GB2349224A

GB2349224A - Improvements in or relating to the monitoring of reinforcing tendons

Info

Publication number: GB2349224A
Application number: GB0004903A
Authority: GB
Inventors: Alan Stuart Bloor
Original assignee: Rock Mechanics Technology Ltd
Current assignee: Rock Mechanics Technology Ltd
Priority date: 1999-03-04
Filing date: 2000-03-02
Publication date: 2000-10-25
Also published as: GB9904825D0; GB0004903D0

Abstract

A method of and apparatus for monitoring the integrity of reinforcing tendons includes the steps of applying a current or a signal to an electrical circuit including the tendon 2, the tendon either being electrically conductive or incorporating a conductor embedded therein, and measuring the resistance and/or the distributed capacitance and/or the distributed inductance of the circuit, changes in these parameters being indicative of deformation or failure of the tendon. The circuit may include a resistor to allow detection of failure modes in which there exists a short-circuit between the tendon and a sensing conductor 4 of the circuit.

Description

IMPROVEMENTS IN OR RELATING TO THE MONITORING OF REINFORCING TENDONS This invention concerns improvements in or relating to the monitoring of reinforcing tendons installed within a host body, for example geological strata.

In our co-pending United Kingdom Patent Applications Nos 95 17173.2 (2 304 417) and 98 14455. 3 we describe methods of monitoring such tendons with the use of radio frequencies to determine the integrity of the relevant tendons which are subject to geological forces occasioned by for example the extraction of minerals by mining operations. For obvious safety reasons, it is at least desirable if not essential to monitor closely the effect of extraction on the strata overlying and surrounding the mine workings.

In our earlier patent applications referenced above, we described a technique which in essence comprised the use of a tendon as a radio frequency transmitter the integrity of which is demonstrated by the signal : any aberrations in the signal frequency indicate a change of status and thus a shift in the strata. Whilst these methods have proved to be effective, we have been investigating ways of improving the efficacy and immediacy of the monitoring with a view to enhancing safety.

An object of the invention is therefore to provide an improved method of and apparatus for monitoring a reinforcing tendon.

According to a first aspect of the invention one method of monitoring the integrity of an electrically conducting reinforcing tendon installed within a host body includes the step of incorporating the tendon within a circuit, applying a current or a signal to the circuit, and monitoring the resistance and/or the distributed capacitance and/or the distributed inductance of the circuit thereby to detect any change in the circuit status indicative of tendon integrity.

According to a second aspect of the invention there is provided apparatus for monitoring the integrity of a reinforcing tendon including in combination an electrically conducting tendon, a circuit in which the tendon is included, means for applying a current or signal to the circuit, and means of monitoring the resistance and/or the distributed capacitance and/or the distributed inductance of the circuit thereby to detect any change indicative of tendon integrity.

Conveniently, the circuit may be an electrical circuit to which in use a current of electricity is fed, the monitoring of the resistance and/or the distributed capacitance and/or the distributed inductance being the current continuity whereby any change from continuous to open-circuit or short circuit is indicative of a change in the integrity of the tendon; in particular, the general indication will be that the tendon has failed.

The host tendon may itself be made from electrically conducting material; for example the tendon may be of solid metal construction or may be constructed of metal strands in the manner of steel wire rope. In the alternative the tendon may be made of non-electrically conducting material and in this case the tendon incorporates at least one conductor element embedded therein thereby to render the tendon electrically conducting.

The method further includes the employment of an electrical conductor provided in association with the tendon, the conductor being insulated from the tendon and being connected to one end of the tendon itself, or to an end of the conductor element implanted therein. The conductor is chosen to display such characteristics of strength as to ensure that rupture thereof will not occur significantly before the failure of the host tendon. A physically small resistor may be included in the circuit to avoid false readings of tendon integrity which a short circuit fault at the terminal or within the tendon might give. For stranded steel tendons in particular, the failure process can result in a short-circuit of the conductor at the failure site. The value of the resistance is chosen so that it will not easily be mistaken for a near-short or near-open circuit. For example the use of a resistance of 120 ohms enables integrity readings to be made with an instrument designed to work with standard electrical resistance strain gauges.

The resistance introduced into the circuit for the purpose of distinguishing short-circuits may be distributed uniformly along the tendon by using a sensing conductor of high specific resistance and/or small cross-sectional area. In the event of the tendon failure process resulting in a short-circuit, the location may be determined from the change in the resistance of the sensing circuit.

In the case where the distributed capacitance of the electrical circuit is monitored, an electrical conducting wire having external insulation of a relatively high dielectric constant is arranged in contact with a conducting surface of the tendon or with a similar parallel wire and the distributed capacitance between the tendon and the wire is monitored to detect any change whereby the integrity of the tendon is determined with the location of any failure being calculated from the change in distributed capacitance value.

The means of measuring the resistance of a sensing circuit may be any commonly available direct-current instrument which measures current and/or voltage, or a DC resistance-bridge.

The means of measuring the resistance, capacitance or inductance of a sensing circuit may be any commonly available alternating-current instrument which measures current, voltage and phase difference or a function of these, known generally as LCR meters or LCR bridges.

