AU626111B2 - Tubular rock bolt - Google Patents

Description

r 0 **6 0 o COMMONWEALTH OF AUSTRAL 2 r 1 Patents Act 1952-1969 2 6 r1 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE: Class Int. Class Application No Lodged Complete Application No Specification Lodged Published Priority: Related art: TO BE COMPLETED BY APPLICANT Name of Applicant: Address of Applicant: Actual Inventors: Address for Service: STELCO INC.

Stelco Tower, 100 King Street West, PO Box 2030, Hamilton, Ontario L8N 3T1, Canada ROBERT B. BROOKS, ALLEN C. HUNSBERGER and ALAN R. BURGESS Care of COLLISON CO., 117 King William Street, Adelaide, South Australia, 5000 Complete Specification for the invention entitled: TUBULAR ROCK BOLT The following statements is a full description of this invention, including the best method of performing it known to us: L i 'il :;il

B

1A BACKGROUND OF THE INVENTION 0 S. 0

S.

@0 S@ a *r 5 0@i 0000 0

*SS@

@000 0*

SS

0 505 S 5g 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 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 10 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, being a thick walled structure, and initial cost is up to about ten times that of a plain, mechanical rock 20 bolt. The Williams rock bolt is used for specialty bolting, such as anchoring a machine to a cement floor or to rock.

t L' '-I One of the dominant factors in selecting rock bolts is that of cost. One of the most commonly used underground bolts are mechanical rock bolts, 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 set into the remote end of a rock bore are anchored within the extended bores, at the back, the wall or the rock face, and a bearing plate and tensioning nut applied the outer end of the bolt for the purpose of compressing an adjacent strata of the rock so as to enhance its strength and stability and to collectively create a virtual 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.

*em* At certain locations in a mine, under circumstances where greater security is required, such as oooo permanent passages or ways and in certain types of strata, rock bolts are grouted into their respective bores.

.Grouting is costly, but enhances the security of the rock bolt, and diminishes movement in the strata by filling the residual bore space or a selected portion, with cement or 'I 3 epoxy cement, thus also providing direct load transfer between the full length of the rock bolt and the adjoined strata, so as to more completely 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, so that rapid bolt deterioration can ensue grouting also affords significant corrosion protection to the rock bolts, to prolong their working life.

10 For a given length of rock bolt at a known unit cost, including hole drilling and bolt setting costs, the apparent cost of purchasing and setting rock bolts to secure a given area can be readily calculated on a theoretical basis, based on the area to be secured, the recommended density of bolting, and hence, the number of 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.

see* Due to a number of factors such as poor initial 20 installation, loss of anchorage due to slippage movement of the shell, including the local effects of blasting and g t other ground tremors and displacement, experience over many 1 i N i r 5 1 -4years in the field has revealed a "loss", in terms of effective load bearing, generally of about 40% of all ungrouted rock bolts, as initially placed.

This situation can be largely remedied by grouting of the installed bolts, with consequently 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 bolt, 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 15 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 t loading and displacements behind the rock face.

*fee*: 00..

"i 0 p SUMMARY OF THE INVENTION The present invention provides a tubular rock bolt that is particularly suited for grouting installations either at the time of installation, or subsequently. The presently disclosed 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.

In one form of the invention there is therefore proposed a hollow, thin walled tubular rock bolt having an axial working load capacity substantially 1 0 equal to a standard solid mechanical rock bolt of predetermined smaller diameter, said tubular rock bolt providing an axial through passage to facilitate *o the optional use of a ground movement indicator rod in cooperation with the rock bolt, said tubular rock bolt consisting of a steel strip of which opposed 1 parallel edges are juxtaposed to curl the strip, and a weld bead securing the edges together.

Furthermore, the subject tubular rock bolt incorporates standard rolled threads to enable use thereof with standard sizes of nuts and more particularly, with substantially 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 in its ultimate loadi characteristics to the common 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.

:i $i

L

~J~I

6@ E*

S

S.

0@

S

S

S

S. S S S *5 In a further form of the invention there is proposed a hollow 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 shell means having an expansible distal head portion to expand 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, ground movement indicator means having a viewable proximal end extending outwardly of the proximal end of the rock bolt, and an elongate support portion extending within the rock bolt passage and supported at substantially said rock bolt distal end, in independently secured relation therein, to provide external visual indication of axial extension of said rock bolt, said rock bolt 15 comprising a steel strip curled so that opposed parallel edges are juxtaposed, and a weld bead securing the edges together.

