Note: Descriptions are shown in the official language in which they were submitted.
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Friction Rock Bolt
15
Field of invention
The present invention relates to expansion or friction rock bolts suitable for
use in the
underground mining and tunnelling industry for use to stabilise rock strata
against fracture
or collapse.
Background art
Expansion rock bolts are installed by drilling a bore into a rock strata,
inserting the rock
bolt into the bore and expanding a part of the bolt to provide a friction lock
against the bore
surface. Expansion rock bolts include an elongate tube which is expandable
radially. This
radial expansion is normally facilitated by the tube being split
longitudinally and by an
expander mechanism being positioned within the tube, normally towards the
leading end of
the tube (being the end of the tube that is inserted first into the drilled
bore in the rock
strata or wall). The expander mechanism is connected to a flexible cable or
solid bar that
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extends to the trailing end of the bolt at which point it is anchored such
that expansion of
the expansion mechanism is effected by pulling or rotating the cable or bar.
The bore that is drilled into the rock strata is intended to be of a smaller
diameter than the
outside diameter of the tube, so that the tube is inserted as a friction fit
within the bore
prior to any expansion of the tube. This maximises frictional engagement of
the rock bolt
via the outside surface of the tube, with the facing surface of the bore. This
method of
insertion is relatively simple, in contrast with other forms of rock bolts
that employ resin or
grout to anchor the rock bolt within the bore.
Resin anchored bolts typically comprise a resin cartridge that is required to
be inserted into
the bore prior to insertion of the bolt. Insertion of the resin cartridge is
sometimes very
difficult, because typically the tunnel walls extend to a significant height,
so that access to
bores into which the cartridge is to be inserted can be inconvenient.
Additionally, the resin
which is employed is relatively expensive and has a limited shelf life.
Cement grouted rock bolts are less expensive than resin anchored bolts, but
application of
the cement is more cumbersome than that of the resin. Cement grouting requires
cement
mixing equipment, as well as pumping and delivery equipment, to deliver the
mixed
cement into the bore.
However, resin or cement anchored rock bolts generally anchor in a bore to
provide greater
levels of rock reinforcement or stabilisation compared to friction rock bolts,
due to a better
bond between the bore wall and the resin or cement, compared to the frictional
engagement
of a friction rock bolt. Also, cement anchored rock bolts typically enable a
bond along the
full length of the rock bolt and the bore wall.
Any form of rock bolt is susceptible to fail if the bolt is exposed to
excessive loading by
the rock strata into which the bolt has been installed. Failure can be tensile
or shear failure
or it can be a combination of tensile and shear failure. In expansion rock
bolts, the bolt can
fail through fracture of the tube. Failure of that kind can often be tolerated
provided the
bar or cable of the bolt does not fail also.
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A particular type of strata which is difficult to bolt is strata that is
either weak or seismic.
Upon fracture of this type of strata, the rock bolt can be subject to dynamic
loading that
tends to cause the bolt to shift outwardly of the bore and to allow the face
of the rock mass
about the rock bolt to also displace outwardly. Contact with the face of the
rock mass
about the rock bolt rock bolt is by a rock plate and in certain territories,
industry set ground
support requirements for seismic conditions such that with ground kinetic
energy of 25 kJ,
in a diameter of about lm about the bore, there should not be a shift in the
positon of the
rock bolt of more than 300mm. In other words, there should not be an outward
displacement of the rock face into the tunnel or underground mine of more than
300mm.
In such conditions resin or cement anchored bolts are not suitable, because
the 25 kJ
energy creates an impact load on the bolts which exceeds their tensile
strength, so that
these types of bolts are known to fail in these conditions.
In some existing expansion rock bolts, the energy created by the movement or
fracture in
the rock strata is transferred straight from the rock plate to the tube of the
rock bolt and if
the friction engagement between the outside surface of the tube and the facing
surface of
the bore above the strata fracture is not sufficient, the rock bolt will
shift. This is
particularly the case in very hard and very weak rock strata because the
frictional ability
for the rock bolt to properly anchor in that strata is poor.
For example, in some existing expansion rock bolts, the rock bolt expands
engagement
members (wedges for example) outwardly to gouge into the bore wall to improve
the
anchor of the bolt in the strata. While the initial gouging might be minor,
any movement
.. of the rock bolt outwardly of the bore under load will cause the members to
gouge further
into the bore wall and to resist further outward movement. However, in very
hard strata,
the members cannot gouge into the bore wall, or can do so only at a minimal
level and so
the contact between the rock bolt and the bore wall is largely frictional
engagement only.
In contrast, in very weak rock, the bore in which the rock bolt is installed
is often "over
drilled", i.e. is of a greater diameter than desired so that the expansion
members cannot
expand sufficiently to gouge into the bore wall to the depth needed to
properly engage the
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bore wall. A rock bolt that addresses one or more of the disadvantages of
prior art rock
bolts would be desirable.
Summary of the Invention
It is an objective to the present invention to provide a friction rock bolt
and a rock bolt
assembly that may be conveniently driven into a borehole formed within rock
strata and is
capable of being clamped in position via a robust and reliable clamping force
resistant to
ground kinetic energy loads and impact loads that would otherwise encourage
dislodgement of the rock bolt from the bore.
It is a specific objective to provide a rock bolt having a clamping mechanism
configured to
apply a radial expansion force within the as-formed bore at or towards a
leading end of the
rock bolt so as to maximise the frictional contact force with which the rock
bolt is secured
within the bore.
It is a further specific objective to provide a rock bolt configured to resist
and to withstand
ground kinetic energy and impact load at the rock bolt due to strata shifts.
It is a specific
objective to provide a rock bolt configured to maintain a fully anchored
position within a
bore in response to ground kinetic energy of the order of 25 kJ and impact
loading on the
rock bolt of the region of 45 t.
The objectives are achieved via a rock bolt (rock bolt assembly) having an
expander
mechanism to provide a symmetrical and controlled expansion at the axially
forward end
of the rock bolt. The objectives are further achieved by providing an expander
mechanism
and a rock bolt arrangement in which the tubular sleeve that at least
initially houses the
expander mechanism is configured to facilitate the symmetrical expansion in
combination
with a plurality of radially outer wedging elements that function
cooperatively with the
specifically configured tubular sleeve to provide the controlled expansion at
the axially
forward end.
Additionally, the objectives are achieved via a loading mechanism provided at
an axially
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rearward end of the rock bolt having a load/shock absorbing configuration to
withstand
impact loading forces transmitted to the rock bolt from the strata. The
loading mechanism
comprises a specific load absorber configured to deform, optionally via
compression,
crushing, crumpling, fracturing, deforming, failing or at least partially
failing in response
to a predefined/predetermined loading force (such as an impact loading force).
