contain
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the sealing member for small charge blasting and a drill and/or impact breaker
for more
effective pressure wave suppression. The enclosure can be a flexible canopy, a
rigid box-
like structure, or combination of rigid and flexible elements. The enclosure
encloses a
sufficient volume such that the gas pressure within the enclosure is
controlled to an
average overpressure of preferably no more than about 15 KPa during formation
and
propagation of a fracture from the hole. The volume of the enclosure
preferably is at least
about 4 cubic meters and more preferably ranges from about 4 to about 100
cubic meters.
The pressure wave layer can be any suitable material including rubber, canvas,
fiberglass
mesh, industrial conveyor belt or commercially available acoustic material and
the impact
resistant layer can be any suitable material including a perforated steel
plate, steel mesh,
fiberglass mesh, industrial conveyor belt, heavy canvas sheet, composite
materials such
as "KEVLAR" or impact resistant plastics. For additional pressure wave
suppression the
enclosure can have a plurality of leakage vents for discharging the gas
released by the
energetic material into the ambient atmosphere at a controlled rate.
Finally, another embodiment of the present invention is directed to a method
for
employing blasting mats for pressure wave and/or flyrock suppression to
efficiently and
inexpensively perform small charge blasting. The steps are:
(a) positioning a blasting mat on at least a portion of the material to be
broken;
(b) drilling or otherwise forming a hole in the at least a portion of the
material
through the blasting mat;
(c) inserting an energetic substance (and/or sealing member) through the
blasting mat and into the hole; and
(d) releasing energy from the energetic substance in the hole to initiate and
propagate a fracture from the hole. The fracture can be a penetrating cone
fracture or
another type of controlled fracture.
The drilling step (b) is typically performed by forming a hole in the mat and
inserting at least a portion of the drill through the hole in the mat. The
hole can be formed
in the mat by cutting through the mat or by deforming a pre-existing opening
in the mat.
The latter approach is used where the blasting mat is composed of one or more
flexible
mats of a mesh material. The flexible mats preferably have differing mesh
sizes to
facilitate removing and/or dispersing of energy from the pressure wave and an
upper layer
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above the perforated mesh material that will inhibit the release of the
pressure waves into
the environment and thereby force the pressure waves to negotiate the tortuous
passages
through the mats to be released from the outer perimeter of the mats.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a decision tree that can used by a small-charge blasting estimator
andlor
operator to select the airblast suppression, noise abatement, and flyrock
control devices
appropriate to a particular excavation job.
Figure 2 is an exemplary plot of flyrock trajectories in both the vertical and
horizontal directions.
Figure 3 is a cutaway side view of a sealing member which includes a series of
high-pressure baffles for disrupting and de-energizing the flow of high
pressure working
fluid in the drill hole.
Figure 4 is a cutaway cross-sectional view of a device for intercepting
flyrock that
originates from the collar region of a drill hole.
Figures SA and SB respectively are a cutaway side view taken along line SA-SA
of Figure SB and a cutaway plan view taken along line SB-SB of Figure SA of a
collar
shroud installed around the collar of a drill hole and secured by the stemming
or gas-
injecting means.
Figure SC is a cutaway side view of an alternative embodiment of a collar
shroud
decoupled from the stemming or gas injecting device.
Figure 6 is a cutaway side view of an alternative embodiment of a collar
shroud
for use with a drill.
Figures 7A and 7B are respectively a cutaway side view taken along line 7A-7A
of Figure 7B and plan view of a ground shroud installed such that several
shots can be
fired before the shroud is moved.
Figures 8A and 8B are respectively a cutaway side view and exploded view of a
section through a wall of an enclosure that is attached to the proximal end of
a boom of
a small-charge blasting machine.
Figure 8C is an isometric view of another configuration of an enclosure.
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Figure 8D is a cutaway side view of an enclosure that is affixed to the
supporting
cable such as might be used on an excavation apparatus for hole drilling,
small charge
blasting, and impact breaking in shaft excavation.
Figure 9 is a cutaway side view of a moveable barrier for pressure wave
5 suppression and flyrock control.
Figure 10 is a plan view of a layout of the relative positions of pressure
wave-
flyrock barriers might be deployed to protect an area from the small-charge
blasting work
site.
Figure 11 is a side view of a small charge excavation machine in firing
position
10 with an atomized spray pattern for pressure wave suppression.
Figure 12 is a cross-sectional side view of a gas generator according to one
embodiment.
Figure 13 is a partially cutaway side view of the gas generator of Figure 12.
Figure 14 is a partially cutaway side view of a gas generator according to
another
embodiment.
DETAILED DESCRIPTION
The Selection of an Appropriate Pressure Wave and Flyrock Suppression Device
Referring to Figure 1, the first step 20 in selecting appropriate pressure
wave and
flyrock suppression devices) is to characterize the unsuppressed pressure wave
and
flyrock emissions 24 of the small charge excavation process. For example, the
drilling
noise characteristics of a particular drilling device, the airblast and fly
rock characteristics
of a particular small-chaxge blasting method, and the noise characteristics of
a particular
impact breaker are generally determined by field measurements.
