Note: Descriptions are shown in the official language in which they were submitted.
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ROCK BOLT
Field of the Invention
[0001] The present invention relates to the field of strata control in civil
and mining operations
and in particular relates to a rock bolt for securing the roof or wall of a
mine, tunnel or other
ground excavation.
Background of the Invention
[0002] One known method of stabilizing the roof or wall of mines, tunnels or
other ground
excavations is to secure a rock bolt into a bore hole drilled into the face of
the rock to be
stabilized. Rock bolts are typically formed of a rigid steel bar with a thread
formed in either the
trailing portion of the bar or along the full length of the bar.
[0003] A known method of installing a rock bolt involves first drilling a bore
hole into the rock
face. A sausage-like two-component resin filled cartridge is then inserted
into the bore hole,
followed by the rock bolt which pushes the resin filled cartridge towards the
top of the bore
hole. The rock bolt is then rotated by the installation rig which also thrusts
the rock bolt
upwardly whilst it is rotating to mix the resin components and shred the
frangible cartridge
casing, pushing it towards the blind end of the bore hole. Various means have
previously been
proposed to be formed with, or mounted on, the leading end portion of the
rigid bar to assist in
mixing of the resin components and/or assisting in anchoring the rock bolt
within the resin.
Rotation of the rock bolt is then stopped for a few seconds to allow the resin
to cure.
[0004] Incomplete resin mixing and the presence of air voids and pockets of
unmixed or "wet"
resin can reduce the load transfer performance of an installed rock bolt. This
concern is further
exacerbated in dynamic ground conditions.
[0005] In one form of installation, the resin only encapsulates the leading
end portion of the
rigid bar, thereby forming a point anchor. In such an installation the rock
bolt may be tensioned
by way of a drive nut mounted on a threaded trailing end portion of the bar
and which bears
against a plate washer that engages the rock face adjacent the bore hole
opening. For additional
load transfer and corrosion protection, it is known to post-grout the annular
cavity about the
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rigid bar extending between the resin encapsulated leading end portion and the
bore hole
opening.
[0006] According to another previously proposed installation configuration,
the entire length of
the rigid bar within the bore hole may be resin encapsulated.
Summary of Invention
[0007] In a first aspect, the present invention provides a rock bolt
comprising:
a bar longitudinally extending between a bar leading end and a bar trailing
end, said bar
comprising:
a bar leading portion longitudinally extending from said bar leading end;
a first anchor portion trailing said bar leading portion, said first anchor
portion
comprising at least one deformation integrally formed in said bar; and
a first debonding portion trailing said first anchor portion, said first
debonding
portion being configured to be debonded from resin encapsulating said rock
bolt, in use,
at least upon application of a predetermined service load to said bar;
said rock bolt further comprising a wire secured to and helically extending
along said bar
leading portion.
[0008] In one or more embodiments, said bar further comprises:
a second anchor portion trailing said first debonding portion, said second
anchor portion
comprising at least one deformation integrally formed in said bar; and
a second debonding portion trailing said second anchor portion, said second
debonding
portion being configured to be debonded from resin encapsulating said rock
bolt, in use, at least
upon application of the predetermined service load to said bar.
[0009] In at least one embodiment, said bar further comprises:
a third anchor portion trailing said second debonding portion, said third
anchor portion
comprising at least one deformation integrally formed in said bar.
[0010] Typically, said bar further comprises a threaded bar trailing portion
longitudinally
extending from said bar trailing end.
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[0011] In a preferred embodiment, each said debonding portion has a smooth
outer surface
configured to debond from resin encapsulating said rock bolt, in use, upon
application of the
predetermined service load to said bar.
[0012] In one or more preferred embodiments, each said anchor portion
comprises a flattened
paddle-like deformation formed in said bar.
[0013] In a preferred form, each said anchor portion comprises a pair of said
paddle-like
deformations, said paddle-like deformations of each said pair being arranged
mutually adjacent
and oriented at least substantially mutually perpendicular.