In the case of distributed resistance, capacitance and inductance, the means of measurement may be any commonly available alternating-current instrument of continuously variable frequency which measures the resonant frequencies of the sensing circuit By way of example only, methods and apparatus for the monitoring of reinforcing tendons installed within a host body according to the invention are described below with reference to the accompanying diagrammatic drawings in which: Figure 1 is a diagrammatic longitudinal sectional view of a tendon; Figure 2 is a cross sectional view on the line II-II of Figure 1; Figure 3 is an electrical circuit for the tendon, showing the circuit intact; Figure 4 shows the circuit of Figure 3 with an open circuit break; and Figure 5 shows the circuit of Figure 3 with a short circuit break.

Referring to Figures 1 and 2, there is shown a tendon 2 in a single circuit arrangement with a conductor 4 in an insulation sheath 6 accommodated within a slot 8 formed in the surface of the tendon in the case of a solid tendon or within the stranding of a steel wire woven tendon. In this example the diameter of the sheathed conductor is < 1mm and the length of the tendon may be in the range of lm to 10m. With these characteristics, the capacitance of the circuit is typically several hundred pF per metre and thus easily measurable by standard techniques.

A jack socket housing 9 is secured to the end of the tendon 2 and provides a socket 10 with connections 12,14 for the conductor 4 and the tendon 2 at 11 respectively, the end of the conductor remote from the socket being connected to the tendon at 13 (Figure 3). A jack plug 16 is shown and is connected via a power cable (not shown) to the means of measurement (not shown).

This particular arrangement facilitates the rapid monitoring of a number of tendons, using a portable measuring means.

Figure 3 shows the tendon 2 in dotted outline and represents the circuit with the distributed resistance of the conductor 4 being shown at 5 and the distributed capacitance at 7.

The initial values of these distributed parameters are measured and known for the intact tendon.

In the event of tendon failure, normally to open circuit 20 as shown in Figure 4, the distributed capacitance of the now shorter tendon would be measured, thus indicating the location of the failure.

In the event of a short-circuit 30 arising within the length of the tendon, as indicated in Figure 5, which is possible with stranded tendons during failure, the measurement of capacitance would be compromised by the short-circuit, but the reduced resistance of the now shorter tendon would still indicate the location of the failure. An added advantage of such an approach is that the conductor will be stretched as the tendon is deformed and will act as a long electric resistance strain gauge, thus providing an indication of the tendon deformation prior to a failure.

In an alternative arrangement, the connection 13 at the remote end of the tendon is omitted, and the capacitance measured routinely until failure, when the location of the failure would be indicated by either a capacitance or a resistance measurement.

The present invention thus offers a relatively simple and yet effective method of monitoring the integrity of a reinforcing tendon by utilising its conductive or parametric characteristics thereby to determine any change indicating failure of the tendon and the site of such failure.

It is to be understood that whilst the host body may be geological strata, the invention is also applicable to civil engineering structures, such as bridges, embankments and the like.

Claims

CLAIMS 1. A method of monitoring the integrity of an electrically conducting reinforcing tendon installed within a host body includes the step of incorporating the tendon within a circuit, feeding a current or a signal to the circuit, and monitoring the resistance and/or the distributed capacitance and/or the distributed inductance of the circuit thereby to detect any change in the circuit status indicative of tendon integrity.
2. A method according to Claim 1 in which the circuit is an electrical circuit to which in use a current of electricity is fed, the property being monitored being the electrical resistance whereby a change in resistance is indicative of a change in the integrity of the tendon.
3. A method according to Claim 1 in which the circuit is an electrical circuit to which in use an alternating current of electricity is fed, the property being monitored being the distributed capacitance of the circuit, whereby any change in capacitance is indicative of a change in the integrity of the tendon.
4. A method according to Claim 1 in which the circuit is an electrical circuit to which in use an alternating current of electricity is fed, the property being monitored being inductance of the circuit
5. A method of monitoring the integrity of an electrically conducting reinforcing tendon installed within a host body substantially as hereinbefore described.
6. Apparatus for monitoring the integrity of an electrically conducting reinforcing tendon installed within a host body including in combination an electrically conducting tendon, a circuit in which the tendon is included, means for applying a current or a signal to the circuit, and means of monitoring the resistance and/or the distributed capacitance and/or the distributed inductance of the circuit thereby to detect any changes indicative of tendon integrity.
7. Apparatus according to Claim 6 in which the circuit includes a sensing conductor disposed on, partially within, or wholly within the tendon and insulated therefrom.
8. Apparatus according to Claim 6 or 7 in which the circuit includes a resistor.
9. Apparatus according to any one of Claims 6 to 8 in which the means of applying a current to the circuit is a direct or alternating current source.
10. Apparatus according to Claim 6 in which the means of applying a signal to the circuit is an alternating current generator of variable frequency.
11. Apparatus according to any one of Claims 6 to 9 in which the means of monitoring the circuit is an LCR type meter.
12. Apparatus according to Claims 6-11 in which the reinforcing tendon is an electrical insulator provided with an electrical conductor for the purpose of completing the monitoring circuit.
13. Apparatus for monitoring the integrity of an electrically-conducting reinforcing tendon installed within a host body substantially as hereinbefore described with reference to the accompanying drawings.