ROCK BOLT

Abstract

A rock bolt which includes a tubular member made from aluminium which is at a fixed position inside or outside a borehole, a cable which goes through the tubular member, and a steel barrel, secured to the cable which deforms the tubular member to give a yielding action with rock movement.

Claims

1. A rock bolt which includes an elongate member, a tube which is made from a first material and which has a bore, an inner surface opposing the bore and an outer surface, and a barrel which is fixed to the elongate member and which is located at least partly in the bore, wherein the barrel is made from a second material which is harder than the first material and includes yield formations, configured to engage with the inner surface of the tube and to deform the first material in response to a tensile force exerted on the elongate member which forces the barrel further into the bore.

2. A rock bolt according to claim 1 wherein the tube has a first end and a second end, and the barrel enters the bore of the tube at the first end and the elongate member extends from the second end of the tube.

3. A rock bolt according to claim 1 which includes an anchor for engaging with a wall of a borehole and wherein the tube and the barrel are in use positioned outside the borehole.

4. A rock bolt according to claim 1 which includes an anchor for engaging with a wall of a borehole and wherein the tube and the barrel are in use positioned inside the borehole.

5. A rock bolt according to claim 1 which includes an anchor which in use is located inside a borehole into which the elongate member extends and which fixes the elongate member to the wall of the borehole, or which fixes the tube to the wall of the borehole.

6. A rock bolt according to claim 1 wherein the first material is aluminum and the second material is steel.

7. A rock bolt according to claim 1 wherein the yield formations comprise ribs on a surface of the barrel which opposes the inner surface of the tube and a respective valley is formed between each adjacent pair of the ribs.

8. A rock bolt according to claim 7 wherein the ribs are tapered, in a radial sense, moving away from an end of the elongate member to which the barrel is fixed.

9. A rock bolt according to claim 7 wherein a thickness of the tube between the outer surface and the inner surface is such that when the first material is deformed, the formations on the barrel extend into the tube, from the inner surface, but do not extend to the outer surface.

10. A rock bolt according to claim 2 wherein the first material is aluminum and the second material is steel.

11. A rock bolt according to claim 3 wherein the first material is aluminum and the second material is steel.

12. A rock bolt according to claim 4 wherein the first material is aluminum and the second material is steel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The invention is further described by way of examples with reference to the accompanying drawings in which:

[0014] FIG. 1 is a side view in section of a rock bolt according to one form of the invention in an installed configuration,

[0015] FIG. 2 shows components of the rock bolt of FIG. 1, disengaged from each other,

[0016] FIG. 3 is a view in perspective of a barrel which is used in the rock bolt,

[0017] FIG. 4 illustrates in cross section the shape of a tube, in the rock bolt, which is deformed by the barrel during use of the rock bolt,

[0018] FIG. 5 is similar to FIG. 1 showing a rock bolt according to a second form of the invention, and

[0019] FIG. 6 which is similar to FIG. 2, shows components of the rock bolt of FIG. 5.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0020] FIG. 1 of the accompanying drawing illustrates a rock bolt 10 according to the invention in an installed configuration. The rock bolt includes an elongate member 12 in the form of a cable or a rod, an expansion assembly 16 and a yielding mechanism 18.

[0021] The expansion assembly or anchor 16 may be of any appropriate kind and, only by way of example, may comprise an expanding shell assembly which is fixed to an end of the member 12 which is located inside a borehole 22 within which the rock bolt is installed. That type of assembly which is known in the art is actuable into engagement with a wall of a borehole into which the elongate member 12 extends.

[0022] The yielding mechanism 18 includes a barrel 30 (FIG. 3) which is made from a hard steel. The barrel has an inner thread 32 which is used to fix the barrel securely to an end 34 of the elongate member 12. On an outer surface the barrel has a plurality of ribs 38. Between each adjacent pair of ribs a respective valley 40 is formed.

[0023] The ribs 38 do not extend the full length of the barrel, in an axial direction. Each rib 38 is tapered in the sense that it decreases in dimension, in a radial sense, moving from what may be referred to as a trailing end 42 of the barrel to a leading end 44 which includes a cylindrical section 46.

[0024] The mechanism 18 also includes an elongate aluminium tube 50see FIG. 2. The tube has a bore 52 which is surrounded by an inner surface 54 and an outer surface 56. A wall 60 of the tube, between the inner surface 54 and the outer surface 56, has a thickness 62. The aluminium in the tube 50 is not as hard as the steel used for the barrel 30 and will yield/deform before the steel, in the barrel when the aluminium and the steel are simultaneously subjected to a compressive force of a high value.

