GB2110275A - Rock anchor - Google Patents

Abstract

A rock anchor consists of a bundle of annular rods enclosing a tube (2). At one end, the ends of the rods pass through an anchor plate (5) and are welded to one side thereof. The anchor plate includes an aperture through which adhesive can be injected into a bored hole (3) into which the rod bundle has been inserted. The tube (2) acts as a vent tube allowing the adhesive to stick the bundle in the hole over its entire length. The bundle is kept together and central in the bored hole by spaced centring rings (15). <IMAGE>

Description

SPECIFICATION Rock anchor The invention relates to a rock anchor, particularly for use as a constructional element, consisting of an anchor rod which is to be adhered over its whole length in a bore hole and an anchor plate designed to be located at the mouth of the bore hole.

Rock anchors used in underground mining serve generally, because of their dowel and pretensioning action, to render the hollow space secure.

As well as rock anchors of the initially noted type which are adhered into the bored hole over their entire length, rock anchors are known which consist of an anchor rod to one end of which an anchor head is fixed which is constructed as a spreadable element which is in the deepest part of the anchor bore hole, tensionable against the wall of the bored hole or can be adhered therein and on the other end of which, at the bore hole mouth, there is arranged an anchor plate which can be tensioned against the rock e.g. with a nut.

In addition to this, so-called cable anchors consisting of wire rope are known which likewise, at least partially stuck in, are tensioned with an anchor plate at the bore hole mouth.

All these rock anchors are to be used in underground hollow spaces as the sole construction or at least as additional construction in dependence on the size of the space.

Such an anchor construction can be of particular importance if the normal gallery construction consisting of copings and pit props reduces the cross section of the gallery too much. Such restriction arises particularly when proceeding to open up low height coal measures, for example having thicknesses between 70 and 130 cm.

If one starts from the fact that in such extraction the shield components necessary for later support devices have to be transported, then it emerges as absolutely necessary to choose a construction which guarantees the greatest possible free gallery cross section.

For extraction with a gallery height between 70 and 1 30 cm the normal construction consisting of copings and hydraulic pit props is no longer in question. The use of rock anchors as are noted initially must accordingly be eliminated, since the known rock anchors cannot be introduced into the slopes in sustainable form which, always starting from the small gallery heights and small gallery widths and the tensionability which is necessary, need a length of, for example, 6 m. In addition to this the ends which carry the pretensioning device and which are arranged below the anchor plates restrict the clear gallery cross section to a not insubstantial degree.

In the light of these difficulties in connection with a construction, particularly for extracting within low thickness measures, the invention faces up to the problem of producing a rock anchor which while taking into account the narrow constraints of space can be prepared in simple fashion and which can be introduced by hand into the rock, which has a sufficient bearing ability having regard to the strong convergence to be expected and which on the other hand does not in any way narrow the gallery cross section.

In accordance with the invention there is provided a rock anchor, consisting of an anchor rod portion designed to be adhered over its whole length into a bored hole and an anchor plate to be arranged at the mouth of the bored hole, wherein the anchor rod portion is constructed as a bundle of anchor rods incorporating in a central position a thin walled tube, the anchor rods being of small cross section, the rods and tube being held essentially parallel to one another, and wherein the ends of the rods are fed through an aperture in the anchor plate and welded to the side of the anchor plate remote from the anchor rod portion.

This construction of the rock anchor allows the anchor rod portion to make an angle with the anchor plate wherein this angle is dependent on the height and breadth of the extraction as well as on the length of the anchor. With the use of rock anchors when extracting with a measure thickness of 70 to 130 cm the angle is generally about 50 .

Distributed along the anchor length there are set on the anchor rods spaced apart centring stars of spring steel sheet, which hold the anchor together during introduction into the bore hole and on the other hand also centre the anchor in the bore hole in such a way that even adhesion is guaranteed.

The way in which the anchor rods are connected with the anchor plate guarantees the greatest possible gallery cross sections so that the construction of the face cannot depend on the anchoring heads, whereby in the one case the construction will be endangered and on the other the shield caps could be damaged.

The anchor rods, which run in a bundle and parallel to one another and are of small cross section, make possible a large flexibility of the anchor and facilitate its introduction without the use of machinery. The flexibility and the large anchor length of the anchor rods in accordance with the invention however prevent the use of adhesive cartridges in the depths of the bored hole.

For this reason the anchor is secured in the rock by the injection of a two component adhesive through an injection opening provided in the anchor plate into the annular space surrounding the anchor rods; during injection the thin walled steel tube provided in a central position between the anchor rods acts as a vent tube.

