Friction rock anchor

Abstract

A friction rock anchor which includes an elongate tubular member with a longitudinal slot which has edges formed into flanges, located inside a bore of the member, which extend between opposed ends of the member.

Description

FRICTION ROCK ANCHORBACKGROUND OF THE INVENTION

[0001] This invention relates to a friction rock anchor.

[0002] A friction rock anchor, also referred to as a stabilizer or as a split set, comprises an elongate tubular member with a longitudinally extending slot. This type of anchor is forced into a borehole formed in rock and, in the process, is radially compressed to some extent so that, once positioned, the anchor tends to expand radially and thereby engages frictionally with a wall of the borehole. The functionality of a split set is susceptible, inter alia to the diameter of the hole into which it is installed. At least two parameters, apart from the nature of the material from which a split set is made, are used to assess the technical characteristics of a split set. One factor is a measurement of the tensile load to which a split set can be subjected before failing. A second factor has to do with the load to which a split set can be subjected, when installed in a borehole, before failing.

[0003] Another issue which can have an adverse effect on this kind of anchor arises when the anchor is forced into a borehole. An impact machine is used for this purpose for as the anchor is advanced into the borehole its diameter must be reduced - it is this feature which gives rise to the frictional engagement of the anchor with the borehole after installation. If the impact force applied to the anchor is too high then a portion of the anchor which is not yet in the borehole can be bent and effective installation thereof is frustrated.

[0004] PCT / DE83 / 00204 describes a modified rock anchor wherein opposed longitudinal edges of a wall of a tubular member, adjacent a slot, are castellated. A shaped solid rod whichacts as a clamping device is inserted into the tubular member and engaged with the castellations to provide a clamping action which pre-strains the castellations and which reduces the effective diameter of the tubular member. An objective in this regard is to reduce the cross sectional area of the member so that it can be placed into a borehole and positioned accurately without using an installation hammer machine. Once the rock anchor is correctly located in the borehole a shaped rod (the clamping device) is dislodged with a slight hammer action and is then withdrawn from the tubular member, which is no longer under the constraint of the shaped solid rod. The rock anchor can expand and engage fractionally with a wall of the borehole. In this respect the rock anchor works in a satisfactory manner but it suffers from the distinct drawback that the frictional force which prevails between the rock anchor and the wall of the borehole is not sufficiently high to generate an acceptable load bearing capacity for the rock anchor.

[0005] Corrosion resistance of the material in the anchor is important. The use of a stainless steel alloy can help but comes at an increased cost.

[0006] An object of the present invention is to provide a friction rock anchor which has increased tensile strength and a higher pull-out force than a conventional split set and which, at least in some embodiments, addresses at least some of the aforementioned considerations.SUMMARY OF THE INVENTION

[0007] The invention provides a friction rock anchor which comprises an elongate substantially tubular member with an internal bore, a first end, an opposing second end and a wall which has an inner surface and an outer surface and which includes a longitudinal slot which extends from the first end to the second end and which is bounded by opposed longitudinal edges of the wall,and wherein each edge includes a respective flange which extends over at least a greater portion of the length of the tubular member between the first end and the second end.

[0008] Preferably the flange extends continuously over at least a greater portion of the length of the tubular member. In a preferred embodiment each flange extends continuously between the first end and the second end i.e. over the full length of the elongate member.

[0009] Preferably each flange projects into the bore. In one embodiment each flange is inside the bore and is in contact with a respective portion of the inner surface of the wall.

[0010] In a different embodiment each flange is outside the bore and overlies and, optionally, is in contact with a respective portion of an outer surface of the wall.

[0011] In one example the member has a substantially constant cross sectional shape from the first end to a location which is spaced from the second end, and is tapered with a cross sectional area which reduces in size from the location to the second end. Preferably the slot has a substantially constant width from the first end to the location.

[0012] Optionally, to increase the strength of the anchor at least one insert comprising a component may be slidably positioned inside the bore, wherein the component is substantially tubular and has an outer surface which is in contact with an inner surface of the elongate member and includes a longitudinally extending slot. In one embodiment edges of said longitudinally extending slot of the insert extend into respective recesses formed between each flange and an inner surface of the wall of the tubular member. In a different example edges of the slot are in contact with surfaces of the flanges, for example edges of the flanges inside the bore.