Such an
arrangement provides an initial stage load absorption. The present rock bolt
arrangement
is further provided with a main load bearing element into which the high
loading forces are
transmitted during/following initial absorption by the load absorber.
Accordingly, in one
aspect the present rock bolt comprises a multi-stage load and shock absorbing
configuration to effectively distribute loading forces across multiple
component
part/features of the rock bolt assembly. Accordingly, a rock bolt arrangement
is provided
to better withstand ground kinetic energy loading and in particular impact
loading due to
elevated and/or sudden strata movement.
According to a first aspect of the present invention there is provided a
friction bolt
assembly to frictionally engage an internal surface of a bore formed in rock
strata, the
assembly comprising: an elongate tube having a leading end, a trailing end and
a
longitudinally extending primary slot; an expander mechanism located within
the tube
towards or at the leading end and configured to apply a radial expansion force
to the tube
to secure the assembly to the rock strata; an elongate tendon extending
longitudinally
within the tube and connected at or towards a first end to the expander
mechanism and at
or towards a second end to a loading mechanism positioned at or towards the
trailing end
of the tube that by adjustment is configured to create tension in the tendon
to act on the
expander mechanism and provide the radial expansion force; characterised in
that: the
expander mechanism comprises: at least two radially outer wedge elements
positionally
secured to the tube; and a radially inner wedge element secured to the tendon
and capable
of axial movement relative to the outer wedge elements to apply the radial
expansion force
to the outer wedge elements; the elongate tube further comprising at least one
secondary
slot positioned axially at the expander mechanism such that the tube is
capable of
deforming radially at the axial position of the expander mechanism via the
primary and
secondary slots in response to axial movement of the inner wedge element and
the
expansion force transmitted by the outer wedge elements.
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Optionally, the outer wedge elements each comprise a radially inward facing
surface that is
oblique relative to a longitudinal axis extending through the assembly and a
radially
outward facing surface of the inner wedge element extends oblique relative to
the
longitudinal axis. Preferably, the inner wedge element comprises a radial
thickness that is
tapered along its respective length so as to comprise a radially thinker
forward end and a
radially thinner rearward end. Similarly, the outer wedge elements comprise a
radial
thickness that is tapered along the respective lengths so as to comprise a
radially thinker
rearward end and a radially thinner forward end.
Optionally, the radially inward facing surface of the outer wedge elements
and/or the
radially outward facing surface of the inner wedge element are at least part
conical or
frusto-conical. The respective surfaces accordingly may be concave in a plane
perpendicular to the longitudinal axis of the rock bolt. Optionally, the
radially inward
facing surfaces of the outer wedge elements and/or the radially outward facing
surface of
the inner wedge element are at least chisel shaped, part-chisel shaped or
wedge shaped
having tapering surfaces (in the longitudinal direction) that are generally
planar.
The relative alignment of the frictional engagement surfaces between the inner
and outer
.. wedging elements being oblique i.e. transverse, angled or alternatively
inclined relative to
the longitudinal axis of the rock bolt, contributes to maintaining the outer
wedges in a
symmetrical configuration as the inner wedge element forces radial expansion
and
distortion of the tube.
Preferably, the secondary slot is positioned diametrically opposed to the
primary slot.
Where the present assembly comprises a plurality of secondary slots,
preferably the
secondary slots are evenly spaced apart in a circumferential direction around
the
longitudinal axis with the outer wedging elements positioned between each
respective slot.
Positioning the secondary slot diametrically opposite the primary slot
specifically provides
.. symmetric expansion of the expander mechanism and maintains the outer wedge
elements
in spaced apart orientation.
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Preferably, an axial length of the secondary slot is less than an axial length
of the primary
slot. Optionally, the axial length of the secondary slot is 0.1 to 50%, 0.5 to
40%, 0.4 to
30% or 2 to 25% of a total axial length of the elongate tube. The secondary
slot extends
axially a short distance beyond the expander mechanism (inner and outer wedge
elements)
in both the axial forward and rearward directions. The primary function of the
secondary
slot is to facilitate expansion of the expander mechanism and to maintain the
circumferential spacing of the outer wedge elements. Accordingly, the
secondary slot is
not required to extend the full length of the tube and accordingly the tube
strength is
optimised to provide sufficient strength during initial installation of the
rock bolt into the
borehole via hammering. Preferably, the secondary slot comprises a width being
less than
a width of the primary slot.
Preferably, the outer wedge elements are spaced apart in a circumferential
direction by an
equal separation distance. This configuration facilitates symmetrical
expansion of the
expander mechanism and ensures the frictional sliding surfaces of the inner
and outer
wedge elements are appropriately aligned relative to one another to avoid
sideways
(torsional) forces and galling.
Preferably, in a circumferential direction, the outer wedge elements are
positioned between
and do not overlap with the primary and secondary slots. It is important the
outer wedging
elements do not hinder expansion of the tube by restricting deformation of the
tube at the
region of the slots. As indicated, the significant advantage with the present
concept is the
extent and control of the radial expansion that is achievable via a
symmetrical sliding
engagement between the inner and outer wedge elements.
Preferably, the outer wedge elements are secured to a radially inward facing
surface of the
tube by welding. More preferably, the outer wedge elements are secured to the
tube
exclusively at or towards an axially rearward end (or face) of each of the
wedge elements.
This attachment mechanism is sufficient to maintain the outer wedge elements
in fixed
position relative to the inner wedge and tube but does not provide an overly
rigid structure
that would be resistant to radial expansion. Accordingly, some degree of
movement of the
outer wedge elements is provided which is beneficial for controlled radial
expansion.
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Optionally, at least a portion of each of the outer wedge elements extends
axially beyond
the leading end of the tube. Optionally, at least a portion of the radially
inner wedge
element extends axially beyond the leading end of the tube. Optionally, a
maximum
.. outside diameter of the inner wedge element is greater than an inside
diameter of the tube.
Optionally, a maximum outside diameter of the inner wedge element is
approximately
equal to an inside or outside diameter of the tube. Such dimensional
relationships may
apply to the tube pre-installed within a bore hole (in the rock strata) of
post installation
within the bore hole (with the latter involving radial compression of the
tube).
Accordingly, it is possible to provide an inner wedge element having a greater
maximum
diameter relative to conventional arrangements so as to strengthen the inner
wedge element
against stress imparted by the elongate bar and contact with the outer wedge
elements.