For sealing the bore mouth, between the anchor plate and the rock there is arrange a several centimetre thick sealing, for example of foam material. Since the rock anchor according to the invention, the ends of which are welded to the underside of the anchor plate are prepared aboveground, their handling below ground is connected with great security on installation.

The individual anchor rods are preferably manufactured from a tough high tensile steel specially developed for anchors, type RM 90, which has a fracture strength of 700 N/mm2 at a total extension of 22%. The high extensibility is achieved by the unusually high elongation for reduction of area. The breaking load of the rock anchors according to the invention can amount, e.g. to 262 KN.

An exemplary embodiment of the invention is illustrated in the accompanying drawings and is described in more detail in the following. In the drawings: Figure 1 is a section through bore hole in which a rock anchor according to the invention is introduced, Figure 2 is a plan view of the anchor plate of the rock anchor of Figure 1, and Figure 3 is a section through the bore hole according to lines I-I on Figure 1.

Referring to the drawings, a rock anchor according to the invention consists of a bundle of anchor rods 1 of small cross section arranged parallel to one another. The number of anchor rods 1, in the exemplary embodiment shown in the drawings, is four, but other numbers may be used.

In the central position, surrounded by the anchor rods 1, there is provided a steel tube 2. The anchor rods 1 pulled together to a bundle including a steel tube 2 are kept together by centring rings 1 5 and are introduced by hand in simple fashion into a prepared bore hole 3.

Ends 9, 10, 11 and 12 of rods 1 are bent round on to the underside 6 of an anchor plate 5 and fixed thereto by welding. The welding seams are denoted at 13. Since the angle a between the introduced bore hole 3 and the attachment is dependent upon the breadth and width of the measure as well as the length of the anchor rods, the anchors, i.e. the connection of anchor rods 1 with the anchor plate 5, can be prepared above ground.

The centring rings 1 5 are held by welded spots 1 6 on to the anchor rods 1.

The construction of the anchor rods 1 with small cross section allows them to be bent round below the anchor plate 5. Bending round is facilitated in that the passage opening 8 broadens out downwardly like a trumpet and is constructed rounded off. In this way any sharp edge bending off the individual anchor rods 1 is prevented. For sealing the annular space 4 in the region of the mouth of the bore hole between the rock and the anchor plate 5 there is introduced a foam seal 14 several centimetres, e.g. 5 cm, thick. Above the injection opening 7 by means of a special injection device the two component adhesive is introduced into the annular space 4, which is vented via the steel tube 2.

The specially spread out arrangement of the ends 9, 10 and 11 or 12, as illustrated in Figure 2, is to be considered only as one example of the way in which the ends 9-12 can be fixed to the anchor plate 5 in loadbearing fashion. Other ways of fixing the ends 9-12 to the anchor plate 5 can be envisaged, but none should cause the rods to project over the edge of the anchor plate.

Claims (9)

1. A rock anchor, consisting of an anchor rod portion designed to be adhered over its whole length into a bored hole and an anchor plate to be arranged at the mouth of the bored hole, wherein the anchor rod portion is constructed as a bundle of anchor rods incorporating in a central position a thin walled tube, the anchor rods being of small cross section, the rods and tube being held essentially parallel to one another, and wherein the ends of the rods are fed through an aperture in the anchor plate and welded to the side of the anchor plate remote from the anchor rod portions.

2. A rock anchor according to claim 1 wherein the bundle consists of at least four anchor rods.

3. A rock anchor according to claim 1 or 2 wherein the tube extends from near the end of the bundle of anchor rods and through the anchor plate.

4. A rock anchor according to any one of claims 1 to 3 wherein the anchor plate includes, at a short distance from the aperture an injection opening through which adhesive may be injected.

5. A rock anchor according to claim 4 and including an annular seal on one side of the anchor plate and surrounding the anchor rods and the injection opening.

6. A rock anchor according to any one of claims 1 to 5 wherein the aperture in the plate has a rounded off broadening trumpet shape in section transverse to the plate.

7. A rock anchor according to any one of claims 1 to 6 wherein the ends of the anchor rods are welded to the underside of the anchor plate in a spread out arrangement, wherein at least three welded on ends forming an obtuse angle with their remainders and at least one welded on end forming an acute angle with its remainder.

8. A rock anchor according to any one of claims 1 to 7 wherein the bundle of anchor rods is provided over its length with a plurality of star shaped centring rings of spring steel acting to hold the anchor rods together in the bundle and to hold them away from the wall of a bored hole into which the anchor is inserted in use.

9. A rock anchor according to any one of claims 1 to 8 wherein the anchor rods are manufactured from grade RM 90 steel.