[0013] The invention further extends to a friction rock anchor which comprises an elongate substantially tubular member with an internal bore, a first end, an opposing second end and a wall which has an inner surface and an outer surface and which includes a longitudinal slot which extends from the first end to the second end and which is bounded by opposed longitudinal edges of the wall and wherein the friction rock anchor further includes at least one insert comprising a component which is slidably positioned inside the bore, wherein the component is substantially tubular and has an outer surface which is in contact with the inner surface of the wall and includes a longitudinally extending slot.

[0014] In one embodiment of the friction rock anchor each said longitudinal edge of the wall includes a respective flange which projects into the bore, and edges of said longitudinally extending slot of the component are in contact with surfaces of the flanges.BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The invention is further described by way of examples with reference to the accompanying drawings in which :Figure 1 depicts an elongate tubular member which is used in a friction rock anchor according to the invention,Figures 2 to 5 are examples of different cross sectional shapes which the tubular member of Figure 1 can have,Figure 6 is a side view of a friction rock anchor according to the invention,Figure 7 illustrates the optional use of an insert with a tubular member of the kind shown in Figure 1 ,Figures 8 and 9 show friction rock anchors according to the invention with different inserts,Figures 10 and 11 depict a possible enhancement to the friction rock anchor of the invention, andFigures 12 and 12A show in cross section preferred dimensions of the friction rock anchor according to the Figure 3 embodiment, and the optional use of an insert as per Figure 7.DESCRIPTION OF PREFERRED EMBODIMENTS

[0016] Figure 1 of the accompanying drawings illustrates an elongate substantially tubular member 10 which is used in a friction rock anchor according to the invention. The tubular member which is shown in cross section in Figure 2 includes a substantially circular wall 12 which encloses a bore 14 and which has an inner surface 16 and an outer surface 18.

[0017] The elongate member 10 has a first end 22 and an opposed second end 24. The member is formed with a slot 28 which extends continuously from the first end 22 to the second end 24. The slot has a nominal width W - see Figure 4. The slot is bounded by opposing edges 30 and 32 respectively of the wall which extend continuously from the first end to the second end. Each edge 30, 32 is bent inwardly towards the bore 14 as is indicated by curved arrows 34, by means of a suitable metal working process under factory conditions. This produces flanges 36 and 38 respectively which extend over a greater portion of the length of the tubular member from the first end 22 to the second end 24. However for maximum strength of the tubular member the flanges are continuous from the first end to the second end i.e. for the full length of the tubular member. The flanges 36 and 38 together with opposing adjacent sections 36A, 38A of the inner surface 16 define respective recesses 36B, 38B adjacent bounding portions of the slot

[0018] It is possible to configure the cross sectional shape of the tubular member 10 in other ways, shown for example in Figures 3 to 5. In Figure 3 the flanges 36 and 38 are produced by bending the edges 30 and 32 of the slot to a lesser extent than in the Figure 2 embodiment so that the flanges are directed more or less to a centre of the bore 14 i.e. are radial relative to a centre 14A of the bore 14.

[0019] In the Figure 4 example the flanges 36 and 38 are bent inwardly towards the inner surface 16 of the wall to a greater extent than in Figure 2. It is possible to bend the flanges so that edges of the flanges abut, i.e. are in contact with, the inner surface 16.

[0020] Figure 5 shows another version of the cross sectional shape of the elongate tubular member 10. The edges 30 and 32 of the wall adjacent the slot 28 are bent outwardly away from the bore 14. The bending is accompanied by a metal working process which effectively deforms the wall 12 adjacent the flanges 36 and 38 so that in outline the outer surface of the tubular member, excluding the slot 28, is effectively circular.

[0021] Figure 6 illustrates a friction rock anchor 42 according to one form of the invention. The anchor includes an elongate tubular member 10 of the kind shown in Figure 1 with a cross sectional shape which, in this instance, is similar to the shape shown in Figure 2 and Figure 3. The tubular member 10 has a length between the first end 22 and the second end 24 chosen according to factors which are known in the art of rock reinforcement and which are not further described herein. A reinforcing ring 44 surrounds the tubular member at the first end 22 and is welded (46) thereto. The cross sectional shape of the tubular member from the first end 22 to a location 48 is substantially constant. However from the location 48 towards the second end 24 the cross sectional area of the bore 14 is tapered, of reducing size, resulting for example from a swaging process, so that the second end 24 has a reduced cross sectional sizecompared to the cross sectional size of the member 10 between the first end 22 and the location48.

