Note: Descriptions are shown in the official language in which they were submitted.
~300940
BACKGROUND OF THE INVENTION
This invention is directed to a rock bolt system,
and embraces the process of manufacture of the rock bolt,
the improved rock bolt thus produced, including a Ground
Movement Indicator embodiment and the method of
manufacture, and of use of the system.
Rock bolting is practiced in many areas of civil
engineering ranging from excavating and tunnelling, to the
attachment by anchoring back of fabricated structures to
rock masses, and to the use of rock bolts in mining.
A wide range of rock bolts are known. In
particular, hollow rock bolts are known, such as:
Canadian Patent 999 762 Williams Nov. 16, 1976
Swiss Patent 631 782 Belloli Aug. 31, 1982
WO 86/02/25 Velikov April 10, 1986
In the case of the familiar Williams bolt, this hollow rock
bolt is manufactured by the pierce-billet method, and
initial cost is up to about ten times that of a plain,
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mechanical rock bolt. The Williams rock bolt is used for
specialty bolting, such as anchoring a machine to a cement
floor or to rock.
One of the dominant factors in selecting rock
bolts is that of cost. One of the most commonly used
underground bolts is a mechanical rock bolt, made from
solid bar and used in conjunction with an anchoring shell.
In mining, the protection and safety of men and
of equipment depends upon the integrity of the strata.
Rock bolts having expansible shell anchors are anchored
within extended bores in the back, the wall or the rock
face, for the purpose of compressing adja ~ tstrata of
the rock so as to enhance ~their strength and stability and to
virtually create a rock beam. Rock bolts also are used for
the attachment of mine screening mesh in area covering
relation primarily with the rock back, for safety purposes,
by containment of loose rock.
At certain locations in a mine, under
circumstances where greater security is required, such as
permanent passages or ways and in certain types of strata,
rock bolts are grouted into their respective rock bores.
Grouting is costly, but enhances the security of the rock
bolt, and diminishes movement in the strata by filling the
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3 ~300940
residual bore space or a selected portion, with cement or
epoxy cement, thus also providing direct load transfer
between the rock bolt and the adjoined strata, so as to
stabilize the strata.
Grouting is not generally carried out in the
working area of a normal stope. However, in areas where
acid ground water is encountered, grouting also affords
significant corrosion protection to the rock bolts.
The apparent cost of purchasing and setting rock
bolts can be readily calculated on a theoretical basis,
based on the area to be secured, the recommended density of
bolting, and hence, the bores to be drilled and the number
of bolts and shells to be procured and set. However, this
does not present a true picture of the situation, nor of
the costs involved.
Due to a number of factors such as poor initial
installation, loss of anchorage due to slippage movement of
the shell, including the local effects of blasting and
other ground tremors and displacement, experience over many
years in the field has revealed a "loss" generally of about
40% of ungrouted rock bo~ts, as initially placed.
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This situation can be largely remedied by
grouting of the installed bolts with conse~uently increased
costs. However, with the solid mechanical bolts in general
use, effective grouting requires to be undertaken at the
outset, owing to the need to provide a vent tube along the
length of the rock bclt, and extending through the bearing
plate at the rock surface, as well as the need to provide a
grout injection access through the bearing plate. Thus,
retroactive grouting cannot be effectively undertaken,
being virtually impossible, on a practical basis while the
cost of grouting on a routine basis greatly increases rock
bolting costs.
One characteristic of all known prior use rock
bolts is a total inability for an observer to ascertain
from the stope floor what is taking place in terms of
loading and displacements behind the rock face.
SUMMARY OF THE INVENTION
The present invention provides a tubular rock
bolt that is particularly suited for grouting
installations. The subject tubular rock bolt construction
lends itself to a family of tubular rock bolts most of
which have a nominal load rating closely approximating the
load ratings of existing mechanical rock bolts.
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i300940
Furthermore, the subject tubular rock bolt
incorporates standard rolled threads to enable use thereof
with standard sizes of nuts and more particularly, with
suhstantially standard shells.