Accordingly, the inner wedge element is less susceptible to cracking during
use.
Additionally, due to the enlarged dimensions of the radially inner wedge
element, not
being restricted by the internal diameter of the tube, a greater radial
expansion is
achievable.
Optionally, the tendon is an elongate bar that is radially enlarged at or
towards the first
end. Preferably, the first end of the bar comprise threads, with the threads
provided at the
radially enlarged first end. Preferably, the inner wedge element is mounted on
the bar via
the threads. Optionally, the second end of the bar may be radially enlarged
and comprise
treads. The radial enlargement reinforces the bar against tensile stress and
mitigates the
creation of stress concentrations due to the presence of the threads formed at
the external
surface of the bar.
Preferably, the assembly comprises a single primary slot, a single secondary
slot and two
outer wedge elements positioned diametrically opposite one another and spaced
apart in a
circumferential direction between the primary and secondary slots. Such a
configuration
provides an expander mechanism that may be manufactured and assembled
conveniently in
addition to providing an effective means for anchoring the rock bolt within
the bore by
maximising the extent and reliability of the radial expansion.
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Optionally, the assembly may further comprise a loading mechanism projecting
radially
outward at the trailing end of the tube so as to be capable of being braced
against the rock
strata at a region around an external end of the bore; a main load element
connected with
the tendon at the second end to brace against the trailing end of the tube and
by adjustment
create tension in the tendon to act on the expander mechanism and provide the
radial
expansion force; the loading mechanism further comprising a load absorber to
absorb load
imposed on the loading mechanism by the rock strata and in response to deform
or fail to
transfer said load to the main load element.
The provision of a multi-stage load support arrangement advantageously allows
a load that
is applied to a rock bolt to be absorbed in separate stages so that individual
components
and stages are required to absorb the full load. This is important as it means
that the full
load is not immediately transferred to the tendon or the tube of the rock
bolt. Rather, the
load is first reacted or partially absorbed by the load absorber (or first
support element) and
if the load is above a predetermined failure load, the load absorber deforms
or at least
partially fails and the remaining load is then reacted or absorbed by the main
load element
(or second support element). Advantageously, the load absorber will absorb
some of the
load or the energy, so that the load that is applied to the main load element
is lower than it
would have been had the full load been applied directly to the main load
element. The
.. energy of the rock displacement is thus dissipated as the load absorber
initially absorbs the
load and then deforms or partially fails. The remaining energy is then
absorbed by the
main load element, because the load applied to the main load element is lower
than the
tensile strength of the tendon. The load is reacted by the tendon by the
tendon applying a
pull load on the expander mechanism tending to expand the expander mechanism.
The
resistance to expansion provides the required reaction.
As an example, the bars typically used for ground support have a tensile
strength of up to
33t. Also, the load absorber could be arranged to deform or partially fail at
10t. Where a
load is applied where ground kinetic energy is in the order of 25 kJ, the
impact load on the
rock bolt could be in the region of 45t. For this, the load absorber will
deform or partially
fail at about 10t and thus will absorb the first 10t of the load. The actual
act of rock
displacement when the load absorber deforms or partially fails also absorbs
displacement
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load or energy (and so diminishes the ground kinetic energy) and so at the
point at which
the load absorber deforms or partially fails, some energy is absorbed via the
movement in
the rock strata itself and via the action of the load absorber deforming or
partially failing.
In fact, the rock displacement can cause some, most or all components of the
loading
mechanism to deform slightly and the expander mechanism to expand (upon
movement of
the tendon) which can each provide for some additional energy absorption,
although these
latter two forms of absorption do not always occur and so are not reliable in
a rock
displacement as absorption mechanisms.
Following energy absorption by the load absorber and associated mechanisms
(rock
displacement, bearing arrangement deformation etc) the bar of the rock bolt
would then
absorb the remainder of the energy, of which the impact load would now be
below the
tensile strength of the bar and so the bar would not fail and thus the rock
bolt would not
fail.
Optionally, the load absorber may comprise a compressible collar positioned in
contact
with the main load element. Optionally, the load absorber may comprise a
curved or bent
region of a flange, plate or washer, the region extending in a direction
axially towards the
main load element.
In certain embodiments, the tube is slotted longitudinally, along at least a
portion of its
length, but preferably fully along its length, to facilitate radial expansion
and contraction of
the tube. Radial contraction is required so that the tube can be driven into a
bore which has
an internal diameter which is slightly less than the external diameter of the
tube. This
advantageously permits the rock bolt to be inserted into firm frictional
engagement with
the internal wall of the bore. The external surface of the tube thus engages
the bore wall
frictionally upon insertion and prior to any expansion of the expander
mechanism.
Expansion of the expander mechanism and radial expansion of the tube is
greatly
facilitated by the provision of the secondary slot or slots that extend
axially along the tube
at the axial position of the expander mechanism. The action of the expander
mechanism is
principally to increase the frictional engagement between the rock bolt and
the internal
surface of the bore. In soft or weak rock, the expansion force of the expander
mechanism
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might exceed the compressive strength of the rock, so that radial expansion of
the tube
could be quite significant. Also, the action of the expander mechanism is to
resist radial
contraction of the tube when subject to an external load applied by the rock
strata. In
addition, where the bore diameter has been over-drilled, the tube can be
radially expanded
to properly engage the bore wall.
Optionally, the tube may have a tapered leading end to assist insertion into a
bore or it can
be of generally constant diameter along its length. Where the tube has a
tapered leading
end, the tapered section can include a slot that opens through the leading
edge of the tube.
This allows the leading end to compress radially as the rock bolt is inserted
into the bore.
Two axial end slots that are diametrically opposed are the preferred
arrangement.
Optionally, the tendon can be a rigid tendon, such as a metal bar, rod or
rigid cable, a cable
which is not rigid, or it can be a hollow bar.
The present rock bolt is adapted for use with a conventional rock plate that
connects to one
end of the rock bolt and that extends into contact with the face of the rock
strata about the
bore. The present rock bolt may comprise any suitable form of rock plate found
in the art.
The expander mechanism may comprise a first pair of expander elements that are
secured
to the tube diametrically opposite each other. These can be fixed in place in
any suitable
manner relative to the tube, but normally would be fixed by welding. The
welding may be
applied to the tube and in particular a short slit formed in the tube that is
filled with weld
and/or the weld could be applied to the inward facing surface of the tube. The
expander
may alternatively include three expander elements that are spaced apart
substantially
equally in the circumferential direction and are secured relative to the tube,
or four or more
expander elements, that are generally all spaced apart substantially equally
in the
circumferential direction.