[0022] The friction rock anchor 42 can be modified through the use of at least one insert 54 as is shown in Figure 7. The insert comprises a component 56 which effectively constitutes a split set of reduced size. The component 56 has a circular outline save for a fairly large slot 60 which extends between a first end 62 and a second end (not shown) of the component. An outer surface 66 of the component 56 is profiled to fit with a fairly tight sliding action directly into the bore 14 of the tubular member 10. The insert 54 can be moved along the tubular member to any chosen position between the first end 22 and the location 48. This has a significant rigidifying effect on the tubular member and helps to increase the frictional force between the anchor and a wall of a borehole in which the anchor is installed. The insert can alternatively be used with a form of the friction rock anchor which does not have the flanges for it can be positioned inside the bore 14 and will provide an increased frictional force when installed.

[0023] in general the insert is used at any location inside the tubular member at which, once installed, additional strength is required. The insert significantly increases the shear value of the anchor.

[0024] Figure 8 illustrates an insert 54A at an intermediate position in a tubular member 10A between the first end 22 and the location 48 of an anchor 42A. Figure 9 shows an insert 54B, inside an anchor 42B, which has a tapered leading end 70 shaped to fit closely inside a tapered portion of an elongate member 10B between the location 48 and the second end 24, of complementary form to the insert. The insert, when used with a tubular member with a cross sectional shape say, as shown in Figure 2 or in Figure 3 i.e. with flanges 36, 38 which project into the bore 14, is shaped so that edges 60A, 60B of the slot 60, in the insert, respectively fitinto the recesses 36B, 38B adjacent the flanges 36, 38 as shown in Figure 7. If the flanges are folded inwardly to a greater extent, possibly into contact with the inner surface 16 of the wall, then the insert is modified, as appropriate, to abut or contact the flanges, possibly at edges of the flanges.

[0025] It is advantageous and desirable for a suitably shaped insert to be located inside the member 22 at the tapered leading end 70. The rock anchor, properly inserted into a borehole, can then provide a point anchor support effect due to the inclusion of the insert.

[0026] Figure 6 illustrates that the slot 28 in the tubular member 10 has a substantially constant width W from the first end 22 to the location 48, and that, from the location 48 to the second end 24 the slot reduces in width. This property is also exhibited by the anchors 42A and 42B shown in Figures 8 and 9.

[0027] Figure 9 illustrates the use of two rings 44A and 44B at the first end of the anchor 42B. These rings are welded to the elongate member 10B and in use act as a stop mechanism for a load spreading washer which is engaged with the rock anchor as is known in the art. This aspect is not further described herein. However it is pointed out that the rings in each instance are welded to an outer surface of the tubular member 10 and, in addition, are welded to adjacent portions of the flanges 36 and 38. The rings are also circumferentially and axially welded together and to the outer surface of the elongate member. The additional resistance force provided by the welding of the rings 44 to the flanges 36, 38 is of significance for the capability of the ring or rings, as the case may be, to hold a load-resisting washer in place during use of the anchor, when installed in a body of rock, is thereby enhanced.

[0028] Figures 10 and 11 illustrate a leading end 74 of an anchor 42X which is substantially of the kind shown in Figures 6, 8 and 9. Where appropriate like reference numerals are used todesignate like structural features. An outer surface 78 of the tapered leading end 74 of the anchor 42X has two, three or more wedge-shaped protrusions 80 welded to or formed in it. The cross sectional width of the anchor adjacent the protrusions, designated by the reference numeral 82 in Figure 11 , is slightly greater than the diameter 84 of a borehole 86 in which the anchor is positioned. This feature is beneficial for the protrusions are shaped and sized to allow the anchor to be driven into the borehole but offer a retentive action which prevents an anchor from sliding out of a borehole, an event which could be caused due to rock movement or other factors.

[0029] The wedge protrusions 80 are on the outer side of the tapered section. The use of the insert does not interfere with a self-drilling application or process - a significant benefit over conventional tubular anchors which include securing / anchoring mechanisms, possibly inside the tubular bolt, to hold the tubular bolt anchor in position in a borehole.

