Note: Descriptions are shown in the official language in which they were submitted.
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1
ROCK BOLT
BACKGROUND OF THE INVENTION
This invention relates to a rock bolt.
In one technique of reinforcing a rock body a shank of a rock bolt is embedded
in a
settable material, eg. a grout, in a hole which is formed in the rock.
The grout, once it has set, provides resistance to movement of the shank, in a
controlled manner, thereby ensuring that a desirable characteristic of yield
versus
load is achieved.
Although the aforementioned technique words satisfactorily it is only
effective once
the grout has set. In other words between the time of placement of the grout
and the
setting thereof very little support is provided by the rock bolt. This can
give rise to a
dangerous situation.
SUMMARY OF INVENTION
The invention provides a rock bolt which includes a tubular member with first
and
second ends, a device at the first end of the tubular member for mechanically
anchoring the tubular member in a hole in a rock face, and an elongate member
which is positioned inside the tubular member and which projects from the
second
end, and wherein at least one load resisting formation is provided in or on
the tubular
member or the elongate member.
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At least a portion of the elongate merz~ber which projects from the second end
of the
tubular member may be threaded, or be furnished in any other suitable way eg.
formed with a hook or eye formations, or the like.
The device at the first end of the tubular member may be of any appropriate
form and
for example may be selected from an expansible anchor, a pivotal wedge, a hook-
shaped formation or formations, a spring washer or the like. An objective in
this
regard is that the device mechanically anchors the tubular member to the hole
and
thereby immediately provides a load resisting capability. On the other hand
the
tubular member is grouted in position and once the grout has set the tubular
member, which is adhesively bonded to the hole, provides a further load
resisting
capability.
The load resisting formation may be provided by means of one or more
deformations
on the tubular member, on the elongate member or on the tubular member and on
the elongate member. In a preferred embodiment of the invention the tubular
member and the elongate member are bent at one or more locations to provide
the
load resisting formation or formations. Preferably the tubular and elongate
members
are bent generally in the shape of a sine-wave.
The invention also provides a method of supporting rock which includes the
steps of
inserting an elongate member at least partially into a tubular member,
engaging the
elongate member with the tubular member in a manner whereby movement of the
elongate member relatively to the tubular member is resisted at least in one
direction
by means of at least one load resisting formation on the tubular member or on
the
elongate member, mechanically anchoring an end of the tubular member in a hole
in
a rock face, and adhesively securing at least part of the tubular member to
the hole.
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The tubular member may be adhesively secured to the hole using any suitable
composition and preferably use is made of a settable material, eg. a
cementitious
material such as a grout, or a resin.
The mechanical anchor is used to secure the tubular member to the hole while
the
load resisting formation constitutes a means of securing the elongate member
to the
tubular member.
Once the tubular member has been adhesively secured to the hole a yielding
characteristic is provided by means of the load resisting formation which
allows
deformation of the elongate member or of the tubular member or both members,
when the load which is exerted on the elongate member exceeds a predetermined
level.
The elongate member may be a rod, formed from wire, which includes a number of
twisted wires, or a cable, and may be made from steel, plastics or any other
suitable
material.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is further described by way of examples with reference to the
accompanying drawings in which:
Figure 1 illustrates from the side and in cross section a rock bolt according
to a first
form of the invention; and
Figures 2 to 7 respectively illustrate rock bolts according to variations of
the
invention.
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DESCRIPTION OF PREFERRED EMB40DIMENTS
Figure 1 of the accompanying drawings illustrates a rock bolt 10 according to
a first
form of the invention which is positioned in a hole 12 formed from a rock face
14 into
a body of rock 16.
The hole 12 is drilled in a conventional manner using techniques which are
known in
the art.
The rock bolt 10 includes an elongate tubular member 18 formed from a length
of
pipe, an anchoring device 20 at one end of the pipe and an elongate member 22,
for
example in the form of round bar, which is partly positioned inside the pipe.
A portion 24 of the round bar is threaded and a nut 26 is engaged therewith.
The
nut, in use, bears against a washer 28 which in turn bears against the rock
face 14.
The device 20 may be of any appropriate type which is capable of anchoring the
pipe
18 to a wall of the hole 12. This aspect is further described hereinafter.
During manufacture of the rock bolt the round bar 22 is coated with a
debonding or
lubricating agent over its full length and is then inserted into the pipe 18
while both
these components are straight ie. undeformed. The components are thereafter
bent
in unison, in a predetermined manner, so that the combined components have a
sinusoidal-type shape 30 of the kind shown in the drawing with a relatively
straight
portion 32 of the bar extending from the pipe. The extent to which the bar and
the
pipe are bent, ie. the depth of each bend and the spacing between adjacent
bends, is
determined substantially by trial and experiment according to the desired
yielding
characteristics.
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Grout 34 is injected into the hole 12 us ng any appropriate technique.
Thereafter the
rock bolt is inserted into the hole 12 through the grout and the device 20 is
actuated
so that the rock bolt is mechanically anchored in position. The way in which
the
device is actuated depends on its construction and this aspect is further
described
hereinafter. It is to be noted that the device 20 acts on the pipe only and
does not
directly act on the round bar. The grout is allowed to set around the pipe and
the
projecting portion 32 of the round bar.
Alternatively the bolt can first be placed in the hole and the device 20 is
actuated
before the grout is injected into the hole 12.
