GB2060109A

GB2060109A - Rock Bolt

Info

Publication number: GB2060109A
Application number: GB8013813A
Authority: GB
Original assignee: Bergwerksverband GmbH
Current assignee: Bergwerksverband GmbH
Priority date: 1979-07-11
Filing date: 1980-04-25
Publication date: 1981-04-29
Also published as: DE2927953C2; DE2927953A1; FR2461093B1; GB2060109B; FR2461093A1

Abstract

A tie rod 3 has an end portion 9 of reduced cross-section surrounded 14 by a casing 4 fitted with a shear lining 8. Studs 10 on the portion 9 project into the shear lining 6. The bolt is anchored in the borehole bottom 14 and mouth 15 by, respectively, an external surface configuration 7 (of the casing 4 or of the tie rod 3) and a stressing and retaining element 13 (on the tie rod 3 or on the casing 4) or an external surface configuration 7 (on the casing 4). The tie rod 3 and casing 4 are relatively slidable under load. <IMAGE>

Description

SPECIFICATION Rock Bolt This invention relates to a rock bolt comprising a casing and a tie rod which are connected to each other and are slidable relative to each other under load.

Such a bolt is described in German Patent Specification 2 411 706 and permits substantial relaxation movements of the rock without losing its retaining and stressing function through fracture of the tie rod.

When using bolts of this kind, the bond strength of the system comprising the rock and the bolt no longer depends on whether the relaxation motion of the rock can be absorbed by the maximum possible elongation of the tie rod material or whether the rock configuration around the roof bolt happens to be suitable. Instead, the extensibility of the bolt can be adapted to the anticipated relaxation motions of the rock, for example as determined by tests, by adjusting the allowable length of sliding travel. The necessary resistance against the relaxation contingency of the rock can be provided by adjusting the sliding resistance of the load supporting elements.

The prior art also discloses rock bolts in which a smooth tie bolt begins to slide in a sleeve of synthetic resin mortar formed between the bolt and the rock when a specific sliding resistance is exceeded.

The important disadvantage of known rock bolts is that initially the tie rod commences sliding against a high sliding resistance but subsequently the sliding resistance drops to a very low value, both the upper and lower sliding resistance values depending on the reaction of the rock and no limitation being possible.

What is desired is a bolt in which the resistance to sliding can be maintained at its initial value despite corresponding relaxation motions of the rock.

The present invention provides a rock bolt comprising a tie rod, which is to be disposed within a borehole, the tie rod being surrounded, on at least one side, by a tie casing, constructed as anchor head and/or stressing element, being providing with at least one tie rod end portion of reduced section and supporting shear cams over its circumference, and the space between the tie rod and the casing tube being completely filled with a shear lining.

According to one embodiment of the invention, the means for anchoring the bolt in the bottom of the borehole is constituted by an external surface configuration of a said casing, and the means for anchoring the bolt in the mouth of the borehole comprises a stressing and retaining element on the opposite end of the tie rod.

In another embodiment, the means for anchoring the bolt in the borehole mouth comprises a stressing and retaining element on one end of a said casing, and the means for anchoring the bolt in the borehole bottom is constituted by an external surface configuration of the opposite end of the tie rod.

In a further embodiment, the tie rod has two said end portions surrounded by respective casings, preferably connected by a zone of preferred frangibility.

By means of the rock bolt of the invention a predefined set load can be made available in a simple and economical manner, given adequate anchoring of the bolt in the borehole, and the magnitude of the load is substantially maintained during shearing of the lining.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a sectioned side view of a rock bolt anchored in a borehole; Figure 2 is an enlargement of the borehole bottom detail; Figure 3 is a sectioned side view of a rock bolt with a casing tube constructed as a stressing and retaining element; and Figure 4 is a similar view of a rock bolt with a tie casing which completely surrounds a tie rod.

Figure 1 shows a longitudinal section through a rock-bolt borehole 1 sunk into the rock 2. The rock bolt disposed in the borehole substantially comprises a tie rod 3 and a casing tube 4 which surrounds the tie rod 3 at a distance. Outside the borehole 1, the tie rod 3 is provided with a stressing and retaining element 13 bearing against an anchor plate 11. The tube 4 has an anchor head 6 embedded in adhesive mortar 5 and thereby firmly anchored in the borehole bottom 14. The surface of the anchor head 6 is provided with a profiled section 7 to increase the resistance to sliding. The profiled section 7 illustrated in Figure 1 or Figure 2 must be regarded as one of many possible configurations.