Such changes in the relative condition of a rock bolt arises as a consequence of a loss in tension in the rock bolt due to non-gripping of the shell within its bore in the rock, or to significant extension of the rock bolt as a consequence of load increase due to strata displacement, which conditions, in the case of prior art rock bolts, were visually undetectable.

These condition changes, taken singly or in combination in the case of the subject TRB tend to cause a relative retraction, inwardly of the GMI visible, projecting outer end towards or into the interior of the TRB.

It has been found that a TRB incorporating the foregoing enumerated advantages can be manufactured at an acceptable increase in cost over that of existing mechanical rock bolts, using fabricated cold drawn mechanical steel tube.

Therefore, in another form of the invention there is proposed a method of manufacturing a tubular rock bolt comprising the steps of: forming an oversize, thick walled tube of vubstantially circular, annular section; i 1k .1 i 6a cold drawing the tube by elongation through dies whereby a tube having an interior diameter of at least about 0.35 inches is produced; cutting a portion of the tube to form a bolt having first and second ends; providing rock anchor attachment means to the first end of the bolt for attaching a rock anchor thereto; Sproviding second attachment means at the second end for attachment of load transfer means thereto; and 1 0 wherein the forming step comprises forming a steel strip having parallel longitudinal edges into a laterally curved form and abutting the edges and joining the edges so as to form the tube.

S S

*S

6

S

555*55 1 i ii'! GAI 0 @6 6 6@

S

7 It has further been found that the fabricated tube can be cold drawn, within very close tolerances, to an outside diameter 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 10 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 thickness, as a function of the tensile strength of the fabricated 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, due to 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 6

S

6e S S 8 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 characteristics, 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.

S

10 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 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. i 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 i"R -9grouting, by avoidance of the air locks frequently experienced with vent tubes, in the prior art. It also makes it possible to retroactively grout a bolt zone, subsequent to bolt installation.

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 having a predetermined outside tubular diameter to 10 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 solid section mechanical rock bolt, wherein the subject tubular rock bolt has a comparable extensibility to that of the referenced solid section mechanical rock bolt, and provides a similar ability of maintaining an effective load and reliability, in use.

Manufacture of the subject cold drawn tube bolt is effected by first manufacturing a larger diameter steel 20 tube.

0

S

t iir i- 10 Three processes are available, the tube being fabricated by: electric resistance welding the edges of a roll formed steel strip; by welding the strip edges by airjet or oxyjet heating, or by forming a seamless tube by the pierce billet and cold draw methods, In the electric resistance welding method a strip of steel is prepared having its width equal to the perimeter of the tube to be formed. The strip is roll formed to the required circular section and the abutting 10 edges welded, using low or high frequency power through electrode wheels, or sliding contacts or by way of an 0I induction coil.

6 In the other continuous weld strip welding process 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, t o provide a high quality continuous butt jointed seamed tube, at comparatively low cost.

In the case of the seamless tube, a heated cylindrical billet is subjected to being pierced axially, and subsequently rolled to produce an intermediate thick walled tube.

I 'L 11 The tubes thus formed by the respective processes are then cold mandrel-drawn through sizing dies until the required outer diameter and wall thickness are achieved, by cold working.

In order to achieve the subject tube bolt, optimized O.D.'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.

Selection of the initial tube forming process is based prin.mrily upon the cost factors involved, which also are influenced by the scale of the operation.

K--

o g.

00 **o 0.

*S.

0 0o 0* 0 00 0 10 In accordance with the present invention a family of six different tubular rock bolts, TRB 1, 2, 3, 4, 5 and 6 have been evolved to-date and are in course of development: Product Designation TRB 1 TRB 2 Dimensions .690"x.150" .690"x.170" (OD x wall thickness) Rock bore size 1-1/4 1-1/4" Replaces 5/8" bolt TRB 3 TRB 4 .920"x.195" .920"x.195" swaged 1-5/8" 1-1/4" 5/8" bolt 5/8" bolt TRB 5 .920"x.260" 1-5/8" 3/4" bolt TRB 6 .920x.260" swaged 1-1/4" 3/4" Bolt

I

:i 1 4' jI 12 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 1-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 10 thickness, provides an initial applied load bearing e capacity approaching that of a 5/8-inch diameter solid rock S. bolt. This tube size also accepts a nominal 1 inch rolled S thread, and exhibits characteristic extensibility under load.