The expander elements can have any suitable shape such as tapered or wedge
shape. The
shape of the expander elements will normally be identical to each other and
when
positioned with the tube, they will be symmetrical about the axis of the tube.
However, the
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invention does not preclude that the expander elements are shaped differently
to each other
or that they are not symmetrical about the axis of the tube.
In some forms, the radially outer wedge elements and the radially inner wedge
elements
that form the expander mechanism are configured such that movement of the
engagement
structure in a first axial direction allows the expander elements to move
towards each other
and thus to allow radial contraction of the tube, while movement of the
engagement
structure in a second and opposite axial direction causes the expander
elements to move
away from each other and thus to provide radial expansion of the tube. To
promote this
form of radial contraction and expansion of the tube, the expander elements
and the
engagement structure can form a wedge whereby the engagement structure engages
diametrically opposed surfaces of the respective expander elements. The
engaging
surfaces can be surfaces of a constant incline. The engaging surfaces can be
flat or planar
surfaces (such as those formed on a cone), or they can be curved mating
surfaces, such as
mating concave and convex surfaces (such as those formed on an ogive.
The radially inner wedge element may have any suitable form. In one form, the
inner
wedge element has a conical form with flat or planar surfaces for tapered
engagement with
the expander elements. Optionally, the radially inner wedge element may have a
central
opening to accept the tendon and the opening can be threaded to threadably
connect to the
tendon. The radially inner wedge element can be otherwise connected to the
tendon as
appropriate. The radially inner wedge element could alternatively comprise a
second pair
of expander elements that are connected to the tendon and that are separate to
each other
but are both connected to the tendon. The second pair of expander elements can
be
connected to each other or can be part of a larger structure that is connected
to the tendon.
Within a wedge-type expander mechanism as described herein, the wedge angle
governs
the length of the cooperating wedge elements, i.e., the shallower the wedge
incline or
taper, the longer the elements need to be for a given amount of expansion. For
greater
expansion, at a set wedge incline or taper, the cooperating wedge elements
need to be
longer. However, long wedge elements are more expensive because they require
more
material, a longer threaded bore for connection to the tendon and the thread
applied to the
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tendon also needs to be longer. In addition, the threads applied to the
components are hot
deep galvanised and need to be specially cleaned and so longer threads require
more
galvanising material and take longer to clean.
In the development of the prior art rock bolt of Australian Patent Application
2010223134,
it was found to be important that the angle of the wedge engagement was
relatively
shallow for the most efficient expansion to be gained using an installation
machine torque
of 400Nm. In Australian Patent Application 2010223134, a single expander
element
cooperating with a single expander at a 5 inclusive angle between the
expander element
fixed to the tube and the expander fixed the tendon was selected for the
optimum
expansion force and length of engagement between the expander elements and the
engagement structure.
In the present invention however, the initial expansion of the expander
mechanism is not
critical, as the expander mechanism can expand further after the rock bolt has
been
installed. This means that the angle of engagement between the cooperating
wedge
elements is not as important and so the inclusive angle between the
cooperating wedge
elements can be increased, and estimates are that it can be increased to 10,
12, 14, 16 or 18,
inclusive with the preferred angle being around 16 . Because of this, the
length of the
20 expander elements can be reduced or will not be excessive.
In the prior art of Australian Patent Application 2010223134, a further
restriction is that
the element attached to the tendon needed to have its threaded bore as close
to the non-
tapered side of the element as possible but still leaving about a 4mm wall
thickness at the
non-tapered side for the structural integrity of the element. This 4mm wall
thickness
requirement limits the maximum expansion as compared to the bore being closer
to the un-
tapered side than 4mm. In the present invention, the bore can be central of
the engagement
structure and so full tapering can be provided. The above advantages mean that
present
invention allows the tube expansion of the rock bolt to be increased by about
2, 4, 6 or
8mm, with 4mm being preferred, which is significant and which was not apparent
until the
second aspect was developed.
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To facilitate tube expansion in the region of the expander mechanism, the tube
includes a
secondary longitudinal expansion slot or slit which extends axially along the
tube for an
axial section corresponding to the location of the expander mechanism.
Preferably, the
secondary expansion slot or slit is positioned diametrically opposite the tube
primary
longitudinal slot that extends fully (or over a majority) of the tube length
(between
repetitive ends). The length of the secondary expansion slot is preferably
much less than
the primary longitudinal slot and may be in region of about 200mm long.
Brief description of drawings
A specific implementation of the present invention will now be described, by
way of
example only, and with reference to the accompanying drawings in which:
Figure 1 is a cross-sectional view of a friction rock bolt according to an
aspect of the
present invention.
Figure 2 is a cross-sectional view through AA of Figure 1.
Figure 2A is a modified version of Figure 2 showing an alternative expander
mechanism.
Figure 3 is a cross-sectional view of the leading end of a friction rock bolt
according to
another aspect of the present invention.
Figure 4 is a cross-sectional view through BB of Figure 1.
Figure 5 is a cross-sectional view of the trailing end of a friction rock bolt
according to
another aspect of the present invention;
Figure 6 is a cross sectional view of an axially forward region of friction
rock bolt
according to a further aspect of the present invention;
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Figure 7 is a cross sectional view of a friction rock bolt according to a
further aspect of the
present invention;
Figure 8 is a cross sectional view of the trailing end of a friction rock bolt
according to a
further aspect of the present invention;
Figure 9 is a cross sectional view of the trailing end of a friction rock bolt
according to a
further aspect of the present invention;
.. Figure 10 is a cross sectional view of the trailing end of a friction rock
bolt according to a
further aspect of the present invention.
Detailed description of preferred embodiment of the invention
Figure 1 is a cross-sectional view of a friction rock bolt 10 according to one
embodiment
of the invention. The rock bolt 10 includes an elongate generally cylindrical
tube 11
(having a circular cross section) with a leading end 12 and a trailing end 13.
The length of
a typical rock bolt be can in the range of about lm to about 5m.
The tube 11 is split longitudinally along its full length via a primary slot
26 so that it can
be expanded radially for improved frictional engagement with the inside
surface 14 of a
bore which is drilled into a body of rock or a rock strata.
For the purpose of expanding the tube 11 radially, or to increase the
frictional contact
between the outer surface of the tube 11 and the surface 14 of the bore with
or without
radial expansion, the rock bolt 10 includes an expander mechanism 15 within
the tube 11
and disposed at or towards the leading end 12 of the tube 11. The expander
mechanism 15
includes a pair of first wedge like expander elements 16 and 17 that are
secured to the
inside surface 18 of the tube 11. Figure 2 also shows this arrangement and in
that figure, it
.. is clear that the expander elements 16 and 17 are secured to the inside
surface 18 of the
tube in positions that are diametrically opposite each other.
