CA2002806C

CA2002806C - Rock anchor and method of manufacture

Info

Publication number: CA2002806C
Application number: CA002002806A
Authority: CA
Inventors: Neville Allan Hedrick
Original assignee: Garford Pty Ltd
Current assignee: Individual
Priority date: 1988-11-14
Filing date: 1989-11-10
Publication date: 1995-08-08
Anticipated expiration: 2009-11-10
Also published as: EP0442935A4; EP0442935B1; ZA898640B; CA2002806F; ATE127550T1; DE68924171T2; WO1990005811A1; EP0442935A1; DE68924171D1; CA2002806A1

Abstract

A rock anchor (10) characterised by a tendon (12) composed of a plurality of strands (14, 15), the tendon (12) having at least one bulbous portion (16), wherein all the strands (14, 15) in the or each bulbous portion (16) are spaced apart from one another substantially around the periphery (17) of each bulbous portion (16) . The tendon (12) is comprised of a plurality of outer strands (14) helically wound around a centre strand (15). In each bulbous portion (16) the outer strands (14) and the centre strand (15) are disposed adjacent the bulb periphery (17) .

Description

200~06 TITLE IMPROVED ROCK ANCHOR AND METHOD OF MANUFACTURE FIELD OF THE INVENTION The present lnventlon relates to an lmproved rock anchor and a method of and an apparatus for manufacture thereof. Rock anchors are steel tendons lnserted down bore holes ln a scree. A portlon of the tendon ls grouted, and a plate attached to the tendon. The tendon ls then stressed; the plate bears upon the scree and thereby stablllses the scree. Tendons typlcally comprlse a plurallty of steel strands wound together to form the tendon. To adequately secure such tendons ln cement, prlor art methods have used machlnes to unravel the strands. Such machines have required people to initiate such unravelling prior to insertion into the machine. This is time consuming and hence expensive. The purpose of unravelllng the strands ls to lncrease the surface area of the tendon ln contact wlth the cement to more securely embed the tendon in the cement. The present lnventlon provides an alternative form of tendon and a method and an apparatus for manufacture thereof. SUMMARY OF THE INVENTION In accordance with one aspect of the present invention there ls provlded a rock anchor comprislng a tendon composed of a plurallty of strands, the tendon havlng at least two spaced bulbous portions, wherein between ad~acent bulbous portions there are a plurality of outer strands hellcally wound around a centre strand, and in each bulbous portlon the outer strands and '~' -- 1 -- 2002~06 the centre strand are spaced from one another substantlally around the perlphery of the bulbous portlon. In accordance wlth another aspect of the present lnventlon there ls provlded an apparatus for manufacturlng a rock anchor, comprlslng a tendon having a plurallty of strands and at least one bulbous portlon ln whlch all the strands are spaced from one another substantlally around the perlphery of the bulbous portion, the apparatus comprlslng a palr of spaced clamp means, each arranged to clamp releasably the tendon thereln at posltlons spaced apart along the length thereof and belng mounted ln the apparatus for there to be relative longltudlnal dlsplaceablllty between the clamp means, and clamp dlsplacement means for dlsplaclng the clamp means towards one another ln order to form a sald bulbous portlon ln the tendon between the two clamp means. In accordance wlth yet another aspect of the present lnventlon there ls provlded a method for manufacturlng a rock anchor comprlslng a tendon havlng a plurallty of stands and at least one bulbous portlon ln whlch all the strands are spaced from one another substantially around the perlphery of the bulbous portion, wherein a tendon ls clamped ln two or more spaced clamp means whlch are moveable relative to one another, the clamp means are displaced towards one another to form at least one sald bulbous portion ln the tendon and the tendon ls then released from the clamp means, and whereln ad~acent clamp means are spaced apart a flrst predetermlned distance apart prior to forming the or each sald bulbous portlon, the flrst predetermlned dlstance correspondlng to an unlnterrupted length 20~?