BACKGROUND OF THE INVENTION
[0001] This invention relates to a rock bolt and is particularly concerned with a rock bolt
which can be used with an anchoring composition.
[0002] The expression "anchoring composition" is herein used to refer to a resinous mixture,
a cementitious mixture or an equivalent mixture which is usable, as is known in the
art, to secure a rock bolt in a borehole.
[0003] An anchoring composition is normally provided in two parts enclosed in separate frangible
containers which can be broken by mechanical action of a rock bolt. The contents of
the containers are mixed, in situ, by rotation of the rock bolt whereafter a setting
process takes place. The rock bolt is then adhered in position to a surface of a borehole
in which the composition and rock bolt are located.
[0004] The container, made from an appropriate flexible material, is pushed into the borehole
and this is followed by insertion of the rock bolt into the borehole. The flexible
material can normally be penetrated with ease by a leading end of the rock bolt. The
material should be adequately shredded so that a maximum release of its contents occurs.
Some materials which are used are, however, resistant to shredding. In one instance
a fabric-type material is used and it can occur that as the rock bolt is inserted
into the borehole the fabric is pushed by the rock bolt to a blind end of the hole.
A build-up of the fabric at this end of the borehole can prevent complete insertion
of the rock bolt into the borehole. Also, it might occur that the anchoring composition
is not fully released from the flexible material. Another problem which can arise
is that the flexible material is pierced by the rock bolt but then wraps around a
shank of the rock bolt and prevents the anchoring composition from bonding directly
to the rock bolt.
[0005] An object of the present invention is to provide a rock bolt which attempts to address
the aforementioned factors.
SUMMARY OF INVENTION
[0006] The invention provides a rock bolt which includes an elongate shank with a leading
end and a trailing end, a shear device fixed to the trailing end, the shank including
a frusto-conical section at the leading end, and a shredding and mixing structure
which extends from the frusto-conical section.
[0007] The shear device at the trailing end of the shank may be of any appropriate kind.
The shear device should shear when it is used to impart torque at a predetermined
level to the shank so that the shear device is then releasable from the shank. The
ability to transmit torque to the shank is required to rotate the shank so that mixing
of an anchoring composition can take place effectively. The device may include a nut
which is theadedly engaged e.g. with a left-hand thread with the shank and which is
then fixed to the shank using a shear pin which traverses at least part of the shank
and the nut.
[0008] Preferably the frusto-conical section is formed integrally with the shank, for example
in a forging process.
[0009] The frusto-conical section may terminate in a substantially planar surface which
is transverse to a longitudinal axis of the shank. The shredding and mixing structure
may project from this surface in a direction which is more or less parallel to the
longitudinal axis.
[0010] The shredding and mixing structure may include a blade which, preferably, is centrally
positioned on the planar surface. The blade, in outline (from one side), may be square
or rectangular. A desirable aspect here is that each corner of the blade, remote from
the planar surface, should form a right angle and, inherently, the corner should be
sharp. Thus the corners are suited for piercing a flexible container which contains
ingredients for an anchoring composition.
[0011] The blade may be flanked by mixing formations so that the structure is of cruciform
shape (cross-shaped), viewed end-on.
[0012] In one preferred form of the invention the shank is formed from round bar with a
diameter of 16 mm. The round bar is made from steel with a minimum yield strength
of 580 mpa. With a shank of this size the frusto-conical section may have a maximum
diameter of the order of 22 mm. The length of the blade measured in an axial direction
of the shank may be of the order of 30 mm.
[0013] The shank may be coated with a release medium, as is known in the art so that it
can debond from an anchoring composition (once set) and can then yield under load.
The frusto-conical section at the leading end of the shank then acts as an anchor
for the shank.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The invention is further described by way of example with reference to the accompanying
drawings in which:
Figure 1 is a perspective view of a rock bolt according to the invention;
Figure 2 illustrates a part of the rock bolt of Figure 1, which is enclosed in a circle
marked 2;
Figure 3 is a side view of the rock bolt of Figure 1;
Figure 4 shows a part of the rock bolt which is enclosed in a circle marked 4 in Figure
3; and
Figure 5 is an end view of the rock bolt taken in the direction of an arrow marked
5 in Figure 4.
DESCRIPTION OF PREFERRED EMBODIMENT
[0015] Figure 1 of the accompanying drawings is a perspective view of a rock bolt 10 according
to the invention. Referring as well to Figure 3, which illustrates the rock bolt from
one side, the rock bolt includes an elongate shank 12 which has a trailing end 14
and a leading end 16. The leading end is shown from one side in Figure 4 and an end
view of the leading end is shown in Figure 5. Figure 2 shows the leading end in perspective.