[0025] The cylindrical section 46 at the leading end of the barrel fits closely into the bore 52. This helps to align the barrel relative to the tube 50, exerts a guiding action on the barrel, and counters any tendency for the tube to collapse inwardly, under load.

[0026] In use of the rock bolt, as shown in FIG. 1, the expansion assembly 16 is inserted into the borehole 22. The expansion assembly or anchor 16 is actuated in a known way and thereby secures the member 12 to a wall of the borehole. The yielding mechanism 18, which is at a trailing end of the elongate member, does not enter the borehole and is exposed. The tube 50, at one end 64, is engaged with a load-spreading washer 66 which bears on a surface 68 of the rock adjacent a mouth of the borehole 22.

[0027] The barrel 30 enters the bore 52 at a first end 76 of the tube, and the member 12 extends from the other end 64 of the tube.

[0028] If, due to rock movement, a tensile force is applied to the member 12 then the barrel 30 is pulled further into the bore 52 of the tube. The tube 50 cannot move in an axial sense because it is restrained by the washer 66 which abuts the rock surface at the mouth of the borehole.

[0029] The barrel, as it moves into the bore 52, exerts an expansive force on the tube but, due to the thickness of the wall of the tube, the dimension of the outer surface 56 is not meaningfully altered in a radial direction. A support function is also created by the cylindrical section 46. The ribs 38 engage tightly with the inner surface 54 and then dig into the wall 60 of the tube. As the barrel 30 is made from a material (steel) which is harder than the material (aluminium) from which the tube is made, each rib 38 forms a respective groove or channel 78 which extends into the wall from the inner surface 54. Material of the tube, displaced by the advancing ribs, flows into the various valleys 40. A yielding action results wherein the yield force is determined by a number of factors including the relative hardnesses of the steel and the aluminium from which the barrel and the tube are respectively made, the number of ribs, the sizes of the ribs, the taper angle of the ribs, and the like.

[0030] The yielding action, on a force versus distance of yield basis, is substantially linear. As the tube 50 has a constant wall thickness 62 and as the barrel 30 is not deformed in any meaningful way during yielding the yielding action is constant and smooth. By increasing the length of the tube the distance over which a yielding action can be achieved is also increased.

[0031] Referring to FIG. 1 the yielding mechanism 18 of the rock bolt 10 according to the first form of the invention, is positioned outside the borehole 22. A reactive force is exerted on the tube 50 due to the washer 66 which bears against the rock surface 68 at the mouth of the borehole.

[0032] FIG. 5 shows a rock bolt 90 according to a different form of the invention in which the yielding mechanism 18 is inside the borehole 22. At the mouth of the borehole (not shown) a load-spreading washer is positioned, bearing against a surface adjacent the mouth of the borehole, and an anchor which acts against an outer surface of the washer is fixed to a protruding end of the elongate member 12.

[0033] The rock bolt 90 includes a tube 50A which is normally longer than the tube 50 described hereinbefore but otherwise is similar save that, at the second end 64, the tube has fixed to it an expansion member 94. The expansion member is configured to engage with an anchor 98, as shown in FIG. 5.

[0034] With the rock bolt 90 assembled and installed in a borehole as shown in FIG. 5, when rock movement takes place a tensile force is exerted on the elongate member 12 acting in a direction 100. This force urges the expansion member 94 into the anchor 98 which is then expanded in a radial direction into engagement with a wall 22A of the borehole 22. The arrangement is one in which the elongate member 12 does not engage directly with the anchor and thus can move through a passage 102 inside the anchor 98. In contrast to the FIG. 1 embodiment the tube 50A, inside the borehole passage is fixed to the wall of the borehole. In FIG. 1 the tube is outside the borehole and the member 12 is fixed to the wall of the borehole.

[0035] The anchor 98 is fixed to the wall 22A of the borehole and does not permit movement of the tube 50A. A tensile force exerted on the member 12 by rock movement in the direction 100 pulls the barrel 30 into the bore 52 of the tube. The yielding action which ensues is similar to what has been described hereinbefore in that the barrel 30 which is made from a hard material advances into the tube 50A which is made from a relatively softer material and, as a consequence, the wall of the tube is deformed producing a substantially constant and smooth yielding action.