U.S. patent application number 10/761872 was filed with the patent office on 2005-07-21 for yielding strata bolt.
Invention is credited to Fergusson, Jeffrey Robert.
Application Number | 20050158127 10/761872 |
Document ID | / |
Family ID | 34750279 |
Filed Date | 2005-07-21 |
United States Patent
Application |
20050158127 |
Kind Code |
A1 |
Fergusson, Jeffrey Robert |
July 21, 2005 |
Yielding strata bolt
Abstract
The present invention discloses a rock bolt having a tendon (1,
201) which is able to yield and thereby control movement of
unstable rock strata into which the bolt is installed. The tendon
has a portion which is provided with a grout slippage mechanism,
preferably in the form of a tube (8) which surrounds the tendon. A
two part grout anchor (10, 100, 200) is clamped to the tendon. The
grout anchor is embedded in grout and is therefore essentially
immovable. The tendon passing through the anchor is worked and
thereby dissipates energy. The tendon can be either a multi-strand
cable (1) or a solid bar (200). Various forms of anchor (10, 200)
are disclosed with either complementary recess(es) (14) and
protrusion(s) (15) or opposed protrusions (115) which form pinch
point(s). A one piece grout anchor (300) is also disclosed.
Inventors: |
Fergusson, Jeffrey Robert;
(Glenorie, AU) |
Correspondence
Address: |
Bell, Boyd & Lloyd LLC
P.O. Box 1135
Chicago
IL
60690-1135
US
|
Family ID: |
34750279 |
Appl. No.: |
10/761872 |
Filed: |
January 21, 2004 |
Current U.S.
Class: |
405/259.5 ;
405/259.1; 405/259.4 |
Current CPC
Class: |
E21D 21/008 20130101;
E21D 21/0033 20130101; E21D 21/0026 20130101 |
Class at
Publication: |
405/259.5 ;
405/259.1; 405/259.4 |
International
Class: |
E21D 020/00; E21D
021/00 |
Claims
1. A yielding grouted rock bolt to control the movement of unstable
rock strata into which the bolt is installed, said bolt comprising
an elongate tendon, a portion of said tendon having a grout
slippage means, and a grout engaging anchor fitted to said tendon
portion and thereby at least partially deforming same, whereby in
yielding said tendon portion passes through said anchor and is
worked thereby.
2. The bolt as claimed in claim 1 wherein said grout slippage means
comprises a tube surrounding said tendon portion.
3. The bolt as claimed in claim 2 wherein said tendon is formed
from metal and said tube from plastics.
4. The bolt as claimed in claim 1 wherein said anchor comprises a
body engageable with said grout and formed in two parts which are
clamped together over said tendon portion to fit said anchor
thereto.
5. (canceled)
6. The bolt as claimed in claim 5 wherein said parts include a
plurality of complementary protrusions and recesses.
7. (canceled)
8. The bolt as claimed in claim 4 wherein said two parts are
substantially identical.
9. The bolt as claimed in claim 4 wherein said parts are maintained
clamped together by keeper rings shaped to mate with said
parts.
10. The bolt as claimed in claim 1 wherein said tendon comprises a
multi-strand cable.
11. The bolt as claimed in claim 1 wherein said tendon comprises a
bar.
12. The bolt as claimed in claim 1 and having a plurality of said
grout engaging anchors.
13. A two part rock bolt anchor adapted to be fitted to a tendon of
a rock bolt, said anchor comprising a body engageable with grout
into which said bolt is embedded, and having two parts shaped to be
clamped together over said tendon, wherein said tendon moves
through said parts in yielding.
14. (canceled)
15. The anchor as claimed in claim 14 wherein said parts include a
plurality of complementary protrusions and recesses.
16. (canceled)
17. The anchor as claimed in claim 13 wherein said two parts are
substantially identical.
18-21. (canceled)
22. A rock bolt anchor adapted to be pressed onto a tendon of a
rock bolt, said anchor comprising a body engageable with grout into
which said bolt is embedded, and said anchor being shaped to be
press fitted to said tendon, wherein said tendon moves through said
parts in yielding.
23. The rock anchor as claimed in claim 22 and comprising a tube
having an interior sized to receive said tendon.
24. The rock anchor as claimed in claim 23 wherein a portion of
said tube is crushed to press fit said tube onto said tendon.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to strata bolts and, in
particular, to a yielding strata bolt intended to control the
movement of unstable rock strata into which the bolt is installed.
The tendon of the bolt can either be a solid rod or a flexible
cable.
BACKGROUND ART
[0002] Rock strata are liable to move as a result of various
developments including mine-induced seismicity, the excavation of
perimeter rock, minor earthquakes, and the like. Some such
movements are termed "rock bursts".