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The expander mechanism 15 further includes an engagement structure 20 in the
form of a
radially inner wedge element that is secured to a tendon on the form of an
elongate bar 21
(which could alternatively be a cable), and is positioned at the leading end
of the bar 21
and for cooperation or engagement with the respective radially outer expander
(wedge)
elements 16 and 17.
It can be seen from Figure 1, each of the generally wedge-shaped expander
elements 16, 17
comprise a radially inward facing surface 22 that is aligned oblique to a
longitudinal axis
67 of the rock bolt 10 so as to be generally tapered. Similarly, the radially
inner wedge
element 20 comprises a radially outward facing surface 23 that is also aligned
oblique to
longitudinal axis 67 and parallel to outward facing surface 22 of the outer
wedge elements
16, 17. Such an arrangement enables the inner wedge element 20 to slide in
frictional
contact with outer wedge elements 16, 17 as the elongate bar 21 is actuated
and the inner
wedge element 20 moved axially relative to the stationary outer wedge elements
16, 17.
The complementary aligned surfaces 22, 23 are advantageous to facilitate
maximum
symmetrical expansion of the expander mechanism 15 and avoid galling of
regions of the
surfaces 22, 23. In particular, it will be evident from Figure 1, that as the
inner wedge
element 20 moves in a direction away from the blind end 25 of the bore, the
relative
movement and engagement that occurs between the outer elements 16 and 17 and
the inner
element 20 will tend to cause the tube 11 to expand radially and force the
tube 11 into
greater frictional contact with the surface 14 of the bore. That radial
expansion is
facilitated by slot 26 (formed longitudinally of the tube 11 as shown in
Figure 2).
Expander elements 16 and 17 may be secured against the inside surface 18 of
the tube 11
in any suitable manner and preferably are secured by weld 68. Likewise, the
inner element
20 can be secured to the bar 21 in any suitable manner. In Figure 1, the
leading end 27 of
the bar 21 is threaded to threadably engage a threaded bore 28 formed in
element 20.
The leading end 12 of the tube 11 is tapered to facilitate insertion of the
rock bolt 10 into a
bore drilled into a rock strata. Figure 1 shows a slot or slit 29 formed in
the leading end 12
to allow the leading end 12 to compress radially if necessary for insertion
into the bore. In
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practice, there could be two slots 29 formed diametrically opposite each other
for this
purpose, or three slots at 120 to each other, or four slots at 90 etc.
The expander mechanism 15 is shown in Figure 1 in an actuated or activated
state, in
which the inner wedge element 20 has been shifted relative to the outer wedges
16 and 17
to cause an expansion load to be applied to the tube 11. However, when the
rock bolt 10 is
to be inserted into the bore, the inner wedge element 20 would be in a
position in which it
would be further towards the leading end 12 of the tube 11. The intention
would be that
wedge element 20 would be positioned so that the expander mechanism 15 is not
imposing
an expansion load on the tube 11. Indeed, it is preferred that inner wedge
element 20 be
positioned such that the tube 11 can radially compress or contract as the bolt
10 is inserted
into a bore by the bore being drilled to a diameter which is slightly smaller
than the outside
diameter of the main portion of the tube 11. This naturally allows the tube 11
to compress
or contract radially as the bolt 10 is forced into the bore and thus allows
the outside surface
of the tube 11 to frictionally engage the inside surface 14 of the bore so
that once the rock
bolt 10 is fully inserted into the bore, there will already be a frictional
engagement between
the tube and the inside surface of the bore.
Once the bolt 10 has been fully inserted into the bore, the expander mechanism
15 can be
.. activated, to impose a radial expansion load on the tube 11 and so to
increase the frictional
engagement between the tube 11 and the inside surface 14 of the bore. As
indicated,
activation of the expansion mechanism 15 causes wedge element 20 to shift
(relative to the
stationary elements 16 and 17) in a direction away from the blind end 25 of
the bore. This
movement may be achieved either by pulling the bar 21 in a direction away from
the blind
end 25, or by rotating the bar 21 so that by the threaded engagement between
wedge
element 20 and the bar 21, wedge element 20 is drawn in a direction away from
the blind
end 25. Rock bolt 10 comprises a nut 30 located at a trailing end 69 of bar 21
to represent
a head of the bar 21 and to be configured to brace against the trailing end of
tube 11 either
directly or indirectly via an axially intermediate washer 48. Nut 30 may be
formed
integrally (i.e., fixed) at the end 69 of the bar 21. Alternatively, nut 30
may be threadably
connected to the end 69 of the bar 21. In that latter arrangement, inner wedge
element 20
would shift relative to the elements 16 and 17 with movement of the bar 21 as
opposed to
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the arrangement where the bar 21 rotates and the inner wedge element 20 shifts
relative to
the bar due to the threaded engagement between the bar 21 and wedge element
20.
In another alternative, the nut can be a blind nut with an internally threaded
bore, so that
the nut 30 can be threaded onto the threaded free end of the bar 21 to the
point at which the
blind end of the threaded opening engages the end of the bar, at which point
no further
threaded movement can take place. Further rotation of the nut then will cause
rotation of
the bar 21.
The expander mechanism 15, comprising a pair of expander elements 16 and 17
contrasts
with earlier arrangements in which only a single wedge element is provided at
the tube
internal surface. In those arrangements, a wedge element that has been fixed
to the bar or
cable interacts with the single wedge element that is fixed to the tube, but
the expansion
available in the arrangements employing a single wedge element is less than
that available
in the arrangement of the present invention. Thus, by the provision of a pair
of expander
elements 16 and 17, which are in diametrically opposed positions against the
inside surface
of the tube 11, there can be an increased level of expansion of the tube 11.
In prior art
arrangements, the maximum expansion of a tube is in the region of 52mm,
whereas in the
new arrangement illustrated in Figure 1, the expansion can be up to 56mm.
While this
increase is only relatively small, the benefits it provides can be
significant. For example,
in very weak rock where the bore diameter is over drilled, the maximum
expansion of prior
art bolts might not be sufficient to frictionally engage the bore surface with
sufficient force
to properly fix the bolt within the bore. However, the extra expansion
facilitated in a rock
bolt according to the present invention enables greater expansion and thus
means it is more
likely that a rock bolt expanded in weak rock will be able to sufficiently
engage the bore
surface to properly anchor the bolt within the bore.