~06 of tendon necessary to form a sald bulbous portion having the correct or desired proportions. BRIEF DESCRIPTION OF THE DRAWINGS The present lnventlon will now be descrlbed, by way of example, wlth reference to the accompanylng drawings, in which:- figure 1 is a view of a rock anchor in accordance with the present invention; figure 2 ls an upper perspectlve view of a section taken along the line 2-2 of Figure l; figure 3 is a side view of a first embodiment of a machine for manufacturing a rock another in accordance with the present invention; figure 4 is a plan view of a second embodiment of a machine for manufacturing a rock anchor in accordance with the present invention in a bulb upsetting condition; figure 5 is a plan view of the machlne of flgure 4 in a tendon dlsplacing condition; flgure 6 ls a plan view of a third embodlment of a machine - 2a - 200Z8C)6 -- 3 for manufacturing a rock ~ncho~ in accordance with the present invention in a bul~ upsetting conditlon; Figuxe 7 is a transverse sectional view along li~es 7-7 of Figures ~ to 6; Fig~re 8 is a transverse sectional ~ie~ along lines 8-8 of Figures 4 to 6; and Figure ~ is a trans~erse sectional view along l~ne ~-~ of Figure ~. DESCRIPTION OF TRE INVENTION Shcwn in Figures 1 and 2 is a rock ancho~ lU ~o~pr7 sing a steel tendon 12. The tendon 1~ is composed of a plurality o~ outer strands 1~ helically wound around a cent~e stran~ 15 to form the tendon 12. As shown, there are six outer strands 1~ wound around the centre str~nds. The tendon 1~ 15 has a pluralit~ o~ bulbcus partio~s 16 spaced ap~rt from one another along the length of the tendon. The portio~s of ~he s~ra~ds 14 ~nd ~S, in the ~ulbous portio~s 16, are spaced apart from each other around the circum~erence of t~e bulbous portions 16, as shown. The sect~o~ line 2-~ has ~een taken through the fattest part o~ the bulbo~s portion 16. As seen in Figure 2, the centre strand 15 ls displaced a~ay fram the centre o~ the tendon 12. Each bul~cus porticn 16 has a ~ulb diameter de~i~ed as the diameter of the smallest tube t~rough which the roc~ 2~ anchor 10 will pass. The ~ulb periphery is indlcated b~ the broken lines, m~r~ed 17 in Flgure 2. The outer strands 14 and the centxe stra~d 15 are all located ad3acent ~nd ~ithin the bulb periphery 17. This enables cement to contact a greater sur~ace ~rea of the 20028~ -- 4 strands 14 in use r as hereinbefore described. The rock a~chor 10 is there~y firmly embedded in the cement. Fur~hermore, the centre stxa~d 15 is displzced away from its normal central pasiti~n, in the bulbous portions 16. When the rock anchor 10 is stressed, ~he load will be t~ken moxe evenly by the strands 14 and 15 th~n if the centre strand 1~ was in its central position. If the centre strand 15 was in i~s centr~l position, more load would be taken b~ the centre strand 15 tha~ ~he outer strands 14. This would then lower the sa~e~ factor of the rock ~cho~ 10 wh~h co~ld then fail a~ a much lower load. Shown in Figure 3 is a ~irst embodiment of an apparatus 20 for ~anufacturing a rock anchor 10. The apparatus 20 compr~ses a plurality of spaced clamp means 22 ~o cl~mp the tendon 12. The sp~cin~ between ~djacent clamp means 22 is initially set at a first predetermined distance~ This first predetermined leng~h corresponds to an uni~terrupted le~gth of tendon 1~ necess~ry to form a ~ulbcus portion 16 having the correct or desired propor~ions. The clamp ~eans 22 are slidably mau~ted on a frame 24 suppor~ed by legs 26 and feet 27. One of the clamp means 22' is fixed to the frame 24. As shown, the fixed cla~p means 22' is located at one end of the ~r~me 24, in this case t~e 7 ~ft hand end, as shc)wn in Figure 3. Each clamp means ~2 co~prises an hydraulic ram 28 having a plate 30 attached thereto to ~ea~ on and thereby clamp the tendon 12 