GB2211259A

GB2211259A - Installation of tensioned rockbolts

Info

Publication number: GB2211259A
Application number: GB8724525A
Authority: GB
Inventors: Stuart Woodlock
Original assignee: Societe dExplosifs et Produits Chimiques SA
Current assignee: Societe dExplosifs et Produits Chimiques SA
Priority date: 1987-10-20
Filing date: 1987-10-20
Publication date: 1989-06-28
Anticipated expiration: 2007-10-20
Also published as: GB8724525D0; GB2211259B; AU605368B2; AU2375688A

Abstract

A rockbolt 1 is rotated to puncture a resin cartridge by means of a tool 6 engaging two (or one or three) radial pins 5 which are adapted to shear when the allied torque increases as the resin sets, so that then the nut 3 can be freely advanced along the thread 2 by the tool 6 into abutment with a rock bearing plate (not shown) 10 apply tension to the rockbolt 1. <IMAGE>

Description

IMPROVEMENTS IN AND RELATING TO THE INSTALLATION OF TENSIONED ROCKBOLTS This invention relates to rockbolts of the kind used for strata reinforcement in mine roofs and the like.

In the installation of such rockbolts, a bonding agent, generally a settable resinous composition contained in one or more frangible cartridges, is placed at the closed end of a borehole drilled in the rock strata and a rockbolt having a threaded end portion is inserted into the borehole with its threaded end portion protruding from the end of the borehole. A washer (or bearing plate) and a nut are positioned on the threaded end portion and the bolt is then rotated so as to be driven into the borehole and thereby puncture the resinous cartridge and cause the settable resinous composition to be distributed around at least the end of the rockbolt. The rockbolt is then held in a fixed position for a short time to allow the resinous composition to harden and thereby fix the bolt in the borehole.Thereafter a tension can be applied on the bolt by tightening the nut on to the bearing plate.

In one known rockbolt system, the initial rotation of the rockbolt is effected by applying a relatively low torque to the nut, which is prevented from running up the threaded end portion of the bolt at the low torque applied. When the rockbolt is fixed in the borehole, a higher torque is applied to the nut which is sufficient to overcome the resistance which prevented the nut from running up the thread during the initial rotation period and is sufficient to tighten the nut on to the bearing plate so as to tension the bolt.

Various ways have been tried of modifying such nuts to prevent the nut from rotating under the low torque initially applied. The nut can, for example, be provided with a thread pitch which is sufficiently different from that of the threaded end portion of the bolt to permit rotation of the nut and the rockbolt as a unitary member below a predetermined torque.

Alternatively, the nut may be provided with deformations, such as by crimping, so as to increase to a predetermined value the torque required to screw the nut on to the bolt. Another way of preventing the nut from rotating below a predetermined torque is to provide the nut with a plastic insert which extends only part of the way into the threaded interior of the nut and which is capable of being retained within the nut until a predetermined torque is applied after the rockbolt has been fixed in the borehole.

The known modified nuts described above all suffer from the disadvantage that, when the predetermined torque is applied to tension the bolt, there is invariably some residual resistance on the nut, since the nut is not entirely free to engage with the threaded end portion of the rockbolt. Consequently a proportion of the torque applied to the nut in order to tension the rockbolt is employed in putting torsion into the bolt. Thus the highest possible tension for a given torque is not achieved and the torsion in the rockbolt reduces the axial applied load on the bolt.

It is an object of the present invention to provide a rockbolt assembly which avoids the disadvantage of the known modified nuts and enables the nut to be free running, so that the maximum tension in a rockbolt for a given torque can be achieved.

According to the invention, there is provided a rockbolt assembly comprising an elongate rockbolt having a threaded end portion and its other end adapted to be anchored in a borehole, a nut capable of threadedly engaging said threaded end portion in a free running manner, a bearing plate having an aperture therein of a size sufficient to allow it to be positioned on the threaded end portion of the rockbolt so as to be capable of being forced against a surface by rotation of the nut to cause tensioning of the rockbolt when in use, and wherein the threaded end portion is provided with shear means enabling the rockbolt to be rotated in use below a predetermined torque without affecting the position of the nut on the threaded end portion but enabling the nut to be freely tightened against the bearing plate when a torque equal to or greater than said predetermined torque is applied to the assembly after the rockbolt has been anchored in a borehole.

In accordance with the invention, the threaded end portion of the rockbolt carrying the bearing plate and nut is provided with shear means enabling the rockbolt to be rotated below a predetermined torque without affecting the position of the nut which is disposed, together with the bearing plate, on said threaded end portion in a location further remote from the end of the threaded end portion than said shear means, the shear means being such as to shear (or deform) at said predetermined torque without affecting the nut which remains free running and can therefore be freely rotated, after the rockbolt has been fixed in a borehole, on to the bearing plate so as to achieve the maximum tension in the bolt for a given torque at or above said predetermined torque.

The shear means can conveniently be a shear pin which is designed to shear or deform at a predetermined load. The shear pin, which may be solid or tubular, may be formed from any suitable material, for example a metal, or plastics or ceramic material, and may have any suitable size and cross-section, for example a circular or rectangular cross-section. If desired, the rockbolt may be provided with a plurality of such shear pins. The shear pin may be attached to the threaded end portion of a rockbolt in any suitable manner. For example, the shear pin can be located in a transverse cylindrical space formed in the threaded end portion with the ends of the pin protruding beyond each end of the cylindrical space. Similarly the pin may be located in a transverse slot formed at the end of the threaded end portion.Alternatively the shear pin may be welded to the end of the threaded portion of the bolt.