[0014] In at least one embodiment, said rock bolt further comprises a cap
mounted on said
leading bar portion and extending longitudinally beyond said bar leading end,
said cap having a
trailing sleeve portion extending partially over said leading bar portion and
a leading nose
portion tapering from said sleeve portion to a leading end of said cap.
[0015] Typically, said wire is located adjacent said cap.
[0016] In a preferred embodiment, the length of the bar leading portion is
less than 500mm. For
example, the length of the bar leading portion is between about 300mm to about
200mm, or
between about 300mm to about 150mm.
[0017] Typically, the first anchor portion is substantially adjacent to a
trailing end of the wire.
[0018] In a second aspect the present invention provides a method of
installing the rock bolt
define above, said method comprising:
drilling a bore hole with a blind end into a rock face to be stabilized;
inserting a two-component resin filled cartridge having a frangible casing
into said bore
hole;
inserting said rock bolt into said bore hole with said bar leading end
leading;
thrusting said rock bolt towards said blind end whilst rotating said rock
bolt;
puncturing said frangible casing;
mixing said resin with said wire and allowing mixed resin to flow along said
bar toward
said bar trailing end; and
stopping thrusting and rotation of said rock bolt, allowing said resin to
cure.
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[0019] Typically, said method further comprises pre-tensioning said bar.
Brief Description of Drawings
[0020] Preferred embodiments of the present invention will now be described,
by way of
example only, with reference to the accompanying drawings wherein:
[0021] Figure 1 is a fragmentary, isometric view of a rock bolt according to a
first embodiment;
[0022] Figure 2 is a fragmentary, front elevation view of the rock bolt of
Figure 1 with end
fittings;
[0023] Figure 3 is a partially cross-sectioned, front elevation view of a
partially completed rock
bolt installation utilizing the rock bolt of Figure 1;
[0024] Figure 4 is a partially cross-sectioned, front elevation view of a
completed rock bolt
installation utilizing the rock bolt of Figure 1;
[0025] Figure 5 is a fragmentary, isometric view of a rock bolt according to a
second
embodiment;
[0026] Figure 6 is a fragmentary, front elevation view of the rock bolt of
Figure 5 with end
fittings.
Description of Embodiments
[0027] Referring to Figures 1 and 2 of the accompanying drawings, a rock bolt
100 according to
a first embodiment comprises a longitudinally extending bar 110 and a cap 120
and a wire 130
each mounted on the bar 110. The bar 110 is here of conventional form and is
typically formed
of structural steel. The bar 110 longitudinally extends between a bar leading
end 111 and a bar
trailing end 112. The bar 100 has a bar leading portion 113 terminating in the
bar leading end
111 and a bar trailing portion 114 terminating in the bar trailing end 112.
[0028] In typical embodiments, the bar trailing portion 114 is threaded for
receipt of a drive nut
140 (shown in Figure 2) provided with a corresponding thread for tensioning of
the rock bolt
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100, as will be discussed below. The bar 110 may have any of various diameters
selected to suit
the particular bore hole in which it is to be installed, and may typically
have a diameter of
between 12 mm and 30 mm, with common diameters being 16, 20, 22 and 24 mm. The
bar 110
will typically have a length of 1.5 to 3 metres, but again the bar 110 may
have any length to suit
the particular bore hole.
[0029] The bar 110 has first and second anchor portions 150, 160 formed along
its length. The
first anchor portion 150 trails the bar leading portion 113 and, accordingly,
trails the wire 130,
although some overlap between the trailing end of the wire and the first
anchor portion is
permissible. The first anchor portion 150 is typically arranged adjacent the
trailing end of the
wire 130. The second anchor portion 160 trails the first anchor portion 150
and is longitudinally
spaced therefrom. Each of the anchor portions 150, 160 comprises at least one
deformation
integrally formed in the bar 110. In the arrangement depicted, each of the
anchor portions 150,
160 comprises a pair of flattened paddle-like deformations 151, 152. Each of
the paddle-like
deformations 151, 152 is formed by locally forging the bar 110 to form a
flattened 'paddle'.