The tie rod end portion 9 disposed within the anchor head 6 is reduced in cross-section and converges towards the end in the interests of improved centring. Shear cams 10 are distributed over the circumference of the reduced portion 9; in the illustrated embodiment the shear cams 10 are in the form of studs attached to the tie rod by a stud welding procedure.

The studs 10 project slightly beyond the periphery of the tie rods 3. The space between the tie rod 3 and the casing 4 is completely filled with a shear lining 8 over the entire length of the casing 4. The lining 8 comprises a material which is poured in liquid form into the casing 4 before or after the introduction of the tie rod 3 and is then cured. A material comprising a synthetic resin or synthetic resin mortar, for example a mixture similar to that of known rock bolt slugs, is provided to this end.

Chipping or shearing of the shear lining 8 along the longitudinal extent of the rock bolt is caused by the studs 10 in the event of relaxation motions of the rock 2 which cause the set load of the roof bolt to be exceeded. This results in grooves 11 being worked by the studs 10 into the shear lining 8. Escape to the rear side of the studs 10 of the material extruded from the shear lining 8 from in front of the studs 10 is ensured by the reduced section of the end portion 9, since without this the chipped or sheared material would accumulate in front of the studs 10 and would cause the casing 4 to bulge or the set load to rise.

The means which allow the chipped or sheared material to flow to the rear of the studs 10 ensures that the magnitude of the initially provided sliding resistance or the available set load is maintained.

In the embodiment shown in Figure 3, the tie bar end portion 9' of reduced section, provided with studs 10' and is surrounded by the tie casing 4, is disposed in the region of the borehole mouth 15. In this embodiment the tie rod end portion in the borehole bottom 14 is constructed as the anchor head 6 provided with the profiled section 7. Outside the borehole 1 the casing 4 is provided with the stressing and retaining element 13.

In the embodiment shown in Figure 4, the tie rod 3 is reduced in cross-section at both end portions 9,9', provided with studs 10,10', and is surrounded by a totally inclosed tie casing 4. In the middle the casing 4 is provided with a circumferential zone of preferred frangibility 1 6 defining two separable half-casings. The end of the casing 4 which is disposed in the borehole bottom 14 is surrounded by adhesive mortar 5 and is constructed as an anchor head 6 provided with a profiled section 7. The end of the casing 4 in the borehole mouth 15 is also provided with a profiled section 7 and is constructed as a stressing and retaining element 13. To assist insertion of the tie casing 4 its front is provided with a cutting edge 18 while the end in the borehole mouth 15 is provided with an insertionaiding member 17.

Claims

1. A rock bolt having means for anchoring the bolt in the mouth and the bottom of a borehole and comprising a tie rod at least one end portion of which is surrounded by a casing, the or each said end portion being of reduced cross-section and supporting shearing elements on its periphery, the peripheral space between the tie rod and the or each casing being completely'filled with a shear lining.

2. A rock bolt as claimed in claim 1, in which the means for anchoring the bolt in the bottom of the borehole is constituted by an external surface configuration of a said casing, and the means for anchoring the bolt in the mouth of the borehole comprises a stressing and retaining element on the opposite end of the tie rod.

3. A rock bolt as claimed in claim 1, in which the means for anchoring the bolt in the borehole mouth comprises a stressing and retaining element on one end of a said casing, and the means for anchoring the bolt in the borehole bottom is constituted by an external surface configuration of the opposite end of the tie rod.

4. A rock olt as claimed in claim 1, in which the tie rod has two said end portions surrounded by respective casings.

5. A rock bolt as claimed in claim 4, in which the said anchoring means are constituted by external surface configurations of the casings.

6. A rock bolt as claimed in claim 4 or 5, in which the casing end which is to be in the borehole mouth is provided with an insertionaiding member.

7. A rock bolt as claimed in any of claims 4 to 6, in which the casing end which is to be in the borehole bottom is provided with a cutting edge.

8. A rock bolt as claimed in any of claims 4 to 7, in which the two casings are connected together by a peripheral zone of preferred frangibility.

9. A rock bolt as claimed in any of claims 1 to 8, in which the or each shear lining comprises a synthetic resin.

10. A rock bolt as claimed in any of claims 1 to 9, in which the shearing elements comprise studs.

11. A rock bolt substantially as described with reference to, and as shown in, Figures 1 and 2, Figure 3, or Figure 4 of the accompanying drawings.