[0003] In general, regular rock bolts are insufficient to withstand
such movement and snap. In the past various proposals have been
made. One such proposal is the so called DURABAR or DURABOLT (South
African Patent No. 94/2177) invented by D Ortlepp which provides a
heavy solid steel bar with a wriggle-like deformation. The bar is
grouted in place at installation. As a consequence, in the event of
ground movement, the deformed portion of the bar pulls through the
grout and this absorbs a considerable amount of energy.
[0004] A similar arrangement is to shape the far end of the bar
into a conical form which is embedded in grout. The shank of the
bar is coated with wax which means that this part of the bar does
not bond with the grout. In the event of excessive forces being
applied to the bar, the conical end is forced or pulled through the
grout. Again this absorbs a considerable amount of energy.
[0005] An alternative arrangement is to insert a mild steel slug
within a multi-strand steel cable. A tapered sleeve is then placed
over the cable. In the event of rock movement, the intention is to
extrude the slug through the cable wires which are held in place by
the tapered sleeve thereby giving a high pull through force and
absorbing a considerable amount of energy. This arrangement is
difficult to use in such a way as to give reproducible results and
is time consuming to assemble.
OBJECT OF THE INVENTION
[0006] The object of the present invention is to provide a yielding
cable bolt which is easy to fabricate and assemble and which
provides a substantially predictable and repeatable result.
SUMMARY OF THE INVENTION
[0007] According to a first aspect of the present invention there
is disclosed a yielding grouted rock bolt to control the movement
of unstable rock strata into which the bolt is installed, said bolt
comprising an elongate tendon, a portion of said tendon having a
grout slippage means, and a grout engaging anchor fitted to said
tendon portion and thereby at least partially deforming same,
whereby in yielding said tendon portion passes through said anchor
and is worked thereby.
[0008] In accordance with a second aspect of the present invention
there is disclosed a two part rock bolt anchor adapted to be fitted
to a tendon of a rock bolt, said anchor comprising a body
engageable with grout into which said bolt is embedded, and having
two parts shaped to be clamped together over said tendon.
[0009] In accordance with a third aspect of the present invention
there is disclosed a method of permitting a grouted rock bolt
having a tendon to yield to control the movement of unstable rock
strata into which the bolt is installed, said method comprising the
steps of:
[0010] (i) providing a portion of said tendon with grout slippage
means;
[0011] (ii) fitting at least one grout engaging anchor to said
tendon and thereby at least partially deforming same;
[0012] (iii) installing said rock bolt in a blind hole drilled in
said rock strata;
[0013] (iv) introducing flowing hardenable grout into said hole to
surround said bolt tendon and said anchor(s); and
[0014] permitting said tendon portion to move through said grout
but be worked by movement of said portion through said anchor(s)
which is/are substantially immobilized in said grout.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Embodiments of the present invention will now be described
with reference to the drawings in which:
[0016] FIG. 1 is a perspective view of a bulge portion of a prior
art multi-strand steel cable;
[0017] FIG. 2 is a longitudinal view, partly in section, of the
cable portion of FIG. 1 installed as a rockbolt;
[0018] FIG. 3 is an exploded perspective view of a grout engaging
anchor;
[0019] FIG. 4 is an exploded perspective view of the anchor of FIG.
3 during assembly onto a portion of a cable;
[0020] FIG. 5 is a perspective view showing the assembled grout
engaging anchor;
[0021] FIG. 6 is a longitudinal sectional view through the
assembled anchor of FIG. 5;
[0022] FIG. 7 is a view similar to FIG. 6 but of a second
embodiment but of the anchor of FIGS. 3 to 6;
[0023] FIG. 8 is an exploded perspective view illustrating a
yielding cable bolt in accordance with a first embodiment of the
present invention;
[0024] FIG. 9 is a longitudinal view, partly in section,
illustrating the bolt of the type shown in FIG. 8 after
installation;
[0025] FIG. 10 is a view similar to FIG. 8 but illustrating a
yielding cable bolt of a second embodiment;
[0026] FIG. 11 is a view similar to FIG. 9 but of a cable bolt of
the type shown in FIG. 10;
[0027] FIGS. 12 and 13 are views similar to FIGS. 4 and 5
respectively but of a still further embodiment of the anchor and
bolt; and
[0028] FIG. 14 is a perspective view of yet another embodiment of a
grout engaging anchor.
DETAILED DESCRIPTION
[0029] Cable bolts are traditionally made from multi-strand steel
cable 1 such as that illustrated in FIG. 1. The cable is
conveniently bulged at 2 in known fashion by gripping the cable 1
at two spaced apart locations and forcing the gripped regions
together to permanently spring out, or bulge, the strands 3.
[0030] As illustrated in FIG. 2, the intention of such a bulge 3 is
to enable grout 5 which is normally used to surround the cable 1,
to better the grip the cable 1 and so provide good keying between
the cable 1 and the grout 5. Such grout 5 is often installed by
means of a tube 6 of inexpensive plastics material through which
the cable 1 passes. An alternative to bolts with a cable tendon are
bolts with a solid tendon. One such bolt is described in Australian
Patent No. 669,393 (WO 94/05900) and known as the "CT" bolt.
