GB2153475A

GB2153475A - Anchoring of rock bolts

Info

Publication number: GB2153475A
Application number: GB08404816A
Authority: GB
Inventors: Edward Victor Byers
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-01-23
Filing date: 1984-02-23
Publication date: 1985-08-21
Also published as: GB8404816D0

Abstract

A highly permanent anchorage to a blind hole in a rock formation capable of withstanding very heavy withdrawal loads utilises a circularly corrugated malleable metal tube 1 which is expanded to fit and grip the sides of the hole by one or more spuds 3 of axially short frusto-conical form, the load being applied to a central tension rod 4 provided with an end plate at the inner end of the plug. A favoured method (Figure 8) involves withdrawing the expanding spud 3 by means of a hydraulic jack, a removeable tube between the jack and near end of the plug accepting the reaction force. The tension rod is screwed into the end plate. The spuds 3 may be force inwards in succession by percussive blows on the tubular drift 5. Alternatively, the spud 3 may be forced in by the spigotal end of a cylindrical drift (Figure 6) which is then removed so that a tension rod can be screwed into the end plate. The spud 3 may be integral with the drift (Figure 7) which also serves as the tension rod. The anchorage will be capable of taking a load of up to 10 tonnes when placed in a 40 millimetre drilling as used for blasting operations. <IMAGE>

Description

SPECIFICATION Devices for providing anchorages in rock formations This specification discloses methods and means additional to those described in the applicant's copending application No. 8401655 entitled "Fastening Device for providing Anchorages in Holes in Hard Materials" which are particularly suitable for rock anchorages which are required permanently to resist very high loadings.

The prior specification describes the use of an expandable plugging means in the form of a tube provided with a variety of possible convolutions around its circumference and which is capable of being expanded to fit and grip the walls of a hole when an expanding means in the form of a short metal spud or ball is driven through it. Similar types of plugging means may be employed in the present invention, but the preferred form is that of a plain metal tube which is deformed by passage through a shaping die or other procrustean method so that it assumes a circularly corrugated shape. The earlier specification discloses a method of using the fastening device as a rock anchor which accepts the axial load at the nearer end of the plugging means, but in the present invention the load is taken by the remote end of the plugging means.There are also certain differences in the types of expanding spud used and additional methods for driving them.

Referring to the drawings accompanying this specification and in which like numerals indicate like parts: Figure 1 shows an end view of a suitable plugging means which is hereinafter termed the plug: Figure 2 shows the shape assumed by the plug when it is expanded to fit and grip the sides of a hole: Figure 3 shows a type of expanding spud which is used for large diameter anchorages: Figure 4 depicts the action of the spuds on the plug: Figure 5 shows a method of placing an anchorage of large diameter: Figure 6 shows an alternative spud driving method: Figure 7 shows the use of a combined spud and force-resisting central rod for anchorages of small diameter: Figure 8 describes a different method of expanded ing the plug.

Figure 1 shows a plug 1 in the preferred form for a rock anchorage and is made by the deformation of a plain circular tube by passage through a shaping die or the radial approach of the appropriate number of Vee-shaped forming tools. The corrugation must have at least three lobes, but will usually have not less than six. A plug of eight lobes is shown. Corrosion-resistant metals such as copper or aluminium are suitable for most purposes, but it will sometimes be necessary to use stainless steel to resist buckling of the end of the plug when subjected to very high forces. Tube thicknesses of three millimetres and upward are envisaged as being suitable for rock anchorages. Plug lengths of 20 centimetres or more will usually be required, but it is possible to use two or more plugs in series down the anchor hole.

Figure 2 shows how plug 1, having a diameter which permits it to be inserted readily into a hole in an unyielding material 2, becomes distorted to fit and grip the sides of the hole when expanded by the forced passage of an expanding spud 3. The proportions of the plug and expanding spud must be chosen in relation to the hole size so that there remains some free space in the annular region between the hole sides and the spud as there is otherwise a risk that the spud will be undriveable. It is also essential that some degree of plastic deformation occurs to that the plug is left with a considerable amount of circumferential compressive stress after the spud has passed by, this stress ensuring that the plug remains firmly sprung outward to continue its grip indefinitely.This type of plug will cater for variations in nominal hole diameter of up to five per cent, which is expectable when drilling through rock.

Figure 3 shows a ring shaped spud for use in plugs of diameter greater than five centimetres.

The spud is made of a hard metal and is of conoidal form so that the minor diameter will readily enter the undistorted plug and the conical form is curvaceously blended into the major diameter. The spud 3 is shown in both perspective and sectional aspects, the latter showing the spud in position to enter a plug 1. The cylindrical surface of the spud must be quite short, preferably not more than two millimetres so that the frictional interaction between the spud and plug does not seriously interfere with the driving process. For instance, a spud having a length of about three plug diameters is virtually undriveable even though the relaxing effect of the plug is very slight.

Figure 4 is a cross-sectional view of the plug 1 under the influence of two spuds 3. A plurality of spuds of successively increasing diameters, driven in one after the other, may be used in this way so that the driving force required for each of them is not inordinately large.

Figure 5 shows a complete anchorage in the course of being fixed in position. A tie rod 4 is first inserted into the hole and is furnished with a screw thread or other attachment means at its upper end, the other being provided with a disc-shaped enlargement to engage the remote end of the plug, which is inserted afterwards. At 3 are shown two spuds of the ring shaped type already described: these are driven through the plug by a tubular drift 5, which is afterwards withdrawn, leaving the anchor ready for use. With this and other types used with a spud-driving drift, force may be applied to the drift by means of percussive blows such as may be generated by a power-driven hammering machine. Alternatively, provided that a suitable reaction resisting abutment is available in the form of, for instance, a weighty vehicle, the drift may be forced home by a hydraulic or other type of jack.