The arrangement of the expander elements 16 and 17 as being diametrically
opposed
within the tube 11 is further advantageous to ensure that there is no
misalignment between
the elements 16 and 17 as the expander mechanism is initially activated and
under
subsequent loading through failure or movement in the rock strata. Where
misalignment
occurs this can develop torsional loading that could negatively affect the
weld connection
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of the elements 16 and 17 to the inside surface 18 of the tube 11. Moreover,
misalignment
between the elements 16 and 17 and the structure 20 can result in reduced
surface
engagement between the respective components which could affect the proper
expansion
of the expander mechanism 15.
To improve the likelihood of complete alignment between the inner and outer
elements 20,
16, 17, a secondary (further) slot or slit 51 is provided opposite the primary
tube slot 26 to
facilitate symmetric tube expansion as the expander mechanism 15 expands as
shown in
Figures 1 and 2. As illustrated in Figures 1 and 2, secondary slot 51
comprises different
dimensions to primary slot 26 and for example, includes a width and a length
that are less
than those of primary slot 26. In particular, slot 51 may comprises a width of
about 5mm
and a length of about 200mm. Such a further slot or slit 51 can also be
provided in the
Figure 3 arrangement.
With reference to Figure 3, an alternative expander mechanism 35 is
illustrated which
includes a pair of outer wedge elements 36 and 37 that are welded to the free
end 38 of the
rock bolt tube 39. The elements 36 and 37 are welded via the annular weld 40
to the free
end 38 of the tube 39 and therefore the elements 36 and 37 are not only
present within the
tube 39, but extend out of the tube 39. An engagement structure (inner wedge
element) 41
is threadably attached to the threaded end 42 of the bar 43 and relative
movement of the
inner wedge element 41 relative to the outer (stationary) elements 36 and 37
can be as
described in relation to the embodiment of Figures 1 and 2 (referring to
elements 20, 16
and 17. The arrangement of Figure 3 facilitates even greater expansion of the
tube 39
compared to the tube 11 of Figures 1 and 2 because the diameter of the inner
wedge
element 35 can be greater than the diameter of the wedge 20 of the figure 1
embodiment.
In particular, inner wedge element 35 is generally frusto-conical along some,
most or all of
its axial length (consistent with the figure 1 embodiment). The inner wedge
element 35
may comprise a maximum diameter (at its thickest axial leading end) that is
greater than an
insider diameter of tube 11 (as defined by tube internal facing surface 18)
with the tube
compressed and squeezed into the as-formed bore hole 14, in contact with bore
surface 14.
Moreover, the maximum diameter of inner wedge element 35 is approximately
equal to an
outside diameter of tube 11 (as defined by tube external surface 71). Such an
arrangement
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is beneficial to strengthen the inner wedge element 35 against compressive
stress encounter
during use and imparted by bar 21. Additionally, the arrangement of Figure 3
is expected
to gain a further 5 to 6mm of tube expansion. Slots (not shown) are provided
in the tube
39 to extend through the free end 38 facilitate that expansion and are to be
considered
consistent with the secondary slot 51 of the embodiment of Figures 1 and 2.
In other respects, the arrangement of Figure 3 is the same as Figure 1, except
that it will be
apparent that the leading end of the tube 39 is not tapered in the manner
shown in Figure 1
as the tube 39 is required to remain of constant diameter to facilitate
attachment of the
elements 36 and 37 to the free end 38 of the tube 39.
While the figures show a pair of expander elements 16, 17 and 36, 37, the
invention covers
arrangements in which an arrangement of three expander elements is provided,
or there
could more expander elements. These expander elements can be wedge elements of
the
kind shown in the figures and they can all be fixed to the tube by welding.
One or two of
the expander elements can be welded in such a position that it or they would
extend into or
over, or even to substantially cover the longitudinal slot (longitudinal slot
26 as shown in
the figures) of the tube. Figure 2A illustrates a tube lla having a primary
longitudinal slot
26a and a pair of secondary slots 51a. An engagement structure (inner wedge
element) 20a
cooperates with three outer wedge elements 44, two of which extend into or at
least
partially over the longitudinal slot 26a. The slots 51a have the same purpose
as the slot 51
described earlier, however because there are three expander elements 44, two
slots 51a are
required.
The arrangement as illustrated in Figure 2A can advantageously act to prevent
the
engagement structure attached to the tendon from being dislodged out of the
tube by
significant impact loading, such as might happen during insertion of the rock
bolt into a
bore. For example, the rock bolt can be subject to significant impact loading
during
manoeuvring of the installation machine where the leading end of the bolt
might strike the
rock surface with a relatively large lateral force. By placing the expander
elements in such
a position that they extend into or over the longitudinal slot, the engagement
structure is
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less likely to, or will actually be prevented from egress out of the tube
during a significant
impact event.
Returning to Figure 1, at the trailing end 13 of the tube 11, a rock plate 45
is shown
bearing against the rock face 46. The plate 45 as illustrated is not
reflective of the shape of
plate that would actually be used in the field, but it is sufficient for the
purposes of this
description. The plate 45 bears against the rock face 46 and against a ring 47
which is
welded to the outside surface of the tube 11. A plate or washer 48 is
positioned axially
between nut 30 and an axially rearwardmost free end 49 of tube 11.
Importantly, a gap G
is provided between ring 47 and washer 48. Figure 4 is a cross-section through
B-B of
Figure 1 and shows spot welds 50 for securing ring 47 to an external surface 1
la of tube
11. In particular, four spot welds 50 are provided.
The arrangement described above at the trailing end 13 of the tube 11 is a
loading
mechanism 70 (alternatively termed a support arrangement) for supporting
loading that is
imposed on the rock bolt 10 by movement or failure in the rock strata and in
particular,
provides a multi-stage load support. In a first stage, load support is
provided by ring 47,
whilst in a second stage, rock support is provided by the washer 48 and the
nut 30. The
operation of the multi-stage loading mechanism 70 is as follows. With the rock
bolt 10
.. inserted within a bore and the expansion mechanism 15 expanded, if a load
is applied to
the rock bolt (normally a dynamic load), then the first stage of support is
provided by
loading mechanism 70 between the rock plate 45 and the ring 47. In the event
that the load
which is applied to the rock bolt exceeds the shear strength of the spot welds
50, then those
welds will fail and the ring 47 will shift to take up the gap G and to bear
against the washer
48. The first stage of load support thus is provided up to the point at which
the spot welds
50 fail. Upon failure of the spot welds 50, the load which is applied to the
rock bolt 10 will
shift to the washer 48 and the nut 30, so that the load will be reacted by the
bar 21 to which
the washer 48 and the nut 30 are connected. That load will tend to shift the
bar away from
the blind end 25 of the bore and thus will cause a shift of inner wedge
element 20 relative
to the outer elements 16 and 17 of expander mechanism 15. This will have the
effect that
there will be a greater expansion load applied by the expander mechanism 15 to
even more
firmly force the tube 11 into frictional engagement with the inside surface 14
of the bore
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and by that increased frictional engagement, the load applied to the rock bolt
10 will be
supported up to the point at which the bar 21 itself fails. In addition, the
tube 11 will be
prevented from movement relative to the surface 14 of the bore (other than
very minor
movement) by the increased frictional engagement between the tube 11 and the
bore wall
as the expander mechanism 15 operates to increase the frictional engagement
load. The
rock bolt 10 is thus restrained against movement within the rock strata, or is
restrained with
acceptable levels of movement.