15 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 swaced, pointed or otherwise reduced to a smaller to accept a 3/4-inch rolled thread thereon, for use with 3/4-inch nuts and 3/4-inch standard shells.

It will 'e understood that the subject invention is not limited to the foregoing family of tubular rock bolts, and that different diameters and wall thicknesses i may be adopted to meet the needs of the industry.

i lir 1 1 *r em

S

B.

9

S

S

R cr L I 13 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 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 15 sensibly and substantially proportionately diminished, to 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 20 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.

i i l

S

S

0 0@

SS

S

S

S. S

S.

S

555 14 Generally, low cost wires or plastic rods are adopted in the role of GMI indicator rods.

Tn 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.

10 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 15 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 and, being readily shortenable by a simple clipping 20 operation, can be provided in a standard length, such as a 9 foot or longer GMI 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 GMI rod is 3 or 4 feet longer than the tube bolt with which it is used.

55.5 5 S 0000 6896 55 .555 0 5.55 5 -I1 i i: i i r i i

I

ii :i L~A 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 S* portion thereof supported independently of the TRB shell, 10 the distal end of the GMI rod can be threadedly disengaged 0* by unscrewing it from its anchor portion, to permit 1 withdrawal of the GMI rod from an installed TRB, so that grouting of the TRB can then proceed without any impedence.

BRIEF DESCRIPTION OF THE DRAWINGS S j| 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; I 16 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 10 associated shell anchor, together with bail and leaves portion of the shell, and a portion of the tubular rock bolt.

i 0£ DETAILED DESCRIPTION OF THE INVENTION S a 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 0 S* 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.

I' 1 1 17 Referring more particularly to Figures 1, 2 and 3, with the bolt assembly 10 inserted into the rock bore 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 The bottom tensioning nut 32 bears against the 10 hardened round washer 34 and bearing plate 36, to draw the 0 tube portion 12 axially outwardly in anchor loading relation. Thus, the tube portion 12 is tensioned, while the associated surrounding rock portion is compressed, and thereby stabilized.

15 In some instances, grout may be injected upwardly 0. 0. through the bore 19 of tube 12, and the GMI 40 is not present.

0 It is preferred to grout, using an offset passage 0* 0" 35' through the bearing plate 36 for upward insertion of a 20 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 requires to be inserted. I i 18 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 20 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 received 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 26 of shell 20. Upon such an occurrence, with the leaves 15 22 being set in the rock bore 30, the bail portion ruptures readily, adjacent to aperture 29, to save dislodgement of the leaves 22.

The GMI rod retaining thread 58, being formed in GMI anchor portion 60, is secured against rotation, and it 20 is possible to unscrew and withdraw the GMI rod portion from out of an installed TRB, so that initial non-grouted i j i t i1S l K

L

~y i i i -L 19 installation of a TRB incorporating a GMI can be followed by removal of the GMI rod 40, and carrying out 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 by application of a predetermined torque on nut 32 to the desired initial value of working load.

In figures 2 and 5, the GMI rod 40 has the inner

S.

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.

J 0 4363b/2-21

I

Claims (10)

1. A method of manufacturing a tubular rock bolt comprising the steps of: forming an oversize, thick walled tube of substantially circular, annular section; cold drawing the tube by elongation through dies whereby a tube having an interior diameter of at least about 0.35 inches is produced; cutting a portion of the tube to form a bolt having first and second ends; providing rock anchor attachment means to the first end of the bolt for 1 0 attaching a rock anchor thereto; providing second attachment means at the second end for attachment of load transfer means thereto; and wherein the forming step comprises forming a steel strip having parallel longitudinal edges into a laterally curved form and abutting the edges and 1 5 joining the edges so as to for the tube.

2. The method of claim 1 wherein joining the edges comprises electric resistance welding of the edges.

3. The method of claim 1 wherein the rock anchor attachment means and the second attachment means comprises threads. 20 4. The method of claim 1 wherein a tube having an exterior diameter between about 0.68 inches and 0.92 inches is produced in the step of cold 0..0e drawing the tube. 0 5. The method of any preceding claim wherein providing the rock anchor attachment means comprises rolling threads onto the first end of the bolt and .0.0 25 providing the second attachment means comprises rolling threads onto the second end of the bolt.

