Note: Descriptions are shown in the official language in which they were submitted.
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ROCK ANCHOR
FIELD OF THE-INVENTION
This invention relates to a rock anchor and, more particularly, to a grouted rock
anchor of the type which is commonly employed in mining applications and
utilities anchoring.
BACKGROUND TO THE INVENTION
Rock anchors are widely used for the stabilisation of rock strata and for the
s anchoring of superstructures in applications such as construction, infra-structure
development, mining, tunnelling and underground space development.
Several types of rock anchors are known, each type having its own advantages
and disadvantages:
1. Mechanical end-anchors consist of elongate tendon members with
expanding shells or similar devices at one end thereof. The expanding shell
can be urged against the periphery of a borehole to provide a frictional
force which retains the tendon in position in the borehole against axial
forces exerted on the tendon. This type of anchor can be easily tensioned
and retensioned, but is unsuitable for use in either very hard or very soft
rock. In very hard rock, the frictional forces generated between the leaves
of the expansion shell and the borehole periphery are often insufficient to
prevent pull-out, while in soft rocks, typical expansion shell assemblies
cannot be secured at all.
2. Frictional anchors comprise an expandable tube which extends along theentire length of an elongate tendon. The tube can be expanded to provide
even friction along the entire length of the borehole. This type of anchor
cannot be effectively tensioned or retensioned and is also sensitive to
variations in borehole diameter.
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3. Groutable anchors typically consist of an elongated tendon which is fixed
into a borehole with cementitious or resin grout. This type of anchor relies
on the adhesive bond strength of the grout. The groutable anchors are
either full column-groutable as illustrated in Figure 1 in which the grout (1) in
the borehole (2) extends over the full length of the tendon (3), or
point-groutable where only the end portion of the tendon remote from the
borehole aperture is grouted, as shown in Figure 2.
~e anchoring strength of this type of anchor is proportional to the surface
area of the elongated tendon (3) which is in contact with the grout (1). The
anchoring strength may thus be increased by increasing the circumferential
dimension of the elongate tendon (3) as indicated in Figure 4 and/or by
increasing the volume of grout (1 ) between the tendon (3) and the periphery
of the borehole (2) as illustrated in Figure 3. Both of these solution methods
are unnecessarily expensive, as they require increased material in the
tendon or increased grout volume.
Point-groutable rock anchors can be conveniently tensioned and
retensioned, and can also be used in wet conditions, such as below water
or in wet rock.
Typically, groutable rock anchors are installed by inserting into the borehole
a grout-containing cartridge. The tendon (3) is then inserted into the
borehole (2) and is rotated by means of a driving tool in order to break the
cartridge and to mix the grout components therein. The grout is then
allowed to set in the borehole, whereafter the anchor can be tensioned as
required.
OBJECT OF THE INVENTION
It is an object of this invention to provide a groutable rock anchor which will, at
least partially, alleviate the above-mentioned difficulties.
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SUMMARY OF THE INVENTION
In accorda,lce-with this invention, there is provided a groutable rock anchor,
com~ri~ing:
an elongate tendon having proximal and distal ends, the tendon being locatable
in a borehole with the proximal end of the tendon projecting outwardly therefrom,
5 characterised in that
the tendon~as an enlarged portion extending over a portion of its length
proximate the distal end of the tendon.
Further features of the invention provide for the enlarged portion to be integral
with the elongate tendon, alternatively for the enlarged portion to be secured to
10 the elongate tendon.
Still further features of the invention provide for the proximal end of the tendon to
be looped, alternatively for the proximal end of the tendon to be threaded, and for
the proximal end of the tendon to have receiving formations for receiving a
driving tool.
Yet further features of the invention provide for the enlarged portion to be a first
endcap co-axially secured over the distal end of the tendon, and for a second
endcap to be co-axially secured over the proximal end of the tendon.
There is also provided for the end of the enlarged portion remote from the
proximal end of the tendon to be substantially sharpened, for the tendon to be a
20 rod, alternatively a rope, and for the enlarged portion to be swaged, alternatively
welded, further alternatively screwed to the tendon.
There is also provided for the rock anchor to include a base plate locatable over
the proximal end of ~he tendon, and a locking and tensioning means for retainingthe base plate on-the tendon and tensioning the tendon in use, and for the
25 locking and tensioning means to be a threaded nut, alternatively a split collet.
