GB2199952A

GB2199952A - Method and apparatus for measuring load on a rock bolt

Info

Publication number: GB2199952A
Application number: GB08701049A
Authority: GB
Inventors: Peter Frank Reinholt Altounyan
Original assignee: Coal Industry Patents Ltd
Current assignee: Coal Industry Patents Ltd
Priority date: 1987-01-16
Filing date: 1987-01-16
Publication date: 1988-07-20
Also published as: GB8701049D0

Abstract

A hollow rock bolt (3) has its distal end (6) secured in a hole (2) in strata (1). A rod (5) is welded to the distal end (6) and extends up the hollow centre (4) of the bolt to actuate a transducer in a housing (10) outside the strata, to give an indication of the load on the bolt. As described the transducer is a linear potentiometer having a member (11) displaced by the rod (5). The transducer housing (10) is screwed onto the projecting end (9) of the bolt, which may be used to support a mine roof. <IMAGE>

Description

METHOD AND APPARATUS FOR MEASURING LOAD ON A ROCK EDLT This invention relates to a method and apparatus for measuring load on a rock bolt and is particularly useful in its application to coal mines where the roof of the strata is bolted to strengthen it in roadways and galleries. The invention however is not limited to this application and may be used anywhere where rock bolting takes place.

The principle of rock bolting is well known. A hole is drilled into the strata to be strengthened and a bolt which may be either metal or a wooden dowelling is inserted and is secured by cementing. Modern bolting techniques use steel bolts which are secured by a resin. Normally the resin is a two part resin which is inserted into the hole and the bolt pierces the resin capsules and mixes the resin which then sets to secure the end of the bolt at the distal end of the hole. It is of course possible for the whole of the bolt to be surrounded by resin. The bolt length can vary from 6 metres to 30 metres depending on the application and the type of strata to be strengthened. As the strata moves the bolt resists the movement and is itself loaded. In order to ensure safe working conditions it is essential that the load on the bolt is known.

The actual load has been determined in many cases by the use of load cells applied to the bolt. However, this type of measurement is not wholly accurate and is also expensive.

It is an object of the present invention to provide a method and apparatus which measures the load on a rock bolt in a simple cheap and reliable manner.

According to a first aspect of the present invention a hollow longitudinally extending bolt has a central bore along its entire length and a rod positioned along the axis of the bore and secured at one end to the corresponding end of the hollow bolt, the other end of the rod being free to move in the bolt and the said other end having means provided thereon for securing a transducer thereto to enable relative longitudinal movements between the rod and the bolt to be monitored and to give an indication of the load on the bolt.

The bolt itself is normally a steel tube and the rod is also of steel of a different nature and maybe welded at the one end to the bolt. Preferably at this end the bolt and the rod are made flush on their outer surface.

The means for securing the transducer to the bolt is preferably by means of a threaded engagement between the means and the bolt and the transducer means suitably is in a rugged housing preferably of metal.

The transducer means is suitably a potentiometer type of device and if the transducer is being used in coal mines the transducer is designed to be intrinsically safe in its operation.

According to a second aspect of the present invention a method of measuring the load in a rock bolt of a kind in which the rock bolt has a hollow outer part and a longitudinally extending co-axial rod within the tube running along its length, the rod being secured to the hollow outer tube at one end, camprises inserting the rock bolt into a borehole in strata, securing the said end, at least, within the borehole, attaching the transducer means to the end of the rod not solidly connected within the borehole and determining from the transducer means differential movement between the rod and the hollow tube of the bolt and thus deriving a measure of the load on the bolt.

In order that the invention may be readily understood one example of apparatus in accordance therewith and the method of operating it will now be described with reference to the two figures of the accompanying drawings. In the figures Figure 1 shows in diagramatic form the apparatus insitu in a hole in strata and Figure 2 is a graphical demonstration of the loading of the apparatus of Figure 1.