As explained above, the increased expansion available with the expander
mechanisms 15
and 35 facilitates improved load support where loads of the above described
kinds occur in
weak rock. Thus in weak rock, if a dynamic load occurred of a magnitude that
caused the
spot welds 50 to shear, there is an improved likelihood of the rock bolt
absorbing the
dynamic load where the ability of the rock bolt to expand radially is greater.
.. The multi-stage (two stage) load support arrangement discussed above is
important and
advantageous for the following reasons. When a rock bolt is subject to a
significant initial
load, such as in seismic rock conditions, the sudden dynamic loading can be
greater than
the tensile strength of the bar or cable which would typically be expected to
absorb the
load. For example, when the rock kinetic energy is at a level of about 25 kJ,
the impact
load may exceed 45t. However, the tensile strength of bars typically used in
rock bolts is
not more than 33t so in such conditions, the bar would break. This obviously
could
compromise the support role that the rock bolt is intended to have. However,
by providing
a multi-stage load support arrangement, the initial load can be partly
absorbed by the ring
47 up to the point of shear which would occur in the region of 2-10t. Some of
the initial
load energy is thus absorbed by the ring up to the point of shearing and
thereafter, the load
energy is transferred via the washer 48 and nut 30 to the bar 21. By absorbing
2-10t of the
overall load energy initially, the energy which is transferred to the washer
and nut is
significantly reduced and is then likely to be of a magnitude which will
develop a tensile
load that is less than the tensile strength of the bar. In the illustrated
embodiment, the gap
G is important, because it allows the spot welds 50 to shear. If the gap G was
not
provided, and the ring 47 rested against the washer 48, there would be no
first stage of load
absorption. The gap G between the ring 47 and the washer 48 is optimally
between 5-
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8mm. According to some installations procedures this allows for some
'mushrooming' of
the trailing end of the tube during impact (hammering) installation, which
typically is
about 2mm, but does not leave the gap G too large to allow excessive rock
displacement as
the ring 47 shears. A rock bolt according to the figures is thus expected to
provide greater
reliability of rock support, particularly in seismic rock conditions or in
weak rock.
The multi-stage load support arrangement of Figure 1 represents just one form
of
arrangement which provides the support required. In alternative arrangements,
multiple
load absorbers (optionally in the form of rings 47) could be provided at the
rearward tube
end 13 to provide further stages of load support or energy absorption. Each of
the multiple
load absorbers (e.g., rings 47) could be spaced apart sufficient to allow
successive energy
absorption (e.g., by a shear of the welds 50). The minimum number of load
absorbers is
one and may comprises one or two rings, while any number of rings beyond two
could be
provided as required.
A further alternative load absorber is a compressible element and such an
arrangement is
shown in Figure 5. In Figure 5, the same components that have been included in
Figure 1
are given the same reference numerals. Thus, Figure 5 illustrates a rock bolt
tube 11, a bar
21, a nut 30, a rock plate 45 and a washer 48. However, Figure 5 also
illustrates a
compressible cylindrical collar 55 which extends axially between the rock
plate 45 and the
washer 48. The rock plate 45 bears against bearing surface 56 of the collar
55, while the
washer 48 bears against bearing surface 57. Between the bearing surfaces 56
and 57 is a
neck 58 and it can be seen in Figure 5, that the outside diameter of the neck
58 is reduced
compared to the outside diameters of the collar 55 at the bearing surfaces 56
and 57.
The compressible collar 55 is intended to compress, crush or crumple at a
particular load
applied to it by the rock plate 45. That load could be the same load that
causes the spot
welds 50 of the rock bolt 10 to fail or it could be a greater or lower load to
cause failure.
Regardless, upon the load being sufficient to cause the element 55 to fail,
collar 55 will fail
by the neck 58 crushing or crumpling. Once the collar 55 has failed to the
maximum it
can, the load energy that has not already been absorbed by failure of the
collar 55 is
transferred to the washer 48. Thus, the load energy that is transferred to the
washer 48 is
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reduced compared to the load energy that the collar 55 was exposed to
initially. Upon that
transfer, the second stage of load support is the same as explained in
relation to the rock
bolt 10 when the ring 47 shears and engages the washer 48.
.. Figure 6 illustrates a further embodiment of the present rock bolt in which
elongate bar 21
is radially enlarged at its leading end 27. In particular, bar 21 may be
divided axially so as
to comprise a main length section 21e having external ribs. Bar 21 then
transitions to a
generally smooth or unribbed region 21a A radially enlarged section 21b
extends axially
from section 21a and comprises threads, as described with reference to figures
1 and 3 to
.. mount the radially inner element 20 (in a form of a conical wedge). As
described, wedge
comprises an internal bore having corresponding threads to mate with the
threads on
radially expanded section 21b. Such an arrangement is advantageous to
strengthen rod 21
at the leading end 27 against tensile forces imposed on bar 21 during use.
Preferably, the
threads on end section 21b are not typical metric threads and are preferably
rounded or
15 .. rope style threads to minimise the creation of stress concentrations
that would otherwise
weaken the bar 21 at leading end 27.
Figures 7 to 9 illustrate further embodiments of the axially rearward loading
mechanism of
the present rock bolt. Referring to figure 7 and in a further implementation,
the loading
20 .. mechanism, alternatively referred to herein as a load support
arrangement, comprises
washer 48 positioned axially intermediate rock plate 45 and nut 30. Washer 45
comprises
an axially forward facing abutment surface 48a that also extends radially
outward beyond a
radially outward facing external surface 71 of tube 11 at the tube rearward
end 13.