[0016] In one preferred form of the invention the shank is made from round bar with a diameter
of about 16 mm and has a predetermined length. The shank preferably has a minimum
yield strength of 580 mpa.
[0017] The trailing end 14 has a left-hand thread and a nut 20 is threadedly engaged therewith.
A shear pin 22 extends through a passage bored radially into the nut and enters an
in-register passage, not shown, at the trailing end 14. In this way the nut and shear
pin form a shear structure which, initially, is fixed immovably to the shank.
[0018] The leading end 16 has an integrally forged frusto-conical section 30. This section,
which typically has an axial length 32 of the order of 30 mm, has a maximum diameter
34 at one end which is of the order of 22 mm. An end of the frusto-conical section
presents a substantially planar surface 36 which is at a right angle to a longitudinal
axis 38 of the shank. Shredding and mixing structure 40, forged integrally with the
section 30, extends from the planar surface 36. This structure includes a shedding
blade 42 and mixing members 44 which project outwardly from and transversely to a
base 46 of the blade. The blade and mixing members, viewed end-on present a cruciform
shape.
[0019] The blade has a length 48, taken from the planar surface 36, which is of the order
of 30 mm. Outer corners 50 of the blade are sharp and each corner defines a right
angle. This is a feature which allows the blade to pierce a flexible housing, which
contains an anchoring composition, with ease.
[0020] The mixing members 44, viewed from one side (see Figure 4) have a relatively short
length 52, of the order of 3 mm or 4 mm i.e. less than about 14% of the length of
the blade. Sides 54 of the mixing members, and sides 56 of the base of the blade are
chamfered (see Figure 5).
[0021] In use of the rock bolt an anchoring composition in a flexible container, not shown,
is inserted into a borehole formed in a body of rock. The leading end 16 of the rock
bolt is then inserted into the borehole and the rock bolt is pushed fully into the
hole. The shear structure at the trailing end 14 is engaged with a device which rotates
the shank. As the shank is pushed home and rotated the blade 42 easily penetrates
the flexible container. Due to its size and the sharp corners 50 the blade, upon rotation
of the rock bolt, rapidly shreds the container irrespective of the material from which
it is made. The anchoring composition inside the container is released and is mixed
by ongoing rotation of the blade. The mixing members 44 which are simultaneously rotated
help substantially in this regard.
[0022] The mixing members are relatively small compared to the blade and the likelihood
that these members, which has chamfered sides 54, will entrain parts of the flexible
container is remote. The blade on the other hand shreds the container and, as noted,
continues with the mixing process.
[0023] The anchoring composition sets fairly rapidly and starts bonding to the shank and
the leading end. The rotational force required to rotate the shank increases and ultimately
a point is reached at which the shear pin 22 shears. It is then no longer possible
to impart torque to the shank. Typically the nut 20 is automatically unscrewed from
the shank. Alternatively the nut is manually released from the shank. A load-spreading
washer, if required, can then be engaged with the shank whereupon the nut is re-engaged
with the shank.
[0024] The structure at the leading end 16 of the shank has been found, in practice, to
be highly effective in shredding a flexible container and, then, in mixing an anchoring
composition released from the container, while addressing the problems referred to
in the preamble hereof.
[0025] If desired at least part of the shank can have a de-bonding agent applied to it.
For example part of the shank can be coated with a thin plastic layer. The anchoring
composition then does not bond directly to the shank but acts primarily against the
frusto-conical section 30. This feature allows the rock bolt to yield under load.
1. A rock bolt which includes an elongate shank with a leading end and a trailing end,
a shear device fixed to the trailing end, the shank including a frusto-conical section
at the leading end and a shredding and mixing structure which extends from the frusto-conical
section.
2. A rock bolt according to claim 1 wherein the frusto-conical section terminates in
a substantially planar surface which is transverse to a longitudinal axis of the shank
and the shredding and mixing structure projects from the substantially planar surface.
3. A rock bolt according to claim 1 or 2 wherein the threading and mixing structure includes
a blade which is centrally positioned on the planar surface.
4. A rock bolt according to claim 3 wherein the blade, in outline, is square or rectangular
and each corner of the blade, remote from the planar surface, forms a right angle.
5. A rock bolt according to claim 3 or 4 wherein the shredding and mixing structure includes
mixing members which project outwardly from and transversely to a base of the blade.
6. A rock bolt according to claim 5 wherein the mixing members have a length in an axial
direction of the shank taken from the substantially planar surface which is less than
14% of the length of the shank, taken in the axial direction, from the substantially
planar surface.
7. A rock bolt according to any one of claims 1 to 6 wherein at least part of the shank
is coated with a release medium.
8. A rock bolt according to any one of claims 1 to 7 wherein the shank is made from round
bar and the frusto-conical section and the shredding and mixing structure are forged
integrally with the shank.