In order to activate the device 20 the washer 28 is threaded over the
projecting end
of the round bar and the nut 26 is engaged with the threaded end of the round
bar
and advanced to force the washer into engagement with the rock face. It is
evident
that a load-bearing mechanical anchor is immediately provided, even though the
grout 34 may not have set, by virtue of the device 20 being urged into
engagement
with the wall of the hole 12 by placing the bar under tension as the nut is
advanced
along the bar. The load which can be carried depends on the nature' of the
device
but, in practice, it is relatively easy to achieve loads of the order of four
or five tons, a
level of magnitude which is quite acceptable for temporary and immediate
support.
Once the grout has set the load resisting capability of the rock bolt is
considerably
enhanced. Initially the device 20 provides the anchoring capability. However
if the
grout abuts the device then the device, in any event, is not capable of
moving. Thus
the only way in which the rock bolt can yield is if the round bar is drawn
through the
restraining sinusoidal formations provided by the pipe 18 and, where
applicable, by
the grout which has set around the projecting portion 32 of the round bar. The
round
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bar, as has been noted, carries a deponding or lubricating agent so that the
grout
does not bond directly, in a meaningful manner, to the round bar portion 32.
The
bonding agent also reduces the likelihood of unwanted bonding between the
round
bar and the interior surface of the pipe. The grout and the pipe provide a
path
through which the round bar must be drawn in order to provide a yielding
action. In
other words the yield characteristic is provided by the round bar 22 deforming
through the shape which is formed in the grout and in the pipe and which is
attributable to the shape to which the round bar and the pipe are initially
formed,
before being embedded in the grout.
The situation should be contrasted to what would be the case if the device 20
were to
be engaged with an end of the round bar 22 and if the pipe 18 were dispensed
with.
In this instance the device and the grout would act accumulatively. The device
20
would, in any event, not be capable of moving and hence would provide an
immovable anchor for the round bar which in all probability would fail
catastrophically
instead of yielding if its breaking point were to be exceeded.
Figures 2 to 7 illustrate variations of the principle shown in Figure 1.
Figure 2 illustrates a plastic head 40 which has hook-shaped formations 42 and
which is designed to be engaged with an end of the pipe 18. The hook-shaped
formations are angled so that the head can be inserted into a hole but when an
attempt is made to withdraw the head the hook-shaped formations dig into a
wall of
the hole. This provides an immediate mechanical anchor.
Figure 3 shows an arrangement which makes use of an expansion anchor or shell
principle, and wherein an end 44 of the pipe 18 is formed into a conical
shape.
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Expansion leaves 46, which are in the? nature of wedges, are engaged with the
cone
and are held in position by means of a band 48. The construction is such that
the
cone and the leaves can be inserted into a hole but when an attempt is made to
withdraw the pipe the leaves wedge against the cone and provide a mechanical
anchor.
Figure 4 shows a spring washer 50 which has a number of hook-shaped formations
52. The washer is attached to an end of the pipe 18 in any appropriate way,
for
example by means of welding, rivets or the like. The arrangement in Figure 4
is
similar to that in Figure 2 in that the spring washer is easily inserted into
a hole 12 for
the hook-shaped formations 52 simply bend or deform to allow this movement. If
an
attempt is made to withdraw the pipe from the hole the hook-shape formations
dig
into the wall of the hole and provide a mechanical anchoring force.
Figure 5 shows that an end of the pipe 18 can, itself, be formed with hook-
shaped
formations 54 which allow the pipe 18 to be moved into a hole 12 but which
inhibit
withdrawal of the pipe from the hole by biting into a surface of the rock.
Figure 6 shows an arrangement 60 wherein the device 20, referred to in Figure
1, is
formed by a cam or wedge 62 which is attached to a leading end of the pipe at
a
pivot point 64. The pipe can easily be inserted into the hole for the wedge 62
collapses onto the pipe. When an attempt is made to withdraw the pipe from the
hole the wedge 62 digs into the surface of the hole and immediately provides a
mechanical anchor.
Figure 7 illustrates a rock bolt 70 which is used in .a similar way to what
has been
described but wherein the round bar, designated 22A, is inserted into a pipe
18A
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which is not bent in the manner showg in Figure 1. Instead the pipe is formed
with a
plurality of indentations or dimples 72 at spaced locations along its length.
A leading end 74 of the bolt 22A is formed with an enlarged or thickened head
which
is shaped into the form of a wedge.
An inner end of the pipe 18A has an anchor device 76 of the kind shown in any
of
Figures 2 to 6.
The rock bolt 70 is used in the manner described in that it is inserted into a
hole in a
body of rock and the anchor is actuated to provide a mechanical anchoring
force.
Thereafter grout is placed in the hole around the outer surtace of the pipe
and the
protruding portion of the round bar 22A. The anchor 76 provides an initial
mechanical anchoring force and once the grout has set the grout bonds to the
outer
surface of the pipe. The yielding action then arises when the bolt is loaded
above a
particular level at which the thickened end 74 is drawn through the interior
of the
pipe. When this happens the deformations 72 provide a load-resisting action
which
restrain movement of the bolt in a controlled manner, relatively to the pipe.
The invention has been described, by way of example only, with reference to
the use
of a round bar 22. In practice the bar can be replaced by a cable.