Abutment surface 48a is annular and is configured to engage, in a butting
contact, a
.. radially inner region of rock plate 45 such that loading forces imposed on
rock plate 45 by
the rock face 46 are transmitted into washer 48 that is axially spaced from
nut 30 by a gap
region G. A conical compressible collar 62 is mounted within the gap region G.
Collar 62
comprises an axially forward end 62a (in contact with an axially rearward
facing face 48b
of washer 48) and an axially rearward end 62b (in contact with an axially
forward facing
.. face 30a of nut 30).
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Collar 62 may be formed from the same material as compressible collar 55 as
described
referring to figure 5 such that collar 62 is capable of compressing via
deformation as
washer 48 is forced axially rearward by loading forces imposed on rock plate
45 (and
hence washer 48) due to movement of the rock surface 46. Collar 62 is
dimensioned such
that a maximum diameter does not exceed an external diameter of nut 30 such
that collar
62 does not extend radially beyond the nut 30. Such an arrangement is
advantageous to
provide a radially accessible region around nut 30 and collar 62 to receive an
axially
forward end 60 of a hammer tool used to deliver and force the rock bolt 10
into the bore
during initial installation. In particular, the axially forward end of hammer
tool 60 is
configured for placement in direct contact against the rearward facing surface
48b of
washer 48 such that the compressive forces delivered to the rock bolt 10 via
the tool 60 are
transmitted directly through washer 48 and into tube 11 importantly without
being
transmitted through nut 30 and compressible collar 62. Such an arrangement is
advantageous to avoid unintended and undesirable initial compression of collar
62 due to
the hammer driven compressive forces by which rock bolt 10 is driven into the
borehole.
The further embodiments of figures 8 and 9 are also configured for avoiding a
compressive
force transmission pathway through the load absorber component (in the form of
a
compressible washer, gasket, seal, flange etc. as described herein).
Accordingly, in some
embodiments, preferably washer 48 extends radially outward beyond tube 11, nut
30 and
the load absorber, so as to present an accessible rearward facing surface 48b
for contact by
the leading end of the hammer tool 60.
A further embodiment of the loading mechanism is described referring to figure
8 in which
flange 48 comprises corresponding surfaces 48a, 48b. However, differing from
the
embodiment of figure 7, a radially inner section 63 of washer 48 is dome-
shaped so as to
curve in the axial direction towards nut 30 (secured at the rearward end of
bar 21). Dome
section 63 occupies the gap region G between the main body of washer 48 and
nut 30.
Accordingly, as load from the rock strata surface 46 is transmitted into rock
plate 45 and
accordingly into washer 48 via surface 48a, dome section 63 is configured to
compress
such that the washer 48 flattens to reduce gap G.
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Figure 9 illustrates a further embodiment of the rock bolt of figure 7 in
which the conical
collar 62 is formed as a generally cylindrical deformable collar 64. As with
the
embodiment of figure 7, collar 64 is dimensioned so as to not extend radially
outward
beyond nut 30 to provide access to the washer surface 48b by the hammer tool
60 and
accordingly avoid compressive force transmission through collar 64 during
initial
hammering of the rock bolt 10 into the borehole as described.
Figure 10 illustrates a further embodiment of the rock bolt 10 corresponding
to the
arrangement of figure 6 having a radially enlarged section of bar 21. As
illustrated in
figure 10, bar 21 at an axially rearward region of main length section 21e
comprises a non-
ribbed generally smooth section 21d. A radially enlarged section 21c extends
from the
rearward end of smooth section 21d and comprises threads to mate with
corresponding
threads formed on a radially inward facing surface (not shown) of nut 30 so as
to secure
nut 32 to bar 21. As described referring to figure 6, the enlarged section 21c
provides
reinforcement of the bar 21 against tensile forces encountered during use with
the thread
configuration at section 21c being preferably the same as described at section
21b.
The expander mechanism as described herein comprising at least two radially
outer
expander elements 16, 17, 44 is advantageous to maximise the radial expansion
force
imposed by the axially rearward movement of the inner wedge element 20. As
indicated,
in contrast to existing rock bolt configurations having a single outer wedging
element, the
present configuration provides a greater maximum radial expansion (combined
radial
movement of wedging elements 16, 17, 44) relative to the corresponding maximum
radial
displacement achievable by a single outer wedging element.
Additionally, the present arrangement, via the plurality of outer wedging
elements 16, 17,
44 provides a desired symmetrical tube expansion. This is achieved, in part,
via the
circumferential spacing between the wedging elements 16, 17, 44, the provision
of a
secondary elongate slot 51 and the oblique alignment of the inward and outward
facing
.. surfaces of the respective outer and inner wedging elements 16, 17, 44 and
20, 20a. The
controlled interaction between and parallel alignment of the mating surfaces
22, 23 (of the
wedging elements 16, 17, 44, 20, 20a) is beneficial to avoid development of
sideways
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(torsional) forces at the region of the expander mechanism 15, 35 that i)
would reduce the
desired frictional contact, ii) lead to possible development of galling of the
wedging
elements 16, 17, 44, 20, 20a and iii) reduce the performance in the clamping
action of the
expander mechanism 15, 35. Additionally, and as will be appreciated, the
provision of a
secondary slot 51 in addition to the primary slot 26 reduces the magnitude of
force
absorbed by the tube 11 as the expander mechanism 15, 35 is expanded which, in
turn,
maximises the efficiency and effectiveness of the expansion mechanism 15, 35
to deform
tube 11 into tight frictional contact with the surrounding rock strata.
As will be appreciated, the present rock bolt may comprise a plurality of
secondary
elongate slots 51 with each slot 51 spaced apart in a circumferential
direction around the
central longitudinal axis 67 of rock bolt 10. Similarly, the present rock bolt
10 may
comprise a plurality of outer wedging elements 16, 17, 44 (optionally
including 2, 3, 4, 5,
6, 7 or 8 separate elements) each spaced apart in a circumferential direction
around axis 67.
.. Preferably, to facilitate radial expansion of tube 11 via the slots 51,
wedging elements 16,
17, 44 are secured to tube 11 at locations between the slots 26 and 51 and do
not bridge or
otherwise obstruct slots 51.
The embodiments illustrated in the figures discussed above are expected
advantageously to
allow for more reliable and secure rock strata support under loading, such as
seismic
loading or loading due to ground swelling. Failure of a bar or cable (for
example due to
the bar or cable being effectively 'pulled-through' the outer wedges) of a
rock bolt
according to the invention is expected to be less likely while the greater
radial expansion
provided in a rock bolt according to the invention is expected to provide more
secure
anchoring of a rock bolt within a bore.