The characteristics of both equipment noise and airblast can be characterized
by
a peak amplitude versus distance curve and an energy amplitude versus
frequency curve.
The former shows the decay of peak amplitude with distance and the latter
shows at which
frequencies the energy of the pressure waves are concentrated.
The unsuppressed pressure wave amplitude for the small charge blasting process
at a given distance from the hole to be drilled/blasted is often a function of
the energy of
the small charge blast. In the case of propellant or explosive based small
charge blasting,
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the dependence of airblast on distance may be expressed in terms of propellant
or
explosive weight used (since their energy densities are usually all in the
range of about 3.5
to about 4.5 MJ/kg). The functional relationship is usually plotted on a log-
log plot
showing either pressure or decibels as a function of scaled distance where
scaled distance
is the actual distance divided by the cube root of the charge energy or
weight.
The characteristics of flyrock can be characterized statistically by a
probability
distribution function that relates charge weight and rock type with the
distribution of
flyrock at varying distances from the hole or by a plot such as that in Figure
2 generated
for a specific charge weight and rock type.
In box 28, the job and operator pressure wave requirements (e.g., job and
operator
airblast requirements 30 and/or job and operator equipment noise requirements
32, if
different), job and operator flyrock requirements 34, and any other pertinent
pressure
wave and flyrock requirements are determined.
Each excavation job and particularly urban excavation work, has a number of
unique job pressure wave restrictions on air-blast and noise. These may be
simple
restrictions such as a noise level not to be exceeded at a certain distance
from the job site.
The restrictions may be more elaborate and contain not-to-exceed levels for
pulsed,
intermittent and continuous air/blast noise levels at several distances and
under certain
atmospheric conditions. Generally the pressure wave restrictions are expressed
as the
peak pressure wave amplitude at one or more distances from the work area or
shot point
and often will specify the range of frequencies where the peak amplitude
requirements
apply.
The operator pressure wave requirements) commonly include the personnel and
machine pressure wave requirements. For example, the operator might have a
near-field
pressure wave limit imposed as a safety measure to comply with the operating
company's
internal requirements. In complying with this, the operator may have to add
additional
pressure wave suppression devices.
After determining all of the pertinent pressure wave specifications, the
operator
selects 36 the most stringent requirements) as the requirements) to be
complied with.
Commonly, the most stringent pressure wave requirements) is the requirements)
that
mandates the lowest pressure wave emissions, and the most stringent flyrock
distance
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requirements) is the requirements) that mandates the lowest flyrock trajectory
distance
from the jobsite.
In some cases, the pressure wave requirements for airblast will be different
from
the pressure wave requirements for equipment noise. In such cases, the most
stringent
S pressure wave requirements) is determined for each type of pressure wave
emissions.
Next, the operator determines 38 the difference between the unsuppressed
pressure wave emissions and the most stringent pressure wave requirement(s).
Because airblast requirements are typically more demanding than noise and
flyrock
requirements, the process first focuses on selecting appropriate pressure wave
devices for
suppressing airblast energy followed by determinations whether additional
devices are
required for attenuating equipment noise and/or flyrock. As will be
appreciated, in some
applications equipment noise or flyrock may be the more demanding requirement
and the
process would be reconfigured to consider that requirement first followed by
verification
that the devices chosen also comply with the less restrictive requirements.
The difference of step 3 8 (or the unsuppressed pressure wave emissions) may
be
adjusted 40 upwards (or sometimes downwards) based on safety factors,
atmospheric
conditions, and other factors. The difference (or the unsuppressed pressure
wave
emissions) may be increased by a suitable safety factor to ensure compliance
with pertinent
regulations. The difference may be adjusted based on the forecasted
atmospheric
conditions received from an outside service. Such special conditions might
include strong
atmospheric temperature inversions, heavy low cloud cover or heavy fog. These
type of
atmospheric conditions can reflect noise and airblast energy initially
radiated upwards back
down toward the ground and may even focus noise and airblast energy in a
specific
location quite far from the work site. The estimator/operator may have access
to services
that provide such information or he may have to use his judgement.
After suitable adjustments of the difference between the unsuppressed pressure
wave emissions and the pressure wave requirement, the operator determines 42
whether
the difference is positive or negative. Ifthe difference is negative or zero,
the small charge
process is airblast compliant 1008 without the need for airblast pressure wave
attenuation
devices. If the difference is positive (i.e., the unsuppressed pressure wave
levels exceed
the pressure wave level requirement), the operator in box 44 selects a
sufficient number
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of cost ei~ective pressure wave suppression devices from a menu of such
devices to
attenuate the diiFerence. If possible, the operator selects the combination of
devices that
minimizes capital and operating costs, controls equipment noise to meet the
job pressure
wave specification, and controls flyrock to meet the job flyrock
specification. For
example, the adjusted dii~erences may be 40 to 50 dB at a first distance from
the jobsite
and 60 to 70 dB at a second distance from the jobsite with the first distance
being less than
the second distance.