Preferably, the paddle-like deformations are cold forged to facilitate work
hardening. Each
flattened paddle-like deformation 151, 152 has a reduced thickness as compared
to the
undeformed diameter of the bar 110 and an increased width as compared to the
undeformed
diameter of the bar 110. The first and second paddle-like deformations 151,
152 constituting
each pair of paddle-like deformation are arranged mutually adjacent and
oriented at least
substantially mutually perpendicular. That is, the second paddle-like
deformation 152 is
flattened in a direction substantially perpendicular to the direction of
flattening of the first
paddle-like deformation. The broadened edges of the first paddle-like
deformation 151 thus
project laterally in one plane and the broadened edges of the second paddle-
like deformation
152 project laterally in a substantially perpendicular plane. For a 22mm
diameter bar, each of
the paddle-like deformations typically has a length of the order of 60 mm, a
width of the order
of 30 mm and a thickness (at the location of minimum thickness) of the order
of 12 mm.
[0030] The bar has first and second debonding portions 170, 180 formed along
its length. The
first debonding portion 170 trails the first anchor portion 150 and extends
between the first
anchor portion 150 and the second anchor portion 160. The second debonding
portion 180 trails
the second anchor portion 160 and extends between the second anchor portion
160 and the
threaded bar trailing portion 114. Each of the debonding portions 170, 180 is
configured to be
debonded from resin encapsulating the rock bolt 100, in use, as will be
further described below.
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Each of the debonding portions 170, 180 may have a length of the order of 1000
mm, and will
typically have lengths of between 700 mm and 1300 mm, depending upon the
length of the rock
bolt 100, although other arrangements are possible. Each of the debonding
portions 170, 180
has a smooth outer surface that results in debonding of the debonding portion
170, 180 from the
resin encapsulating the rock bolt 100 in use, upon application of a
predetermined service load to
the bar 110 which exceeds the shear bond strength between the resin and the
smooth outer
surface of the debonding portion 170, 180. Accordingly, whilst when initially
installed the
debonding portions 170, 180 may be relatively lightly bonded to the
encapsulating resin, the
resin readily debonds from the debonding portions 170, 180 upon the
application of a
predetermined service load. Such a load may result from movement or fracture
of the rock that
is resisted by yielding deformation of the debonding portions 170, 180. It is
also envisaged that
a surface coating, such as a low-friction paint surface coating, may be
applied to the debonding
portions 170, 180 to further promote debonding. Suitable surface coatings
include low-friction,
lubricating paints such as polyolefin based paints. It is also envisaged that
the debonding
portions 170, 180 may be provided by locating debonding sleeve on portions of
the bar 110, the
debonding sleeves each debonding the underlying portion of the bar from the
encapsulating
resin.
[0031] The wire 130, typically formed of steel, is secured to and helically
extends along the bar
leading portion 113, from adjacent the cap 120 towards the bar trailing end
112. The wire 130
here extends from a wire leading end 131, offset from the bar leading end 111
adjacent to and
trailing the trailing end of the cap 120, to adjacent the first anchor portion
150. It is envisaged,
however, that the wire leading end 131 may be located under the cap 120,
between the trailing
end of the cap 120 and the bar leading end 111. The wire 130 preferably
helically extends in a
direction opposing the intended direction of rotation of the bolt 100 during
installation.
Accordingly, for installation of the rock bolt 100 with a standard left-handed
installation rig, the
wire 130 will typically helically extend in a right-handed direction as
depicted. The wire 130
will typically extend over a length of the bar of the order of about 200 mm to
about 300 mm.
[0032] The wire 130 may be secured to the bar 110 by tack welding, typically
at the wire
leading end 131 and wire trailing end 132. In some embodiments a further weld
may be placed
part way along the length of the wire 130. It is also envisaged, however, that
the wire 130 may
be secured to the bar 130 by use of one or more ferrules crimped onto the bar
110, with the wire
130 welded to the ferrules. This arrangement will avoid potential weakening of
bar 110 formed
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of high tensile steel by annealing the steel as a result of heat generated
through the welding
process. If the wire 130 is secured to the bar 110 by welding, it is
preferable that the welds are
applied only to the bar leading portion 113. This is because the leading
portion 113 of the bar
does not typically have much geotechnical load carrying benefit such that the
effective length
essentially starts from the first anchoring portion 150 continuing toward the
trailing end 112.