In reviewing the contribution afforded by the
present invention over the prior art, in terms both of cost
and of load bearing capability performance,the subject
tubular rock bolt most closely approximates ~e ox~n
mechanical rock bolt of solid bar, and accordingly the
latter is herein adopted as the valid basis for prior art
comparison. The subject tubular rock bolt (TRB) makes
possible the adoption of a visible projecting rod-like
ground movement indicator (GMI) for installation with the
TRB when in a non-grouted condition, to provide a readily
seen visual indicator for indicating relative condition
changes having taken place between the TRB and its
associated strata subsequent to the placement of the TRB.
Such changes in the relative condition of a rock
bolt arise as a consequence of a loss in tension in the
rock bolt due to non-gripping of the shell within its bore,
or to significant extension of the rock bolt as a
consequence of strata displacement, which conditions, in
the case of prior art rock bolts, were visually
undetectable.
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6 1300940
These condition changes, ta~en singly or in
combination in the case of the subject TRB tend to cause a
relative retraction of the GMI visible outer end inwardly
into the interior of the TRB.
It has been found that a TRB incorporating the
foregoing enu~erated advantages can be manufactured at an
acceptable increase in cost over that of existing
mechanical rock bolts, using continuous weld,cold drawn
mechanical steel tube.
It has further been found that the fabricated
tube can be cold drawn, within very closs tolerances, to an
outside diameter (O.D.) particularly suited to the
formation of rolled threads thereon, for use with existing
standard nuts and shells presently used in rock bolting.
In carrying out the process, a range of sizes of
tubular bolts having a range of wall thicknesses has been
achieved, so as to provide bolts having a significant range
of load capabilities. In view of established rock bolting
practice, wherein 3/4 inch and 5/8 inch solid mechanical
rock bolts are used, the subject tubular rock bolts have a
selected tube O.D. to facilitate the rolling of a
predetermined standard thread thereon, and a selected wall
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thickness, as a function of the tensile strength of the
continuous weld cold drawn steel tube, to provide an as
installed load bearing capacity equivalent to that of a
respective one of the existing equivalent standard
mechanical rock bolts. However, it has been found ~at, due bo
the material selected, and possibly influenced by the
tubular form adopted, the subject tubular rock bolt
demonstrates great axial resilience and a capacity for
extension under cold working, such that effective
longitudinal bolt extensions under working tensile loads
are readily achieved. The strength and ductility of the
TRB are significant in maintaining the compression load in
the rock strata in the event of strata disturbance, such as
blasting in the vicinity of the installed bolt, thereby
providing acceptable performance, in use.
In addition to the foregoing recited charac-
teristics, the subject tubular rock bolt (TRB), by virtue
of its larger diameter, presents a larger peripheral
"wetted area", to which grouting can bond itself, while the
volume of grouting required is significantly diminished.
Also, the required volume of grout, on the basis
of a standard bored rock hole, is significantly less for
grouting a subject .920 inch diameter tubular rock bolt
than for grouting a 3/4 inch diameter equivalent strength
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i3009AO
plain rock bolt; and is much less for grouting a subject
tubular rock bolt than for grouting a 5~8 inch diameter
plain rock bolt. These material reductions represent
significant cost savings to off-set the higher initial
tubular rock bolt costs.
In addition to making available significant
savings in grouting quantities, the subject TRB no longer
requires the provision of an air vent tube, being self
venting through its centre, thereby assuring more reliable
grouting, by avoidance of the air locks frequently
experienced with vent tubes, in the prior art.
In the case of a 1_5/8 inch diameter rock bore, a
subject TRB effects a worthwhile savings in grout quantity,
compared with the corresponding standard 3/4 inch diameter
bolt. The present invention thus provides a tubular rock
bolt of continuous weld cold drawn steel having a
predetermined outside tubular diameter to accept a
predetermined rolled thread form. The subject tubular rock
bolt has a wall of predetermined thickness, to afford an
initial tensile load capacity substantially equal to that
of an existing predetermined solid section mechanical rock
bolt, wherein the subject tubular rock bolt has a
comparable extensibility to that of the referenced solid
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section mechanical rock bolt, and provides a si~ilar
ability of maintaining an effective load and reliability,
in use.