[0031] Turning now to FIGS. 3 to 6, a grout engaging anchor 10
proposed which is fabricated from two complimentary shells 11 and
12, each of which forms half of the anchor 10. The shell 11 is
provided with a single recess 14 whilst the shell 12 is provided
with a single protrusion 15. The recess 14 and protrusion 15 are of
complimentary shape. In addition, each shell 11 and 12 is provided
with a half boss 17 at each end.
[0032] As indicated in FIG. 4, the anchor 10 is assembled by
passing the cable 1 through a close fitting tube 8 of inexpensive
plastics material. The tube 8 is preferably manufactured from high
density polyethylene (HDPE) and is widely used for irrigation
purposes. The two shells 11 and 12 are then positioned as indicated
in FIG. 4 and clamped together so as to securely grip the cable 1
within the anchor 10. With the two shells 11 and 12 clamped
together, a keeper ring 19 is passed over the two adjacent half
bosses 17 at each end of the anchor 10. With both keeper rings 19
in place, the clamping force on the anchor 10 can be released since
the half bosses 17 are then maintained within the keeper rings 19.
This is the situation illustrated in FIGS. 5 and 6.
[0033] FIG. 7 illustrates a second embodiment of the anchor 100 in
which the keeper rings 19 are as before but each of the shells 111
and 112 is provided with a recess 114 and a protrusion 115. As will
become apparent hereafter, the purpose of the anchor 100 of FIG. 7
is to provide a greater degree of work before the cable 1 can be
passed there through.
[0034] Turning now to FIG. 8, a first embodiment of a yielding
cable bolt 21 is illustrated. The tendon 22 of the bolt 21 is
fabricated from the multi-strand steel cable 1 and the near end is
provided with the threaded end fitting 23 which cooperates with a
load plate 24, grout injector 25 and nut 26. The grout injector 25
works in the general manner described in the abovementioned
Australian Patent No. 669,393 (WO 94/05900). In the embodiment
illustrated in FIG. 8, a single anchor 10 is secured adjacent the
free end of the bolt 21, however, in the embodiment illustrated in
FIG. 9 a pair of spaced apart anchors 10 are so secured.
[0035] In the particular embodiment illustrated in FIG. 9, three
strata 30, 31 and 32 are illustrated and strata 30 and 32 are
relatively strong whereas stratum 31 is relatively weak and liable
to movement. The tube 8 covers the cable 1 essentially throughout
the stratum 32 but does not cover the cable 1 essentially
throughout the strata 30 and 31. As a consequence, there is good
keying between the cable 1 and the grout 5 in the area of strata 30
and 31 but the far end of the cable bolt 21 is itself able to move
relative to the grout 5 if necessary, notwithstanding that the two
anchors 10 are securely fixed within the grout 5 within the stratum
32.
[0036] As a consequence, in the event that strata 30 and 31, for
example, move to the left as seen in FIG. 9, the cable 1 at the
near end of the cable bolt 21 will move with the strata 30 and 31
due to the keying between the cable 1 and the grout 5. However at
the far end of the cable bolt 21 the cable 1 covered by tube 8 is
free to move relative to the grout 5 but the grout anchors 10
remain firmly secured relative to the grout 5. As a consequence,
the cable 1 is plastically deformed by the anchors 10 as the cable
1 moves past the interengaged recesses 14 and protrusions 15.
[0037] As a result, considerable mechanical work is performed in
moving the far end of the cable bolt 21 through the anchors 10. In
this way, a considerable amount of energy is able to be rapidly
dissipated thereby ensuring that the cable bolt 21 yields and
absorbs the energy, but does not break.
[0038] FIGS. 10 and 11 illustrate a second embodiment which is
essentially as before save that prior art tube 6 is provided
adjacent the near end of the cable bolt 21 and the far end of the
cable bolt 21 is provided with an expansion anchor 35 which is
formed as part of the anchor 10, or anchor 10 closer(est) to the
far tip of the cable. The expansion anchor 35 enables installation
of the bolt so as to permit post tensioning deforms the tendon 301
thereby creating a pinch point. When a sufficient tensile load is
applied to the tendon 301, the tendon 301 pulls through the anchor
300 and is plastically deformed thereby dissipating energy. The
deformed anchor 300 remains fixed in the surrounding grout (not
illustrated in FIG. 14). If desired, the anchor 300 can be deformed
at a number of longitudinally spaced apart locations thereby
forming a series of pinch points which are preferably of increasing
severity moving away from the far end of the bolt.
[0039] The foregoing describes only some embodiments of the present
invention and modifications, obvious to those skilled in the art,
can be made thereto without departing from the scope of the present
invention.
[0040] The term "comprising" and its grammatical variations as used
herein are used in the sense of "including" or "having" and not in
the exclusive sense of "consisting only of".
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