Generally, the welds are not applied to the first anchoring portion 150, or
any other part of the
bolt 100 intended to carry a substantial geotechnical load, so that the load
bearing capability of
the first anchoring portion 150, which may be formed of high tensile steel is
not impacted by the
welds. However, for ease of manufacture, it is permissible to place a tack
weld on the trailing
end of the wire 130 at an uppermost (leading) broadened edge of the first
paddle-like
deformation 151 of the first anchor portion 150.
[0033] The cap 120 is mounted on the bar leading portion 113 and extends
beyond the bar
leading end 111. The cap 120 has a trailing sleeve portion 121 that extends
partially over the
bar leading portion 113 and a leading nose portion 122 that tapers from the
sleeve portion 121 to
a leading end of the cap 120. The sleeve portion 121 will typically extend
over a length of the
bar leading portion 113 slightly less than the offset distance of the wire
leading end 131 from the
bar leading end 111, with the wire leading end 131 located adjacent to the
sleeve portion 121,
although it is envisaged that the wire leading end 131 may be located within
the sleeve portion
121. In one example, the sleeve portion 121 extends over a length of about 50
mm of the bar
leading portion 113. The sleeve portion 121 is typically of cylindrical form
whilst the nose
portion 122 is typically of a generally conical form defining a rounded cap
leading end.
Alternatively, the nose portion 122 could be generally hemispherical or of an
alternate tapered
form.
[0034] The sleeve portion 121 may have a diameter equal to or greater than a
maximum
transverse dimension defined by the wire 130 helically extending along the bar
110. This
maximum transverse dimension is generally defined by the diameter of the bar
110 plus twice
the diameter of the wire 130. The thickness of the wall of the sleeve portion
121 will thus
typically be equal to or greater than the diameter of the wire 130. The wire
diameter may vary,
and will typically be of the order of 3 to 6 mm. With typical wire diameters
being
3.15 mm, 4.5 mm and 6.0 mm. Whilst the sleeve portion 121 may have a diameter
equal to or
exceeding the maximum transverse dimension defined by the wire 130, it is also
envisaged that
the diameter of the sleeve portion 121 may be less than the maximum transverse
dimension
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defined by the wire 130, such that the thickness of the wall of the sleeve
portion 121 is less than
the diameter of the wire 130.
[0035] The sleeve portion 121 may have an interference fit with the bar
leading portion 113
such that it may be mounted on the bar leading portion 113 by simply pushing
the cap 120 onto
the bar leading portion 113. Alternatively, the sleeve portion 121 may have a
clearance fit on
the bar 110 and be secured thereto by adhesive or other means. An adhesive may
also be
utilised in conjunction with other types of fit.
[0036] The cap 120 will typically be formed of plastics material, with
polypropylene and nylon
being particularly suitable materials. The cap 120 may also be formed with a
high visibility
colour, such as bright yellow or orange, for enhanced visibility of the
leading end of the rock
bolt 100 in underground applications. Embodiments are also envisaged in which
the cap 120 is
omitted.
[0037] As shown in Figure 2, a drive nut 140 is threaded onto the threaded bar
trailing portion
114. The drive nut 140 is typically provided with a mechanism which fixes the
drive nut 140
onto the threaded bar trailing portion 114 up to a predetermined torque at
which the mechanism
fails, enabling the drive nut 140 to be threadingly advanced along the
threaded bar trailing
portion 114. Such mechanism may take the form of a disc shaped insert (not
depicted) located
in the trailing end of the aperture extending through the drive nut 140. The
insert engages the
bar trailing end 112 as the drive nut 140 attempts to advance, locking the
drive nut 140 onto the
bar 110 until a predetermined torque is exceeded, at which the insert is
ejected from the drive
nut 140, allowing the drive nut 140 to advance along the threaded bar trailing
portion 114. Such
a drive nut 140 housing an insert is described in Australian Patent
Publication No. AU
2007201848A1, the entire contents of which are incorporated herein by cross-
reference. Other
known mechanisms for selectively fixing the drive nut 140 to the bar 110 may,
however, be
utilised as desired, for example a shear pin. An anti-friction washer 141 and
dome washer 142
are also mounted on the bar 110 in the usual manner.
