GB2202600A

GB2202600A - Yielding rock bolt

Info

Publication number: GB2202600A
Application number: GB08706925A
Authority: GB
Inventors: Peter David Bloomer; Maurice Richard Stokes
Original assignee: BOART
Current assignee: BOART
Priority date: 1987-03-24
Filing date: 1987-03-24
Publication date: 1988-09-28
Also published as: GB8706925D0

Abstract

A rock bolt rod 22 is anchored at its inner end in a hole in a rock face, the rod 22 being mounted inside a ductile tube 28, and having an enlargement 26 of larger diameter than the tube 28 in its unstressed condition, so that the enlargement 26 can only pass along tube 28 by expanding it. The bolt yields in response to rock movements while supporting the rock face. The extent of yielding can be detected by inserting a probe through hole 34. The rod 22 is anchored by resin (as shown); alternatively anchoring is done by applying tension to rod 22. In the latter case, the outer end of rod 22 is screw threaded to receive a screw threaded enlargement 26 which is gripped by the tube 28. The tube 28 can be rotated to tension rod 22 by means of a hexagonal head. The fixed plate 32 is replaced by a loose washer engaged by a hemispherial formation on the hexagonal head. <IMAGE>

Description

TITLE: A Rock Bolt This invention relates to rock bolts for securing exposed rock faces in mines.

Rock bolting is a known technique wherein an elongate hole is drilled in the rock face to be secured, and an elongate rod is secured in the hole either through an expanding, mechanical anchor or through a resin mixture which sets in the hole. A support plate is then secured at the outermost end of the rod, so that the plate is held against the rock face, and this serves to tie the rock face back to the solid rock where the innermost end of the rod is secured.

One problem with known rock bolts is that their failure mode can be sudden, and there is no easy way to tell by inspection whether a particular bolt is near to failure. Various proposals have been made for yielding bolts which will allow some rock movement without failure of the bolts, but these have all been less than satisfactory either because there has been no visible indication that yield has taken place, or because of undue complexity.

According to the present invention there is provided a rock bolt comprising a rod adapted to be anchored in a hole in a rock face, a radial enlargement at one end of the rod, a tube for surrounding the rod at said one end and means connected to the tube for abutting the exposed rock face, wherein the external diameter of the radial enlargement is greater than the internal diameter of the rod by an amount which allows the enlargement to be forced into the tube thereby to radially expand the tube.

With such a bolt, yielding can take place by movement of the enlargement along the length of the tube.

However this movement will be accomplished by radial expansion and thus deformation of the tube to absorb energy and to provide mechanical resistance to such yielding.

Preferably the end of the bolt which is exposed at the rock face has a central opening, so that a probe can be inserted to determine the position within the tube of said one end of the rod. This will indicate the extent to which yielding has already taken place.

The radial enlargement may be a collar threaded onto the end of the rod, or may be an enlargement formed in the material of the rod, by upsetting the rod end.

The material of the tube is preferably mild steel which possesses a suitable degree of ductility. In one practical embodiment, the external diameter of the enlargement can be between one and five per cent greater than the internal diameter of the tube.

However the actual ratio selected for a particular application will depend on the mechanical properties of the tube material and on the load at which yield is designed to take place, bearing in mind the particular rock conditions in question.

there the bolt is to be anchored by resin in the hole, no tensioning of the bolt is required. However where a mechanical anchor is to be used, it is necessary to apply a torque to the end of the bolt which is exposed at the rock face in order to activate the anchor. In order to do this, a torque receiving head, such as a hexagonal bolt head, can be attached or formed on the exposed face of the tube, and in this case the radial enlargement will be a collar threaded onto the end of the rod so that as the tube is rotated, the collar being normally held fast in the tube, will rotate on its thread and apply a tension to the rod.

In order to cater for the insertion of bolts other than at right angles to the rock face, the rock face abutment means may comprise a surface which is allowed to take up an angle to the bolt axis suitable to provide a wide bearing area on the rock face. It is possible to provide a hemispherical surface on the outer end of the tube, and a part-spherical seat on a loose support plate.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 illustrates installation of a conventional rock bolt: Figure 2 is a section through part of a bolt in accordance with the invention Figure 3 is a section through part of an alternative form of bolt in accordance with the invention and Figure 4 shows a detail of the bolt of Figure 3.

Figure 1 shows a bolt 10 secured in a hole 12 by means of a set resin mixture 14. The bolt has a head 16 and a support plate 18 behind the head. The plate 18 butts up against an exposed rock face 20 which can for example be the rock face in a mine tunnel or roadway.

In use, the hole 10 is drilled and then a cartridge containing the two components of a resin mixture is inserted in the hole. On subsequent introduction of the rod 10, the two components are mixed whereupon the resin hardens and holds the bolt firmly in place. The effect of this is that the support plate 18 holds the exposed surface of the rock back against the bulk of the rock in the area away from the face itself.

Figure 2 shows a simple form of bolt in accordance with the invention. The bolt comprises an elongate rod 22 which has a rearmost end 24 which will be at the deepest point of the hole 12 in the rock, and which will be actually anchored in the rock either by resin or by a mechanical anchor. At the opposite end of the rod 22 is a terminal enlargement 26 which can be formed typically by upsetting the rod end. The external diameter of the enlargement 26 is slightly greater than the internal diameter of a tube 28 in which the rod is fitted. The tube 28 surrounds the end of the rod and the rod itself will normally extend very much further into the hole than the tube. Figure 2 shows, at the region 30, how the tube is expanded slightly when the enlargement 26 is pulled into the tube.