The menu must characterize the devices at least in terms of the amount of
pressure
wave attenuation they provide at specified distances from the drill
hole/jobsite. This is
usually expressed in terms of dB of attenuation at the specified distance.
Most well
designed passive devices will provide in the range of about 10 to 20 dB of
pressure wave
attenuation at a specified distance relatively close to the hole/jobsite. Some
devices will
of course provide an amount of attenuation that is indirectly proportional to
the distance
from the jobsite. The pressure wave attenuations determined for each pressure
wave
suppression device may contain a safety factor (i.e., be stated
conservatively) to ensure
that the devices will suppress the stated amounts of pressure wave energy. An
example
of such a menu might be:
AMOUNT OF PRESSURE
PRESSURE WAVE SUPPRESSION DEVICE WAVE ATTENUATION
(dB)
Down Hole Baffle Device 20
Collar Shroud 20
Ground Mats 10
Atomized Fluid Spray 10
Flexible Rig Canopy 15
Rigid Rig Enclosure 20
Wall Sound Absorbing Barriers (for 10
example,
liners for shafts)
Stand Alone Sound Barriers 5
Referring to Figure 1, once the pertinent pressure wave suppression
requirements
have been met for airblast, the operator must verify 48 that equipment noise
and other
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types of pressure wave emissions do not exceed the pertinent pressure wave
specifications) when the devices selected above are in place. This will
require the
operator to determine whether unsuppressed equipment noise (e.g., noise from a
drill or
impact breaker) or other types of pressure wave emissions exceeds, for the
specific type
of pressure waves, the more stringent of the job pressure wave and operator
pressure
wave requirements. If not, additional pressure wave attenuation is
unnecessary. If so and
if the devices are inadequate to suppress the type of pressure waves, a second
menu of
pressure wave attenuation devices, which is similar to (but typically not the
same as) the
table described above, is reviewed and additional devices selected to suppress
the type of
pressure waves.
Although many pressure wave suppression devices can suppress both equipment
noise and airblast, some devices will provide differing degrees of attenuation
for airblast
and equipment noise. For example, some devices will have higher attenuation of
airblast
and have little or no attenuation of equipment noise. The downhole baffle
device and
ground mats attenuate at most only a small fraction of the drilling noise
while a collar
shroud, flexible rig canopy, rigid rig enclosures wall sound absorbing
barriers, and stand
alone sound barners are effective in attenuating equipment noise such as
drilling noise.
Other devices that are able to suppress the type of pressure waves but not
airblast
energy may not be listed in the menu. For example, devices not listed in the
menu above
include, additional muffling for engines; a noise suppression collar for the
rock drill; and
a separate noise abatement enclosure for an impact hammer.
Once the pressure wave requirements have been met for airblast, equipment
noise,
and/or other types of pressure wave emissions, the operator must verify 50
that flyrock
control requirements are also met. The flyrock verification step 50 requires
the operator
to compare the more stringent of the personnel, machine, and job flyrock
distance
requirements with the unsuppressed flyrock distances) to determine if flyrock
protection
is necessary. If the pertinent flyrock distance requirements) is less than the
unsuppressed
flyrock distance(s), flyrock control equipment is required. In that event, the
devices
selected above are analyzed to determine whether they are able to suppress
flyrock and
comply with the requirements, If not, yet another menu of flyrock control
devices is
considered to select additional devices to suppress the flyrock. Usually the
use of ground
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mats, collar shrouds and barriers will be sufficient to meet flyrock control
requirements.
However, if only barners are required to achieve the job and operator pressure
wave
requirements, then additional devices such as ground mats or a collar shroud
may be
required to achieve adequate flyrock control. The operator may also require
some close-
s in flyrock control to protect the excavating machine or some other piece of
equipment
used at or near the work face.
After applying the above-noted steps, the operator should be in full
compliance 52
with job and operator pressure wave and flyrock requirements and all pressure
wave
requirements.
10 As will be appreciated, the above-described process steps may vary somewhat
depending on the job type (surface excavation, basement excavation, partially
enclosed
excavation, shaft, tunnel, drift or cavern). The process steps were described
generally
with reference to pressure wave and flyrock restrictions in an urban
environment.