[0038] Installation of the rock bolt 100 of the first embodiment will now be
described with
reference to Figures 3 and 4 of the accompanying drawings. The anti-friction
washer 141, dome
washer 142 and drive nut 140 are mounted on the threaded bar trailing portion
114, along with a
standard plate washer 143. A bore hole 201 is drilled into the rock face 200
to be stabilised,
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optionally with one or more sheets of reinforcing mesh held against the rock
face 201. A two-
component resin filled cartridge 210 is then inserted into the bore hole 201.
The rock bolt 100,
with the assembled anti-friction washer 141, dome washer 142, drive nut 140
and plate washer
143 is mounted on an installation rig and maneuvered toward the bore hole 201.
As the rock
bolt 100 is guided toward the bore hole 201, the nose portion 121 of the cap
120 assists in
ensuring the blunt bar leading end 111 and protruding wire leading end 131 do
not catch on the
reinforcing mesh or edge of the bore hole 201.
[0039] Once the cap 120 is guided into the bore hole 200, the rock bolt 100 is
thrust deeper into
the bore hole 201 using the installation rig, pushing the resin filled
cartridge 210 toward the
blind end of the bore hole 201. As the rock bolt 100 is thrust toward the
blind end of the bore
hole, the installation rig also rotates the drive head 140 of the rock bolt
100, here in an anti-
clockwise direction. The insert (or other mechanism for selectively fixing the
drive nut 140 to
the bar 110) retains the drive nut 140 in a fixed relationship to the bar 110,
such that rotation of
the drive head 140 results in rotation of the bar 110 in unison with the drive
nut 140. When the
cartridge 210 advances to the blind end of the bore hole 201, the cap 120
ruptures the frangible
casing of the cartridge 210, resulting in the resin flowing over the bar
leading portion 113 in the
narrow annulus defined between the bar 110 and the wall of the bore hole 201,
where the two
components of the resin are thoroughly mixed by the wire 130. The wire 130
also assists in
shredding the casing of the cartridge 210. With the wire 130 helically
extending in an opposing
direction to the direction of rotation of the rock bolt 100, the resin is
actively encouraged via an
Archimedean pumping action towards the blind end of the bore hole 201,
although sufficient
resin will typically be utilized to substantially fill the annulus, thereby
fully resin encapsulating
the rock bolt 110. As the resin flows past the anchor portions 150, 160, the
paddle-like
deformations 151, 152 assists in further mixing of the resin, although they
are typically less
effective than the wire 130 in mixing the resin. The resin is then allowed to
cure for a few
seconds.
[0040] After the resin has cured, the drive nut 140 is driven by the
installation rig at an
increased torque that results in failure of the insert fixing the drive nut
140 onto the bar 110.
Further torque applied to the drive head 140 thus threads the drive nut 140
along the threaded
bar trailing portion 114. Continued driving of the drive nut 140 bears the
drive nut 140 against
the anti-friction washer 141 which in turn bears the dome washer 142 and plate
washer 143
against the rock face 200, thereby resulting in tensioning of the bar 110. In
a particularly
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preferred installation method, the two-component resin filled cartridge 210
contains two
separate resins, with a relatively fast setting resin at the top of the
cartridge 210 used to point
anchor the bar leading portion 113, and a slower setting resin at the bottom
of the cartridge used
to encapsulate the anchor portions 150, 160 and debonding portions 170, 180.