against an opposed ~}ate 32. Spr~n~s 34 are provided to help bias the plates 30 a~d 32 apart when the ram 28 is released. Each ram 28 ls supplied with hydraulic 2002806 p~essure through a pipe 36 co~nected to a common mani~old 38 and controlled by a valve 40. Each clamp ~eans 22 is provided with a stop 42 extending to the left, as shown, to limit movement of ~he plu~alit~ of clamp means 22 relative to e~ch othex. A cl~mp d~splacement ~eans such as an hydraulic ram 44 is fixed to the xight hand end of the frame ~4 ~nd is controlled ~y a valve 4~. The ram 44 has a plunger 45 which ~ears on the clamp means 22 on the right, as shown in Figure 2. The ram 44 is able to displa~e the plurality of cl~p ~eans 22 relative to one another. A~ hydra~lic power source 48 supplies hydraulic pressure to the rams 28 and ~4 a~d is controlled ~y a valve 50. ln use, a te~don 1~ is placed between the plates 30 and 32, ~5 of e~ch clamp means 22. The valves 40 and 50 are then ope~ed to oper~te the rams 2~ The rams 28 move the plates 30 to bear on the tendon 12 and thereby clamp the te~don 12 against the opposed pl~tes 32. The ~alve 46 is then opened to operate the ram 44. The ram 2~ 44 beaxs on the clamp means 22 on the right and displace~ it tcwards the left. Since the clamp means ~2' i~ ~ixed to the frame 24, the other cl~mp me~ns 22 are displaced relative to one another. Such r~lative displacement causes the tendon 12 to be interrup~ed and the ~ul~ous portions 16 to be 2~ formed in the tendon 12, between adjacent clamp means 22. Thus, i~ is apparent that the r~ht hand cl~mp 22 is moved to the left as seen in Figure 3 kefore ~ny of the other clamps 22 commence to move. This causes the first bul~ous portion 1~ to ~e formed i~ the tendon 1~. Su~sequently the 2f'lr~2~0~ stop 42 o~ the right hand clamp 22 contacts the next clamp 2~ to the left~ Thus, the next clamp 2~ is caused to mov~ towar~s the left as seen in ~igure 3 and to cause the second ~ul~ous portian 1~ to be formed in the tendon 12. This sequential ~ode o~ operaticn is continued until all of the steps 42 are in contact with the next clamps 22 to the left as seen in Fi~ure 3 and untll ~he left hand stop 42 contacts ~he fixed clamp 22'. Thus, the bulbous portions 16 are prod~ced in se~uence by the sequential move~ent of t~e clamps 22. The stops 42 l~mit the displace~ent of one clamp means 22 relative to ad3acent clamp means 22. Conveniently, the stops 42 l~mit the distance between a~ja~ent clamp means 22 to a second predetermined distance. This second predetermined dist~nce corresponds to the bul~ous portions 16 reaching thei r co~rect or desired proportions. ~isplacement of the ram 44 is continued until bulbous portions 16 of su~ficlent si7e and shape are formed. This may occur when each stop 42 ~ontacts an adjace~t c~amp ~eans 20 22 and movement of the clamp mea~ 22 relative to one another is no longer possible. When the tendon 12 is axially compressed in the abo~e manner to form the bu~bous portions 16, ~he strands 14 are forced ap~rt and the strand 15 is forced out~ardly ~o a ~osition 2~ adjacent the bul~ periphery 17 as described above and a.rock anchar 10 in accordance with the present inven~icn is formed. The valves 40 and 4~ are then closed. The springs 34 o~ the clamp means ~2 urge the plates 30 away from the plates 32, 2002806 -- 7 thereby rele~sing t~e tendon 12. The ro~ a~cho~ 10 is then removed and may ~e used in kncwn manner. Shown ~ Figures 4, 5, 7 and 8 is a second em~cdiment of an apparatus 52 for manufacturing a rock an~hor ~0. Reference S numerals used in Figuxe 3 denote like parts in ~igures 4, 5, 7 and 8. As is apparent, there is only one slida~le clamp means 22~ This clamp means 22 is held in a carriage 54 slidable along the frame 24. The hydraul~c ram 28, associ~ted ~ith the fixed clamp me~ns 2~', is supplied with ~0 hy~raulic pressure through a P~pe 56. The ram ~8, associa~ed ~ith the slidable colamp means 22, is s~pplied with hydr2ulic pressure throush a separate pipe 58. Si~ilarly, the ram 44 o~ the dispiacement means, has a separate pipe 60 to supply hydraulic pressure~ The pipes 5~, 58 and 60 axe connected to an hydraulic ti~er ~not shown) which co-ordlnates t~e act~o~ of both ~ams 28 2nd the ram 44. As shown, the tendon 12 passes through the cen~e of the ram 44. The plu~ger 4~ is at~hed ~o the plates 30 and 32 of the fixed cl~mp ~eans 22, on the right z~ shown in Fig~re 4. A straight pipe 61 is disposed to the left of the fi~ed clamp 22 ~d is ~rranged to receive ~he rock anchor 10. The pipe 61 has a d~dmeter at least th~t of the bulb diameter, as defined herein~efore. 