When, in accordance with the invention, the shear means is a shear pin, rotation of the rockbolt, at least in the initial stage when the rockbolt is rotated at a torque below the predetermined torque at which the pin will shear, can conveniently be effected by means of an adaptor tool (commonly known in the art as a dolly) one end of which is formed with a recess capable of accommodating the shear pin and at least a major portion of the nut adjacent thereto and the other end of which can be attached to a bolting machine.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a side view of the threaded end of a rockbolt assembly provided with a shear pin in accordance with the invention; Figure 2 is a side view taken at right angles to that of Figure 1; Figure 3 is a side view of the threaded end of another rockbolt assembly in accordance with the invention; Figure 4 is a side view of the threaded end of a further rockbolt assembly in accordance with the invention; Figure 5 is a side view taken at right angles to that of Figure 4; Figure 6 is a side view of the threaded end of another rockbolt assembly in accordance with the invention; Figure 7 is a bottom view of the threaded end of another rockbolt assembly in accordance with the invention;; Figure 8 is a bottom view of the threaded end of a further rockbolt assembly in accordance with the invention; Figure 9 is a cross-sectional view of an adaptor tool designed for use in rotating a rockbolt assembly as shown in Figure 1; and Figure 10 is a plan view of the adaptor tool shown in Figure 9.

Referring now to the drawings, one embodiment of a rockbolt assembly in accordance with the invention is shown in Figures 1 and 2 thereof. The assembly comprises a rockbolt 1 having a threaded end portion 2, the other end of the rockbolt (not shown) being adapted to be anchored in a borehole by means of a bonding agent, such as a settable resinous composition. The assembly further comprises a bearing plate (not shown) which is normally adjacent to a nut 3 threadedly engaged with the threaded end 2 of the rockbolt 1.

Between the nut 3 and the end 4 of the rockbolt, there is a solid cylindrical shear pin 5 which is located in a cylindrical bore drilled through the bolt. The shear pin 5 is made of nylon and is designed to shear under an applied torque of 60pdl ft (2.5Nm).

In use the assembly shown in Figures 1 and 2 (including a bearing plate) is inserted in a conventional manner in a borehole drilled in rock strata, for example, a mine roof, after a bonding agent, normally a cartridged settable resinous composition, has been located at the inner closed end of the borehole. A relatively low torque below the predetermined shearing torque of the shear pin 5 is applied to the assembly by means of an adaptor tool 6 as shown in Figures 9 and 10 of the drawings, which is attached to a bolting machine. The resulting rotation of the rockbolt drives the bolt into the borehole and into contact with the bonding agent cartridges which are consequently destroyed so that the contents thereof are mixed and allowed to set around the end of the rockbolt which, after a short time, becomes fixed in the borehole.

A higher torque above the said predetermined shearing torque of the shear pin, for example a torque of 100 pdl ft (4.2Nm), is then applied to the bolt by means of the adaptor tool 6 so as to cause the shear pin 5 to shear off and thereby allow the nut 3 to rotate, with respect to the bolt and thereby force the adjacent bearing plate against the rock face. Because the nut 3 is free running, substantially all the torque applied to the nut is converted into tensioning the bolt, so that the highest possible tension in the bolt for a given torque can be achieved and there is substantially no torsion in the bolt which would reduce the axial loading of the bolt.

Another embodiment of the invention is shown in Figure 3, in which like numerals indicate like parts with respect to Figures 1 and 2. In the embodiment shown in Figure 3, the shear pin 5 is located in a slot 7 formed in the end of the bolt 1. This embodiment can be used in a manner similar to that described with reference to Figures 1 and 2.

Figures 4 and 5 show another embodiment similar to that shown in Figure 3, the shear pin 5 having a rectangular cross-section rather than a circular crosssection as in the embodiments shown in Figures 1 and 3.

Figure 6 shows another alternative embodiment in which a solid cylindrical shear pin 5 is located in a cylindrical bore which only extends about half way across the diameter of the bolt 1.

Figure 7 shows a further embodiment in which the shear pin 5 is welded to the end of the bolt 1, whilst Figure 8 shows a similar embodiment in which three shortened shear pins 5 are welded on to the end of the bolt 1.

Claims

CLAIMS:

1. A rockbolt assembly comprising an elongate rockbolt having a threaded end portion and its other end adapted to be anchored in a borehole, a nut capable of threadedly engaging said threaded end portion in a free running manner, a bearing plate having an aperture therein of a size sufficient to allow it to be positioned on the threaded end portion of the rockbolt so as to be capable of being forced against a surface by rotation of the nut to cause tensioning of the rockbolt when in use, and wherein the threaded end portion is provided with shear means enabling the rockbolt to be rotated in use below a predetermined torque without affecting the position of the nut on the threaded end portion but enabling the nut to be freely tightened against the bearing plate when a torque equal to or greater than said predetermined torque is applied to the assembly after the rockbolt has been anchored in a borehole.

2. A rockbolt assembly as claimed in Claim 1, wherein the shear means is a shear pin located in a borehole formed perpendicularly to the axis of the rockbolt.

3. A rockbolt assembly as claimed in Claim 2, wherein the shear pin is cylindrical.

4. A rockbolt assembly substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.