Manufacture of the subject continuous weld, cold
drawn tube bolt is effected by first manufacturing a larger
diameter cold drawn steel tube. A steel ribbon is roll
formed at high temperature into a tube formation, and the
closure edges thereof compressed together, generally under
airjet or oxyjet heating to form a continuous butt welded
tube wall, to provide a high quality continuous butt
jointed seamed tube, at comparatively low cost. The
thus-formed tube is then cold mandrel-drawn through sizing
dies until the required outer diameter (O.D.) and wall
thickness are achieved, by cold working. In order to
achieve the subject tube bolt, optimized OoD~ ~s have been
adopted, wherein, working within the constraints of
standard rock bolt practice, threads can be rolled thereon
for use with standard sized nuts and shells as used with
prior art mechanical rock bolts.
In accordance with the present invention a family
of five different tubular rock bolts, TRB 1, 2, 3, 4 and 5
have been evolved to-date and are in course of development:
1300940
Product
Designation TRB 1TRB 2 TRB 3 TRB 4 TRB 5
Dimensions .690"x.150" .920"x.195".920"x.195".920"x.260" .920x.260"
(OD x ~all thickness) swaged swaged
Rock bore size 1-1~4"1-5/8" 1-1~4" 1-5/8" 1-1/4"
Replaces 5/8" bolt5/8" bolt3/4" bolt3/4" bolt
Adoption of a 0.26 inch wall thickness with the
large .92 inch diameter tube provides a high initial load
capacity tubular bolt having substantially the same initial
load bearing capacity as a 3/4~inch solid bar mechanical
rock bolt, while accommodating standard rolled threads to
receive a standard l-inch threaded rock bolt shell, for
anchoring purposes.
A further embodiment of the .920 inch diameter
tubular bolt having a thinner wall, namely of 0.195-inch
thickness, provides an initial applied load bearing
capacity approaching that of a 5/8-inch diameter solid rock
bolt. This tube size also accepts a nominal 1 inch rolled
thread, and exhibits characteristic extensibility under
load.
In a further embodiment, it has been established
that an end of a tubular rock bolt, such as the 0.92-inch
tube, may be swaged, pointed or otherwise reduced to a
smaller O.D., to accept a 3/4-inch rolled thread thereon,
for use with 3/4-inch nuts and 3/4-inch standard shells.
11 1300940
It will be understood that the subject invention
is not limited to the foregoing family of tubular rock
bolts, and that different diameters and wall thicknesses
may be adopted to meet the needs of the industry.
A further aspect of the present invention is the
incorporation, with a tubular rock bolt, of a low cost,
effective Ground Movement Indicator (GMI). Thus, an
elongated rod-like GMI having the rod portion thereof
freely suspended within the bore of the tubular rock bolt
has the remote distal end thereof independently secured
within the distal end of the strata bore, and the free
proximal end of the rod portion initially extending
outwardly from the bolt proximal end, as the visible GMI.
Upon initial loading, by tensioning the bolt, in applying a
predetermined working load thereto, a viewable portion of
the GMI rod can be left projecting at the proximal end.
Upon significant subsequent extension of the subject bolt
under load, the projecting length of the GMI rod becomes
sensibly and substantially proportionately diminished, to
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provide visual indication of significant change in the
condition of the rock bolt relative to its strata, in its
position at the rock back, wall or face, with a
corresponding indication of a possible significant change
having taken place in the related rock strata. Thus, the
extent or degree of disappearance of the GMI rods in a
given area of a stope could indicate to some extent the
probable degree of danger of a rock fall occurring.
Generally, low cost wires or plastic rods are
adopted in the role of GMI indicator rods.
In the preferred embodiment the GMI includes an
indicator rod having its own anchor located above the shell
to optimize its integrity, by enabling the rod to be
independently anchored in the strata. The bridge portion
of the bail of the shell is recessed to accommodate the
distal bolt end of the TRB, and has a central aperture
therein through which the GMI rod extends, when installed.
The apertured bail bridge serves a valuable purpose in
facilitating rupture of the bail in the event that the TRB
tube is accidentally screwed upwardly through the shell, so
as to rupture the bridge.