2S As seen in Figures 7 and 8, each pla~e 30 and 32 has a recess 62. ~he plates 3C and 32 have a centralising ~i~ 64 located ~elow the recesses 62 and extendi~g between the plates 3~ and 32. The pin 64 serves to support the ~endon 12 and to guide the plates 30 and 32 toward one another so 2002806 thz~ ~he recesses ~2 clamp the tendon 12 therein. Th~s, the plate 30 contains a recess (not s~own) which receives the pin ~4 when the plates 30 and ~2 are moved together by th~ ram 28. In use, a ten~on 12 is flrst fed through the r~m 44, between the plates 30 and 32, of the c~amp means 2~ and 22', ~nd into the pipe ~1. The hydraulic timer operates and activates the ram 28, of the ~ixed cl~pin~ means 22'. This results i~ the clamping means 22' securely holding t~e tendon 1~ by means of the pl~tes 30 and 32. At this poi~t, the rzm 28, of the s~idable clamp means 22, is deactiva~ed to a~l~w the tendon 12 ~o slide between its plates 30 a~d 32. The r~ 44 is then acti~ated and causes the plunger 45 to move t~ the left, toward the fixed clamp means 22'. This mo~7es th~ carria~e 54, with th~ sli~hle ~lAm~ m~n~ ~, t. the left, toward the fixed clamp means 22'. When ~he dista~ce ~etween the clamp ~eans 22 and 22' reaches the first predetermined length as de~ined he~einbefore, the ram 44 is deactivated. The ram 28, of the slidable clamp means 22, is then activated to cl~mp the tendon 12 between the plates 30 and 32. The r~m 44 is the~ reactivated. This causes the plunger 45 to b~:ar upon the pleLte9 3 0 and ~ ~, of the slid~ble cl~mp means 22, and displace the slid~le clamp means 22 towards the fixed clamp means 22'. Such re~a~i~e displacement causes the ~ul~cus portions 1~ ~o ~e formed in the tendon 12 200Z806 g between the clamp means 22 z~d 22'. When the distance between the clamp means 22 and 22' reaches the secon~ predetermined distance ~as defined hereinkefore~ the ram 28, of the slida~le clamp means 22, and the ram 44 are t~e~ deactivated to release the te~don 12. The ram 44 then withdraws the plunger 45 a~ay fxom the fixed clamp means 22'. This ~o~es the carriage 54 wi~h the slidable c~a~p means 22, to the right, away from the fixed clamp me~s 22'. The r~m 28, of the slidable clamp means 22, is ~hen ~ctivated ~o clamp the tendon 12 by mea~s o~ the plates 30 and 32. The ram 28, of the f~xed clamp means 22', ls then deactivated to re~ease the tendon 12. The ram 44 is the~ a~tivated, the p}~nger 45 exten~s and moves the carriage 54 and slidable clamp means 22 towards the fixed clamp means 22'. Thus, in this ccndition, the slid~e clamp means 22 and the ram 44 ~unction as a te~don dlsplacement means to mo~e the tendo~ 12~ This also moves ~0 the rock anchor 10 to the le~t and into the straight pipe ~. The pipe 61 generally straighte~s out any bends that may h~ve occurred ~hen the bulbous ~ortion 16 was formed. When the rock anchor 10 has been moved a distance equal to the spacing ~etween adjacent bulbou~ portions 1~, ~he ram 44 is then deactivated to p~event any fur~hex movement o~ ~he tendon 12. The ram ~8, of the fixed clamping means 22' is then activated a~ the process is repeated as many t~es as is required. 