15 Down Hole Pressure Wave Suppression Devices
Down hole pressure wave suppression devices are aimed at intercepting and
controlling the airblast adjacent to the source of the small charge blast
event in the drill
hole and around the barrel or sealing member. In one configuration, rigid
impact resistant
annular housing fits around and is connected to the sealing member and
functions like a
silencer on a gun. The housing is substantially open at the both the downhole
and uphole
ends, and has a series of internal baffle plates and/or dead-end chambers
and/or
attenuation material that cause the pressurized working fluid that leaks up
the drill hole
to negotiate a series of separate paths of unequal length (nonlinear
pathways). In this
way, the coherence of the airblast that exits the silencer is eroded such that
the peak
pressure ofthe airblast is diminished and its pulse width is lengthened,
resulting in a lower
amplitude, lower frequency air-blast than would be otherwise generated. The
baffles
and/or dead-end chambers and attenuation material will also increase the
surface area over
which the energetic working fluid must flow thereby enhancing heat transfer
from the
working fluid to the baffle plates. This acts to de-energize the pressurized
working fluid
that leaks up the drill hole which, in turn, acts to reduce the peak amplitude
and pulse
width of the resultant airblast wave.
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An example of such a down hole airblast silencer device is shown in Figure 3 .
The
silencer 2001 is firmly connected to the sealing member 2002 which is inserted
into a drill
hole 2003. The small-charge blasting agent (or working fluid) pressurizes the
bottom
portion 2004 of the drill hole 2003. The sealing member 2002 seals the bottom
portion
2004 by a sealing surface or other sealing device 2005 near the hole bottom.
The silencer
2001 is located uphole ofthe sealing surface 2005 and substantially fills the
cross-sectional
area of the hole. The down hole end 2006 of the silencer 2001 is open or
perforated to
allow the flow of high-pressure gases to enter. The inside of the silencer
includes a
plurality of baffles 2007, a plurality of dead-end chambers 2008 and/or
attenuation
material 2009. The uphole end 2010 of the silencer 2001 is also open or
perforated to
allow the high-pressure gases to escape in a less coherent and less energetic
state than
they entered. The gas pressures that the silencer device is exposed to are
typically in the
range of about 5,000 to about 50,000 psi.
Collar Pressure Wave and Flyrock Suppression Devices
Collar pressure wave and flyrock suppression devices intercept and control the
energetic working fluid that escapes from the collar (i.e., opening area) of
the drill hole
and/or stop small pieces of relatively high velocity flyrock that originate
from the collar
of the drill hole. The collar pressure wave suppression device can have the
same features
as the downhole silencer device except that the collar devices are commonly
much larger,
have a larger internal volume and are positioned outside the drill hole over
the collar of
the drill hole instead of inside the hole.
An example of a collar shroud device for airblast suppression is shown in
Figure
SA. The collar shroud 5001 is typically rigidly connected to the proximal end
of the
sealing member 5002 such that when the sealing member 5002 is inserted in a
drill hole
5003, the body of the collar shroud 5001 and a flexible skirt 5005 contact the
region
around the collar of the drill hole 5006 to form a rough seal. The down hole
end 5007 of
the collar shroud 5001 is open or perforated to allow the flow of high-
pressure gases to
enter from the hole. The inside of the collar shroud includes a plurality of
baffles 5008,
a plurality of dead-end chambers 5009 and/or the attenuation material 5010
which
together form non-linear pathways of different lengths and a large surface
area for heat
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transfer. The uphole end 5011 of the collar shroud 5001 is also open or
perforated to
allow the high-pressure gases to escape in a less coherent and less energetic
state than
they entered. The heavy flexible outer skirt 5005 further contains the
escaping gases.
This outer flexible skirt 5005 may be made from flexible air conditioning duct
material,
heavy canvas or industrial conveyor belt, and the like. This outer flexible
skirt 5005 may
or may not be included as part of the collar shroud 5001 and may be attached
to the
uphole end 5011 of the collar shroud 5001 and the downhole end 5007 by any
number of
means (such as by a heavy chain as shov~m)or directly to the sealing member.
The collar
shroud 5001 is shown in plan view in Figure 5B which shows the hole for the
sealing
member 5013, perforations 5014 on the uphole end SO l 1 and the flexible outer
skirt 5005.
The collar shroud 5001 shown in plan view 5015 may be either round,
rectangular, or any
other shape.
An alternate embodiment of a collar shroud device is shown in Figure 5C. This
embodiment shows a collar shroud 5021 that is decoupled from the sealing
member 5023
by any suitable device 5022, such as a spring. It also shows internal baffles
5024 that are
shock-isolated from the main body 5025 by rubber or shock isolation elements
5026. The
nonlinear escape path of the working fluid is shown by arrows.
Since the rigid collar shroud must contain substantially undiluted but
expanded
energetic working fluid, it must be of robust construction. The gas pressures
are in the
range of about 1,000 to about 5,000 psi. The internal volume of the collar
shroud is
preferably in the range of about 0.03 m3 to 2 m3, more preferably in the range
of about
0.06 m3 to 1 m3, and most preferably in the range of about 0.1 m3 to 0.5 m3.
