WO2006105557A1

WO2006105557A1 - Rock anchor

Info

Publication number: WO2006105557A1
Application number: PCT/ZA2006/000042
Authority: WO
Inventors: Johann Smit
Original assignee: Grinaker-Lta Limited
Priority date: 2005-03-29
Filing date: 2006-03-27
Publication date: 2006-10-05

Abstract

A rock anchor which is made under factory conditions and which has an elongate body of circular cross section formed from a yieldable material in which is embedded a part of an elongate shank which has a conical formation at one end and a threaded opposing end which projects from the yieldable material.

Description

ROCK ANCHOR

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to the reinforcement of rock and more particularly is concerned with a rock anchor for use for this purpose, and to a method of forming a rock anchor.

[0002] A large variety of rock anchors, rock bolts and the like exist. Mechanical anchors which make use of an expanding head which is positioned in a hole in a rock face provide a point anchor system. Friction stabilisers and the like provide frictional engagement of an elongate anchor which usually is of tubular form, over the entire length of the device. In another approach elongate shanks or reinforcing rods are secured in position by means of adhesives, resins, grouts and so on.

[0003] Each of the aforementioned systems has its own advantages, such as ease of manufacture or use, and disadvantages such as high cost, time of installation and the like.

[0004] A drawback in many instances is that although a rock bolt or rock anchor may provide a yield characteristic, as movement of the rock in which the rock bolt is installed takes place, the yield characteristic is not necessarily repeatable. This means that it is not possible, with a fair degree of accuracy, to relate the extent to which a rock bolt has yielded to the loading force applied to the rock bolt.

[0005] A related problem which is associated with a bolt which is installed using a grout or resin is that the bolt performance is dependent on the quality of the grout or resin mix. If the mix is too "strong" the bolt can break and not exhibit a yielding characteristic whereas, if the mix is too "weak", the bolt's load-bearing capacity is reduced. Factors of this type can imply a significant degree of unreliability.

[0006] Other negative issues which arise with grout and resin bolts are the time needed for installation, transport problems which can be pronounced in the arduous conditions which can be encountered underground, and the cost of the resin or grout, an aspect which is exacerbated if a bolt is materially smaller than a hole in which the bolt is installed.

[0007] The invention is concerned with a rock anchor which at least partly addresses these problems.

SUMMARY OF INVENTION

[0008] The invention provides a rock anchor which includes a body which is formed at least from an elongate member which is at least partly embedded in a yieldable material.

[0009] Preferably the embedding process takes place at a location which is remote from the location at which the rock anchor is used e.g. under factory conditions.

[0010] The body may be elongate and may have a substantially round external cross section.

[0011] The body may include retaining, keying, gripping or similar formations on an outer surface. Preferably these formations, or additional formations, are adapted to facilitate an effective and efficient mixing of a settable material, e.g a resin or grout, when the body is placed in a hole in rock. [0012] The elongate member may include a shank, cable, tendon or the like. A first end of the elongate member may be located at one end of the body and project from the yieldable material. An outer surface of the elongate member, i.e. a surface in contact with the yieldable material, is preferably smooth.

[0013] A de-bonding agent may be located between the elongate member and the yieldable material to facilitate yielding movement of the elongate member through the yieldable material, in use of the rock anchor.

[0014] A fastener may be attachable to the first end in any appropriate way. The first end may for example be threaded and a nut may be threadedly engagable with the first end.

[0015] A restraining component or formation such as a cone or wedge may be attached to, or be formed integrally with, the elongate member, for example at a second end thereof which is remote from the first end. The restraining formation may be of any suitable shape and size. Its function is to exert force on the yieldable material thereby to provide a yielding characteristic for the rock anchor during use thereof.

[0016] The invention further extends to a rock anchor installation which includes a rock anchor of the aforementioned kind which is secured in a hole in a rock face using a settable material.

[0017] The settable material may be a resin, an adhesive or a cementitious grout or the like and it may at least partly surround an external surface of the body thereby substantially filling all voids which otherwise would exist between a wall of the hole and an opposed external surface of the body. A primary function of the settable material is to create a "self-actuating" frictional lock, or engagement, of the body with a surrounding wall of a hole in which the body is placed.

[0018] The invention further extends to a method of forming a rock anchor which includes the steps of placing an elongate member in a mould, introducing a fluent material into the mould so that the elongate member is at least partly embedded in the material, allowing the material to set and, once the material has set, removing the material and the elongate member from the mould.

[0019] The mould may be of any suitable shape and size and preferably includes two or more mould components which, in combination, form an enclosed tubular cavity which accommodates a portion of the elongate member, with a first end of the elongate member projecting from the cavity.

[0020] The material in which the elongate member is embedded is chosen so that in use it can exhibit a desired yielding action, as is explained hereinafter.