At the mouth of the tube, à support plate 32 is attached and this support plate will butt up against the face of the rock surrounding the drilled hole.

Once the bolt has been correctly inserted and anchored, if the rock yields then the distance between the inner end 24 of the rod and the support plate 32 will need to lengthen. Movement of the rock face will pull the tube, with the support plate 32, over the enlargement 26. Since this movement will require plastic deformation of the tube, there will be a resistance to the movement, but the bolt will continue to support the required lead over a wide range of dimension changes.

This therefore provides the first objective of the invention which is to provide an easily constructed bolt which permits controlled yielding.

Furthermore however it is possible to tell at any time the extent to which yielding has taken place by inserting a probe into the tube 28 through the hole 34 at the mouth of the tube. Using such a probe the position of the end 26 of the rod can be determined at any time and thus it is easy to know to what extent yielding has taken place and whether the rock is still safely held in place by the bolts or whether it has become dangerously unstable.

Figure 3 shows a form of the bolt which is particularly suitable where a mechanical anchor is used and where the bolt has to be tensioned after insertion in the drilled hole. In this embodiment, the outer end 26a of the rod is threaded and carries a threaded collar 36 which forms the enlargement at the end of the rod. At the outer end of the tube 28, a hemispherical bearing surface 38 is secured, and this co-operates with a part-spherical seat 40 in a support plate 42. This allows the support plate 42 to be correctly positioned against the rock face even if the axis of the bolt is not at right angles to the rock face. Furthermore, a hexagonal head 44 is secured to the end of the tube so that a spanner or other torque applicator can be connected to the tube to rotate the tube.When the tube is thus rotated, the collar 36 is gripped by the tube and turned on the thread on the end 26a of the rod 22. This will cause the mechanical anchor on the rod to be activated, and the plate 42 to be drawn up against the rock surface. Thereafter the bolt will function as described before, and it is to be noted that the hexagonal head 44 has a central passageway 46 through it to allow a measuring probe to be inserted.

Figure 4 shows the collar 36 and the tube 28 before they are assembled to one another. The collar 36 has an internal thread 48 which will engage with an external thread on the end 26a of the rod 22 (not shown in Figure 4). Note that the end of the collar 36 facing the tube 22 has an annular chamfer 50 to facilitate lead-in.

The external dimension (indicated by 52) of the collar 36 is about 3t greater than the internal dimension 54 of the tube 22. In one practical example, the diameter 52 amounted to 22 mm whilst the internal diameter 54 of the tube 22 amounted to 21.3 mm. A tube of 14 gauge was found to be suitable, and with these parameters the bolt started to yield (ie the collar 36 started to move along the length of the tube 22) when a load in the region of 2 - 3 tons was applied between the ends of the bolt.

The bolt described can be made from readily available materials. Notably the tubing is a standard commercial product. Assembly is straightforward, as is the manner of installation. The operation, whilst the bolt is in use, of checking the amount of yield that has taken place is also simple to carry out.

It will be apparant to a skilled man that the force required to produce yield can be varied by varying the relative dimensions of the tube and of the collar or enlargement at the end of the rod, or by suitably selecting the material of the tube.

Claims

1. A rock bolt comprising a rod adapted to be anchored in a hole in a rock face, a radial enlargement at one end of the rod, a tube for surrounding the rod at said one end and means connected to the tube for abuting the exposed rock face, wherein the external diameter of the radial enlargement is greater than the internal diameter of the rod by an amount which allows the enlargement to be forced into the tube thereby to radially expand the tube.

2. A rock bolt as claimed in Claim 1, wherein that end of the bolt which is exposed at the rock face has a central opening, so that a probe can be inserted to determine the position within the tube of said one end of the rod.

3. A rock bolt as claimed in Claim 1 or Claim 2, wherein the radial enlargement is a collar threaded onto the end of the rod.

4. A rock bolt as claimed in Claim 1 or Claim 2, wherein the radial enlargement is an enlargement formed in the material of the rod, by upsetting the rod end.

5. A rock bolt as claimed in any preceding claim, wherein the material of the tube is mild steel.

6. A rock bolt as claimed in any preceding claim wherein the external diamter of the enlargement is between 1% and 5% greater than the internal diameter of the tube.

7. A rock bolt as claimed in any preceding claim1 wherein the rod is adapted to be anchored in the hole by mechanical means, wherein a torque receiving head is provided on the exposed face of the tube, and wherein the radial enlargement is a collar threaded onto the end of the rod.

8. A rock bolt as claimed in Claim 7 wherein the torque receiving head is a hexagonal bolt head.

9. A rock bolt as claimed in any preceding claim, wherein the means for abutting the exposed rock face comprises a surface which is allowed to take up an angle to the bolt axis suitable to provide a wide bearing area on the rock face.

10. A rock bolt as claimed in Claim 9, wherein a hemispherical surface is provided on the outer end of the tube, and a part-spherical seat is provided on a loose support plate.

11. A.rock bolt substantially as herein described with reference to any one embodiment shown in the accompanying drawings.