Figure 6 depicts a collar shroud for suppressing pressure waves generated
during
the drilling of the hole. The drill has a drill steel that is decoupled from
the collar shroud
102 so that the drill steel may rotate freely from the shroud 102. This shroud
102 is
designed to substantially attenuate the noise emanating from the drill hole
104 as a result
of the percussive hammering of the rock by the drill bit 105 at the downhole
end of the
drill steal. The shroud, like the other collar devices discussed above,
surrounds and
encloses the hole from the exterior environment. The proximal end 106 of the
device is
rigid and made from a suitable high strength material such as steel plate. The
distal end
107 of the device is a flexible shroud such that when the drill slide 103 is
positioned next
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to the rock face 108 for drilling, the flexible end 107 can conform to the
rock face 108 to
seal the pressure wave emissions in the shrouded area 109 and thereby impede
the release
of or attenuate the pressure waves in the exterior environment. The flexible
end 107 may
be made from a deformable material such as a heavy industrial plastic, rubber,
canvas and
the like. The flexible end 107 will also allow drilling fluids (water and air)
and rock dust
to escape through the rough seal formed with the rock face 108. The outside of
the noise
suppression device may be further covered with one or more additional layers
110 of
acoustic material such as heavy industrial plastic, rubber, canvas or other
commercially
available acoustic materials. Noise from the drill motor piston hammering on
the proximal
end of the drill steel can be suppressed by wrapping the drill motor and
proximal end of
the drill steel with a noise absorbing material such as a heavy rubber sheath.
A collar shroud can also be used only for intercepting and controlling small,
higher
velocity flyrock originating in the drill hole or at the collar of the drill
hole used for the
small charge blast. The collar of the drill hole, in particular, is often the
source of smaller,
higher velocity flyrock.
If the collar shroud is used only for flyrock control, its internal
construction may
be simplified by removing the internal baffles, dead-end chambers and internal
attenuation
material. The flyrock will be substantially stopped by the bottom plate and
flexible outer
skirt.
An exemplary flyrock control device is depicted in Figure 4. Figure 4 shows a
cross-sectional view of a collar flyrock suppression shroud 201 positioned
around and
enclosing the collar 202 of the hole 203 for intercepting flyrock that
originates from the
collar region of the drill hole during small-charge blasting. A sealing member
204 is
shown inserted into a drill hole 203 in firing position. Since some or all of
the gases
generated in the drill hole when the gas-generating device 204 is fired can
escape up the
drill hole 203, there is a strong potential for these gases to accelerate at
high velocities
broken or partially broken rock 205 in the collar region 202. The collar
shroud 201 is
solidly attached at its proximal end 206 to the sealing member 204 such that
when the
sealing member 204 is positioned in the drill hole for firing, the shroud 201
substantially
blocks the line-of flight of the flyrock that would originate in the collar
region 202 of the
drill hole 203. The flyrock shroud 201 may be made from a heavy impact
resistant
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material such as steel, impact resistant plastics or composite materials, and
the like. The
shape of the flyrock shroud 201 may be convex as shown or in the shape of a
cylindrical
housing with its open end facing towards the entrance of the drill hole 203.
For shooting
holes drilled vertically into the rock, a shroud of mesh or chain links 207 or
another
deformable impact resistant material may be attached to the distal end 208 of
the flyrock
shroud 201 so as to further intercept flyrock that is accelerated laterally.
Ground Pressure Wave and Flvrock Suppression Devices
Ground pressure wave and flyrock suppression devices intercept and control the
air-blast that escapes from fissures or fractures existing or created around
the working
face centered on the drill hole used for the small charge blast. A series of
perforated mats
are laid on top of each other over an area around the hole collar. The
perforations in the
adjacent, stacked mats are of differing sizes and/or are misaligned to form a
labyrinth of
gas passageways. For pressure wave suppression (but not for flyrock
suppression alone),
the mats are covered by a mat that is substantially impermeable to the flow of
gas to force
the working fluid through a labyrinth formed by the overlapping and
interlinked meshes
and/or perforations. The labyrinth forces the pressurized working fluid to
negotiate a
series of intricate passage ways of differing lengths and de-energizes the
pressurized fluid
by forcing the fluid to contact a large surface area to create conditions for
significant heat
transfer to the mesh. The gas pressures that this apparatus is exposed to are
typically in
the range of about 1,000 to about 10,000 psi.
One of the principal features of this embodiment is that it can be set in
place and
used without moving for several or many shots. One of the advantages of the
mesh
structure is that a hole for inserting the sealing member or drill can be
readily formed by
simply deforming the mesh to create a suitable insertion hole.
An example of such a ground shroud device is shown in Figures 7A and 7B. The
ground mat 6001 may be formed from a plurality of separate mats 6003a-c and
6004a-c
covered by the impermeable mat 6000 such as shown in Figure 7A. The mats may
be
formed from compliant industrial mesh mats 6003 and/or perforated rubber or
canvas
6004 or other similar types offlexible, deformable, perforated mats. The
impermeable mat
can be made from heavy rubber, conveyor belt or plastic sheet or from a
lighter gauge
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sheet metal or from a combination of such materials layered together to form a
strong but
impermeable mat. Referring to Figure 7B, several holes 6006 are formed in the
mat as
needed for drilling of successive holes and placement of successive small
charge blasting
shots in the holes.