[0021] The material may be of any appropriate kind. It may for example be cementitious or be plastic-based. The invention is not limited in this regard. A requirement however is that it should be possible to shape the material in a mould under factory conditions.

[0022] In forming the rock anchor of the invention a de-bonding agent may be applied to the elongate member before it is embedded in the material which is referred to herein as "yieldable material". Thus once the body is fixed in position in a hole in a rock face the yieldable material is adhered, or frictionally locked, to a surface of the hole and the elongate member is movable relatively to the material in a manner which is determined by the yield characteristics of the yieldable material without a strong influence being exerted thereon by a bond between the yieldable material and the elongate member.

[0023] The invention also provides a method of supporting rock which includes the steps of fixing a preformed body of yieldable material inside a hole in the rock, engaging an end of an elongate member which is at least partly embedded in the yieldable material with a surface of the rock around a mouth of the hole, and allowing the elongate member to move through the body which exerts a yielding action on the elongate member, with movement of the rock surface relatively to a remainder of the rock.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The invention is further described by way of example with reference to the accompanying drawings in which:

Figure 1 is a perspective view of a mould which is used for the manufacture of a rock anchor according to the invention; Figure 2 is a plan view of a mould section which is one of two substantially identical sections used in the mould of Figure 1 ;

Figure 3 depicts a rock anchor which is made using the mould of Figure 1 ;

Figure 4 shows in cross section the rock anchor of Figure 3 installed in a hole in a rock face; and Figure 5 is similar to Figure 4 and shows the rock anchor in use.

DESCRIPTION OF PREFERRED EMBODIMENT

[0025] Figure 1 of the accompanying drawings illustrates a mould 10 which is made to a suitable length 12 and diameter 14. The mould includes two half sections designated 16 and 18 respectively. Figure 2 shows the section 18 in plan, from an inner side, with the section 16 detached therefrom. Each mould section essentially comprises an elongate trough of semi-circular cross section.

[0026] The section 18 is described hereinafter but it is to be understood that the section 16 is substantially the same as the section 18.

[0027] At one end the section 18 has an end wall 20 in the nature of a semi-circular disc which seals the end of the trough-shaped channel. At an opposing end a semicircular disc 22 is attached to the mould section but this disc is formed with a semi¬

circular cut out 24 which accommodates half of the cross sectional dimension of a shank 26 of an elongate member 30, as is described hereinafter.

[0028] The mould section 18 is formed with a number of externally projecting ribs 32 which, on an inner surface of the mould section, define complementary grooves.

[0029] The two mould sections can be attached to each other by means of one or more hinge connections 34, see Figure 1 , or can otherwise be detachably secured to each other in any appropriate way. When the section 16 is placed on the section 18 an enclosed cavity of tubular form is defined. The projecting ribs 32 on the mould sections are, if desired, substantially coincident to form continuous bands around the mould 10.

[0030] The mould section 16 can include one or more filling apertures and gas escape apertures 38, according to requirement. [0031] The elongate member 30 is placed in the mould section 18. It is to be understood that the construction of the elongate member may be varied according to requirement and that the following description is exemplary only and not restrictive.

[0032] The elongate member includes, as noted, an elongate shank 26. A first end of the shank, designated 40, has external threads 42. This end projects through a hole which is formed by registering cut outs 24 in the mould sections 16 and 18. A second end 44 of the elongate member has a conical formation or member 46 which is integrally formed with the shank. This formation extends over a length 48, the magnitude of which determines an angle 50 of the conical member. This angle can be varied according to requirement in accordance with design and empirical data.

[0033] Before the elongate member 30 is placed in the mould section 18, it is coated externally with a de-bonding agent such as a light film of grease, silicone or the like. Any appropriate agent can be used in this regard.

[0034] When the sections 16 and 18 are engaged with each other as is shown in Figure 1 the elongate member 30 is automatically centrally positioned inside the mould 10. A yieldable material 52, from a suitable source which is represented by a block 54 in Figure 1, is mixed and then introduced through the filling apertures into the mould. The yieldable material 52 may be of any appropriate type and may for example comprise a cementitious material or grout, which may be aerated, a plastics material such as polyurethane and so on. Different materials can be used to obtain different yield characteristics.

[0035] The entire free volume inside the mould around the elongate member is filled with the yieldable material which, as noted, is initially fluent. With the passage of time, the^*yieldable material sets and the elongate member is then firmly embedded in the yieldable material. Setting of the material may be accelerated by the use of suitable additives, or by increasing the ambient temperature. Once the material has set the two mould sections are opened and the resulting rock anchor 60, shown from one side in Figure 3, can then be removed from the mould.

[0036] The rock anchor thus includes an elongate body 62 of generally circular cross section, of a desired length, with the threaded end 40 of the shank protruding from the body. The body has a number of outwardly projecting ridges 64 which were formed by the corresponding groove formations 32.