[0030] The anchor of the invention in its various embodiments presents a number of advantages over conventional split sets. In each case the elongate slot which extends continuously from one end 22 of the tubular member to the other end 24 is flanked by the flanges 36 and 38. The tubular member and the flanges can be formed in a tube mil) or may be bent, pressed, moulded or formed in any other appropriate way.

[0031] As indicated the flanges 36, 38 can extend into the bore (Figures 2, 3, 4) or out of the bore (Figure 5). The Figure 2 and Figure 3 embodiments are however preferred. These flanges impart significant additional strength to the anchor. The formation of each flange is accompanied by the formation of a curved junction 90 at which the material from which the wall 12 is made is work-hardened and stiffened during formation of the flanges. The torsional resistance of the wall is increased. As the flanges are displaced from the circular outline of thewall 12, the flanges also longitudinally reinforce the tubular member. The tapered leading end of the anchor allows for easy initial placement of the anchor into a borehole and thereafter the anchor is hammered into position using a suitable impact tool. The increased stiffness of the tubular member, produced by the flanges, enhances the resistance of the anchor to bending, which can arise due to the hammer action with an impact tool. The protrusions 80 shown in Figures 10 and 11 allow the anchor to be moved into the hole but as these protrusions are slightly oversized compared to the diameter 84 of the borehole they assist greatly in retaining the anchor in an installed position.

[0032] In some respects installation conditions relating to the use of split sets have been standardised. For example the diameter of a borehole should not normally exceed 46mm. However the diameter of the borehole in practice typically varies say from 34mm to 45mm, according to rock conditions and to the wear of a drill steel used to drill the borehole. A split set must be usable therefore with boreholes with diameters of a predetermined range of sizes. This characteristic has allowed for the cross sectional shape of the anchor of the invention to be optimized, with a preferred set of dimensional values for the example in Figure 3, as shown in cross section and in greater detail in Figure 12, for an anchor 42P wherein the “circular part” of the anchor has a diameter D = 46mm and the slot has a width S = 21mm. The minimum cross sectional dimension of the tubular member is H = 40, 1 mm. The width of each flange 36, 38 is F = 6mm to 8mm and the radius of the curved junction 90 is R = 3,5mm. The radius value should be in the range of 3,2 < R < 3,8 but R = 3,5mm has been found through tests to be of particular benefit for the Figure 3 example in which the flanges 36, 38 are radially directed to the centre 14A of the bore 14. The radial alignment of the flanges provides greater radially directed frictional forces between the outer surface 18 and an opposed wall surface of a borehole.

[0033] Figure 12A is similar to Figure 12 but depicts how an insert 54, of the kind referred to hereinbefore, can be positioned inside the elongate member 22, There is a close fit between these components and the insert adds materially to the shear strength of the anchor. The insert is situated at a position at which, in use, additional strength is required of the anchor.

[0034] The anchor of the invention has improved structural capabilities. The pull-out resistance after installation is increased by a factor of from 20% to 50% compared to a conventional split set. Additionally the tensile strength of the anchor is increased due to the work hardening aspect resulting from the formation of the flanges.

[0035] The flanges of the rock anchor of the invention offer increased torsional strength compared to a conventional split set. This allows for higher grade, lighter and thinner materials to be used in its manufacture. This means that materials including carbon steel, stainless steel and aluminium, or suitable alloys of the aforegoing, which may be corrosion-resistant, optionally suitably reinforced as appropriate with optional inserts 54, can be used with safety and effectiveness.

[0036] The inserts shown in Figures 7, 8 and 9 are used when required to add additional strength to a rock anchor installation, in each instance the insert is positioned at a chosen location inside the elongate tubular member prior to installation of the rock anchor. The insert acts as a split set mechanism of reduced size and in a region adjacent its installation location (inside a borehole) the pull-out force of the anchor is meaningfully increased.

[0037] It should be borne in mind that the rock anchor of the invention exhibits its best characteristics if the flanges are continuous and extend over a greater portion of the length of the tubular member. The flanges do not however necessarily have to be included in the tapered leading end of the tubular member.