2002806 -- 10 -- Shown in Figur~s ~ and 9 is a third embodiment of an apparatus 66 for manu~ac~ux7 ng a rock a~chox 10 ~hich is a modifica~ion of the apparatus 52 of Figures 4, 5, 7 and 8. Like numerals denote like parts. The apparatus 66 additio~ally comprises a tendon displacement means 68 to the right of the slidable clamp me~ns 22 in Figu~e ~. The tendon displacement means ~ comp~ises a slida~le clamp means 70 car~ied b~ ~ c2rriage 72 slidable along the frame 24. The clamp means 70 comprises an hydrauliC ram 74 supplied ~ith hydxaulic pressure hy a pipe 7~. The ram 74 h~s a plate 78 attached thereto to bear on and thereby ~lamp the tendon 12 agains~ an opposed plate 80. Each plate 78 a~d 80 has a recess 8~, ln a similar manner to the maohine 52 of ~igures 4, 5, 7 and 8. ~he plates 78 and 80 have a ce~tral~sing pin 84 loc~ted below the half recesses 82, in si~ila~ manner to the machi~e 52 of Figures 4, 5, 7 and 8. The tendon displ~emen~ means 68 further comprises a clamp displa¢ement ~eans in the form of an hydraulic ram 86 supplied with hy~rauliC pressure through a p~pe 88. The ram ~ has ~ plunger gO which ~ears on ~he carriage 72 and is arranged to displace the sliding ~lamp means 70 to the 1e~t, as shown in Fig~re 6. As is a~parent, the tendon displacement means 68 is a separate item, as compared to ~he tendon displacement means o~ the appaxatus 52. The pipes 58 an~ 60 are connected to a ccmmon mani~old ~2 supplied with hydraul~c pressure by a pipe 94~ ~hus, the f~xed clamp means 22 t ~d the slidable clamp mea~s 22 are activated simultanecusly. .. . , .. . _ .. . . . . . .. 2002806 }n use, a tendon 12 is fi~st fed through between the pl~tes 78 and 80, through the ram 44, between the pl~tes 30 and 32, of the clamp means 22 and 22l and lnto the plate ~1. An hydraulic t~mer operates and simulta~eausly activates both of the rams Z8 so that the clamps 22 and 22' securely hold the tendon 12. At this poi~t, the ram 86 is reactlvated to al~ow t~e tendon 12 to slide between the plates 78 and 80. Further, at this stage, the clamp means 22 and 22' are the firs~ predetermined dl'stan~e ~as defined hereinbefore~ apa~. The r~m 44 is then activ~ted to move the c~rriase 54 to the le~t, towa~ds the fixed clamp means 22'. This relative displacement in~errupts the tendon 12 and for~s a bulbous portion 1~. When the distance between the clamp means ~2 and 2~' reaches the second predetermi~ed dist~nce (as defined hereinbefore) both rams ~8 ~nd ~he ram 44 are deactivated. Ram 74 is ~hen activated so that the tendon 1~ is securely hel~ in the slidable clamp means 70 by the plates 7~ and 80. The r~m ~ is then ~ctivated and the plunger 90 moves the carria~e 72 to the left, towards the fixed clamp means 22'. This also moves the rock anchor 10 to the left and intc the straigh~ pipe ~1. When the ~ock ~nchor 10 has been moved a d}s~ance equal to the spacings between adjacent bul~ous portions 1~, the ~ams 74 and 86 are deactivated. ~oth rams 28 are then simul~aneously activated and the process ~s repeated as many times as desired. T~e present invention provides a rock anchor which is easily Z002806 - 12 - manufactured and dGes not require un~avellln~ of the strands, and more roGk anchors may be produced i~ the same time as compared to priox ~t systems. With ~he rock anchor of the present inve~tionr the strands in the bul~ous portions are separated a~d he~ce the rock anchor will ~e firmly e~bedded in t~e cement in the bore hole. Finally, it is envisaged ~hat the tendon displacement means may alternate~y comprise an electric motsr driving a wheel. An idler wheel would bea~ upon the driven wheel and the tnedon would pas~ betwee~ the id~er ~nd driven wheels. A timer would acti~ate an electric ~otor to rotate the driven wheel and thereby mcve the tendon. Modific~tions and variations such as wauld ~e appare~t ta a skilled addressee are deemed within the scope of the preser.t inve~tion. For example, there may be a num~er of each of the machines 2~, S~ and 6~ operating simult~neou~ly together in parallel. Further, the machine 20 may be provided wit~ z ~ydraulic timer to co-ordinate the ~c~ivation o~ the rams 2 and 4~. Even furt~er, the ram 44 may have an ad~ustment ~eans in the ~orm cf a t~readed column to vary the position cf the ram 44 along the f~ame.