Rock bolts are provided in a range of lengths and
sizes, and 5 foot and 6 foot rock bolts are generally
considered standard production bolts. The subject GMI rods
can be provided as a standard item, having a rod size,
diametrically compatible with a range of tube bolt ID's,
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13~0940
and, being readily shortenable by a simple clipping
operation, can be provided in a standard length, such as a
9 foot or longer rod, suitable for use with a 6 foot or
longer bolt, and then shortened, if required to be used
with 5 foot or shorter bolts. Generally the rod is 3 or 4
feet longer than the tube bolt with which it is used.
When it is recognized that stope roof heights in
the range of 12 to 20 feet are commonplace, it will be
understood that to be effective the protruding rod end of a
GMI rod must be clearly visible, and a significant relative
displacement thereof must be plainly evident, preferably
when viewed from stope floor level, by suitable
colouration, such as fluorescent paint.
In the preferred GMI embodiment, having the rod
portion thereof supported independently of the TRB shell,
the distal end of the GMI rod can be threadedly disengaged
by unscrewing it from its anchor portion, to permit
withdrawal of the GMI rod from an installed TRB, so that
grouting of the TRB can then proceed without any impedance.
13 1300940
More particularly, this invention provides the
method of manufacturing a tubular rock bolt comprising
the steps of: forming a steel strip in heated condition
into a substantially circular section; joining the
adjacent ends thereof in welded relation: cold drawing
the section to a predetermined smaller diameter, having
a predetermined wall thickness; applying a rock anchor
means in secured relation adjacent one end of the rock
bolt by rolling threads on a surface portion of the rock
bolt, said threads being engageable by said rock anchor
means, and providing load transfer means in secured
relation with the rock bolt other end by rolling further
threads on said rock bolt other end, said load transfer
means including nut means secured in threaded adjustable
relation with said further threads, and load transfer
washer means encompassing said other end, in thrust
transfer relation with said nut means.
Further this invention provides a method of forming
a tubular rock bolt, comprlsing the steps of:
a) forming a steel strip ha~ing parallel
longitudinal edges into a laterally curved shape,
abutting the edges, and welding the abutting edges by
electric resistance welding, in order to form an
oversize, thick-walled tube of substantially annular5 section,
b) cold drawing the tube by elongation through
dies to produce a tube with smooth inner and outer walls
and an internal diameter of at least about 0.34 inches,
c) cutting a portion of the tube to form a bolt0 having first and second ends,
d) rolling threads on both ends of the bolt,
e) providing a rock anchor attachment which
includes a portion having an internally threaded passage
matching the rolled thread on said first end of the
bolt, and removably threading said attachment onto said
first end, and
f) providing load transfer means for said second
end, said means comprising an internally threaded member
13b ~3~940
for threadably receiving said second end, and for
transferring a load from the bolt, through a suitable
means, to the surrounding rock structure.
Still further, this invention provides a hollow,
thin walled tubular rock bolt having an axial working
load capacity substantially equal to a standard solid
mechanical rock bolt of predetermined smaller diameter,
said tubular rock bolt providing an axial through
passage, in use to facilitate the optional use of a
ground movement indicator rod in cooperation with the
rock bolt, said rock bolt being fabricated from a steel
strip having parallel longitudinal edges by forming the
strip into a laterally curved shape, abutting the edges,
and welding the abutting edges in order to form an
oversize, thick-walled tube of substantially annular
section, cold drawing the tube by elongation through
dies to produce a tube with smooth inner and outer
walls, and cutting a portion of the tube to provide said
tubular rock bolt.
Finally, this invention provides, in a rock bolt
for use in inserted, tensioned relation within a bore in
a rock face, said rock bolt being of predetermined
length and including a combination with a shell means
having an expansible distal head portion to expand, in
use, into gripping relation within a remote inner
surface of said bore, and a bearing plate and nut means
on the proximal end of the rock bolt for tensioning the
rock bolt and pulling the bearing plate in compressed
relation against the rock face; the rock bolt having a
passage therein extending axially for a major portion of
said predetermined length, the improvement comprising
ground movement indicator means having a viewable
proximal end extending outwardly of the proximal end of
the rock bolt, and an elongated support portion
extending within the rock bolt passage and supported, in
use at substantially said rock bolt distal end, in
independently secured relation therein, to provide
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i300940
13c
external visual indication of axial extension of said
rock bolt.