[0038] Tests conducted under controlled conditions have given the following results, without the use of inserts i.e. normal installation techniques:TABLE 1 - A conventional split 3CR12 Stainless Steel:Thickness wall of 3,0mm - Maximum of kN 185 tensile force46mm outer diameter - Pull-out 40kN per meter.TABLE 2 - Stabilizer as per invention - 3CR12 Stainless Steel:Thickness of material 3,0mm - Maximum of kN 220 tensile force = 20% improvement 46mm outer diameter - Pull-out 60kN per meter = 50% improvement.TABLE 3 - Stabilizer as per invention - U-2205 Stainless Steel:Thickness of material 3,0mm - Maximum of kN 300 tensile force = 62% improvement compared to Table 1 figures46mm outer diameter - Pull-out 124kN per meter = 210% improvement compared to Table 1 figures.The benefits of the stabilizer of the invention can be summarised:(a) lipped (flanged) profile - structurally stronger causing a high torsion, tension spring-back effect i.e. greater frictional anchor force; and(b) higher shear resistance.Additionally the friction anchor force which is a measure of the pull-out force is increased by at least 30% in the area at which an insert is positioned (when used).

Claims

CLAIMS1. A friction rock anchor which comprises an elongate substantially tubular member with an internal bore, a first end, an opposing second end and a wall which has an inner surface and an outer surface and which includes a longitudinal slot which extends from the first end to the second end and which is bounded by opposed longitudinal edges of the wall, and wherein each edge includes a respective flange which extends over at least a greater portion of the length of the tubular member between the first end and the second end.

2. The friction rock anchor of claim 1 wherein each flange extends continuously.

3. The friction rock anchor according to claim 1 wherein each flange extends continuously between the first end and the second end.

4. The friction rock anchor of claim 1 wherein each flange projects into the bore.

5. The friction rock anchor of claim 4 wherein each flange is in contact with a respective portion of the inner surface of the wall.

6. The friction rock anchor of claim 1 wherein each flange is outside the bore and overlies a respective portion of the outer surface of the wall.

7. The friction rock anchor of claim 6 wherein each flange is in contact with said respective portion of the outer surface.

8. The friction rock anchor of claim 1 wherein the member has a substantially constant cross sectional shape from the first end to a location which is spaced from the second end, andis tapered with a cross sectional area which reduces in size from the location to the second end.

9. The friction rock anchor of claim 8 wherein between the location and the second end each edge does not include a respective said flange.

10. The friction rock anchor of claim 8 wherein each flange extends between the location and the second end.11 . The friction rock anchor of claim 8 wherein the slot has a substantially constant width from the first end to the location.

12. The friction rock anchor of claim 1 which includes at least one ring around the member at the first end which is welded to an outer surface of the member and to an adjacent portion of each flange.

13. The friction rock anchor of claim 1 which includes at least one insert comprising a component which is slidably positioned inside the bore, wherein the component is substantially tubular and has an outer surface which is in contact with the inner surface of the wall and includes a longitudinally extending slot.

14. The friction rock anchor of claim 8 which includes an insert located inside the tapered tubular member at least between the location and the second end.

15. The friction rock anchor of claim 13 wherein edges of said longitudinally extending slot of the component are in contact with surfaces of the flanges.

16. The friction rock anchor of claim 15 wherein edges of said longitudinally extending slot of the component extend into respective recesses formed between each flange and the inner surface of the wall.

17. The friction rock anchor of claim 1 which includes at least one protrusion on the outer surface of the wall between the first end and the second end.

18. The friction rock anchor of claim 8 which includes at least one protrusion on the outer surface of the wall between the location and second end wherein each protrusion is configured to provide a retentive action which acts against the elongate member from sliding out of a borehole in which the elongate member is located.

19. The friction rock anchor of claim 4 wherein an outer surface between each flange and an adjacent portion of the outer surface of the wall has a radius of curvature R where 3,2mm < R < 3,8mm and preferably R = 3,5mm.

20. A friction rock anchor which comprises an elongate substantially tubular member with an internal bore, a first end, an opposing second end and a wall which has an inner surface and an outer surface and which includes a longitudinal slot which extends from the first end to the second end and which is bounded by opposed longitudinal edges of the wall and wherein the friction rock anchor further includes at least one insert comprising a component which is slidably positioned inside the bore, wherein the component is substantially tubular and has an outer surface which is in contact with the inner surface of the wall and includes a longitudinally extending slot.

21. The friction rock anchor of claim 20 wherein each said longitudinal edge of the wall includes a respective flange which projects into the bore, and wherein edges of said longitudinally extending slot of the component are in contact with surfaces of the flanges.

Images