Claims

WHAT IS CLAIMED IS: 1. A rock anchor comprising a tendon composed of a plurality of strands, the tendon having at least two spaced bulbous portions, wherein between adjacent bulbous portions there are a plurality of outer strands helically wound around a centre strand, and in each bulbous portion the outer strands and the centre strand are spaced from one another along the periphery of the bulbous portion, wherein the bulb diameter of said bulbous portion is less than three times the diameter of the tendon between adjacent bulbous portions.
2. An apparatus for manufacturing a rock anchor, comprising a tendon having a plurality of strands and at least one self-supporting bulb of strands in which all the strands are spaced from one another along the periphery of the bulb of strands, the apparatus comprising a pair of spaced clamp means, each arranged to clamp releasably the tendon therein at positions spaced apart along the length thereof and being mounted in the apparatus for there to be relative longitudinal displaceability between the clamp means, and clamp displacement means for displacing the clamp means towards one another past an elastic limit of said strands in order to form a said self-supporting bulb of strands in the tendon between the two clamp means in order to suit said tendon as a rock anchor, wherein the apparatus also includes tendon displacement means for displacing the tendon in the longitudinal direction thereof relative to the clamp means after a bulb of strands has been formed in the tendon, comprising longitudinal displacement clamp means arranged releasably to grip the tendon therebetween.
3. An apparatus according to claim 2, wherein the longitudinal displacement clamp means of the tendon displacement means is one clamp means of the said pair of clamp means.
4. An apparatus according to claim 2, wherein the longitudinal displacement clamp means of the tendon displacement means is a clamp means additional to said pair of spaced clamp means and arranged to grip the tendon therein when both of said pair of spaced clamp means is not acting on the tendon.
5. An apparatus according to claim 2, wherein only one clamp means of the said pair of clamp means is displaceable in said longitudinal direction.
6. An apparatus according to claim 3, wherein the longitudinal displacement clamp means of the tendon displacement means is displaceable by hydraulic ram means.
7. An apparatus according to claim 4, wherein the longitudinal displacement clamp means of the tendon displacement means is displaceable by hydraulic ram means.
8. An apparatus according to claim 2 wherein a substantially straight pipe is provided to receive and substantially to straighten the rock anchor as it emerges from the clamp means after formation of the bulb of strands.
9. An apparatus according to claim 2, wherein the longitudinal displacement clamp means is carried in a carriage slidable along a frame of the apparatus.
10. An apparatus for manufacturing a rock anchor, comprising a tendon having a plurality of strands and at least one self-supporting bulb of strands in which all the strands are spaced from one another along the periphery of the bulb of strands, the apparatus comprising a pair of spaced clamp means, each arranged to clamp releasably the tendon therein at positions spaced apart along the length thereof and being mounted in the apparatus for there to be relative longitudinal displaceability between the clamp means, clamp displacement means for displacing the clamp means towards one another past an elastic limit of said strands in order to form a said self-supporting bulb of strands in the tendon between the two clamp means in order to suit said tendon as a rock anchor and a substantially straight pipe to receive and substantially to straighten the rock anchor as it emerges from the clamp means after formation of the bulb of strands.
11. An apparatus according to claim 2 or claim 10, wherein each clamp means has a pair of opposed plates and hydraulic ram means bearing on one said plate to bring it into opposition against the other said plate.
12. An apparatus according to claim 2 or claim 10, wherein each clamp means has a plate with a recess arranged to receive the tendon therebetween.
13. An apparatus according to claim 12, wherein a centralising pin is associated with the plates of the clamp means to support the tendon and to guide recesses of the plates towards one another.
14. An apparatus according to claim 2 or claim 10, wherein the clamp displacement means is operable to space said pair of clamp means by a first predetermined distance prior to forming the bulb of strands and by a second predetermined distance after forming the bulb of strands.
15. A method for manufacturing a rock anchor comprising: clamping a tendon in two or more spaced clamps; displacing each pair of adjacent clamps towards one another past an elastic limit of said strands to form a self-supporting bulb of strands in which all the strands are spaced from one another along the periphery of the bulb of strands; releasing the tendon from the clamps; wherein adjacent clamps are spaced apart a first predetermined distance prior to forming the or each said bulb of strands, the first predetermined distance corresponding to an uninterrupted length of tendon necessary to form a said bulb of strands having the correct or desired proportions to suit said tendon as a rock anchor; after releasing the tendon from the clamps, displacing the tendon in the longitudinal direction with a clamp.