In this
configuration, the rock bolt 100 will be pretensioned once the first, faster
setting resin has cured,
anchoring the bar leading portion 113, the first anchoring portion 150 and at
least a portion of
the first debonding portion 170, but prior to the second, slower setting resin
curing. This
enables elongation of the anchor portions 150, 160 and debonding portions 170,
180 within the
second, slower setting resin during the pretensioning process. The separate
fast and slow setting
resins could alternatively be housed in separate cartridges.
[0041] In the completed installation, depicted in Figure 4, the first and
second anchor portions
150, 160 are firmly anchored within the resin, whilst the first and second
debonding portions
170, 180 are debonded from, or only lightly bonded to, the resin by virtue of
the smooth outer
surface of the debonding portions 170, 180. As a result, in the event of any
movement or
fracture of unstable rock the first and second debonding portions 170, 180 are
able to yield over
their full length, effectively "slipping" within the encapsulating resin,
allowing the bar 110 to
absorb movement of the rock over the debonding portion 170, 180. This can be
contrasted
against an installation where the bar remains bonded to the resin along its
entire length such that
the bar must endeavor to absorb any movement locally at the failure plane of
the rock over a
very short distance, often resulting in catastrophic failure of the bar. In
the event of a significant
shock load or slow yielding ground movement through the strata, the first and
second anchor
portions 150, 160 are able to be drawn through the encapsulating resin,
absorbing further energy
without catastrophic failure of the bar 110.
[0042] A rock bolt 300 according to a second embodiment is depicted in Figures
5 and 6 of the
accompanying drawings. The rock bolt 300 is of an identical configuration to
the rock bolt 100
of the first embodiment, apart from the provision of an additional anchoring
portion 350 on the
bar 310, trailing the second debonding portion 180 and extending between the
second debonding
portion 180 and the threaded bar trailing portion 114. The remaining features
of the rock bolt
300 that are identical to the rock bolt 100 will thus not be further described
and are provided
with identical reference numerals in Figures 5 and 6. The rock bolt 300 is
installed in the same
manner as the rock bolt 100 of the first embodiment as described above, and
behaves under load
in substantially the same manner as the rock bolt 100 of the first embodiment,
except that the
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additional anchor portion 350 provides additional anchoring of the rock bolt
300 within the
encapsulating resin. It is also envisaged that further debonding portions
and/or further anchor
portions may be formed in conjunction with a longer length of the bar 110.
Embodiments are
also envisaged incorporating a single anchor portion only trailed by a single
debonding portion.
[0043] As discussed above, the paddle-like deformations 151, 152 provide a
secondary mixing
action to that of the wire 130. In isolation, the secondary mixing action of
the paddle-like
deformations 151,152 typically provides for a weak or less effective mixing
action than the wire
130. However, it has been surprisingly found that the combination of the wire
130 and the
paddle-like deformations 151,152 provides for good mixing results, so much so
that the helical
extension of the wire 130 can be reduced while still providing for sufficient
mixing of the resin.
Being able to reduce the helical extension of the wire 130 along the bar
leading portion 113
allows the length of the bar leading portion 133 to in turn be reduced.
[0044] For example, for a typical 35mm borehole, it would generally be thought
that a bar
leading portion 113 carrying the wire 130 extending substantially along this
length would need
to be about 500mm long to provide for sufficient mixing of the resin. However,
with the wire
130 combining with the paddle like deformations 151,152, it has surprisingly
been found that
the bar leading portion 113 carrying the wire 130 can be substantially less
than 500mm, with
good mixing results observed for a bar leading portion 113 with 300mm length
and 200mm
length. It is envisaged that that a bar leading portion 113 of 150mm carrying
the wire 130 that
substantially extends this length may also provide for sufficient mixing
results. It is further
envisaged that similar results can be achieved for a larger borehole by
increasing the gauge of
the wire 130 beyond the typical 4.5mm size and/or increasing the diameter of
the bar 110.