Referring now to Figure 1 of the drawings strata in a coal seam is indicated by the numeral 1 and a hole 2 is drilled in the strata to accomodate a rebar type rock bolt indicated generally at 3 having a hollow centre 4 in which a steel rod 5 extends co-axially with the tubular bolt 3. The rod 5 is welded at 6 to the end of the bolt 3 and the rod and the bolt are machined to give a flush face at the distall end of the bolt in the hole 2.

The bolt 3 is secured in the hole by a resin grout which firmly holds the bolt to the walls of the hole.

At the other end of the bolt the bolt extends beyond the face 8 of the strata for a short distance and has at its end 9 a thread formed on it onto which the housing 10 of a transducer is secured.

The transducer is basically a 10 kilo-ohm linear potentiameter and the active part thereof is a linear member 11 which goes against the end of the rod 5. For use in a coal mine the transducer is designed to be intrinsically safe and to operate at suitable intrinsic voltages.

In use the bolt is secured in the hole 2 by the resin 7 and the housing 10 is screwed onto the end 9 of the bolt projecting fram the wall 8. As the load on the bolt increases so the member 11 is moved and the potentiometer notes the change in voltage. A cable 12 carries the voltage to a remote indicating device which gives a linear readout of the load.

It will be appreciated that in order to ensure that a correct reading is given it is necessary to set a reading for the apparatus first of all a zero reading is taken and the bolt displacement can be calculated from the following formula: Displacement (nan)

where: Rz= Zero reading resistance across potentiometer rz= Zero reading resistance across wiper Ra= subsequent reading resistance across potentiometer ra= subsequent reading resistance across wiper The average load (in KN) on the bolt can be obtained from either a table or Figure 2 using the displacement obtained above and the bolt length.The approximate equation for calculating the load on a bolt is given below: LOAD (KN) = 90040 x bolt extension (trip) bolt length (mm) In this particular example the figures were as follows: Rz= Zero reading resistance across potentiometer = 9.76 K# rz= Zero reading resistance across wiper = 1.27 K# Ra= subsequent reading resistance across potentiometer = 9.65 K# ra= subsequent reading resistance across wiper = 2.43 K; Displacement (mn) D = 10 x (2.43 ~ 1-274 (9.65 9.76) Displacement = 1.216 mn which for a 1.8 metre bolt and using Figure 2 gives a mean load on the bolt of approximately 60 KN or 6 Tonnes.

It will be appreciated from this that it is very easy constantly to monitor the load on a rock bolt and to determine any changes as they take place.

Claims

CLAIMS:

1. Apparatus comprising a hollow longitudinally extending bolt having a central bore and a rod extending along its length and secured to one end, the other end of the rod being free to move in the bolt, the said other end of the bolt having means for securing a transducer thereto to enable relative movement between the rod and the bolt to be monitored and to give an indication of the load on the bolt.

2. Apparatus as claimed in claim 1 in which the rod is steel and is welded to the said one end of the bolt.

3. Apparatus as claimed in claim 1 or claim 2 in which the means for securing the transducer to the bolt includes a threaded connection between the transducer and the bolt.

4. Apparatus as claimed in any preceding claim in which the transducer is of the potentiometer type.

5. Apparatus substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

6. A method of measuring the load on a rock bolt wherein the rock bolt comprises a hollow outer part and a longitudinally extending rod co-axial with the outer part and running along its length, the rod being solidly connected to the hollow tube at one end, comprising inserting the solidly connected end into a bore hole in strata, securing the said end, at least, within the bore hole, attaching transducer means to the end of the rod not solidly connected and determining from the transducer means the differential movement between the rod within the hollow tube and thus the load on the bolt.

7. A method as claimed in claim 6 in which the transducer is electrically operated and is attached to an indicating device.

8. A method as claimed in claim 7 in which the indicating device is arranged to give a linear indication of the load on the bolt.

9. A method of measuring the load on a rock bolt substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.