Figure 6 shows a type in which the tie rod is in

Claims

two separate parts, a disc 6 provided with a central threaded hole capable of accepting the threaded end of a tie rod 8. The ring-shaped spud 3 is shown about to be engaged by the spigoted end of a cylindrical driving drift 7. Tie rod 8 is passed through the hole in the spud and screwed into disc 6 after the removal of the drift 7. Figure 7 shows a type which is suitable for holes of small diameter when it becomes difficult to use a sufficiently robust ring-shaped spud owing to the consequent reduction in diameter and tensile strength of the tie rod. In such a case, the tie rod 10 may bear a conoidal enlargement 3 which does not differ in any other respect from the spud types already described. The upper end of the rod is threaded or otherwise fashioned to accept external load and the lower end is threaded to screw into the centrally threaded hole of a disc 6 which applies the load to plug 1 by transmitting it through a tubular spacing member 9 which is sufficient size to accept the expanding enlargement without hindrance. After the rod has travelled sufficiently far to force enlargement 3 right through plug 1 it becomes freely rotatable and may be screwed into disc 6.Alternatively, provided that a jacking means is available for at least withdrawing the expanding means, the latter may be in the form of a plain rod with end enlargement as shown for rod 10, but without the threaded end portions. After the expander of this type has been driven through plug 1, it may then be withdrawn and a tie rod as shown at 8 in Figure 6 screwed into disc 6. In such a case, spacer tube 9 need not necessarily be used. The force required to remove the expanding means is only a small fraction of that needed to drive it home. This type of anchor is suitable for use in holes of 40 millimetre diameter which is a size in very common use for the explosive cartridges used in blasting operations and a retention capability of 10 tonnes is expectable in such a hole. Figure 8 shows a method of expanding a plug by means of a spud 3 which is drawn through plug 1 by tensile force. A force-applying disc 6 with a threaded central hole for the eventual acceptance of a tie rod as shown at 8 in Figure 6 is first inserted into the drilled hole and is, desirably, provided with serrations on its lower face to bite into the rock and provide a torque reaction when the tie rod is screwed in. The plug 1 is slipped over a jacking rod 12 bearing a threaded portion 11 at its lower end so that an expanding spud 3 with corresponding central thread may be screwed on. The other end of rod 12 terminates in piston 14 which works in a hydraulic jacking cylinder 15, this being retained in an appropriate position relative to plug 1 by means of an interposed reaction tube 13.The spud is forced through the plug by the action of hydraulic pressure applied to the lower face of piston 14 and the assembly is then removed to permit the insertion of the tie rod. The jack rod and reaction tube may both be of considerable length to permit the plug 1 to be very deeply inserted into a hole in the rock formation to obtain the advantage of a great weight of overburden. Only a single expander spud need be used as the jack cylinder may be designed to provide a very high withdrawal force. The withdrawal force may also be provided by other types of jack utilising screws or ratchets and racks. CLAIMS

1. An anchoring device for providing an axial force resistant connection to a blind circular hole in a body of rock or other rock-like material comprising an expandable plugging means made of malleable metal and an expanding means consisting of one or more hard metal spuds which are driveable through a central aperture in the plugging means to cause it to increase in external size to engage and grip the sides of the hole and a centrally located tension rod having at its remote end an integral or attachable enlargement to engage the remote end of the plugging means.

2. An anchoring device as in Claim 1 in which the plugging means is formed from a plain cylindrical metal tube which is forced to assume a circularly corrugated shape of not less than three lobes by forced passage through a shaping die or by other procrustean method.

3. An anchoring device as in Claims 1 and 2 in which the expanding spud is an axially short ring of frusto-conical external shape.

4. An expanding spud as in Claim 3 in which the central hole is additionally provided with a screw thread.

5. An anchoring device as in Claim 1 in which the tension rod is threaded or otherwise fashioned at its near end to accept a load-bearing connection.

6. A tension rod as in Claims 1 and 5 on the remote end of which is formed an integral discshaped enlargement to engage and transfer axial tension to the remote end of the plugging means.

7. A tension rod as in Claims 1 and 5 which is provided at its remote end with a screw thread to engage the correspondingly threaded hole in a disc which engages the remote end of the plugging means.

8. An anchoring device as in Claim 1 in which a tension rod as in Claim 5 is combined with an expanding means consisting of a frusto-conical enlargement near its remote end beyond which a threaded central extension is provided to engage and connect to a central threaded hole in a disc which transfers force to the remote end of the plugging means by means of an interposed tube having dimensions which permit free rotation of the rod after the expanding enlargement has been driven clear of the plugging means.

9. A tubular driving drift for applying force to an expanding spud as in Claim 3 for an anchoring device as in Claim 1.

10. A cylindrical driving drift for engaging on a central spigot an expanding spud as in Claim 3 for an anchoring device as in Claim 1.

11. A plugging means expanding spud for an anchoring device as in Claim 1 having a centrally threaded hole as in Claim 4 which is used to connect said spud to a withdrawal rod by means of a jacking device outside the anchorage hole, reaction force between the jack and near end of the plugging means being provided by a subsequently removeable tube, a tension rod as in Claim 7 then being engaged to the previously inserted disc which engages the remote end of the plugging means.