[0045] The above result appears contrary to the conventional approach that a
relatively long
leading portion 113 with the wire 130 extending therealong is required to
achieve sufficient
resin mixing, and that the paddle-like deformations 151,152 in isolation only
provide for a weak
or limited mixing effect. For example, a leading portion 113 length of at
least about 500mm has
typically been thought necessary to achieve thorough mixing for rock bolts 100
without any
paddle-like deformations 151,152. It is thought that the combination of the
paddle-like
deformations 151,152 and the wire 130 may enhance the mixing contribution of
the paddle-like
deformations 151,152 so as to provide a synergistic mixing effect beyond what
may be expected
from the individual mixing effects of the wire and the paddles.
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[0046] Without wishing to be bound by theory, it is thought that the combined
interaction of the
wire 130 and the paddle like deformations 151,152 may actually improve the
mixing effect of
the paddles by limiting the amount of shredded cartridge casing in the
vicinity of the paddle-like
deformations 151,152. As the bolt 100 is inserted into the borehole, the wire
130 ruptures and
gathers the cartridge casing toward the blind end of the borehole. As the
catalyst and resin
mastic within the cartridge is pumped toward the blind end and then down the
borehole annulus
toward the trailing end 112 of the bolt 100, the catalyst and resin mastic are
mixed as a
predominantly fluid resin that passes the paddle-like deformations 151, 152 of
the first anchor
portion 150 with limited cartridge casing. Limiting the cartridge casing in
the vicinity of the
paddle like deformations 151, 152 may increase their mixing effectiveness by
avoiding the
casing wrapping around the paddles which may otherwise reduce mixing
effectiveness.
Furthermore, increasing the mixing effectiveness of the paddle-like
deformations 151,152
through combination with the wire 130 is thought to be additionally beneficial
in that once the
rock bolt is installed within the borehole and continues rotating for a short
period, the enhanced
operation of the paddle-like deformation 151,152 is thought to provide an
enhanced radial
mixing action in the vicinity of the anchoring portions. In this manner,
mixing of resin in the
vicinity of the anchoring portions may be improved, increasing the
effectiveness of the rockbolt.
[0047] Furthermore, it was surprisingly found that a wire 130 helically
extending about 200mm
along a shortened bar leading portion 113 may actually provide for an enhanced
mixing effect
compared to a longer bar leading potion 113 with a wire 130 helically
extending about 300mm
along the length thereof. This result was further unexpected since the reduced
length leading
portion 113/wire 130 facilitated an increased insertion speed of the rock bolt
100 into the
borehole, which would typically be expected to reduce mixing effectiveness.
The increased
insertion speed is thought to come about due to a lower back pressure within
the borehole being
generated by the bolt 100 with the 200mm length leading portion/wire as
opposed to a 300mm
length leading portion/wire. The suprising and counter intuitive result the
shorter leading
portion/wire producing in a better mixing of the resin with a shorter
installation time provides
substantial benefits in operational efficiency.
[0048] The bar leading portion 113 carrying the wire 130 does not contribute
substantially to the
geotechnical load carrying benefit of the rock bolt. Instead, the effective
geotechnical length of
the rock bolt starts at the first anchor portion 150. Regardless of the exact
mechanism resulting
in the combined mixing effectiveness of the wire 130 and the paddle-like
deformations 151,152,
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reducing the extension of the wire 130 due to the combined effect with the
paddle-like
deformations 151,152 allows the bar leading portion 113 length to be reduced
with sufficient
resin mixing being achieved, which can result in shorter rock bolts 100 and
boreholes to produce
the same or similar geotechnical load carrying benefit.
[0049] In an alternative embodiment to that described above, each or any of
the anchor portions
may have more than a pair of flattened paddle-like deformations. For example,
embodiments
with four or five paddle-like deformations comprising a first anchoring
portion may serve to
increase resin mixing and subsequent anchoring proximal to the leading end of
the bolt.
Furthermore, although it is preferred for a pair of paddle-like deformations
to be orientated
mutually perpendicular, they are not limited to this arrangement. When an
anchoring portion
comprises more than a pair of paddle-like deformations, they may be orientated
at various
angles to one another.
[0050] A person skilled in the art will appreciate various other modifications
and alterations of
the rock bolt may be made to suit specific applications.