If the ground shroud is used only for flyrock control, its layered
construction may
be simplified by using only one or two layers of material. For example, if
used only for
fly rock, the ground shroud can be made from one layer of heavy chain link
mesh, or from
two layers of lighter chain link mesh, or from a layer of lighter chain link
mesh and a layer
of heavy canvas or rubber mat and the upper impermeable layer omitted.
Enclosure for Pressure Wave and Flyrock Suppression
Enclosures for pressure wave and flyrock suppression are aimed at intercepting
and controlling the expanded air-blast in the immediate vicinity of the small
charge blast
around the undercarner or rig stationed at the working face as well as
equipment noise
and flyrock. An enclosure is erected over the boom of the undercarrier or
around the
entire undercarrier or, in the case of a shaft rig, over the entire shaft rig.
The enclosure
may not be strongly coupled to the rock to be broken (as is the case for the
collar or
ground shrouds) but can be suspended over the work area from an independent
structure
that may or may not be the undercarner or rig. Thus the enclosure can be of a
lighter
weight construction than a collar shroud. Its function is to contain the
expanded working
fluid that escapes from the hole collar and/or ground fractures and fissures,
and then
slowly dissipate this expanded gas to the outside world. The enclosure may
include
controlled leakage vents to facilitate controlled dissipation of the gas.
The enclosure contains enough volume of atmospheric air to dilute and mix with
the expanded working fluid. In the case of a flexible canopy, the mass of
ambient air
contained within the enclosure is substantially greater than the mass of
energetic working
fluid so that the diluted energy of the mixture is only slightly greater than
the ambient
energy of the air. In the case of a rigid enclosure, the volume of ambient air
may be much
less so that substantial momentary over-pressures may be developed but these
can be
contained by the enclosure.
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Figure 8A shows a cutaway side view of a rigid type canopy 7001 that is
attached
to the end of a boom 7002 of a small-charge blasting machine. A sealing member
7003
is mounted on the boom 7002. There may be a rock drilling apparatus 7004 also
attached
to the boom 7002. The drilling apparatus 7004 is located outside of the canopy
7001.
S As shown in Figure 8B, the enclosure 7001 is constructed of a heavy rigid
outer shell
7005 (such as metallic plate) which may be lined on the inside with one or
more thick
rubber or canvas membranes 7006a, b which can both mitigate sound transmission
through the shell 7005 and absorb flyrock impacts. The rigid canopy 7001 may
have
several large or many small holes or vents 7008 on its body 7009 or in its top
7010 to
allow controlled venting of the gases during and after a shot. The enclosure
7001 may
also have a flexible skirt 7011 that can form a rough seal 7012 with the
ground to deflect
the airblast and flyrock into the enclosure 7001. The inside of the enclosure
7001 may be
further lined with a impact resistant layer 7013 for flyrock protection and/or
with a
pressure wave absorbent material 7014 for pressure wave mitigation.
Figure 8C depicts another configuration of a rigid-type canopy 300 that is
designed to be attached to the feed holder or boom assembly of a small-charge
blasting
machine. In this embodiment, both the rock drill apparatus and the sealing
member are
housed in within the canopy 300 so that all pressure waves and machine noise
as well as
flyrock from the collar region are captured in the enclosure. The outside of
the canopy
301 may be fabricated from sheet metal, light steel plate, or another heavy
rigid material
attached to a frame structure fabricated from wood, aluminum, steel, or other
types of
structural members. One or more pressure wave absorbing, dissipating, and/or
reflecting
layers 307 are used to line the interior of the enclosure. The layer 307 may
be held in
place and protected by a impact resistant layer 306 of heavy industrial
plastic, rubber,
canvas or metallic mesh 307 and the like that is impact resistant and capable
of resisting
flyrock. There is a hinged door 304 for access to the rock drill and/or small
charge
blasting device. In addition, there may be a relatively heavy layer or layers
of industrial
plastic, rubber, canvas, or other deformable material secured to the bottom of
the canopy
305 to form a shroud around the drill hole.
3 0 Figure 8D shows a cutaway side view of a heavy flexible type canopy 8001
shown
affixed to the supporting cable 8002 such as might be used on a small-charge
blasting
apparatus 8003 used for shaft excavation. The flexible canopy formed from
heavy flexible
materials such as industrial conveyor belt, heavy canvas or a flexible
sandwich such as
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heavy aluminum foil on either side of an acoustic material (e.g. ethyl vinyl
acetate, poly
vinyl chloride or other such plastic or foamed materials) such that it will
absorb and
deaden the airblast and equipment noise. The enclosure should form a heavy
flexible bag
type structure to contain the over-pressure. The enclosure may have a flap or
door that
can be secured shut during firing of the shot. The canopy 8001 is attached to
a rigid
support frame 8004 that may be constructed from wood or structural steel
members. The
canopy 8001 is draped onto the ground to form a rough seal 8005. The volume
8006
inside the canopy is sufficient to dilute the energy of the airblast as
described above. The
canopy 8001 may have several large or many small holes on its body 8007 or in
its top
8008 to allow controlled venting of the gases during and after a shot.