16. A method for manufacturing a rock anchor according to claim 15, wherein the tendon is additionally displaced longitudinally a distance equal to the spacing between adjacent bulb of strands after releasing the tendon from the clamps.
17. A method for manufacturing a rock anchor comprising: clamping a tendon in two or more spaced clamps; displacing each pair of adjacent clamps towards one another past an elastic limit of said strands to form a self-supporting bulb of strands in which all the strands are spaced from one another along the periphery of the bulb of strands; releasing the tendon from the clamps; wherein adjacent clamps are spaced apart a first predetermined distance prior to forming the or each said bulb of strands, the first predetermined distance corresponding to an uninterrupted length of tendon necessary to form a said bulb of strands having the correct or desired proportions to suit said tendon as a rock anchors: substantially straightening the rock anchor with a substantially straight pipe as it emerges from the two or more spaced clamps after formation of one or more bulbs of strands.
18. A method for manufacturing a rock anchor according to claim 15 or claim 17, wherein the clamps of each pair of adjacent clamps are displaced towards one another until they are a second predetermined distance apart, the second predetermined distance corresponding to the or each said bulb of strands reaching the correct or desired proportions.
19. An apparatus for manufacturing a rock anchor, comprising a tendon having a plurality of strands and at least one self-supporting bulb of strands in which all the strands are spaced from one another substantially around the periphery of the bulb of strands, the apparatus comprising a pair of spaced clamps, each arranged to clamp releasably the tendon therein at positions spaced apart along the length thereof and being mounted in the apparatus for there to be relative longitudinal displaceability between the clamps, and a ram for displacing the clamps towards one another past an elastic limit of said strands in order to form a said self- supporting bulb of strands in the tendon between the two clamps in order to suit said tendon as a rock anchor, wherein the apparatus also includes a longitudinal displacement clamp arranged releasably to grip the tendon for displacing the tendon in the longitudinal direction thereof relative to at least one of said pair of clamps after a bulb of strands has been formed in the tendon.
20. An apparatus for manufacturing a rock anchor, comprising a tendon having a plurality of strands and at least one self-supporting bulb of strands in which all the strands are spaced from one another substantially around the periphery of the bulb of strands, the apparatus comprising a pair of spaced clamps, each arranged to clamp releasably the tendon therein at positions spaced apart along the length thereof and being mounted in the apparatus for there to be relative longitudinal displaceability between the clamps, a ram for displacing the clamps towards one another past an elastic limit of said strands in order to form a said self- supporting bulb of strands in the tendon between the two clamps in order to suit said tendon as a rock anchor, and a straight pipe to receive and to straighten the rock anchor as it emerges from the pair of clamps after formation of the bulb of strands.
21. An apparatus according to claim 19, wherein the longitudinal displacement clamp is one clamp means of the said pair of clamps.
22. An apparatus according to claim 19, wherein the longitudinal displacement clamp is a clamp additional to said pair of spaced clamps and arranged to grip the tendon therein when both of said pair of spaced clamps is not acting on the tendon.
23. An apparatus according to any one of claim 19 to claim 22, wherein only one clamp of the said pair of clamps is displaceable in said longitudinal direction.
24. An apparatus according to claim 21, wherein the longitudinal displacement clamp is displaceable by a ram.
25. An apparatus according to claim 22, wherein the longitudinal displacement clamp is displaceable by said ram.
26. An apparatus according to any one of claim 19 to claim 25, wherein the ram is carried in a carriage slidable along a frame of the apparatus.
27. An apparatus according to any one of claim 19 to claim 26, wherein each of said clamps has a pair of opposed plates and a hydraulic ram bearing on one said plate to bring it into opposition against the other said plate.
28. An apparatus according to any one of claim 19 to claim 27, wherein each of said clamps has a plate with a recess arranged to receive the tendon therebetween.
29. An apparatus according to claim 28, wherein a centralising pin is associated with the plates of the clamps to support the tendon and to guide recesses of the plates towards one another.
30. An apparatus according to claim 21 or claim 24, wherein the longitudinal displacement clamp is operable to space said pair of clamps by a first predetermined distance prior to forming the bulb of strands and by a second predetermined distance after forming the bulb of strands.
31. An apparatus according to claim 19, wherein a substantially straight pipe is provided to receive and substantially to straighten the rock anchor as it emerges from the pair of clamps after formation of the bulb of strands.