[0037] The rock anchor of the invention is used in the manner shown in Figures 4 and 5. Each of these Figures depicts a rock body 70 in which is formed a hole 72 using techniques which are known in the art. The rock body has a face 74. The hole 72 is formed to a desired length. The rock anchor 60 is inserted into the hole and a settable material 76 is used to fill an annular space between an external surface of the body 62 and a surrounding surface of the hole 72. This material may be a resin, grout or the like which is contained in a pre-inserted capsule which is broken upon installation of the body into the hole. The formations 64, on the body, are designed to facilitate efficient and effective mixing of the resin when the body 70 is rotated. Once the material 76 has set a load-distributing washer 78 can be engaged with the end 40 and a nut 80 can be threaded onto the thread 42 to provide a load-distributing characteristic at the face 74.

[0038] Ideally the annular gap between opposing surfaces of the body 70 and the hole 72 is small so that the resin requirement is reduced. Generally this can be ensured as the body 70 is usually formed under controlled factory conditions. It is to be borne in mind that a principal function of the resin is to fill the annular gap effectively, so that a strong frictional lock is formed between the rock anchor and the wall of the hole, an attribute which is enhanced by the provision of the outwardly projecting ridges 64 on the body.

[0039] If the rock body 70 is placed under stress and relative movement of portions thereof takes place, the shank 26 is subjected to an increased tensile loading. At a particular point which is determined inter alia by the nature and size of the yieldable material which makes up the bulk of the body 62 and by the size and shape of the conical restraining member 44, yielding occurs. When this happens the conical member moves through the yieldable material at a substantially controlled and predictable rate which, due to the use of the de-bonding agent, is not meaningfully affected by a direct bond between the elongate member and the yieldable material.

[0040] The yielding action results from the capability of the conical formation 46 to pass through the yieldable material at a steady rate which is determined by the various parameters which have been referred to.

[0041] The yielding action is dependent, at least, on the capability of the yieldable material to be compressed or otherwise deformed in a controlled manner so as to permit the passage of the conical member. This action can however only take place in a predictable manner if the body 62 is firmly frictionally locked to the wall of the hole by the resin.

[0042] The rock anchor of the invention is made under factory conditions and is capable of a predictable and controlled yielding action. A further advantage is that the amount of settable material, such as resin or grout, used to fix the rock anchor in position,- is reduced, due to the capability of forming the body accurately to a predetermined diameter which allows the body to fit with a close tolerance in the hole in the rock face.

[0043] In a variation of the invention the yieldable material 52, which may be a foamed perlite or cementitious mixture, or an equivalent composition, is enclosed in a thin-walled lightweight tube, e.g. of steel, which could have the general external shape shown in Figure 3. The tube is then frictionally fixed in position using a suitable resin and the conical formation 46 is drawn through the yieldable material, inside the tube, when rock movement takes place.

Claims

1. A rock anchor which includes a body which is formed at least from an elongate member which is at least partly embedded in a yieldable material.

2. A rock anchor according to claim 1 wherein the body is elongate with a substantially round external cross section and has keying formations on an outer surface.

3. A rock anchor according to claim 1 or 2 wherein a de-bonding agent is located between the elongate member and the yieldable material to facilitate yielding movement of the elongate member through the yieldable material.

4. A rock anchor according to any one of claims 1 to 3 wherein the elongate member is selected from a shank, a cable and a tendon.

5. A rock anchor according to any one of claims 1 to 4 which includes a retaining formation, on the elongate member, which is embedded in the yieldable material.

6. A rock anchor according to any one of claims 1 to 5 wherein the body includes a tube in which the yieldable material is located.

7. A rock anchor installation which includes a rock anchor according to any one of claims 1 to 6 which is secured in a hole in a rock face using a settable material which at least partly surrounds an external surface of the body thereby substantially filling a space between a wall of the hole and an opposed external surface of the body.

8. A method of forming a rock anchor which includes the steps of placing an elongate member in a mould, introducing a fluent material into the mould so that the elongate member is at least partly embedded in the material, allowing the material to set and, once the material has set, removing the material and the elongate member from the mould.

9. A method according to claim 8 wherein the mould includes two or more mould components which, in combination, form an enclosed tubular cavity which accommodates a portion of the elongate member, with a first end of the elongate member projecting from the cavity.

10. A method according to claim 8 or 9 which includes the step of applying a de- bonding agent to the elongate member before it is embedded in the material.

11. A method of supporting rock which includes the steps of fixing a preformed body of yieldable material inside a hole in the rock, engaging an end of an elongate member which is at least partly embedded in the yieldable material with a surface of the rock around a mouth of the hole, and allowing the elongate member to move through the body which exerts a yielding action on the elongate member, with movement of the rock surface relatively to a remainder of the rock.