The internal volume of the flexible canopy is preferably in the range of about
50
m3 to 200 m3, more preferably in the range of about 50 m3 to 100 m3, and most
preferably
in the range of about 75 m3 to 100 m3. The internal volume of the rigid
enclosure is
preferably in the range of about 1 m3 to 10 m3, more preferably in the range
of about 2 m3
to 10 m3, and most preferably in the range of about 2 m3 to 5 m3. .
The enclosures described above can be used to contain equipment other than a
small charge blasting device to suppress pressure waves. For example, the
enclosures can
contain a drill, an impact breaker, and the like.
Pressure Wave and Flyrock Suppression Barriers
The pressure wave and flyrock suppression barners intercept, absorb and/or
deflect the noise or substantially weakened air-blast in the intermediate to
far-field region
of the small charge blast. Barriers of sound absorbent material are erected
between the
work area and the areas to be protected from noise (buildings, residences,
playgrounds
etc). The barriers are located such that they absorb and/or deflect the noise
energy up and
over or away from the areas to be protected. In the case of a shaft work area,
the noise
barriers may be assembled as a lining for the shaft.
This same apparatus can also be used for intercepting any flyrock that escapes
the
immediate working area. In addition to being located to best absorb and
deflect noise, the
barriers are located such that they will also intercept any direct line of
flight, high velocity
flyrock; or lower velocity flyrock that is follows an arched trajectory. It
may be necessary
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to protect any layers of noise absorbing material lining the inside of the
barrier with a layer
of mesh or other material that can absorb the impact of flyrock while
protecting the noise
absorbent material.
Figure 9 shows a cutaway side view of the construction for a pressure wave and
flyrock suppression barrier 9001. The barrier frame 9002 may be constructed
from wood
beams or structural steel members. The vertical members may be inserted into
holes 9003
in the ground for support if the barriers are free-standing. The main barner
structure or
skin 9004 may be made from plywood or sheet metal and covered with sound
absorbent
material such as acoustic tile, heavy canvas or industrial conveyor belt.
Figure 10 shows
how such barriers 10001 may be deployed around a work site 10002 to protect
airblast
and flyrock sensitive areas 10003. The angle "8" defined by a line extending
from the
work site 10002 to the structure and a line extending from the work site to
the end of the
barrier 10001 is typically at least about 30 and no more than about 90 degrees
with about
45 degrees being preferred.
Stand alone barriers may be typically 2 to 4 meters high and anchored
typically 0.5
to 1 meters into the ground.
Atomization Device for Suppressing, Airblast
The airblast that escapes from the drill hole or fragmented rock around the
shot
point, moves at a velocity equal to or greater than the speed of sound. The
pulse width
of the airblast immediately around the shot point is on the order of a few
milliseconds. It
is possible to extract energy from the airblast or escaping working fluid by
causing it to
pass through a cloud of atomized fluid particles or spray. The spray must be
atomized to
increase the surface area of spray particles. For a given mass of fluid, the
total surface
area is proportional to the cube root of the number of droplets. The mechanism
of
extracting energy is by convective and conductive heat transfer of hot gases
to fluid
particles. If the amount of spray is large enough and the atomization is fine
enough, then
the resultant fog cloud can extract significant energy from the airblast and
expanded
working fluid, and therefore reduce the amplitude and pulse width of the
airblast near the
source. The atomized fluid spray has the advantage that it is relatively easy
to generate
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and apply compared to some of the mechanical airblast suppression apparatuses.
Further,
the spray has the beneficial side effect of helping to suppress dust.
To achieve effective heat transfer of a significant portion of the airblast
energy, it
is necessary to envelope the work face with a large volume of fluid such as
water in a
highly atomized state. The droplet size ranges preferably from about 1 mm to
0.01 mm,
more preferably from about 0.5 mm to 0.01 mm and most preferably from about
0.1 mm
to 0.01 mm. The volume of fluid suspended around the working face at the time
of firing
is dependent on the charge weight of the explosive or propellant used. The
volume of
fluid ranges preferably from about 10 liters per kg of charge to 2000 liters
per kg, more
preferably from about 50 liters per kg to 2000 liters per kg and most
preferably from
about 100 liters per kg to 2000 liters per kg.
Figure 11 shows a typical small charge excavation machine 11001 in firing
position
at the work face 11002. A small charge blasting device 11003 is positioned in
a drill hole
11004. A spray system 11005 for atomizing a fluid such as water may be mounted
on the
machine 11001 and used to create a spray pattern 11006 that envelopes the
working area
11007 between the machine 11001 and the drill hole 11004.