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14 13 ~ 940
BRIEF DESCRIPTION OF THE DRAWINGS
Certain embodiments of the invention are
described, reference being made to the accompanying
drawings, wherein;
Figure 1 is a side view of a section of rock face, showing
an embodiment of the subject tubular rock bolt ~TRB)
installed in tensioned, anchored relation therein;
Figure 2 is a like view, showing a tubular rock bolt
embodiment incorporating a ground movement indicator (GMI)
therewith;
Figure 3 is an end view, from below, showing a TRB
installation having a grouting tube inserted upwardly
through the plate washer of the installation;
Figure 4 is a plan view of the GMI rod anchor arrangement;
and
Figure 5 is a side view showing a GMI rod and associated
anchor, together with bail and leaves portions of the
shell, and a portion of the tubular rod bolt.
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i3009A0
DETAILED DESCRIPTION OF THE INVENTION
Referring to Figures 1, 2 and 5, the rock bolt
assembly 10 comprises a cold drawn tube portion 12 having
threaded end portions 14, 16 with rolled threads 18
thereon, and an axial through passage 19. A standard type
of shell 20 has leaves portions 22, having serrated
gripping surfaces 24 on the exterior thereof. Shell wedge
body portion 26 is threadedly mounted at the distal end, on
the rolled threads 18 of the tube end 16.
Referring more particularly to Figures 1 and 2,
wi~h the bolt assembly 10 inserted into the rock bore 30,
the leaves portions 22 of the shell 20 bear against the
rock bore, and upon downward retraction of the tube portion
12, the wedge body 26 is drawn downwards axially into
expanding relation with the leaves 22 of shell 20, causing
them to engage the gripping serrations 24 thereof with the
inner surface of rock bore 30.
The bottom tensioning nut 32 bears against the
hardened round washer 34 and bearing plate 36, to draw the
tube portion 12 axially outwardly in anchor loading
relation. Thus, the tube portion 12 is tensioned, while
the associated surrounding roc~ portion is compressed, and
thereby strengthened.
1~ ~ 300940
In some instances, grout may be injected upwardly
through the bore 19 of tube 12, and the GMI 40 is not used.
It is preferred to grout, using an offset passage
35' through the bearing plate 36 for upward insertion of
grouting tube 35 therethrough, with displaced air exiting
downwardly through the centre passage 19 of tube 12. In
such an arrangement, only a short length of grouting tube
35 requires to be inserted.
Referring to Figures 4 and 5, showing particulars
of a first GMI embodiment, the threaded upper (distal) end
portion 16 of a subject rock bolt 12 is shown.
The elements of the shell are referred to above.
The rod 40 of GMI 54, having a cruciform anchor
portion 60 secured to the rod 40 by threads 58 is inserted
downwardly into passage 19 of bolt 12. The bail portion 28
of shell 20 has a central aperture 29 therein, for passage
of rod 40 therethrough. The surface 40' of bail portion 28
is recessed to receive the distal end 16 of tubular bolt
12, in the event of misadvertant rotation of bolt 12
causing the bolt 12 to advance axially through wedge body
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17 i300940
26 of shell 20. Upon such an occurrence, with the leaves
22 being set in the rock bore 30, the bail portion 40'
ruptures readily, adjacent to aperture 29.
The rod retaining thread 58 being formed in
anchor portion 60 is secured against rotation, and it is
possible to unscrew and withdraw the GMI, rod portion 40
from out of an installed TRB, so that initial non-grouted
installation of a TRB incorporating a GMI can be followed
by removal of the GMI rod 40, and completion of normal
grouting procedures.
The proximal ends of the GMI rods 40 can be
clipped off at a standard exposed length, on initial
installation, subsequent to the tubular rock bolt 12 being
tensioned to the desired initial value of working load.
In Figures 2 and 5, the GMI rod 40 has the inner
end portion 42 thereof supported independently above the
distal end 16 of tube portion 12 by way of cruciform spring
clip 60 which engages the rock bore 30. This has the
benefit of optimizing the independence of GMI 54.