6. The method of claim 5 wherein the rock anchor comprises a shell and the load transfer means comprises a bearing plate. I V- p i i1 1 11 1 -21

7. A method of forming a tubular rock bolt, comprising the steps of: a) forming a steel strip having parallel longitudinal edg:,s into 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 annular 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; 1 0 c) cutting a portion of the tube to form a bolt having first and second ends; d) rolling threads on both ends of the bolt; e) providing a rock anchor attachment which includes a portion S.I :having an internally threaded passage matching the rolled threading .0 1 5 said attachment onto said first end; and f) providing load transfer means for said second end, said means comprising an internally threaded member for abutting receiving said second end, and for transferring a load from the bolt, through a suitable plate, to the surrounding rock structure.

8. The method claimed in claim 7, in which the step of cold drawing under b) gives the tube an outer diameter between about 0.68 inches and about 0.0: 0.92 inches, and an inner diameter between about 0.34 inches and about 0.53 inches. -To

9. 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 to facilitate the optional use of a ground movement indicator rod in cooperation with the rock bolt, said tubular rock bolt consisting of a steel strip of which opposed parallel edges are juxtaposed to curl the strip, and a weld bead securing the edges together. The hollow rock bolt as set forth in claim 9, having a thread on at least one end thereof. i

11. The hollow rock bolt as set forth in claim 9, in combination a shell anchor secured to one end thereof. I N 'Jr 0' I*

22- 12. The combination as set forth in claim 11, said axial through passage permitting the passage of air and grout therethrough. 13. The hollow rock bolt as set forth in claim 9, having one end thereof reduced to a smaller diameter, and having a corresponding diameter thread thereon. 14. The hollow rock bolt as set forth in claim 9, said standard solid rock bolt being a 3/4-inch diameter size, said tubular rock bolt having an outside diameter precisely sized for a 1-inch nominal size thread thereon. g:'o 10 15. The hollow rock bolt as set forth in claim 9, said standard solid rock bolt i being a 5/8-inch diameter size, said tubular rock bolt having an outside o diameter precisely sized for a 3/4-inch nominal size thread thereon. a4 4 16. The hollow rock bolt as set forth in claim 9, in which said curled and @ao: welded steel strip is in cold-worked condition, having been drawn down to a 1 5 smaller diameter. 17. The hollow rock bolt as set forth in claim 9, in combination with ground o. movement indicator means having a viewable proximal end, an elongated rod portion extending internally of said tubular bolt and having attachment means for sec'Jring the distal end of said rod portion, remote from said proximal end, in secured, self-supporting relation whereby, on installation, variations in the effective position of said rock bolt proximal end, in use, under load, are inversely represented by the extent of exposure of said rod proximal end .:i portion outwardly beyond said rock bolt proximal end. 18. The hollow rock bolt and indicator means combination as set forth in claim 17, said attachment means permitting the securement of, and the removal of said ground movement indicator means from within said rock bolt. il 19. The combination as set forth in claim 18, said ground movement indicator means including an elongate cylindrical body. Ii The combination as set forth in claim 19, wherein said cylindrical body A comprises a hollow plastic section. ?A 0 T0 o -23 21. A hollow 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 shell means having an expansible distal head portion to expand 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, ground movement indicator means having a viewable 1 0 proximal end extending outwardly of the proximal end of the rock bolt, and an elongate support portion extending within the rock bolt passage and supported at substantially said rock bolt distal end, in independently secured 0 relation therein, to provide external visual indication of axial extension of said rock bolt, said rock bolt comprising a steel strip curled so that opposed parallel 1 5 edges are juxtaposed, and a weld bead securing the edges together. 22. The hollow rock bolt as set forth in claim 21, in which said rock bolt is in cold-drawn condition and has a smaller diameter than when initially formed.

23. The method of manufacture of a rock bolt substantially as described in the specification with reference to and as illustrated by the accompanying 20 drawings. Dated this 1st day of April 1992. STELCO INC. By their Patent Attorneys, COLLISON CO. u.J I s ''N c -i