An embodiment of a novel gas generator device that may be used as part of the
present invention to introduce a pressurized working fluid rapidly into a
portion ofthe drill
hole and to seal the hole is shown in Fig. 12. It includes a cartridge 14004
containing a
propellant charge 14008 which is hand-inserted into a cartridge housing 14012.
The
cartridge 14004 may be contained completely inside the cartridge housing 14012
or the
distal end of the cartridge 14004 may protrude a small distance beyond the
muzzle end
14016 of the cartridge housing 14012 (typically about one third or less of the
overall
cartridge length protrudes beyond the muzzle end 14016 of the cartridge
housing). The
cartridge 14004 may be made with a metallic base 14020 attached to a plastic
cartridge
body 14024. Alternately, the cartridge 14004 may be formed from only one
material such
as a plastic, compressed paper, or any other suitable material including
combustible
material used for consumable ammunition.
When the cartridge 14004 has been inserted, the cartridge housing 14012 is
then
attached to the end of a long stemming bar 14028 by means of a full thread, an
interrupted
thread, a bayonet type lug, or another suitable attachment mechanism. The
stemming bar
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14028, which is usually attached to an undercarrier by means of an extension
cylinder, is
inserted into a drill hole 14032 such that the cartridge housing 14012 comes
to rest at or
near the bottom of the hole. It can be appreciated that the stemming bar can
be mounted
to any suitable undercarnage, that may or may not include a drill performing
the drilling
S function.
When the device is fully inserted, the propellant 14008 in the cartridge 14004
is
initiated and the propellant 14008 is burned to completion generating a
controlled high
pressure in the bottom portion of the hole. The propellant 14008 may be
initiated by a
mechanical firing pin 14036, which is itself actuated by a firing pin assembly
14040,
10 striking a percussion primer 14044 inserted in the cartridge base 14020.
Alternately, an
electric primer may be used and initiated by a current pulse transmitted
through an
electrical contact with a wire pair running down the stemming bar. The
initiator can
utilize any other initiation method, including inductive coupling.
Currently, the drill hole 14032 is formed by a reamer/pilot bit combination
such
15 that the distal portion 14048 ofthe drill hole 14032 is a smaller diameter
than the proximal
portion 14052 of the drill hole 14032. The outside of the cartridge housing
14012 has a
slight taper 14056 (smaller diameter towards the distal end) so that the
insertion will be
stopped when the outside of the cartridge housing 14012 comes to rest on the
step or
ridge 14060 formed between the distal portion 14048 and the proximal portion
14052 of
20 the drill hole 14032. The taper 14056 is preferably in the range of 0.5 to
3 degrees and
most preferably in the range of 0.5 to 1.5 degrees.
As illustrated in Fig. 13, the ridge 14060 of the stepped drill hole 14032 and
the
taper 14056 of the cartridge housing 14012 form a seal 15004 restricting the
flow of
pressurized gas in the hole bottom 15008 during the rock-breaking process. The
partial
25 cut-away at the distal end ofthe cartridge housing 14012 illustrates that
the cartridge body
14024 and the propellant 14008 are positioned within the cartridge housing
14012.
Alternate sealing techniques are also possible. For example, as illustrated in
Fig.
14, the cartridge housing 14012 may have a straight, constant diameter portion
16004 at
its tip that is a reasonably tight fit in the distal portion 14048 of the
drill hole 14032. This
sealing method provides a gap 16008 that remains roughly constant, even as the
device
recoils away from the hole bottom 15008 after firing.
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The diameter of the distal portion 14048 of the drill hole 14032 is preferably
in the
range of 30 to 150 mm and most preferably in the range of 50 to 120 mm. The
amount
of propellant 14008 is preferably in the range of 100 to 750 grams and most
preferably in
the range of 200 to 450 grams. The length (L) of the pilot hole (distal
portion 14048 of
the drill hole 14032), expressed in terms of bottom hole diameters (D), is
preferably in the
L/D range of 0.5 to 6 and most preferably in the L/D range of 1 to 3. The
total volume
available to the high pressure propellant gas products is such that the
average density of
the gas is preferably in the range of 100 to 750 kg/m3 and most preferably in
the range of
200 to 500 kg/m3.
The foregoing description ofthe present invention has been presented for
purposes
of illustration and description. Furthermore, the description is not intended
to limit the
invention to the form disclosed herein. Consequently, variations and
modifications
commensurate with the above teachings, and the skill or knowledge of the
relevant art,
are within the scope of the present invention. The embodiments described
hereinabove
are further intended to explain best modes known for practicing the invention
and to
enable others skilled in the art to utilize the invention in such, or other,
embodiments and
with various modifications required by the particular applications or uses of
the present
invention. It is intended that the appended claims be construed to include
alternative
embodiments to the extent permitted by the prior art.