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BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of-the invention are described below, by way of example only, and
with refer~i ,ce to the acco,n,u~nying drawings in which:
Figure 1 is a sectional view of a prior art full column-grouted rock anchor
shown in conjunction with a borehole;
Flgure 2 is a sectional view of a prior art end-grouted rock anchor shown in
conjunction with a borehole;
Figure 3 is a sectional view of a prior art grouted rock anchor with increased
grout volume relative to the end-grouted anchor of Figure 2;
Figure 4 is a sectional view of a prior art grouted rock anchor with increased
tendon diameter relative to the end-grouted anchor of Figure 2;
Figure 5 is a sectional view of a rock anchor according to the invention;
Figure 6 is a sectional view of the rock anchor of Figure 5 shown in
conjunction with a borehole; and
Figures 7a
to 7k are side views of alternative embodiments of a rock anchor
according to the invention.
DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS
Referring to Figures 5 and 6 in which like features of the invention are indicated
by like numerals, a rock anchor is indicated generally by reference numeral (10).
The rock anchor (10) comprises an elongate tendon (3) in the form of a rod or a
20 length of wire rope. The tendon (3) has a distal end (4) and proximal end (5).
The tendon (3) has a portion (6) of enlarged diameter extending from the distal
end (4) towards the proximal end (5) of the tendon for a portion of its length.
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--5--
A ~o, liol~ (7) of the tendon (3) cl~ses( to the proximal end (5) is threaded.
The rock anchor (10) includes a sll~.s~ ially circular base plate (8) having an
axial aperture (not shown) extending therelhr~ugh. The aperture (not shown) on
the base plate (8) is sized to receive the threaded portion (7) of the tendon (3)
therethrough.
The rock anchor (10) also includes a retaining~ r~Teans (5) in the form of a nutcomplementarily threaded to engage the threaded portion (7) at the proximal end
(5) of the tendon (3).
In use, the proximal end (4) of the tendon (3) is inserted into a borehole (2) of
sufficient diameter to accommodate the thickened portion (6) of the tendon. The
thickened portion (6) of the tendon (3) is grouted into position in the borehole (2)
in the conventional manner as described above. Once the grout has set, the
base plate (8) is engaged on the threaded portion (7) at the proximal end (5) ofthe tendon (3), and the nut (9) is tightened to establish a desired tensile force in
the tendon between the base plate (8) and the grouted enlarged portion (6) of the
tendon.
It will be appreciated by those skilled in the art that the rock anchor (10) hasnumerous advantages over prior art grouted rock anchors, these being:
a) The rock anchor (10) exhibits high pull-out strength due to the increased
grout contact area provided by the enlarged portion (6) of the tendon (3) at
its distal end;
b) The rock anchor (10) enables anchoring to be effected in both soft and very
hard rock;
c) The increased pull-out strength is achieved without a concomitant increase
2s in the volume of grout required to set the rock anchor, and with a relatively
minor increase in the volume of material in the tendon; and
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d) The end-grouted rock anchor (10) can be conveniently tensioned and
rel~,lsio.~ed to suit the requir~n)e"~s of a particular application.
Numerous modifications are rossible to the above embodiment without
departing from the scope of the invention. These modifications are discussed
5 below with reference to Figures 7a to 7k, in which Figures 7a to 7e illustratevariation of the rock anchor (10) in which the tendon (3) is a rod, while Figures 7f
to 7k show variations in which the t~n~ ~t3t is a wire rope.
In Figures 7a to 7e, the enlarged portion (6) of the tendon (3) may be an
integrally formed enlargement on the tendon or, aller,lalively, may be welded or
10 screwed to the tendon at (20). The proximal end (5) of the tendon (3) may be
looped in a hook shape instead of threaded, as illustrated in Figure 7a, in order to
facilitate the attachment of cables thereto.
Where the tendon (3) is a wire rope, the thickened portion (6) is a terminating
member (21) in the form of a first endcap (21) which is co-axially swaged, at (22),
1s over the distal end (4) of the tendon (3) as illustrated in Figures 7f to 7k. A
second endcap (23) is co-axially swaged, at (24), over the proximal end (5) of the
tendon (3), as illustrated in Figures 79 to 7k. The second endcap (233 is
part-threaded.
The end of the thickened portion (6) remote from the proximal end of the tendon
20 (3) may be sharpened, at (25), to facilitate rupturing of a grout-containing
cartridge in the borehole (2) and to promote adequate mixing of the grout in theborehole.
Further, the proximal end of the tendon (3) has receiving formations (26) for
engaging a driving tool to facilitate rotation of the tendon in the borehole (2) in
2s order to mix the grout therein. The retaining means (9) may be a split collet (27)
instead of a nut.
The invention therefore provides a groutable end-anchor with increased pull-out
strength relative to prior art equivalents.
